GB1587725A

GB1587725A - Thermal storage

Info

Publication number: GB1587725A
Application number: GB25546/76A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-09-02
Filing date: 1977-09-02
Publication date: 1981-04-08

Description

(54) THERMAL STORAGE (71) I, DONALD JAMES HIGHGATE, a British subject, formerly of Brambles, Wonham Way, Gomshall, Guildford, Surrey and now of Meopham Trading Estate, Meopham, Gravesend, Kent DA13 OLT, 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: The present invention relates to thermal storage apparatus.

It is sometimes desired to have a reservoir of low or high temperature matter which, through heat exchange, can be used to cool or heat, respectively, other matter. For high temperature storage it is known to use bulk solid materials, e.g. ceramic brick, as the heat storage or reservoir material; for example, for industrial purposes it is known to heat an array of bricks by contact with a hot gas and then to pass a cool gas around the bricks in order to heat this gas. Such known systems rely on the specific heat of the storage material and have the disadvantage that, as the storage material is a bulk solid, the rate of heat input and output is severely restricted by the thermal conductivity of the storage material.For low temperature storage it is known to make use of the latent heat of a solid-solid eutectic phase change but, again, such systems have the above-mentioned disadvantage arising from the use of a bulk solid as the storage material.

Thermal storage apparatus according to the invention comprises (1) a storage vessel containing a fluid in which are suspended particles impregnated with a heat reservoir substance that can undergo, while remaining within the particles, a reversible phase change with release or absorption of latent heat or a reversible chemical reaction with release or absorption of heat of reaction, (2) means for maintaining the particles in mobile suspension and (3) a first heat exchanger for applying or withdrawing sufficient heat to cause the reversible phase or chemical change.

This apparatus permits a high rate of heat output and input. In the invention heat is stored and transferred using this apparatus by transferring heat between the mobile suspension and a material that can be cooled or heated, thereby effecting a reversible phase change or reversible chemical reaction in the heat reservoir substance while it remains with the particles, and subsequently transferring heat between the suspension and the material that can be heated or cooled, and thereby effecting the reverse phase change or chemical reaction, the heat reservoir sub stance still remaining within the particles and thus being ready for reuse.

The apparatus can be used for high tem perature storage i.e. above ambient tempera ture or for low temperature storage i.e. below ambient temperature.

The first heat exchanger mentioned is pre ferably within the storage vessel, e.g. there may be ducting through the vessel through which a hot or cold fluid may be passed for heating or cooling the suspension. Alter natively, the heat exchanger may be exter nal to the storage vessel e.g. there may be ducting for leading the particles to an exter nal heat exchanger for heating or cooling suspension and ducting leading back from the heat exchanger to the vessel; in this case it will usually be desirable for the apparatus to include a pump for circulating the suspen sion through the heat exchanger.

Once the suspension has been brought to a suitably high or low storage temperature, it may be used for heating or cooling other material. For this purpose, the apparatus may comprise a second heat exchanger whereby heat can be transferred to or ex tracted from other matter. For example, if ft is desired to heat or cool a fluid, the fluid in question may be passed through ducting in the storage vessel; if it is desired, for example, to cool air in a chamber, the duct ing may be open-ended, a fan being used to draw in warm air and to expel cooled air.

Alternatively, the matter in the storage vessel may be passed, e.g. pumped, through an external heat exchanger where it transfers heat to or extracts heat from the fluid to be heated or cooled.

It is necessary for the particles to be maintained in suspension, at least during those periods when heat is being transmitted in or out of the suspension and so the apparatus includes suitable means for maintaining the particles in suspension.

The means must be such as to maintain the particles in a mobile suspension, i.e.

moving freely within the fluid of the suspension, in order to ensure rapid heat exchange.

The means may be a stirrer or other mechanical agitator if, as is preferred, the fluid of the suspension is a liquid and its nature should be chosen to suit the nature of the suspension in question as the tendency of solid particles to separate out from suspensions in liquids depends both on the particles and on the liquids. The means may be operated continuously or only during periods of heat extraction or input and continuous operation has the merit or avoiding any risk of the particles separating out and subsequently being difficult to re-suspend in the liquid.

Instead of using a mechanical agitator, the particles may be maintained in mobile suspension by suitably circulating by means of a pump, the matter in the vessel through internal or external ducting.

If the suspending fluid is a gas, usually air, the solid particles are kept in suspension by virtue of the gas being made to circulate through the vessel and in this case the means for maintaining the particles in suspension comprises a pump and ducting, usually external, for circulating the gas. Some or all of the particles may be circulated together with the gas.

The impregnant into the particles is preferably material that can undergo a reversible phase change with release or absorption of latent heat or fusion, that is to say it can melt or solidify.

If the apparatus is to be used for high temperature storage, heat is supplied to the suspension in the storage vessel and initially this serves merely to increase the temperature of the suspending fluid and supended solid particles. However, once the phase change temperature is reached, the input of further heat supplies the heat necessarv to cause melting of the phase change material.

Once all this has melted, further heat may be supplied in order to increase the temperature further if it is desired that the storage temperature should be above the melting point of the material.

In the case of high temperature storage, when the stored heat is used to heat other material the above process is reversed and, as at least part of the available stored heat is stored in the form of the latent heat of fusion of the phase change material, when the temperature of the stored matter falls, through the extraction of the heat, to the melting point of the solid, heat can then be extracted without a further fall in temperature until the matter in the original particles has solidified again.

For low temperature storage, the changes involved in obtaining the desired storage temperature and using the suspension to extract heat from other material are analogous to the changes discussed above in connection with high temperature storage except that, of course, the initial stage is extraction of heat from the storage matter and the use stage is input of heat to the storage matter.

In the case of low temperature storage, the melting point of the phase change material should be below ambient temperature.

When the suspending fluid is a liquid the pressure in the vessel is normally substantially atmospheric.

Since in the invention the material storing the heat, namely the phase change substance, is present as a suspension this greatly facilitates rapid heat input and extraction since there can be excellent contact between the particles containing the substance and the gas or liquid to be heated or cooled and this means that, although a solid is used, the rate of heat input and output is little affected by the thermal conductivity of the solid. Furthermore, the fact that the heat or "cold" is stored partly by virtue of the latent heat of fusion of the solid means that the thermal storage capacity of the apparatus is greater than if only the specific heat of the storage matter was utilised. Thus, for a given mass of storage matter, a greater thermal storage capacity is obtained than if the storage capacity resulted only from the specific heat of the storage matter.

The storage apparatus is particularly useful in circumstances where a source of heat or a means of extracting heat is required intermittently and is particularly beneficial where the availability and/or price of power, e.g. electric power, varies intermittently. For example, for heating domestic, office or factory accommodation the bulk of the demand is usually during daytime and it is just then that there is usually the greatest demand on power, e.g. electric power, supplies. Likewise, for air-conditioning systems it is usually again during the daytime that the maximum capacity for extracting heat is required. Accordingly, the apparatus according to the invention is particularly valuable in such circumstances, where the storage matter can be brought to the desired storage temperature using the more available and/-or cheaper power supply available at night.In these circumstances the fact that the apparatus of the invention permits rapid rates of heat input and output is particularly advantageous.

Since it is preferred that the phase change occurs exclusively within the impregnated solid particles when the phase change substance is a substance which reversibly changes from solid to liquid phase it is preferred that any suspending liquid used for forming the suspension is immiscible with the liquid form of the phase change substance.Furthermore, the phase change substance must normally have a greater affinity for the solid particles in which it is impregnated than is the case for any suspending liquid since, otherwise, then phase change substance when in its liquid state would tend to be displaced from the solid particles by the suspending liquid and thus, subsequently, the phase change would not occur within the solid particles, However, if the impregnated particles are given a coating, preferably flexible, of a material, e.g. an epoxy resin, that is impervious to the liquid form of the phase change substance and/or to any liquid intended to be used as suspending fluid, the above conditions concerning immiscibility and affinity need not be met.

For low temperature storage particularly satisfactory materials for the solid particles of the modified storage matter are polymers that are swellable in, but not soluble in, the liquid form of the phase change substance.

With water as the phase change substance, a wide variety of hydrophilic polymers are suitable, e.g. poly(hydroxyethyl methacrylate) and methyl methacrylate-vinyl pyrrolidone polymers. Such polymers may be made in powder form, hydrated with water and then, to form the storage matter, suspended in a suitable liquid, e.g. a light oil.

Whilst hydrophilic polymers may be swollen in water, as is useful for low temperature storage at about 0 0C, they are generally also swellable in a variety of organic solvents, especially polar ones, in particularly hydroxy compounds such as alcohols. Accordingly, for low temperature storage at temperatures below 0 C, hydrophilic polymers swollen in solvents such as alcohols may be used. In this case the suspending fluid could be a silicone oil or fluorocarbon oil. Furthermore, as the solid to be impregnated, a variety of polymers that are not hydrophilic e.g. rubbers may be used as long as the polymer is so selected as to be swellable, but insoluble, in the liquid form of the phase change -substance and, in this case, the phase change substance will usually be a non-polar one e.g. xylene.For such nonhydrophilic polymers, oils e.g. silicone oils could be used as the suspending fluid.

The use of swellable polymers as the solid to be impregnated in the modifield storage matter is very convenient for low temperature storage but suitable solids (and impregnating substances) are also available for high temperature storage. For example, for high temperature storage the solid may be - a microporous material such as porous ceramic or a metal foam, Tn this case the phase change substance may be a metal salt e.g.

sodium chloride and the fluid may be a liquid that has a higher boiling point than the melting point of the metal salt or other impregnant and that is immiscible with the liquid form of the impregnant. Alternatively, the suspending fluid may be a gas. Another example for high temperature storage is an alloy or low melting metallic element impregnated into, e,g. ceramic, particles, the particles being suspended in a high boiling immiscible liquid, or preferably, a gas.

In order to maximise the benefit obtained by utilising the phase change heat it is desirable that the proportion of the solid particles in the suspension should be as high as possible but in practice this has to be balanced against the fact that in the case of suspensions with very high solids contents there may be practical difficulties involved in keeping the solids in suspension.

Furthermore, with suspensions of very high solids contents it may be difficult to pump the suspension through any ducting forming part of the apparatus. In the case of liquids as the suspending fluid it is usually desirable for the volume concentration of the particles in the suspension to be at least 60%, e.g.

6080% although higher concentrations e.g.

up to 90% may sometimes be practicable.

High solids concentrations are facilitated if the ratio of the diameter of the largest particles to that of the smallest is from 6:1 to 10:1. A wide range of average particle sizes are suitable and in the case of liquids as the suspending fluid it is usually preferred that the size should not exceed 5 mm, 1-2 mm being a preferred range. With gases as the suspending fluid, the average particle size is preferably from 0.01-2 mm.

The concentration of the phase change substance in the particles is desirably as high as possible and, in the case of polymers, e.g.

hydrophilic polymers, swollen by a liquid, e.g. water, there may be as much as four parts by weight of the impregnating substance per part by weight of the solid in its unimpregnated form. Where the solid that is impregnated is other than a polymer, there are normally no more than two parts by weight of the impregnating substance per part by weight of the solid before impregnation.

When the fluid of the suspension is a gas usually slightly more energy is required to keep the particles in suspension than in the case of liquid suspensions but this is un important as this energy is usually only about 5 to 10% of the energy that can be stored. Where the- suspending fluid is a gas it is normaly advantageous to allow the particles to separate out if the apparatus is not in use or being ''charged", as this saves energy and the separated mas of particles is a good insulator and only loses or gains heat slowly. Furthermore the use of a gas as the suspending fluid can have the advantage, for high temperature storage systems, that it permits higher storage temperatures because, in the case of liquids as the suspending fluid, the maximum- storage temperature is limited by the boiling point of the liquid.

Instead of using as the phase change substance a material that will release or absorb the latent heat of fusion during the phase change similar effects, especially for high temperature storage, are obtained when using a heat reservoir substance that will undergo a reversible chemical change with release or absorption of heat of reaction. Normally a substance is used that can be dehydrated with the evolution of heat and the hydrated form being convertible to the dehydrated form by raising the temperature. In this form of the apparatus the essential property of the substance used to impregnate the solid particles is that it should be reversibly hydratable, hydration being accompanied by evolution of heat. Thus, in this form of the apparatus the thermal storage capacity is provided, at least to some extent, by the heat of hydration of the impregnant.

The suspending fluid must contain, at least when the storage matter is being used to impart heat to other matter, water or water vapour. On the other hand, when the suspension is being brought up to the desired storage temperature, it is not necessary for the suspending fluid to contain water or water vapour as, indeed, at this stage the object is to remove water from the hydrated impregnant.

It is desirable for the impregnant to have a high heat of hydration and many ionic inorganic compounds are suitable. For example, the impregnant may be sodium hydroxide or calcium hydroxide and in such cases the hydration results in a relatively minor change in the chemical properties of the dehydrated impregnant. However, the dehydrated form of the impregnant may be a compound which reacts with water to give what would normaly be regarded as an entirely distinct chemical compound rather than merely a hydrated form of the same compound. The solid to be impregnated should remain a solid throughout the range of temperatures to which it may be desirable to subject the storage matter and the impregnant should not be vaporised within this range although it is harmless if it undergoes a solid-liquid phase change in this range.

Preferably the suspending fluid is a gas but if a liquid is used it is usually desirable to coat the particles with a hydrophilic polymer in order to prevent loss of the impregnant from the particles into the suspending liquid whilst still permitting loss or absorption of water.

A specific Example of the invention is given below.

In a storage vessel having a capacity of about 75 gallons the cooling element of a 1.5 - Kw deep freezer unit was fitted in order to provide a heat exchanger for cooling matter in the vessel. The vessel was also fitted with a water circuit, part of this circuit being in the form of ducting passing through the vessel, this ducting providing a heat exchanger for cooling water in the circuit.

The external part of the circuit provides the "output" part of the apparatus e.g. serves as a heat exchanger for cooling the air which the apparatus is ultimately intended to cool e.g. air in an air conditioner. The water in the circuit may circulate by convection or if desired a pump may be provided in the circuit.

The vessel was filled with a light liquid hydrocarbon in which were suspended particles of an acrylonitrile-vinyl pyrrolidone polymer hydrated with water in an amount of two parts by weight of water per part by weight of anhydrous polymer. The average particle size of the particles was about 5 mm and they formed about 60% of the volume of the suspension. At the top of the vessel was an outlet leading into external ducting which in turn led, via a pump, to an array of inlet holes in the base of the vessel. In use, the pump is operated and serves to circulate the suspension from the top of the vessel to its base and this circulation of the suspension serves to maintain the particles in suspension in the hydrocarbon. To use the apparatus, the deep freezer unit is operated and at this stage a valve in the water circuit may be closed. The temperature of the circulating suspension gradually falls and in due course the water in the hydrated polymer freezes, at this stave the latent heat of fusion of the water being extracted. The stored matter is then at a temperature of about 0 C and can be used to cool water in the water circuit, any valve in the water circuit being open at this stage and circulation of the suspension being continued. The apparatus has a storage capacity of about 20 Kwh. i.e.

this is the amount of energy needed to bring the cold stored matter to ambient temperature.

WHAT I CLAIM IS:- 1. Thermal storage apparatus comprising (1) a storage vessel containing a fluid in which are suspended particles impregnated

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. important as this energy is usually only about 5 to 10% of the energy that can be stored. Where the- suspending fluid is a gas it is normaly advantageous to allow the particles to separate out if the apparatus is not in use or being ''charged", as this saves energy and the separated mas of particles is a good insulator and only loses or gains heat slowly. Furthermore the use of a gas as the suspending fluid can have the advantage, for high temperature storage systems, that it permits higher storage temperatures because, in the case of liquids as the suspending fluid, the maximum- storage temperature is limited by the boiling point of the liquid. Instead of using as the phase change substance a material that will release or absorb the latent heat of fusion during the phase change similar effects, especially for high temperature storage, are obtained when using a heat reservoir substance that will undergo a reversible chemical change with release or absorption of heat of reaction. Normally a substance is used that can be dehydrated with the evolution of heat and the hydrated form being convertible to the dehydrated form by raising the temperature. In this form of the apparatus the essential property of the substance used to impregnate the solid particles is that it should be reversibly hydratable, hydration being accompanied by evolution of heat. Thus, in this form of the apparatus the thermal storage capacity is provided, at least to some extent, by the heat of hydration of the impregnant. The suspending fluid must contain, at least when the storage matter is being used to impart heat to other matter, water or water vapour. On the other hand, when the suspension is being brought up to the desired storage temperature, it is not necessary for the suspending fluid to contain water or water vapour as, indeed, at this stage the object is to remove water from the hydrated impregnant. It is desirable for the impregnant to have a high heat of hydration and many ionic inorganic compounds are suitable. For example, the impregnant may be sodium hydroxide or calcium hydroxide and in such cases the hydration results in a relatively minor change in the chemical properties of the dehydrated impregnant. However, the dehydrated form of the impregnant may be a compound which reacts with water to give what would normaly be regarded as an entirely distinct chemical compound rather than merely a hydrated form of the same compound. The solid to be impregnated should remain a solid throughout the range of temperatures to which it may be desirable to subject the storage matter and the impregnant should not be vaporised within this range although it is harmless if it undergoes a solid-liquid phase change in this range. Preferably the suspending fluid is a gas but if a liquid is used it is usually desirable to coat the particles with a hydrophilic polymer in order to prevent loss of the impregnant from the particles into the suspending liquid whilst still permitting loss or absorption of water. A specific Example of the invention is given below. In a storage vessel having a capacity of about 75 gallons the cooling element of a 1.5 - Kw deep freezer unit was fitted in order to provide a heat exchanger for cooling matter in the vessel. The vessel was also fitted with a water circuit, part of this circuit being in the form of ducting passing through the vessel, this ducting providing a heat exchanger for cooling water in the circuit. The external part of the circuit provides the "output" part of the apparatus e.g. serves as a heat exchanger for cooling the air which the apparatus is ultimately intended to cool e.g. air in an air conditioner. The water in the circuit may circulate by convection or if desired a pump may be provided in the circuit. The vessel was filled with a light liquid hydrocarbon in which were suspended particles of an acrylonitrile-vinyl pyrrolidone polymer hydrated with water in an amount of two parts by weight of water per part by weight of anhydrous polymer. The average particle size of the particles was about 5 mm and they formed about 60% of the volume of the suspension. At the top of the vessel was an outlet leading into external ducting which in turn led, via a pump, to an array of inlet holes in the base of the vessel. In use, the pump is operated and serves to circulate the suspension from the top of the vessel to its base and this circulation of the suspension serves to maintain the particles in suspension in the hydrocarbon. To use the apparatus, the deep freezer unit is operated and at this stage a valve in the water circuit may be closed.The temperature of the circulating suspension gradually falls and in due course the water in the hydrated polymer freezes, at this stave the latent heat of fusion of the water being extracted. The stored matter is then at a temperature of about 0 C and can be used to cool water in the water circuit, any valve in the water circuit being open at this stage and circulation of the suspension being continued. The apparatus has a storage capacity of about 20 Kwh. i.e. this is the amount of energy needed to bring the cold stored matter to ambient temperature. WHAT I CLAIM IS:-

1. Thermal storage apparatus comprising (1) a storage vessel containing a fluid in which are suspended particles impregnated

with a heat reservoir substance that can undergo, while remaining within the particles, either a reversible phase change with release or absorption of latent heat of fusion or a reversible chemical reaction with release or absorption of heat of reaction, (2) means for maintaining the particles in mobile suspension and (3) a first heat exchanger for applying or withdrawing sufficient heat to cause a reversible phase or chemical change.

2. Apparatus according to claim 1 also comprising a second heat exchanger for effecting transfer of the latent heat or the heat of reaction between the suspension and matter to be cooled or heated.

3. Apparatus according to claim 1 or claim 2 in which the means for maintaining the particles in mobile suspension comprise a mechanical agitator in the vessel.

4. Apparatus according to claim 1 or claim 2 in which the means for maintaining the particles in mobile suspension comprise ducting, inside or outside the vessel, through which the suspension in the vessel may be circulated.

5. Apparatus according to any preceding claim in which the particles are formed of polymeric material and the heat reservoir substance is a liquid at ambient temperature.

6. Apparatus according to claim 5 in which the polymer is a hydrophilic polymer and the heat reservoir substance is water.

7. Apparatus according to claim 6 in which the polymer is polyhydroxyethylmethacrylate or is a methylmethacrylate vinyl pyrrolidone copolymer.

8. Apparatus according to any of claims 5 to 7 in which the fluid of the suspension is an oil.

9. Apparatus according to any of claims 1 to 4 in which the impregnated particles are ceramic or of a metal that will remain solid during the phase change.

10. Apparatus according to claim 9 in which the heat reservoir substance is a fusible metal salt.

11. Apparatus according to claim 9 in which the heat reservoir substance is a fusible metal or metal alloy.

12. Apparatus according to any of claims 1 to 4 or 9 in which the heat reservoir material is an inorganic compound that can be reversibly hydrated or dehydrated, hydration occurring upon heating and dehydration occurring with evolution of heat upon contact with water and the suspension includes sufficient water for hydration.

13. Apparatus according to claim 12 in which the heat substance is calcium hydroxide.

14. Apparatus according to claim 1 substantially as herein described.

15. A method of storing and transferring heat using apparatus according to any preceding claim comprising transferring heat between the mobile suspension and a material that can be cooled or heated and thereby effecting the reversible phase change or reversible chemical reaction in the heat reservoir substance while it remains within the particles, and subsequently transferring heat between the mobile suspension and a material that can be heated or cooled, and thereby effecting the reverse phase change or chemical reaction.