• Energy Research

Abstract Concentrated photovoltaic is a promising route to achieve high efficiency solar conversion. Concentration of sunlight increases the irradiation on panel along with its temperature. At higher temperature, the efficiency of the cells drops necessitating active cooling. However, flow of a coolant consumes pumping-power leading to reduction in net power output. These two counter-acting factors lead to a two-stage optimization for designing an advection cooled concentrated photovoltaic (ACCPV) system. Here, we attempt developing a design methodology where standard numerical scheme for solving a multi-level conjugate heat problem is utilized to obtain optimal operating conditions in terms of the coolant flow rate, and the sunlight concentration ratio for a series of widely used photovoltaic materials: a-Si, m-Si, p-Si, CIGS, CdTe. For the chosen ACCPV geometry, the estimated concentration ratios are computed to be 7, 9, 9, 16 and 20 for m-Si, p-Si, CIGS, p-Si and CdTe cells, respectively. Even though the estimated values of optimal parameters depend on the choice of PV material, coolant and the geometry of the system, the methodology presented here is universally applicable for any such ACCPV system. The general intuition - higher the irradiation, better the performance - is demonstrated to be false through the present analysis.

Abstract Improving efficiency and yield of a parabolic trough collector (PTC) field is of paramount technological importance. Here, an attempt toward this goal is made through combined optical and thermal simulation. Two major factors that affect the performance of an individual trough are mass flow rate and aperture width, which scales with rim angle. Performance of PTC-field is observed to have a non-linear scale-up rule as opposed to the same for an individual trough. Wider aperture leads to higher heat collection for individual trough; however, the same results in higher inter-trough shading limiting the number of troughs in a field and the effective unshaded hours. It also depends on insolation and latitude. Therefore, there lies a possibility of trade-off where flow rate, rim angle and design hours of operation require careful tuning to maximize annual energy yield. Here, a step by step procedure is demonstrated for Kanpur, India where optimal values of flow rate, rim angle and hours of operation are obtained to be 1.1 kg/s (for Syltherm 800 as the heat transfer fluid), 70° and 4 h around solar noon on winter solstice day, respectively. The methodology is illustrated to be generally applicable to any location or working fluid.

Abstract Solar-PV is the most mature technology for the solar energy utilization. The design of such PV-based systems are done in terms of the rated power of the panels while the rating is determined at the standard test condition of a specified temperature, irradiance level and for the standard AM1.5G solar spectrum. However, such rated output is never achieved as the actual operating conditions vary widely from the standard one. Hence, it is critical to estimate the actual power output under certain operating conditions in a given location for a proper design. Here, we provide a comprehensive method for estimating the actual power output which is applied to a particular location (Kanpur, India). Three major factors are computed which cause deviation of the power output from the rated value: spectral factor, temperature factor and irradiance factor. The combined effect is expressed through power factor which is observed to be dominated by the irradiance factor. It is observed that the actual power is significantly less - from 40% to 75% of the rated value - for the whole year which needs to be accounted for during the sizing-decision. The current study applies the estimation method on five commonly used PV materials and identifies amorphous silicon panel to be best suited for Kanpur for the prevailing condition in spring and summer seasons because it provides the highest output for the same rated power. The case study of Kanpur clearly demonstrates the effectiveness of the present method and its usefulness in the design process.

In this paper, an annual comparative analysis for the coefficient of performance (COP) and the energy consumption are presented for two direct expansion heat-pump water heating systems: solar-assisted heat pump, and air-source heat pump. Two main research questions are addressed: (1) How does each system's performance differ from the other system under the same conditions? and (2) How does the climate affect the performance of an individual system to help the designer choose the more energy efficient water heater for a specific location? To make those water heaters comparable, all design parameters for both heat pumps are assumed to be the same with identical components. A sensitivity analysis is also carried out for various factors such as irradiance, ambient temperature, collector/evaporator area, wind speed, and compressor rotational speed considering three different hot water tank temperatures. The results for both heat pump water heating systems show that the climate (different months) plays the key role on the annual performance. Changes in irradiance and ambient temperature as well as the wind speed throughout the year can increase the COP by up to 33 % (from 2.1 to 2.8) under certain conditions for the case study with moderate weather. Therefore, the benefits from the local weather should be investigated before choosing the design variables and proper source (solar thermal, air) for the heat pump. Nearly the same results were obtained for both type of water heaters with slightly better performance of air heat pump in colder months of the year (December to February).