• Energy Research

Abstract Photovoltaic technology seems to be one of the fastest growing industries in the world. Many studies have been conducted to consider methods of increasing the efficiency of these systems. The efficiency of the solar cell decreases with temperature increasing. So, one of the proposed solutions is cooling the panel that is named PV/T combining system. In this study, a water PV/T collector with a serpentine half pipe instead of a full circle pipe has been evaluated. Using this configuration led to the removal of several thermal resistances. A precision mathematical model is provided and validated with experimental data from a laboratory scale setup. There is a good comparison between theoretical and experimental data and the maximum value of the errors is under 15%. In addition, some parameters such as inlet water temperature and flow rate, hydraulic diameter of half pipe and the number of rows are evaluated. In this novel design, the thermal efficiency of 70% and electrical efficiency of higher than 11.5% is obtained. The thermal efficiency of the proposed model is greater about 10–13% and the electrical efficiency of about 0.4–0.6% compared to conventional models.

AbstractRenewable energies, specifically solar energy has been employed in numerous applications while being CO2 emission free energy in comparison with fossil fuel resources. The main purpose of this study is to predict thermal efficiency of photovoltaic‐thermal (PV/T) setups in regard with input temperature, recirculation flow rate, and solar irradiation by modifying multilayer perceptron artificial neural network (MLP‐ANN), adaptive neuro‐fuzzy inference system (ANFIS), and least squares support vector machine (LSSVM) approaches. For this goal, more than 100 empirical measurements were performed on a fabricated water‐cooled PV/T setup. Several numerical analyses are also carried out to assess the validity of the presented models. It is confirmed that there is a great agreement between predictive models and actual data. The proposed ANN model provided the best performance due to the mean squared error (MSE) and determination coefficient (R2) values of 0.009 and 1.00, respectively. Also, numerical comparisons with other recently developed models were performed.

In this paper, retrofitting of a hydroxy (HHO) cell which is coupled to a gasoline engine is performed by investigating the effects of electrodes’ shape and distances and the impacts of improvement technologies ultrasonic waves, DC pulsed power with various frequencies and magnetic field on the cell’s bubble pumping efficiency and rate of HHO gas production. Subsequently, an energy analysis was performed to provide a comparison between different improvement technologies. As a result, by applying DC pulsed power with 300 kHz frequency to the HHO cell, highest HHO gas production rate, energy efficiency and lowest energy intensity is achieved among other technologies, however, it will allocate lowest surface intensity which leads to requiring more electrode surface area. In addition, employment of ultrasonic waves improves HHO gas production rate from approximately 6.4% to 52.4%, and it is possessed the highest surface intensity which result in requiring lowest electrode surface among other methods. Finally, the effects of superior improvement methods on the overall cost and weight of an HHO generator and its impacts on engine parameters are studied.

AbstractA new geometry of photovoltaic/thermal (PVT) system is introduced in this study, and the application of Al2O3/water nanofluid as the coolant on the electrical and thermal behavior of a solar system is experimentally evaluated. To intensify the thermal ability of the solar collector, a serpentine half‐pipe cooling system is designed and fabricated behind the solar panel for better cooling. The cells of the panel are assembly on a 3‐mm‐thick aluminum plate, and the Tedlar removes from the system to maximize the heat transfer in the cooling section. A special layer arrangement for the composite panel is used to minimize thermal resistance of the system. Different concentrations of nanofluid samples (0.05–0.5 wt.%) prepared. To increase the stability of prepared nanofluids, suitable surfactants are also used. Based on the obtained results, adding the nanoparticles to the pure water can remarkably raise the efficiency of the PVT apparatus. The best behavior of the solar collector is observed for 0.5 wt.% of nanofluid samples. 126.71% and 7.38% enhancement in terms of thermal and electrical efficiency can be gained by the application of nanofluid in the system compared with pure water, respectively. The best obtained value of the overall efficiency is around 93.73% for the studied system. The maximum temperature rise for water and nanofluid is around 5.5 and 9.1°C, respectively, which confirms the better cooling ability of PVT system by nanofluid compared with water.

The heat pipes are found as one of the promising technologies, which provide massive improvement of the energy performance in the energy systems. However, there are still some challenges in the employment of this technology in various applications, such as the ones with moderate-temperature operating conditions. The purpose of this study is the development of a moderate-temperature heat pipe on heat recovery, especially on replacing Ljungstrom in steam power plants with heat pipe heat exchangers. The temperature of the smoke passing through the Ljungstrom in the steam power plant is around 320 °C. For this purpose, 304 stainless steel vertical wickless heat pipe filled with special oil as working fluid is analyzed theoretically and experimentally. An experimental setup is fabricated, and the effects of aspect ratio, filling ratio, and exhaust temperature on the heat transfer rate and temperature distribution along the moderate-temperature wickless heat pipe are studied. Moreover, the operating limitations of the wickless heat pipe are investigated. A simple but accurate mathematical model is developed based on the equivalent thermal resistances and various physical limitations. By increasing the filling ratio of the heat pipe, the thermal performance of the system in all aspect ratios is reduced. The amount of heat transfer increased by increment of exhaust flow temperature up to 375 °C for aspect ratios of 11 and 25; however, for aspect ratio of 18, it is changed minimally. Based on the obtained results, the maximum heat transfer to cooling water, which is equal to 262.13 W, is achieved when FR and AR equal 30% and 18, consecutively. Furthermore, the average error for the equivalent thermal model is approximately 21% for a moderate-temperature wickless heat pipe, based on the assessment performed in this study.

The purpose of this study is the fabrication and performance evaluation of a new type of solar humidification–dehumidification (HD) desalination unit to supply sufficient fresh water for a seawater greenhouse in the MAKRAN coast in southeast Iran. In the proposed design, a particular type of air-to-air condenser is used. The cold air coming out of the greenhouse ventilation system (fan and pad) in summer and the cold ambient air in winter is used to supply the required cooling of the system. In this way, when cold air passes over the pipes in air-to-air condensers, condensation of water vapor occurs in the moist air inside the pipes, and fresh water is produced. Greenhouse fans, which have an air flow rate of around 20,000 m3/hr, are used to create this air flow. By fabricating two condensers, each using 42 rows of PVC pipes with a diameter of 75 mm, it is possible to produce 400 L of fresh water per day in a 400 m2 greenhouse. The required heating is provided by the solar farm, which includes 96 square meters of flat plate collectors. The steps of unit fabrication are described in detail in this research. However, the effect of greenhouse air temperature and circulating seawater flow rate on freshwater production, energy consumption, and energy intensity are also investigated. By increasing the flow rate of circulating seawater and decreasing the greenhouse air temperature, the production rate of the system increases. When the hot seawater and greenhouse air temperature are 61.7 °C and 26 °C, respectively, the maximum instantaneous production is estimated to be 80 L/h. The energy intensity of the HD desalination unit is varied between 3192 and 4382 kJ/L, and the gain output ratio of the system is around 0.6. The proposed system can be easily paired with conventional greenhouses employing a fan and pad cooling system and produces around 1.25 (L/m2·day) fresh water.

A heat pipe is an energy-efficient heat transfer device that relies on evaporation and condensation processes for energy transfer. The main purpose of this study is to simulate a two-phase closed thermosyphon, at moderate temperature, that can be used in industrial applications such as steam power plants. After creating a computational network in the Gambit software, the thermosyphon is simulated in Fluent software using the VOF model. Special oil is employed as the working fluid. Based on the CFD results, the efficiency of the system reaches approximately 96%, and the thermal resistance decreases to 0.54 K/W. The contours of the boiling and evaporation process at differing filling ratios, ranging between 30–90%, is visually investigated and the best performance is obtained for 30% of the filling ratio in thermosyphon. At higher filling ratios, more giant bubbles are generated in thermosyphon, which can attach to the inner wall of the system and reduce the thermal performance. The steady-state condition is obtained 84 s after the start of the process.

The drinkable water storages shortage is one of the main obstacles that many societies are envisaged with. In addition, the water content of humid air is known as the main storage of drinkable water for the future. In this paper, the energy and exergy analysis of a solar driven atmospheric water generator which contains an absorption refrigeration cycle coupled to a compound parabolic collector is performed. The mathematical model of the proposed system is developed in MATLAB software. In the solar thermal system provided in this paper, the storage tank is employed for extending the working hours of the system leading to improvement of total water generation rate. Furthermore, the performance of the system is evaluated in various climates included arid, semi-arid and Mediterranean, and for this purpose three cities of Iran named Bandar Abbas, Ramsar and Tehran were selected. Moreover, by comparing the water production in various months during the year, the highest value of water production during the year is held by Bandar Abbas city leading to owning the lowest values of specific power consumption and specific collector area. The maximum rate of water production of proposed system is approximately 400 lit per month in tropical climates with specific energy consumption of 3 kWh/lit. Based on the results of exergy analysis of whole system, the most exergy destruction rate belongs to compound parabolic collector component, therefore, large amount of energy is wasted through compound parabolic collector which can be recovered for generation of chilled water. Moreover, the impacts of geographical and environmental parameters on the energy and exergy efficiencies of the atmospheric water generator are also assessed.