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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%.

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.

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.