Ahmed Hamed

Ejector refrigeration systems (ERS) are promising refrigeration systems that can replace conventional vapor compression systems, because of their ability to operate by thermal energies of low grades and their capability to be integrated with renewable energy sources. However, there are restrictions concerning its low system COP, limited range of operating conditions, and high sensitivity of the performance to the variation of operating conditions. Therefore, this present paper introduces a detailed mathematical model to design and investigate the performance of a novel double-stage in-series ERS, which is believed to overcome the abovementioned restrictions of the single-stage ERS. The present work investigates the critical operating points of the double-stage in-series ERS to reach the maximum COP. Four refrigerants are employed in the analysis, R600a and R134a are compared to each other as they are widely used; additionally, R1234yf and R290 are compared to each other, as they represent the future refrigerants. The comparison shows that double-stage in-series ERS improves the system performance and ability to withstand a high range of operating conditions with low COP variations. Double-stage in-series ERS can result in performance improvements ranging between 3 % to 160 % depending on the refrigerant and its operating condition. Moreover, the usage of double-stage in-series ERS gives the chance to operate at low evaporator temperatures below 0 °C and high condenser temperatures above 50 °C, which is not reachable by single-stage systems. Furthermore, the analysis shows that R600a and R290 have higher performance than R134a and R1234yf, respectively.

Desiccant air-conditioning systems are considered a competitive alternative to conventional vapor compression cycles. However, the literature lacks detailed studies of such systems when coupled with thermally active buildings and powered by solar energy due to the complexity of modeling and controlling the system. In the present study, a novel hybrid desiccant-dehumidification-absorption system, driven by external compound parabolic concentrators (DDA-XCPC), is used to serve a typical office space in a hot climate zone. The entire system is modeled and simulated dynamically in the TRNSYS environment. The current study investigates the proposed system performance in terms of the achieved thermal comfort, energy consumption, solar fraction, life cycle costs, and carbon emissions. The results reveal superior thermal comfort levels with percentages of people dissatisfied below 5.6 %. The daily solar fraction ranged between 41 and 90 %, with an average of 49 %, whereas the coefficient of performance varied between 0.46 and 1.38 while having a season average of 0.86. Compared to a conventional system, driven by a gas boiler, the proposed system increased the life cycle costs by 6,824 USD due to the large capital investment of the solar thermal loop. However, each 1,000 USD of additional expenses was accompanied by a cutdown of 4,619 kg of CO2 emissions. Increments in the solar system’s capacity (i.e., solar collector area and storage tank volume) have adverse and favorable impacts on the economic and environmental performances, respectively. Therefore, the system can be a viable option whenever green energy production is prioritized and considerably incentivized.

Compact installations of solar photovoltaic (PV) systems to maximize the use of land are a necessity in urban regions, where the available installation areas exposed to solar irradiance are limited. Large-scale commercial buildings are typically surrounded by open areas and have busy roofs with mechanical and air conditioning equipment. This study assesses three candidate installation schemes of dual-row near-wall ground-mounted PV systems. The baseline configuration (BC) comprises the classic parallel PV rows. The overhead reflector (OR) configuration adds reflectors above the panels of the second row, to be supported by the wall. The overhead panel (OP) configuration moves the panels of the second row to be above the panels of the first row. Mathematical models are developed to evaluate and optimize the techno-economic performance (in terms of total annual energy production “EPV” and levelized cost of electricity “LCOE”, respectively) based on the tilt angles of the two rows, as well as the length and tilt angle of the reflector in the OR configuration. The results show maximum energy production of 12.54, 12.77, and 12.53 MW for the three configurations, respectively. When the land cost is excluded, the minimum LCOE is 0.0696, 0.0698, and 0.0696 USD/kWh, which correspond to 0.1318, 0.1332, and 0.1182 USD/kWh, respectively, when the land cost is included. The EPV-LCOE Pareto fronts are dominated by different designs of the OR and the OR/OP configurations when excluding and including land cost, respectively, which demonstrates the favorableness of the two proposed schemes, compared to the baseline one.

Supercritical carbon dioxide is becoming a hot research topic as a potential heat transfer fluid in parabolic trough concentrators since it enables operating the solar system at high temperatures for a higher quality of energy. However, the corresponding inflated thermal losses necessitate alternative receiver designs. This work examines four double-glazed receivers, with each annular space being evacuated or non-evacuated, in terms of the absorber tube's diameter (53–80 mm) and the diameter ratios of the two glass shells (1.2–2.0). An analytical model is developed and validated for this purpose, and the four designs are further examined using ground-level solar and meteorological measurements. The results demonstrate higher performance in the case of fully evacuating the receiver and using the smallest possible diameters of the three concentric cylinders, where the energy and exergy efficiencies reach 65.3 and 40.3%, respectively. Yet, evacuating only the inner annular space is sufficient to achieve virtually the same performance level. This energy efficiency decreases to 62% in case of increasing the tube diameter to 80 mm. As the operating temperature increases from 423 to 850 K, the specific thermal losses increase by 3.98–4.34 folds, depending on the receiver design. Double glazing the receivers is favorable only at high operating temperatures of sCO2, where the reduction in thermal losses overcomes the drop in optical efficiency. For an inlet sCO2 temperature of 850 K, thermal losses are reduced by 33.64 and 53.92%, compared to evacuated and non-evacuated single-glazed receivers, respectively. Throughout the year, the fully evacuated and fully non-evacuated double-glazed receivers have energy efficiencies of 54.96 and 52.39%, exergy efficiencies of 33.64 and 32.06%, and thermal losses of 348.8 and 402.5 W/m, respectively.

With the growth in the solar photovoltaic (PV) market, there is a renewed interest in increasing the system’s annual produced energy. Most of the proposed solutions in the literature require special additional equipment, which might adversely affect the cost of the produced energy. This paper proposes an integrated PV-reflector system that augments the solar irradiance on already installed PV modules. A new mathematical model has been developed in MATLAB R2020a to simulate and techno-economically assess the proposed integrated PV system during the entire year using real meteorological data. A genetic algorithm has been employed in MATLAB R2020a to obtain the optimal geometric and optical parameters that maximize the annual produced energy and minimize the Levelized Cost of Electricity (LCOE) at two selected locations (Cairo in Egypt and Ma’an in Jordan) and for three module technologies (monocrystalline, polycrystalline, and thin film) and different commercial reflectors. The optimal system configurations increased the annual energy production by up to 6.05, 5.14, and 8.34% for the three PV technologies, respectively. For both locations, the optimal tilt angles of the PV module and reflector range between 24° and 34° for Cairo, and between 19° and 36° for Ma’an, depending on the PV technology. Finally, all the system configurations have favorable and low payback periods (∼3 and 6.5 years for Ma’an and Cairo, respectively), as well as attractive LCOE, which varies between 0.0369 and 0.044 USD/kWh in Ma’an and between 0.0543 and 0.065 USD/kWh in Cairo.