Ashok Kumar Bhargava

Study of a hybrid solar system—solar air heater combined with solar cells

Abstract In a conventional solar thermal system, electrical energy is needed to circulate the fluid. This energy is supplied by an electrical source. A hybrid system, a combination of thermal and photovoltaic systems can generate sufficient electrical energy to turn the pump. The solar cells are pasted directly over the absorber plate. The part of the solar radiation falling on the cell area is converted into electrical energy, and part is collected by the flowing air. A hybrid system is, thus, operated solely by the solar radiation. In this paper, a hybrid system, which is a combination of an air heater and photovoltaic system is analysed. The optimum area of the solar cells necessary to generate sufficient electrical energy for the pump is calculated for different configurations of the air heater. A linear relation has been used to calculate the variation of efficiency of the solar cells with temperature. It is shown that the hybrid system is self sufficient only for certain design parameters and flow rates.

The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer

Abstract We present a theoretical study of a solar photovoltaic-thermal (hybrid) system consisting of a flat-plate solar air heater mounted with solar cells and a plane booster. A conventional flat-plate collector is converted into a hybrid system by mounting solar cells directly on the absorber plate. A hybrid system is self-sufficient in the sense that the electrical energy required by the pump is supplied by the panel. Such systems are well suited to applications such as solar drying. The combined system is analysed for the case when the radiative and absorptive properties of the cell surface and the absorber plate are nearly the same. The solar cell efficiency is a linearly-decreasing function of the absorber plate temperature. The performance of the system has been evaluated for various combinations of boosters. The minimum area of the solar cells required to run the pump at a given flow rate has been calculated as a function of time, with and without boosters. The minimum cell area required decreases with the use of boosters. High cost cells may be replaced by low cost reflectors. The solar air heaters presently available on the market are not suitable for direct conversion to hybrid systems.

Evaluation of the performance of air heaters of conventional designs

Abstract Air heaters, because of their simplicity, are more amenable to theoretical investigations than the other solar thermal systems like solar stills, etc. In air-heating collections various parameters on which its performance depends can be identified and their effect can be studied theoretically. In this paper the transient equations of three types of air heater are written and their solutions are attempted. In Type I air heater there is one air channel above a metallic plate, in Type II, the air channel exists between two metallic plates and in Type III, there are two air channels one above the plate and another between two metallic plates. The transient equations of the model are solved within the framework of periodic analysis. The temperature of any component of the air heater is represented by a Fourier series in time, the coefficients being coordinate dependent. The effect of flow rate, plate length, selective coating, etc. are studied. It is shown that for a given rate of flow, there is an optimum value of plate length for which efficiency is maximum. The optimum value of plate length in Type II air heater increases as rate of flow increases. For a given plate length, the efficiency increases with rate of flow. The selective coating will be useful only when plate length is large and rate of flow is small. For a flow rate of 100 kg m−2hr−1 and plate length of 7 m, the difference between outlet air temperature from a selectively-coated plate and that of black-painted plate, at midday, is 33°C. The selectively-coated air heater is 21 per cent more efficient than the black pained one at the same condition. It is found that an efficient configuration has lower glazing temperature.

Theory of multiple-pass solar air heaters

We have developed the theory for a four-pass air heater. We have also studied the system performance with one, two, and three passes. The effect of the number of glass cover plates on one, two, and three passes has also been investigated. It is shown that the multiple-pass air heater performs well for low flow rates and large plate lengths. For small plate lengths and high flow rates, the addition of cover plates does not affect the performance of two- or three-pass air heaters.

A solar water heater based on phase-changing material

A water heater utilising a material which changes phase for storage of solar energy is discussed. The transient analysis is carried out by replacing the solid-liquid block of the phase-changing material by a fictitious solid and assuming that the solid-liquid boundary remains stationary throughout the day. The efficiency of the system and the outlet water temperature during the evening hours increase with the increase in the thermal conductivity of the solid-liquid phases of the materials. Hot water can be obtained throughout the day if water pipes are placed near the surface of the storage material. The outlet water temperature curve becomes flat if the pipes are placed near the bottom of the storage material.

Performance studies on a finned-air heater

We have investigated a fin-type air heater, both experimentally and theoretically. In the experimental set-up, the fins are integrated with the upper plate of the conventional duct-type air heater with single glazing. The experiments were performed on two clear days for two different mass-flow rates. A transient model is then developed to study the performance of the finned air heater and, by adjusting heat-transfer coefficients, the theoretical data are fitted to the experimental results. Agreement between theoretical predictions and experimental values is found to be good. We have used the resulting values of the heat-transfer coefficient to predict the performance of the system when the fins are placed in the lower metallic plate or in both the upper and lower plates. The dependence of efficiency on fin density has been studied.

Performance of a multiple-wick solar still with condenser

In a conventional still, because the water condenses underneath a glass cover, its temperature becomes quite high. During the period of maximum sunshine, the glass temperature is higher than the dew point of the air-vapour mixture inside the still. Thus the yields of these kinds of stills are low. In this communication we have investigated experimentally the performance of a still with a condenser. Most of the condensation now takes place in the condenser, consequently the glass temperature remains low causing less heat loss to the ambient environment. The yield of the condenser-type still is higher than that of the non-condenser-type still.

Transient analysis of a solar air heater of the second kind

Abstract This paper presents a theoretical analysis along with the experimental validation study of a solar air heater of the second kind. The heater consists of a flat passage between two metallic plates through which the heat transfer fluid air is made to pass by some auxiliary means. Study of the periodic response of different parameters of this solar air heater is attempted. The heat balance equations governing the behaviour of the system are solved explicitly. The results obtained from the analytical expressions for the transient variation of outlet air temperature compare well with experimental data. Predictions are also made regarding effects of different performance parameters of the air heater with variations of air mass flow rate and plate emissivity with the hope of optimizing the collector configuration.

Functional aspects of a porous bed solar air heater

We have developed the theory of a matrix air heater with a rear plate, investigated the role of individual components, and studied the performance differences for air flowing downwards and air flowing upwards through the matrix. The system is compared with an air heater of conventional design with flow between 2 plates.

Analysis of a solar air heater with thermosyphon flow in one channel and forced flow in other channel

Abstract In a conventional solar air heater where two metal plates and one glazing are used, air is forced in the lower channel, while stagnant air between the plate and glazing prevents heat loss. In many cases, the two ends of the upper channel may not be properly sealed. As a consequence, a thermosyphon flow will occur, causing a loss of efficiency. In this work, the analysis of an air heater, where flow is forced in the lower channel, and thermosyphon flow occurs in the upper channel, is presented. It is shown that, if the upper channel is kept completely open, the efficiency loss in the lower channel due to the natural flow in the upper channel is