GB2143316A

GB2143316A - Method and means for heat storage

Info

Publication number: GB2143316A
Application number: GB08414972A
Authority: GB
Inventors: Douglas William Bingham
Original assignee: Hawker Siddeley Power Engineering Ltd
Current assignee: Hawker Siddeley Power Engineering Ltd
Priority date: 1983-06-18
Filing date: 1984-06-12
Publication date: 1985-02-06
Also published as: GB8414972D0; GB2143316B

Abstract

Heat is periodically charged into a heat storage medium 12 in an excavated hole 10 in the ground having heat-dissipating side and bottom walls and a heat-insulating top cover provided by insulating blocks 14 and soil layer 15. The heat storage medium is at least partly in the form of a material, e.g. slack wax, which is liquid over a range of working temperatures at which heat is to be stored and solid at the containment temperature and which acts as heat insulation and a liquid barrier when solid. A solidified layer 20 of the material is thus created between the side and bottom walls 19 and 18 of the hole 10 and a body of liquid material in the central part of the containment, heat being charged into and discharged from the body of liquid material. A sheet steel barrier 21 protects the solidified layer 20 at the side walls 19 from liquid circulation in the region of heat input and output pipe coils 16 and 17. The heat storage wax may be impregnated in washed gravel. <IMAGE>

Description

SPECIFICATION Method and means for heat storage This invention is concerned with heat storage.

Heat stores using a liquid heat storage medium in the form of water or an aqueous solution have been proposed for storing heat (e.g. heat from solar heat collectors or waste heat recovered from industrial processes) for subsequent use.

It is an object of the present invention to provide an improved method and means for heat storage using a liquid heat storage medium.

The invention provides a method of heat storage in which heat is periodically charged into a heat storage medium in a containment having a heat-dissipating side and bottom walls and a heat-insulating top cover and heat is periodically discharged from the medium, and in which the heat storage medium is at least partly in the form of a material which is liquid over a range of working temperatures at which heat is to be stored and solid at the containment temperature and which acts as heat insulation when solid, whereby a solidified layer of the material is created between the side and bottom walls of the containment and a body of liquid material in the central part of the containment, heat being charged into and discharged from said body of liquid material. The solidified layer therefore acts as heat insulation to reduce loss of heat from the liquid material to the containment.The solidified layer is maintained in solidified form by heat dissipation through the containment.

Normally, the body of material in the central part of the containment will be kept at working temperatures continuously during a plurality of consecutive heat charging and discharging cycles.

The invention further provides heat storage means comprising a heat-dissipating containment having heat-dissipating side and bottom walls and a heat-insulating top cover, a heat storage medium in said containment, which heat storage medium is at least partly in the form of a material which is liquid over a range of working temperatures at which heat is to be stored and solid at the containment temperature and which acts as heat insulation when solid, and means for charging heat into and discharging heat from the heat storage medium in a central part of the containment, whereby material in said central part can be kept liquid while a solidified layer of said material separates said central part from the side and bottom walls of the containment.

The heat charging and discharging means will usually be located in said central part of the containment.

Thus, for low energy flux applications, they may comprise submerged pipes (e.g. in the form of coils) containing a flowing process fluid which loses or gains heat to or from the heat storage medium by natural convection and conduction. For high energy flux applications, the liquid heat storage material may be pumped across the pipes to achieve forced convective and conductive heat transfer.

Alternatively, also for high energy flux applications, the heat charging and discharging means may comprise a heat exchanger and a pump for pumping the liquid heat storage material through the heat exchanger.

If desired, the heat charging means may comprise electric heating means located in the central part of the containment.

Since the liquid material will tend to circulate when the heat is charged into or discharged from it, preferably an upright barrier is provided round the inside of the containment spaced away from the side walls thereof. This barrier, which preferably terminates short of the bottom wall of the containment, prevents the main body of cirulating liquid material from contacting and breaking down the solidified layer round the side walls.

The invention has the important advantage that porous material, e.g. sand, gravel or earth, may be used for the containment since the solidified layer, as well as providing heat insulation, also provides an impervious barrier preventing escape of liquid material into the porous containment. Thus, the containment may be in the form of a hole in the ground with substantial cost savings corn pared with a containment fabricated from metal or other non-porous material. Excavated soil from the hole may be used in forming the cover for the containment, preferably over a layer of insulation.

The aforesaid heat storage material is preferably an organic material, e.g. a wax. It is particularly preferred to use slack wax. Slack wax is a petroleum refinery by-product. Typically, it may have the following properties: Working temperature 250-300"C Flash point 300"C Spontaneous combustion approx. 375"C Phase change range 60-70"C Full solidity (candle wax type consistency) 50"C Specific gravity (liquid) 0,8 Specific heat (liquid) approx..42 Viscosity at 100 C 20 Centistoke Approx thermal conductivity (solid) (0,145 Btu/Ft/h/"F) (0,251 Wlm/ K) The heat storage medium may also comprise solid granular or particulate material (e.g. washed gravel) impregnated by the heat storage material (e.g. slack wax).

The foliowing is a description, by way of example, of one embodiment of the invention, reference being made to the accompanying schematic drawing of a heat store.

The heat store comprises a containment 10 in the form of a hole excavated in the ground. The hole is filled up to level 11 with heat storage medium 12 consisting of washed gravel impregnated with slack wax. Above level 11,the hole contains a layer 13 of washed gravel alone. A blanket 14 of insulation blocks lies on top of the layer 13 and a cover 15 of excavated soil is provided on the blanket 14. Heat input coils 16 are provided near the bottom of the heat storage medium 12 and heat extraction coils 17 are provided near the top thereof. The slack wax in the central part of the containment is kept at working temperatures at which it is liquid.However, owing to heat losses to the heat sink constituted by the ground at the bottom wall 18 and round the side walls 19 of the containment hole, the slack wax round the bottom and sides of the hole is maintained in the form of a solidified layer 20 which provides insulation for the liquid wax and which prevents escape of liquid wax into the ground. Extending round inside the hole is a vertical barrier 21 (e.g. of light sheet steel). The barrier extends down through the gravel layer 13 and the heat storage medium 12 and terminates adjacent but short of the top of the solidified layer at the bottom of the hole. The barrier is disposed between the coils 16, 17 and the side walls 19 of the hole, being spaced away from the sides 19.The spaces between the barrier and the sides contain liquid wax nearer the barrier and the solidified layer 20 of wax nearthe side walls 19 but circulation of liquid wax which is most vigorous near the coils 16, 17 is prevented by the barrier from damaging the soldified layer.

The heat store described with reference to the drawings is suitable for storage of high grade heat at a temperature adequate for heat to mechanical or electrical energy conversion via either an organic Rankine cycle or low pressure stream turbine. Using slack wax the working temperature would, as previously indicated be 250-300"C. Solidification of the wax would occur at 50-60"C and a suitable ground temperature to cause solidification would be in the range 10-25"C, although lower temperatures could of course be used.

Suitably, the store might contain at least 300 m3 of slack wax in a washed gravel bed about 3 m deep.

Preferably, the store would be charged and discharged regularly within a reasonably short period, e.g. 24 hours. Suitable applications are for storing heat from solar heat collectors or from process industries where a continuous but variable waste heat production is in imbalance with energy demand from recovery systems.

Claims

1. A method of heat storage in which heat is periodically charged into a heat storage medium in a containment having heat-dissipating side and bottom walls and a heat-insulating top cover and heat is periodically discharged from the medium, and in which the heat storage medium is at least partly in the form of a material which is liquid over a range of working temperatures at which heat is to be stored and solid at the containment temperature and which acts as heat insulation when solid, whereby a solidified layer of the material is created between the side and bottom walls of the containment and a body of liquid material in the central part of the containment, heat being charged into and discharged from said body of liquid material.

2. A method according to claim 1 wherein the body of material in the central part of the containment is kept at working temperatures continuously during a plurality of consecutive heat charging and discharging cycles.

3. Heat storage means comprising a heat-dissipating containment having heat-dissipating side and bottom walls and a heat-insulating top cover, a heat storage medium in said containment, which heat storage medium is at least partly in the form of a material which is liquid over a range of working temperatures at which heat is to be stored and solid at the containment temperature and which acts as heat insulation when solid, and means for charging heat into and discharging heat from the heat storage medium in a central part of the containment, whereby material in said cental part can be kept liquid while a solidified layer of said material separates said central part from the side and bottom walls of the containment.

4. Heat storage means according to claim 3 in which said heat charging and discharging means are located in said central part of the containment.

5. Heat storage means according to claim 4 in which the heat charging and discharging means comprise pipes for the passage of fluid which loses or gains heat to or from the heat storage medium.

6. Heat storage means according to claim 3 in which said heat charging and discharging means comprise a heat exchanger and a pump for pumping the aforesaid liquid material through the heat exchanger.

7. Heat storage means according to any of claims 3 to 6 in which said heat charging means comprise electric heating means located in said central part of the containment.

8. Heat storage means according to any of claims 3 to 6 in which an upright barrier is provided round the inside of the containment spaced away from the containment side walls.

9. Heat storage means according to claim 8 in which the barrier terminates short of the containment bottom wall.

10. Heat storage means according to any of claims 3 to 9 in which the containment is a hole in the ground.

11. Heat storage means according to claim 10 in which the containment is a hole excavated in the ground and the cover includes excavated material.

12. A method according to either of claims 1 or 2 or heat storage means according to any of claims 3 to 11 in which said material is an organic material.

13. A method or heat storage means according to claim 12 in which said material is wax.

14. A method or heat storage means according to claim 12 in which said material is slack wax.

15. A method according to any of claims 1,2, 12, 13 or 14 or heat storage means according to any of claims 3 to 12, 13 and 14 in which the heat storage medium also comprises solid granular or particulate material impregnated by the first-said material.

16. A method of heat storage substantially as hereinbefore described with reference to the accompanying drawing.

17. Heat storage means substantially as hereinbefore described with reference to the accompanying drawing.