Cooling the Solar Cells

A comprehensive study on harnessing wind energy and fluidics principle to enhance the cooling rate of solar modules.

In the present scenario the focus is on harnessing the environmentally friendly energy sources particularly on solar energy. Solar energy can be harnessed either directly or indirectly. Photon energy is harnessed by using solar cells. In this article, a new method of cooling the solar cells, solar modules or solar arrays by incorporating a novel wind energy harnessing chimney and fluidics principle is discussed in detail.

Introduction

Basic photovoltaic cells or solar cells (SC) were bulky, breakable and costly. Semiconductor technology SC is in the form of wafers resulting in reduction in cost, size and higher safety in transportation, installation and maintenance. SCs are available in the form of individual SC, solar panels (SPs), solar modules (SMs) and solar arrays (SAs).

SC is a silicon wafer sawn from semiconductor, amorphous or polycrystalline crystal blocks. A number of SCs are connected together in series to form a SM or SP and many such SMs interconnected to form SA to generate higher (Kilowatts or even Megawatts) wattage. Theoretically the efficiency of SC is limited to 44 per cent. In the laboratories the highest efficiency achieved is 40 per cent. However, in practice even 15 per cent efficiency cannot be achieved due to a number of practical problems. One of the problems is the rise in temperature of SC. Though the testing temperature is 250-degree C, in actual practice the solar cell operates at 500-degree C to 900-degree C depending upon the location and the solar radiation received.

Figure 1

To reduce the temperature and operate SCs at enhanced conversion efficiency, different types of devices have been used and these devices are difficult to use and uneconomical. To overcome the problem of cooling SC, the new design concept of “A comprehensive study on harnessing wind energy and fluidics principle to enhance the cooling rate of solar modules” is developed.

Advantages are: simple and economic, no moving parts hence, no wear and tear, characteristic and performance are not affected, fluidic device has smaller collector surface area but has the capacity of enhanced rate of heat transfer and to get the same capacity with any other devices the collector surface area would be three times larger than the surface area of the fluidic device, there is no shadowing effect. Solar tracking devices can be conveniently used. It can be manufactured as an integral part of SM. It will not import any obstructions to the installation of the solar concentrator to SM.

Figure 2

Theory

Solar cell, solar panel, solar module & solar arrays

In SM or SA, SCs (connected in series) are embedded in ultra violet stabilised ethyl vinyl acetate (EVA), are sandwiched (Figure 1) in between a thin glass plate (high impact resistant, highly transparent and tempered) and Tedlar (which in turn covered by aluminium cover). A corrosion resistant anodised aluminium alloy frame provides the required structural support. SC responds to the solar radiation ranging from ultraviolet to infrared. SC wafer can be square, circular, and semi-circular or quadrant shaped. The circular or square ones are generally 100 mm in diameter or have 100 mm edges respectively. Solar thermal energy and the heat energy generated within SM are transmitted to Tedlar or aluminium cover by conduction. The heat energy from the Tedlar dissipates on its own to the surroundings mainly by convection. The thermal energy of SCs can be effectively dissipated by installing the fluidic vortex diode (FVD) with a wind harnessing chimney (WHC), at the bottom open surface of SM.

Fluidics

The word ‘fluidics’ is combination of fluid and logics. American industry trade association National Fluid Power Association has defined fluidics as one in which sensing, controlling, information processing and logic operation are performed primarily through fluid dynamics, without the use of mechanical moving parts.

Fluidics systems are similar to electronic systems. Almost all the counter parts of the electronic systems are found in fluidics systems. In the fluidics amplifiers, the flow of a high energy fluid jet (power jet) is controlled by a low energy fluid jet (control jet). There are mainly five types of fluidics amplifiers and their classification is based upon the principle of operation.

FVD is one of them and is used as a fluidics diode in a number of applications. FVD consists of a vortex chamber or chamber with the tangential power nozzle to produce power jet at the inlet port. The chamber is a cylindrical chamber as shown in figure 2. The outlet port is at the centre of the bottom circular surface of chamber and the top circular surface is the bottom aluminium cover of SM. Fresh and cool air is issued tangentially in the form of a jet within the chamber and it swirls within the chamber before it flows out. This results in the longer flow path for the air stream facilitating enhanced heat transfer.

Wind harnessing chimney

WHC is a tubular chimney, with its bottom end fixed or welded at the centre of the bottom circular surface of the chamber and the top end is covered with a pivoted cowl or conical cover as shown in figure 2.

Methodology

The integration of FVD and WHC is shown in the figure 2. FVD is installed in such a way that the bottom cover of SM will be the top circular surface FVD chamber. FVD can be manufactured as the integral part of SM and WHC can be welded at FVD`s output. The whole assembly is erected in the given location by using proper supports. There should be no air gap in between the aluminium cover of SM and the FVD, all the joints should be airtight and the other open surfaces of FVD should be insulated.

Working

SM or SA installed with FVD and WHC are erected at any good location. As the solar radiation falls on SM or SA, the photon energy is absorbed and is converted into useful electrical energy. The heat generated within the SM and the solar thermal energy impinging on SM results in heating SM. This heat is transmitted to the aluminium cover closely fixed with the top surface of the chamber and subsequently will be absorbed by the swirling ambient air. The required low pressure to get the air stream enter and swirl in the chamber is done by venting a partial amount of air from the chamber and it is carried out by using the design concept of WHC. WHC is shown in figure 2. It is tubular chimney. One of its ends is welded to the outlet of the chamber and the other one is covered with a pivoted light material cowl.

When there is no wind the cowl will be in its normal position, covering the top end of the chimney. When there is wind, the cowl tilts along the direction of the wind and partially opens the top end of the chimney and this position of the cowl facilitates the creation of a low-pressure region at the partially opened chimney end. Due to this low-pressure region at top end of the chimney, air is partially evacuated out from the chamber via the chimney. The venting of even a small amount of air from the chamber via the chimney will facilitate the ambient air to rush into the chamber and swirl within it. The swirling fresh air cools aluminium cover and SM itself by absorbing effectively the excess heat from them. This hot air is continuously flows out into the atmosphere through the chimney. Thus, by adapting FVD and WHC, SM can be continuously cooled economically and effectively by harnessing the wind energy. The effect can be further enhanced by optimising the configuration of the cowl, the height at which the cowl is pivoted, the height, diameter, and resistance of the chimney and also the site of erection.

Maintenance

SM or SA installed with wind harnessing device does not require much maintenance. The general maintenance required are periodical cleaning of the top surfaces of SMs or SAs, maintenance electrical elements, sun tracking devices, solar energy concentrators and batteries, cleaning and oiling the pivot of the cowl fixed at the tip of the chimney.

Conclusion

It is observed that output or efficiency of solar cell, SM, SA can be conveniently enhanced effectively and economically by using FVD and WHC. Further enhancement of output or efficiency is achieved by having effective convective heat transfer and also by incorporating water lens system over the top surface of SM or SA as described earlier. Thus, comparative enhancement of efficiency or output can be realised by incorporating the discussed simple concepts in SMs or SAs systems.

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