GB2154729A - Solar energy collection system - Google Patents

Abstract

A series of reflectors 1 are pivotally mounted beneath a double glazed roof 4 so as to continually focus solar radiation onto an absorber 2. Air to be heated is drawn through ducts 5 in the roof floor and passed through the absorber 2 to be ducted to a drying process. The upper surfaces of the ducts 5 are matt black to absorb unreflected radiation and the absorber 2 is formed as a duct with a lower surface formed by a plurality of layers of coated mesh (13, Fig. 3). The air supply to the ducts 5 may be passed through a flat plate solar collector. <IMAGE>

Description

SPECIFICATION Improved solar energy collection system This invention relates to solar energy collection and is particularly relevant to air heating systems for drying processes where temperatures in excess of 80"C are required.

The use of simple flat plate solar collectors for low temperature heating systems is well known. It is also known that the efficiency of these collectors falls off rapidly as the operating temperature increases. This is due to increasing losses by re-radiation and heat conduction to the environment. Also, where the medium to be heated is a gas, e.g. air, the flow velocities through the collector, necessary in most practical applications in order to achieve the temperature required, result in poor heat transfer characteristics and consequently higher collector surface temperatures for given output temperatures. This also adversely effects the efficiency.

The object of this invention is to provide a more efficient solar air heating system for drying processes particulariy within the earth's tropical zones.

This has been achieved by designing a solar collector consisting of a series of reflectors pivotally mounted so as to focus solar radiaiton onto an absorber the whole being located within a transparent enclosure, the fluid to be heated being drawn through or from within the said enclosure and passed through the said absorber. The heated fluid, which may be air or any other suitable fluid, is then ducted to a drying process.

The invention will now be described by reference to the following figures: Figure 1 indicates a general arrangement of the solar energy collection system.

Figure 2 gives details of one possible arrangement of reflectors.

Figure 3 gives details of one particular design of absorber.

In Figs. 1 and 3 arrows are used to indicate both air flow routes and solar energy paths.

Long narrow reflectors, 1, are pivotally mounted to rotate about their longitudinal axis in a frame which is pivotally supported on the floor of a glazed ridged roof, (Fig. 1 ). The reflectors track and focus the sun onto a long tubular absorber, 2, located in the apex of the roof. Air from the roof space is drawn into the absorber along its length and passes via duct 6 to the drying process machinery or heat store and is replenished by air at ambient temperatures entering the roof space via inlets, 3, at eaves level.

Heat "iosses" due to radiation from the absorber and inefficiencies of the reflectors will tend to warm the air and structure in the roof space. However, since this air is being continuously drawn into the absorber and replenished, the temperature of the air in the roof space will be maintained at a relatively low level and therefore heat losses by conduction through the roof glazing, 4, which may be double glazed, will be minimal. Also since the glazing tends to be opaque to longwave radiation (the "greenhouse" effect) heat losses by radiation from the roof will be small.

The optimum orientation of the reflectors and roof ridge will depend on latitude. Within the tropics it will be preferably, but not essentially, north/south, so that the daily motion of the sun relative to the roof can be tracked by rotating the reflectors about their longitudinal axes.

Seasonal variations in the sun's altitude and azimuth can be accommodated by variably tilting the reflector mounting frame in a plane normal to the longitudinal axis of the reflectors.

Each reflector, positioned so as to project the sun's rays onto the absorber will rotate through an angle of 90 during a 1 2 hour period. This rotation can be achieved by various known means, a simple mechanical method is described by way of example, and shown in Fig. 2. A cam 7, driven by a geared electric motor, 8, to rotate at 1 revolution in 24 hours, displaces a connecting link, 9, which is pivotally connected to crank arms, 10, rigidly connected to the spindle, 11, of the reflectors pivotally mounted in a frame, 1 2.

The movement of the connecting link, 9, rotates the crank arms, 10, thrugh an arc of 90 every 1 2 hours at a constant rate of 7.5 degrees per hour. The reflector mounting frame, 12, is pivotally supported from the roof floor so that the reflectors can be tilted about an east/west axis to correct for seasonal variations in the sun's elevation.

In a preferred embodiment the absorber consists of a horizontal duct (Fig. 3) the lower surface of which is formed by several layers of selectively coated mesh, 1 3. Air entering the absorber passes through this mesh and subsequently through a louvred plate, 14, which directs the flow along the duct to its exit.

The collector system described may be referred to as an enclosed concentrating system and would be most effective during peiods of direct solar radiation since diffuse radiation would not, for the most part, be reflected onto the absorber. Diffuse radiation would therefore make only a small contribution by warming the air and internal structure of the roof space.

The performance of the enclosed concentrating system can be further enhanced by passing the air entering the roof space through ducts 5 in the floor of the roof. The upper surfaces of these ducts would be matt black or selectively coated to absorb radiation, both diffuse and direct beam, which did not impinge on the reflectors.

With such a collecting system and using suitably useful flow rates of air, temperature rises of up to 50"C can be achieved with insolation levels in the region of 750 watts/m2 (See example).

In a tropical situation with an ambient temperature of 30"C and an insolation level of 1000 W/m2 air temperatures well in excess of 80"C should therefore be readily achievable.

A further feature of the present invention is the use of a simpler type of solar collector to preheat the air supply to the enclosed concentrating system hitherto described. For example a simple flat plate collector operating at high efficiency may be used to raise the air temperature by 20-30"C before feeding to the enclosed concentrating system. In this way it can readily be seen that temperatures in excess of 1 00 C can be achieved.

Example Tests were carried out in ambient conditions using natural solar radiation. The collector was mounted at an angle to the horizontal equal to the angle of latitude (51 40') and facing due south. During the tests the reflectors were rotated to track the sun and maintain the focus of solar radiation on the absorber.

Air at a series of flow rates was passed through the absorber and measurements were taken, at frequent intervals, of solar radiation, ambient temperature, absorber inlet and outlet temperatures, and airflow rates.

The results obtained are summarised in the following Table: where T1 = ambient temperature, T2 = temperature inside the enclosure, T3 = temperatures of air leaving collecting system.

Efficiency Air flow Solar radiation T T T Collector Overall Kg/s W/m2 1 2 3 only system Day 1 9.45 0.016 546 12 26 44.5 32 56 10.05 0.016 612 13 27 47.5 32 53 10.25 0.016 680 13.5 28 51.5 33 53 Day 2 10.40 0.015 659 5 19 49 40 58 11.00 0.015 715 5.5 19 51 40 55 11.30 0.013 748 6 22 55.5 34 51

Claims (8)

1. A solar collector consisting of a series of reflectors pivotally mounted so as to focus solar radiation onto an absorber the whole being located within a transparent enclosure, the fluid to be heated being drawn through or from within the said enclosure and passed through the said absorber.

2. Apparatus as described in Claim 1 in which the reflectors are moved in such a way as to maximise the amount of radiation falling on the absorber.

3. Apparatus as described in Claim 1 in which the fluid to be heated is air.

4. Apparatus as described in Claim 1 in which the fluid entering the enclosure passes through ducts the surface of which are selectively coated to absorb radation thereby raising the temperature of the fluid before it passes into the absorber.

5. Apparatus as described in claim 1 in which the absorber consists of a horizontal duct one surface of which is formed by several layers of selectively coated metal mesh.

6. The use of apparatus as described in Claim 1 in series with any other form of solar energy collector in order to maximise efficiency.

7. A process for dehydration using apparatus according to Claim 1.

8. Products made using the process according to Claim 7.