GB2463704A

GB2463704A - Solar-powered absorption refrigeration system with phase-change heat store

Info

Publication number: GB2463704A
Application number: GB0817409A
Authority: GB
Inventors: Robert Eaton Edwards; Michael Graham Reid
Original assignee: Solar Polar Ltd
Current assignee: Solar Polar Ltd
Priority date: 2008-09-23
Filing date: 2008-09-23
Publication date: 2010-03-24
Also published as: WO2010034991A1; GB0817409D0

Abstract

Cooling apparatus comprises solar collector 2 for converting radiant energy from sun into heat, absorption refrigeration system 1, and phase-change heat store 10, the refrigeration system connected to be driven by heat supplied from heat collector via heat store. System is arranged to cool air in air conditioning system or within enclosed refrigerator cabinet. System may have heat pipe 8 connected to transfer heat from solar collector to heat store. Method comprises converting radiant energy from sun into heat using solar collector, applying heat during the day to solid/liquid phase-change heat store causing phase change of medium, and applying heat from heat store to absorption refrigeration system so that during evening and night latent heat from heat store used to drive refrigeration system during phase change of medium from liquid to solid. The device is used to power cooling systems in remote parts of world without mains electricity.

Description

Coo1in Apparatus This invention relates to a cooler, particularly an air cooler, and is applicable to the cooling of air in rooms e.g. as part of an air conditioning system; or the cooling of air in a confined space such as a refrigerator.

Many parts of the world that suffer high climatic temperatures, where refrigeration and air conditioning are therefore important, are too remote to have a mains electricity supply. Solar power has the potential to resolve this problem but most solar powered refrigeration systems rely on moving mechanical compressors and other parts that are liable to failure. This makes existing solar powered systems unsuitable for prolonged use in regions where there is no facility for repair and maintenance.

Another problem is that most existing designs are electrically driven and employ photoelectric panels to generate the electricity. Unless provided with electric storage facilities, such designs are unable to function during the evening when there is no sunlight. Compounding these problems is often a lack of local skilled maintenance personnel.

The invention provides cooling apparatus comprising a solar collector for converting radiant energy from the sun into heat, an absorption refrigeration system, and a phase-change heat store, the refrigeration system being connected to be driven by heat supplied from the heat collector via the heat store.

By employing the simplest of absorption refrigeration systems the invention makes it possible to eliminate the need for moving parts, allowing the system to function for many years without maintenance. Furthermore, heat stored in the phase change heat store is able to drive the refrigeration system long into the evening after sunset.

The invention can be used for cooling air in an air conditioning system, where there is normally a need for continued operation into the evening but not throughout the night. The invention is also applicable to refrigeration systems for storage of food or medicines. In such a system, since the space to be cooled is relatively small, the stored latent heat of the phase change material may be sufficient to last throughout the night or at least for sufficient time to ensure that the temperature of air within the relevant space does not rise unacceptably.

The invention is not limited to environments where it is used for cooling air. It could be used for cooling liquids such as drinks; and fluids that require cooling in industrial processes.

It would be possible for the phase-change material to be in direct contact with, or included within, the solar collector but this is not preferred because it would be difficult, without recourse to powered fans or the like, to ensure that heat is efficiently transferred into and out of the phase-change medium. It is therefore proposed that a heat pipe be included having its hot end (ie the end from which heat is delivered) within the solar collector. The relatively cold end of the heat pipe (ie the end to which heat is delivered) and an appropriate part of the refrigeration system can then be arranged so that they are both immersed (or otherwise thermally connected to) the phase-change medium. The latter is preferably contained within a vessel that also contains the generator and the cold end of the heat pipe.

The invention can also be expressed in terms of a method and thus, according to a second aspect of the invention, there is provided a method of cooling comprising converting radiant energy from the sun into heat using a solar collector, applying the said heat during the day to a phase-change heat store so as to cause a phase change from solid to liquid or other solid phase change transformation of a phase-change medium, and applying heat from the heat store to an absorption refrigeration system 50 that, during the night, latent heat from the heat store is used to continue to drive the refrigeration system during a change from liquid to solid of the phase change medium.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which: -Fig 1 is a schematic illustration of the components of an air conditioning system constructed in accordance with the invention; Fig 2 shows a vertical cross-section through a house having a pitched roof and fitted with the system of Fig 1; Fig 3 is a perspective view of a solar collector and a module into which most of the other components shown in Fig 1 are contained; Fig 4 shows a variation of the module design for use on a flat roof or on a rectilinear cabinet for use as a refrigerator; and Fig 5 shows a perspective view of a group of similar modules connected in parallel.

Referring firstly to Fig. 1, there is shown a refrigeration module 1 comprising a solar collector 2 exposed to sunlight on the outside of a roof 3 of a building and a housing 4 mounted inside a roof space defined between the roof 3 and a ceiling 5. The solar collector 2 is formed in this particular example by three evacuated tubes 6 (only one shown for simplicity of description) each having a seal 7. Arrangements having a different number of tubes 6, eg two or four would also be suitable.

The module 1 also includes heat pipes 8, one for each collector 2, containing, in this particular example, water as its operating fluid. The hot end of each heat pipe is located within the heat collector tube and it passes through the seal 7 and through the roof 3 to its cold end within the housing 4.

A heat store is formed by an insulated vessel 9 containing a phase-change material 10 having a melting point of about 200°C in this particular example. Other materials having melting points in the range of 190°C and 220°C would also be suitable. The heat pipe 8 passes through the wall of the heat store vessel 9 so that its colder end is immersed in the phase-change material 10.

A generator 11 containing strong ammonia solution, is immersed in the phase-change material 10 and is connected via a bubble pump 12 and collector 13 to a condenser 14, a trap 15, an evaporator 16, a heat exchanger 17, a heat exchanger 18 and a reservoir 19.

The housing 4 is formed from pressed metal sheet and defines an air duct 20. The evaporator 16 is located in a heat exchange chamber 24 where hot air drawn through port 24A is cooled and flows by convection down through port 24B into a living area of the building. The heat exchange chamber is defined between side walls of the housing 4 and partition walls as shown on Fig 1. These partition walls extend downwardly towards an exit 24B for cool dry air and an exit port 24C for condensed water. The latter can be drained away via a flexible pipe (not shown).

The housing 4 is formed with holes 22 and 23 and with circular lines of weakness defining disc shapes 21, 24A and 24B that can be pushed out to define holes as required. The shapes 21 are formed on opposite parallel vertical faces of the housing 4 in the region of the heat exchange chamber 24. In a single module system the shapes 24A and 24B are pushed out to allow entry of air to be cooled into the chamber 24, and exit of cooled air from it. Where additional modules are connected to the first module, the shapes 21 are removed on the contiguous faces of all adjoining modules so that the heat exchange chambers 24 of all modules are connected, whilst sharing a common entry and exit 24A and 24B provided by just one of them.

It is necessary to heat the generator 11 to a temperature of about 230°C to start the refrigeration cycle but, once started, it will continue to operate unless the temperature of the generator 11 drops to about 190°C or below. Operation is as follows.

Sunlight during the day heats the hot, lower, end of the heat pipe 8. The pipe 8 contains water, which acts as a refrigerant. The resulting water vapour rises to the upper, relatively cold, end of the heat pipe, where it condenses, giving up its heat to the phase change material 10.

The temperature of the phase change material increases until it reaches its phase change temperature of 200°C at which point it remains at that temperature whilst continuing to absorb heat from the heat pipe as it changes phase. When the phase change material has become entirely liquid, its temperature continues to rise again until it reaches 230°C, the start-up temperature of the refrigeration system. The refrigeration system then starts to operate and the temperature of the phase change material drops, say to 2 10°C, as the heat is drawn from it to drive the refrigeration system.

The refrigeration cycle itself is entirely conventional in operating principles as follows.

The generator 11 contains a strong solution of ammonia in water. Heat from the phase change material boils the solution, releasing bubbles of ammonia gas and resulting in weakening of the solution. The bubbles raise the weakened solution to the separator 13 by the action of the bubble pump 12.

In the separator 13, the ammonia gas is separated from the weak ammonia solution and travels to the condenser 14 where heat is released to the air in duct 20 causing the ammonia gas to condense as liquid ammonia. The latter passes through trap 15 into the evaporator where it is exposed to hydrogen gas. The hydrogen environment, lowers the vapour pressure of the liquid ammonia sufficiently to cause the ammonia to evaporate, extracting heat from air in the duct 24. This produces cool, dehumidified air for air conditioning purposes and pure water, which exits from port 24C and can be collected for use.

The ammonia gas and hydrogen mixture passes to the heat exchanger 17 and gains heat from the rising returning hydrogen and then into a heat exchanger 18 where it loses its heat to air within the duct 20 and the ammonia dissolves in the weakened solution from the separator 13, producing a more concentrated solution which flows into the. reservoir 19 and thence to the generator 11 whereupon the cycle is complete.

When the power of the sun becomes insufficient to retain the phase change material above 200°C, the latter starts to solidify and the latent heat of fusion maintains the generator 11 at a sufficient temperature to sustain the refrigeration cycle. In this way the refrigeration mechanism can remain operational throughout the night or at least a sufficient part of it to ensure that cooling is maintained until the ambient temperature drops to an acceptable level.

Figure 2 shows how the various parts that have been described are installed in a building having a pitched roof 3. From this drawing it can be seen that the solar collector 2 lies against the roof surface, on the outside of the building whilst the housings 4 and their contents are in the roof space isolated from the main living area of the building (i.e. the area to be cooled). A chimney 27 connects to the port 23 (or each of the ports where there are multiple modules) to provide improved draft of cooling air.

Figure 3 shows how the housing 4 is formed with parallel flat faces 4A, a sloping edge 4B arranged to accommodate installation close to a pitched roof, a short horizontal top edge 4C formed with vent hole 23 and adapted to be connected to the chimney duct 27 and a relatively long, bottom horizontal edge 2D formed with vent hole 22. The faces 4A have gaskets 72 which provide a seal between adjoining units when they are connected together in the manner described below to give the required power depending on the installation.

Fig. 4 shows a variation where the tubes 2 are angled so as to be perpendicular to the bottom faces 2D of the modules to permit mounting on a wall. Fig 5 shows a modular construction comprising a battery of housings connected physically together, face to face by clips 28.

A system as shown in Fig 4 or 5 can readily be adapted for use as a refrigerator instead of an air conditioning system. In such an arrangement, one or more modules would be mounted on an outer surface (eg the top surface) of an insulated cabinet with pipes analogous to those shown at 25A and 26A on Fig 2 extending through that surface into the cabinet interior so as to circulate and cool air in the cabinet. In this arrangement it is envisaged that the cabinet would normally be located inside a building with the tubes 6 projecting through the outside wall and fixed on and parallel to the outside of the wall to collect solar heat.

It is emphasised that the particular systems that have been described and illustrated are just examples of an unlimited number of variations that are possible within the scope of the invention as defined by the accompanying claims.

Claims

CLAIMS1. Cooling apparatus comprising a solar collector for converting radiant energy from the sun into heat, an absorption refrigeration system, and a phase-change heat store, the refrigeration system being connected to be driven by heat supplied from the heat collector via the heat store.
2. Cooling apparatus according to claim 1 in which the refrigeration system is arranged to cool air in an air conditioning system.
3. Cooling apparatus according to claim 1 in which the refrigeration system is arranged to cool air within an enclosed refrigerator cabinet.
4. Cooling apparatus according to any preceding claim comprising a heat pipe connected to transfer heat from the solar collector to the heat store.
5. A method of cooling comprising converting radiant energy from the sun into heat using a solar collector, applying the said heat during the day to a solid/liquid phase-change heat store so as to cause a phase change of a phase-change medium, and applying heat from the heat store to an absorption refrigeration system so that, during the night, latent heat from the heat store is used to continue to drive the refrigeration system during a change from liquid to solid of the phase change medium.