WO2004065877A1

WO2004065877A1 - Thermal storage apparatus

Info

Publication number: WO2004065877A1
Application number: PCT/GB2004/000186
Authority: WO
Inventors: Geoffrey Barker; Adrian Hutchings; Iain Henshaw
Original assignee: Thermetica Limited
Priority date: 2003-01-21
Filing date: 2004-01-20
Publication date: 2004-08-05
Also published as: GB0301312D0

Abstract

The thermal storage apparatus comprising a vessel (100) , a charging loop (200) and discharge loop (300) is provided. In use, the thermal storage apparatus includes a heat transfer fluid (10) and a thermal storage medium (20), such that the heat transfer fluid (10) can be drawn from the vessel (100) and into the charging loop (200). A sump (115) is provided at a lower part of the vessel (100) and arranged to collect heat transfer fluid (10) in the vessel (100) and supply the collected heat transfer fluid (10) to the charging loop (200).

Description

Thermal Storage Apparatus

The present invention relates in general to a thermal storage apparatus .

GB 2 283 307 discloses a known thermal storage apparatus comprising a vessel which in use contains a thermal storage medium, such as water, together with a heat transfer fluid. The thermal storage medium and the heat transfer fluid are immiscible and of differing densities, such that they settle by gravity to form distinct layers. The vessel is coupled to a charging loop that cools the heat transfer fluid during a charging cycle, and to a discharge loop that draws cooled thermal storage medium from the vessel during a discharge cycle.

The thermal storage apparatus is useful in applications such as air conditioning or process chilling such as for milk. In particular, the apparatus allows electrical load to be time shifted, such as by charging the apparatus using a lower-cost overnight power supply, and then discharging the apparatus during peak times.

It is desired to achieve efficient thermal transfer between the cooled heat transfer fluid and the thermal storage medium. It has been observed that efficient thermal transfer is improved by mixing together of the heat transfer fluid and the thermal storage medium to increase their direct contact. For example, GB 2 283 307 discloses that a small amount of thermal storage medium should be drawn into the charging loop immediately upstream of an inlet nozzle, in order to aid mixing. A problem arises in that if a sufficiently low temperature is achieved, thermal storage medium present in the charging loop can solidify, thereby reducing efficiency of the charging loop or causing a blockage. When water is used as the thermal storage medium and the temperature in the charging loop falls below 0°C an ice blockage can occur. The charging loop typically contains a heat exchanger, which is particularly vulnerable to blockage. Hence, it is desired to promote separation of the heat transfer fluid from the thermal storage medium, following thermal transfer. In particular, separation is desired in order to prevent thermal storage medium being drawn into the charging loop .

The heat transfer fluid is relatively expensive, compared to the thermal storage medium (e.g. water) . Hence, it is desired to minimise a quantity of heat transfer fluid required during operation of the thermal storage apparatus. Further, it is desired to minimise loss of the heat transfer fluid during prolonged use of the apparatus.

An aim of the present invention is to provide a thermal storage apparatus that inhibits thermal storage medium being drawn from a vessel into a charging loop. A preferred aim is to provide a thermal storage apparatus that in use promotes efficient separation of a heat transfer fluid from a thermal storage medium. Other preferred aims include achieving efficient thermal transfer, minimising a total volume of heat transfer fluid, and minimising loss of heat transfer fluid during use . According to a first aspect of the invention there is provided a thermal storage apparatus, comprising: a vessel for in use containing a heat transfer fluid and a thermal storage medium being immiscible and of differing densities; and a charging loop including an outlet nozzle arranged to draw heat transfer fluid from the vessel for cooling in the charging loop; characterised by: a sump arranged to collect heat transfer fluid in the vessel and to supply the collected heat transfer fluid to the outlet nozzle of the charging loop.

Preferably, the sump is arranged to collect a barrier layer of heat transfer fluid, which in use inhibits thermal storage medium being drawn into the outlet nozzle of the charging loop.

Preferably, the sump is arranged such that in use heat transfer fluid moves from the vessel into the sump and the outlet nozzle of the charging loop along a smooth velocity gradient.

Preferably, the vessel comprises a floor, and the sump is arranged between the floor of the vessel and the outlet nozzle of the charging loop.

Preferably, the floor is sloped toward the sump.

Preferably, the charging loop comprises an inlet nozzle arranged to return cooled heat transfer fluid into the vessel, and the sump is arranged distal from the inlet nozzle . Preferably, the sump is arranged opposite to the inlet nozzle .

Preferably, the sump is shaped as a frustum, having an upper cross sectional area that engages a floor of the vessel and a lower cross sectional area that engages the outlet nozzle of the charging loop.

Preferably, the thermal storage apparatus comprises at least one agitation nozzle arranged to inject an agitation fluid into the vessel.

Preferably, the at least one agitation nozzle is arranged to inject the agitation fluid in use to cause agitation at a boundary layer between the heat transfer fluid and the thermal storage medium.

Preferably, the agitation fluid comprises thermal storage medium drawn from the vessel and/or atmospheric air.

According to a second aspect of the invention there is provided a vessel adapted for use in a thermal storage apparatus, the vessel comprising: a main tank including a floor, the tank for containing a heat transfer fluid and a thermal storage medium being immiscible and of differing densities; and a sump communicating with the floor of the tank to collect the heat transfer fluid, the sump being coupleable at its lower end to an outlet nozzle of a charging loop.

Preferably, the floor of the main tank is sloped toward the sump . For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Figure 1 is a schematic overview of a preferred thermal storage apparatus ;

Figure 2 is a perspective view of a lower portion of a vessel;

Figure 3 is a plan view of the vessel in a rest condition;

Figure 4 is a plan view of the vessel in use during a charging operation;

Figure 5 is a sectional side view of a lower portion of the vessel; and

Figure 6 is an end view of the lower portion of the vessel .

Figure 1 is a schematic overview of a thermal storage apparatus according to a preferred embodiment of the present invention. The thermal storage apparatus comprises a vessel 100, a charging loop 200, and a discharge loop 300.

In use, the vessel 100 contains a heat transfer fluid (HTF) 10, such as a hydrofluoroether or other suitable fluid, and a thermal storage medium 20, such as water.

Conveniently, the heat transfer fluid 10 and the thermal storage medium 20 are immiscible and of differing densities, such that they settle by gravity to form distinct layers. The HTF 10 is of greater density than the thermal storage medium 20 so as to settle below the thermal storage medium 20 by gravity.

The charging loop 200 supplies cooled HTF 10 into the vessel 100. The charging loop includes a charging loop outlet nozzle 201 arranged to draw HTF from the vessel 100, a charging heat exchanger 203 arranged to cool the HTF, and a charging loop inlet nozzle 205 arranged to return the cooled HTF to the vessel 100.

The discharge loop 300 draws thermal storage medium 20 from the vessel, for use in a cooling load such as an air conditioning unit or a refrigerator. The discharge loop 300 suitably comprises a discharge loop outlet nozzle 301 arranged to draw cooled thermal storage medium 20, particularly an ice slurry surface layer 21, from the vessel 100, through a discharge heat exchanger 303, and to return the thermal storage medium to the vessel through a discharge loop inlet nozzle 305.

If any thermal storage medium (i.e. water) is drawn into the charging loop 200, then an ice blockage may occur, particularly in the_, charging heat exchanger 203. A blockage hinders operation of the charging loop 200 and ultimately causes the charging loop 200 to malfunction. Hence, it is strongly desired to avoid drawing thermal storage medium into the charging loop 200. As shown in Figure 1, it is preferred that the outlet nozzle 201 is arranged at a lower portion of the vessel 100, suitably engaging a bottom surface of the vessel. Meanwhile, the inlet nozzle 205 is arranged higher, suitably in a side wall of the vessel. As a result, cooled heat transfer fluid enters the vessel through the inlet nozzle 205 and falls, due to gravity settling, down towards the outlet nozzle 201. The dwell time of the heat transfer fluid in the vessel 100 allows thermal exchange between the cooled HTF and the thermal storage medium, thereby cooling the thermal storage medium and charging the thermal storage apparatus . In the preferred embodiment, the thermal storage medium (water) is cooled to form ice crystals, which tend to float towards a layer of ice slurry 21. Water is particularly suitable for use as the thermal storage medium because it has a liquid phase of greater density than its solid phase. Further, this ice slurry system has a large latent heat of melting, and the melting temperature of ice is suitable for many commonly-required cooling applications.

When water is used as the thermal storage medium 20, materials having suitable physical characteristics for the heat transfer fluid 10 include 1, 1, 1-trichloroethane, 1, 1, 2-trichloroethylene, and perfluorohexane . More preferably, the HTF comprises a hydrofluoroether, which is environmentally relatively benign. The heat transfer fluid 10 has a lower freezing point than that of the thermal storage medium 20.

It has been realised that competing demands are placed upon the thermal storage apparatus shown in Figure 1. On the one hand, it is desired to achieve an efficient thermal transfer between the cooled HTF introduced through inlet nozzle 205 and the thermal storage medium in the vessel 100, which is enhanced by turbulent mixing of these two fluids. On the other hand, once thermal exchange has been achieved, it is then desired to efficiently separate the fluids, particularly so that HTF is not drawn up toward the ice slurry layer 21 (resulting in depletion of available heat transfer fluid) , and avoiding thermal storage medium being drawn into the charging loop 200 (resulting in ice blockages) . Ideally, it is desired to address both of these problems simultaneously.

The preferred embodiment of the present invention employs a vessel 100 which has been shaped in order to enhance thermal transfer between the HTF and the thermal storage medium, and also to enhance subsequent separation of these fluids .

Figure 2 shows a lower portion of the vessel 100 in more detail. This example embodiment employs a generally square lower section 110, which is arranged to communicate with a generally cylindrical main body section 112, shown by dotted lines in Figure 2. Alternatively, the upper and lower portions 110, 112 are integrally manufactured. Alternative embodiments can employ circular or other shaped lower sections.

As shown in Figure 2, a sump 115 is provided in the lower portion 110. The sump 115 is arranged generally between the vessel 100 and the outlet nozzle 201 of the charging loop 200. Advantageously, the sump 115 allows a concentrated, localised volume of heat transfer fluid to collect immediately above the outlet nozzle 201. This deep layer of HTF helps to prevent the formation of vortices which would otherwise tend to draw thermal storage medium

(water) into the charging loop 200. The sump is shaped to produce a smooth velocity gradient of HTF moving from the interface with the thermal storage medium into the sump

115 and through the outlet nozzle 201. By ensuring a smooth velocity gradient is produced in the HTF movement of thermal storage medium into the charging loop is further inhibited. As a result, minimal thermal storage medium (water) is drawn toward the outlet nozzle 201, and the risk of ice blockage in the charging loop 200 is reduced.

Using water as the thermal storage medium is relatively inexpensive, whereas suitable materials for use as the heat transfer fluid are comparatively highly expensive. The sump 115 allows a relatively small volume of HTF to provide a relatively deep barrier layer between the outlet nozzle 201 and the thermal storage medium (water) . Only a relatively small volume of HTF is required in a remainder of the vessel above the sump 115, sufficient for thermal transfer. This significantly reduces the total quantity of HTF required by the apparatus, whilst maintaining efficient operation during charging.

As shown in Figure 2, the sump 115 is preferably arranged in the vessel 100 distal from the charging loop inlet nozzle 205, ideally being positioned opposite to the inlet nozzle 205. Hence, there is a maximum separation between the HTF inlet nozzle 205 and the sump 115 across a horizontal dimension of vessel 100. This arrangement increases dwell time of the HTF within the vessel 100. The HTF therefore has an increased opportunity for thermal exchange, and then has an increased opportunity to settle by gravity toward the sump 115.

Flow velocity of the HTF is greatest upon entry into the vessel 100 through the inlet nozzle 205, and slows significantly as the HTF settles towards the sump 115. It has been found that separation of ice crystals from the HTF occurs more readily at lower flow velocities. The vessel configuration shown in Figure 2 therefore promotes efficient separation in a slower-moving region of HTF at and near the sump 115.

The sump 115 is preferably shaped as a frustum. Suitably, the sump 115 is an inverted truncated pyramid or an inverted truncated cone. As can be readily envisaged, the sump 115 can comprise other similar shapes, for example having a curved or other non-linear profile. At a lower end, the sump 115 has a lower cross-sectional area which is approximately matched to the cross section of the outlet nozzle 201, and at an upper end has an upper cross sectional area matched to an aperture in a floor 101 of the vessel 100. This frustum shaped sump 115 allows a smooth velocity gradient of the heat transfer fluid, between a relatively static pool near a floor 101 of the vessel 100, gradually increasing in velocity toward the outlet nozzle 201. Hence, in use, the heat transfer fluid is drawn from the vessel 100 at a relatively high flow rate, whilst avoiding significant turbulence and disruption in the region of the outlet nozzle 201, which would otherwise tend to increase unwanted contamination of thermal storage medium (water) into the charging loop. Preferably, the floor 101 of the vessel 100 is sloped toward the sump 115. In the preferred embodiment of Figure 2, the floor 101 comprises three sloping sections 102, 104 and 106. The first sloping section 102 slopes from the inlet nozzle 205 toward the sump 115. The second and third sloping sections 104, 106 each slope perpendicular to the first sloping section 102. These three planar sloping sections 102, 104, 106 are convenient to manufacture, but other more complex floor forms are also possible, such as a smooth elliptical cone.

Figure 3 is a plan view of the lower portion 110 of the vessel 100, looking along the line of arrow B in Figure 2. Figure 3 shows the apparatus in use in a rest condition, where the heat transfer fluid 10 settles by gravity over the sump 115 as a static coalescence pool .

Figure 4 is a view equivalent to Figure 3, but this time showing the apparatus in use during a charging operation. The heat transfer fluid 10 flows into the vessel 100 through the inlet nozzle 205, at a relatively high velocity, causing turbulent mixing with the thermal storage medium (water) 20. The inlet nozzle 205 defines a generally linear flow axis, illustrated by arrow F in Figure 4. The heat transfer fluid disburses across the lower portion of the vessel 100 and is directed by the sloping floor 101 toward the sump 115. Flow velocity of the HTF tends to decrease toward the sump 115, leaving a relatively static pool of HTF at the sump 115. This static pool provides optimal conditions for drawing HTF 10 from the vessel, through the outlet nozzle 201, without contamination by any thermal storage medium (water) . The sloped floor sections 102, 104, 106 help to concentrate the heat transfer fluid within the vessel and thereby minimise the required volume of HTF in this dynamic charging operation.

Figure 5 illustrates a further preferred embodiment of the present invention including further optional features to enhance separation of ice crystals from the heat transfer fluid. Figure 5 is a sectional side view of the lower portion 110 of the vessel in the region immediately above the sump 115. An interface layer 120 is formed at an upper boundary between the heat transfer fluid 10 and the thermal storage medium 20. It has been found that ice crystals may become trapped at this interface layer 120 due to surface tension of the relatively dense heat transfer fluid. As a result, the ice crystals are not released to float upwards to join a layer of ice slurry, and there is an increased possibility that the ice crystals will be drawn into the charging loop 200 through the outlet nozzle 201. Therefore, it is preferred to apply fluidic agitation directed generally at or in the region of the interface layer 120, in order to release the ice crystals. Applying fluidic agitation in the region above the interface layer improves release of the ice crystals and encourages the ice crystals to move upwards and away from the interface. Jets of an agitation fluid are directed at the approximate position of the interface layer 120 above the sump 115. Suitably, the agitation fluid comprises thermal storage medium (water) drawn from the vessel 100 and/or atmospheric air drawn from an upper portion of the vessel 100.

Figure 6 is an end view of the vessel 100, looking along the line of arrow A of Figure 2. Agitation nozzles 121 may be provided in any suitable position and in any suitable number, according to the needs of a particular vessel 100. In the preferred embodiment, four agitation nozzles 121 may be provided through a side wall of the vessel 100, to create a horizontally extending agitation region across an interface layer between the HTF 10 and the thermal storage medium 20. Also, as shown in Figure 4, optional agitation nozzles 121 may be provided at either side of the vessel 100, arranged generally perpendicular to a flow direction F of the HTF introduced through the inlet nozzle 205. Again, the agitation nozzles 121 are directed to enhance separation of the HTF from the thermal storage medium.

The thermal storage apparatus described above has many advantages. The apparatus minimises contamination of thermal storage medium (e.g. water) in the HTF cooling loop, but also allows a reduction in a volume of heat transfer fluid required in use. Loss of HTF into an ice slurry layer is minimised by efficient separation. Meanwhile, efficient thermal transfer is achieved between the heat transfer fluid and the thermal storage medium. Other advantages will be apparent from the description above, and by practice of the invention.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A thermal storage apparatus, comprising:

a vessel for in use containing a heat transfer fluid and a thermal storage medium being immiscible and of differing densities; and

a charging loop including an outlet nozzle ^• arranged to draw heat transfer fluid from the vessel for cooling in the charging loop;

characterised by:

a sump arranged to collect heat transfer fluid in the vessel and to supply the collected heat transfer fluid to the outlet nozzle of the charging loop.

2. The apparatus of claim 1, wherein the sump is arranged to collect a barrier layer of heat transfer fluid, which in use inhibits thermal storage medium being drawn into the outlet nozzle of the charging loop.

3. The apparatus of claim 1, wherein the sump is arranged such that in use heat transfer fluid moves from the vessel into the sump and the outlet nozzle of the charging loop along a smooth velocity gradient .

4. The thermal storage apparatus of claim 1, wherein the vessel comprises a floor, and the sump is arranged between the floor of the vessel and the outlet nozzle of the charging loo .

5. The apparatus of claim 4, wherein the floor is sloped toward the sum .

6. The apparatus of any preceding claim, wherein the charging loop comprises an inlet nozzle arranged to return cooled heat transfer fluid into the vessel, and the sump is arranged distal from the inlet nozzle.

7. The apparatus of claim 6, wherein the sump is arranged opposite to the inlet nozzle.

8. The apparatus of any preceding claim, wherein the sump is shaped as a frustum, having an upper cross sectional area that engages a floor of the vessel and a lower cross sectional area that engages the outlet nozzle of the charging loop .

9. The apparatus of any preceding claim, comprising at least one agitation nozzle arranged to inject an agitation fluid into the vessel.

10. The apparatus of claim 9, wherein the at least one agitation nozzle is arranged to inject the agitation fluid in use to cause agitation at a boundary layer between the heat transfer fluid and the thermal storage medium.

11. The apparatus of claim 9 or 10, wherein the agitation fluid comprises thermal storage medium drawn from the vessel and/or atmospheric air.

12. A vessel adapted for use in a thermal storage apparatus, the vessel comprising: a main tank including a floor, the tank for containing a heat transfer fluid and a thermal storage medium being immiscible and of differing densities; and

a sump communicating with the floor of the tank to collect the heat transfer fluid, the sump being coupleable at its lower end to an outlet nozzle of a charging loop.

13. The vessel of claim 12, wherein the floor of the main tank is sloped toward the sump.

14. A thermal storage apparatus substantially as hereinbefore described with reference to the accompanying drawings .

15. A vessel substantially as hereinbefore described with reference to the accompanying drawings .