• Energy Research

In this paper, theoretical investigation of an integrated solar-powered cooling system consisting of commercial photovoltaic/thermal (PVT) collectors and an adsorption chiller is performed. The performance is investigated under different climate conditions of the Middle East such as Alexandria in Egypt, Dubai in United Arab Emirates and Riyadh in Saudi Arabia. System performance parameters including cooling capacities, chiller COP, solar COP, generated electric power and total system efficiency have been investigated. The results of the study for the month of July show that the maximum generated electric power is about 12.55 kW in Alexandria, while the maximum cooling capacity and COP are 8.1 kW and 0.43, respectively in Dubai. Furthermore, the average total system efficiency in a typical day in July is about 0.248, 0.288 and 0.275 in Alexandria, Dubai and Riyadh, respectively.

Abstract Adsorption working pairs are the vital main components in the adsorption refrigeration machines. Therefore, the key for the further development is focusing on the adsorption pairs, which lead to the improvement of the adsorption refrigeration machines. In this study, an overview of both classical and modern adsorption pairs of the adsorption refrigeration systems is presented, compared and summarized. It was found that the maximum adsorption capacity for the classical working pairs was 0.259 kg/kg for activated carbon/methanol pair and that for the modern working pairs was 2 kg/kg for maxsorbIII/R-134a pair. This study concluded that, further investigations are still necessary to improve the performance of the adsorption working pairs of adsorption cooling systems as well as to develop the adsorption pairs with higher sorption capacity while with low or no impact on environment, in order to build compact, efficient, reliable, and long-life adsorption chillers. It was additionally found that activated carbon powder adsorbent has not been paid much attention so far, and hence, the study and application of it are to be of great interest. Further researches need to be focused on designing the adsorption system that provides efficient heating and cooling for the adsorbent materials by distributing the adsorbent material over heat exchanger surface, to allow good heat and mass transfer between the adsorbent and the refrigerant.

Abstract This study investigated experimentally the performance due to automatic cooling and surface cleaning of Photovoltaic (PV) module installed on the roof of a building in hot arid area as compared with that of a module without cooling and cleaning. The module cooling is controlled automatically according to the rear side temperature via rejection of none-converted solar-energy to the ambient to keep the PV module surface temperature always close to the ambient temperature. In addition, this system controls the cleaning period of the module front surface. The results showed a decrease of about 45.5% and 39% in module temperature at front and rear faces, respectively. Consequently, the cooled and surface cleaned module has an efficiency of 11.7% against 9% for the module without cooling and cleaning. Moreover, the maximum output power produced by cooled and cleaned module is 89.4 W against 68.4 W for non-cooled and non-cleaned module.

This paper examines the effect of microwave drying on biomass characteristics and subsequent dry pyrolysis and characteristics of produced biochar from rice straw, sugarcane bagasse, rice husk and cotton stalk compared to oven drying at 105 °C. Dried samples from both methods are torrefied at 250 and 300 °C with 30-minutes residence time. Drying time reached 60 times faster with microwave. The fast and violent microwave drying ruptured the biomasses' surface, releasing more volatiles and having lower crystallinity; these lowered the heating value, energy yield and elemental carbon compared to oven drying except for cotton stalk only due to its woody nature which reduced devolatilization. Sugarcane, rice husk and cotton stalk have the most promising values of elemental carbon, energy yield and heating value reaching that of the bituminous coal. Torrefied rice straw showed high crystallinity of 50.7% while sugarcane and rice husk were completely amorphous.

Abstract In this study, a comprehensive exergetic performance investigation of a single slope a passive solar still system is theoretically presented. Energy and exergy methodologies have been applied for all components of the solar still comprising glass cover, brackish water, and basin-liner. Also, exergy irreversibility analysis was conducted to identify and localize the sources responsible for the exergy destruction and losses in the system for further analysis and improvement. The theoretical model was solved numerically by using fourth-order Runge–Kutta method and the program was written by MATLAB. To examine the validity of the model, the numerical results were verified with the available experimental data in the literature. The numerical results were in good correspondence with the experimental data for the components’ temperatures and output productivity. The results showed that the maximum energy and exergy efficiencies of the proposed system are 32.5 % and 2.23 %, respectively. It is observed that the exergy efficiency has much lower value than the energy efficiency. The maximum irreversibility or exergy destruction in each component, i.e. glass cover, saline water, and basin-liner, has been estimated as 61.1, 50.2 and 717 W/m2, respectively, related to the maximum solar exergy input of 1005 W/m2. Furthermore, the results showed that exergy destructions rates in the solar still is proportional to the received solar insolation. From irreversibility analysis, it is found that the basin liner accounts for the highest exergy destruction (86% of the total exergy destruction).

There are no data in the literature on the energy valorization of globe artichoke (GA) leaves. Thus, an extensive lab-scale experimental torrefaction, carbonization, and coking study was performed. Operative temperatures of 200 °C-1000 °C with 30-120 min residence times were considered. Nonisothermal thermogravimetric analysis was performed at 10, 20, and 40 °C/min heating rates. Pyrolysis and combustion kinetics of raw and thermally treated samples using the Ozawa-Flynn-Wall (OFW) isoconversional method were investigated. All samples exhibited three-stage thermal decomposition behavior: first, moisture and light volatiles evolution common under air and nitrogen; second, carbohydrate fraction decomposition under nitrogen and volatiles combustion; third, lignin decomposition under nitrogen and char combustion. Average activation energy ranges are 54-223 kJ/mol and 223-503 kJ/mol for combustion and pyrolysis, respectively. Some irregular trends appeared when carbonization exceeded 500 °C due to the occurrence of secondary reactions between residual char and evolved gas and the decomposition of some ash components at temperatures reaching 1000 °C. Negative temperature kinetic coefficient appeared at 800-1000 °C as the temperature approached ash softening/fusing temperatures. SEM images indicated amorphous nature and increased porosity from 600 °C, which explains the pyrolysis and oxidation behavior observed in biochar samples produced over this range. Samples pyrolyzed for 30 min showed better elemental and energy results compared to longer times.

Abstract For the safe and efficient operation of concentrator photovoltaic cells and electronic chips, low and uniform temperature should be attained. Therefore, the prime focus of this study is to design the optimal headers and to evaluate the performance of a monolithic double-layer microchannel heat sink (MDL-MCHS) operating under forced convective boiling conditions. The designed and fabricated heat sink was proved to attain a uniform temperature distribution over the entire surface of the MCHS heated wall, in a narrow temperature range around the coolant boiling point. The designs of the MCHS inlet and outlet headers were computationally optimized to avoid flow maldistribution over 10 parallel channels in each layer. Subsequently, an MDL-MCHS with an optimized header was fabricated using a metal 3D printer, and its thermal characteristics were experimentally evaluated in counterflow and parallel-flow operations under single-phase liquid flow and forced convective boiling conditions. The supplied heat flux was varied from 1.0 to 9.2 kW/m2. Ethanol and acetone with a boiling point of 78.4 °C or 56 °C were identically fed into each layer in a flowrate ( V ) range of 15–400 ml/h. At 9.2 kW/m2 (11.5 suns), the counterflow operation of forced convective boiling attained temperature uniformity below 1.6 °C and 1.8 °C in the V range of 25–100 ml/h for ethanol and 50–300 ml/h for acetone, respectively. The resultant wall temperature was nearly identical with the boiling point of operated coolant.

Abstract The high solar light concentration onto the photovoltaic cell leads to extremely high cell temperature, which significantly decreases the cell efficiency and degrades its lifetime due to the thermal stresses. One of the main challenges of these types of solar cells is to propose an efficient cooling technique that allows the cells to operate under its recommended operating conditions. Therefore, the focus of this study was to develop a comprehensive three-dimensional model for the high concentrator photovoltaic/thermal (HCPV/T) system. This model comprises a thermal model for a triple-junction solar cell integrated with a thermo-fluid model for four distinct designs of confined jet impingement heat sinks. The results showed that the cell electrical efficiency increased with the coolant flow rate, and sufficient temperature uniformity can be achieved by the jet impingement configurations. Additionally, the use of jet impingement configurations consumed a slight pumping power less than 1% of the generated power in the solar cell. The maximum local temperature of uncooled solar cell was predicted to reach 1360 °C under solar concentration ratio of 1000 Suns. Under the same conditions, the single jet design reduced the maximum local temperature to about 65 °C with coolant mass flow rate of 50 g/min. It should be noted that the thermal stress substantially decreased with the increasing coolant mass flow rate. Exergetic analysis showed that the single jet design attained the maximum total exergy efficiency of 53.25% at the flow rate of 25 g/min.