Search Results (82)

This study conducts a comparative performance analysis of three different low-temperature solar collector systems: flat plate solar collectors (FPCs), heat pipe evacuated tube solar collectors (HPETCs), and heat pipe flat plate solar collectors (HPFPCs). Key performance parameters, such as heat transfer coefficients, useful heat, and thermal efficiency, are analyzed under varying mass flow rate, fluid temperature, and antifreeze concentration. The objective is to evaluate the thermal performance of these systems using different heat transfer fluids, specifically water, and mixtures of 30% and 50% ethylene glycol and propylene glycol. The performance data indicate that the heat transfer coefficient in the HPFPC diminishes by 28% and 41% when antifreeze is employed at concentrations of 30% and 50%, respectively. Furthermore, the integration of heat pipes with water in a flat plate solar collector results in efficiency enhancements, with respect to FPCs, of up to 13% at a fluid temperature of 30 °C, and up to 21% at 80 °C. At the elevated fluid temperature of 80 °C, an efficiency increase of 13% is observed with a 30% ethylene glycol concentration. The incorporation of heat pipes leads to an efficiency improvement of up to 6.5% in comparison to traditional flat plate solar collectors. This study highlights the significant impact of fluid properties, affecting the convective heat transfer coefficient, on the overall efficiency of solar collectors, emphasizing the importance of optimizing fluid composition and operating conditions for enhanced thermal performance. Full article

Treatment of extremely saline water such as the brine rejected from reverse osmosis water desalination plants, and produced water from shale oil and non-conventional gas extraction, is considered a global problem. Consequently, in this work, hollow fiber membrane distillation (HFMD) is experimentally evaluated for desalinating extremely saline water of a salinity ranging from 40,000 to 130,000 ppm. For the purpose of comparison, the HFMD is also tested for desalinating brackish (3000–12,000 ppm) and sea (25,000–40,000 ppm) water. Firstly, the HFMD is tested at two values of feed water temperature (65 and 76 °C) and flow rate (600 and 850 L/h). The experimental results showed that the HFMD productivity significantly increases when the temperature of feed water increases. Increasing the feed water flow rate also has a positive effect on the productivity of HFMD. It is also concluded that the productivity of the HFMD is not significantly affected by increasing the salt concentration when brackish and sea water are used. The productivity also slightly decreases with increasing the salt concentration when extremely saline water is used. The decrement in the productivity reaches 27%, when the salt concentration increases from 40,000 to 130,000 ppm. Based on the conducted economic analysis, the HFMD shows a good potential for desalinating extremely saline water especially when the solar collector is used as a heat source. In this case, the cost per liter of freshwater is reduced by 21.7–23.1% when the evacuated tube solar collectors are used compared to the system using electrical heaters. More reduction in the cost per liter of freshwater is expected when a high capacity solar-powered HFMD plant is installed. Full article

This research aims to determine the primary thermofluidic correlations describing the thermosiphon effect under idealized steady-state conditions, considering water-in-glass evacuated-tube geometry, tilt angle, and heat flux. A numerical model based on Computational Fluid Dynamics (CFD) was developed to obtain these correlations for water-in-glass evacuated-tube solar collectors. Initial validation against experimental velocity and temperature profiles was necessary. With a validated CFD model, thermofluidic correlations were determined, expressed as dimensionless parameters such as Re, Gr, and Pr, water-in-glass evacuated-tube dimensions, and tilt angle. Symmetry was exploited in the water-in-glass evacuated-tube geometry for both validation simulations and the development of thermofluidic correlations. Contrary to correlations recorded in the literature, the correlations obtained in this study indicate an increase in water flow and a decrease in mean temperature with increasing tilt angle. These correlations are crucial for the energy–exergy balance formulations used in the analysis and design of such thermal systems. Full article

The goal of this research is to examine the applicability of the Hottel–Whillier–Bliss model, developed for flat-plate collectors, to evacuated tube solar collectors. During this study, the model is applied to an evacuated tube collector, and then the identification and validation of the model are made with the help of measurements performed on the collector. This research also includes the application, identification and validation of the energy balance model for the investigated solar collector. This model works for both flat-plate and evacuated tube collectors. The results obtained with the two different models are then compared. By comparing the modelled results with the measured values, the accuracy and applicability of the models can be determined. Based on the results, the Hottel–Whillier–Bliss model works excellently with evacuated tube solar collectors for predicting the outlet temperature of the medium from the solar collector. It is important to note that the identification gives negative heat transfer parameter values. According to the validation, the average absolute error is 0.8 °C, and the average relative error is 1%. For the energy balance model, these values are 0.87 °C and 1.1% respectively, indicating that the accuracy of the Hottel–Whillier–Bliss model is very similar, and even slightly higher. Additionally, the research provides further proof of the applicability of the energy balance model to evacuated tube collectors. Full article

In the context of Concentrated Solar Power (CSP) technology, this paper presents a comparison between the Parabolic Trough Collector (PTC) and the Linear Fresnel Collector (LFC), considering both evacuated and non-evacuated receiver tubes. The comparison was carried out in terms of the Levelized Cost of Electricity (LCOE) considering a reference year and four locations in the world, characterized by different levels of direct normal irradiation (DNI) from 2183 kWh/m²/year to 3409 kWh/m²/year. The LCOE depends on economic parameters and on the net energy generated by a plant on an annual basis. The latter was determined by a steady-state 1D model that solved the energy balance along the receiver axis. This model required computing the incident solar power and heat losses. While the solar power was calculated by an optical ray-tracing model, heat losses were computed by a lumped-parameter model developed along the radial direction of the tube. Since the LFC adopted a secondary concentrator, no conventional correlation was applicable for the convective heat transfer from the glass cover to the environment. Therefore, a 2D steady-state CFD model was also developed to investigate this phenomenon. The results showed that the PTC could generate a higher net annual energy compared to the LFC due to a better optical performance ensured by the parabolic solar collector. Nevertheless, the difference between the PTC and the LFC was lower in the non-evacuated tubes because of lower heat losses from the LFC receiver tube. The economic analysis revealed that the PTC with the evacuated tube also achieved the lowest LCOE, since the higher cost with respect to both the LFC system and the non-evacuated PTC was compensated by the higher net energy yield. However, the non-evacuated LFC demonstrated a slightly lower LCOE compared to the non-evacuated PTC since the lower capital cost of the non-evacuated LFC outweighed its lower net annual energy yield. Finally, a sensitivity analysis was conducted to assess the impact on the LCOE of the annual optical efficiency and of the economic parameters. This study introduces key technical parameters in LFC technology requiring improvement to achieve the level of productivity of the PTC from a techno-economic viewpoint, and consequently, to fill the gap between the two technologies. Full article

Air conditioning is vital for indoor comfort but traditionally relies on vapor compression systems, which raise electricity demand and carbon emissions. This study presents a novel thermo-mechanical vapor compression system that integrates an ejector with a conventional vapor compression cycle, incorporating a thermally driven second-stage compressor powered by solar energy. The goal is to reduce electricity consumption and enhance sustainability by leveraging renewable energy. A MATLAB^® model was developed to analyze the energy and exergy performance using R1234yf refrigerant under steady-state conditions. This study compares four solar collectors—evacuated flat plate (EFPC), evacuated tube (ETC), basic flat plate (FPC), and compound parabolic (CPC) collectors—to identify the optimal configuration based on the collector area and costs. The results show a 31% reduction in mechanical compressor energy use and up to a 44% improvement in the coefficient of performance (COP) compared to conventional systems, with a condenser temperature of 65 °C, a thermal compression ratio of 0.8, and a heat source temperature of 150 °C. The evacuated flat plate collectors performed best, requiring 2 m²/kW of cooling capacity with a maximum exergy efficiency of 15% at 170 °C, while compound parabolic collectors offered the lowest initial costs. Overall, the proposed system shows significant potential for reducing energy costs and carbon emissions, particularly in hot climates. Full article

This study introduces the use of compressed air as a heat transfer fluid in small-scale, concentrated linear solar collector technology, evaluating its possible advantages over traditional fluids. This work assumes the adoption of readily available components for both linear parabolic trough and Fresnel collectors and the coupling of the solar field with Brayton cycles for power generation. The aim is to provide a theoretical analysis of the applicability of this novel solar plant configuration for small-scale electricity generation. Firstly, a lumped thermal model was developed in a MatLab^® (v. 2023a) environment to assess the thermal performance of a PT collector with an evacuated receiver tube. This model was then modified to describe the performance of a Fresnel collector. The resulting optical–thermal model was validated through literature data and appears to provide realistic estimates of temperature distribution along the entire collector length, including both the receiver tube surface and the Fresnel collector’s secondary concentrator. The analysis shows a high thermal efficiency for both Fresnel and parabolic collectors, with average values above 0.9 (in different wind conditions). Th5s study also shows that the glass covering of the Fresnel evacuated receiver, under the conditions considered (solar field outlet temperature: 550 °C), reaches significant temperatures (above 300 °C). Furthermore, due to the presence of the secondary reflector, the temperature difference between the upper and the lower part of the glass envelope can be very high, well above 100 °C in the final part of the collector string. Differently, in the case of PTs, this temperature difference is quite limited (below 30 °C). Full article

In this paper, the experimental results of the thermal sub-system of a reliable and cost-effective polygeneration solar system are presented. This polygeneration system produces heating, cooling, and electricity from solar energy, which is used in an existing test building. Heat is generated in four evacuated tube solar collectors (ETCs). The heat may be used for space cooling through a variable geometry ejector (VGE) heat pump. In order to reduce the mismatches between generation and consumption, two thermal storage tanks were added. The performance of a new thermal storage, with 400 L, able to store both sensible and latent heat, was tested. The heating performances of the test building were assessed. Ejector cycle tests were also performed, and the variation of the cooling coefficient of performance (COP) was calculated for different flow rates. For heating, the results showed that the heat storage was capable of heating the test building for 8 h, with temperatures between 22 °C and 26 °C. All results showed that this polygeneration prototype could be capable of meeting the heating and cooling needs when applied to a real building. Full article

The recent post-pandemic period has economically affected many business sectors. One of these is the hotel industry. As a result of this economic crisis, it is necessary to act on the economic costs of running energy-intensive buildings such as a hotel. The thermal and electrical energy consumption of an accommodation facility weighs heavily on the economic balance. Governments around the globe have moved to help those activities in need. To improve the sustainability of the hospitality sector from an environmental and economic point of view, the introduction of energy retrofit solutions is mandatory. Following European sustainability laws, the impact and efficiency of the building were calculated using smart readiness indicators. The purpose of this paper is to present a case study of a 5-star hotel located in southern Italy characterized by high energy consumption. Precisely these consumptions are due to air conditioning, lighting, hot water, catering, and all other utilities. The entire building and the systems serving it were characterized by means of software that studies consumption with dynamic models, Trnsys. The same software made it possible to model the case study by replacing the existing air conditioning system with one supplied by renewable energies. Two energy retrofit hypotheses were chosen to obtain the best economic and environmental results. First, the choice was to install solar cooling powered by flat solar panels, and the second choice was solar cooling powered by evacuated tube collectors. This paper reports the technical and economic characteristics of both proposed solutions, quantifying the energy and economic savings to identify the best solution. Full article

In this study, transient modelling for a solar-powered adsorption-based multi-generation system working under the climatic conditions of the Gulf Cooperation Council (GCC) countries is conducted. Three cities are selected for this study: Sharjah in the United Arab Emirates, Riyadh in Saudi Arabia, and Kuwait City in Kuwait. The system comprises (i) evacuated tube solar collectors (ETCs), (ii) photovoltaic-thermal (PVT) solar collectors, and (iii) a single-stage double-bed silica gel/water-based adsorption chiller for cooling purposes. A MATLAB code is developed and implemented to theoretically investigate the performance of the proposed system. The main findings of this study indicate that among the selected cities, based on the proposed systems and the operating conditions, Riyadh has the highest cooling capacity of 10.4 kW, followed by Kuwait City, then Sharjah. As for the coefficient of performance (COP), Kuwait City demonstrates the highest value of 0.47. The electricity generated by the proposed system in Riyadh, Kuwait City, and Sharjah is 31.65, 31.3, and 30.24 kWh/day, respectively. Furthermore, the theoretical results show that at 18:00, the overall efficiency of the proposed system reaches about 0.64 because of the inclusion of a storage tank and its feeding for the adsorption chiller. This study analyzes the feasibility of using a combination of ETCs and PVT collectors to drive the adsorption chiller system and produce electricity in challenging weather conditions. Full article

This work aimed to investigate temperature polarization (TP) and concentration polarization (CP), which affect solar-powered air-gap membrane distillation (SP-AGMD) system performance under various operating conditions. A mathematical model for the SP-AGMD system using the experimental results was performed to calculate the temperature polarization coefficient (τ), interface temperature (T_fm), and interface concentration (C_fm) at various salt concentrations (C_f), feed temperatures (T_f), and flow rates (M_f). The system of SP-AGMD was simulated using the TRNSYS program. An evacuated tube collector (ETC) with a 2.5 m² surface area was utilized for solar water heating. Electrical powering of cooler and circulation water pumps in the SP-AGMD system was provided using a photovoltaic system. Data were subjected to one-way analysis of variance (ANOVA) and Spearman’s correlation analysis to test the significant impact of operating conditions and polarization phenomena at p < 0.05. Statistical analysis showed that M_f induced a highly significant difference in the productivity (P_r) and heat-transfer (h_f) coefficients (p < 0.001) and a significant difference in τ (p < 0.05). Great F-ratios showed that M_f is the most influential parameter. P_r was enhanced by 99% and 146%, with increasing T_f (60 °C) and M_f (12 L/h), respectively, at a stable salt concentration (C_f) of 0.5% and a cooling temperature (T_c) of 20 °C. Also, the temperature increased to 85 °C when solar radiation reached 1002 W/m² during summer. The inlet heat temperature of AGMD increased to 73 °C, and the P_r reached 1.62 kg/(m²·h). Full article

In this paper, the effect of solar intensity on the heat pipe tip temperature in a heat pipe type—evacuated-tube solar collector (HP-ETSC) was investigated. A simple relation was proposed, relating the solar intensity to the heat pipe tip temperature generated from the experimental data. This simple empirical relation was applied in a set of heat transfer equations derived to predict the heating medium temperature at the manifold outlet of the evacuated-tube solar collector. The calculated results corresponding to two types of heating medium, i.e., palm oil and water, were compared with experimental results from the literature. The results show that the average error was 6.41% for the case of palm oil and 4.66% for the case of water. Based on the case of water as a heating medium fluid, it was found that the flow rate of the heating medium fluid affected the accuracy of prediction, as the percentage error increased with the heating medium flow rate. The maximum percentage error increased from only 1.83% for a water inlet flowing at a Reynolds number of about 2.4 × 10³ to 15.23% for a water flow rate at a Reynolds number of about 2.6 × 10⁴. The correction factor was added into the correlation to predict the heat transfer coefficients of heating medium fluids. With this correction factor, the maximum error could be reduced from 11.78% to 7.29% for the palm oil case and from 15.23% to 5.57% for the water case. The average errors corresponding to palm oil and water cases could be reduced to 0.74% and 1.26%, respectively. Full article

Buildings represent approximately two-thirds of the overall energy needs, mainly due to the growing energy consumption of air conditioning and water heating loads. Hence, it is necessary to minimize energy usage in buildings. Numerous research studies have been carried out on evacuated tube solar collectors, but to our knowledge, no previous study has mentioned the combination of an evacuated tube solar collector with a parabolic trough collector and a helical coil heat exchanger. The objective of this paper is to evaluate the thermal behavior of an innovative evacuated tube solar collector (ETSC) incorporated with a helical coil heat exchanger and equipped with a parabolic trough collector (PTC) used as a domestic water heater. To design the parabolic solar collector, the Parabola Calculator 2.0 software was used, and the Soltrace software was used to determine the optical behavior of a PTC. Moreover, an analytical model was created in order to enhance the performance of the new model of an ETSC by studying the impact of geometric design and functional parameters on the collector’s effectiveness. An assessment of the thermal behavior of the new ETSC was performed. Thus, the proposed analytical model gives the possibility of optimizing ETSCs used as domestic water heaters with lower computational costs. Furthermore, the optimum operational and geometrical parameters of the new ETSC base-helical tube heat exchanger include a higher thermal efficiency of 72%. This finding highlights the potential of the heat exchanger as an excellent component that can be incorporated into ETSCs. Full article

The new energy context since 2021 has led to dramatic increases in the energy bills of agribusinesses, affecting the price of foodstuffs. A considerable part of energy consumption is due to the heating of water at high temperatures. The present study analyzed the feasibility of using a Solar Water Heating System (SWHS) with an evacuated tube collector. In particular, the required sizing changes, potential savings and cost-effectiveness were analyzed. The results show that the new energy context makes the SWHS investment highly attractive: a payback of less than 4 years in most of the scenarios analyzed; energy savings of more than 60% in the scenarios with higher irradiation; a reduction in total energy expenditure of more than 50% in the favorable scenarios close to the current reality. The new context especially favors cold and temperate climates, with very sharp drops in payback compared to the previous situation. To achieve these values, it is necessary to design an optimized sizing of the SWHS, reducing the risk of future variations in the price of energy. The results of the study should serve as a reference for decision making in the agroindustrial sector to reduce the energy bill and strategic dependence on fossil fuels from third countries. Full article

In recent years, building energy consumption has increased every day due to population growth and an increased human desire for a healthy and pleasant lifestyle, and this is responsible for a crisis of energy shortages worldwide. Therefore, use of solar water heating (SWH) systems in buildings for hot water demand is the prime need of the hour to maintain sustainability. The novelty of this work was in developing a phase change material (stearic acid)-filled U-tube based evacuated tube solar collector (collector A). In addition, another collector B, left without energy storage material, was considered a reference unit for comparing the energy and exergy outputs. The study’s main aim was to examine the energy, exergy, enviro- and exergoeconomic analysis of newly developed water heating systems. The findings of study revealed that the maximum daily energy outputs of collector A were found to be 85.86% (simultaneous mode) and 84.27% (midday charging mode) at a high mass flow rate (0.5 LPM), and exergy outputs were 19.41% and 21.35%, respectively, at a low flow rate. The thermal output of collector A was higher than that of collector B. The per liter cost of hot water produced from collector A with PCMs was found to be INR 0.1261 and INR 0.1276, respectively, under both modes, which is less compared with the electric geyser (0.325 INR). The levelized energy cost, net present worth, and the payback time of the developed collector A obtained were 4.61 INR/kWh, INR 49710, and 4.49 years (simultaneous), and 4.67 INR/kWh, INR 48130, and 4.64 years (mid-day charging), respectively. Furthermore, the amount of CO₂ mitigation from the energy and exergy perspective for collector A was found to be 24.30 and 23.76 tCO₂/lifetime and 5.31, 5.58 tCO₂/lifetime, respectively. Full article