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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 aim of the present work is the analysis of a solar-driven unit that is located on the non-interconnected island of Kythnos, Greece, that can produce electricity and green hydrogen. More specifically, solar energy is exploited by parabolic trough collectors, and the produced heat is stored in a thermal energy storage tank. Additionally, an organic Rankine unit is incorporated to generate electricity, which contributes to covering the island's demand in a clean and renewable way. When the power cannot be absorbed by the local grid, it can be provided to a water electrolyzer; therefore, the excess electricity is stored in the form of hydrogen. The produced hydrogen amount is compressed, afterward stored in tanks, and then finally can be utilized as a fuel to meet other important needs, such as powering vehicles or ferries. The installation is simulated parametrically and optimized on dynamic conditions, in terms of energy, exergy, and finance. According to the results, considering a base electrical load of 75 kW, the annual energy and exergy efficiencies are found at 14.52% and 15.48%, respectively, while the payback period of the system is determined at 6.73 years and the net present value is equal to EUR 1,073,384. [ABSTRACT FROM AUTHOR]

The exploitation of solar irradiation in the building sector is a promising way to achieve decarbonization and reduce the operating costs of the building. The potential of solar energy in Greece is high and therefore this idea can lead to excellent results. In this direction, the goal of the present analysis is the detailed investigation of different solar thermal systems for domestic hot water production in the different climate zones of Greece. Four typical cities are studied in order to examine the climate zones A, B, C, and D, while three different solar thermal collectors coupled to insulated storage tanks are studied for every city. The simple flat plate collector, the advanced flat plate collector, and the evacuated tube collector are the selected solar systems for the present work. The climate data and the characteristics of the solar thermal systems follow the Greek Chamber regulations known as "KENAK". The analysis is conducted by following the well-known f-chart method and every design is optimized by using energy and financial criteria. The final design is determined by conducting a multi-objective evaluation methodology. It is notable to state that the collector slope and the collecting area are important parameters of this work, while the study case regards a building with 30 residents. According to the final results, the advanced flat plate collector is the best choice according to the multi-objective evaluation procedure. [ABSTRACT FROM AUTHOR]

The purpose of this simulation study is the examination of a solar-driven power cycle that is driven by collectors with evacuated tubes. The power cycle is an organic Rankine cycle (ORC) that works with cyclopentane. The novelty of the cycle is the use of reheating in order to enhance its thermodynamic efficiency, while the cycle also has a recuperator. Moreover, the cycle is compared with the conventional ORC in order to determine the performance enhancement with the examined idea. The analysis is done with a developed program in Engineering Equation Solver and both in steady-state and transient conditions for a typical year. The analysis was conducted in terms of energy and economic performance. According to an optimization procedure for maximizing the net present values of the investment, it is found that the novel design leads to a net present value of 68 k€, a simple payback of 10 years, and a yearly energy efficiency of 7.0%, while the respective values for the conventional ORC are 44 k€, 10.8 years, and 5.1% for its optimal design according to the net present values maximization. Thus, it is obvious that the suggested design increases both energy and financial performance, compared to the usual design. [ABSTRACT FROM AUTHOR]

The goal of the present paper is the investigation of a solar desalination system with an organic Rankine cycle system for power and freshwater production. This system is an environmentally friendly technology that is able to utilize solar energy properly in a novel cogeneration application. A parabolic trough concentrator with a smooth and corrugated receiver was employed as the heat source of the desalination system. A humidifier–dehumidifier desalination technology was used for producing freshwater. The electricity is produced by an organic Rankine cycle which is fed both by the solar field and by the hot brine. The present analysis is performed by using a detailed numerical model which is validated by experimental literature data. Based on the final results, the corrugated tube has a maximum performance of 66.59%, and it is more efficient than the smooth tube with 63.11%. The average freshwater productions were estimated equal to 13.09 kg hr−1 and 12.71 kg hr−1 for the corrugated and smooth tubes, respectively. The maximum net work production is found at 7.57 kW with R113, while the less efficient working fluid is R134a. It was found that the application of the developed desalination system leads to the production of high amounts of fresh water and a significant reduction of the equivalent CO2 emissions. [ABSTRACT FROM AUTHOR]

In this study, a solar parabolic trough concentrator (PTC) was evaluated as a heat source of a power generation system based on energy (E1), exergy (E2), environmental (E3), and economic (E4) analyses. Various configurations of power generation systems were investigated, including the solar SRC (SRC) and solar ORC (ORC). Water and R113 were used as heat transfer fluids of SRC and ORC system, respectively. It should be mentioned that the proposed solar systems were evaluated for providing the required power of a mobile house in an emergency condition such as an earthquake that was happened in Kermanshah, Iran, in 2016 with many homeless people. The PTC system was optically and thermally investigated based on sensitivity analysis. The optimized PTC system was assumed as a heat source of the RC with two various configurations for power generation. Then, the solar RC systems were investigated based on 4E analyses for providing the power of the mobile house based on various numbers of solar RC units. It was concluded that the solar SRC system could be recommended for achieving the highest 4E performance. The highest value of its energy efficiency was found at 24.60% and of his exergy at 26.37%. On the other hand, the ORC system has energy and exergy efficiencies at 17.64% and 18.91%, respectively, which are significantly lower than the efficiencies of the SRC system. The optimum heat source temperature for the SRC system is found at 650 K, while for the ORC system at 499 K. Moreover, the best economic performance was found with the SRC system with a payback period of 7.47 years. Finally, the CO2 mitigated per annum ( φ CO 2 ) was estimated at 5.29 (tones year−1), and the carbon credit ( Z CO 2 ) was calculated equal to 76.71 ($ year−1). [ABSTRACT FROM AUTHOR]

The objective of this paper is the investigation of the annual performance of a solar power plant with linear Fresnel reflectors in the El-Oued region at Algeria. The solar collectors produce water steam that feeds a turbine to produce electricity. The System Advisor Model (SAM) tool is used for simulation. The mean net daily electricity production rate from 8:30 am to 5:30 pm is 48 MWe, and the respective annual production is 210,336 MWh/year. The mean daily optical efficiency of the solar field was close to 52%, while the mean thermal efficiency was about 39%. The net daily cycle efficiency is found to be 24%. The net capital cost of the examined system is $393 million, and the developer net present value is $47 million; the investor net present value is $15 million, the entire period of capital recovery is 11 years, and the levelized cost of electricity is 0.0382 $/kWh. The solar power plant leads to the yearly avoidance of 420,672 tons carbon dioxide emissions (operational cost savings of $6.1 million). Based on the obtained results, linear Fresnel reflectors can be used to achieve satisfying, energetic, financial, and environmental performance that can lead to sustainability. [ABSTRACT FROM AUTHOR]

The objective of this study is the investigation of different solar concentrating collectors for application in a trigeneration system. Parabolic trough collectors, linear Fresnel reflectors and solar dishes are the examined solar concentrating technologies in this work. The trigeneration unit includes an organic Rankine cycle coupled with an absorption heat machine that operates with LiBr/water. The analysis is performed throughout the year by using the weather data of Athens in Greece. The results of this work indicate that the selection of parabolic trough collectors is the best choice because it leads to the maximum yearly system energy efficiency of 64.40% and to the minimum simple payback period of 6.25 years. The second technology is the solar dish with the energy efficiency of 62.41% and the simple payback period of 6.95 years, while the linear Fresnel reflector is the less efficient technology with the energy efficiency of 35.78% and with a simple payback period of 10.92 years. Lastly, it must be stated that the thermodynamic investigation of the system is performed with a created model in Engineering Equation Solver, while the dynamic analysis is performed with a code in the programming language FORTRAN. [ABSTRACT FROM AUTHOR]

The solar collector (reflector and receiver) is the primary device being used in the concentrating solar power technologies for tapping the solar energy to meet various objectives. The performance of the solar collector is influenced by the type of reflector and receiver being selected, and its material also has significant impact. The choice of the heat transferring medium, storage material, receiver design and its performance parameter also influences the solar collector's performance. In this regard, a review has been carried out for these parameters and presented in this paper to highlight the challenges encountered, new methodologies identified and proposed, and future research needs by various researchers to improvise the thermal efficiencies of the different types of collectors used in the concentrating solar power technologies. [ABSTRACT FROM AUTHOR]

Solar-driven trigeneration systems are able to cover all the building energy needs in heating, cooling and electricity using only the solar energy. The objective of this paper is to evaluate a solar trigeneration system under different criteria in order to make clear that there are numerous factors which have to be taken into consideration in the trigeneration system design. The examined system consists of parabolic trough solar collectors, a storage tank, an organic Rankine cycle and an absorption heat pump. The system is evaluated using different evaluation criteria which are associated with the energy, the exergy and the financial performance of the system. Moreover, a new criterion which takes into account the building energy needs is introduced and is investigated in different scenarios. The final results of this work clearly indicate that the optimisation of the solar driven trigeneration systems is depended on the desired design conditions and goals. Abbreviations: COP, Coefficient of performance; ECO, Economizer; EES, Engineering Equation Solver; EVAP, Evaporator; ORC, Organic Rankine Cycle; PTC, Parabolic trough collector; SC, Scenario [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]

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]

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]

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 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]

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 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 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]

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 work was to optimize and to evaluate a solar-driven trigeneration system which operates with nanofluid-based parabolic trough collectors. The trigeneration system includes an organic Rankine cycle (ORC) and an absorption heat pump operating with LiBr-H2O which is powered by the rejected heat of the ORC. Toluene, n-octane, Octamethyltrisiloxane (MDM) and cyclohexane are the examined working fluids in the ORC. The use of CuO and Al2O3 nanoparticles in the Syltherm 800 (base fluid) is investigated in the solar field loop. The analysis is performed with Engineering Equation Solver (EES) under steady state conditions in order to give the emphasis in the exergetic optimization of the system. Except for the different working fluid investigation, the system is optimized by examining three basic operating parameters in all the cases. The pressure in the turbine inlet, the temperature in the ORC condenser and the nanofluid concentration are the optimization variables. According to the final results, the combination of toluene in the ORC with the CuO nanofluid is the optimum choice. The global maximum exergetic efficiency is 24.66% with pressure ratio is equal to 0.7605, heat rejection temperature 113.7 °C and CuO concentration 4.35%. [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]

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]

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]

The integration of solar energy systems is crucial to achieving the global goals for sustainability as well as the reduction of greenhouse gases and pollutants emissions. In this direction, the development of multigeneration systems is very important as they can cover the increasing and numerous demands of modern society in a high-efficient way. In parallel, hydrogen is a promising alternative fuel that has gained attention recently because it can substitute conventional energy sources. So, the objective of the present work is the review, classification, discussion, and comparison of solar-driven multigeneration systems that include hydrogen production. This study is a novel one because it is focused on the integration of hydrogen generation methods into polygeneration systems, while existing publications present the useful outputs of the units, without specialization. In parallel, it meets the increasing need for the investigation of hydrogen energy potential. Apart from hydrogen, other common useful outputs are electricity, heating, cooling, domestic hot water, and freshwater. In this work, the plants are classified according to the hydrogen production method. In parallel, different types of solar devices, such as thermal collectors, photovoltaics, and thermal photovoltaics as well as other additional energy sources are also examined. So, numerous configurations are investigated comprehensively, which can produce a plethora of useful outputs, as well as include thermodynamic cycles, such as Rankine cycles, organic Rankine cycles, and other devices, such as absorption chillers, or desalination units. All the units are evaluated and compared in terms of energy, and exergy performance as well as economic viability, and hydrogen production rate. In general, solar tower-based and parabolic trough collector-based units are high-efficient energetically and exergetically, while concentrating photovoltaic thermal plants achieve high energy efficiency. Finally, suggestions for future research and development are made. • Solar-driven multigeneration systems with hydrogen production are investigated. • The systems are mainly classified according to the hydrogen generation methods. • Solar collectors, photovoltaics, and hybrid solar devices are examined. • Hybridization with auxiliary energy sources is also presented. • Solar towers, PTC, and CPVT are the most efficient systems. [ABSTRACT FROM AUTHOR]

• Solar-driven polygeneration systems are summarized and discussed in detail. • Photovoltaics, concentrating and non-concentrating solar systems are examined. • Parabolic trough collectors and concentrating thermal photovoltaics are the best cases. • The use of vapor compression heat pumps leads to viable investments. • High exergy efficiency is found in the systems with electrolyzer or gasifier. Solar energy exploitation is an important weapon for facing critical problems such as fossil fuel depletion, global warming and the increasing energy demand due to the new lifestyle trends. Moreover, the increase in the performance of energy systems is another way of achieving sustainability. Polygeneration systems are highly efficient systems that can produce multiple useful energy outputs like electricity, cooling, heating, fresh water and hydrogen. So, combing solar energy systems and polygeneration units is an optimal way of producing many useful outputs with high efficiency and by utilizing a clean energy source. In this direction, the objective of the present work is to review and to discuss the present literature studies about the exploitation of solar irradiation in polygeneration systems. The present review paper investigates the use of different solar energy technologies in any kind of polygeneration system. More specifically, flat plate collector, evacuated tubes, parabolic trough collector, linear Fresnel reflector, solar dish concentrator, solar tower and thermal photovoltaics are the investigated systems in this work. The examined polygeneration systems include various engineering devices like organic Rankine cycle, gas turbine, absorption heat pump, distillation unit, electrolyzer and heat exchanger. The final results indicate that solar-driven polygeneration systems with parabolic trough collectors, concentrating thermal photovoltaics and solar tower can be sustainable and viable systems for application in the future energy commodities. [ABSTRACT FROM AUTHOR]

• A nanofluid-based thermal PV coupled a heat pump is investigated. • This system produces space heating and net electricity. • The system is optimized with a multi-objective procedure. • Water/Cu nanofluid leads to 4.80% energy efficiency enhancement. • The heating production is increased by 5.75% with water/Cu nanofluid. The objective of this paper is to investigate a solar assisted heat pump which is driven by nanofluid-based hybrid PV collectors for space-heating and electricity production purposes. The use of nanofluids, as working fluids in the solar collector, enhances both the thermal and electrical performance in order to have higher useful outputs. Practically, the use of nanofluids enhances the heat transfer rates inside the flow and so higher thermal output are produced in the collector and the photovoltaic cell produces higher electricity because it is cooled efficiently by the nanofluids. Water/Cu and water/Al 2 O 3 are found to be the most efficient nanofluids and they are tested in the total system. The system is optimized for steady-state conditions and the optimum design is examined on a daily basis for the pure water case and the nanofluids cases. The optimization procedure is a multi-objective procedure which is conducted using energy, exergy and financial criteria. According to the final results, the use of water/Cu nanofluid is the best case which presents 4.80% higher energy system efficiency and 0.66% higher exergy system efficiency compared to the pure water case. The use of water/Al 2 O 3 leads to an enhancement of 4.34% and 0.19% for energy and exergy efficiency respectively. For the case with water/Cu, the heating production during the winter is found 7423 kWh th and the net electricity production 1068 kWh el. The analysis is performed with a developed thermal/thermodynamic model in Engineering Equation Solver. [ABSTRACT FROM AUTHOR]

The thermal enhancement of parabolic trough collectors is a critical issue and numerous ideas have been applied in the literature on this domain. The objective of this paper is to investigate some usual thermal enhancement techniques for improving the performance of evacuated and non-evacuated receivers of parabolic trough solar collectors. More specifically, the use of twisted tape inserts, perforated plate inserts, and internally finned absorbers are compared with the reference case of the smooth absorber. The analysis is conducted with a developed and validated thermal model in Engineering Equation Solver. The collector is investigated for a typical flow rate of 100 L/min and for inlet temperatures between 50 °C and 350 °C with Syltherm 800 as working fluid. According to the final results, the use of internally finned absorber leads to the highest thermal efficiency enhancement, which is up to 2.1% for the non-evacuated collector and up to 1.6% for the evacuated tube collector. The perforated plate inserts and the twisted tape inserts were found to lead to lower enhancements, which are up to 1.8% and 1.5%, respectively, for the non-evacuated collector, while they are up to 1.4% and 1.2%, respectively, for the evacuated collector. Moreover, the pressure drop increase with the use of the thermal enhancement methods is investigated and the use of internally finned absorber is found again to be the superior technique with the performance evaluation criterion to be ranged from 1.5 to 1.8 for this case. [ABSTRACT FROM AUTHOR]

Highlights • A conical and a spiral cavity receiver are examined for a solar dish collector. • The analysis is optical, thermal and exergetic for different operating conditions. • It is found that the conical design is more efficient than the spiral one. • The optical efficiency of the conical design is 1.38% higher than the spiral one. • The thermal efficiency enhancement with the conical design is 5.63% at 100 °C. Abstract The objective of this work is to compare two cavity receivers for a solar dish concentrator. The spiral and the conical cavities are investigated using a developed thermal model. The analysis is optical, thermal and exergetic for different operating temperatures and flow rates. The developed thermal model is combined with an optical tool in order to simulate properly the solar dish collector and it is validated for the case of the spiral absorber with experimental results. Every receiver is separated in the different coil and every coil is simulated separately in order to increase the model accuracy. Totally, 13 coils are used for the spiral design and 11 for the conical design. The location of the receiver in every case is optimized in order to achieve maximum optical efficiency. The results show that the conical design leads to a 1.38% increase in the optical efficiency due to the increased intercept factor. The thermal efficiency enhancement with the use of conical design is found to be 5.63% at 100 °C and 40.45% at 200 °C, while the exergy efficiency enhancement 6.67% at 100 °C and 42.06% at 200 °C. [ABSTRACT FROM AUTHOR]

From the last few decades, due to the escalating requirement of heat and electricity, the relative utilization of Photovoltaic/thermal (PV/T) system has enlarged as compared to the photovoltaic or solar thermal system alone due to better performance of combined PV/T systems when compared to the conventional ones. PV/T hybrid systems generate both electrical and thermal energy concurrently, giving it a broad variety of applications. Recently, different nanoparticles mixed with base fluid (Nanofluid) have been used in these hybrid systems resulting in attracting the attention of numerous researchers. Due to its improved thermal conductivity nanofluid can be used as an optical filter and proficient coolant in PV/T systems. In the last years, much research has been focused on this domain because it is crucial for the design of sustainable and environmentally friendly energy systems. The primary focus of this review is to accomplish the briefing of diverse PV/T technologies at the outset together with aspects of their efficiencies, structure, thermal governing expressions and their applications as well as the latest technologies and parameters which profoundly affect the PV/T collector performance. We have also reviewed the various positive and adverse effects on the environment safety level by applying these solar techniques for the energy production as compared to the conventional energy sources techniques. As the solar techniques are tremendously helpful for the environment in several ways but at the same deployment of these resources on the wide-scale may lead to adverse implications for the environment. Moreover, the variations in the performance efficiency of the PV/T system using nanofluids, and finally, the suggestions for future research directions are presented. The outcome of this review paper would give an overview of further enhancements in solar PV/T systems with high practicability for extensive relevance in energy supply at all levels in the vicinity of future. [ABSTRACT FROM AUTHOR]

The application of phase change materials (PCMs) in the building envelope can improve the thermal performance and the indoor thermal comfort conditions because they allow the proper storage or release of energy. In this work, three different solar assisted heat pump underfloor heating systems with and without the use of phase change material are designed, simulated and evaluated energetically for a building of 100 m 2 floor area. Different kinds of solar collectors (flat plate, photovoltaic, thermal photovoltaic) are coupled to a storage tank which feeds a heat pump for space heating purposes. The PCM is used on the underfloor heating system in order to increase the storage capacity and to avoid the operation of the heat pump during the peak electricity demand period. Furthermore, this work presents and compares the coefficient of performance, the electricity consumption, the solar coverage and the indoor temperature distribution of a building in Athens (Greece), with and without a PCM layer on the floor. More specifically, the PCM is placed on the underfloor heating system and different cases are examined by changing the collecting area, the collector type and the thickness of the insulation layer in the floor. The results prove that the use of the PCM layer on the underfloor heating system reduces the heating load by about 40%. The electricity consumption can be reduced between 42% and 67% using the solar-driven systems. It is also found that the system with the thermal PV consumes the lowest grid electricity. Moreover, the lowest simple payback period is found to be 9.61 years for the system with flat plate collectors without PCM. The simulation is conducted with TRNSYS for all the winter period. [ABSTRACT FROM AUTHOR]

• This article offers a comprehensive overview of adsorption cooling systems aided by solar, ultrasonic, and microwave energy sources. • A comprehensive discussion of thermal and mathematical modelling, as well as the issues posed by an integrated adsorption system employing ultrasound and microwave energy have been included. • Discussion on status and challenges in designing efficient adsorption cooling system has been included. The present article regards a systematic review of adsorption cooling systems assisted by solar, ultrasound, and microwave energy sources. Numerical and experimental studies are included in this work, while the emphasis is given to the presentation of the mathematical formulation techniques. Due to the exceptional capability of utilizing low-grade energy, the adsorption cooling system attracted researchers amid increasing demand for cooling applications, decreasing the consumption of rapidly lasting fossil fuels. A significant quantity of energy is available as waste heat from industry, power stations, solar radiation, etc. The critical review is done on adsorption cooling systems to enhance the existing system's efficiency, which contributes to the reduction of pollution and global warming. Moreover, the efficient system is expected to have low adsorption and desorption time. This review presents different types of desiccants and various integrated techniques available to enhance the effectiveness of the adsorption-based cooling system, including ultrasound, microwaves, solar energy, etc. The present study critically reviews different modeling techniques of integrated adsorption cooling systems, using recent advancements in power techniques like microwave energy and ultrasound. [ABSTRACT FROM AUTHOR]

• A solar-driven polygeneration unit with CO 2 as the working fluid is investigated. • Initial optimization in steady-state conditions and secondary dynamical analysis. • The payback period is found to be around 6.4 years and the total efficiency is 55%. • The use of an extra ORC can be used for enhancing electricity production. • The exergy efficiency with ORC is found 23.63%, while without 23.37%. The object of this study is the investigation of polygeneration plants utilizing solar energy in order to produce power products on the scale of a few MW. Parabolic trough collectors are used in order to exploit solar irradiation and to provide the generated heat to the polygeneration/trigeneration cycle in a high-temperature range (500–570 °C). The plants under consideration additionally consist of a storage tank, a Brayton cycle with s-CO 2 , a LiBr-H 2 O absorption chiller and also the installation of a suitable thermodynamic organic Rankine cycle (ORC) cycle is examined in order to utilize waste heat for further power generation. The systems are first investigated for operation under steady-state conditions and then for dynamic conditions on an annual basis. Based on the final annual results of the plants, the plant in which ORC was not used shows annual electrical, exergy and energy efficiencies equal to 18.33%, 23.37% and 54.91% respectively, while the plant in which ORC was used similarly shows 19.05%, 23.63% and 49.00%. According to economic analysis, the payback period was found at 6.37 years without ORC and 6.51 years with ORC. Taking into account the operational and techno-economic results of the plants it can be concluded that the plants produce satisfactory energy products ensuring also the economic viability of the investment. [ABSTRACT FROM AUTHOR]

Solar energy is an auspicious renewal energy source because of its abundant availability and its flexibility to be converted into heat or into electricity. Concentrated solar power technologies are promising units for producing electricity in large scale applications and parabolic trough solar collector is the most matured technology for these plants. The objective of this paper is to summarize and to discuss the current status of parabolic trough collector commercial power plants around the world and the technological developments in various constituents of such concentrated solar power plants. More specifically, this review includes various developments in the concentrators and receiver tubes in order to enhance both the thermal and the optical efficiencies. Moreover, the different existing and newly developed heat transfer fluids are discussed in this work and the emphasis is given in the discussion of the different operating temperature levels, the storage system and the applications that use different fluids. Furthermore, different tracking options of the parabolic trough solar collector are examined, different storage technologies, as well as various power cycles and their modifications to enhance the overall efficiency of the parabolic trough collector-based power plants are discussed in detail. The final conclusions of this work indicate the recent developments and the future designs of the parabolic trough solar collector power plants. [ABSTRACT FROM AUTHOR]

The concentrating solar systems of linear Fresnel reflectors offer the ability to generate electricity from solar energy. The main objective of this paper is to perform a transient numerical simulation on a linear Fresnel solar reflector directed to produce superheated water steam for the power plants, in order to determine its efficiencies (optical and thermal), where the heat transfer fluid mass flow inside the copper absorber tube is 0.045 kg/s. The dimensions of the studied linear Fresnel solar collector are the same dimension as the modular system Nova-1 of NOVATEC SOLAR Company. The dimension of the studied model is 1D. For this purpose, El-Oued region has been designated as a study field. The twelve recommended average days for months have been selected for the study. This paper was based on a numerical solution using the finite differences method. MATLAB has been used as a software language. The optical efficiency of the LFR solar receiver has reached 53.7% where the thermal efficiency has reached 37.5%, while the pressure drop within the absorber pipe has reached 02 bars. As for superheated steam temperatures variations, it ranged between 233.35 °C and 263.75 °C, knowing that the water steam inlet temperature when entering the absorber tube is equal to 140 °C. As for the overall coefficient of heat loss, its mean value is 5.84 W/m2.°C. The results obtained are very encouraging to invest in this steam power plant, especially in the desert areas, which are rich in solar energy. Image 1 • Optical efficiency of the solar reflector obtained was 53.7% • Thermal efficiency of the solar reflector achieved was 37.5% • An annual electricity capacity of more than 2000 MW can be produced • This technique is 100% clean and has no negative environmental impact [ABSTRACT FROM AUTHOR]

Highlights • A solar organic Rankine Cycle was considered using dish collector as the heat source. • The performance of three cavity receivers was modeled and compared. • A hemispherical cavity receiver was experimentally investigated. • Alumina/thermal oil was investigated as the solar working fluid. • The system was evaluated at different sizes and concentrations of nanoparticles. Abstract The objective of this work is the investigation of a solar-driven organic Rankine cycle with nanofluid-based solar dish concentrator. Three different cavity receivers are tested with the alumina/oil nanofluid in order to determine the most effective choice. More specifically, the examined cavities are hemispherical, cubical, and cylindrical, while the organic Rankine cycle is a non-regenerated system with superheating, R601 working fluid and heat rejection temperature at 310 K. The analysis is perfumed using the first and the second thermodynamic laws. The organic Rankine cycle is investigated for different pressure in the turbine inlet and different nanoparticle concentrations are also studied. According to the results, the thermal performance of the cavity and the electricity production are improved by decreasing alumina nanoparticle size, and by increasing the nanofluid concentration. Moreover, the optimum pressure in the turbine inlet was found at 2 MPa. Finally, the hemispherical cavity receiver has been found to be the best choice with cubical to be the second one and the cylindrical to be the less effective choice. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • It was an experimental study of nanofluid in a solar dish collector. • A hemispherical cavity receiver was used as the dish absorber. • MWCNT/thermal oil nanofluid was tested as the solar working fluid. • Performance of the nanofluid was compared with the pure oil. • Two thermal efficiency models were suggested for the nanofluid and the pure oil. Abstract Nowadays, nanofluids are introduced as an effective technique for enhancing the thermal efficiency in solar collectors. In this work, the MWCNT/oil nanofluid with 0.8% nanoparticle mass fraction is investigated experimentally in a solar dish concentrator coupled to a hemispherical cavity receiver. The main objective of this research is to examine the effect of the nanofluid application for improving the solar thermal performance of the hemispherical cavity receiver. The results revealed that thermal efficiency of the hemispherical cavity receiver has averagely increased equal to 12.93%, using application of the MWCNT/oil nanofluid compared to the application of the basefluid. In the steady state periods, the average thermal efficiency of 76.23% and 67.50% were calculated for the hemispherical cavity receiver using the MWCNT/oil nanofluid and pure oil, respectively. It was conducted that the receiver heats gain and the thermal efficiency of the cavity receiver has a similar trend with the working fluid difference temperature. Two experimental models were suggested based on the overall performance versus T in - T amb I beam for the MWCNT/oil nanofluid and the pure oil. Finally, it is found that the use of the examined nanofluid is able to enhance the thermal performance of the examined collector about 13%. Also, in this study, group method of data handling (GMDH) artificial neural networks is applied in order to model the dependency of thermal efficiency and the cavity heat gain of the Hemispherical Cavity Receiver on the mentioned factors. [ABSTRACT FROM AUTHOR]

The organic Rankine cycle (ORC) is an effective technology for power generation from temperatures of up to 400 °C and for capacities of up to 10 MW el. The use of solar irradiation for driving an ORC is a promising renewable energy-based technology due to the high compatibility between the operating temperatures of solar thermal collector technologies and the temperature needs of the cycle. The objective of this review paper is to present and discuss the operation principles of solar-ORC technology and the wide range of solar-ORC systems that have been studied in the literature. Various solar thermal technologies that can drive the ORC are investigated, such as the flat plate collector, evacuated tube collector, compound parabolic collector, parabolic trough collector, linear Fresnel reflectors, dish concentrators and solar towers. Both simulation studies and experimental investigations are included in the study. Hybrid systems and different thermal storage techniques are also examined in detail. Moreover, systems with ORC which produce many useful outputs such as cooling, heating and fresh water are studied because they present high sustainability indexes. The limitations of the technology are also highlighted, along with critical suggestions aimed at steering future research in this field. The final conclusions indicate that the development of trigeneration and polygeneration systems with ORC sub-systems is a promising avenue, not only for the future development of solar-ORC technology but also for the development of renewable and sustainable energy systems in a broader context. • Solar driven organic Rankine cycles are summarized and discussed in detail. • Concentrating and non-concentrating solar thermal systems are included. • Parabolic trough collector is the best solar technology for power production. • The use of solar organic Rankine cycles in polygeneration is a promising idea. • There is a need for conducting future experimental studies in a great scale. [ABSTRACT FROM AUTHOR]

• Latest developments in solar thermal applications using nanofluids are reviewed • Low to medium temperature solar collectors are summarized • Improvements in solar collectors using nanofluids as working fluid is compiled • Application of solar collectors in buildings is reviewed The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors' performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given. [ABSTRACT FROM AUTHOR]