GB2089024A

GB2089024A - Solar energy collector

Info

Publication number: GB2089024A
Application number: GB8115165A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-12-06
Filing date: 1981-05-18
Publication date: 1982-06-16
Also published as: GB2089024B

Abstract

A solar collector has energy absorption apparatus e.g. metal tube 3 within an evacuated insulation box with a transparent coverplate 6. Reflectors 2 may be provided beneath the tube and the evacuated space may be provided with a getter arrangement 11, 12, 13. The solar energy collector may be adopted for such industrial use as power generation or hydrogen production. <IMAGE>

Description

SPECIFICATION Solar energy collectors The present invention relates to solar energy collectors.

As a result of recent appreciation of the energy savings to be had, solar heating and hot water systems based on solar heating are becoming increasingly common. However, a fundamental weakness of the conventional solar heating systems has been lack of efficient means of converting the received solar energy into usable heat in a thermal transfer medium. The methods generally employed in conventional solar heating systems to prevent the loss of heat from the receiving side of the collectors, where the solar energy is trapped, comprise; preventing heat loss by radiation using the spectral radiation characteristic of a selective absorption film, and thermally insulating the collector with reinforced glass placed on the upper part of the collector with an air layer formed between the glass and an absorption device, both the air layer and the glass functioning as thermal insulators.In addition to the fact that the thermal insulation achieved is not perfect, the heat loss caused by convection in the air layer cannot be prevented, thus resulting in the encountered sharp increase in heat loss as the temperature of the thermal transfer medium rises.

Because of this, it becomes difficult to effectively raise the temperature of the thermal transfer medium higher than a certain range, for example 500C-700C.

As a possible solution to the problem of heat loss from a solar energy collector, collectors have been proposed which comprise cylindricallyshaped passages for the flow of a heat transfer medium surrounded by transparent, glass cylinders, with the space between the passages and glass cylinders being evacuated. However, since solar rays arrive parallel, a significant disadvantage of a cylindrical incident window of this radius is that the incident ratio of the sun's radiation is extremely low. Furthermore, since each pipe along which the thermal transfer medium flows is designed to enable the vacuum insulating of each structure to be independent, and also since the solary energy collector comprises a number of these cylindrical vacuum insulation structures, its manufacturing cost seriously reduces the cost advantages of solar energy over other energy sources.

In addition, since many independent vacuum insulation structures are employed in this known system, the chances of malfunctions related to evacuation are increased, and so too are the difficulties arising from the maintenance and servicing of the collector.

The present invention is concerned with providing a solar energy collector in which the aforementioned difficulties are reduced.

According to the present invention there is provided a solar energy collector, in which a thermal transfer medium is heated by solar radiation received by the collector, comprising radiation-absorption means including said thermal transfer medium, the absorption means being housed in an evacuated insulation vessel, the vessel having a transparent wall through which solar radiation is received which is either planar or has a large radius of curvature relative to the width of the vessel over which it curves.

In a preferred embodiment the transparent wall is substantially planar and the collector as a whole may comprise a generally shallow structure.

The absorption means may be spaced from the transparent wall by an insulation structure which may comprise layers of elongate members of thermally insulating material the members being parallel in a layer and angled to those in adjacent layers.

Thermal insulation panels as described in the Japanese published patent application No. (S55) 1980 --01 1794 may also be utilised.

A vacuum maintaining device may be included within, or in contact with, the interior of the vessel.

This device may include a getter substance or an absorbent material.

In a further aspect the present invention provides a method of manufacturing a solar energy collector as aforesaid which includes providing an absorbent material in contact with the interior of the insulation vessel, heating the absorbent material to an elevated temperature, evacuating said vessel while the absorbent material is at an elevated temperature, and, following evacuation, cooling the absorbent material to below its normal temperature.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings:- Figure 1 is a vertical section view illustrating a first example of a solar collector in accordance with the present invention; Figure 2 is a sectional view taken on line a-a of Figure 1; Figure 3 is a vertical section view illustrating a second example of a solar collector according to the present invention.

Figure 4 is a vertical section view illustrating a third example of a solar collector according to the present invention; Figure 5 is a schematic illustration showing part of a solar collector in use; Figures 6 and 7 are vertical section views of respective solar energy collectors each equipped with a vacuum-maintaining device including a getter; and Figures 8 and 9 are vertical section views of a respective further solar collector each equipped with a vacuum-maintaining device including an absorbent.

Referring to Figures 1 and 2 a solar energy collector comprises energy-absorbing apparatus A housed within an evacuated, shallow insulation box B.

The insulation box B is formed by a stainless steel, open box structure 1 having four rectangular side walls and a rectangular rear wall, all integrally formed; and a glass or plastics transparent cover plate 6. The cover plate Q is fitted across the open front of the box 1 and located in a rebate adjacent the inner edges of the side walls remote from the rear wall. Forward and rear spacing structures 5 made from thermally-insulating material are included adjacent respectively, the rear wall and the cover plate 6. These will be described in more detail below.

The energy-absorbing apparatus comprises a metal pipe 3 which enters the box B perpendicu larly through one sidewall and extends across the box parallel and close to an adjacent side wall. At a short distance from the side wall opposite the one through which it enters the box, pipe 3 bends through 900 to provide a short perpendicular section before bending through a further 900 so that the following length of pipe extends across the box parallel to the length which enters the box.

A further three such bends of the pipe 3 occur so that five parallel lengths of the pipe are disposed across the interior of the box B. The pipe 3 leaves the box B through the side wall opposite the one through which it entered.

The parallel lengths of the pipe 3 are situated just frontward of the mid-hsight of the box ana each length is located in front of an elongate mirror 2 of parabolic cross-section. A number of support struts 4 of thermally-insulating material connect each one of the parallel lengths of pipe 3 with the longitudinal edges of its associated mirror 2.

The pipe 3 is covered with a selective radiationabsorbing film and conveys a fluid thermaltransfer medium.

The spacing structures are formed by rods of material 7a, 7b and 7c arranged in layers. The rods in each layer are parallel, and the rods in adjacent layers orthogonal. The rods 7 are made of glass, quartz or heat-resistant plastics. The spacing structures 5 are designed to support the coverplate Sand outer frame 1 against atmospheric pressure on the outside of the insulation box.

In this example, the spacing structures 5, comprise heat resistant plastics rods 7a, 7b and 7c of rectangular cross section, 2 mm thickness, and 5 mm width aligned at 1 5-50 mm intervals in each layer and integrally formed as a 3-layer structure. The positions P and Q where a rod of material e.g. 7b in the middle layer contacts respectively rods in the forward and rear layers e.g. rods 7c and 7a do not overlap. Further details of the spacing structures are disclosed in Japanese Patent Applications Nos. (554) 1979-082714 and (S55)1980-012527.

Selection of materials, and the determination of sectional dimensions of each rod of material in the spacing structure, the number of layers, the intervals between the rods may be chosen to suit the particular collector in which they are employed. In other embodiments the layers which make up the spacing structures may be formed as rectangular lattices or as a 'honeycomb' lattice.

The area of the front-face of the collector that is shielded from radiation by the rods of material in the spacing structure is small, and it has been found that more than 83% of solar rays penetrate through to the absorbing apparatus A.

Figure 3 illustrates a second embodiment of the present invention. An energy-absorbing apparatus A is enclosed within an evacuated insulation box B as described with reference to Figures 1 and 2. In this embodiment the energy-absorbing apparatus includes a metal pipe 3 for conveying the heat transfer fluid, successively bent as in the above described embodiment and attached to a metal plate 8a which has channel-shaped recesses to accommodate the portions of the pipe and is located forwardly of the pipe. A selective absorption film 8b covers the forward facing surface of the plate 8a.

Figure 4 illustrates a third embodiment of the present invention. An energy-absorbing apparatus A similar to that described with reference to Figure 3 is enclosed within an evacuated insulation box B as aforesaid. In this embodiment the energyabsorbing apparatus is sandwiched and supported between spacing structures 5 which each comprise two sheets of insulation material 9 each in the form of a honeycomb lattice. The two sheets 9 are mounted one on top of the other and in such a manner that corresponding parts of the lattices do not overlap. In this case the honeycomb lattice shape of the layers of the spacing structure result in a somewhat worse insulation characteristic compared with the structures described with reference to Figures 1-3. However, this difference is sufficiently small that it can be ignored in practice.

The operation of and the results given by the solar energy collectors the structure of which has been described above will now be explained.

It has been found that in the above-described embodiments the heat loss from energy-absorbing apparatus A is remarkably reduced. For example, in one experiment a solar energy collector with heat collection pipes 20 mmo x 7 pcs and water at 900C as a thermal transfer medium were used.

The spacing structures 5 comprised transparent acrylic rods 7 arranged in layers as in Figures 1 and 2 the rods having 2 mm thickness and 5 mm width aligned in parallel in a layer at 30 mm intervals. The energy-absorption device included a flat-type selective absorption film body 8 employed between the rear and forward spacing structures 5 and housed in an evacuated insulation box (120 cm x 90 cm x 10 cm) comprising a 2 mm thick glass cover plate 6 on a stainless-steel box 1. The box was evacuated to a pressure of 4 x 1 O-4Torr. The quantity of heat loss became approximately 130 kcal/m2 hr with a heat collection efficiency of approximately 64% when the temperature of the thermal transfer medium was 90 C. In contrast, in a conventional type of solar energy collector the quantity of heat loss is typically 341 kcal/m2 hr (heat collection efficiency - 6%) when the temperature of the water or other themal transfer medium is 900 C.

Thus, it has been discovered that heat loss in the embodiment of the invention compared with that of a conventional type is significantly reduced. The conditions in which measurements were made in these experiments were at an air temperature of 200C with dead calm.

Particularly, in the first and second examples of the present invention described with reference to Figures 1 to 3 the thermally-insulating spacing structures which comprise transparent, thermallyinsulating rods of rectangular cross-section arranged in successive layers so that the intersections of rods do not overlap, result in a heat loss path through the layered rods of the spacing structures which is relatively long with a relatively small narrow cross-sectional area. So, the evacuated box B is equipped with an excellent insulation characteristic and can be produced even as a flat type of collector requiring a large area.

Therefore, a solar energy collector, even one with a large area, can be manufactured easily and at lower cost, and its insulation characteristic can be fully ensured.

A number of substances, such as water, freon and helium gas can be used as the thermal transfer medium within the pipe 3. Using these as the transfer medium and utilizing a large size of concave mirror 10, which allows solar rays to fo',-us, temperatures of the thermal transfer medium easily reach around 2000C-4000C. As a result, power generation with a steam or freon vapour cycle, or power generation with a helium turbine for which high temperature helium gas is utilized, or production of hydrogen gas, or distillation of sea water become possible. It is envisaged that the present invention can provide solar energy collectors applicable to a wide range of industrial fields.

As stated above a solar energy collector in accordance with the present invention and in which an absorption apparatus A is housed inside an evacuated insulation box B, results in a remarkable reduction of heat loss from the energy absorption apparatus A, achievement of efficient heating of the thermal transfer medium, and a large reduction of manufacturing cost regardless of the size.

Figure 6 illustrates the fourth embodiment of the present invention. In this embodiment, to prevent the degradation of the vacuum in the evacuated box B, a getter 11 which is to be heated by solar rays S from a reflecting mirror is housed inside a heat-resisting container 12 and fixed at a suitable place in the box B. The getter 11 may be composed of a substance such as titanium, titanium alloy or barium. Titanium is used as a getter in this example, and the quantity used is approximately 0.05-0.1 g peril of inner volume of the box housing B. Alternatively, the getter may be scattered in the box or hung without a container.

Figure 7 illustrates a fifth embodiment of the present invention which is similar to that shown in Figure 3. A heat-resistant container 1 2 houses a getter substance 11 and can either be placed, as shown, on the upper corner of the selective absorption film plate 8 or fitted with a suitable supporter in the evacuated insulation box B. Solar rays S are focussed by a lens 14 onto the getter substance 11.

When the temperature of the titanium getter 11 is raised to 4000C-6000C by focussed solar radiation S, the getter is gradually sublimated and adheres to the inner walls of the evacuated insulation box B. The titanium getter is heated to a high temperature in the container 12 and high temperature titanium adhering to the walls keep the inside of the evacuated insulation box B in the state of high evacuation by reacting with gas molecules which are residual in the insulation box B, and adsorbing them.On the basis of past experience it is possible that by placing 1-10 g/l of titanium getter heated to 4000C-6000C with solar rays during day time, the initial value of the degree of evacuation (approximately 5 x 10-4 Torr) can be maintained over 1 0-20 years with an evacuated insulation box B of size 1200 x 900 x 120 mm. Since in this embodiment of the invention a getter 11 is designed to be heated by solar rays S so that the remaining gas molecules are absorbed, no extra heating equipment is required and a high degree of evacuation can be maintained for a long period.

This makes it a very practical device.

Figures 8 and 9 are vertical section views illustrating sixth and seventh embodiments of the present invention. An evacuated insulation box B is constructed as in the above described embodiments and is attached to a container D in which an absorbent is kept. An electrical heating furnace 1 5 is disposed around the outer side of the container. A terminal 20 for connecting with an ionization-type vacuum meter communicates with the interior of the box B via a side wall of the box. On the opposite side wall is a vacuum outlet 1 9. An absorbent material 1 6 is housed inside the container D and the interior of the evacuated insulation box B and the container D are connected via two pipes 1 7 and 18 through the rear wall of the insulation box B.

The absorbent container D is attached to the evacuated insulation box B with the two pipes 1 7 and 1 8 in a by-pass configuration. However, its effectiveness of action remains the same even when the container D is attached to the evacuated insulation box B with one pipe in a branch configuration. Though not illustrated in the drawing, it is also possible for the absorbent container D to be located inside the evacuated insulation box B.

The outer dimensions of the container D for the absorbent are 2.8 mm thick, 500 mm long with an outer diameter of 48.6 mm. It is filled with 500 g of 5A type molecular sieve. The absorbent 1 6 may also comprise palladium hydride and palladium oxide together with the molecular sieve. By using palladium along with the molecular sieve, oxygen in the residual gas turns to H20 on the surface of the palladium hydride and is absorbed within the molecular sieve. Hydrogen in the residual gas turns to H20 on the surface of palladium oxide, and is absorbed within the molecular sieve.

The absorbent container D is heated by causing the electric furnace 1 5 to raise the temperature of absorbent 16 of the molecular sieve to 5000C, and the absorbent is kept at this temperature during the evacuation stage.

After the absorbent has been heated the box housing B is evacuated through the vacuum outlet 19 with a vacuum pump (not shown in the figures) in one example for 96 hours at room temperature, to reach a degree of evacuation of 2 x 10-4 Torr.

Further evacuation was made for another 24 hours until the degree of evacuation reached 6 x 10-5 Torr, after which the vacuum outlet 1 9 was closed and the vacuum pump separated.

Later, the furnace for the molecular sieve was taken off and cooled. At this stage, the degree of vacuum reached 3 x 10-5 Torr. Hence, by keeping the absorbent at high temperatures while evacuating the insulation box B, water and gas molecules absorbed in the absorbent 1 are eliminated, and by lowering the temperature of the absorbent 1 6 below the ordinary temperature, absorption action was initiated, thus resulting in the remarkable improvement in absorption capability of the absorbent 1 6, and abrupt absorption of internal residual gas. The excellence of the absorption action of the molecular sieve absorbent 1 6 is proved by the fact that in the example the degree of vacuum in the insulation box B remained almost the same after four months elapsed.

In those embodiments in which the absorbent 1 6 is not used, or in which it is not maintained at high temperatures during evacuation it has been found that the degree of vacuum becomes approximately 5 x 10-5 Torr if a similar insulation box B is employed under the same conditions and the evacuation procedures are the same.

Furthermore, after four months, it has been found that the degree of vacuum goes down to around 2 x 10-3 Torr. In contrast, in the former example, the degree of vacuum remained somewhere around 3 x 10-5 Torr after four months, while in the latter cases, the degree of vacuum goes down to around 2 x 10-3 Torr. This demonstrates the effectiveness of the evacuation method described with reference to Figures 8 and 9.

Claims

1. A solar energy collector, in which a thermal transfer medium is heated by solar radiation received by the collector, comprising radiation absorption means including said thermal transfer medium, the absorption means being housed in an evacuated insulation vessel, the vessel having a transparent wail through which solar radiation is received which is either planar or has large radius of curvature relative to the width o, the vessel over which it curves.

2. A solar energy collector as claimed in claim 1 in which the transparent wall is substantially planan

3. A solar energy collector as claimed in claim 1 or claim 2 wherein said radiation-absorption means includes a plurality of lengths of pipe conveying fluid thermal transfer medium, the lengths of pipe either being separate or continuous.

4. A solar energy collector, as claimed in any one of claims 1 to 3 in which the radiation absorption apparatus is spaced from the transparent wall by an insulation structure.

5. A solar energy collector as claimed in claim 4 in which the radiation absorption apparatus is sandwiched inside the vessel by said insulation structure and a further insulation structure.

6. A solar energy collector as claimed in claim 4 or claim 5 wherein the or each insulation structure comprises layers of elongate thermal insulation members, the members being parallel in a layer and angled to those in adjacent layers.

7. A solar energy collector as claimed in claim 6 in which the overlying junctions between the elongate members do not overlap.

8. A solar energy collector as claimed in claim 4 or claim 5 in which the or each insulating structure comprises two or more sheets of lattice-shaped thermally insulating material mounted one on top of the other, but so that corresponding parts of the lattices do not overlap.

9. A solar energy collector, as claimed in any one of the preceding claims, which is equipped with a vacuum maintaining device comprising a getter substance housed within the insulation vessel and a reflecting mirror arranged to direct solar radiation onto the getter substance to heat it.

10. A solar energy collector, as claimed in any one of the preceding claims, which is equipped with a vacuum maintaining device comprising a getter substance housed within the insulation vessel and a lens arranged to direct solar radiation onto the getter substance to heat it.

11. A solar energy collector, as claimed in claim 9 or claim 10 wherein the getter substance is titanium, titanium alloy or barium.

12. A solar energy collector, as claimed in any one of claims 1 to 8 which includes an absorbent material in contact with the interior of the insulation vessel and means for heating the absorbent material so that it is at an elevated temperature during evacuation of the insulation vessel.

13. A solar energy collector, as claimed in claim 12 in which the absorbent material is stored in a separate container which communicates with the interior of the insulation vessel.

14. A solar energy collector, as claimed in claim 12 or claim 13 in which the absorbent material comprises a molecular sieve or a molecular sieve together with the mixture of palladium hydride and palladium oxide.

1 5. A method of manufacturing a solar energy collector as claimed in any one of claims 1 to 8 which includes providing an absorbent material in contact with the interior of the insulation vessel, and further comprises heating the absorbent material to an elevated temperature, evacuating said vessel while the absorbent material is at an elevated temperature, and following evacuation, cooling the absorbent material to below its normal temperature.

1 6. A solar energy collector substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.