GB2030282A

GB2030282A - An integrated system for collecting and storing solar energy

Info

Publication number: GB2030282A
Application number: GB7928519A
Authority: GB
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1978-08-17
Filing date: 1979-08-16
Publication date: 1980-04-02
Also published as: IT1105458B; GB2030282B; IT7850758A0; ES483036A1; BE878007A; PT70058A; FR2433716A1; FR2433716B1

Abstract

An integrated system for collecting and storing heat energy, comprising a modular green-house- effect solar collector having a selective energy-receiving surface 4, each module comprising the selectively-absorbing receiving surface, a duct (6) for conveying a heat-conveying fluid, and a material (5) which undergoes a reversible thermochemical reaction (with or without a change of physical state) at a high specific density of energy storage, wherein the material is disposed in close contact with the surface and surrounds the duct which, at the ends of the module, is connected to common collectors into and from which the fluid is supplied and withdrawn, respectively. In a second embodiment, Fig. 3 (not shown), each module comprises a double-walled, generally cylindrical vessel containing the heat storage material through which the duct passes. The outer wall is transparent and the inner wall is blackened. <IMAGE>

Description

SPECIFICATION An integrated system for collecting and storing solar energy The object of the invention is to construct an integrated system for collecting and storing heat energy obtained from solar energy.

Known systems for storing heat energy of solar origin are based on the use of hot water or another fluid. It is clear, however, that the functions of collecting and storage cannot be combined in a single component, in view of the large volumes required for storing an appreciable quantity of energy.

More recent research has been done on the use of chemical reagents capable of storing and returning the absorbed energy by means of a reversible chemical reaction at a relatively low temperature in the range between about 50 and 50 CC, associated, if required, with heat pumps in order to raise the temperature level when necessary for use.

Systems have been developed on the basis e.g.

of reversible reactions between double salts in an aqueous mixture, which can store lowtemperature energy at a high specific density, typically of the order of 5 times that which can be stored in normal hot-water systems.

During the day, under favourable sun conditions, the reaction is endothermic and the system absorbs heat. During the night the reaction is exothermic and the stored heat is released from c the solution.

The problem arises, however, of arranging the space in the reacting system in the most suitable position for maximum release of stored heat.

The fundamental problems -- i.e. the space required for storage and the required area of the receiving surface -- must be reconciled in order to obtain a completely integrated system. It is also important for the region where energy is stored to be fairly near the receiving surface, i.e. the energy generator in the present case.

The invention relates to an integrated system which can solve this problem in a simple, elegant and extremely efficient manner.

According to the main feature of the invention, the integrated system for collecting and storing heat energy, comprising a modular greenhouseeffect solar collector having a selective energyreceiving surface preferably protected by a screen transparent to light and being thermally insulated, each module comprising the selectively-absorbing receiving surface, a duct for conveying a heatconveying fluid and a thermochemically-reacting material which stores energy at a high specific density, is in close contact with the surface and surrounds the duct which, at the ends of the module, is connected to common collectors into and from which the fluid can be supplied and withdrawn respectively.

According to another feature of the invention, the selectively-absorbing surface is corrugated.

According to another feature of the invention, each modular collector is made up of a cylindrical element comprising two coaxial, corrugated, parallel glass vessels enclosing an evacuated space, the inner cylindrical vessel being blackened and the common interior being filled with a thermochemically-reacting material capable of storing energy at a high specific density, and wherein the duct for conveying the heatconveying fluid is disposed in a helix around the axis of the cylindrical element all along its length, is embedded in the thermochemically-reacting material and is connected, at the ends of the cylindrical element, to common collectors into and from which the fluid can be supplied and withdrawn, respectively.

Two embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a set of four modular collectors of the invention fitted together in parallel; Figure 2 is a partial vertical section through one of the modular collectors of Figure 1 at right angles to the major axis of the collector; and Figure 3 illustrates a variant of the embodiment shown in Figures 1 and 2.

Each modular collector has a transparent surface 1 which can be made up of one flat glass piate 1 or two such plates 1 and 2 and separates the undemeath components from the environment, thus reducing heat losses. Heat losses can be further reduced by suitably treating the surface 3 of pane 2 facing the receiving surface 4 so that it reflects infra-red radiation backwards.

The receiving surface 4 must be capable of picking up the maximum amount of solar radiation. It can be made of stainless steel or another metal alloy and must be suitably treated so that it selectively absorbs solar radiation but reemits little or no infra-red radiation; thus further reducing heat losses. The surface can be given a suitable shape, e.g. corrugated, in order to facilitate transmission of heat energy to the underneath parts and allow local variations in the volume of the storage material. The advantage of corrugating the receiving surface is two-fold; it absorbs any thermal expansion and also facilitates the expansion of the reacting material surrounding ducts 6; thus improving the heat exchange therewith in both directions.

A reacting substance 5 is disposed down to a depth-of 10 cm below the receiving surface and in contact therewith. The resulting volume, which extends under the entire receiving surface, is for storing and subsequently releasing heat energy.

A system of ducts 6 is provided for conveying the heat-conveying fluid (water or a mixture of water and glycol or another suitable fluid). The ducts are suitably disposed below the receiving surface 4 and, if required, are incorporated in or in contact with surface 4, or are completely immersed in the reacting substance in the storage space. The ducts must be disposed so that the heat energy absorbed from the received energy can be directly used during the hours of sunshine and the previously-stored heat energy can be extracted from the reacting material during the night or when there is no sun.

The receiving surface is the top surface of the module, which also has side surfaces 12, end surfaces 1 3 and a bottom 14. The reacting substance is disposed inside the modular collector and fills all of its interior.

The bottom of all of the modular collectors is protected by a heat-insulating system 1 5 made of expanded plastics or fibrous material such as glass wool. The entire assembly is enclosed in a supporting structure for holding all the aforementioned components and materials.

The integrated system described hereinbefore can be mainly constructed as a modular system in which the storage space is obtained by placing elementary prismatic spaces side by side; all the elements having the same dimensions. A suitable number of elementary units placed together can yield a receiving surface and simultaneously provide a heat-storage space having the desired dimensions and capacity.

Each elementary unit can e.g. have a rectangular cross-section 20 cm wide, 10 cm deep under the receiving surface, and from 2 to 3 m long, i.e. the same length as for normal solar collectors. The elementary prismatic units can be connected in groups, e.g. of 10, to form a subassembly held in a single supporting structure and covered by a single transparent surface, in exactly similar manner to that used for constructing ordinary solar collectors.

Figure 1 diagrammatically illustrates one possible embodiment of the integrated system of the present invention comprising a fluid inlet collector 7 and a fluid outlet collector 8 collecting the ends of each elementary unit.

The integrated system can operate in two ways, i.e in the presence of sun and in the absence of sun.

Operation during the hours of sunshine depends on the capacity, i.e. the mean temperature of the heat conveying fluid. Take, for examp!e, a reacting material (i.e. a mixture of doubie salts in water) in which the combination and decomposition reactions occur at about 220C, which is thus the storage temperature.

If the mean temperature of the heat-conveying fluid is below the storage temperature, i.e. 220C in the present case, the system will supply the user with the energy collected from the sun and with any excess energy previously stored.

If the mean temperature of the fluid is equal to the storage temperature, the system supplies the user with the energy collected from the sun, whereas then the mean temperature of the fluid is greater than the storage temperature, the system supplies the user with only part of the collected solar energy and stores the rest for the night hours or, in general, for sunless periods. If the flow of heat-conveying fluid is completely stopped, operation of the system will be limited to storing all the energy collected from solar radiation.

In the absence of sunlight, the heat-conveying fluid operates at a temperature below the storage temperature, so that the stored heat energy can be used.

During the night or during the prolonged absence of sun, the system stabilises at the storage temperature, e.g. at 220C. This temperature is fairly low and, under temperate climatic conditions, the heat losses can easily be restricted so as to ensure a given duration of storage (e.g. a predetermined duration of 100 hours).

During daylight hours when the sun irradiates the receiving surface and the space underneath has to absorb the incident energy, the temperature gradient should be such that incident energy at 22 OC can also be absorbed by the lower layers of the absorbent material. Accordingly, the maximum temperature variation At through the thickness of the storage material must not be sufficient to produce excessive temperatures at the receiving surface; on the other hand the absorbent material must not be thick enough to disperse heat irrecoverably within its deeper layers.

The extremes or maximum-temperature conditions occur when the sun is in the zenith and the energy collected by the receiving surface is not used directly but is all stored in the lower layers of the absorbing material, i.e. up to 10 cm from the surface in the illustrated embodiment.

We shall assume that the incident radiation has a maximum heat flux of about 800 W/m2, that good solar collectors having a selective surface have an efficiency of 75% corresponding to a heat flux of 600 W/m2, and that the thermal conductivity of the absorbing material at the temperature in question, in the present case of an aqueous solution of inorganic salts, is 600 x 10-3W/m2C.

In that case, the maximum possible temperature variation, relative to the storage temperature, through the entire thickness of 10 cm of reacting material, is: 0.1 mx 600 W/m2 (1)AT= = 100 C 600 x 1 0-3 W/mOC, i.e. the temperature of the absorbing surface can reach a maximum of about 1200C, i.e. the maximum temperature intended for flat collectors having selectively absorbent surfaces.

Other reacting systems capable of storing thermal energy by means of a chemical reaction which is reversible over a temperature range between 50 and 500C, can be designed in a similar manner to the previously-described system. In the case of systems used at about 5 C, i.e. near the average air temperature in winter, the systems can be installed without protective glass and with their receiving surface directly exposed, so as to absorb and store not only solar radiation but also the heat energy supplied by the ambient air during the warmest hours of the day. These systems can be associated with a heat pump for using the heat energy at an acceptable temperature level.

Further heat-insulation problems arise in the case of systems capable of storing heat energy at a temperature of 500 C.

Figure 3 shows an embodiment of an integrated system of the present invention operating at 500 C. The reacting substance 5, which surrounds oppositely-disposed ducts 6 for a heat-conveying fluid, is enclosed in a doublewalled cylindrical glass vessel.

The two concentric cylindrical parts 9, 10 can thus form a single structure made completely of glass and enclose a space 11 which can easily be evacuated when the container is constructed. The outer wall 10 is preferably transparent whereas the inner wall 9 can be blackened and made opaque in order to improve the absorption of incident radiation. The two walls can be corrugated to improve the absorption of differential thermal expansion. The inlet collector 7 and the outlet collector 8 for heat-conveying fluid are preferably disposed on the outside.

The elements shown in Figure 3 can be connected in parallel orin series as required.

Claims

1. An integrated system for collecting and storing heat energy, comprising a modular greenhouse-effect solar collector having a selective energy-receiving surface, each module comprising the selectively-absorbing receiving surface, a duct for conveying a heat-conveying fluid and a material which undergoes a reversible thermochemical reaction (with or without a change of physical state) at a high specific density of energy storage, wherein the material is disposed in close contact with the surface and surrounds the duct which, at the ends of the module, is connected to common collectors into and from which the fluid is supplied and withdrawn, respectively.

2. An integrated system as claimed in claim 1 wherein the receiving surface is corrugated.

3. An integrated system as claimed in claim 2 wherein the duct for conveying the heatconveying fluid is mechanically secured to or made integral with the receiving surface along a frame comprising the lower portions of the corrugations of the receiving surface.

4. An integrated system as claimed in any one of the preceding claims wherein the thermochemically-reacting material fills the space inside the modular collector.

5. An integrated system as claimed in any one of the preceding claims wherein the space filled with the thermochemically-reacting material extends to a distance of about 10 cm from the receiving surface.

6. An integrated system as claimed in any one of the preceding claims wherein the duct for conveying the heat-conveying fluid is completely immersed in the thermochemically-reacting material.

7. An integrated system as claimed in any one of claims 1 to 5 wherein the thermochemicallyreacting material is completely in contact with the receiving surface and with the duct for supplying the heat-conveying fluid.

8. An integrated system as claimed in any one of the preceding claims wherein the receiving surface is protected by a screen which is transparent to light and which comprises two superposed glass plates separated by a gap; the glass surface nearer to and facing the receiving surface being treated so as to reflect infra-red radiation emitted by the receiving surface.

9. An integrated system as claimed in any one of claims 1 to 7 wherein the receiving surface is protected by a screen which is transparent to light and which comprises a glass plate whose surface facing the receiving surface is treated so as to reflect infra-red radiation emitted by the receiving surface.

10. An integrated system as claimed in any one of the preceding claims wherein each modular collector is made up of a cylindrical element comprising two coaxial, corrugated, parallel glass vessels enclosing an evacuated space, the inner cylindrical vessel being blackened and the common interior being filled with a thermochemically-reacting material capable of storing energy at a high specific density and wherein the duct for conveying the heatconveying fluid is disposed in a helix around the axis of the cylindrical element all along its length, is embedded in the thermochemically-reacting material, and is connected, at the ends of the cylindrical element, to common collectors into and from which the fluid can be supplied and withdrawn, respectively.

11. An integrated system for collecting and storing heat energy substantially as hereinbefore described with reference to and as illustrated in either Figures 1 and 2 or Figure 3 of the accompanying drawings.