GB2470400A

GB2470400A - Heat energy collection and storage device comprising a phase change material

Info

Publication number: GB2470400A
Application number: GB0908797A
Authority: GB
Inventors: Douglas Nangle
Original assignee: GEOHEAT Ltd
Current assignee: GEOHEAT Ltd
Priority date: 2009-05-21
Filing date: 2009-05-21
Publication date: 2010-11-24
Also published as: GB0908797D0

Abstract

A heat energy collection and storage device 6 for placing in a volume, such as the ground 4, forming a source or sink of heat energy. The device comprises a container 7 containing a phase change material 8, such as a substance based on paraffin. Pipework 10 may be provided for passing a heat transfer medium, such as a water and anti-freeze mixture, through the phase change material to transfer heat between the heat transfer medium and the phase change material. The phase change material is in a first, preferably liquid phase, at the ambient temperature of the volume and is in a second, preferably solid phase, at the temperature of the heat transfer medium when it enters the device. Heat may be transferred from the volume in contact with the container's surface to the phase change material, or vice versa. Preferably, the device comprises part of a cooling or heating system, such as a heat pump 2, where geothermal energy is used as a heat source. The ambient temperature surrounding the device may be increased by introducing additional solar heat energy.

Description

Heat energy storage device This invention relates to a heat energy storage and release device, more particularly used for storing geothermal energy and subsequently releasing that energy for use on demand. The invention also relates to a heat source for use in a heat pump system.

With the advent of global concerns regarding climate change and escalating energy prices, new sources of energy for use in heating systems are being considered. One such source is geothermal, taken from the ground or areas of water. Systems employing geothermal energy sources use refrigeration technology in the form of heat pumps to remove low-level heat from the ground and compress this heat into higher more usable temperatures for use in domestic central heating and for heating water.

Currently, these systems offer one of the best, most environmentally friendly and practical renewable energy sources available as for every 1 KW of electrical energy used to compress refrigeration gas, almost 4 KW of heat energy can be moved from underground to where required in the home or workplace.

However, such systems can be expensive and recuperation of funds used to install such systems can take many years. A particularly expensive area of the installation is the part of the system used to collect the low level heat from the ground. Several methods are available to the system designer. These include; burying tens of metres of heat transfer pipes below ground level where there is a constant temperature, laying heat transfer pipes in a small lake or river, drilling a bore hole and installing pipes vertically and pumping water directly from below the water table removing the heat from it and then pumping it back again. All the above-mentioned methods involve expensive procedures to install especially when noting that the heat transfer pipes to be positioned underground could be up to and even longer than 200 metres. Also it is to be considered that the majority of homes in a country such as the United Kingdom do not have the available land or lake or river from which to extract heat. For such houses the only available option being to drill a bore hole, up to and possibly beyond 100 metres deep. This latter is expensive, and at present recovery of the capital costs in installing such systems takes very many years.

The reason such lengths, depths or areas under ground or in water are required for extraction of heat is that over extraction must be avoided. If too small a collection area is used, that the temperature of that immediate area will drop significantly in too short a time period to be able to absorb the load from the heat transfer system.

Typically existing ground source energy collection systems only collect heat when it is required. In other words, as and when heat is required a pump may be switched on which will pump water (the heat transfer medium) from the heat pump, down, through the ground source pipes and back to the heat pump collecting a few degrees of heat energy on the way. When no heat is required in the home, no water is pumped and therefore no heat is being removed from the ground. In turn, this means that the length of pipe used or the area of ground collection to be used is critical, having to absorb 100% load when required.

This invention seeks to provide a more efficient method for collecting heat energy from below ground or under water where the temperature is at a constant; and storing enough of that energy to be extracted at the required time for the required time period thus avoiding the deep or very deep boring or pipe laying over large areas.

The present invention provides a heat collection and storage device for placing in a volume forming a source or sink of heat energy, comprising a container containing a phase change material, a means of passing a heat transfer medium through the material to transfer heat to or from heat transfer medium from or to the said material, and in which the phase change material is in a first phase at the ambient temperature of the volume containing the source or sink and, in operation, would be in a second phase at the temperature of the heat transfer medium when entering the device.

In a preferred embodiment heat may be transferred from the volume in contact with the container's surface to the said material or vice versa. Preferably in such a device heat may be disposed equally throughout the said material.

In such a heat collection and storage device conveniently the first phase is the liquid phase and the second phase is the solid phase. Alternatively the first phase is a gaseous phase and the second phase is a liquid or solid phase.

In one aspect in heat collection and storage device, the phase change material is contained in individual sacs, and the heat transfer medium may be passed around the outside of the sacs.

Such a device may comprise an outer layer and an inner layer, forming a jacket there between and the inner layer describing a chamber holding the individual sacs and in which the heat transfer medium may be passed around the jacket and into the inner chamber.

For best effect the heat sink or thermal store is placed at depth below the ground where the ground temperature does not vary markedly with changing seasons. In such a situation, when used as part of a heating system the device uses geothermal energy as a heat source. Instead of being placed in the ground, the device may be place below the surface of a lake or other body of water, which is large compared to the device Such a device can be used as part of a heating or cooling system. This in another aspect of the invention a heating and or cooling system comprising a heat pump to pump a heat transfer medium into a heat collection and storage device, in which the heat transfer medium may be conducted though a material whose phase changes from one phase to another, and in which, when the heat pump is operating heat is transferred from or to the phase change material to the heat or cool the transfer medium and when the heat pump is not working the temperature of the phase change material tends towards that of the surroundings and the phase change material stores heat O absorbed from the surroundings or the heat transfer medium by changing phase.

The invention makes it possible to collect and store ground source energy on a continuous basis and therefore reduce the size of the required collection system significantly and therefore its cost and the cost of installation.

Figure 1 shows a diagram of a typical ground heat energy collection and storage system for domestic premises employing this invention; Figure 2 shows a detailed view of a ground heat energy collection and storage device according to the invention; Figures 3A to 3D show an alternative embodiment of the invention, figure 3A being an overall view of the embodiment, figure 3B being a partially cut-away side view of the embodiment, figure 30 being an enlarged detail view of the cut-away portion A of figure 3B, and figure 3D being an enlarged detail view of the cut-away portion B of figure 3B.

In figure 1, geothermal heat energy collection and storage devices 6 are buried in the ground 4 at a depth at which the surrounding ground 4 is at an almost constant temperature regardless of surface weather conditions or season. Arrow(s) 12 depicts a continuous ambient ground temperature available to recharge the storage device.

A heat pump central heating system 2 supplies heat energy via connecting pipe work 3 to the required locations in building 1. Connecting pipe work 5 carries heat energy transport medium, in a typical installation, a water and antifreeze mixture, to and then away from the geothermal heat energy collection and storage device 6.

Conveniently the pipe work 5 may have non-return valves 11 to prevent the backf low.

When heat energy is required by the heat pump 2 energy is drawn from the storage devices 6. When no heat energy is required by the heat pump 2, heat energy depicted by arrows 12 continues to flow into the heat energy collection and storage devices 6. Depending on the size of the building I and the load required by the heat pump 2, a number of geothermal heat energy collection and storage devices 6 can be connected as shown as required to provide the necessary thermal capacity.

In figure 2, the geothermal heat energy collection and storage device 6 shown in figure 1 has an outer shell or casement 7 of high thermal conductivity so that heat energy can freely pass from earth 4 through the casement 7 and into the device.

Aluminium, which is used typically to manufacture the casement 7, has both high thermal conductivity and resistance to corrosion. The casement 7 contains and hermetically seals within a quantity of a phase change material 8, in this case a commercially available substance based on paraffin whose phase changes from liquid to solid (or vice-versa) at about 6°C. As the phase change material's phase changes at about 6°C centigrade, which is a lower temperature than the surrounding earth 4, its normal state is liquid, and its temperature will tend towards the temperature of the surrounding ground. When a heat transfer medium is passed through into and out of feed pipes 9, heat energy is removed from the device via a heat transfer coil 10 which in this example is a metallic tube, such as copper, of low thermal resistance containing and transporting the water/anti-freeze heat transfer medium. In use the water/anti-freeze mixture entering the transfer coil from the heat pump 2 will typically be low, say 2°C. As a result of the flow of the cold heat transfer medium, heat energy is removed from the phase change material 8 as the temperature of the phase change material begins to fall towards its phase change temperature. When the phase change occurs in the phase change material, and it begins to solidify, latent heat energy is released, this is energy previously stored when the phase change medium's state was changed from solid to liquid (as the device recharged). It is a well known principle of physics that when a material is transitioning its phase change more heat energy is required to change its state than would be to simply change its temperature by the same amount if it was not at its phase change temperature band.

The result is the useful storage of latent heat energy, which can be drawn upon as required and, as heat is drawn from the storage device at or around the phase change material's phase change temperature, heat is drawn off at a temperature that is steady within a band that is more useful than a fluctuating temperature release as a simple water based heat storage device would provide.

Because the heat energy collection and storage devices 6 is situated in the ground 4, which is at a constant temperature, it is continuously being replenished with heat energy by means on conduction from the ground 4. It will be seen therefore, that when the heat transfer medium ceases flowing within coil 10, the phase change medium will warm, at about 6°C, and the phase change material that has solidified will absorb latent energy and revert to liquid, at which point its temperature will continue to rise tending towards the ambient temperature of the surrounding ground.

In figures 3A to 3D an alternative heat energy collection and storage device 6 is shown. It is placed in the ground in the same way was the device of figure 2 and connected to a heat pump system as shown schematically in figure 1. This device does overcome potential corrosion issues involving the coil 10 (in figure 2) and the possible need to replace it from time to time.

In figures 3A to 3D, the heat transfer medium (water anti/freeze mix in this example) flows into the device through inlet 37 and leaves via outlet 47. The device itself comprises an outer tube of aluminium 31 with a top 33 and bottom 32.

Inside the outer tube 31 is an inner aluminium shell 41 with a top 35 and bottom 36.The volume between the outer tube 31 and inner shell 41 forming a jacket 43.The interior of the inner shell 41 forms a chamber 34 which is filled with sacs in the form of balls 40 containing a phase change medium. In this case it is a paraffin/wax based phase change material that solidifies at about 6°C or any other phase change material such as metal salt based materials. The outer skin of the balls is rubber or a suitable substitute. There are a number of commercially available materials that will change from solid to liquid form at about 6°C, the only criteria for selection being the temperature of the phase change and suitability for encapsulation in the balls. The jacket 43 connects through the outer top 32 to the inlet 37 and the outlet 47 passes through outer top 32 and inner top 35 to the interior chamber 34.

A large tube 30 passes through the outer top 32 and inner top 35, enabling the chamber 34 to be filled to the phase change material containing baIls 40 (or to enable the balls to be removed and replaced if degradation occurs over time). The tube 38 is plugged with an end cap 39. A number of apertures 42 in the inner bottom enable fluid to flow from the jacket 43 into the chamber 34.

In use when no heat is required, the device, including the materials in the balls 40 assumes an equilibrium temperature near that of the surrounding ground. When heat is required in the building 1 (see figure 1), the heat pump 2 (figure 1) is turned on and cold heat transfer fluid (typically around 2°C) from the heat pump 2 (figure 1) enters the device through inlet 37 and flows around the jacket 43. This fluid will tend to warm by absorption of heat though the outer tube 31. The heat transfer fluid enters chamber 34 through the apertures 42 and passes around the balls 40.

In doing so the balls 40 will cool and if their temperature reduces to around 6°C, the material within the balls will begin to solidify giving up latent heat to the heat transfer medium passing through the chamber 34. The heat transfer medium, now at the same temperature as the balls 40, leaves the interior 43 though the outlet 47 to return to the heat pump.

Assuming that on the pumping cycle equilibrium is reached, it will be seen that the temperature of the heat transfer medium passing though the device is raised by about 4°C, providing an increase in the heat content of the transfer medium usable in a heat pump. Once pumping ceases, the device will continue to absorb heat from its surroundings, and the material in balls 40 will liquefy again absorbing latent heat from the ground, effectively storing heat until the next pumping cycle starts, when once again energy stored in the balls 40 will be transferred to the heat transfer medium. It should be noted that pumping can be allowed to continue independently of the heat pump's circulation circuit pump so that good transfer of heat between earth and battery core takes place over a maximum period as required.

One or more expansion chambers may be used in a closed cycle system to allow for displacement of the heat transfer fluid when the material within the balls 40 expands or contracts on heating or cooling.

As in figure 2 a number of the devices shown in figure 3 can be buried in the ground to ensure that the necessary heat capacity is available for larger buildings.

A number of variations will be apparent to the skilled reader. Although a water/anti freeze mixture will normally be the cheapest heat transfer medium, other fluids can be used. The device has been described as being in the ground, in appropriate circumstances it could be below water in a lake, for example.

If the device is to be placed in the ground, a depth of a few metres (depending on ground conditions) will normally be enough to ensure that the ground is at a near constant temperature in all weathers and seasons.

As with existing heat pump systems, the unit can be run in reverse, thereby removing heat from a building and transferring it in the ground. Heat transfer is most efficient if there is a high temperature gradient between the source and the sink. In the case of a heat pump, for a building cooling application, heat must be rejected by the compression cycle of the heat pump. One problem encountered with existing systems that deposit rejected heat to the ground is the earth's slow thermal conductive properties, so the heat deposited cannot dissipate quickly enough and the local area temperature around the heat exchanger increases, reducing the effectiveness of the system. Using a phase change heat exchanger design as described with, a suitably selected phase change material temperature band, will ensure that the store is held at a constant low temperature of say 12°C until the latent heat capacity of the store has been reached, at which point the surrounding ground acts as the heat sink and continues to do so until the phase change material has expelled all latent heat and returned to the temperature of the surrounding earth. The storage and heat exchange system as described acts as a buffer between the heat pump (load) and the earth, smoothing out a series of thermal load peaks to a continuous deposit to or draw of thermal energy from ground.

Essentially, the earth's surface, of which thermal energy is extracted by ground source heat pumps, is a natural store of solar energy. To further reduce the size and cost of geothermal heat exchangers solar energy can be used to replenish thermal energy removed from the earth by the heat exchanger. Solar collectors that are mounted on the ground surface above the thermal storage heat exchanger can be connected either directly using conductive metal rods or be connected and be part of the heat transfer fluid circuit. Any means of solar collector can be used and sized according to the systems thermal load calculations.

When applying the described geothermal heat exchanger and storage device to an application many variables need to be considered. These variables will include the type of ground the device is to be buried, the available depth that it can be practically buried in, the thermal conductivity and thermal capacity of the earth and the available ground area. The load induced on the ground will also be a factor in deciding on the number of devices used and if solar collection should be considered to replenish thermal energy removed.

As an additional option to the application engineer, in situations where ground thermal conductivity is poor, is that additional thermal conductive area can be used. However, using additional devices, as described may be expensive when it is only additional thermal conductive area in contact with the ground and no extra phase change material storage is required. For this reason it has been identified that additional devices can be used that are identical in design except they contain no phase change material and are water filled only. These water only devices continue to benefit from their double skin design by quickly and efficiently removing thermal energy from the contacting earth and since they will be connected via the heat transfer fluid circuit they can deposit this heat in devices that do contain phase change material thermal storage. They also have some addition storage capacity in the volume of water within although this amounts to approximately only 118th of the capacity of the phase change material filled device.

Claims

CLAIMS1. A heat collection and storage device for placing in a volume forming a source or sink of heat energy, comprising a container containing a phase change material, a means of passing a heat transfer medium through the material to transfer heat to or from heat transfer medium from or to the said material and in which the phase change material is in a first phase at the ambient temperature of the volume containing the source or sink and, in operation, would be in a second phase at the temperature of the heat transfer medium when entering the device.
2. A heat collection and storage device according to claim 1 in which heat may be transferred from the volume in contact with the container's surface to the said material or vice versa.
3. A heat collection and storage device as claimed in claim 2 in which heat may be disposed equally throughout the said material.
4. A heat collection and storage device according to any one of claims 1 to 3 in which the first phase is the liquid phase and the second phase is the solid phase.
5. A heat collection and storage device according to any preceding claim in which the material is contained in individual sacs (40), and the heat transfer medium may be passed around the outside of the sacs.
6. A heat collection and storage device according to any preceding claim comprising an outer layer and an inner layer, forming a jacket there between and the inner layer describing a chamber holding the individual sacs (40) and in which the heat transfer medium may be passed around the jacket and into the inner chamber.
7. A heat collection and storage device according to any one of claims 1 to 6 comprising part of a heating or cooling system.
8. A heat collection and storage device according any one of claims 1 to 7 using geothermal energy as a heat source.
9. A heat collection or storage device according to claim 8 in which the volume is the ground.
10. A heat collection or storage device according to any preceding claim in which the ambient temperature surrounding the device may be amplified or boosted by using passive means to introduce additional heat energy by natural conduction.
11. A heat collection or storage device according to claim 10 in which the passive means is heat conducting rods attached to a mechanism to absorb solar energy.
12. A heat collection or storage device according to claim 10 in which the passive means comprises pipes with a circulating fluid connected to a means of absorbing or focusing the solar energy.
13. A heat collection or storage device according top any one of claims 1 to 12 used as thermal sink.
14. A heat collection or storage device according to any preceding claim connected by a heat transfer circuit to one or more further containers in the volume, which further containers may contain material to absorb heat from the volume.
15. A heat collection or storage device according to claim 14 in which the one or more further containers comprise an outer layer and an inner layer, forming a jacket there between and the inner layer describing an inner chamber in which the heat transfer medium may be passed around the jacket and into the inner chamber.
16. A heating and or cooling system comprising a heat pump to pump a heat transfer medium into a heat collection and storage device according to any one of claims ito 15.
17. A heat collection and storage device substantially as hereinbefore described with reference to the accompanying drawings.
18. A heating system substantially as hereinbefore described with reference to the accompanying drawings.
19. A cooling system substantially as hereinbefore described with reference to the accompanying drawings.