• Energy Research

Abstract Solar radiation can be considered as one of the most suitable energy source to be used for cooking food. It is therefore important to promote advancement in solar cooking systems, in particular as concerns their efficiency and cooking times. In this work, we designed, realized and tested a novel low-cost solar cooker that uses a high-performance light-concentrating Fresnel lens, which is able to concentrate onto a small focal area a large amount of solar radiation. The radiation is then reflected by a mirror towards a cooking surface, where a pot can be located. The cooker has a geometrical concentration ratio of 40.97. In order to characterize the thermal performance of the cooker, we carried out several outdoor tests with a dedicated test setup, using water and silicone oil as absorbing media. Respect to other solar cookers, results show that the proposed cooker is able to reach high temperatures with good efficiency and reduced heating times: 3 kg of water can reach 90 °C in around 30 min, while 3 kg of silicone oil can be taken from 40 to 170 °C in less than one hour.

In this paper, the procedure of finding the coefficients of an equation to describe the thermal conductivity of refrigerants low in global warming potential (GWP) is transformed into a multi-objective optimization problem by constructing a multi-objective mathematical model based on the Pareto approach. For the first time, the NSGAII algorithm was used to describe a thermophysical property such as thermal conductivity. The algorithm was applied to improve the performance of existing equations. Two objective functions were optimized by using the NSGAII algorithm. The average absolute relative deviation was minimized, while the coefficient of determination was maximized. After the minimization process, the optimal solution located on the Pareto frontier was chosen through a comparative analysis between ten selection methods available in the literature. The procedure generated a new set of coefficients of the studied equation that decreased its average absolute relative deviation by 0.24%, resulting in better performance over the entire database and for fluids with a high number of points. Finally, the system model was compared with existing literature models to evaluate its suitability for predicting the thermal conductivity of low-GWP refrigerants.

In this study, a novel solar dryer is presented and analyzed experimentally and numerically. The proposed device is a small, passive, indirect solar dryer that works in an unconventional way. The product is mainly heated by irradiation from the walls of the drying chamber, while its moisture is removed by an airflow caused by natural convection. In addition, it is a low-cost solar dryer made of readily available materials and has a variable geometry that allows it to increase its thermal performance. Two types of experimental tests were conducted to analyze its performance. Thermal tests without load were carried out to assess the suitability of the drying chamber temperatures. Load tests with various masses and types of food were carried out to evaluate its drying performance. The results of the experimental tests demonstrated that the solar dryer achieved temperatures suitable for food drying and was able to dry the tested foods. Finally, a Computational Fluid Dynamics (CFD) model was developed to predict the performance of the proposed solar dryer. The validation of the numerical model with experimental data confirms their reliability in accurately predicting the temperatures within the dryer.

Abstract In this paper, a literature survey of the available experimental surface tension data of both low global warming potential (GWP) refrigerants and binary systems containing low GWP refrigerants is provided. The experimental data were collected for 14 halogenated alkene refrigerants and 5 blends. The data were analyzed with the most reliable semi-empirical correlation models designed for pure refrigerants and blends, based on the corresponding states principle (CSP). Recent scaled correlations based on the CSP and developed for other kinds of fluids were re-fitted and tested, too. The coefficients of these equations were subdivided for ethylene derivative refrigerants, propylene derivative refrigerants and blends. The absolute average deviations (AADs) of the new re-fitted equation for the ethylene derivatives are in agreement with the AADs of the correlations existing in the literature, while the proposed equations for the propylene derivatives and the binary systems give a considerable improvement respect to existing relations.

During their lifetime, photovoltaic (PV) plants are subject to a normal degradation of their components, and they are consequently characterized by decrease of the expected production. In order to prevent and evaluate failures and loss of production, specific tests can be carried out on the PV modules. Non-destructive methods, such as visual inspection and infrared thermography, can be performed in order to determine production failures or defects on the PV modules. I-V curves allow to estimate the performance of photovoltaic modules and strings, estimating the deviation between the power of the examined module and that declared by the manufacturer. The aim of this work is to evaluate the efficiency loss of photovoltaic modules associated to specific defects, causing in a systematic way some faults on a set of brand-new modules and assessing the relative decrease of power. The set of brand-new photovoltaic modules, after being damaged, was experimentally characterized determining their I-V curves by means of an indoor solar flash test device based on a class A+ AM 1.5 solar simulator. Using the I-V curves as a dataset, it was possible to estimate the incidence of different defects on the power of the photovoltaic module being considered.

Abstract Clear-sky conditions are an essential need to allow proper high-temperature solar cooking. Moreover, it is not an easy task to accomplish evening cooking, especially during wintertime when solar radiation is available only for a few hours. A solution to bypass these drawbacks lies in adopting a cooker provided with a thermal storage unit. The storage unit proposed in this work is a double-walled vessel composed by two stainless steel cylindrical pots assembled concentrically. The annular space between the pots was loaded with 4 kg of phase change material (PCM) based on a ternary mixture of nitrite and nitrate salts (solar salt: 53 wt% KNO3, 40 wt% NaNO2, 7 wt% NaNO3). The thermal storage unit was characterized by means of a test rig including a high-concentration-ratio (10.78) solar box cooker. Four different sets of 14 experimental tests, divided into a heating and a cooling phase, were carried out to assess the performance of the solar cooker with the storage unit. It was found that the PCM thermal storage significantly improves the load thermal stabilization when solar radiation is not available: the load cooling time in the range 170–130 °C was determined to be from 65.12% to 107.98% higher than that without the solar-salt-based PCM thermal storage, proving the effectiveness of the proposed solution.