Abstract In this paper, due to its advantages of high accuracy and moderate computational cost, detached eddy simulation (DES) is adopted to observe detail flow characteristics of a kind of vertical axial wind turbine equipped with four helical blades. The numerical results obtained from the method have been validated by experiments as well as been compared with those from unsteady Reynolds-averaged Navier–Stokes equations (U-RANS). Several critical parameters, such as the torque, power coefficient, and tangential force, are studied in detail. It is found that the results from DES are closer to the measurement data than those from U-RANS. And, the characteristics of the flow field in the turbine can be clearly revealed by DES. Furthermore, the effects of tip vortex, Reynolds number, and pitch angle on turbine performance have also been analyzed and discussed.

Abstract A water-based silver nanofluid (Ag/PW) was prepared from gelatin-stabilized silver nanoparticles (Ag NPs) of about 15 nm and further used as a working fluid in an evacuated U-tube solar collector (EUSC) to investigate the variation in the collector efficiency. An Ag/PW nanofluid having 0.035 wt% was prepared and demonstrated a good promise of colloidal stability when dispersed in pure water. Collector efficiency measurements were carried out at outdoor conditions with four different mass flow rate values (0.063, 0.051, 0.033, and 0.02 kg/s). Results showed that Ag/PW have superior heat transfer properties than that of pure water as the base fluid. It was found that the efficiency of the collector was directly proportional to the mass flow rate of the working fluid until an optimum value was attained. Experimental results show that the highest collector efficiency was 72.2% at 0.051 kg/s mass flow rate, which is 21.3% higher than that of the pure water.

Abstract With increasing energy consumption in buildings, energy efficiency measures are matter of prime concern. A huge portion of energy consumed in buildings is used for regulating the thermal comfort. A solution to this is to incorporate phase change material (PCM) within the building elements which increases their overall thermal capacity. In the present study, the temperature of inner room surface, with and without PCM incorporation, is calculated for composite climate of Delhi. The analysis of PCM sandwiched walls has been performed. The performance analysis of five PCMs, having different melting temperatures, is carried out with nodal temperatures as the output. The results show that a phase change temperature range of 34–38 °C is suitable for peak summer conditions of Delhi. It is also observed that due to the low thermal conductivity of PCMs, they act as both storage medium and insulation, thus reducing temperature fluctuation during summer/winter. Based on the simulation, three PCMs were found suitable and hence were experimentally tested for their characteristic charging and discharging properties and performance, using differential scanning calorimeter (DSC). Based on the characterization results, it is concluded that two commercially available PCMs (Eicosane and OM35) are suitable for Delhi. All the other PCMs have also been simulated for different climatic conditions in India and their impact on heat gain has been assessed.

Abstract The paper reviews the present understanding of the analysis of the heat and mass transfer processes in single-slope solar stills. By using the results of published experiments, it is proposed that the heat and mass transfer phenomena from the basin water to the glass cover are coupled. This coupling makes it possible to derive the dependence of the heat transfer coefficient for condensation on the inclination of the glass cover of the still. The derived relation, i.e., Nucon = 0.738 (Grcon*Prcon*sin β/Ja*)¼ A−1 where A is the aspect ratio, has been demonstrated to be an important expression for predicting the heat transfer coefficient for condensation hcon necessary for a more realistic evaluation of the overall efficiency of single-slope solar still of a given cover angle β.

Solar drying is one of the most important processes of preserving agricultural products. This review paper focused mainly on the enhancement of efficiency a solar drying system. The establishment of different techniques and factors, which may affect the performance of solar dryers, helps to improve solar dryers’ effectiveness. Different types of solar dryers were described here; moreover, various performance analyses of solar drying systems (SDSs) were presented. Factors and techniques for improving efficiency of solar dryers were discussed as well. The effect of operating conditions, geometrical conditions, adding of reflectors, heat exchanger, heat pump, photovoltaic source, air circulation mode, and phase change material (PCM) on the efficiency of a solar drying system were studied and discussed. Results showed that climatic conditions such as ambient temperature and solar radiation have an important influence on the solar dryer performance. The chimney integrated in solar dryer increases the buoyant force applied on the air stream to maintain a greater air flow velocity, which removes one side of moisture. The concentrators found to be effective in reducing the drying time by increasing the air temperature inside the dryer. Photovoltaic panels provides electricity source to run electrical components such as the fan to provide a forced air circulation that removes more moisture from the product compared with the natural convection or the heat pump to ensure the drying process at night. PCMs store the thermal energy during sunshine hours and release it after sunset, which can reduce the heat losses and improve the thermal efficiency of the drying system.

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m2 K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.

To improve the solar energy utilization in the photovoltaic power, the sun ray double axis tracking device is designed and its tracking method is introduced. Using the finite element method, the parameterized analysis model is built and the static calculation is performed in different conditions for the device. The design feasibility of the device is verified by evaluating the stress. The lightweight of the device is made on the premise that the design satisfies the intensity and this provides the basis for manufacturing the prototype. The sun's rays tracing experiment is carried out by the prototype. Results show that the device design is reasonable and meets the design requirements; the key working conditions for the solar tracking design are found; the lightweight is obvious and the weight is reduced by 14%; the average errors of azimuth angle and height angle are within 5 deg; compared with the fixed device, the increasing proportion of solar energy one day is up to 52.6%; and the device works steadily and has good mechanical properties.

This paper presents an evaluation of alternative particle heat-exchanger designs, including moving packed-bed and fluidized-bed designs, for high-temperature heating of a solar-driven supercritical CO2 (sCO2) Brayton power cycle. The design requirements for high pressure (≥20 MPa) and high temperature (≥700 °C) operation associated with sCO2 posed several challenges requiring high-strength materials for piping and/or diffusion bonding for plates. Designs from several vendors for a 100 kW-thermal particle-to-sCO2 heat exchanger were evaluated as part of this project. Cost, heat-transfer coefficient, structural reliability, manufacturability, parasitics and heat losses, scalability, compatibility, erosion and corrosion, transient operation, and inspection ease were considered in the evaluation. An analytic hierarchy process was used to weight and compare the criteria for the different design options. The fluidized-bed design fared the best on heat transfer coefficient, structural reliability, scalability, and inspection ease, while the moving packed-bed designs fared the best on cost, parasitics and heat losses, manufacturability, compatibility, erosion and corrosion, and transient operation. A 100 kWt shell-and-plate design was ultimately selected for construction and integration with Sandia's falling particle receiver system.

Buildings' energy consumption has a great energetic and environmental impact worldwide. The architectural design has great potential to solve this problem because the building envelope exerts influence on the overall system performance, but this is a task that involves many objectives and constraints. In the last two decades, optimization studies applied to energy efficiency of buildings have helped specialists to choose the best design options. However, there is still a lack of optimization approaches applied to the design stage, which is the most influential stage for building energy efficiency over its entire life cycle. Therefore, this article presents a multi-objective optimization model to assist designers in the schematic building design, by means of the Pareto archived evolutionary strategies (PAES) algorithm with the EnergyPlus simulator coupled to evaluate the solutions. The search process is executed by a binary array where the array components evolve over the generations, together with the other building components. The methodology aims to find optimal solutions (OSs) with the lowest constructive cost associated with greater energy efficiency. In the case study, it was possible to simulate the process of using the optimization model and analyze the results in relation to: a standard building; energy consumption classification levels; passive design guidelines; usability and accuracy, proving that the tool serves as support in building design. The OSs reached an average of 50% energy savings over typical consumption, 50% reduction in CO2 operating emissions, and investment return less than 3 years in the four different weathers.

The solar energy is a renewable source that has a great potential for conversion into thermal energy or for generation of electric power through photovoltaic panels in Brazil. Concerns about environmental impacts and the fossil resources scarcity have motivated the technological development of renewable alternatives to fill out the energy matrix. The flat-plate solar water heater is an equipment used for domestic or commercial applications to heat fluids, which can minimize the demand for electric energy and, consequently, decrease the electrical bill. However, the development of technologies to increase the conversion of solar energy into thermal energy remains a challenge in order to increase the efficiency of these devices. Thus, passive techniques to enhance heat transfer have been applied and those results seem to be promissory. Among them, delta-winglet longitudinal vortex generator (VG) is a consolidated passive technique currently applied on compact heat exchangers, although few works have been applied this technique on the solar water heater. By a computational fluid dynamics approach, in this work, we analyze the augmentation of heat transfer through delta-winglet longitudinal vortex generator inside a tube of a flat-plate solar water heater. For the Reynolds numbers 300, 600, and 900, the better ratio between the heat transfer and the pressure drop penalty is found for the attack angle of the delta-winglet of 30 deg, while the highest heat transfer was to the attack angle of 45 deg. Moreover, the first vortex generators showed significant impact only on the friction factor and could be eliminated of the solar water heater with no penalty to the heat transfer.