GB2267962A - Energy store capsules and heat exchange systems - Google Patents

Abstract

An energy store capsule for a heat exchange vessel is of substantially rounded, preferably substantially spherical shape, and has a through passage (23) to increase its surface to volume area and decrease the distance from any internal point to a surface. The capsule contains phase change material (21) preferably so that it is at least 96% full of such material. Many such capsules are located in a vessel preferably so that their passages are substantially parallel to the flow of heat exchange fluid through the vessel. <IMAGE>

Description

ENERGY STORE CAPSULES AND HEAT EXCHANGE SYSTEMS This invention relates to energy store capsules and heat exchange systems using such capsules and more particularly to such capsules containing Phase Change Material (PCM) which store and transfer latent thermal energy.

Systems for the storage of latent thermal energy in liquid or solid materials - during periods when useful energy is available and not required and/or when primary fuel tariffs are cheap for use when the energy is required and/or when primary fuel tariffs are high - are well known.

When any liquid PCM crystallises isothermally (for example changes from a liquid to a solid) a relatively large amount of energy is released. In reverse, when the material fuses or melts isothermally the same quantity of energy is absorbed.

If a PCM is chosen with a convenient fusion temperature relative to a thermal load, energy can be stored in the latent phase of crystallisation of this PCM with a high volumetric efficiency, for example by crystallising one unit mass of water the same quantity of thermal energy can be stored in this latent phase as could be stored sensibly by reducing its temperature by 800C.

Conventionally an energy storage system comprises a plurality of capsules containing PCM stacked in a vessel so that heat transfer fluid is passed through the vessel and heat exchanges with the capsules. The capsules are most conventionally of substantially spherical form although those of slab form are known.

Any encapsulation used within a system should normally be assessed on the following attributes: 1 Heat exchange rate performance 2 Latent energy storage density 3 Practicality of installation of capsules within the containment vessel 4 Physical strength and durability of the capsules, and 5 Cost.

Spherical capsules benefit from their ease of installation within the containment vessel and have good strength properties for the wall thickness of the capsule.

Other geometries require time consuming and complex loading procedures or allow less circulation of heat exchange fluid around the capsules.

However the spherical shape has two main disadvantages: (a) the sphere has the lowest surface area for contained volume and the heat exchange rate is dependent on the surface area, and (b) for a given volume the thickness of crystallised PCM that needs to be created to fully solidify the PCM is greater. Since the rate of heat transfer into the capsule is dependent on the thickness of crystallised PCM, the greater this thickness the lower the heat transfer.

This results in the spherical capsules having to be relatively small (conventionally about 75-105mm) with a relatively high number of them and this is expensive.

In one aspect the present invention provides energy store capsules for a heat exchange system, the capsules being of rounded shape, preferably substantially spherical, and each including at least one passage therethrough through which heat transfer fluid can pass. The passage increases the surface area for heat exchange and reduces the distance from any point in the interior of the capsule to a surface which will be contacted by heat exchange fluid.

In one preferred form each capsule has an elongate sectioned slot extending therethrough substantially on a diameter thereof. Preferably the slot has a length of at least three-quarters the diameter and a width to permit sufficient passage of heat transfer fluid for effective heat transfer and this increases the ratio of the capsule surface area to enclosed volume by about 508. Further it reduces by more than 50% the maximum distance from any point in the interior of the capsule to a surface which will be contacted by heat exchange fluid.

In one form the slot is formed dur-ing moulding or forming of a spherical container.

In another form each substantially spherical capsule is formed from two substantially hemispherical portions permanently joined together by spacial locators to define a passage between them.

Because there may be a substantial expansion/contraction between the solid and the liquid condition of the PCM (eg for water 8 to 10%), conventional substantially spherical capsules are left with an air pocket to accommodate this volume change. Conventionally the air pocket is approximately 4-8% of the volume of the capsule.

Because the walls of the slot will be substantially straight they can easily deform to bulge into or away from the centre of the slot thus accommodating some or all of the required expansion. This allows a smaller air volume to be left within the capsule. Preferably the air is reduced in volume by at least 50% from the conventional volume. The sides of the slot could initially be concave to further accommodate expansion on increasing volume.

The invention extends to a heat exchange system including a plurality of such capsules located in a vessel, with the vessel adapted for heat exchange fluid to flow therethrough.

During crystallisation of the PCM, (eutectic liquid such as water), the PCM will itself expand thus expanding the capsules and displacing the heat transfer fluid around the capsules. This expansion can be absorbed within an inventory tank to give a visible and measurable indication of the proportion of energy stored within the latent phase of crystallisation of the PCM. Elimination of a proportion of the air volume in the capsules makes this measurement more accurate.

It will be preferable for the passages in at least the majority of capsules to substantially align (be angled at less than 300) with the overall direction of flow of heat transfer fluid through the vessel. To achieve this each capsule may be weighted on one side remote from the passage and/or may be slightly elongated parallel to the passage and/or the capsules may be loaded into the vessel when it is filled or partially full of liquid in which the capsules float or nearly float so that the capsules will tend to settle in the tank with the passages horizontal.

Embodiments of energy storage capsules and a heat exchange system will now be described, by way of example only with reference to the accompanying drawings of which: Figure 1 is a diagrammatic view of a heat exchange system, Figures 2 and 3 are sectional views through alternative embodiments of energy store capsules, and Figure 4 is an enlarged sectional view of an alternative form of the wall of a capsule.

A heat exchange system comprises a substantially cylindrical, circular sectioned, vessel 12 having rounded ends and a horizontal axis and being filled with a stack of, generally round, energy store capsules 13 which can be inserted through a sealable entrance 14 in the top of the vessel. The vessel has an axial inlet 15 and outlet 16 at opposite ends for circulation of heat exchange fluid through the vessel and through an operating system 17 by means of a pump 18. Such a system is known and is therefore only indicated diagrammatically. The vessel need not be this shape but could, for example, be rectangular or could have its axis vertical. The heat exchange fluid circulates in a closed system to which is connected an open topped inventory tank 19 having a level indicator 19a. The level indicator will record the expansion and contraction of the liquid in the system and thus give a measurable indication of the energy stored within the system.

Each of the capsules 13 comprises a substantially spherical container 20 substantially filled with a PCM 21, for example water, and a sealed nipple 22 through which the PCM is initially introduced. The capsules do not have to be exactly spherical provided that they are generally rounded, for example, elliptical. Any suitable material may be used to form the capsules and the PCM contained within the capsules will be chosen in accordance with the requirements of the heating or cooling process for which energy is stored.

The capsule material may be of uniform thickness shown in Figures 2 and 3 or of variable castellated thickness as shown in Figure 4. This latter form increases the heat transfer surface and strength of the capsule.

In accordance with the invention each capsule 13 is formed with at least one through passage 23. In the examples of Figures 2 and 3 there is a single passage of elongate section on a diameter of the capsule and having a length of about 90% of the diameter and a width of about 10% of the diameter. In the example of Figure 2 this is formed by initially making the capsule in two halves each in the form of a substantially hemispherical container 24 each with a nipple 22. The two halves are connected by solid spacial locators 25 so as to form a substantially spherical capsule with the passage 23 between the two halves 24. In the case of Figure 3 the passage 23 is formed during moulding or forming of the spherical container.Because passage 23 as shown is substantially straight sided, the walls 26 defining this passage can deform into or away from the centre of the passage as indicated in broken line in Figure 3 to accommodate a change of volume of the PCM during heat exchange cycles. This means that the normal air volume left within the capsule to accommodate expansion and contraction, as shown in Figure 2, can be substantially eliminated or substantially reduced as shown in Figure 3. This has the advantage that volume changes indicated on the level indicator 19a will be a more accurate measure of the energy stored.

Ideally the passage 23 would align with the general direction of flow through the vessel 12. In order to assist the capsules to settle in the vessel with the passages 23 to be aligned they may be weighted at the bottom as shown for example at 30 in Figure 3. They could additionally be slightly elongated in the direction of the passages.

Conventionally the capsules are loaded into the vessel without liquid in the vessel. Preferably capsules in accordance with the invention will be loaded into the vessel when it has liquid therein and preferably liquid flowing therethrough. This will assist the capsules to settle in the vessel with the passages aligned with the general direction of heat exchange fluid flow.

By increasing the heat exchange characteristics of substantially spherical capsules by introducing the passages 23, the capsules can be made larger and therefore fewer are required and the overall cost is reduced. For example conventional capsules have a diameter of 77-105mm and capsules in accordance with the invention can have a diameter of 130-165mm to achieve similar performance characteristics.

Claims (16)

1. An energy store capsule for a heat exchange system, the capsule being substantially of rounded shape and including at least one passage therethrough.

2. A capsule according to claim 1 which is substantially spherical in shape.

3. A capsule according to claim 1 or claim 2 in which the passage is a substantially elongate sectioned slot extending through the capsule substantially on a diameter thereof.

4. A capsule according to claim 3 in which the slot has a length of at least three-quarters of the length of the capsule at the position of the slot and a width to permit sufficient passage of heat transfer fluid for effective heat transfer.

5. A capsule according to claim 3 or claim 4 in which the slot has dimensions such as to increase the capsule surface by about 50% compared with the capsule surface with no slot.

6. A capsule according to any of claims 1 to 5 in which the exterior surface of the capsule is castellated.

7. A capsule according to any of claims 1 to 6 formed from two substantially hemispherical portions joined together by spacial locators to define the passage between them.

8. A capsule according to any of claims 1 to 7 containing a liquid phase change material.

9. A capsule according to claim 8 in which the phase change material occupies at least 96% of the volume of the capsule.

10. A capsule according to claim 8 in which the phase change material substantially fills the capsule.

11. A capsule according to any of claims 1 to 10 weighted on one side remote from the passage.

12. A capsule according to any of claims 1 to 11 having external dimensions in the range 130 to 165 millimeters.

13. An energy store capsule substantially as described herein with reference to or as illustrated in Figures 2, 3 or 4 of the accompanying drawings.

14. A heat exchange vessel containing a plurality of energy store capsules, each according to any of claims 1 to 13.

15. A heat exchange vessel according to claim 14 in which the energy store capsules are located in the vessel with the majority of their passages aligned with the direction of heat exchange fluid flow.

16. A heat exchange vessel substantially as described herein with reference to Figure 1 of the accompanying drawings.