AU688392B2

AU688392B2 - Solar-powered hot water systems

Info

Publication number: AU688392B2
Application number: AU78951/94A
Authority: AU
Inventors: Aliakbar Akbarzadeh; Peter Johnson; Ioan Sauciuc
Original assignee: Technisearch Ltd
Current assignee: Technisearch Ltd
Priority date: 1993-11-22
Filing date: 1994-11-21
Publication date: 1998-03-12
Anticipated expiration: 2014-11-21
Also published as: AU7895194A

Description

-1- Rcgult:ion 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

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Name of Applicant: Actual Inventors: Address for Service: Invention Title: TECHNISEARCH LTD.

Aliakbar Akbarzadeh Peter Johnson loan Sauciuc DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

"Solar-Powered Hot Water Systems" Details of Associated Provisional Application(s) No(s): PM2565/93 filed 22 November, 1993 The following statement is a full description of this invention, including the best method of performing it known to us: Q:\OPER\RSH\TECHN.358 24/12/97 j SOLAR-POWERED HOT WATER SYSTEMS The present invention relates to solar-powered hot water systems and more particularly to a solar-powered hot water system having means for dissipating excess heat.

Solar-powered hot water systems conventionally comprise a storage tank connected to a solar collector. In an open system, cooler water from a lower part of the tank flows into the collector, is heated in the collector and is returned to an upper part of the tank. The circulation normally occurs naturally by means of a thermosyphon action. In a closed system, the collector is connected in a closed circuit to a heating coil within the tank and water circulates by a natural thermosyphon action through the collector and coil to thereby heat the water within the storage tank. When the system is installed in a region subject to high radiant 15 energy input at relatively high ambient temperatures, the temperature of the hot water stored within the tank might exceed a desirable maximum temperature, particularly if the system is subject to a relatively low or intermittent draw-off rate of hot water and hence replenishment with cold water. This can result in hot water temperatures which can be so high as to cause scalding. It is not considered practical to incorporate a temperature control valve within the circulation system to prevent circulation through the collector when the temperature within the tank exceeds a predetermined temperature as this can result in the gen _ration of high water temperatures within the collector which can lead to reduced collector life and also, in an open system where the tank water flows through the collector, the water within the collector when the circulation valve is closed can stagnate and this may o result in fouling of the water.

The potential for excessive heating of the water within a solar hot water system is known and designers of such systems sometimes deliberately create inefficiencies within the system in order to minimise this possibility. This might involve the omission of thermal insulation in certain parts of the system, the use of insulation of reduced effectiveness, or the incorporation of less-efficient solar 94112,p:\oper\rshsolarhotwater.spe,I I -I, -2collectors. However even if such steps are taken, circumstances can still arise when excessive heating occurs. Further, the efficiency of the system is inherently reduced and this is undesirable when the system is being used in conditions when excessive heating is unlikely to occur.

There has been proposed a means for dissipating heat from the system when a predetermined water temperature has been reached, comprising an external pipe having an inlet connected to the hotter part of the storage tank and an outlet leading into the cooler part of the tank. Flow through the pipe is controlled by means of a temperature-responsive valve which opens when a predetermined water temperature is reached to enable a natural thermosyphon flow of the water through the pipe.

During passage of the water through the pipe externally of the tank the temperature of the water will reduce by heat transfer through the pipe wall. This previously proposed means for effecting heat dissipation requires the use of a temperature- 15 responsive valve which is relatively expensive and may also be subject to failure in operation.

According to the present invention, there is provided a solar-powered hot Q' inC--to- water system eej si*Pr storage tank for hot water, a solar collector for heating the water, and means for controlling the temperature of the water within the tank, said means comprising a heat pipe (as hereinafter defined) having a portion within the tank and a portion externally of the tank, said heat pipe being operative to •dissipate heat from the water within the tank to atmosphere externally of the tank 0 by evaporation and condensation of the working fluid of the heat pipe when the water temperature exceeds a predetermined temperature.

By the term "heat pipe" as used herein there is meant a closed tube of a heat conductive material containing a working fluid which is in a liquid state at a lower end portion of the tube to be heated to boiling point by heat input through the lower part of the tube, the vapour of the liquid travelling upwardly to a part of the tube at a cooler temperature at which the vapour condenses to provide high thermal dissipation through the wall of the tube and the condensate then flowing back to the 941121,p:\operrshsolar-hotwater.spe,2 -3lower end portion of the tube. The condensate return flow can be a natural gravity flow achieved by the inclination of the pipe and/or a capillary flow achieved by means of a wick if the pipe is operating at an angle close to the horizontal. The nature of the working fluid and the internal pressure within the heat pipe will determine the temperature at which the working fluid boils and hence the operating characteristics of the heat pipe.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 shows schematically a solar-powered hotwater system embodying the principles of the invention; Figure 2A shows schematically a hot water storage tank of the system and incorporating a heat pipe in accordance with a first embodiment of the invention; 15 Figure 2B shows the heat pipe of Figure 2A to an enlarged scale; and Figures 3A to 3C show schematically a hot water storage tank of the system incorporating a heat pipe in accordance with a second embodiment of the invention, the figures showing the heat pipe in different modes of operation.

o 20 With reference to Figure 1 the basic system is substantially conventional and comprises a hot water storage tank 2 and a solar collector 4. The system may be an open system in which cooler water from a lower part of the tank 2 flows through the o collector 4 by a natural convection or thermosyphon action to be returned at a S"higher temperature to an upper part of the tank. Alternatively, the system may be a closed system in which the collector 4 is connected to a heating coil within the tank 2 and water flows by means of a natural convection or thermosyphon action in a closed circuit through the collector and the coil to be heated within the collector and thence to heat the water stored in the tank 2 by thermal conduction through the coil.

In accordance with a preferred embodiment of the invention, dissipation of excess heat from the system is achieved by means of a heat pipe 6 when the water temperature in the hotter part of the water tank exceeds a predetermined 9412lp:\oper\rshsolar-otwater.spe,3 III I a temperature, for example a temperature of the order of 80-85 C. More particularly, the heat pipe 6 extends through the wall of the tank from within the interior of the tank in which it contacts at least the water in the hotter part of the tank to a position externally of the tank 2 in which the heat pipe can dissipate thermal energy to the surrounding atmosphere.

The operating characteristics of the heat pipe 6 used in the system are influenced by the presence of a non-condensable gas within the pipe as will be described hereinafter, so that operation of the heat pipe does not occur until a predetermined temperature is attained within the water storage tank 2. It is to be noted that as the heat pipe 6 is constructed of a heat-conductive material there will always be some heat transfer by conduction along the length of the pipe, but the conductive heat transfer (and hence heat loss) which occurs in this manner will be almost negligible in comparison with the substantive heat transfer which occurs by 15 the continuous process of evaporation and condensing of the working fluidwithin the heat pipe when the temperature of the water within the tank 2 is sufficient to cause boiling of the working fluid.

In one embodiment of a suitable heat pipe as shown in Figures 2A and 2B, the heat pipe 6 contains the working fluid and also a non-condensable gas. The working fluid and the gas pressure within the heat pipe 6 are such that boiling of the working fluid does not occur until the water within the tank 2 reaches a prescribed maximum temperature, which will normally be of the order of 80-85 *C whereby S" operation of the heat pipe 6 to effect significant heat transfer from the water stored 25 within the tank 2 to the external atmosphere only occurs when the stored water temperature is at or above the desired maximum temperature. In Figure 2B, the reservoir of working fluid is shown at 6a at the lower end of the heat pipe 6 and condensed fluid which condenses on the pipe wall externally of the tank 2 flows back to the reservoir 6a as a film of a stream of droplets shown at 6b.

In an alternative embodiment as shown in Figures 3A to 3C, the heat pipe 6 contains the working fluid and also a non-condensable gas at such a pressure that the 941121,p:\oper\rshsolar-otwater.spe,4 0 gas creates, under lower temperature conditions, a barrier between the evaporated working fluid and the outer end of the pipe. This barrier is defined by an interface zone between the vapour and the gas. In Figure 3A, the reservoir of working fluid is shown at 6a, the vapour is shown at 6c, the gas is shown at 6d and the interface zone is shown at 6e. As the water temperature and the vapour pressure increases, the interface zone 6e will be progressively displaced towards the outer end of the heat pipe. In this embodiment the position of the interface zone 6e along the heat pipe can be a primary determinant for heat dissipation to the surrounding atmosphere rather than the actual boiling temperature of the working fluid. More particularly, the boiling temperature of the working fluid may be below the temperature at which heat dissipation is required, but at boiling temperature and at other temperatures below the prescribed temperature at which heat dissipation is required, the interface zone 6e is positioned so as to confine the vapour 6c to within the portion of the heat pipe 6 lying within the tank 2 so that there is substantially no external heat dissipation. As the water temperature within the tank increases, the interface zone 6e will be displaced progressively outwardly so that when the prescribed maximum water temperature is attained within the tank 2, the interface zone 6e will lie outside of the tank 2 to permit substantive heat dissipation from the water within the tank to the surrounding atmosphere externally of the tank by the continuous cycie of evaporation and condensation of the working fluid (see Figure 3B). If tewater teprtr Lfhe icrass the inefc zone 6wilbefute displaced towards the remote end of the pipe to further increase the rate of heat dissipation (see Figure 3C).

The gas used in the heat pipe in the preferred embodiments must be such that it does not condense at the lowest temperatures encountered in operation of the system and, by way- of example only, suitable gases include nitrogen, argon, or air.

The working fluid may comprise a refrigerant or another liquid such as -"ater. It is to be understood however that these examples of gas and working fluid are given purely for illustrative purposes and that a wide variety of working fluids and gases may be used.

941121,p:\oper\rsh,solar-hotwater.spe,5 When the heat pipe is such as to operate by means of a progressively displaceable interface zone as described above, the remote end of the heat pipe may incorporate a reservoir into which subhantially all of the gas can be displaced at higher water temperatures to thereby ensure that the maximum length of exposed surface area of the heat pipe is available for heat dissipation.

Although in the embodiments illustrated, the heat pipe is shown as a rectilinear pipe, the pipe does not need to be rectilinear and, in practice, a bent pipe is likely to be preferred so that the pipe can be shaped to fit neatly within the confines of the overall system without excessive projection to one side.

Preferably, to facilitate maximum heat dissipation from the exposed part of the heat pipe, this part of the heat pipe will be located in an area which is not subjected to direct solar radiation, for example an area which will always or mostly 15 be in shadow. This may be achieved by bending the external portion of the heat pipe so that it is shadowed by the structure of the hot water tank itself. The external part of the heat pipe maybe finned or otherwise constructed to increase its external surface area for heat dissipation. Under certain climatic conditions it may be .eo.

desirable to provide a forced air flow across the external surface of the exposed portion of the pipe for increased heat dissipation and this can be achieved by means of a fan, preferably powered by a solar cell.

The heat pipe can be produced relatively inexpensively and is simple to S"install, only a single fitting being needed at the point at which the heat pipe passes through the wall of the tank. A heat pipe has no mechanical working parts and can have a working life considerably in excess of that of the solar hot water system itself.

When operating, the heat pipe can provide heat dissipation from the tank at a rate as high as the heat input to the tank via the solar collector and hence the heat pipe can, in a simple and inexpensive way, provide effective temperature control for the system. The effective heat dissipation which can be achieved by means of the heat pipe is such that solar hot water systems can be designed with improved solar collectors and improved insulation in order to provide improved efficiency in cooler 94112,p:\oper\rshsolar-otwater.spe,6 -I rll-~~ or overcast weather, as the heat pipe can cope with the heat dissipation required to maintain the water temperature within the prescribed limits when operating under climatic conditions where there is high radiant energy and/or ambient temperatures and particularly when there is an irregular usage of hot water from the system.

The embodiments have been described by way of example only and modifications are possible within the scope of the invention.

Throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

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Claims

1. A solar-powered hot water system including a storage tank for hot water, a solar collector for heating the water, and means for controlling the temperature of the water within the tank, said means comprising a heat pipe (as hereinbefore defined) having a portior within the tank and a portion externally of the tank, said heat pipe being operative to dissipate heat from the water within the tank to atmosphere externally of the tank by evaporation and condensation of the working fluid of the heat pipe when the water temperature exceeds a predetermined temperature.

2. A hot water system according to claim 1, wherein the characteristics of the heat pipe are such that the working fluid of the heat pipe boils at a temperature substantially equal to the temperature at which heat dissipation is to commence.

3. A hot water system according to claim 1, wherein the characteristics of the heat pipe are such that the working fluid of the heat pipe boils at a temperature substantially below the temperature at which heat dissipation is to commence and substantive heat i transfer between the portion of the pipe internally of the tank and the portion externally of the tank does not occur until that water temperature is attained. 99

4. A hot water system according to claim 3, wherein the heat pipe contains a non- condensable gas at such a pressure that the gas defines a barrier between evaporated working fluid and the outer end of the pipe, said barrier moving towards the outer end as the water temperature increases such that the barrier is within a portion of the pipe internally of the tank at water temperatures below said predetermined temperature, and is within a portion of the pipe externally of the tank at temperatures above the predetermined temperature.

A hot water system according to claim 4, wherein the outer end of the pipe incorporates a reservoir into which substantially all of the gas can be received at (4J temperatures substantially above said predetermined temperature. 971224,p:\oper\rshsolar- otwater.sp,8 -r I

6. A hot water system according to any one of claims 1 to 5, wherein the portion of the heat pipe externally of the tank is positioned in an area not subject to significant direct solar radiation.

7. A hot water system according to claim 6, wherein the portion of the heat pipe externally of the tank is bent so that it is within an area which is at least for the most part shadowed from direct solar radiation.

8. A hot water system according to any one of claims 1 to 7 comprising fan means for providing a forced air flow over the portion of the heat pipe externally of the tank.

9. A solar powered hot water system substantially as hereinbefore described with reference to the accompanying drawings. DATED this 21st day of November, 1994. TECHNISEARCH LIMITED By its Patent Attorneys DAVIES COLLISON CAVE 941121,p:\oper\rsh,solar-htwater.spe,9 L ABSTRACT A solar powered hot water system comprises a heat pipe projecting from the storage tank to dissipate excess heat to atmosphere when the water temperature in the tank exceeds a predetermined temperature. 2 I,p, \oper\mkbsoarbtwatersc,