WO1991007626A1

WO1991007626A1 - Transfer of heat within water storage tank by the use of heat pipes

Info

Publication number: WO1991007626A1
Application number: PCT/AU1990/000555
Authority: WO
Inventors: Kenneth Ian Guthrie; William Edward Jones; Peter Miles Chapman; Henry Ronald Mendelsohn
Original assignee: Renewable Energy Authority Victoria
Priority date: 1989-11-16
Filing date: 1990-11-16
Publication date: 1991-05-30

Abstract

An apparatus for heating fluids in which hot water (1) circulates through piping (2) where it initially flows past end (3) of heat pipe (4). Heat pipe (4) communicates with upper warmest section (5) of water tank (6). If the temperature in hot water (1) which circulates past end (3) is higher than section (5) heat energy is transferred via heat pipe (4) to section (5). Hot water (1) subsequently flows past ends (3) of heat pipes (7) and (8) respectively. Heat pipes (7) and (8) communicate with cooler section (9) and cold section (10) of water tank (6). If the temperature of the hot water (1) flowing past ends (3) of heat pipes (7) and (8) is higher than sections (9) and (10) of water tank (6), heat energy is transferred via heat pipes (7) and (8) to sections (9) and (10).

Description

TRANSFER OF HEAT WITHIN WATER STORAGE TANK BY THE USE OF HEAT PIPES

FIELD OF THE INVENTION

The invention relates to improvements in heating fluids and in particular, the invention has application to fluid heaters which use solar power as a source of heat energy.

BACKGROUND OF THE INVENTION

In a conventional hot water storage tank there is a natural temperature stratification of the water. The hottest water is located at the top of the tank as hot water has a density which is lower than that of cold water. Consequently, it is normal to take off the hottest water from the top for use and to introduce cold water into the bottom of the tank so that any heat energy in higher temperature regions is not lost to the cold water. It is important to ensure that the water at the outlet is of usable temperature .

In most conventional solar water heating systems, particularly those which utilise a remote, usually roof-mounted, solar collector in conjunction with a roof -mounted or ground level storage tank, the method of heat energy transfer into the solar hot water system has been achieved by two principal methods .

The circulating fluid, more particularly water, is pumped from the storage tank at ground level through the collector to become heated and is then returned at a higher temperature to the storage tank. Alternatively, the heated circulating fluid which flows through the collector exchange heat indirectly whereby the heat energy in the circulating fluid can be transferred either to another fluid medium or directly into the storage tank through a heat exchanger.

In the former system, the water returning from the collector is usually returned at least half-way up the tank on the basis that it is less hot than the upper-most temperature stratum and will serve to top-up the hot water in the tank. Problems arise when the returning water is significantly cooler than the water in the region of the tank into which it is injected. Thus, the hot water in that region which may have been heated by conventional means or by solar energy earlier in the day is degraded to a lower temperature .

Heat exchange results through a difference in temperature between two fluids. In the latter system, the use of a heat exchanger has been limited by the fact that the energy carrying fluid moves in a direct heat exchange relationship with a relatively stationary storage fluid. This direct relationship allows heat to be exchanged from the circulating fluid to the storage tank and vice -versa. For this reason, the circulating fluid is usually contacted with the uppermost part of the storage tank first so that when it is at its highest temperature it is exposed to the hottest water in the tank. Any heat energy at a lower temperature which is not transferred can be transferred to lower temperature stratum areas. Problems arise therefore, when the temperature of the circulating fluid is lower than the temperature of the hottest part of the tank. Heat energy in this case is lost from the highest temperature regions of the tank as it is transferred from the tank to the circulating fluid. The temperature of the circulating fluid is increased and is able to transfer heat energy to lower temperature stratum. In this way, the heat energy in the upper part of the tank is distributed into the lower parts of the tank. Although the temperature of the lower stratum may be increased, the temperature of the upper heat stratum is degraded. This has important ramifications as it is desirable to use the heat energy efficiently and more significantly it is desirable to keep the temperature at the top of the tank where the water is drawn off for use as high as possible .

The problem associated with both systems is that advantage is not taken of the natural stratification of the storage tank. Consequently the heat energy collected and stored is used inefficiently . It is well recognised that the higher the temperature difference the easier it is to transfer heat energy as the larger temperature difference constitutes a more effective heat transfer driving force . In addition, transferring energy to the colder region of the storage tank only marginally raises the overall temperature of the storage tank and does little to ensure that the temperature of the water drawn off from its upper region is as high as possible which is a condition that is desired in a hot water system.

Accordingly, investigations have been undertaken to determine a more efficient manner of carrying out heat energy transfer which avoids the abovementioned problems and may be readily adapted for use with solar energy. Accordingly, it is an object of this invention to take advantage of the stratification by transferring heat only where temperatures match and where it can be transferred efficiently . Where the heat source is not hot enough to heat the fluid strata in the heat sink, it should only be utilised where it can be matched to heat fluid at a lower temperature stratum in the sink. Heat energy could also be more efficiently used by taking advantage of the natural stratification of the stationary fluid in the tank by utilising the stationary fluid as a heat source to heat circulating fluid of a heat sink by heat exchange with the stratum of lowest temperature first. Tfte invention may also include temperature controls to ensure that the heat transfer by heat exchange maintains the fluid in the tank to below a predetermined maximum. Once the operating mode or performance of the particular heat exchange system under the influence of temperature differences is known a predetermined maximum could be set. In this respect, the heat exchange to a particular stratum may be selected so that it only permits heat exchange if the circulating fluid is above particular temperature .

STATEMENT OF THE INVENTION

According to a first embodiment of the invention, an apparatus is provided for heating fluids comprising:

(a) a fluid heat source;

(b) a heat sink separated from the heat source; and

(c) two or more heat exchange members each having a first portion located in the heat source and a second portion remote from the first portion located in the heat sink.

According to a second embodiment of the invention, a method is provided for heating a stream of fluid comprising the steps of :

(a) establishing a reserve of fluid with differing temperatures;

(b) transmitting to the stream of fluid a portion of heat energy derived from a portion of the reserve; and

(c) subsequently transmitting to the stream of fluid a portion of heat energy derived from a second portion of the reserve which portion has a temperature more than the temperature of the first portion. According to a third embodiment of the invention, a method is provided for heating a reserve of fluid having a temperature which decreases with descending depth comprising the steps of :

(a) transmitting to the reserve a portion of heat energy derived form a portion of the stream of fluid; and

(b) subsequently transmitting to a lower portion of the reserve a portion of heat energy derived from a second portion of the stream of fluid which second portion has a temperature less than the temperature of the first portion .

Accordingly energy transfer can be more efficiently utilised by either:

(a) transferring heat to a stratified heat sink only when the temperature of the heat source exceeds that in the various strata; or

(b) utilising the stratified temperature profile of a stationary heat source to transfer heat from a lower temperature stratum to the fluids stream prior to transferring heat from a higher temperature stratum to the fluids stream.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferably each first portion is located at a different position in the heat source and each second portion is located at a different height in the heat sink.

In another preferred embodiment heat exchange members allow heat to substantially travel in one direction only . As such there will be no substantial flow of energy from the heat sink to the heat source . In another preferred embodiment of the apparatus, the heat exchange members are inclined at an angle towards the heat sink . The heat exchange members in this arrangement are typically sealed and evacuated and contain a small quantity of liquid, the liquid transferring heat from the heat source to the heat sink. The Hquid may be chosen with a specific boiling temperature. If an excess in energy is transferred into the heat exchange member the liquid will gasify and because no condensation can take place (ie the heat sink temperature is not low enough) it will severely limit or stop energy transfer to the heat sink. This may be important for domestic water storage applications where the temperature of the drawn off water must not be a scalding temperature.

Where the heat source is supplied by solar energy, that energy may be insufficient to maintain the heat source at the desired temperature. Accordingly, additional heat energy may be provided into the reserve of fluid in the heat source from an auxiliary heating source.

DESCRIPTION OF THE INVENTION

The invention will now be illustrated with reference to the accompanying drawings in which:

Fig 1 is a cross-sectional view of first form of the invention.

Fig 2 is a further cross-sectional view of a second form of the invention.

Fig 3 is yet a further cross -sectional view of a third form of the invention.

Figure 1 shows the use of circulating hot water 1 and the use of heat pipes or heat exchangers 4, 7 and 8 to heat or supplement the heating of a water tank 6. More specifically, heat pipes 4, 7 and 8 communicate between piping 2 and water tank 6. The heat pipes 4, 7 and 8 have ends 3 which are in constant contact circulating with hot water 1 and their other ends are located in the water tank 6.

As previously stated, to efficiently utilise the heat energy in water 1 it is desirable to transfer the greatest heat energy to the upper warmest section 5 of water tank 6. Water tank 6 will be naturally stratified into a temperature profile in which temperature decreases with increasing depth of the tank 6. Accordingly, section 9 is cooler than section 5 and section 10 is cooler than section 9. To achieve this aim, incoming hot water 1 flows initially over the cold end 3 of the heat exchange pipe 4. Heat pipe 4 is located in section 5 of the water tank 6. Accordingly, a selective transfer of heat energy takes place by matching the temperature differential of both the circulating water 1 and the water tank 6.

Subsequent heat pipes 7 and 8 are thereafter sequentially contacted by the hot water 1 and energy is extracted into cooler section 9 and cold section 10 of the water tank 6.

The heat pipes 4, 7 and 8 are selected from a variety of one way heat exchangers which only permit heat energy which is at or above the temperature of the section of the water tank to which energy is to be transferred to pass to that section. As such the heat pipes will not allow energy in the water tank 6 to be retransmitted into the circulating water 1.

In Figure 2 a similar arrangement is shown to that depicted in Fig 1 and like numbers are adopted for like elements . A modified contact zone 11 is formed about the heat pipes 4, 7 and 8. Zone 11 is an expanded manifold which enables the velocity of the circulating water 1 about the heat pipes end 3 to slow down to maximise heat exchange.

With the present invention, an energy of the highest capacity in circulating exchange medium is introduced into the storage tank in the most thermally efficient way. That is, to the extent desired, heat energy is first introduced into the hottest section of the tank. This maximises the efficiency of the heating of the water tank G.

In Figure 3, solar energy 12 is introduced into heat store 13 via a solar transparent panel 14. The heat store 13 includes an inner chamber 15 comprising high temperature oil surrounded by insulation 16. The temperature of the heat store 13 is capable of being boosted by auxiliary heating means 17.

Heat pipes 18 are situated partially in the heat store 13 and extend from the inner chamber 15 through the insulation 16 into a conduit 19 which houses fluid 20 to be heated. As shown, conduit 19 is physically separated from the heat store 13 but is in thermal contact with it.

The heat pipes 18 are preferably inclined upwardly away from the heat store 13 and encapsulate a small amount of heat transfer fluid (not shown) . As this fluid heats up it evaporates and carries energy to the upper part of the heat pipes 18 where energy is transferred to fluid 20. The heat depleted transfer fluid condenses and runs down the inside surface of the heat pipe 18 for reuse. If the temperature of fluid 20 maintains the transfer fluid as a gas which subsequently cannot be condensed by the heat sink (because the sink temperature is getting too high) then heat transfer to fluid 19 is drastically reduced ensuring that fluid 19 is not over-heated. Of course, such heat pipes are usable in the embodiments of the invention shown in Figures 1 and 2.

Fluid 20 flows upwardly into conduit 19 and there contacts in turn ends of heat pipes 18 before flowing out the upper outlet of conduit 19. Fluid 20 initially receives low heat energy from the lowest heat pipe 18 and increasing heat energy as it flows upwardly. This energy input profile corresponds to that existing in inner chamber 15.

Accordingly, this system utilises the concept of a relatively stationary heat store 13 in combination with a heat exchanger of heat pipes 18 which ensures that the fluid being heated for final use is usable . It more closely aligns energy transfers and more effectively utilises the heat energy available .

Claims

The Claims defining the invention are as follows:

1. An apparatus for heating fluids comprising:

(a) a fluid heat source;

(b) a heat sink separated from the heat source; and

2. An apparatus according to claim 1, wherein a first portion is located at a different position in the heat source and each second portion is located at a different height in the heat sink.

3. An apparatus according to either claims 1 or 2, wherein the heat exchange members allow heat to substantially travel in one direction only.

4. An apparatus according to claim 2, wherein the heat exchange members allow heat to travel from the heat source to the heat sink but substantially not from the heat sink to the heat source.

5. An apparatus according to any one of claims 1 to 4, wherein the heat exchange members are inclined at an angle towards from the heat sink.

6. An apparatus according to claim 1, wherein the heat exchange members are sealed and evacuated and contain a small quantity of liquid, the liquid being the heat transferring medium from the heat source to the heat sink.

7. An apparatus according to claim 6, wherein the liquid has a specific boiling temperature above which substantially no heat energy transfer will occur.

8. A method of heating a stream of fluid comprising the steps of:

(a) establishing a reserve of fluid with differing temperatures;

(c) subsequently transmitting to the stream of fluid a portion of heat energy derived from a second portion of the reserve which portion has a temperature more than the temperature of the first portion.

9. A method according to claim 8, further comprising the step or steps of subsequently transmitting to the stream of fluid a portion of heat energy derived from a third portion of the reserve of fluid which portion has a temperature more than the temperature of the second portio .

10. A method according to claim 8, further comprising the additional step of transmitting additional heat energy into the reserve of fluid from an auxiliary heating source.

11. A method of heating a reserve of fluid having a temperature which decreases with descending depth comprising the steps of:

(b) subsequently transmitting to a lower portion of the reserve a portion of heat energy derived from a second portion of the stream of fluid which second portion has a temperature less than the temperature of the first portion.

12. A method according to claim 11, comprising the further step or steps of repeating step (b) after completion of step (a) .

13. An apparatus for carrying out the method of claims 8 to 12 comprising:

(a) a fluid heat source;

(b) a heat sink separated from the heat source; and