Search

The primary scope of the present work is the comprehensive investigation of a novel multigeneration system that exploits renewable energy sources, i.e. solar energy, geothermal energy, and biomass, and produces four useful outputs, i.e. electricity, heating, cooling, and green hydrogen. Solar irradiation is the primary energy source, which is exploited by parabolic trough collectors and powers a supercritical CO 2 regenerative Brayton cycle, while a biomass boiler serves as an auxiliary energy input. In addition, the whole installation includes a proton exchange membrane water electrolyzer, a water heater, as well as an absorption chiller, where a geothermal well is used for heat rejection. Apart from the thermal energy storage tank, which is coupled with the collectors, hydrogen production is used as a secondary energy storage method, as the excess produced renewable electricity feeds the electrolyzer. Consequently, the installation is examined parametrically, and optimized on steady-state conditions. Then, a dynamic simulation study is conducted for the climate data of Athens, Greece. According to the results, at the optimum operation point on steady-state conditions, the energy efficiency is determined at 37.89%, and the exergy efficiency at 20.86%. Moreover, the annual energy and exergy efficiencies are defined at 32.52%, and 18.51%, respectively. The economic evaluation analysis indicates that the payback period is about 11 years, while the internal rate of return, and the net present value are determined at 10.62% and 7,753,905 €, respectively. In addition, the system is estimated to save 3072.6 tons of CO 2 equivalent per annum. In conclusion, the proposed multigeneration system can address the various energy needs and recent environmental challenges, as a totally renewable, economically feasible, and environmentally friendly solution. • A multigeneration system that produces four useful outputs is examined. • The unit exploits three renewable sources, solar, biomass, and geothermal. • Both steady-state and dynamic analyses are carried out. • The annual energy and exergy efficiencies are defined at 32.52% and 18.51%. • The system payback period is found at 11 years, while it saves 3072.6 tn CO2-eq. [ABSTRACT FROM AUTHOR]

The focus of the current study is on a cogeneration unit designed to produce both electricity and hydrogen, which is made up of a parabolic trough collectors' field associated with a storage tank, a recuperative organic Rankine cycle module, and a proton exchange membrane water electrolyzer. The latter system operates using electricity generated by the organic Rankine cycle to produce hydrogen, which can subsequently be used as either a fuel or a storage medium for seasonal storage. The study begins with a thermodynamic investigation under both steady-state and dynamic conditions. The solar field is simulated using the Engineering Equation Solver, while the organic Rankine cycle and the electrolyzer are modeled in Aspen Plus. The analysis extends to economic and environmental evaluations, along with an optimization procedure based on techno-economic criteria. Regarding the optimum design case, the annual net electricity yield is determined at 333.8 MWh, the yearly hydrogen production amount is equal to 6236.6 kg, while the yearly energy efficiency is calculated at 14.9% and the yearly exergy efficiency at 15.9%. Finally, the economic investigation indicates the net present value is about 645,555 €, while the system contributes to saving 198.1 tn CO2-eq during the yearly operation. [Display omitted] • A solar-driven electricity and hydrogen cogeneration unit is investigated. • Thermodynamic, economic, and environmental aspects are examined. • The net electricity production is found at 333.8 MWh per year. • The hydrogen production amount is determined at 6236.6 kg per year. • The investment NPV is 645,555 €, while it saves 198.1 tn CO2-eq per year. [ABSTRACT FROM AUTHOR]

• An innovative in-line water heater solar thermal collector is proposed. • PCM function was validated through experimental results from literature (<5% deviations). • The collector was optically optimized and exceeded another one from literature by up to 30%. • The implementation of fins enhances significantly the performance of the collector. • The 8x8 plus geometry ensures 96% more useful output (2.78 kWh) than the no-fin one. In the present work, an innovative asymmetrical evacuated tube compound parabolic collector (CPC) with integrated Phase Change Material (PCM) is proposed as an in-line water heater. This solar collector was examined numerically as regards its thermal and optical operation. The optical efficiency was firstly optimized by modifying two independent design variables of the reflector. Then the optical performance was calculated for the finally selected geometry, considering 15 different transversal incident angles. As regards the thermal operation of the collector, the charging and discharging of PCM were investigated in detail. Four different internal fin arrangements were compared to the no-fin case and each other and the optimum one was revealed. The simulations were conducted for the city of Athens in Greece considering the typical clear day in July. It was considered that there is a charging period of five hours from 09:00 to 14:00 (solar time) and then a discharging one with a duration of one hour. For the thermal optimization, a volumetric flow rate of 4 L/min was selected during the discharging phase. The water temperature from the grid was considered to have a temperature of around 20 °C. The main parameters which were evaluated during charging and discharging were the useful power output, the outlet water temperature and the mean PCM temperature. For validation purposes, experimental results from a previous study were used. In the previous study, a PCM heat exchanger with water as the working fluid was examined experimentally. The same heat exchanger was designed in the present study for comparing the results. It was found that the present numerical results are very close to the experimental of the previous study with a mean deviation of 1.60 % and a maximum of 4.24 %. The design and the numerical simulations were conducted with SolidWorks Flow Simulation software. The final results show that the best design leads to 96 % higher daily useful heat production compared to the baseline case without fins. [ABSTRACT FROM AUTHOR]

• A novel Carnot Battery system with double waste heat source exploitation is studied. • Organic Rankine cycle, high-temperature heat pump and latent storage are used. • Parametric analysis and optimization are performed in steady-state conditions. • For the benchmark scenario, the roundtrip efficiency of 64 % was calculated. • Performance enhancements up to 174 % compared to ambient source unit were found. Carnot batteries are critical technologies for increasing the penetration of renewable energy sources because they enable suitable energy storage for long-term periods. The achievement of a high round-trip efficiency is an important point that determines the sustainability of the Carnot batteries. In this direction, the present work comes to investigate a Carnot battery with latent storage that exploits properly a waste heat stream. The examined unit includes a high-temperature heat pump, an organic Rankine cycle, a latent storage system and the proper heat exchanger for the waste heat stream exploitation. The novelty of the studied idea lies in the exploitation of the waste heat stream in two ways. Specifically, the waste heat stream feeds firstly the latent storage system with heat, and secondary it feeds the evaporator of the heat pump with heat input for enhancing the heat pump's coefficient of performance. This design makes it possible to achieve high round-trip efficiency values and also to exploit properly a waste heat stream. For the benchmark scenario with 100 kW electricity input, 350 kW waste heat input at 200 °C and 160 °C storage temperature, the round-trip efficiency was found at 64 %, the system's energy efficiency at 14.2 %, the system's exergy efficiency at 36.7 % and the produced electricity at 64 kW. [ABSTRACT FROM AUTHOR]

• The use of an absorption chiller for subcooling in a CO 2 system is studied. • The absorption chiller is fed by waste heat after the compressor. • In all the cases, the examined system is more efficient than the reference. • The mean COP enhancement is about 23% while the maximum is 75%. • The enhancements are greater in the cases with higher compressor pressure ratio. The objective of this work is to examine a transcritical CO 2 refrigeration system coupled to a single-effect absorption chiller. The role of the absorption machine is to create a subcooling after the gas-cooler in order to increase the COP. The absorption system is fed by waste heat after the CO 2 compressor and so there is not any need for any external energy source. The analysis is conducted with a validated numerical model which is developed in Engineering Equation Solver. The system is studied and optimized for different heat rejection temperatures from 35°C up to 50°C and for different refrigeration temperatures from -35°C up to 5°C. It is found that there is COP enhancement in all the operating scenarios and especially in the cases with higher heat-rejection temperature and lower refrigeration temperature. The mean COP enhancement is about 23.4% which is an important enhancement for designing more efficient systems. [ABSTRACT FROM AUTHOR]

• Various ideas for the optical enhancement of a linear Fresnel collector are studied. • The results are evaluated on a yearly basis for the climate of Athens – Greece. • The use of an extended receiver enhances the yearly performance up to 50.3%. • The use of a displaced receiver enhances the yearly performance up to 20.2%. • The combination of these methods enhances the yearly performance up to 48.7%. The objective of this work is the investigation of some alternative ideas for enhancing the optical performance of a linear Fresnel reflector (LFR) with North-South orientation. More specifically, the examined methods aim to reduce the optical end losses which are crucial for the short LFR plants in particular during the winter period. The first studied idea is the extension of the receiver after the concentrator which is able to enhance the yearly mean incident angle modifier up to 50.3%. The second examined idea is the displacement of the receiver in order to eliminate the non-illuminated area at the beginning of the receiver. This idea proved that the mean yearly incident angle modifier can be enhanced up it 20.2% for a displacement equal to 20% of the concentrator length. The third examined idea is the hybrid design with the extended and displayed receiver in combined. This idea leads to intermediates enhancements compared to the previous cases but its advantage is the lower investment cost compared to the simple receiver extension case. The analysis is performed with a developed optical model in SolidWorks Flow Simulation and the yearly evaluation has been done for the location of Athens (Greece). [ABSTRACT FROM AUTHOR]

Highlights • In this study, a solar driven cooling system with absorption chiller is investigated. • The system is examined without and with phase change materials into radiant walls. • The best location of the PCM layer is found to be in the south wall. • The auxiliary energy consumption can be reduced from 30% up to 63%. • The application of the PCM layer can decrease the indoor temperature on average 0.6 °C. Abstract This work investigates energetically and financially a building solar cooling system with radiant walls which includes phase change materials (PCMs). The examined building has a floor area of 100 m2 and it is located in Athens (Greece). The solar cooling system includes evacuated tube collectors coupled to a single-effect absorption chiller for producing cold water for the building radiant walls. The use of PCM is examined in all the building's outer walls and different scenarios with PCM in only some walls are also investigated in details. Different combinations of collecting areas and storage tank volumes are examined in order to determine the optimum design for every scenario. The study is performed with the commercial software TRNSYS which uses an active layer to simulate the radiant wall. The most important calculated parameters of this work are the auxiliary energy consumption, the solar coverage and the indoor temperature of the building. The results indicate that the best location of PCM layer is in south wall with a reduction of the auxiliary energy 30%, an increase of the solar coverage 3.8% and a reduction of the total system cost of about 3%. [ABSTRACT FROM AUTHOR]

Abstract Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and high-temperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC configurations regard the thermal efficiency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modifications are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modifications include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat flux. The thermal modifications regard the use of nanofluids, turbulators and the use of thermally modified receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with flat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies. [ABSTRACT FROM AUTHOR]

Highlights • A secondary booster reflector is examined in a parabolic trough collector. • The optical enhancement is found to be higher in greater incident angles. • The optical enhancement is greater during the winter period. • The yearly enhancement is found to be 21.7% for Athens, Greece. • The thermal enhancement is more intense in higher operating temperatures. Abstract The objective of this work is the investigation of a novel booster reflector in a parabolic trough collector (PTC). More specifically, a vertical secondary booster reflector is located at the end of the trough in order to enhance the optical performance of the solar collector. This extra concentrator reduces the end optical losses of the PTC by reflecting back them in the receiver. Moreover, it acts as an extra reflecting area which captures extra solar beam irradiation in the cases with a non-zero incident angle. The collector is examined optically and thermally in order to determine both the optical and the thermal gain by the use of this novel idea. The overall analysis is conducted for steady-state conditions, as well as the yearly evaluation of the suggested idea is given. According to the final results, the use of the extra booster leads always to optical and thermal enhancement, especially in greater incident angles. The yearly optical enhancement, for a collector with a focal distance to length ratio of 0.236, is calculated at 21.7% and it is more intense in the months with greater incident angles. More specifically, it was calculated at 8.7% in June, 24.2% in September and 77.1% in December. The thermal analysis proved that the thermal enhancement is greater than the optical and it is higher when the system operates with great fluid temperature levels. [ABSTRACT FROM AUTHOR]

Highlights • The use of nanofluids increases both the thermal and electrical performance. • The collector can operate up to 200 °C with oil and nanofluid. • At 50, 100 and 150 °C, the thermal efficiency was 45.0%, 46.8% and 39.7% respectively. • At 50, 100 and 150 °C, the electric efficiency was 5.8%, 6.6% and 3.9% respectively. • At 100 °C, the thermal and electric enhancements were 1.66% and 5.17% respectively. Abstract The objective of this paper is the investigation of a thermal photovoltaic concentrating collector operating with and without nanofluids. The use of nanofluid Syltherm 800/copper oxide with 5% volumetric concentration is examined and it is compared with the base fluid. The examined thermal photovoltaic has a parabolic concentrator and an inverted flat insulated receiver. The analysis is conducted with SolidWorks Flow Simulation which is able to perform the thermal, optical and flow analysis. According to the present study, the system is able to operate for inlet fluid temperatures up to 200 °C which is a promising result. Moreover, it is found that the use of nanofluid leads to enhancement both in thermal and electrical performance for all the examined cases (different combinations of flow rates and inlet temperatures). For an inlet temperature at 100 °C and flow rate 540 L/h with nanofluid, the thermal efficiency and the electrical efficiency were found at 46.84% and 6.60% respectively. For this case, the enhancements in the thermal, electrical, total and exergy efficiencies were found at 1.66%, 5.17%, 2.08% and 3.05% respectively. [ABSTRACT FROM AUTHOR]

Abstract In this work, exergy of incoming solar radiation is estimated using empirical models. Long-term meteorological data (1986–2000) consisting of monthly mean diffuse and global solar radiation on a horizontal plane, ambient temperature and sunshine hours for 23 selected stations were available from Indian Meteorological Department (Pune), India. Exergy of direct and diffuse radiations was calculated using Petela model and summed to obtain exergy of global solar radiation. Beam and diffuse exergy efficiencies were in the range of 0.9286–0.9365 and 0.7189–0.7499 respectively. Estimated global exergy efficiencies were found in the range of 0.7700–0.8912. Empirical models were developed by correlating global solar exergy (in the form of global exergy efficiency and global exergy efficiency factor) in terms of clearness index and relative sunshine period. Four categories of single predictor models were defined with each consisting of eight models. Statistical analysis was used to compare and evaluate the accuracy of the models. Further, Global Performance Indicator was calculated and used to rank the models within the individual categories of models as well as the overall group of the 32 developed models. Overall assessment proved that power model with the clearness index is the best one for estimation of global exergy efficiency factor. Highlights • Estimation of monthly average global solar exergy over India is performed. • Solar exergy efficiency and exergy efficiency factor are described and calculated. • Solar radiation exergy is correlated in terms of clearness index & sunshine period. • 32 models are developed under four categories using long term solar radiation data. • Statistical error analysis is performed to evaluate the accuracy of the models. [ABSTRACT FROM AUTHOR]

The objective of this work is the investigation of a polygeneration system which is driven by solar energy and a biomass boiler. Parabolic trough solar collectors coupled to a storage tank are used in order to produce useful heat at high-temperature levels (∼350 °C). The system includes an organic Rankine cycle and a vapor compression cycle in order for electricity and cooling to be produced respectively. Moreover, useful heat is produced at two temperatures levels (50 °C and 150 °C) and so there are totally four useful energy outputs. The system is optimized in steady-state conditions and then the most suitable design is investigated in dynamic conditions for all the year period. According to the final results, the yearly energetic efficiency of the system is 51.26% while the yearly exergetic efficiency is 21.77%. The net present value is found 165.6 k€, the payback period 5.13 years, the internal rate of return 21.26%, while the yearly CO 2 avoidance to 125 tn. Finally, the results of this work indicate that the suggested configuration can provide various useful outputs with high efficiency and the total investment is characterized as viable due to the suitable financial index values. [ABSTRACT FROM AUTHOR]

Highlights • The incident angle modifiers of a linear Fresnel collector are calculated. • Analytical and simple expressions are suggested and validated. • Six different collectors are used for validating the developed model. • The deviations are generally low and up to 5%. • The developed formulas can be used also for optimization procedures. Abstract Linear Fresnel reflector is one promising solar concentrating technology for medium and high temperatures (200–400 °C) because of its simple design and its low cost. However, the greatest drawback of this collector is the increased optical losses and thus the emphasis is usually given to this issue. In this direction, the objective of this work is the development of analytical expressions for the incident angle modifiers of a linear Fresnel reflector for the longitudinal and the transversal directions. The goal is to create a simple and accurate formula for all the possible solar angles ranges. The developed equations are based on reasonable assumptions and on the geometric analysis of a simple collector with flat primary mirrors. The developed equations are tested with literature data from other studies and from commercial collectors. It is found that the developed equations lead to accurate results with mean deviations up to 5%. The developed analytical expressions can be used for the quick calculation of the optical performance of a linear Fresnel reflector, as well as they can be used for the geometry optimization of the collector. [ABSTRACT FROM AUTHOR]

Highlights • A linear Fresnel reflector is examined on daily, monthly and yearly basis. • The collector is examined thermally and optically with SolidWorks Flow Simulation. • The mean yearly incident angle modifier is found to be 37%. • The optimum operating temperature level is close to 450 K exergetically. • This collector operates efficiently from March up to September. Abstract The main objective of this study is to investigate the daily performance of a linear Fresnel reflector in energetic and exergetic terms. The collector is designed and examined in SolidWorks Flow Simulation in order to obtain the optical and thermal efficiency curves. These data are inserted in a developed numerical dynamic model for the prediction of the daily performance of the solar collector which is coupled to a storage tank. The collector has total aperture 27 m2, concentration ratio 20.46 and the storage tank volume is 3 m3. The system is investigated for twelve typical days, one for each month, and its yearly performance is determined by taking into consideration only the sunny days of every month. According to the final results, the mean yearly incident angle modifier is found to be 37% while the maximum yearly exergy efficiency is 8.0% when the system starts with 450 K inlet temperature at the morning. For this case, the mean yearly thermal efficiency is found to be 18.5% and also June is the month with the highest thermal and exergy outputs. Furthermore, it is important to state that the mean daily incident angle modifier is found to be the 81% of the maximum daily incident angle modifier for all the months. [ABSTRACT FROM AUTHOR]

Concentrating solar technologies are promising renewable energy systems for exploiting incident beam solar irradiation with high exergy efficiency values. These systems provide the possibility for producing useful heat at high temperatures that can be utilized by highly efficient power cycles or producing directly solar fuels with receiver reactor technology. In the last years, the concept of beam-down concentrating solar technology gains more and more attention due to a series of advantages associated with this idea. This concept is based on the use of two-stage reflectors for concentrating solar irradiation close to the ground, something that leads to a more compact system with reduced height. Furthermore, the high-temperature heat production and the chemical processes take place on the ground and not at a great height, increasing the safety levels of the system. Moreover, this design leads to compact configurations with lower materials use, lower wind loads and without the need to move the receiver for tracking the sun. The objective of this review is to present the recent progress on beam-down solar concentrating technology and to highlight the need for giving attention to this direction. Critical advantages of this technology are demonstrated and the associated limitations are discussed. The emphasis is on the presentation of the different technologies that can be coupled with the beam-down technology. Thermodynamic power cycles (Brayton, Rankine and Stirling), photovoltaics, thermochemical processes, as well as other applications are included and discussed. Practically, power production and solar fuels are the major useful outputs that can be generated by beam-down solar concentrating configurations. The reviewed technologies are critically discussed and compared in terms of energy, economic and environmental aspects. Future steps in the field are suggested based on the existing literature. [ABSTRACT FROM AUTHOR]

Solar energy exploitation is one of the most important weapons for facing the recent environmental and energy management dangers. Concentrating solar collectors can produce useful heat in medium and high-temperature levels. So, they can be used in a great variety of applications as space-cooling, industrial heat, chemical processes and electricity production. The use of nanofluids is one of the most effective ways for enhancing the performance of the concentrating collectors. In this paper, the use of nanofluids as working fluids in concentrating solar collectors is investigated in detail. The examined collector types are the following: Evacuated tube collectors with booster reflector, concentrating thermal photovoltaics, compound parabolic concentrator, parabolic trough collector, linear Fresnel reflector and solar dishes. Moreover, the use of nanofluid-based solar concentrating collectors in applications (heating, cooling, electricity, and trigeneration) is investigated in this work. The emphasis is given to the determination of the thermal efficiency enhancement of the collector with the use of the nanofluids in every case. The recent trends in nanofluid utilization are given in this work. Furthermore, the challenges and the future work about the nanofluid-based solar systems are discussed in details. [ABSTRACT FROM AUTHOR]

The objective of this work is to compare and to evaluate the performance parabolic trough solar collector (PTC) and the linear Fresnel solar reflector (LFR) for the climate conditions of Athens, Greece. These collectors are examined under the same operating conditions and they are evaluated energetically, exergetically and financially. The analysis is conducted for all the year period using twelve typical days, one for every month. Performance curves for the solar systems are taken from the literature in order to estimate the collector performance for different operating temperature levels, solar irradiation level and solar angles. According to the final results, the mean yearly incident angle modifier for the PTC is about 83.73%, while for the LFR is 58.66%, at the examined climate conditions. It has been found that the mean yearly thermal efficiency at 350 °C is 52.25% for the PTC and 35.39% for the LFR, while the mean yearly electrical efficiency of the equivalent power production system is 16.11% with the PTC and 10.92% with the LFR. The simple payback period for process heat production was found close to 3–4 years, while for power production close to 8–9 years. Moreover, it is proved that the specific cost of the LFR has to be lower than 185 €/m 2 in order to be a more attractive choice financially compared to the PTC. [ABSTRACT FROM AUTHOR]

Highlights • The secondary concentrator of a linear Fresnel reflector is optimized. • Bezier polynomial parametrization is applied for the optimization of the concentrator. • The optimum design has 19.39% higher optical performance compared to the initial. • The optimum design leads to a relatively uniform heat flux over the absorber. • The thermal efficiency of the optimum design is enhanced close to 20.50% Abstract Linear Fresnel reflector (LFR) consists a promising solar concentrating technology for medium and high temperatures (150–400 °C). Usually, these collectors are designed with a secondary reflector in order to maximize the exploitation of the incident solar irradiation. The objective of this work is to optimize the secondary reflector of an LFR using Bezier polynomial parametrization which is something new in this domain. This parametrization is based on the movement of some control points (three in this case) in order to determine the optimum secondary reflector geometry. According to the final results, the optimum optical efficiency of the LFR is found to be 72.84%, while the optical efficiency in the initial design is 61.01%. Moreover, the optical efficiency is calculated for different solar angles and it is found that the optimized concentrator has superior performance compared to the initial design in all the cases. Furthermore, it is found that the optimum design leads to a relatively uniform heat flux distribution over the absorber periphery compared to the initial design. The last step is the determination of the thermal efficiency performance curve and it is found 20.50% mean thermal efficiency enhancement with the optimum design. [ABSTRACT FROM AUTHOR]

Parabolic trough collector is one of the most usual solar collectors for applications up to 400 °C. The thermal enhancement of this collector concentrates a lot of interest and various techniques are tested in order for the thermal efficiency to be maximized with a reasonable penalty in the pressure drop. The use of nanofluids as working fluids, as well as the use of flow turbulators, mainly inserts and internal fins or tube dimples are the main techniques which are examined. The objective of this work is to give a complete literature review of the existing studies on this domain and to present a numerical comparative analysis between the most usual thermal enhancement techniques. More specifically, the use of oil-based nanofluids with 6% CuO is compared with the use of internal rectangular fins in the absorber, while the combination of these techniques is also examined. The analysis is conducted with a validated CFD model in SolidWorks Flow Simulation for various fluid temperature levels. According to the final results, the use of nanofluids leads to 0.76% thermal efficiency enhancement, the use of internal fins to 1.10% and the combination of these techniques to 1.54%. Moreover, emphasis is given in the pressure drop of the examined cases and in the evaluation criteria which are used in every case. [ABSTRACT FROM AUTHOR]

Solar cooling is one of the most promising technologies for covering the increasing needs of our society with a clean and sustainable way. This paper examines an ejector-absorption cooling system operating with the LiBr-H 2 O working pair which is driven by parabolic trough solar collectors. The main objective of this investigation is to optimize the total solar cooling system and to compare it with the respective optimum system with conventional absorption chiller and parabolic trough collectors. The generator temperature and pressure, as well as the pressure drop in the ejector, are the optimization parameters, while the maximization of the system performance is the objective function. According to the final results, the system with the ejector leads to higher COP (coefficient of performance) for the majority of the examined operating scenarios. Moreover, the optimized ejector-absorption system presents performance enhancement up to 60.9% compared to the conventional absorption system for the case with 12.5 °C evaporating temperature and 30 °C condenser temperature. For this optimum case, the system COP is about 1.01, the chiller COP is 1.65 and the system exergy efficiency is 4.76%. Generally, the system COP is ranged from 0.473 to 1.01 for the system with ejector, while the demanded specific collecting area from 0.987 to 2.11 m 2 /kW cooling . The analysis is performed with a developed model in Engineering Equation Solver which is validated with literature results. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the daily and the instantaneous performance of a linear Fresnel reflector (LFR) experimentally and numerically. The examined LFR has an inverted flat plate receiver with a 36 m 2 total collecting aperture area and is coupled to a storage tank of 1 m 3 volume. This system is investigated experimentally for daily operation in the period from May to October for the climate conditions of Athens (Greece), using water as the working fluid (temperature levels up to 100 °C). Moreover, a detailed mathematical model for the prediction of the instantaneous and of the daily performance is developed. This model is validated with the experimental results for 52 operating days and it estimates the daily useful energy production with a mean error of 3%. This modeling is based on the energy balance on the collector loop and the storage tank. The emphasis is given on the determination of the optical losses of the collector, which are associated with the sun position during the day. It is found that the maximum useful heat production is about 8.4 kW, while the maximum daily heat production is about 260 MJ. The results of this work can be exploited for the evaluation of integrated solar concentrating systems and especially of linear Fresnel reflectors. [ABSTRACT FROM AUTHOR]

The objective of this study is to optimize a solar driven trigeneration system under various optimization criteria. More specifically, parabolic trough collectors are selected to feed with heat the generator of a trigeneration system. The produced vapor is expanded in a turbine where electricity is produced. The system also includes an ejector device, a condenser where the heating is produced and an evaporator where the cooling is produced. The cooling load is produced at 10 °C and the heating load at 50 °C, typical temperature levels for building applications, as hotels. This system is optimized under steady-state conditions with different criteria which are based on the exergetic, energetic and economic performance. The optimization is performed separately with every criterion, as well as with combined goals performing multi-objective optimization procedures. Different criteria lead to different optimum system designs. According to the final results, using all the examined criteria together, the optimum system presents 11.26% exergy efficiency, 87.39% energy efficiency and 7.694 €/h energy savings cash flow. The electricity, cooling and heating productions are 4.6 kW, 7.1 kW and 59.4 kW respectively. These results are obtained for turbine pressure ratio 3.6, turbine inlet temperature 195.5 °C and R141b as working fluid. [ABSTRACT FROM AUTHOR]

Solar linear Fresnel reflector (LFR) is a promising concentrating technology, which presents important advantages such as the low investment cost, the reduced wind loads and the relatively low land utilization factor. The objective of this work is to investigate an innovative way for enhancing the thermal performance of LFR, especially at high temperatures. The utilization of nanofluid as heat transfer fluid is the investigated thermal enhancement method and more specifically the use of CuO nanoparticle dispersed on Syltherm 800 (6% volumetric concentration). The examined collector has total net aperture equal to 154 m 2 and concentration ratio of 58.36. The primary reflectors are curved mirrors, the secondary reflectors have compound parabolic shape and the receiver is an evacuated tube. The operation with nanofluid is compared to the operation with pure thermal oil for various inlet temperatures from 350 K up to 650 K and flow rate equal to 200 L/min. According to the final results, the maximum thermal efficiency enhancement with the nanofluid is close to 0.8%, while the pumping work demand is increased up to 50% with the nanofluid. Various criteria like the exergy efficiency, overall efficiency and entropy generation are applied in order to evaluate the nanofluid utilization properly. Finally, the operation with nanofluid is found to be beneficial, especially in high-temperature levels. The analysis is conducted with SolidWorks Flow Simulation with a validated model. [ABSTRACT FROM AUTHOR]

Solar energy utilization is vital in order to achieve the sustainability and reduce the utilization of conventional energy sources. The use of nanofluids in concentrating solar collectors is a promising solution in order to achieve high performance. This paper examines the use of various nanofluids as working fluids in a solar dish collector with smooth and corrugated absorber tube. The use of Al 2 O 3 , Cu, CuO and TiO 2 dispersed on thermal oil and water is presented in this work. The examined concentrations of these nanoparticles are up to 5% in this study. The solar collector is investigated exergetically in order to evaluate together the useful thermal output and pressure losses. The analysis is performed with a developed 1-D thermal model and experimental results are used for the model validation for operation with water. According to the final results, the use of oil-based nanofluids leads to higher exergetic efficiency, while the use of water-based nanofluids leads to higher thermal performance. Moreover, the use of Cu nanoparticle is the most suitable exergetically among the examined cases. Generally, the exergetic efficiency of the examined collector is maximized for Cu-oil based nanofluid with the corrugated absorber and it is about 12.29%. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate a solar cooling system with an absorption chiller driven by nanofluid based flat plate collectors. Pure water and water/Cu nanofluid (2% volumetric concentration) are the examined working fluids on the solar field and they are compared for steady-state conditions, as well as for daily operation. The system is optimized for steady-state conditions using a multi-objective procedure with energetic and exergetic criteria. The mean thermal efficiency enhancement by the use of nanofluids it is found close to 2.5%. The optimized system is further examined on a daily basis for operation with pure water and nanofluid. According to the final results, the use of nanofluids in the solar collector is able to enhance the exergetic performance of the system close to 4.0% and to increase the refrigeration production approximately 0.84% on a daily basis. The analysis is conducted with a developed model in Engineering Equation Solver. [ABSTRACT FROM AUTHOR]

The objective of this work is to present a hybrid Organic Rankine Cycle (ORC) driven by solar energy and waste heat. Parabolic trough collectors coupled to a storage tank feed the heat recovery system which also utilizes waste heat of low-grade temperature (150 °C – 300 °C). Four different working fluids (toluene, cyclohexane, MDM and n-pentane) are examined in the regenerative ORC. The system is designed properly in order the maximum electricity production to be produced with the highest possible exploitation of the waste heat source. The examined model is investigated under steady-state conditions with a developed thermodynamic model in EES (Engineering Equation Solver). According to the final results, toluene leads to the highest electricity production, with cyclohexane, MDM and n-pentane to follow. The electricity production with toluene is found to be ranged from 479 kW to 845 kW and the system efficiency from 11.6% to 19.7%, with waste heat source temperature variation from 150 °C to 300 °C respectively. Moreover, it is found that higher waste heat source temperature leads to a higher fraction of the waste heat input in the ORC for all the working fluids. [ABSTRACT FROM AUTHOR]

The objective of this work is to examine a solar ejector refrigeration system under various operating scenarios. Evacuated tube collectors of 100 m 2 collecting area coupled to a storage tank of 4 m 3 are selected for feeding the generator of the refrigerator. The system is examined for different evaporator temperatures from −10 °C to 10 °C and for different heat rejection temperatures from 30 °C to 50 °C. In every case, the system is optimized by selecting the optimum ejector design and the optimum generator temperature. Different refrigerants are examined as R123, R245fa, R600a and R134a, while the emphasis is given to the R141b. The innovation of this work is based on the systematic examination and optimization of a great range of operating scenarios giving a clear image for the system performance. According to the final results, the R141b is found to be the best candidate in all the operating scenarios. More specifically, the maximum system COP is found to be 0.234 when the system produces refrigeration at 10 °C and rejects heat to the environment at 30 °C. The optimum generator temperatures are found to be from 114 °C to 157 °C and the cooling capacity is ranged from 1.85 kW up to 23.39 kW. The analysis is performed with a developed model in EES (Engineering Equation Solver) under steady-state conditions. [ABSTRACT FROM AUTHOR]

The main objective of this work is to investigate the utilization of nanofluids in the solar field in order to achieve higher system performance. An Organic Rankine Cycle (ORC) driven by solar parabolic trough collectors (PTCs) is the examined system. Four different nanoparticles are examined (Al 2 O 3 , CuO, TiO 2 and Cu) in the base fluid (Syltherm 800), as well as the pure thermal oil is examined as working fluid. The examined ORC is a regenerative cycle and four organic fluids are also tested (toluene, MDM, cyclohexane and n-pentane). In every combination between the organic fluid in ORC and working fluid (nanofluid) in the solar field, an optimization procedure is followed. The concentration of every nanoparticle and the pressure ratio (pressure in the turbine inlet to critical pressure) are the optimization parameters. According to the final results, toluene is the best organic fluid and CuO is the most suitable nanoparticle. The combination of these two working fluids leads to 167.05 kW electricity production and to 20.11% system efficiency with concentration 4.16%. The enhancement by the use of nanofluids is found up to 1.75% compared to the respective case with pure thermal oil and this result indicates that the use of them is able to improve the performance of solar driven ORCs. For the other nanoparticles and Toluene in the ORC, Cu, Al 2 O 3 and TiO 2 lead to 166.18 kW, 165.72 kW and 165.60 kW electricity productions respectively with optimum concentrations 3.98%, 2.51% and 2.57% respectively. [ABSTRACT FROM AUTHOR]

The objective of this study is to estimate the yearly thermal and electrical enhancement of a hybrid PV operating with nanofluids. The examined nanofluid is Cu/water and it is compared with water as working fluid for various operating conditions. An integrated solar thermal system with a hybrid PV and a storage tank is examined for twelve typical days in order its yearly performance to be determined. Different storage tank volumes are investigated because of the storage capacity influence on the thermal and the exergetic performance of the system. According to the final results, the storage tank of 150 L is found to be the most suitable solution for the hybrid PV of 2 m 2 collecting area, using exergetic criteria. For this case, the yearly thermal performance enhancement is about 4.35%, while the electrical and exergetic enhancements are 1.49% and 3.19% respectively. Moreover, it is found that the yearly enhancement is higher in the cases with greater storage tanks. The study is performed with a developed thermal model in EES (Engineering Equation Solver) which is validated with experimental literature results. The climate data has been taken from the literature for Athens, Greece. [ABSTRACT FROM AUTHOR]

Parabolic trough collector (PTC) is the most mature concentrating technology which is used in a great variety of thermal applications. In this study, the daily performance of a PTC is investigated for all the year period. The module of Eurotrough parabolic trough collector is examined with various storage tanks from 3 m 3 to 15 m 3 and the collector performance is investigated in energetic and exergetic terms. An Input/Output method is applied for the daily evaluation of the solar thermal system, something innovative for concentrating solar collectors. According to the final results, different storage tank volumes lead to similar thermal outputs but to extremely different exergetic outputs because of the high dependence of the mean storage tank temperature with the storage tank volume. For the storage tank of 9 m 3 , the mean thermal efficiencies in December and in June are found to be 37.1% and 68.7% respectively, while the mean exergetic efficiencies 2.0% and 8.5% respectively. Moreover, it is proved that the Input/Output performance method in concentrating collectors has to take into consideration the solar incident angle at solar noon in order high accuracy to be achieved. [ABSTRACT FROM AUTHOR]

Parabolic trough collector is one of the dominant emerging solar technologies for producing heat at high temperatures (usually 200–400 °C). The objective of this study is to investigate the thermal performance of internally finned absorbers. Twelve different fin geometries are examined and compared with the smooth absorber case for various operating scenarios. More specifically, the investigated internal fins have thicknesses 2 mm, 4 mm and 6 mm, while their lengths are 5 mm, 10 mm, 15 mm and 20 mm. The examined parameters for the evaluation of the internally finned absorbers are the thermal efficiency, the Nusselt number, the pressure losses, as well as the thermal enhancement index. According to the final results, higher fin thickness and length lead both to higher thermal performance and simultaneously to higher pressure losses. The impact of the length on the results is found to be more intense than the thickness. According to the thermal enhancement index, the case with 20 mm length and 4 mm thickness is found to be the optimum case. For this absorber, the increase in the thermal efficiency and the thermal enhancement index are found 1.27% and 1.483 respectively for 600 K inlet temperature, while the Nusselt number is proved to be 2.65 times greater than in the smooth case. [ABSTRACT FROM AUTHOR]

Among the solar concentrating technologies, parabolic trough collector (PTC) is the most mature and cost-effective technology for medium and high-temperature levels (150–400 °C). This paper investigates the utilization of internally finned absorbers in LS-2 PTC module for various operating conditions. Twelve different longitudinal fins are tested and compared with the smooth case. The analysis is performed with SolidWorks Flow Simulation, using a validated model by literature results. Generally, it is proved that both greater length and thickness lead to higher thermal enhancement and to higher pressure losses. Various methods are presented for evaluating together the thermal efficiency or Nusselt number enhancement versus the increase in pressure drop or in the friction factor. Taking into consideration four different criteria, the absorber with 10 mm fin length and 2 mm fin thickness is found to be the overall optimum case. For this case, the thermal efficiency is enhanced about 0.82%, the Nusselt number increase 65.8%, while the friction factor and the pressure losses are about the double compared to the smooth case. [ABSTRACT FROM AUTHOR]

The objective of this study is to present a detailed exergetic analysis of the commercial parabolic trough collector LS-2. A complete thermal model is developed in EES (Engineering Equation Solver) and it is validated with literature results. The solar collector is examined for operation with Therminol VP1 and air in order to examine the most representative liquid and gas working fluids. In the exergetic analysis, detailed presentation of the exergetic losses and the exergy destruction is given for various operation cases. More specifically, different combinations of flow rates and inlet temperature levels are tested for both working fluids and the results indicate the reasons for the exergetic reduction in every case. According to the final results, the global maximum exergetic efficiency for operation with air is 25.62% for an inlet temperature of 500 K and flow rate of 10,000 l/min, while for Therminol VP1 is 31.67% for 500 K and 100 l/min. Moreover, it is proved that the exergy destruction is more intense in the thermal oil case, while the exergetic losses are more important in the air case. The final results and conclusions clearly present the exergetic analysis of parabolic trough collector for a great range of operating conditions. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the yearly performance an absorption-compression cascade refrigeration system driven by solar energy. Different operating scenarios are examined by testing different combinations of refrigeration and condensation temperatures. Evacuated tube collectors coupled to a storage tank are selected to be used for feeding the refrigeration system with the proper heat demand. LiBr-H 2 O and R134a are selected as working pair in the absorption cycle and refrigerant in the compression cycle respectively. The analysis is performed in energetic, exergetic, economic and environmental terms with a developed and validated model in EES (Engineering Equation Solver). According to the final results, the cascade system, in cases with low refrigeration temperatures is better economically while in higher refrigeration temperatures presents the greater percentage of electricity savings. On the other hand, higher heat rejection temperature leads both to higher economic indexes and to higher electricity savings. For the case of heat rejection at 40 °C and refrigeration at −20 °C, the electricity savings are found 53.98%, the IRR 6.6% and the payback period close to 14 years. [ABSTRACT FROM AUTHOR]

In this study, a solar driven trigeneration system is investigated and optimized in energetic and exergetic terms. Parabolic Trough collectors (PTC) are used for supplying the demanded heat input in the present configuration which includes a storage tank, an Organic Rankine Cycle (ORC) module and an absorption heat pump operating with the LiBr-H 2 O working pair. The heat source from solar collectors drives the ORC which rejects heat to the generator of the absorption heat pump. This heat pump produces heating and cooling demand simultaneously, while electricity is produced from the ORC. The examined system is analyzed parametrically in steady state conditions with Engineering Equator Solver (EES) tool by keeping the collecting area equal to 1000 m 2 in all the examined cases. Eight different working fluids are tested in the ORC, while six extra design parameters are studied. The inlet temperature in the heat recovery system, the superheating, the maximum pressure in ORC, the heat rejection temperature of ORC, the evaporating temperature of the heat pump, as well as the solar beam irradiation are the examined parameters. According to the final results, toluene is the working fluid which leads to maximum exergetic output with n-octane and MDM to follow with 29.42%, 28.50% and 28.35% respectively. In the optimum case, the electricity production of the system is 177.6 kW el , while the cooling and the heating outputs are 398.8 kW and 972 kW respectively. [ABSTRACT FROM AUTHOR]

The objective of this study is to present and to examine a novel solar assisted mechanical compression refrigeration system. Evacuated tube collectors are used in order to partially thermally compress the refrigerant, after the mechanical compressor. This design aims to reduce the electricity consumption using a renewable energy source, creating a sustainable system. The suggested design is analyzed parametrically in order to investigate its energetic and exergetic behaviour in various operating conditions and it is examined with EES (Engineering Equator Solver) in steady state conditions. The temperature levels in the evaporator and in the condenser, as well as the thermal compression fraction (expressed with the pressure ratio parameter), are the examined parameters. The final results proved that the optimum values of the pressure after the mechanical compressor is the 75% of the maximum pressure and for this case, energy savings from 15% to 25% can be achieved. Moreover, the specific collecting area is found to be relatively low, close to 2 m 2 /kW for the optimum cases. The final results proved that the new design leads to energy savings in all the examined cases and especially in cases with higher evaporating temperature levels, a fact that makes it ideal for space cooling applications. [ABSTRACT FROM AUTHOR]

The objective of this study is to compare energetically and financially a solar assisted heat pump heating system powered by flat plate collectors with a conventional air source heat pump system for twenty European cities. The examined buildings have 100 m 2 floor area and they are identical among the examined cases in order to perform a comparison under the same terms. Only the insulation thickness varies, examining two insulation scenarios with 4 cm and 8 cm insulation layer thickness. According to the final results, the electricity savings using the solar assisted heat pump system are ranged from 30% to 40%, indicating great sustainability in both insulation scenarios. The COP in the solar assisted systems is found close to 4 and for the conventional air source systems close to 2.5. The solar assisted heating system is proved to be also the financially optimum solution for the majority of the examined cities and especially in low insulation cases. More specifically, this system is proved to be ideal for cities with high solar potential and not extremely low heating loads. The simulation tool is TRNSYS and the analysis is performed for all the winter period. [ABSTRACT FROM AUTHOR]

The objective of this study is to analyze and optimize a solar assisted gas turbine system. Parabolic trough collectors are used in order to supply a part of the demanded heat input, reducing the natural gas consumption and leading to an environmental friendly system. Emphasis is given in the pressure losses in the collector field loop which reduce the pressure level in the turbine inlet, leading to lower electricity production. In the first part of this study, the gas turbine and the collector field are investigated separately and in the second part, the solar assisted gas turbine is investigated parametrically and it is optimized. In the optimization part, a multi-objective optimization is performed by setting as goals the minimization of the collector area, of the fuel consumption and of the inversed electricity production. The final results proved that 1050 collector modules lead to 0.3389 kg/s natural gas consumption and to electricity production equal to 14.81 MW el . This optimum solution leads to 64% fuel savings with a 2.8% penalty on the produced electricity. The system is analyzed in steady state conditions with the Engineering Equator Solver (EES). [ABSTRACT FROM AUTHOR]

In this study, a solar cooling system of 100 kW is analyzed parametrically in dynamic basis for the city of Athens, Greece. The objective of this study is the design of a sustainable system, using energetic, exergetic and financial criteria. The examined system includes evacuated tube collectors, storage tank and a single stage absorption chiller operating with LiBr-H 2 O working pair. Different combinations of collecting areas and storage tank volumes are tested in order to determine the most suitable cases exergetically. These optimum cases are evaluated financially and finally the system with the higher financial indexes is selected as the most suitable. More specifically, the collecting area is analyzed from 150 m 2 to 600 m 2 and the storage tank from 6 m 3 to 16 m 3 . Finally, 450 m 2 of evacuated tube collectors with a 14 m 3 storage tank was proved to be the optimum solution financially with 15 years payback period and 67 k€ net present value. [ABSTRACT FROM AUTHOR]

In this study, the use of internal fins in parabolic trough collectors operating with gas working fluids is examined. Air, helium and carbon dioxide are the investigated working fluids, while Eurotrough ET-150 is the examined solar collector. The design and the simulation of this solar collector are performed with the commercial software Solidworks Flow Simulation. The internal fins lead to higher thermal efficiency but also to higher pressure losses; something very important for the solar fields of Concentrated Power Plants. Thus, the collector is examined energetically and exergetically in order to take into account the increase in the useful output with the simultaneous greater need of pumping power. Various fin lengths are examined and finally the fin of 10 mm was proved to be the most appropriate exergetically. In working fluid comparison, helium is the most efficient working fluid exergetically for all the examined cases. In the thermal efficiency comparison, helium performs better up to 290 °C, while carbon dioxide is the best choice in higher temperature levels. Moreover, the optimum mass flow rate for the helium was proved to be 0.03 kg/s and for the other working fluids the value of 0.015 kg/s seems to lead to the most satisfying results. [ABSTRACT FROM AUTHOR]

In this study a linear Fresnel solar collector with flat plate receiver is investigated experimentally and numerically with Solidworks Flow Simulation. The developed model combines optical, thermal and flow analysis; something innovative and demanding which leads to accurate results. The main objective of this study is to determine the thermal, the optical and the exergetic performance of this collector in various operating conditions. For these reasons, the developed model is validated with the respective experimental data and after this step, the solar collector model is examined parametrically for various fluid temperature levels and solar incident angles. The use of thermal oil is also analyzed with the simulation tool in order to examine the collector performance in medium temperature levels. The experiments are performed with water as working fluid and for low temperature levels up to 100 °C. The final results proved that this solar collector is able to produce about 8.5 kW useful heat in summer, 5.3 kW in spring and 2.9 kW in winter. Moreover, the operation of this collector with thermal oil can lead to satisfying results up to 250 °C. [ABSTRACT FROM AUTHOR]

The new technological trends are focused on the development of sustainable systems for covering the energy needs in buildings, using renewable energy sources and minimizing the carbon dioxide emissions. In this direction, the development of innovative trigeneration systems is an intelligent solution for creating energy efficient buildings which can satisfy their energy needs. In this paper, an innovative trigeneration system which uses low temperature level heat sources is analyzed and optimized. This system is consisted of an absorption heat pump and a steam turbine which consumes a part of the produced steam in the generator. This system is able to produce electricity, heating, cooling and hot water, covering the typical energy needs of buildings. Various scenarios are examined parametrically in order to present the system performance for various operation conditions. The final results proved that the exergetic efficiency is about 72% and the electrical output close to 9 kW, when a heat input of 100 kW is given. By selecting the proper parameters, the outputs of the system can be distributed according to the building energy demands, something very important for the sustainability of this system. The analysis is performed with the Engineering Equation Solver (EES). [ABSTRACT FROM AUTHOR]

The use of solar thermal energy for electricity production is a clean and sustainable way to cover the increasing energy needs of our society. The most mature technology for capturing solar energy in high temperature levels is the parabolic trough collectors (PTC). In this study, an Organic Rankine Cycle (ORC) coupled with PTC is analyzed parametrically in order to be optimized financially and energetically. The first step is the thermodynamic investigation of the ORC by using various working fluids. The second step is the energetic and financial investigation of the total system which includes the solar field, the storage tank and the ORC module. By testing many combinations of collecting areas and storage tank volumes, finally cyclohexane proved to be the most suitable working fluid for producing 1 MW el with PTC. Specifically, in the optimum situation a solar field of 25,000 m 2 with storage tank of about 300 m 3 leads to a payback period of 9 years and to an internal rate of return (IRR) equal to 13.46%. Moreover, an economic comparison for different commercial collectors is presented, with Eurotrough ET-150 being the financially optimum solution for this case study. [ABSTRACT FROM AUTHOR]

This paper investigates numerically the deep renovation of a multi-family building in Greece to reduce dramatically its energy demand and also to incorporate renewable energy sources, rendering it a positive one; thus in position on an annual basis to offer net electricity to the grid. The examined building has 8 apartments of 75 m2 floor area each and is located in Moschato, a suburb of Athens in Greece. The goal of the present investigation is to determine the energy savings, but also to calculate the financial and environmental benefits through a life cycle analysis. The energy simulation of the building is conducted on annual basis by using a novel and detailed dynamic software tool (INTEMA.building), which is developed in the Dymola environment using the Modelica modeling language. This tool makes possible the detailed simulation of both passive and active systems in the building environment. Furthermore, it includes the control of the energy systems and can provide accurate enough results, encompassing detailed numerical models for the systems investigated, accounting for an adjustable time step of the dynamic analysis. According to the calculations, the proposed retrofitting scenario can achieve a reduction of the heating loads by 93% and of cooling loads by 78% respectively. The electrical demand for domestic hot water can be decreased by about 79%, while the electricity demand for appliances and lighting by about 60%. In terms of specific thermal needs, the specific heating demand can be reduced from 151.5 kWh/m2 down to 10.7 kWh/m2, while the cooling specific demand from 112.6 kWh/m2 to 24.4 kWh/m2. Moreover, it is calculated that the reduction in the primary energy demand after the renovation can be up to 88%, with the building providing around 5.3 MWh of net electricity to the grid on a yearly basis through a net-metering connection. Finally, the life cycle cost analysis indicated 622 k€ savings and specific CO 2 avoidance per renovated floor area in the range of 2.64 tons CO 2 /m2. • A new tool for dynamic simulations of buildings and energy systems is presented. • A holistic renovation scenario of a multi-family Greek building is investigated. • The primary energy reduction after the renovation was found at 88%. • The renovation payback period is 3.8 years and the specific lifecycle gains 1037€/m2. • The specific CO 2 avoidance was calculated at 2.64 tons CO 2 /m2. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the use of an alternative working pair in a solar absorption cooling system. LiCl-H 2 O is the new examined pair and it is compared energetically and exegetically with the conventional pair LiBr-H 2 O, which is the most usual in air-conditioning applications. The simplest solar cooling system is analyzed in order to focus in the comparison between these working fluids. Specifically, flat plate collectors, coupled with a storage tank, feed the single effect absorption chiller which produces 250 kW cooling at 10 °C. The two pairs are examined parametrically for various heat source temperature levels and for three ambient temperature levels (25 °C, 30 °C and 35 °C). The minimization of the collecting area, which means maximum exergetic efficiency, is the optimization goal in every case. The final results show that LiCl-H 2 O pair performs better in all cases by giving greater exergetic efficiency. More specifically, about 8% lower collecting area is required to cover the demanded cooling load with this working pair. Another interesting result is that the optimum heat source temperature for the LiCl-H 2 O is roughly lower than the respective for the LiBr-H 2 O. The system is analyzed in steady state with the commercial software Engineering Equator Solver (EES). [ABSTRACT FROM AUTHOR]

Using solar energy for space heating purposes consists an alternative way for substituting fossil fuel and grid electricity consumption. In this study, four solar assisted heat pump heating systems are designed, simulated and evaluated energetically and financially in order to determine the most attractive solution. The use of PV collectors with air source heat pump is compared to the use of FPC, PVT and FPC with PV coupled with a water source heat pump. A sensitivity analysis for the electricity cost is conducted because of the great variety of this parameter over the last years. The final results proved that for electricity cost up to 0.23 €/kW h the use of PV coupled with an air source heat pump is the most sustainable solution financially, while for higher electricity prices the coupling of PVT with an water source heat pump is the best choice. For the present electricity price of 0.2 €/kW h, 20 m 2 of PV is able to drive the air source heat pump with a yearly solar coverage of 67% leading to the most sustainable solution. Taking into account energetic aspects, the use of PVT leads to extremely low grid electricity consumption, fact that makes this technology the most environmental friendly. [ABSTRACT FROM AUTHOR]