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The unpredictability of heat demand–supply is one of the major challenges in our future sustainable energy system. Thus, coming up with new methods of heat storage for operating solar systems during low solar irradiance is among the advanced technical approaches in this field. The compact integrated solar thermal collector-storage systems have recently attracted the researchers’ attention due to the direct usage of stored thermal energy as well as the reduced space occupation. The current literature lacks a review paper on investigating various existing designs of such compact PCM-solar systems; thus, this paper analyzes the latest developments on experimentally investigated single-unit PCM-based solar collectors regarding both solar water and air heaters. Various types of solar collectors, including flat plate, evacuated tube, concentrating, and photovoltaic/thermal collectors have been thoroughly reviewed and discussed and the thermodynamic correlations for the system analysis have been given. The investigations revealed that a large proportion of integrated flat plate storage systems have been designed to provide space heating and it was deduced that paraffin and other hydrocarbons were basically used for the low-temperature heat production for domestic uses. Also, the concentrating solar collectors have mostly been utilized for providing the medium temperature for the industrial usage employing salt hydrates and nano-composite based phase change materials possessing higher melting points are adopted. Last but not least, it was concluded that the exergy efficiencies of solar collectors are remarkably low (below 5%), and the hybrid photovoltaic-thermal collectors can encounter this drawback by exploiting the high electrical exergy.

In this paper, the thermal characteristics of an evacuated tube solar collector for different volumetric flow rates of the fluid (10, 30 and 50 l/h) was experimentally improved by using copper oxide/distilled water (Cu2O/DW) nanofluid, and parabolic concentrator. Moreover, the effect of different volume fractions of the utilized nanofluid on the fluid properties, such as convective heat transfer coefficient, Nusselt number, and the useful gain of the collector was experimented. Finally, three artificial intelligence (AI) techniques namely, multi-variate adaptive regression spline (MARS), model tree (MT) and gene-expression programming (GEP) have been employed to predict the energy efficiency ( η І ) and inlet-outlet water temperature difference (ΔT). The input variables were volume of the storage tank (V), volume fraction of the nanofluid (VF), and mass flow rate of the fluid ( M ˙ ). The proposed AI methods presented robust formulations for prediction of η І and ΔT with an acceptable level of precision. The statistical results of AI models demonstrated that the MARS method can make a more accurate prediction of the collector performance than GEP and MT. It was also concluded that increase in both flow rate, and concentration of the nanofluid, lead to an increase in the thermal performance of the solar collector.

In this study, the effect of various gases filling the space between the cover and the absorber coil on the thermal performance of a cylindrical solar water heater (CSWH) is experimentally investigated. Since the Prandtl number for argon gas is less than that for the air, the thermal performance of the CSWH increases. The experiments are conducted for mass flow rates of 2.5, 3, 3.5 and 4 kg/h. The results show that an increase in the flow rate leads to a decrease in the temperature difference the of water between the inlet and outlet of the coil due to reduction of the residence time of the fluid. The energy efficiency of the collector reaches its maximum at the mass flow rate until 3.5 kg/h, and then reduces at higher values of mass flow rate because the water temperature difference decreases dramatically. Hence, the optimum mass flow rate for a cylindrical collector is reported as 3.5 kg/h. The maximum energy efficiencies for argon and air are 52.14% and 48.17%, respectively. Finally, the constructed cylindrical solar water heater is also economically compared to a flat plate collector (FPC) and an electric water heater with similar thermal efficiency.

Many factors impact the thermal efficiency of the evacuated tube solar collectors (ETSCs) and numerous studies have been carried out to increase the efficiency of ETSCs. In this study, the stagnant region at the bottom of the evacuated tube has been removed using a bypass tube to connect the storage tank to the bottom of the modified evacuated tube. The thermal performance of a modified evacuated tube solar collector (METSC) has been investigated experimentally and numerically. The thermal characteristics of the METSC were compared to those of a commercial sample under equal environmental conditions. Furthermore, the impact of the diameter of the bypass pipe on the velocity profile, and temperature distribution of the fluid inside the modified collector has been investigated numerically to achieve the optimum diameter. The results demonstrate that this structure modification improves the collector performance due to removing the stagnant region at the bottom of the evacuated tube and the storage tank. This modification made the temperature distribution inside the pipe and tank more uniform, and increased the efficiency up to 11%. Moreover, useful gain of the solar collector in the modified model was 25% more than that of the typical model over a one-hour period. In addition, for the same duration, the average temperature of water inside the tank enhanced by 1.5 °C in the METSC in comparison to the typical model under equal environmental conditions.

In this study, in order to enhance the thermal efficiency of the solar water heating system, the effect of reflector, parabolic concentrator, and the nanofluid copper oxide/distilled water (Cu2O/DW) for 0.01 to 0.08 volume fractions on thermal performance of an evacuated tube solar collector (ETSC) has been both numerically and experimentally investigated. The numerical analysis has been undertaken to develop the experimental setup. The TESCs were simulated for different diameters of the evacuated tube to show how diameter of the tube affects the circulation rate, and the temperature distribution inside the ETSC. To summarize the experimental results the results have been shown for pure water, 0.04 volume concentration, and 0.08 volume fraction of the nanofluid. By use of 0.08 volume fraction of the nanofluid the energy and exergy efficiency of the system were enhanced 10% and 12.7%, respectively. Moreover, the mean temperature of the tank increased dramatically by use of nanofluid. In addition, the impact of different volume concentrations on the thermo-physical properties of the working fluid, such as density, heat capacity, viscosity, and conductivity was theoretically conducted.

Providing sufficient potable water using solar energy is a promising method; however, existence of legionella bacteria, causing pneumonia is prevalent in the solar systems operating at low temperature range, such as solar stills. This work is the first report of applying the antibacterial property of a hybrid nanofluid for disinfection, and using its magnetic property for increasing the heat transfer rate, simultaneously. In this research, the performance of a modified single slope solar still using a thermoelectric-equipped channel integrated with a concentrating evacuated tube collector applying an antibacterial-magnetic Ag@Fe3O4/deionized water hybrid nanofluid was examined experimentally, and was compared to that of a traditional solar still. The results indicated that both productivity and energy efficiency were improved by integrating the thermoelectric cooling channel solar still with the concentrating solar collector and adding Ag@Fe3O4 nanoparticles to the fluid. The modified solar still integrated with the solar collector utilizing the proposed hybrid nanofluid at a volume concentration of 0.08% ameliorates the daily productivity and energy efficiency rates by respectively 218% and 117% compared to a traditional solar still. Also, the cost of producing freshwater by the proposed modified solar still is about 0.019 ($/L/m2) and the payback period was calculated as 369 days.

In the present study, the thermal and thermohydraulic characteristics of different types of double pass solar air heaters (DPSAHs) containing three flow channels were analytically investigated. The analyses were conducted at air mass flow rates of 0.01, 0.015, 0.025 kg/s and different reflux ratios of 0.1 to 1. The effects of upstream and downstream recycling patterns were analyzed. Moreover, the impacts of matrix placed between the absorber plate and the second glass cover with various porosities, and variation of solar radiation intensity on the DPSAH performance were examined. Furthermore, the pressure drop due to the existence of matrix was considered to obtain more realistic outcomes. The results demonstrated that as to the downward recycling pattern, using matrix leads to an increase in the DPSAH thermal efficiency presenting the thermal efficiency of 79%; however, it brings about a reduction in its thermohydraulic efficiency at high mass flow rates and reflux ratios and high fan power cost is incurred, subsequently. The most compelling result is: if we consider an upstream recycling pattern, it is true that we have overlooked a certain amount of the DPSAH efficiency compared to a matrix-based downward recycling DPSAH (about 7% less efficiency); however, at high mass flow rates and reflux ratios the thermal efficiency of DPSAH is similar to that of a matrix-based upward recycling DPSAH, and in this way, not only the hot air demand is supplied, also the extra fan power cost and the cost of providing a suitable matrix are eliminated.

This study is the report of modelling the evacuated tube solar collector (ETSC) by Gene-Expression Programming (GEP). The data were gathered by simulating the ETSC for different volumes of the thermal storage tanks (10-50 Lit) with various solar radiation intensities through Computational Fluid Dynamics (CFD). In order to obtain the most accurate mathematical model (expression) and better predicting the efficiency of system, a trial and error approach was employed. The GEP model was tested and trained based on the numerical data. On the other hand, the numerical data were verified by comparing the results with the previous experimental investigations. The numerical results indicated that as the solar radiation absorption increases, the water temperature difference and the energy efficiency of the ETSC also increase. Moreover, for the ETSC possessing three evacuated tubes, the optimum volume of the thermal storage tank was obtained 26 Lit, and the maximum energy efficiency of the optimized ETSC for the numerical investigations was reported roughly 72%. In the previous published work, the maximum amount of efficiency of the ETSC was 70% showing that the CFD-based approach could acceptably model the performance of ETSCs, and it can be concluded that the GEP-based model for the ETSC performance is reliable and trustworthy.

The effect of variation of longitude and latitude on the amount of solar radiation is investigated in this work in the Iranian climate context on different solar collectors and storage tank geometries. An equation for prediction of solar radiation intensity is presented through regression analysis (R2 = 0.9976). The effects of different types of storage tanks, change of Grashof number, and change of time of experiments on the thermal characteristics of the evacuated tube solar collectors are numerically investigated. Solar irradiance on the collector surface was simulated via P-1 method. Results demonstrated that changing longitude does not influence the amount of solar radiation, whereas change of latitude affects such amount, and regions with higher latitudes (northern areas) receive more beam radiation around solar noon. In addition, the tank with longer height absorbs more heat, and presents a better efficiency. Also, the tank geometry affects its performance; the useful heat gain of a collector possessing a vertical square tank is about 10% more than that of a collector with a cylindrical tank without consideration of heat loss from the tank. Nonetheless, considering such heat loss, the collector with cylindrical tank presents the highest performance due to minor heat loss area of the tank.

In this study, the exact amount of the rate of irradiance within the ambit of a tube in the evacuated tube solar collector (ETSC) has been measured in various hours of the day by means of a solar meter. Some novel correlations for determining the distribution of insolation intensity around the tube have been proposed by the analysis of experimental data. A correlation is offered by using the results of previous studies and mathematical analysis. The offered correlation is utilized to calculate the direction and the amount of the maximized insolation intensity in cross section area of the tube concerning geographical circumstances, hour angle, and the slope of the collector by denoting the solar radiation distribution function around the section of the tube. Moreover, by integrating around the tube, a correlation for calculation of the whole amount of the absorbed radiation through the absorber has been presented. In the end, the error analysis is conducted and the maximum amount of error for the proposed model is estimated to be 8% before 5 p.m., and at solar noon the amount of error is reported less than 1%. Furthermore, the energy efficiency of the constructed water heater is obtained as 65%.

Prediction of friction factor plays a prominent role in determination of head losses along pipelines or water distribution systems. Numerous traditional equations-based regressive models have been extracted for prediction of the friction factor using experimental and numerical studies. In the present research, a model tree (MT) was developed to present formulations for evaluation of friction factor in pipes. The effective parameters on the friction factor (f), included pipe diameter (D), average flow velocity in pipe (V), kinematic viscosity (ν), and roughness height of the inner surface of pipe (ɛ). Two non-dimensional parameters of the fluid, Reynolds number (Re) and relative roughness (ɛ/D) were employed to improve the proposed model. The training and testing of MT approach were carried out using datasets obtained from the approximate solution. The testing results for the MT were compared with those obtained using gene-expression programming (GEP), evolutionary paradigm regression (EPR), and co...

In the present study, the modified evacuated tube solar collector (METSC) with a bypass pipe utilizing copper oxide/distilled water (Cu2O/DW) nanofluid is experimented. Then, the performance of METSC was predicted through Artificial Neural Networks (ANNs) techniques. The input variables were different volumes of the storage tank from 5 to 8 l, various diameters of the bypass pipe from 6 to 10 mm, and various volumetric concentration of the nanofluid from 0 to 0.04. Also, the output variables were the temperature difference of fluid in 1-h period and the energetic efficiency of METSC. The results demonstrated that the METSC performance was mostly impacted by the tank volume alteration. Moreover, the optimum bypass tube diameter value was obtained, and it was denoted that using the Cu2O/DW nanofluid enhances the daily energy efficiency of METSC up to 4%. Furthermore, it was shown that both MLP and RBF techniques are two reliable algorithms to predict the thermal characteristics of an METSC. The maximum amounts of mean relative percentage error for MLP and RBF algorithms were reported as 0.576 and 0.907, respectively. Hence, two mathematical models were reported for formulating the output variables in terms of the input variables using the MLP technique.

In this study, an air based ejector (ABE) was designed and manufactured, and the experiments were conducted for air as the working fluid with several motive pressures at two different nozzle exit positions. Furthermore, a simulation was carried out through the Realizable k-e turbulence model to predict effect of the inlet primary pressure, and the nozzle exit position on the first and second laws of Thermodynamics within the ABE. The hydrodynamic behavior of the flow field as well as the exergy dissipation values within the ABE at different inlet primary pressures, and positions of nozzle were assessed. Moreover, the results revealed that the strain rate value and the situation of the vortex inside the ABE determine the alteration of energy type, i.e. the kinetic energy enhances and a vacuum is created. Ultimately, the highest first and second law efficiencies of the ABE were 37% and 82%; respectively, at nozzle exit position equal to −10 mm.