The concept of the Typical Meteorological Year (TMY) was firstly introduced in the late 1970s in USA (Crow 1970; 1980; 1983), as a design tool for approximating expected climate conditions at specific locations, at a time when computers were much slower and had less memory than today. A TMY is a collation of selected weather data for a specific location, listing usually hourly values of meteorological and solar radiation elements for one-year period. The values are generated from a data bank of at least 10 years in duration. It is specially selected so that it presents the range of weather phenomena for the location in question, while still giving annual averages that are consistent with the long-term averages for the specific location. TMY sets remain in popular use until today consisting a handy tool between building designers and renewable energy systems engineers, providing them with a relatively concise set of data for system performance estimates, without the need of incorporating large amounts of data into simulation models. Commercial software packages such as TRNSYS, Energy+, PV*SOL, PVscout and PVsystand support simulations using TMY data.
Current trends in the energy production sector call for alternative energy production methods with a high focus on renewable energy sources. Most of the countries in the world, and especially the developed countries, fund research towards distributed generation and zero energy balance communities. In order to eliminate the consumption of fossil fuels, a crucial role is taken by hydrogen as a fuel, as, if it is produced from renewable energy sources, it could contribute in substituting the fossil fuels used in transport or building's thermal energy sectors. Moreover, it is well known that electrolysis-fuel cells can also be used as a storage medium in autonomous renewable energy systems. In this case, fuel cells need to be carefully sized in order to optimize the storage system both in energy and economic aspects. In this respect, a theoretical model was developed, able to simulate at any time step the operation of a Proton Exchange Membrane Fuel Cell, by using as input data the technical specifications of the cell and the hydrogen flow. The developed model is based on theoretical, experimental and semi-empirical models in order to provide a flexible algorithm in terms of fuel cell sizing. The model is validated with an existing fuel cell experimental system (Nexa 1200) at different hydrogen flow profiles. The results showed high precision which verifies the reliability of the proposed model for using it in optimization procedures.
To confront problems concerning large-scale integration of renewable energy sources, introduction of energy storage constantly gains ground. Benefits stemming from the adoption of energy storage include exploitation of otherwise rejected energy, increased reliability of energy supply and improved operation of a given power system overall. In this regard, contribution of such systems in achieving large-scale integration of wind energy into island grids is currently considered. More precisely, fuel cells and hydrogen storage (FC–HS) are investigated, in comparison with conventional batteries. For this purpose, a simulation algorithm is developed to study the energy performance of different FC–HS configurations used to recover wind energy curtailments. The developed algorithm is then applied to a representative Aegean island of medium–high quality wind potential. Results obtained indicate that FC–HS may become attractive in comparison with conventional batteries, only in the case that the use of hydrogen surplus to cover other energy flows is also put forward.
Increased interest is demonstrated recently in the emergence of prosumer schemes for the residential sector on the basis of combined RES and storage configurations. Primarily, such schemes aim to increase energy autonomy for end users. Despite providing an alternative supply solution that may secure end users from volatile energy prices, RES–battery configurations also suggest costly and, in most cases, capital-intensive solutions. As such, exploring the generation of additional revenue through market participation is an exercise worth undertaking, noting at the same time that decongestion management services may also be provided to the local grid. In this context, the current study introduces an operational framework for the market participation of RES–battery prosumer schemes, seeking to determine the optimal balance between self-consumption and market integration. For that purpose, we use typical demand patterns and perform an extensive parametrical analysis concerning system size, spot price levels and degree of market integration in the context of the Greek electricity market, with our results indicating areas of optimum balance for the minimization of similar schemes’ levelized cost of electricity.
The objective of the present work is the medium and short term forecasting of load demand (LD) in Tilos Island, Greece. For this purpose, Artificial Neural Network (ANN) models were developed to predict the LD in Tilos Island 24 hours ahead in hourly intervals (medium-term prognosis) and 24 hours ahead in 10 minutes intervals (short-term prognosis). For training the ANNs, meteorological data, covering the 2015-2017 period, were used. These data have been recorded in 1 minute intervals by a meteorological mast which has been installed in a specific location in Tilos Island. Furthermore, a biometeorological human thermal comfort-discomfort index was calculated and used also during the training procedure. For the validation of the developed ANN forecasting models well established statistical evaluation indices were applied. Results show that in all cases, for both medium and short-term LD prognoses, the developed ANN forecasting models present a remarkable ability to predict LD with high accuracy. The proposed load demand forecasting models enable the design of an energy demand information tool for end-users and transmission system operators.
Battery storage is nowadays considered a key component not only in off-grid applications but also in the context of grid-tied, residential-scale systems, facilitating the broader use of RES even in heavily congested distribution grids. Since batteries normally comprise the costliest part in similar configurations, their optimal sizing is a priority. Informed decisions to that end should not be limited to rough, initial-cost estimations alone; instead, they should also take into account the life-cycle costs of batteries, which, in turn, relate to battery aging mechanisms and the gradual, cycling-based fading of the batteries’ useful capacity. Acknowledging the above, the impact of battery cycling aging on the performance of typical, residential-scale, RES-based battery storage configurations is investigated herein, considering also the utilization of second-life lithium-ion EV batteries. To that end, we used a literature-informed empirical aging model and performed a simulation exercise for a broad set of different RES-battery configurations, with our results indicating the importance of the aging factor while also designating areas of optimum sizing with regard to the long-term energy and economic performance of similar solutions.
The exploitation of Renewable Energy Sources (RES) has been rapidly increased during the last decades, with the most significant increase being recorded in solar photovoltaic (PV) plants. The design of a PV installation is mainly based on energy yield estimates, with the site-specific solar potential being the dominant parameter. The use of solar energy measurements can lead to accurate predictions of the annual energy yield, which is the case in interconnected PV installations. However, in the case of autonomous PV systems, and more precisely in remote dwellings, matching electricity generation and demand is of high importance. Especially when equipped with relatively expensive energy storage systems, an optimum sized PV generation system will lead to a better understanding of the needs, hence reduced installation and energy production cost. In this context, the present study examines the utilization of solar radiation and air temperature values from Typical Meteorological Years (TMYs), as meteorological data input for sizing stand-alone PV systems. The advantage of using TMYs compared to the use of time series from individual years is that the values of a TMY represent the climatic conditions that are considered characteristic over a long period of time for a specific location. Therefore, by using TMY, one may design PV systems that can provide a high level of energy autonomy to remote domestic consumers with the minimum capital expenditures, in respect of long-term reliability. In this respect, the specific work examines the reliability of sizing a stand-alone PV system able to satisfy the electricity needs of a domestic consumer by using solar radiation and air temperature values from TMY time series. The results are compared to the system’s sizes resulting from simulations by using actual meteorological measurements from individual years of a 15 year period.
It is generally accepted that a climatic data set of meteorological measurements with true sequences and real interdependencies between meteorological variables is needed for a representative climate simulation. In the late 1970s the Typical Meteorological Year (TMY) concept was introduced in USA as a design tool for approximating expected climate conditions at specific locations, at a time when computers were much slower and had less memory than today. A TMY is a collation of selected weather data for a specific location, listing usually hourly values of meteorological and solar radiation elements for one-year period. The values are generated from a data bank much longer than a year in duration, at least 10 years. It is specially selected so that it presents the range of weather phenomena for the location in question, while still giving annual averages that are consistent with the long-term averages for the specific location. Each TMY data file consists of 12months chosen as most “typical” among the years present in the long-term data set. Although TMYs do not provide information about extreme events and do not necessarily represent actual conditions at any given time, they still reflect all the climatic information of the location. TMY sets remain in popular use until today providing a relatively concise data set from which system performance estimates can be developed, without the need of incorporating large amounts of data into simulation models. TMY sets for 33 locations in Greece distributed all over the country were developed, covering for the first time all climatic zones, for both historical and future periods. Historical TMY sets generation was based on meteorological data collected from the Hellenic National Meteorological Service (HNMS)network in Greece in the period 1985-2014, while the corresponding total solar radiation values have been derived through the Meteorological Radiation Model (MRM).Moreover, the generation of future TMY sets for Greece was also performed, for all 33 locations. To this aim, bias adjusted daily data for the closest grid point to the HNMS station’s location were employed from the RCA4 Regional Climate Model of the Swedish Meteorological and Hydrological Institute (SMHI) driven by the Earth system model of the Max Planck Institute for Meteorology (MPI-M). Simulations were carried out in the framework of the EURO-CORDEX modeling experiment, with a horizontal RCA4 model resolution of 0.11 o (~12 x 12 km). We used daily data for four periods: the1985-2014 used as reference period and the 2021-2050, 2046-2070 and 2071-2100 future periods under RCP4.5 and RCP8.5 scenarios. This work was carried out in the framework of the “Development of synergistic and integrated methods and tools for monitoring, management and forecasting of environmental parameters and pressures” (KRIPIS-THESPIA-II) Greek national funded project.
Recent trends in the energy sector require the transition to alternative technologies, with interest focusing on renewable energy technologies and zero-emission fuels. In order to eliminate conventional fuel consumption and decarbonize the energy sector, hydrogen keeps a vital role as a fuel. More specifically, if hydrogen production utilizes energy provided by the Renewable Energy Sources (RES), it could reduce fossil fuel in major energy consumption sectors such as buildings and transportation (i.e. fuel cell vehicles). One of the main barriers of the RES is their stochastic nature which, combined with demand fluctuations, significantly decreases the reliability of the energy systems. In these cases, an electrolysis-fuel cell system can be used as a storage medium in stand-alone electricity grids to eliminate the energy fluctuations supplied from RES. To achieve that, an electrolysis-fuel cell system must be optimally sized to benefit both energy and the economy. The scope of this work is the experimental investigation of a hydrogen-based system by utilizing an advanced experimental unit of the Soft Energy and Environmental Protection Laboratory (SEALAB) at the University of West Attica. In more detail, the system consists of a storage module of three metal hydride canisters, a Proton Exchange Membrane Fuel Cell (PEMFC) of 1.2 kW, a power management module (DC/DC converter), a battery bank module, a DC load simulator, and a computer control unit. The DC load module can simulate load profiles up to 1.5 kW under different patterns. After studying the performance of the PEMFC, a theoretical model was developed, able to simulate the operation of PEMFC by using as input data the cell’s technical specifications and the available hydrogen flow. The model is based on theoretical, experimental, and semi-empirical equations describing the FC’s operation. The developed model was experimentally validated under different hydrogen flow scenarios as applied to the PEMFC configuration. The results showed high accuracy between observed and predictive values verifying the reliability of the model’s calculations for sizing procedures.
The building sector’s contribution to the energy consumption reaches -for most countries- up to 40%. Buildings account for almost a third of the global final energy consumption while being an equally important source of CO2 emissions. Considering the energy saving potential of the building sector along with the technological progress met in many fields of construction and design, energy reduction in the building sector is of great importance to tackle climate change, promote energy security and preserving the finite non-renewable energy reserves of our planet. In this context, the building sector can contribute to reducing the environmental degradation by minimizing energy consumption in existing and newly designed buildings. The accurate prediction of building’s energy consumption requires, among others, detailed weather data in the installation area. The most common weather datasets used in models for simulating the energy balance of the buildings are Typical Meteorological Years (TMYs), which represent the long-term climate conditions of a specific location. In this respect, the present work investigates the influence of using TMY on the calculation results of the thermal energy consumption of a building. As a case study, a residential building is simulated for the heating period of the year. During the simulation, actual weather data, along with TMY produced through the implementation of a fifteen-year period real weather data, have been used. The results have shown that the thermal energy consumption is slightly underestimated when using TMY weather datasets meanwhile the error would be lower than the one resulting from using a single-year actual weather data.
This study investigates the interventions in the electromechanical equipment of an office building at the University of West Attica to minimise CO2 emissions and to classify the building as a nZEB (nearly Zero Energy Building). The research includes the process that has been followed for the energy study of the building following different scenarios with all possible combinations of altera-tions in the building's electromechanical equipment, which includes the addition of a new highly efficient HVAC system (variable refrigerant flow -VRF), LED lights, daylight & occupancy controls, and photovoltaic panels (PV) sized according to the energy needs of each scenario. A comparison of all the scenarios with the different combinations of alterations was carried out, and results of energy consumption, annual operating costs and CO2 emissions were obtained. Finally, the payback period for each scenario was calculated, providing a clear aspect of the cost gradient according to the level of intervention.
The contemporary university research teams are often urged to act as facilitator and intermediator between citizens and several innovative areas as for instance in the renewable energy ecosystem. In this context, the present specific study explores the design of a Citizens Awareness Energy Park (CAEP) for demonstrating and disseminating knowledge to mobilize local community on promoting behavioral change and attitude related to energy and environmental issues. A key concern of local societies is to manage the energy challenges facing modern cities and communities by improving the use of available resources and reducing pollution to achieve sustainability. The Energy Transition adjusts the way we produce and consume energy, and this implies new habits adoption and behavioral changes. The main purpose of the planning and creating the CAEP is to inform and raise awareness among citizens and students about the modern, important topic of Renewable Energy Sources (RES), energy efficiency, environmental protection, and sustainable development. A research questionnaire was prepared first and a sufficient number of questionnaires were completed with live interviews, to capture the knowledge of citizens on energy and environmental issues. At the same time the researchers collect the citizens queries and their interest to better understand several energy and environmental related specific topics. This step was followed by specialized planning research for the implementation of a CAEP, considering the questions raised and the actors involved, as local society, municipal authority, local businesses, and academic institutions. Subsequently, sizing and siting of selected RES application examples was carried out in an open existing space in the Municipality of Ilion. In order to promote the CAEP implementation, a related digital virtual visualization of the CAEP space was created. An action plan for citizens’ participation was drawn up and field action was implemented with citizens’ participation to measure environmental pollution and the heat island effects as “citizen science” practice. Results of this specific study identified the possibilities as well as the difficulties of creating an effective Citizens Awareness Energy Park while assessing critical commitment factors of the actors’ involved.
The discussion of the energy transition is more relevant than ever in the 21st century. The main challenge humanity faces concerns the establishment of carbon-neutral societies, utilizing Renewable Energy Sources (RES) to replace fossil fuels. For this purpose, ambitious targets have been set to reduce greenhouse gas emissions between 2035-2050. Some of the long-term energy strategies of various European countries include targets regarding the diversification of energy sources, maintenance of security supply, and reduction of import dependency. Thus, hydrogen (H2) is an energy carrier that constantly gains momentum in the current global situation. Green hydrogen has the potential to create clean cycles regarding the future renewable-based electricity grids, providing the proper flexibility to power systems and acting as a buffer to balance the stochastic character of other technologies such as wind power and photovoltaics. Undoubtedly, the excess power produced by non-dispatchable renewable power plants can be stored in the form of hydrogen and then employed to produce electricity via fuel cells or other power systems, which can drastically minimize greenhouse gas emissions. Hydrogen storage technologies have a vital role in making the cycle of excess electrical energy-hydrogen-production of green electricity more efficient, further promoting the capabilities of hydrogen as a storage medium. In this respect, the current work will present the various methods from which hydrogen can be produced. The storage and distribution methods, and the corresponding challenges will be further discussed. Emphasis will be given on the opportunities of hydrogen in the transport, building, and power sectors, considering possible policies that could be applied to support relevant hydrogen applications.
Assessment of the electricity generation status for Non-Interconnected Islands (NIIs) of the Aegean Sea region, excluding the electricity systems of Crete and Rhodes, is undertaken in the current study. The authors focus on the long-term analysis of thermal power generation characteristics and also on the challenges so far limiting the contribution of Renewable Energy Sources (RES) in covering the electricity needs of the specific area. According to the present analysis, due to the existing technical limitations, the annual RES shares in the electricity balance of NIIs of the Aegean Sea have since 2010 stagnated in the range of 15% to 18%. Moreover, the performance of thermal power stations for all 30 NII systems is evaluated on the basis of their utilization factor, associated fuel consumption and electricity production costs. The vast majority of these stations is characterized by low capacity factors in combination with high specific fuel consumption and high operational expenses that in the case of smaller scale island regions could even exceed 600€/MWh. At the same time, the authors discuss on the alternatives and encourage further investigation of novel, intelligent energy solutions, such as the smart microgrid and battery-based hybrid power station that are currently developed on the island of Tilos under the implementation of the TILOS Horizon 2020 program.
A rapid development of solar energy systems has been noticed in the last decade worldwide, with the photovoltaic systems' installed global power capacity exceeding 100GW e. Moreover, the European Union has realized that one of the key sectors for the implementation of energy saving techniques is the building sector for which Member States have already adopted measures for reducing energy consumption. A key parameter for energy calculations with regards to photovoltaics and building structures, along with the external temperature, is the possession of long-term solar radiation data. Nevertheless, due to the frequent lack of real field measurements in the area under investigation, the use of validated solar radiation models comprises a key step in the process allowing analysts and scientists to obtain the required background information. In this respect , the present work investigates the reliability of a theoretical model's (MRM) solar radiation estimations through experimental 35,064 hourly solar radiation data points obtained in the area of Ioannina, NW Greece. The MRM routine was ran throughout a four (4)-year period, separating also each year into two main phases in order to demonstrate seasonal patterns between cloudy (cold season) and less cloudy (hot season) sky conditions. According to the results obtained, in general, a fairly good agreement was observed between experimental data and model's results, however, deviations derived mostly in cases of partly cloudy weather mainly due to the use of recorded-daily instead of recorded-hourly values of sunshine duration.
During the last three years the installed wind power in Greece has significantly risen from 40MW to almost 300MW. This important wind power ascension was based on contemporary wind turbines of remarkable size, concentrated in few relatively restricted geographical areas. In certain areas, however, this outstanding wind power penetration provokes serious local population reactions, which, in several cases, lead even to the complete wind power projects cancellation by claiming important environmental impacts. In this context, "visual intrusion" is found to be one of the major factors determining opposition to wind energy. In order to examine this problem, an extensive study is carried out concerning the visual impact of the existing wind parks in Greece. For this purpose, also a public opinion survey is carried out throughout Greece, concerning the local habitants' attitude towards a wind park, as far as the "visual intrusion" of existing wind turbines is concerned. The results collected are analyzed in view of the machines' general acceptability. According to the data analyzed, -sample of 417 respondents in three representative Greek territories-, an important part of local people including wind energy supporters claims remarkable visual annoyance of existing wind turbines. Thus, if the target is to accelerate wind power penetration in the local energy market, more attention should be paid on the visual incorporation of new installations in the local landscape.
Wind energy is the fastest growing energy sector for electricity production in various European countries. A substantial wind power penetration is also expected in the Greek energy market. This significant number of new wind turbines provokes serious reaction of local people, pretending important environmental impacts. For this purpose, an introductory survey is carried out to validate the real size of the wind energy applications' impact on human societies and local ecosystems. During the present investigation, several important parameters, like visual impact, noise emissions, avian mortality, land usage and energy payback period-materials' requirements are taken into account. On the other hand, the wind energy contribution to air pollution reduction is also considered.
This study investigates the performance of isotropic and anisotropic diffuse models to estimate the total solar energy received on flat-plate collectors fixed on dual-axis trackers. These estimations are applied at twelve sites selected in both hemispheres with different terrain and environmental conditions. The diffuse (or transposition) models used in this study are the isotropic Liu-Jordan (L&J), Koronakis (KOR), Badescu (BAD), and Tian (TIA), and the anisotropic Hay (HAY), Reindl (REI), Klucher (KLU), Skartveit and Olseth (S&O), and Steven and Unsworth (S&U). These models were chosen because of their simplicity in the calculations and minimum number of input values. The results show that a single transposition model is not efficient for all sites; therefore, the most appropriate models are selected for each site under all, clear, intermediate, and overcast conditions in skies. On the other hand, an increase in the ground albedo in the vicinity of the solar installation can increase the annual inclined solar availability on a two-axis tracker by at least 9% on average. Further, a linear dependence of the annual inclined solar energy on the variation of the ground albedo was found. Also, a linear relationship exists between the annual diffuse-fraction and cloud-modification factor values at the 12 sites.
This research conducted a bibliometric analysis of scholarly literature on fruit sorting and grading using machine vision, identifying primary themes, sources, most-cited publications, and countries. The literature and bibliometric analysis were thoroughly evaluated to consolidate knowledge, identify research trends, and propose specific research opportunities within the context of machine vision for fruit sorting and grading. Research articles from 2011 to 2023, indexed in the main collections of the Dimensions, Web-of-science, and Scopus databases, were examined. Findings were presented quantitatively, using tables and graphs to emphasize the key performance factors for article writing and citation. Upon applying inclusion and exclusion criteria, 129 out of 1812 discovered articles were included for examination, while 1683 studies were excluded due to non-compliance with the requirements and duplicates. Thirty-four (34) case study publications on machine vision applications for fruit sorting and grading were comprehensively examined to identify the adopted methodologies and future research opportunities. Covered methodologies include fruit varieties, data volumes, data collection, classification methods, and accuracy metrics. The study's findings indicate a significant increase in deep learning applications for fruit recognition in the recent five years (2019–2023), with excellent results achieved either by utilizing new models or with pre-trained networks for transfer learning. The research also identifies gaps and future directions for machine vision in fruit sorting and grading, such as enhancing system robustness, scalability, and adaptability, integrating multiple sensors and technological methods, and developing evaluation and comparison standards and criteria. The paper concludes that machine vision holds promise as a potent tool for fruit quality assessment, but further research and development are needed to address existing challenges and meet the growing demands of the fruit industry.
Battery storage is nowadays considered a key component not only in off-grid applications but also in the context of grid-tied, residential-scale systems, facilitating the broader use of RES even in heavily congested distribution grids. Since batteries normally comprise the costliest part in similar configurations, their optimal sizing is a priority. Informed decisions to that end should not be limited to rough, initial-cost estimations alone; instead, they should also take into account the life-cycle costs of batteries, which, in turn, relate to battery aging mechanisms and the gradual, cycling-based fading of the batteries’ useful capacity. Acknowledging the above, the impact of battery cycling aging on the performance of typical, residential-scale, RES-based battery storage configurations is investigated herein, considering also the utilization of second-life lithium-ion EV batteries. To that end, we used a literature-informed empirical aging model and performed a simulation exercise for a broad set of different RES-battery configurations, with our results indicating the importance of the aging factor while also designating areas of optimum sizing with regard to the long-term energy and economic performance of similar solutions.
Increased interest is demonstrated recently in the emergence of prosumer schemes for the residential sector on the basis of combined RES and storage configurations. Primarily, such schemes aim to increase energy autonomy for end users. Despite providing an alternative supply solution that may secure end users from volatile energy prices, RES–battery configurations also suggest costly and, in most cases, capital-intensive solutions. As such, exploring the generation of additional revenue through market participation is an exercise worth undertaking, noting at the same time that decongestion management services may also be provided to the local grid. In this context, the current study introduces an operational framework for the market participation of RES–battery prosumer schemes, seeking to determine the optimal balance between self-consumption and market integration. For that purpose, we use typical demand patterns and perform an extensive parametrical analysis concerning system size, spot price levels and degree of market integration in the context of the Greek electricity market, with our results indicating areas of optimum balance for the minimization of similar schemes’ levelized cost of electricity.
The exploitation of Renewable Energy Sources (RES) has already been proved as a promising power source for a sustainable future in energy supply, to cover the world’s energy demand. Solar energy is considered the most abundant RES of all, while being available in every part of our planet makes it favorable for distributed RES energy production systems. Solar-based stand-alone power systems, installed in remote areas far from the main local electricity networks, can be cost efficient and even provide energy security given that they have been carefully sized. Moreover, stand-alone photovoltaic systems can highly contribute to distributed renewable energy and microgrid networks even without other RES combination if they integrate with an appropriate energy storage (ES) system. During the sizing procedure of a photovoltaic (PV) plant, the solar potential of the area is the dominant parameter. The use of solar energy values, which may deviate from the actual values, has a relatively small impact on the overall energy assessment of interconnected photovoltaic installations. However, in the case of stand-alone PV systems, where the user requires energy autonomy, it is necessary to use hourly solar radiation along with air temperature values with increased reliability in order to determine the most cost effective combination of PV-ES system. Although there is an extended literature on stand-alone PV systems sizing procedures, the effect of using different input meteorological data on system’s size has not been sufficiently addressed. In this context, the present study examines the use of different meteorological input data of solar radiation and air temperature with data series from 12 individual years, a Mean Year (MY) and a Typical Meteorological Year (TMY). Although the TMY has been mainly used in building’s energy calculations, the results of the present work prove that the use of a TMY increases the power system reliability if compared with sizing the system by using time series of individual years or a MY. The case study presented in the specific work concerns an application of combined PV-ES able to provide energy autonomy to a domestic consumer, in a remote area in the island of Rhodes.
The building sector consumes 36% of the world’s energy and produces around 40% of energy-related carbon emissions. While the building industry moves towards a zero net greenhouse-gas emission policy, ventilation is, and will be, a necessity for the preservation of air quality especially in climates defined by unsavoury conditions. Therefore, a “mixing mode” cooling system was employed to lower the required energy consumption at an earthen building situated in the premises of Istanbul Technical University. A room of the high-mass earthen building was monitored under different ventilation and shading conditions. Night ventilation was conducted using two modes, 3.2 and 2.3 air changes per hour, and the air conditioning unit, operating from 08:00 to 17:00, had a set temperature of 23 ∘C. Night ventilation was somewhat impactful, reducing the average expected cooling energy demand up to 27%. Furthermore, the earthen building proved to be extremely effective on moderating extremes of temperature under non-ventilated conditions. During a rather hot day, with an outdoor maximum temperature of 35 ∘C, the indoor maximum temperature of the high-mass building was only 25 ∘C, namely within thermal comfort levels. The diurnal temperature proved to be key in the effective application of night ventilation.
Variability of energy production is considered to be the main shortcoming in the operation of renewable energy systems. Combination of different Renewable Energy Sources (RES), employment of energy storage and application of Demand Side Management (DSM), are all elements used to encounter the problem of RES variability. Exploitation of such elements in an effective manner challenges the development of advanced Energy Management Systems (EMSs), especially in the case of island microgrids with high shares of RES, lacking the flexibility and capacity of centralized electricity systems to facilitate increased RES penetration. In this work, and in the framework of the Horizon 2020 TILOS project, an advanced Forecasting System (FS) has been developed, able to provide reliable predictions of load demand, wind power and solar power production. The specific variables are independently predicted through a set of forecasting models that produce both deterministic and probabilistic results for different time horizons and time resolutions, fully adjustable to the requirements of any given island microgrid. The developed FS has been deployed and tested considering the smart microgrid of Tilos island, in the SE Aegean Sea, with results obtained demonstrating its ability to provide sufficient and accurate forecasts for all studied variables.
The environmental degradation caused by the increased use of fossil fuels in the energy sector, call for an urgent switch to alternative energy sources which can assure sustainability, energy security and reliability. So far, solutions based on Renewable Energy Sources (RES) have been proved reliable in providing energy even in autonomous networks, if they combine different sources (e.g. solar and wind) or combined with conventional energy generators. Among the promising solutions to the energy problem is the geothermal energy which consists of stored energy in the earth in the form of water or steam at high pressure and temperature. The countries which are most likely to possess a high geothermal potential are those located in seismic zones close to destructive and constructive plate margins. Greece is among the countries that, due to location, possesses a remarkable geothermal potential of medium and high enthalpy fields. The interest in exploiting geothermal energy in Greece for electricity generation is mainly focused on non-interconnected islands, where energy is produced from oil-based generators with high energy cost, mainly during the summer months which is a period of rapid increase in energy demand. In this respect, the present study investigates, through an economic evaluation, the optimum sizing of a hybrid solution, which includes a geothermal power plant and a solar field of concentrated collectors able to cover the energy needs of an isolated island. The combination of solar and geothermal energy source aims to maintain the long-term productivity of the geothermal field by reducing its degradation ratio. The thermal energy, generated by the solar installation, increases the temperature of the geothermal brine before injected it back into the well and therefore reduces its energy degradation. In this study, different cases of solar field sizes are evaluated in order to achieve the optimum combination of the geothermal power plant and the concentrated solar array. According to the results, the proposed installation can guarantee energy autonomy with increased economic efficiency and competitive energy production cost, compared to the current prices in small isolated Aegean Sea islands.
There is a significant number of Greek islands in the Aegean Sea that still meet their electricity demand on the basis of the local autonomous thermal power stations, using considerable amounts of imported oil. In addition, many of these islands suffer from water scarcity, covering their potable water needs with water imports at extremely high costs. Under the framework of PHAROS research project, and in an effort to support energy and water self-sufficiency, an integrated software tool has been developed for the optimal planning of hybrid renewable energy systems (HRES), capable of investigating the energy balance of autonomous island regions. In this context, a planning methodology for HRES combined with a sizing algorithm for desalination plants were devised and integrated in the advanced software tool named Energy System Analysis (ESA). ESA enables the detailed analysis of systems that include components such as wind, PV and desalination plants, energy storage devices and diesel engines. To this end, the long-lasting problems of the Aegean Sea islands, mainly stemming from their state of remoteness and isolation, require the examination of long-term energy strategies, which ESA can support in an efficient manner. In this context, the current work provides a short presentation of the ESA tool main features, using two discrete case studies that correspond to a North and a South Aegean Sea island respectively.
The Aegean Archipelagos is an island region possessing excellent wind potential, with areas where the long-term average wind speed even exceeds 9m/s. Despite the excellent wind and solar potential of the area, most of the inhabited islands cover their electricity needs by utilizing thermal power stations (mainly diesel or heavy oil based units). In this context, wind power contribution is limited due to the local grid stability constraints, with zero new wind parks installed across the Aegean Archipelago during the last years. To this end, and in an effort to establish the "Green Island" electrification model for the numerous remote islands of Europe, a theoretical model is currently presented for estimating wind energy curtailments of existing and new wind parks. The model is then applied to an existing wind park of 3.6MW, operated in the island of Kos, and results obtained are compared with actual wind energy curtailments faced during a representative year of operation.
A large number of various sized islands are spread throughout the south-east Mediterranean Sea. Most of these small islands face serious infrastructure problems, like the insufficient power supply and the low quality of electricity available at very high production cost. In an attempt to improve the life quality of all these isolated communities, an investigation concerning the financial viability of an integrated electrification solution based on one or more photovoltaic generators and an appropriate energy storage system is described. The main target of a similar solution is to maximize the contribution of the photovoltaic generator and minimize the life-cycle electricity generation cost of the remote island networks investigated. In addition, special emphasis is given in order to select the most cost-efficient energy storage configuration available. According to the results obtained for high and medium–high solar potential regions, the proposed configuration is found to be more cost-effective than the existing thermal power stations. Several side benefits like the improved electrical network reliability and the minimization of the environmental and macroeconomic impacts resulting from the replacement of the imported oil should also be considered.
The oil-dependent electricity generation situation met in the Aegean Archipelago Islands is in great deal determined by increased rates of fuel consumption and analogous electricity production costs, this being also the case for other island autonomous electrical networks worldwide. Meanwhile, the contribution of renewable energy sources (RES) to the constant increase recorded in both the Aegean islands’ annual electricity generation and the corresponding peak load demand is very limited. To compensate the unfavorable situation encountered, the implementation of energy storage systems (ESS) that can both utilize the excess/rejected energy produced from RES plants and improve the operation of existing thermal power units is recommended. In the present study, a techno-economic comparison of various RES-ESS configurations supported by the supplementary or back-up use of existing thermal units is undertaken. From the results obtained, the shift of direction from the existing oil-dependent status to a RES-based alternative in collaboration with certain storage technologies entails – apart from the clear environmental benefits – financial advantages as well.
The progress met in the world market of photovoltaics underlines the maturity of investments realized, guarantees the reliability of the technology utilized and designates the variety of applications in covering the energy demands of both stand-alone and grid connected consumers. Concerning stand-alone systems, the incorporation of photovoltaic systems in water pumping applications is thought to be one of the most popular and ideal uses of solar energy exploitation, especially under the common allegation of coincidence between insolation and water demand. In this study, an attempt to investigate the opportunities of a PV powered water pumping system able to meet additional – apart from the water pump – electricity loads, results in the development of an optimum sizing methodology which is accordingly validated by experimental measurements. From the results obtained, it becomes clear that a properly designed PV-pumping configuration of 610 Wp is capable of covering both the electricity (max 2 kWh/day) and the water (max 400 L/h) management demands of a large variety of remote consumers.
The entirety of Aegean Sea Islands, including Crete, is characterized during the last decade by a considerable annual increase of the electrical power demand exceeding the 5% in annual basis. This continuous amplifying electricity consumption is hardly fulfilled by several outmoded internal combustion engines usually at a very high operational cost. On the other hand most of the islands possess high wind potential that may substantially contribute in order to meet the corresponding load demand. However, in this case some wind energy absorption problems related with the collaboration between wind parks and the local electricity production system cannot be neglected. In this context, the present study is devoted to realistically estimating the maximum wind energy absorption in autonomous electrical island networks. For this purpose a new reliable and integrated numerical algorithm is developed, using the available information of the corresponding electricity generation system, in order to calculate the maximum acceptable wind power contribution in the system, under the normal restrictions that the system manager imposes. The proposed algorithm is successfully compared with existing historical data as well as with the results of a recent investigation based almost exclusively on the existing wind parks energy production.
The electrification of autonomous electrical networks is in most cases described by low quality of electricity available at very high production cost. Furthermore, autonomous electrical networks are subject to strict constraints posing serious limitations on the absorption of renewable energy sources (RES)-based electricity generation. To bypass these constraints and also to secure a more sustainable electricity supply status, the concept of combining photovoltaic (PV) power stations and energy storage systems comprises a promising solution for small scaled autonomous electrical networks, increasing the reliability of the local network as well. In this context, the present study is devoted in developing a complete methodology, able to define the size of an autonomous electricity generation system, based on the maximum available solar potential exploitation at minimum electricity generation cost. In addition special emphasis is given in order to select the most cost-efficient energy storage configuration available. According to the calculation results obtained, one may clearly state that an optimum sizing combination of a PV generator along with an appropriate energy storage system may significantly contribute on reducing the electricity generation cost in several island electrical systems, providing also abundant and high quality electricity without the environmental and macro-economic impacts of the oil-based thermal power stations.
In this work, a new approach is tested by applying neural networks treatment to meteorological time-series data sets, recorded during 1991–2000 at certain Greek locations, in order to create fully appropriate solar data information. Neural networks, in this case, are used for creating missing mean, maximum and minimum global and diffuse solar irradiance hourly data, when educated with other known meteorological time-series hourly values. For this purpose, hourly data of air temperature, relative humidity, sunshine duration, clouds’ octals, as well as local latitude are used with regard to these sites. Neural networks’ education process outputs are checked against known hourly values of solar irradiance, based upon the mentioned meteorological hourly raw data necessary for this action recorded at the National Observatory of Athens, the actinometric station at the Technological Education Institute (TEI) of Piraeus, and six other locations. Selection of these sites is representative of the climatic conditions in Greece, from north to south and east to west. Following the same scheme, the produced hourly global and diffuse mean hourly solar irradiance values are in a very good agreement (p<0.01) with actual measurements.
Wind power and photovoltaic driven stand-alone systems have turned into one of the most promising ways to handle the electrification requirements of numerous isolated consumers worldwide. In this context, the primary target of the present work is to estimate the appropriate dimensions of either a wind power or a photovoltaic stand-alone system that guarantees the energy autonomy of several typical remote consumers located in representative Greek territories. For all regions examined, long-term wind speed and solar radiation measurements as well as formal meteorological data are utilized. Accordingly, special emphasis is put on the detailed energy balance analysis of the proposed systems on an hourly basis, including also the battery bank depth of discharge time evolution. Finally, comparison is made between the wind and the solar based systems investigated, proving that in most Greek regions either a wind or photovoltaic driven stand-alone system is able to cover the electrification needs of remote consumers, at a moderate first installation cost, without any additional energy input.
More than one third of world population has no direct access to interconnected electrical networks. Hence, the electrification solution usually considered is based on expensive, though often unreliable, stand-alone systems, mainly small diesel-electric generators. Hybrid wind–diesel power systems are among the most interesting and environmental friendly technological alternatives for the electrification of remote consumers, presenting also increased reliability. More precisely, a hybrid wind–diesel installation, based on an appropriate combination of a small diesel-electric generator and a micro-wind converter, offsets the significant capital cost of the wind turbine and the high operational cost of the diesel-electric generator. In this context, the present study concentrates on a detailed energy production cost analysis in order to estimate the optimum configuration of a wind–diesel-battery stand-alone system used to guarantee the energy autonomy of a typical remote consumer. Accordingly, the influence of the governing parameters—such as wind potential, capital cost, oil price, battery price and first installation cost—on the corresponding electricity production cost is investigated using the developed model. Taking into account the results obtained, hybrid wind–diesel systems may be the most cost-effective electrification solution for numerous isolated consumers located in suitable (average wind speed higher than 6.0 m/s) wind potential regions.
A vital problem encountered in Greek islands within the Aegean Archipelago is insufficient electricity generation. Wind generated electricity appears to be a fiscally viable solution, as the area has excellent wind potential. Yet despite the technological improvement, the penetration of substantial wind power to autonomous electrical grids is still limited, mainly due to existing technical barriers. One of the most interesting autonomous electrical network cases is the island of Crete. Here, with an excellent wind potential, about 180 wind turbines of 120 MW total installed rated-capacity are in operation or under construction. However, even in this relatively strong electrical system, grid instability and mismatching of supply and demand have led to significant wind energy rejection. This “spilt” wind power corresponds to an annual financial loss of income of 30,000 € per MW of wind power installed. The present study seeks an integrated methodology with pumped-hydro storage for maximizing the contribution of wind energy in the Crete electricity supply. In addition, the objective is to improve grid stability. An analysis of the wind-hydro electricity production cost is presented and compared with the corresponding operational cost of the existing thermal power plants.
In many small- or medium-sized remote islands, there is a significant electrical power shortage, where the water resources are quite limited. This unfavourable situation results in the operation of high cost autonomous thermal power stations and the transportation of fresh water of questionable quality at extremely high prices. Wind energy can definitely contribute on solving these problems at a rational investment and operational cost. To face the intermittent and stochastic wind behaviour, a combined wind-hydro configuration is proposed in collaboration with an appropriate desalination plant. The proposed solution leads to high wind energy penetration rates and bounds the operational hours of the existing internal combustion engines, additionally contributing to the air pollution reduction. Besides, significant water quantities can be produced, remarkably reinforcing the water reserves of the local community with fresh water of desired quality. Consequently, the configuration investigated can efficiently fulfil the electrical energy and the clean water requirements of numerous remote communities on the basis of clean and low cost wind energy, overcoming the intermittent and stochastic behaviour of the wind.
Solar thermal applications have been acknowledged among the leading alternative solutions endeavouring to face the uncontrollable oil price variations, the gradual depletion of fossil fuel reserves and the chain environmental consequences caused by its excessive usage. Almost 30 years after the initial emergence of the commercial domestic solar water heating system (DSWHS) in the European market, the corresponding technology is qualified as quite mature. On top of this, the European Commission expects that 100,000,000 m2 of solar collectors are to be installed in Europe by the year 2010 to facilitate durable and environment-friendly heat. In this context, the Greek DSWHSs market is highly developed worldwide, having a great experience in this major energy market segment. The present study is devoted to an extensive evaluation of the local DSWHSs market, including a discerning analysis of its time variation, taking seriously into account the corresponding annual replacement rate. Accordingly, the crucial techno-economic reasons, limiting the DSWHSs penetration in the local heat production market, are summarized and elaborated. Subsequently, the national policy measures—aiming to support the DSWHSs in the course of time—are cited, in comparison with those applied in other European countries. Next, the financial attractiveness of a DSWHS for Greek citizens is examined in the local socio-economic environment. The present work is integrated by reciting the prospects and mustering certain proposals that, if applied, could stimulate the local market. As a general comment, the outlook for penetration of new DSWHSs in the local market is rather grim, as the current techno-economic situation of solar heat cannot compete with oil and natural gas heat production, unless the remarkable social and environmental benefits of solar energy are seriously considered. Hence, the Greek State lacks stimulus to further DSWHSs installations, being strongly in support of the imported natural gas. As a result, the future of domestic solar thermal market and the survival possibilities of the local manufacturers are at stake.
Official statistics estimate that almost two billion people have no direct access to electrical networks, 500,000 of them living in European Union and more than one tenth of them in Greece. An autonomous photovoltaic system is one of the most interesting and environmental friendly technological solutions for the electrification of remote consumers or entire rural areas. The primary objective of this current study is to determine the optimum dimensions of an appropriate stand-alone photovoltaic system, able to guarantee the coverage of remote consumers energy demand located in typical Greek territories using long-term measurements, under the restriction of minimum initial cost. Accordingly, the impact of acceptable reliability level on the stand-alone photovoltaic system energy behaviour and initial cost is also examined. Finally, special emphasis is laid on the detailed energy balance analysis of selected stand-alone photovoltaic system configurations, on an hourly basis at least. According to the results obtained, a properly sized stand-alone photovoltaic system is a motivating prospect for the energy demand problems of numerous existing isolated consumers all around Greece.
During the last 4 years, a substantial wind energy penetration was encountered mainly in the Greek mainland. At the same time, limited size new wind-parks were built in the numerous Greek islands, although their wind potential is clearly higher than the one for the mainland and their wind energy generation could be used to replace electricity produced by diesel engines and gas turbines at moderate and high cost, respectively. This negative conclusion for the Greek island communities underlines the inability of the local weak autonomous electrical networks to entirely absorb the gradually increased wind-energy production. Thus, in the present work, special attention is paid to correctly estimate the annual income loss of existing wind-parks, due to the wind energy rejection by the autonomous island electrical grids. For this purpose a complete and reliable method is developed, able to realistically calculate the instantaneous wind-power production rejected by the local electricity-generation system, according to the information provided by the system operator and the wind-park owners. The present analysis is based on extensive time series of real data and measurements. Applying the proposed method to the Crete-island network situation, a remarkable amount of wind produced electric-energy rejection is predicted for the last 3 years, which is definitely rising in the course of time. Calculation results are well in agreement with the official monthly data provided by the local power-utility, in view of the existing power-purchase agreement between the private wind-power investors and the local network management.
For the majority of the Greek islands, water resources are quite restricted, limiting the economic development of the local societies. To face increased potable water requirements, more than 2,500,000 m of clean water is transferred annually to these islands at a cost approaching the value of 7 /m3 On the other hand, the final cost of the locally produced water from renewable energy sources (RES) based desalination plants is expected to be quite lower than this value. The main purpose of the present study is to examine the economic viability of several representative desalination plant configurations based on the available renewable energy sources using an integrated cost-benefit analysis. In the proposed analysis all the cost parameters of the problem are taken into consideration, including the capital cost of the desalination plant, the annual maintenance and operation cost, the energy consumption cost, the local economy annual capital cost index and the corresponding inflation rate. The calculation results obtained definitely support the utilization of RES-based desalination plants as the most promising and sustainable method to satisfy the fresh, potable water demands of the small- to medium-sized Greek islands at a minimal cost, without disregarding the considerable environmental and macro-economic benefits.
The possibility to create a combined wind–hydro energy station in a medium size island of the Aegean Archipelago is investigated on a techno-economic basis. The proposed solution may be used to face the extremely high electricity production cost in these regions, taking also advantage of the excellent local wind potential. Additionally, a parametrical analysis is performed on a techno-economic basis in order to select the appropriate number of wind turbines and the optimum size of water reservoirs. The calculation results obtained, based on real measurements and on experimental data, validate the proposed solution. More precisely, the electricity demand of the remote system is covered in any case, the imported fuel is minimized, the renewable energy sources penetration exceeds 90%, and the negative environmental effects are remarkably reduced.
Energy shortage and clean-water deficit, especially during the summer, are among the main factors delaying the economic development of Aegean Sea islands. All these islands possess an outstanding wind potential. However, the stochastic behaviour of wind speed leads to significant disharmony between wind energy production and electricity demand. Hence, the prospect of creating a combined wind-hydro energy production station is found to be a vital solution for all these islands, under the preconditions of maximum energy autonomy and limited first installation cost. Accordingly, a methodology of optimal wind-hydro solution estimation is developed and subsequently applied to several typical Aegean Sea island cases, in order to define the most beneficial configuration of the proposed renewable station. All numerical calculations are based on real data, like long-term wind speed measurements, demanded electrical-load and operational characteristics of the system components. In all cases analyzed, the renewable energy sources penetration exceeds 85%, while a significant part of the system's wind energy surplus is forwarded to a desalination plant for clean-water production.
The aim of the present work is to investigate the efficiency of flat-plate solar panels in Greece for delivering solar energy. In this study, the solar panels are mounted on a two-axis tracker, which follows the daily path of the sun. In this context, the annual energy sums are estimated on such surfaces from hourly solar horizontal radiation values at forty-three locations, covering all of Greece. The solar horizontal radiation values are embedded in the typical meteorological years of the sites obtained from the PVGIS tool. All calculations use near-real surface-albedo values for the sites, and isotropic and anisotropic models are used to estimate the diffuse-inclined radiation. The analysis provides non-linear regression expressions for the energy sums as a function of time (month, season). The annual energy sums are found to vary between 2247 kWhm−2 and 2878 kWhm−2 under all-sky conditions with the anisotropic transposition model. Finally, maps of Greece showing the distribution of the annual and seasonal solar energy sums under all- and clear-sky conditions are derived for the first time, and these maps constitute the main innovation of this work.
In the present study, the most prevalent space heating (SH) systems are assessed by means of probabilistic economic method, for Athens (Greece) under the ongoing energy crisis. The examined SH systems are the air source heat pump, the natural gas boiler and the oil boiler. An apartment of 85 m2 living area is chosen as the standard Athenian dwelling type for the assessment of the SH systems. In addition, two economic scenarios are set related to the annual energy inflation rate, the pessimistic and the optimistic. Simulations are executed in TRNSYS for the physical system and in MATLAB for the economic analysis. The economic analysis is carried out with the combination of the Monte Carlo simulations and the Life Cycle Cost (LCC) method. It turned out that, the LCC (as net present value) for all systems is much higher than the pre-crisis cost and is not feasible under the existing obscure economic period. It was proved that, the energy cost subsidy may be a solution on the volatility of the energy market, however, this can be achieved with rather high subsidy rates. The air source heat pump system was found to obtain better economy than the conventional fuel-based systems and the pairing of the heat pump with the PV system proved as the only economic feasible solution. The developed methodology can be utilized for evaluating the investment risk associated with the uncertain of the future cost of energy.
A three-dimensional steady-state model has been developed to study the phenomena that occurs during Proton Exchange Membrane Fuel Cell’s (PEMFC) operation. Electrochemical and transport phenomena on both the anode and cathode sides were investigated. Particular emphasis has been given to the composition and structure of the catalyst layers (CLs), considering parameters such as the metal loading, the most effective specific metal surface, the agglomeration, and the particle size. In this context, two types of CLs were investigated. The first type concerns conventional CLs consisting of Pt/C, while the second type refers to bimetallic CLs consisting of Pt-Ru/C. In both cases, the CLs were examined for various loadings of Pt, Ru, and C to define the optimum atomic ratio for an enhanced PEMFC performance, while, in parallel, possible challenges are intedified. The mathematical model for simulating the entire phenomena and the method for modeling the bimetallic catalyst layers are presented. The results show a good agreement between the model and the experimental data reported in the literature. Additionally, the scenario of bimetallic CLs consisting of Pt-Ru/C with a ratio of 50-50 (Pt-Ru) significantly improved the overall PEMFC electrochemical performance.
Solar azimuth may become indefinite at sunrise and sunset. This discrepancy is corrected in the present work. The correction concerns application of Fourier–series analysis to the solar paths over a site during days when the problem arises. The result is the derivation of a new curve that is fitted with great accuracy to the daily solar azimuth values, thus bridging the gap of the discontinuity. A demonstration for the solar azimuth correction is given for 3 sites around the world (Athens, Stockholm, and Sydney). The correction can be applied to any solar geometry code; in this work the algorithm of the XRONOS code is selected without (XRONOS.bas, a BASIC programme) and with (XRONOS.m, a MATLAB code) the correction proposed. A corrected expression for the atmospheric refractive index at various altitudes is given in Appendix A as the refraction of solar right is treated in XRONOS.
The main categories of solar water heating systems (SWHSs) are the thermosyphon and the forced circulation (FC). This paper presents an experiment carried out with the aim to compare the energy performance of the FC with a thermosyphon SHWS. Identical SWHSs were installed side by side at the University of West Attica in Athens, Greece. Domestic hot water load was applied to both systems via a microcontroller-based dispensing unit which mimics the demand profile. The trial period comprised the last two months of spring (April and May). For the first law assessment, two energy indicators were utilized: the solar fraction (SF) and the thermal efficiency of the system (ηth). On days with distinctive weather conditions, both systems obtained approximately equal SF and ηth values, without a specific preference between the ambient conditions and the type of SWHS. Regarding a four-day nonstop operation, the FC overperformed the thermosyphon system at both energy indicators. Namely, for the FC and the thermosyphon SWHS, the SF was calculated to be 0.62 and 0.48, and the ηth was 68.2% and 53.3%, respectively.
The constant increase in energy demand and related environmental issues have made fuel cells an attractive technology as an alternative to conventional energy technologies. Like any technology, fuel cells face drawbacks that scientific society has been focused on to improve and optimize the overall technology. Thus, the cost is the main inhibitor for this technology due to the significantly high cost of the materials used in catalyst layers. The current discussion mainly focuses on the fundamental electrochemical half-cell reaction of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) that are taking place in the catalyst layers consisting of Platinum-based and Platinum-non noble metals. For this purpose, studies from the literature are presented and analyzed by highlighting and comparing the variations on the catalytic activity within the experimental catalyst layers and the conventional ones. Furthermore, an economic analysis of the main platinum group metals (PGMs) such as Platinum, Palladium and Ruthenium is introduced by presenting the economic trends for the last decade.
Hydrogen (H2) can be a promising energy carrier for decarbonizing the economy and especially the transport sector, which is considered as one of the sectors with high carbon emissions due to the extensive use of fossil fuels. H2 is a nontoxic energy carrier that could replace fossil fuels. Fuel Cell Electric Vehicles (FCEVs) can decrease air pollution and reduce greenhouse gases when H2 is produced from Renewable Energy Sources (RES) and at the same time being accessible through a widespread network of Hydrogen Refueling Stations (HRSs). In this study, both the sizing of the equipment and financial analysis were performed for an HRS supplied with H2 from the excess electrical energy of a 10 MW wind park. The aim was to determine the optimum configuration of an HRS under the investigation of six different scenarios with various numbers of FCEVs and monthly demands, as well as ascertaining the economic viability of each examined scenario. The effect of the number of vehicles that the installation can refuel to balance the initial cost of the investment and the fuel cost in remote regions was investigated. The results showed that a wind-powered HRS could be a viable solution when sized appropriately and H2 can be used as a storage mean for the rejected wind energy. It was concluded that scenarios with low FCEVs penetration have low economic performance since the payback period presented significantly high values.
Fuel cells are promising energy conversion devices exhibiting high electrical efficiencies and zero emissions when green hydrogen is employed as a fuel feedstock, with applications in both the mobility and stationary sectors. This paper presents a comprehensive review on anode and cathode layer macroscopic modelling studies for proton exchange membrane fuel cells (PEMFCs) that incorporate in a coupled manner both the electrochemical and transport (mass, heat and momentum) phenomena taking place at each compartment. The reviewed models have been classified according to their spatial dimensions into one-dimensional, two-dimensional and three-dimensional, giving particular emphasis on the examination of both catalyst layers. For each examined case, valuable information is provided regarding the modelling technique applied, the assumptions that have been made, and the validation procedure followed. This review includes essential information regarding the suitability of each simulation method to understand the impact of electro-catalysts’ physicochemical properties on the overall PEMFC electrochemical performance. In this sense, the requirement to simulate PEMFCs operation by investigating several alternative electrode material composites is underlined to provide a credible pathway to improve cell performance and minimize or even eliminate the incorporation of costly materials such as platinum or platinum group metals (PGM) in the anode and cathode electrodes.
To help stakeholders plan, research, and develop Hybrid Renewable Energy Systems (HRES), the elaboration of numerous modelling techniques and software simulation tools has been reported. The thorough analysis of these undoubtedly complex systems is strongly correlated with the efficient utilisation of the potential of renewable energy and the meticulous development of pertinent designs. In this context, various optimisation constraints/targets have also been utilised. This specific work initially carries out a thorough review of the modelling techniques and simulation software developed in an attempt to define a commonly accepted categorisation methodology for the various existing HRES simulation methods. Moreover, the widely utilised optimisation targets are analysed in detail. Finally, it identifies the sensitivity of two commercial software tools (HOMER Pro and iHOGA) by examining nine case studies based on different wind and solar potential combinations. The results obtained by the two commercial tools are compared with the ESA Microgrid Simulator, a software developed by the Soft Energy Applications and Environmental Protection Laboratory of the Mechanical Engineering Department of the University of West Attica. The evaluation of the results, based on the diversification of the renewable energy potential used as input, has led to an in-depth assessment of the deviances detected in the software tools selected.
The energy cost fluctuations along with the gradual decrease of natural resources (i.e. oil, natural gas, coal) and the environmental issues raised by the extended use of fossil fuels, leads to the urgent development of advanced and clean energy systems. On top of that, the European Union requests new climate and energy targets to be supplied from Renewable Energy Sources until 2030. One of the main barriers to reach these targets is the stochastic character of RES combined with the demand fluctuations during the operation of the energy systems. In this direction, hydrogen technology can contribute as an energy carrier that can be produced by supplying electricity from RES to electrolysis modules without emitting hazardous gasses and then storing it at the metal hydride canisters for future use via fuel cell (FC) systems. Modeling these systems is important for achieving optimal sizing and being beneficial in both energy and economic aspects, while at the same time RES penetration is increased. For the accurate understanding of the hydrogen systems and especially FCs, it is important to comprehend the operating principles and both thermodynamic and electrochemical phenomena. In the present study, an integrated mathematical model is proposed concerning the electrical and thermal behavior of the FC. The model developed is able to simulate the operation of FC by using FC’s technical specifications. The reliability of the model was validated under three different hydrogen flow patterns applied in the experimental configuration NEXA 1.2 kW. The proposed model was applied to a specific case study for optimum sizing of a FC system in terms of maximum hydrogen absorption under a hydrogen production pattern from electrolysis utilizing excess energy from a wind park installed in an autonomous grid island network.
A typical meteorological year (TMY) is a set of meteorological and solar radiation parameters usually consisting of hourly values in a year for a given geographical location. Such TMYs have been derived in many countries for various applications. In Greece, TMYs have been generated mainly for Athens. The present work derives TMYs for 33 locations in Greece distributed all over the country and covering its 4 climatic zones defined for energy purposes. The TMYs are based on meteorological data collected from the Hellenic National Meteorological Service network in Greece in the period 1985–2014. The solar radiation needed in the generation of the TMYs is derived by using the meteorological radiation model. The TMY at every one of the 33 locations is provided in 5 versions to adapt to the applications of meteorology-climatology, bio-meteorology, agro-meteorology-hydrology, photovoltaics, and energy design of buildings. The 5 versions of the TMYs are generated following the modified Sandia National Laboratories method. After their generation, each version of the TMYs at any of the 33 locations is compared with the annualand monthly-averaged values of the selected parameters of the initial 30-year database. The comparison shows that the errors (differences) do not usually exceed 5%, thus supporting the fact that the so-derived TMYs are representative of the sites selected. Also, the distribution of the errors follows the normal distribution curve.
The objective of the present work is the medium, short and very short-term prognosis of load demand (LD) for the small-scale island of Tilos in Greece. For this purpose, Artificial Neural Network (ANNs) models were developed to forecast the LD of Tilos for different prediction horizons and time intervals, these covering the cases of 24 h ahead in hourly intervals (medium term prognosis), 2 h ahead in 10-min intervals (short term prognosis) and 10-min ahead in 1-min intervals (very short term prognosis). At the same time, stochastic/persistence autoregressive (AR) models were also developed and compared with the respective ANN models with regards to the LD prediction results obtained. For the training of the developed ANNs, meteorological data covering the period 2015 e 2017 were used, which had been recorded in 1-min intervals by two meteorological masts installed on the island Tilos. Furthermore, the biometeorological human thermal comfort-discomfort index, known as the cooling power index (CP), was also estimated and introduced in the training procedure of the forecasting models, while, for the evaluation of both AR and ANN forecasting models, well established statistical evaluation indices were applied. To this end, results show that in all cases covered, i.e. for both medium and short-term prognoses, the developed ANN forecasting models present a remarkable ability to predict the local LD of the island with high accuracy, enabling in this way the development of advanced energy management tools for both end-users and the system operators.
The rapid increase of world energy demand over the past decades has led to an extensive exploitation of the planet’s natural resources. Given that the population growth will continue for the next decades, there is an urgent need for a switch to Renewable Energy Sources (RES). One of the promising solutions to the energy problem is the geothermal energy which consists of stored energy in the earth in the form of water or steam at high pressure and temperature. Countries located in seismic zones possess a high geothermal potential due to their position at destructive and constructive plate margins. Greece is one of the countries with a remarkable potential in geothermal energy of medium and high enthalpy fields. Interest in the exploitation of geothermal fields for electricity generation is mainly focused on non-interconnected Greek islands, where energy is produced from oil-based generators with high energy cost and low-quality electricity, mainly during the summer months which is a period of high energy demand. In this respect, the present study investigates the possibility of combining a geothermal power plant with a concentrated solar array to provide energy to an autonomous island grid. Since the productivity of geothermal fields is gradually decreasing over the years, the combination of solar with geothermal energy source aims to the extension of the productive lifetime of the field, achieved by reheating the geothermal fluid during reinjection to the injection well. The thermal energy, generated by concentrated solar collectors, increases the temperature of the geothermal brine and reduces the energy degradation of the field. In this study, the energy efficiency of different reheat values are studied in order to achieve the combination of a geothermal power plant and a concentrated solar field that can meet specific targets for lifetime energy production. According to the results, the proposed installation can merely contribute to the energy autonomy of the island grid during periods of low demand and provide energy security in periods of high energy consumption.
Assessment of the electricity generation status for Non-Interconnected Islands (NIIs) of the Aegean Sea region, excluding the electricity systems of Crete and Rhodes, is undertaken in the current study. The authors focus on the long-term analysis of thermal power generation characteristics and also on the challenges so far limiting the contribution of Renewable Energy Sources (RES) in covering the electricity needs of the specific area. According to the present analysis, due to the existing technical limitations, the annual RES shares in the electricity balance of NIIs of the Aegean Sea have since 2010 stagnated in the range of 15% to 18%. Moreover, the performance of thermal power stations for all 30 NII systems is evaluated on the basis of their utilization factor, associated fuel consumption and electricity production costs. The vast majority of these stations is characterized by low capacity factors in combination with high specific fuel consumption and high operational expenses that in the case of smaller scale island regions could even exceed 600€/MWh. At the same time, the authors discuss on the alternatives and encourage further investigation of novel, intelligent energy solutions, such as the smart microgrid and battery-based hybrid power station that are currently developed on the island of Tilos under the implementation of the TILOS Horizon 2020 program.
The European Union’s 2020 climate and energy package (known as “20–20–20” targets) requests, among other key objectives, 40% of the electricity production in Greece to be supplied from Renewable Energy Sources by 2020. The main barriers for reaching this target is the intermittency of renewable energy sources combined with the penetration limits in the local electrical grids and the high seasonal demand fluctuations. In this context, the introduction of energy storage systems, comprises one of the main solutions for coping with this situation. One of the most promising technologies for storing the excess energy, that would be otherwise lost, is the production and storage of hydrogen through water electrolysis. Hydrogen can be used for supporting the electricity grid during periods of high demand but also as transportation fuel for hydrogen-based automobiles (e.g. fuel cell vehicles). For this purpose, a simulation algorithm has been developed, able to assess the specifications of the optimum sizing of hydrogen production storage systems. For the application of the algorithm, the area of the Aegean Sea has been selected, owed to the considerable renewable energy sources curtailments recorded in the various non-interconnected islands in the region. More specifically, the developed algorithm is applied to an autonomous electricity network of 9 islands, located at the SE area of the Aegean Sea and known as the “Kos-Kalymnos” electricity system. The results obtained designate the optimum size of the hydrogen-based configuration, aiming to maximize the recovery of otherwise curtailed renewable energy production.
The European Union’s 2020 climate and energy package (known as “20-20-20” targets) requests, among other key objectives, 40% of the electricity production in Greece to be supplied from Renewable Energy Sources (RES) by 2020. The main barriers for reaching this target is the intermittency of RES combined with the penetration limits in the local electrical grids and the high seasonal demand fluctuations. In this context, the introduction of energy storage systems (ESSs), comprises one of the main solutions for coping with this situation. One of the most promising technologies for storing the excess energy, that would be otherwise lost, is the production and storage of hydrogen through water electrolysis. Hydrogen can be used for supporting the electricity grid during periods of high demand and as transportation fuel for H 2 -based automobiles (e.g. fuel cell vehicles). For this purpose, a simulation algorithm has been developed, able to assess the specifications of the optimum sizing of hydrogen production storage systems. For the application of the algorithm, the area of the Aegean Sea has been selected, owed to the considerable RES curtailments recorded in the various non-interconnected islands in the region. More specifically, the developed algorithm is applied to an autonomous electricity network of 9 islands, located at the SE area of the Aegean Sea and known as the “Kos-Kalymnos” electricity system. The results obtained designate the optimum size of the hydrogen-based configuration, aiming to maximize the recovery of otherwise curtailed RES production.
Autonomous photovoltaic systems have already been proved as one of the most reliable ways to handle the electrification requirements of remote consumers in isolated areas. The technology improvement in the field of building integrated photovoltaic systems, along with the governmental financial incentives for boosting the corresponding energy sector, have increased the interest of installing small scale photovoltaic systems not only in remote dwellings but also in grid-connected households. The penetration of photovoltaic systems in densely populated areas has made people even more familiar with them and therefore people with environmental consciousness are more likely to adopt the specific technology. In this context, the present work aims in highlighting the capabilities of building integrated photovoltaic systems along with battery storage devices in covering the electrical needs of a typical dwelling using real electricity demand and meteorological data based on Typical Meteorological Year time series. The system is simulated on an hourly basis by using an integrated numerical code that has been developed by the Soft Energy Applications & Environmental Protection Laboratory of the Mechanical Engineering Dept. of Piraeus University of Applied Sciences in Greece. The proposed solution guarantees zero load rejections under different solar potential schemes investigating also the possibilities of minimizing the storage system capacity.
The realization of Renewable Energy Sources (RES) for power generation as the only environmentally friendly solution, moved solar systems to the forefront of the energy market in the last decade. The capacity of the solar power doubles almost every two years in many European countries, including Greece. This rise has brought the need for reliable predictions of meteorological data that can easily be utilized for proper RES-site allocation. The absence of solar measurements has, therefore, raised the demand for deploying a suitable model in order to create a solar map. The generation of a solar map for Greece, could provide solid foundations on the prediction of the energy production of a solar power plant that is installed in the area, by providing an estimation of the solar energy acquired at each longitude and latitude of the map. In the present work, the well-known Meteorological Radiation Model (MRM), a broadband solar radiation model, is engaged. This model utilizes common meteorological data, such as air temperature, relative humidity, barometric pressure and sunshine duration, in order to calculate solar radiation through MRM for areas where such data are not available. Hourly values of the above meteorological parameters are acquired from 39 meteorological stations, evenly dispersed around Greece; hourly values of solar radiation are calculated from MRM. Then, by using an integrated spatial interpolation method, a Greek solar energy map is generated, providing annual solar energy values all over Greece.
Human thermal comfort sensation is defined as the conditions in which human expresses satisfaction with the thermal environment, absence of thermal discomfort, or conditions in which a great percentage of the population (more than 80%) do not express dissatisfaction. The assessment of these conditions can be accomplished by the application of a large number of theoretical and empirical indices estimated using meteorological parameters such as air temperature, wind speed, air humidity and solar radiation. The aim of this work is to investigate the human sensation of heat stress in 30 different sites across the Greek territory, during the warm period of the year, for the period 1995-1999. For that purpose, one of the widely used thermal index, Heat Index (HI), which was adopted by the USA’s National Weather Service, is calculated. HI is an index that is also called "apparent temperature". This index is a measure of how hot someone feels when relative humidity is added to the actual air temperature. For the estimation of HI values, hourly values of air temperature and relative humidity were used. The aforementioned meteorological data have been recorded by the network of meteorological stations of the Hellenic National Meteorological Service. Results indicate a great variability of heat stress conditions, at the same time in different regions across Greece, within the warm period of the year. Furthermore, it seems that the height above sea level, geographical coordinates and the distance from the sea plays an important role in the establishment of asychronous heat stress conditions. In general terms, during the warm period of the year neutral human thermal conditions along the Greek territory are prevailing. These conditions, allow people to have a com- fort thermal sensation as well as outdoor activities (agricultural activities, livestock, tourism etc).
Although efficiency of photovoltaic (PV) modules is usually specified under standard test conditions (STC), their operation under real field conditions is of great importance for obtaining accurate prediction of their efficiency and power output. The PV conversion process, on top of the instantaneous solar radiation, depends also on the modules’ temperature. Module temperature is in turn influenced by climate conditions as well as by the technical characteristics of the PV panels. Taking into consideration the extended theoretical background in the field so far, the current study is focused on the investigation of the temperature variation effect on the operation of commercial PV applications based on in-situ measurements at varying weather conditions. Particularly, one year outdoor data for two existing commercial (m-Si) PV systems operated in South Greece, i.e. an unventilated building-integrated (81 kWp) one and an open rack mounted (150 kWp) one, were collected and evaluated. The examined PV systems were equipped with back surface temperature sensors in order to determine module and ambient temperatures, while real wind speed measurements were also obtained for assessing the dominant effect of local wind speed on the PVs’ thermal loss mechanisms. According to the results obtained, the efficiency (or power) temperature coefficient has been found negative, taking absolute values between 0.30%/ oC and 0.45%/ oC, with the lower values corresponding to the ventilated free- standing frames.
Increased interest is recently demonstrated in the promotion of distributed generation based on renewable energy sources (RES). Contrariwise, owed to RES intermittency, most remote areas rely on oil-fired power generation. To facilitate further penetration of RES, the concept of demand side management (DSM) has lately emerged. To this end, a new DSM algorithm is currently developed. Peak shaving and load shifting are applied, considering implementation levels that the residential sector may comply with. Finally, a small-medium scale island grid is used as case study, with our results indicating that the appropriate level of DSM application may yield considerable benefits.
To confront problems concerning large-scale integration of renewable energy sources, introduction of energy storage constantly gains ground. Benefits stemming from the adoption of energy storage include exploitation of otherwise rejected energy, increased reliability of energy supply and improved operation of a given power system overall. In this regard, contribution of such systems in achieving large-scale integration of wind energy into island grids is currently considered. More precisely, fuel cells and hydrogen storage (FC–HS) are investigated, in comparison with conventional batteries. For this purpose, a simulation algorithm is developed to study the energy performance of different FC–HS configurations used to recover wind energy curtailments. The developed algorithm is then applied to a representative Aegean island of medium–high quality wind potential. Results obtained indicate that FC–HS may become attractive in comparison with conventional batteries, only in the case that the use of hydrogen surplus to cover other energy flows is also put forward.
Across the entire Greek territory one may encounter several remote consumers that could cover their needs on the basis of PV-based stand-alone applications exploiting the high quality local solar potential. In this context, optimum sizing of such installations also involves investigation of the optimum panels’ tilt angle, which opposite to grid-connected applications is required to provide year-round energy autonomy rather than maximization of the annual energy yield. Considering the above, investigation of the optimum panels’ tilt angle for stand-alone applications is the aim of the specific study, in both theoretical and experimental terms. Theoretical investigation is based on the validation of the assumption that the optimum angle for such applications coincides with the angle that provides maximum exploitation of solar potential during winter months, while following, the optimum angle determined in the area of 60o is also experimentally validated with the conduction of long-term winter measurements for the area of Athens.
On the numerous small and medium-sized Greek islands one may encounter several thousands of remote consumers unable to appreciate a direct electricity utility supply. For this purpose, remote consumers usually cover their electricity needs based on the operation of small diesel-generator sets. On the other hand, most of these areas appreciate high quality solar potential that comprises a stimulus for the use of stand-alone photovoltaic (PV)-based configurations. In this context, the primary objective of the present study is to determine the optimum dimensions of a stand-alone PV-diesel system, under the restriction of minimum long-term electricity generation cost, and accordingly obtain a comparison with diesel-only systems. For this purpose, the developed methodology is applied to a representative Greek island, with results obtained being rather encouraging for the implementation of the proposed solution.
Renewable energy sources (RES) based stand-alone systems employing either wind or solar power and energy storage comprise a reliable energy alternative, on top of conventional diesel-electric generator sets, commonly used by remote consumers. However, such systems usually imply the need for oversizing and considerable energy storage requirements leading to relatively high costs. On the other hand, hybrid configurations that may exploit both wind and solar potential of a given area may considerably reduce energy storage capacity and improve the economic performance of the system. In this context, an integrated techno-economic methodology for the evaluation of hybrid wind– photovoltaic stand-alone power systems is currently developed, aiming at the designation of optimum configurations for a typical remote consumer, using economic performance criteria. For the problem investigation, the developed evaluation model is applied to four representative areas of the Greek territory with different wind potential characteristics in order to obtain optimum configurations on the basis of minimum initial investment, 10-year and 20-year total cost. According to the results obtained, the proposed solution is favorably compared with all other stand-alone energy alternatives, reflecting the ability of hybrid systems to adjust even in areas where the local RES potential is not necessarily of high quality.
Renewable energy sources (RES) based stand-alone systems employing either wind or solar power and energy storage comprise a reliable energy alternative, on top of conventional diesel-electric generator sets, commonly used by remote consumers. However, such systems usually imply the need for oversizing and considerable energy storage requirements leading to relatively high costs. On the other hand, hybrid configurations that may exploit both wind and solar potential of a given area may considerably reduce energy storage capacity and improve the economic performance of the system. In this context, an integrated techno-economic methodology for the evaluation of hybrid wind– photovoltaic stand-alone power systems is currently developed, aiming at the designation of optimum configurations for a typical remote consumer, using economic performance criteria. For the problem investigation, the developed evaluation model is applied to four representative areas of the Greek territory with different wind potential characteristics in order to obtain optimum configurations on the basis of minimum initial investment, 10-year and 20-year total cost. According to the results obtained, the proposed solution is favorably compared with all other stand-alone energy alternatives, reflecting the ability of hybrid systems to adjust even in areas where the local RES potential is not necessarily of high quality.