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The partial evaporation organic Rankine cycle (PE-ORC) is an emerging technology for the efficient exploitation of low-temperature heat sources. The purpose of the present study is the energy and thermodynamic analysis of a solar-fed PE-ORC with a solar field of evacuated flat plate collectors coupled to a sensible storage tank. The studied PE-ORC operates with the environmentally friendly working fluid R1233zd(E) with variable evaporation percentage in the expander inlet. The thermodynamic investigation is done with a validated model in Engineering Equation Solver, and the transient analysis is performed with the TRNSYS tool by coupling it with the thermodynamic model. According to the dynamic analysis, the annual electrical yield is found at 6243 kWh, while the maximum obtained value is found at 3.81 kW. Moreover, the annual mean solar field efficiency was calculated at 45.0%, the annual mean thermodynamic cycle efficiency at 8.71% and the annual mean system efficiency at 3.68%.

Energy retrofit solutions that concern a building’s roof structure play a significant role in the enhancement of a building’s thermal behaviour. This study investigates the integration of phase change materials (PCMs) with cool coatings (CCs) or thermochromic coatings (TCCs), namely, a PCM roof, a PCM-CC roof, and a PCM-TCC roof, as alternative and novel tactics for the simultaneous control of solar heat transfer and solar heat reflection. An energy simulation analysis with the DesignBuilder tool is conducted for a one-story residence and the climatic conditions of Athens. The simulation results indicate that, compared to the existing concrete roof construction, the PCM roof, PCM-CC, and PCM-TCC roof systems demonstrate energy savings that reach up to 13.55%, 16.04%, and 21.70%, respectively. The systematic analysis reveals that the increase in PCM’s thickness leads to an increase in the total electricity savings of the buildings, but in the case of PCM-CC and PCM-TCC roof systems, they merely effect the cooling thermal loads. The mean phase transition temperature that favours the cumulative electricity savings is 28 °C in the case of PCM and PCM-TCC roof systems and 35 °C in the case of PCM-CC roof systems. The methodology of this study allows the design of efficient, integrated roof systems with advanced thermal and optical properties as energy retrofit solutions for Mediterranean climatic conditions.

The exploitation of solar irradiation is a critical weapon for facing the energy crisis and critical environmental problems. One of the most emerging solar technologies is the use of solar towers (or central receiver systems) coupled with high-performance thermodynamic cycles. In this direction, the present investigation examines a solar tower coupled to a closed-loop Brayton cycle which operates with supercritical CO2 (sCO2) as the working medium. The system also includes a storage system with two molten salt tanks for enabling proper thermal storage. The sCO2 is an efficient fluid that presents significant advancements, mainly reduced compression work when it is compressed close to the critical point region. The novelty of the present work is based on the detailed dynamic investigation of the studied configuration for the year period using adjustable time step and its sizing for achieving a continuous operation, something that makes possible the establishment of this renewable technology as a reliable one. The analysis is conducted with a developed model in the Modelica programming language by also using the Dymola solver. According to the simulation results, the yearly solar thermal efficiency is 50.7%, the yearly thermodynamic cycle efficiency is 42.9% and the yearly total system efficiency is 18.0%.

The goal of this investigation is the thorough analysis and optimization of a solar-assisted heat pump heating unit for covering the space heating demand for a building in Athens, Greece. The novelty of the studied system is the use of a high-temperature heat pump that can operate with radiative terminal units, leading to high thermal comfort standards. The examined system includes flat-plate solar thermal collectors, an insulated thermal storage tank, auxiliary electrical thermal resistance in the tank and a high-temperature heat pump. The economic optimization indicates that the optimal design includes 35 m2 of solar thermal collectors connected with a storage tank of 2 m3 for facing the total heating demand of 6785 kWh. In this case, the life cycle cost was calculated at 22,694 EUR, the seasonal system coefficient of performance at 2.95 and the mean solar thermal efficiency at 31.60%. On the other hand, the multi-objective optimization indicates the optimum design is the selection of 50 m2 of solar field connected to a thermal tank of 3 m3. In this scenario, the life cycle cost was calculated at 24,084 EUR, the seasonal system coefficient of performance at 4.07 and the mean solar thermal efficiency at 25.33%.

The exploitation of renewable energy sources in the building sector is a challenging aspect of achieving sustainability. The incorporation of a proper storage unit is a vital issue for managing properly renewable electricity production and so to avoid the use of grid electricity. The present investigation examines a zero-energy residential building that uses photovoltaics for covering all its energy needs (heating, cooling, domestic hot water, and appliances-lighting needs). The building uses a reversible heat pump and an electrical heater, so there is not any need for fuel. The novel aspect of the present analysis lies in the utilization of hydrogen as the storage technology in a power-to-hydrogen-to-power design. The residual electricity production from the photovoltaics feeds an electrolyzer for hydrogen production which is stored in the proper tank under high pressure. When there is a need for electricity, and the photovoltaics are not enough, the hydrogen is used in a fuel cell for producing the needed electricity. The present work examines a building of 400 m2 floor area in Athens with total yearly electrical demand of 23,656 kWh. It was found that the use of 203 m2 of photovoltaics with a hydrogen storage capacity of 34 m3 can make the building autonomous for the year period.

High-temperature heat pumps consist of a promising choice for substituting conventional boilers and producing hot water with temperature levels in the range of 70–80 °C. The use of high-temperature heat pumps makes it possible to exploit the existing radiators in existing buildings to achieve effective heating of the indoor space and keep thermal comfort at high standards. The goal of this work is the detailed investigation of a high-temperature heat pump, with the R1234ze(E) as the working medium for producing heating at 80 °C for space-heating purposes, in Greek climate conditions. The analysis was conducted at two locations, one in the south (Athens—Zone B) and one in the north (Thessaloniki—Zone C), in order to examine the performance of the study’s technology in the most representative locations of Greece. The thermodynamic analysis of the heat pump was performed with a developed model in Engineering Equation Solver by exploiting real data from the compressor manufacturer and by practically using a gray box model. The heating thermal loads were calculated with a dynamic analysis using TRNSYS software. According to the final results, the high-temperature heat pump was beneficial when compared to a conventional oil boiler from energy, financial, and environmental points of view. The net savings were found at EUR 6448 for Athens and EUR 13,161 for Thessaloniki; the simple payback period resulted in 8.03 years for Athens and 6.11 years for Thessaloniki, while the CO2 emissions avoidance was found at 35% for Athens and 32% for Thessaloniki. Last but not least, the lifetime CO2 emissions reduction was calculated at 20,882 kgCO2 for Athens and 35,278 kgCO2 for Thessaloniki.

Hydrogen has received increased attention in the last decades as a green energy carrier and a promising future fuel. The integration of hydrogen, as well as the development of cogeneration plants, makes the energy sector more eco-friendly, and sustainable. The aim of this paper is the investigation of a solar-fed cogeneration system that can produce power and compressed green hydrogen. The examined unit contains a parabolic trough collector solar field, a thermal energy storage tank, an organic Rankine cycle, and a proton exchange membrane water electrolyzer. The installation also includes a hydrogen storage tank and a hydrogen compressor. The unit is analyzed parametrically in terms of thermodynamic performance and economic viability in steady-state conditions with a developed and accurate model. Taking into account the final results, the overall energy efficiency is calculated at 14.03%, the exergy efficiency at 14.94%, and the hydrogen production rate at 0.205 kg/h. Finally, the payback period and the net present value are determined at 9 years and 122 k€, respectively.

An innovative parabolic trough concentrator coupled to a twin cavity receiver (PTC-TC) in evacuated tube conditions is investigated thermally and optically. The suggested design is compared with a PTC design with a flat receiver (PTC-F) in order to evaluate the efficiency of the proposed configuration. In the very first stages of the study, the optical efficiency was calculated for both collectors, and the optimum design was determined in the PTC-TC case. Then a mass flow rate independency procedure was conducted to ensure accurate results and to make a suitable comparison. The collectors were examined in a wide range of inlet temperatures ranging from 20 °C to 200 °C, and the thermal performance was calculated. Through the comparison process, it was revealed that the proposed collector appears to have higher thermal performance than the typical collector. In particular, there was a mean enhancement of approximately 8%, while the minimum enhancement was found to be greater than 5%. The simulation results regarding both configurations were verified through two models based on theoretical equations. In both geometries, the mean deviations in the verification procedure were lower than 5.6% in both the Darcy friction factor and the Nusselt number. The design and the simulations were conducted with the SolidWorks Flow Simulation tool.

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.

Renewable energy exploitation in the building sector can lead to significant energy savings and carbon dioxide emission avoidance. The objective of this study is the detailed investigation of a biomass-driven absorption heat pump for heating and cooling. The heat pump is practically a single-effect absorption chiller operating with the Lithium-bromide/water solution and it has been properly modified for heating production during the winter. This system is a novel one and its combination with a biomass boiler was examined for the first time, especially for covering both heating and cooling needs. For the present study, a typical building in Athens, Greece, with a 400 m2 floor area is selected to be coupled with the suggested heating/cooling configuration. The analysis was conducted by using TRNSYS software for the estimation of the building’s thermal loads and with the Engineering Equation Solver for determining the heat pump behavior. According to the results, the yearly biomass consumption is found to be 3.76 tons covering a heating demand of 9136 kWh and cooling demand of 8168 kWh. The seasonal energy cooling performance was found to be 0.751, while the seasonal energy heating performance was at 1.307. Moreover, the proposed configuration was found to have economic and environmental benefits compared to conventional units with an oil boiler and heat pump for cooling. Specifically, the present system leads to 10.8% lower operational costs and 4.8% lower primary energy demand, while there are significant amounts of CO2 avoidance.

Heat and flow enhancement is a critical weapon for the design of highly efficient, compact, and cost-effective devices. The objective of this analysis is the detailed examination of the implementation of different fin shapes on the internal side of a tube aiming the heat and flow enhancement. The reference empty tube is examined, while the use of circular, rectangular, and triangular fins is also studied. Different simulations were conducted with a developed computational fluid dynamic model, and the results were expressed in heat and flow terms. The developed model was validated by comparing the results with the theoretical expressions for the Nusselt number, as well as for the friction factor. The obtained results were evaluated by calculating the heat convection coefficient, the pressure drop, the Nusselt number, the friction factor, the pump work, the mean wall temperature, and the Bejan number. According to the calculations, the use of rectangular fins is the best solution; the use of triangular fins is the second choice, while the less efficient improvement method is the use of circular internal fins. Regarding the heat transfer enhancement, the rectangular fins lead to 36% enhancement, circular fins to 25%, and triangular fins to 23% compared to the reference tube.

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.

The objective of this investigation is to present a novel concept for the optimum exploitation of volatile electricity from renewable energy sources. The idea of the Carnot battery is extended to a general concept for trigeneration which can be called “power to XYZ”. This idea is applied for the building sector where there are needs for cooling production, space-heating production/domestic hot water production and electricity. More specifically, volatile electricity feeds a multi-stage heat pump that produces cold storage at 0 °C for cooling, medium heating storage at 50 °C for space heating and high thermal storage at around 115 °C for future utilization in an organic Rankine cycle for electricity production. The storage is performed in three different temperature levels, with latent storage proposed for proper long-term and efficient storage. The use of ice is suggested especially for cold storage in order to make the design a cost-effective one. This work is a theoretical preliminary thermodynamic analysis performed with a model created in Engineering Equation Solver. The results indicate the system’s maximum exergy efficiency is found at 45.28%, while the respective energy efficiency is found at 322.16%. Moreover, this work includes parametric studies and calculations about the operating margins of the suggested system.