GB1597459A

GB1597459A - Apparatus for extracting heat from solar energy

Info

Publication number: GB1597459A
Application number: GB37978/76A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-12-14
Filing date: 1977-12-14
Publication date: 1981-09-09

Description

(54) IMPROVEMENTS IN OR RELATING TO APPARATUS FOR EXTRACTING HEAT FROM SOLAR ENERGY (71) I, FREDERICK JOHN LEONARD ROBINSON, a British Subject of 'Blackthorne', 151 Woodfield Road, Northgate, Crawley, Sussex, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to devices for heating liquid by solar energy.

In harnessing solar energy for heating it is important to convert the radiant energy from the sun into heat without re-radiating energy or losing heat by convection or conduction due to poor heat insulation.

A substantial portion of the solar energy made use of in such devices is radiated with a wavelength of between 0.35 and 0.75 micrometres in the visible band). The remainder of the energy so used is in the infra-red band with a wavelength greater than 0.75 micrometres. When a body adsorbs radiation, there is a redistribution of the energy, and any emitted radiation will be at a different wavelength distribution from that of the absorbed radiation, the distribution being determined by the temperature of the body.

For high efficiency a high value of absorptivity is required. So-called black bodies have high values of absorptivity but they also have high emissivity values and this can result in low efficiency in a solar energy conversion unit. In order to obtain further improvement in efficiency a selective absorber is required, the absorber providing high absorption for the shorter wavelength solar energy with high emissivity for the consequent long wave re-radiation.

According to one aspect of the present invention, a device for heating liquid by solar energy comprises a rigid sealed hollow member adapted to float in the liquid, the hollow member when floating, having an upper surface which is transparent to solar energy and a lower surface, a portion at least of the interior of which has a layer of thermally absorbtive material, the interior of the hollow member being either evacuated or filled with a gas which is nonreactive with the absorbtive material, the external configuration of the device being such that the device can nest with a plurality of sub stantially identical devices to cover substan tially completely the surface of the liquid over which they are floating.

A second layer of material may be pro vided on the said portion between the layer of thermally absorbtive material and the lower surface, which second layer is ther mally reflective, each layer being of thick ness which is transparent to solar radiation, the materials of the layers being selected to provide a ratio of absorption to emission substantially in excess of unity in the wavelength spectrum of the sun's radiation.

Preferably, the said ratio is substantially 9.1.

According to a further aspect of the pres ent invention, a cover for a panel, that is, a vessel containing a liquid to serve as a heat source, comprises a plurality of juxtaposed devices each as defined above, floating on the surface of the liquid and partially or wholly covering the latter.

According to yet a further aspect of the present invention, a solar pond comprises a cover as defined above in which the vessel is provided with a plate opaque to solar radia tion and disposed above the liquid level and a thermostatically controlled pump is pro vided to cause the hollow members to move under the opaque plate when the tempera ture of the liquid exceeds a predetermined value.

Embodiments of the invention will now be described, by way of example, with refer ence to the accompanying diagrammatic drawings, in which: Figure 1 is a diagrammatic sectional ele vation of a float device in accordance with the invention for extracting heat from solar energy; Figure 2 is a diagrammatic sectional ele vation of a system for household heating by the use of heat extracted from solar energy by means of a plurality of devices according to Figure 1; Figures 3A and 3B respectively show two diagrammatic geometrical configurations of devices in accordance with the invention than can be arranged to extend over an area; Figure 4 is a diagrammatic sectional elevation of a further solar heating system employing devices according to Figures 5A and SB;; Figures 5A and SB respectively show a diagrammatic sectional elevation and a plan of a modification of the float of Figure 1; Figures 6A and 6B respectively show a diagrammatic sectional elevation and a plan of a further modification of the float of Figure 1; and Figure 7 is a plan of a solar pond presenting a surface covered by a large number of devices in accordance with the invention.

Referring to the drawings, Figure 1 shows a generally spherical bulb-shaped member 1 made of glass of low coefficient of thermal expansion and high transmittivity in the range of solar radiation wavelengths. It is formed with a sealed exhaust tube 3 tipped-off after the use of a vacuum pump for evacuating the bulb-shaped member 1.

The member 1 may be partly of doublewalled construction in the manner of a vacuum flask. The tube 3 is wound with a coil 4 providing a weight to maintain the float in the position shown. The portion of the float 1 above the float line in a liquid 5 is transparent but may be provided with a layer of quarter-wavelength thickness material the internally facing side of which is reflective to low heat (eg. up to 100"C) long-wave radiation. The portion of the float 1 below the float line is coated internally with a selective radiation adsorber 2 consisting of a highly absorptive thin oxide film of, for example, an oxide of nickel or copper, disposed next to the glass and covered by a thin reflective film, for example, of silver. The thickness of the film is chosen to be so thin as to be transparent to the solar radiation.The films may be deposited by vacuum deposition or ba a suitable chemical deposition. The absorptive film serves to adsorb the shorter wavelength solar energy, and with the aid of the reflective film, to emit much of the long wavelength radiation.

The use of a vacuum inside the float serves practically to eliminate the deterioration of the delicate films. The evacuation of the space inside the bulb-shaped float may be completed by gettering.

In place of the vacuum the bulb-shaped float may contain an inert gas at reduced pressure. In either case, heat losses due to convection and conduction are substantially reduced.

The member 1 is designed for use on so-called solar ponds, wherein a plurality of juxtaposed members are arranged to float on the liquid 5, such as a high density salt solution, by which the heat is transmitted for a useful purpose. The spherical shape maximises the heat transfer area. Thus, as shown in Figure 2, this liquid may be contained in a tank lined with insulation 13 having a pro tective lining 12. The surface of the liquid is covered by a layer 3A of the members and the tank may, if required, be closed on top by a layer 4A of solar glass capable of transmitting a wide range of solar radiation, ie direct radiation 7A and diffused and reflected radiation 8A. The comparatively cold liquid 10 at the top of the tank, when heated by and through the floats 1 sinks to provide a comparatively hot layer 9 at the bottom of the tank.This layer 9 transfers heat via a heat exchanger 11 to a circulating system 8 containing a circulating medium such as oil or water that passes through a nonreturn valve 7 to a further heat exchanger 5A which may be a water heater.

The shape of the members in plan view may take various forms chosen so that with the aid of surface tension they fit together to provide a substantially continuous cover for the liquid. Thus, they may be hexagonal, square, oblong or triangular. Figures 3A and 3B show alternative patterns of hexagonal and oblong members, respectively.

In Figure 3B the ratio of the length of the longer side to the shorter side is 2:1. By the use of such members the surface of the liquid is almost totally covered by an insulating covering that cuts down losses due to evaporation of the liquid and convective air currents. The floats are held together by surface tension.

In prior solar ponds with glass covering plates for cutting down convection, evaporated liquid condenses on the glass and the droplets of condensate absorb radiation before reaching the collector. Moreover, in such ponds it is difficult to cover large areas while leaving a small air gap for ventilation.

With the present invention, using the glass members 1, as droplet absorption cannot occur there is no ventilated space.

Moreover, heat loss by convection from the liquid is cut to a minimum because air currents can pass only through any small gaps that may exist between the juxtaposed floats.

Where there is a lot of cloud cover, the radiation is diffuse. For such conditions it is advantageous to design the exposed surfaces of the bulbs to an optical shape that can collect the greatest amount of diffuse radiation and focus it on to the absorptive layer.

This, of course, provides a substantial advantage over prior art arrangements using flat plates. Moreover, the arcuate base of each member presents a large contact area to the liquid with the hottest part below the top level of the liquid and because the heated liquid sinks, the liquid presents a very small radiation surface at a low temp erature.

During summer months or periods of high radiation known units for extracting energy from solar radiation tend to overheat. This can be avoided, for example, as shown in Figure 4. In this arrangement the members 18 do not cover the entire liquid surface of the solar pond 14 and the tank is provided with a lateral extension 14A covered by an opaque plate 15. A thermostat bulb 19 is located close to a heat exchanger 20 and controls a thermostat 24 which in turn controls a reversible pump 22 connected by pipes 23 to opposite sides of the liquid a little below its surface. When conditions are too hot, the pump 22 causes the liquid to move so as to cause the members to drift under the plate 15 so that they are no longer exposed to solar radiation. When conditions are too cold the members are caused to drift in the opposite direction so as to become wholly exposed to the radiation.

Figures 5A and 5B respectively show an alternative way of controlling the amount of heat derived from the system in sympathy with the quantity of radiant energy received.

In this case, a member 30 is substantially ellipsoidal with a joint 32 between its top and bottom portions. The exposed part is made of a known form of glass 31 that changes its transmission factor according to variations in the radiant energy received.

Some thin films also have this effect and, therefore, as shown in Figures 6A and 6B, such a film 35 can be vacuum or chemically deposited on the inside surface of the exposed part of the bulb, the bulb in this case being made wholly of ordinary low expansion glass.

Figure 7 shows a large circular solar pond 40 substantially covered by hexagonal members 41. These are exposed to the atmosphere and therefore require periodic cleaning. For this purpose, a pump or impeller (not shown) can be actuated to cause the peripheral portion of the top of the liquid to circulate in the direction of arrow A so that the outermost members pass in succession into a cleaning machine 42 extensible into and retractable from the pond 40. In the machine 42 the members 41 are cleaned and discharged between guides 43 that lead them to the centre of the tank.

WHAT WE CLAIM IS: 1. A device for heating liquid by solar energy comprising a rigid sealed hollow member adapted to float in the liquid, the hollow member when floating, having an upper surface which is transparent to solar energy and a lower surface, a portion at least of the interior of which has a layer of thermally absorbtive material, the interior of the hollow member being either evacu ated or filled with a gas which is non reactive with the absorbtive material, the external configuration of the device being such that the device can nest with a plurality of substantially identical devices to cover substantially completely the surface of the liquid over which they are floating.

2. A device as claimed in claim 1, in which a second layer of material is provided on the said portion between the layer of thermally absorbtive material and the lower surface, which second layer is thermally reflective, each layer being of thickness which is transparent to solar radiation, the materials of the layers being selected to provide a ratio of absorption to emission substantially in excess of unity in the wavelength spectrum of the sun's radiation.

3. A device as claimed in claim 2, in which the said ratio is substantially 9:1.

4. A device as claimed in claim 1, 2 or 3, in which the hollow member is made of glass of low thermal expansion and high transmissivity for solar energy ranging from a wavelength of 0.35 micrometres to a wavelength of 2.0 micrometres.

5. A device as claimed in claim 4, in which the hollow member is of double walled construction, with the gap between the walls being evacuated.

6. A device as claimed in any one of claims 1 to 5, in which the upper surface of the hollow member is optically shaped to increase its efficiency when subjected to diffused radiation.

7. A device as claimed in any one of claims 1 to 6, in which the upper surface of the hollow member is made from light sensitive glass that automatically regulates the solar energy conversion by altering its transmission factor.

8. A device as claimed in any one of claims 1 to 6, in which the internal surface of said upper surface of the hollow member is coated with a light sensitive film that alters its transmission factor in sympathy with variations in the density of the solar energy received.

9. A device as claimed in any one of claims 1 to 8, in which the hollow member has an external configuration which is either hexagonal, square, oblong or triangular in plan view.

10. A cover for a solar pond, that is, a vessel containing a liquid to serve as a heat source, the cover comprising a plurality of juxtaposed devices each as claimed in any preceding claim, floating on the surface of the liquid and partially or wholly covering the latter.

11. A solar pond, including a cover as claimed in claim 10, in which the vessel is provided with a plate opaque to solar radiation and disposed above the liquid level an,d a thermostatically controlled pump is provided to cause the hollow members to move under the opaque plate when the tempera

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. erature. During summer months or periods of high radiation known units for extracting energy from solar radiation tend to overheat. This can be avoided, for example, as shown in Figure 4. In this arrangement the members 18 do not cover the entire liquid surface of the solar pond 14 and the tank is provided with a lateral extension 14A covered by an opaque plate 15. A thermostat bulb 19 is located close to a heat exchanger 20 and controls a thermostat 24 which in turn controls a reversible pump 22 connected by pipes 23 to opposite sides of the liquid a little below its surface. When conditions are too hot, the pump 22 causes the liquid to move so as to cause the members to drift under the plate 15 so that they are no longer exposed to solar radiation. When conditions are too cold the members are caused to drift in the opposite direction so as to become wholly exposed to the radiation. Figures 5A and 5B respectively show an alternative way of controlling the amount of heat derived from the system in sympathy with the quantity of radiant energy received. In this case, a member 30 is substantially ellipsoidal with a joint 32 between its top and bottom portions. The exposed part is made of a known form of glass 31 that changes its transmission factor according to variations in the radiant energy received. Some thin films also have this effect and, therefore, as shown in Figures 6A and 6B, such a film 35 can be vacuum or chemically deposited on the inside surface of the exposed part of the bulb, the bulb in this case being made wholly of ordinary low expansion glass. Figure 7 shows a large circular solar pond 40 substantially covered by hexagonal members 41. These are exposed to the atmosphere and therefore require periodic cleaning. For this purpose, a pump or impeller (not shown) can be actuated to cause the peripheral portion of the top of the liquid to circulate in the direction of arrow A so that the outermost members pass in succession into a cleaning machine 42 extensible into and retractable from the pond 40. In the machine 42 the members 41 are cleaned and discharged between guides 43 that lead them to the centre of the tank. WHAT WE CLAIM IS:

1. A device for heating liquid by solar energy comprising a rigid sealed hollow member adapted to float in the liquid, the hollow member when floating, having an upper surface which is transparent to solar energy and a lower surface, a portion at least of the interior of which has a layer of thermally absorbtive material, the interior of the hollow member being either evacu ated or filled with a gas which is non reactive with the absorbtive material, the external configuration of the device being such that the device can nest with a plurality of substantially identical devices to cover substantially completely the surface of the liquid over which they are floating.

ture of the liquid exceeds a predetermined value.

12. A solar pond, including a cover as claimed in claim 10, a pump for keeping, when actuated, the hollow members in continuous motion, and a cleaning machine so disposed and arranged as to capture the moving hollow members, clean them and return them to the pond.

13. A method of utilising the heat of solar energy, comprising disposing a cover according to claim 10 on a solar pond, and providing a heat transfer medium circulation system between the pond and a point of use, the system including heat exchanging means, ducting for hot and cold heat transfer medium and flow regulation means.

14. A device, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the Figures of the accompanying drawings.

15. A cover for a solar pound constructed and arranged substantially as hereinbefore described with reference to Figure 2 or Figure 3A or Figure 3B or Figure 4 or Figure 7 of the accompanying draw ings.

16. A solar pond constructed and arranged substantially as hereinbefore described with reference to Figure 2 or Figure 4 or Figure 7 of the accompanying drawings.

17. A method of utilising the heat of solar radiation substantially as hereinbefore described with reference to and as illustrated in the Figures of the accompanying drawings.