GB2086563A

GB2086563A - Energy transfer apparatus

Info

Publication number: GB2086563A
Application number: GB8131431A
Authority: GB
Original assignee: Boyle & Associates R Solar Eng
Current assignee: Boyle & Associates R Solar Eng
Priority date: 1980-10-21
Filing date: 1981-10-19
Publication date: 1982-05-12

Abstract

Energy transfer apparatus, intended primarily as a solar energy collection system, has a collector panel with relatively thin sheet metal outer walls (3,4) and a relatively rigid, corrugated inner wall (2). The walls (2, 3, 4) define an evaporation space (8) and a condensate return passage (9), both of which are in communication with a vapour collection vessel (12) at the top of the panel. A single pipe (5) connecting the vapour collection vessel (12) with a coiled tube (6) in a preheat vessel (7) provides a path for upward transmission of vapour and downward return of condensate. The system operates by transfer of latent heat energy of vapourisation, the preferred heat transfer fluid being de-ionised water. Normally the system operates with a negative internal pressure so that the outer walls (3) and (4) are drawn into engagement with the corrugations of the wall (2). <IMAGE>

Description

SPECIFICATION Energy transfer apparatus This invention relates to energy transfer apparatus in which heat is transferred from an energy collector to an energy emitter by the evaporation and condensation of a heat transfer fluid. The invention is concerned particularly with the collection of solar radiation energy and its conversion into heat energy for heating a hot water system, but it is also applicable as a heat transfer or transport device, for example, for the recovery of heat from waste fluids.

Many different designs have been proposed for solar water heating systems, the most common systems in use being those which use water as a heat transfer fluid which circulates between a solar radiation collector and a heat exchanger vessel in a hot water supply. The collector generally comprises a radiation absorbing plate in contact with the circulating water. Radiation energy absorbed by the collector is converted into heat energy which is transferred by the circulating water to the heat exchanger to heat incoming cold water in the hot water system. Circulation is brought about either by natural convection or with the aid of an electrical pump. If a pump is used, a temperature sensing device is needed to determine when circulation should start or stop. Only a proportion of the total radiation energy supplied to the collector reaches the hot water system in the form of heat energy.The remainder is lost through re-radiation or by convective heat transfer to the surroundings from the collector and the connecting pipework. This heat loss to the surroundings rises as the temperature of the circulating water relative to the ambient temperature rises. However, with the known systems described above, a good rate of heat transfer to the hot water system is only obtained if there is a relatively high temperature difference between the circulating water and water to be heated. Therefore, such a system has the disadvantage that a compromise must be made in the design between the two conflicting requirements of (a) minimising heat loss to the surroundings and (b) maximising the heat transfer to the hot water system.

Other problems which are encountered with such a system include the prevention of corrosion when for example different metals are used in the construction of the sytem. This can be overcome to some extent by using special materials and plating corrosion-prone surface, and by dissolving a corrosion inhibitor in the circulating water. However, inhibitors tend to deteriorate with time and must be renewed periodically if their effectiveness is to be maintained.

In many cases it is necessary to take precautions against freeze damage such as draining the system if freezing conditions are expected, or adding antifreeze to the circulating water. The use of antifreeze is not recommended, since there may be a risk of such a fluid inadvertently entering the domestic water main.

In efforts to obtain the maximum energy conversion and energy transfer efficiency, designers have used large amounts of metals of high thermal conductivity, e.g. copper, which is relatively expensive. The use of large amounts of copper or other metals also increases the thermai capacity of the system and therefore limits the speed with which the system can respond to short periods of sunlight.

This is particulariy important in temperature regions such as North Western Europe where alternate short periods of sunlight and short overcast periods are frequently encountered.

It is an object of this invention to provide energy transfer apparatus which at least mitigates these disadvantages and offers greater efficiency.

According to this invention energy transfer apparatus includes a sealed container which comprises an energy collector coupled to a heat exchanger element, wherein the collector comprises a hollow element containing a liquid condensate and having an evaporation space in which the condensate is in contact with an energy absorbing wall, a vapour collection space coupled to the heat exchanger element, and a return passage for returning condensate from the vapour collection space-to the evaporation space. In operation of the apparatus, heat energy transferred through the wall of the collector element to the liquid causes vapourisation of the liquid adjacent the wall. The vapour rises to the vapour collection space and thence to the heat exchanger where it condenses and gives up its latent heat energy. The condensate formed in the heat exchanger is fed back to the collector element.

Liquid which is carried directly into the vapour collection space by the rising vapour is allowed to overflow into the return passage of the element, to be returned to the lower region of the element together with any condensate arriving from the heat exchanger. In this way, circulation of the condensate between the upper and lower parts of the element is maintained in normal conditions so long as vapourisation is taking place at the energy absorbing wall.

This creation of natural circulation in the collector element has the advantage that a relatively even temperature distribution is obtained in the element, which benefits efficiency. In addition, the possibility of condensate being carried into the connection between the vapour collection space and the heat exchanger is minimised.

When the apparatus is used as a solar energy conversion system, the collector element in the form of a panel is mounted in an inclined position on, for example, the roof of a building so that radiation from the sun is incident on the energy absorbing wall surface. This surface is preferably treated to maximise the net absorption of energy in the wavelength range of solar radiation. The radiation energy is converted to heat energy in the treated surface and transferred to the condensate in the panel. The vapour migrates to the heat exchanger, which may be part of a domestic hot water system. The heat exchanger acts as a preheat vessel where the latent heat energy of the vapour is transferred to the water to be heated which may then be fed to a further conventional, non-solar heating means.Since the apparatus operates by the transfer of latent heat energy, it is not necessary to have a large tempera ture difference between the liquid in the collector and the water in the preheat vessel. In fact, in a practical system a temperature difference of 2 degrees centigrade can be sufficient. Therefore, compared with the known systems referred to above, the collector temperature can be relatively low, with a corresponding reduction in heat loss to the surroundings.

One alternative use of the apparatus in accordance with the invention is its application to the recovery of heat from waste fluids. In industrial and domestic situations, heated water or other fluids are commonly allowed to flow into a drainage system, thereby representing a considerable wastage of heat energy.

By positioning the collector element (which may comprise an array of hollow tubes) so that the warm waste fluids pass over the energy absorbing wall, the condensate can be made to vapourise in the same way as it does when a collector panel is exposed in solar radiation, and the vapour is then conducted as before to a heat exchanger where the latest heat of vapourisation is released and heats, for example, cold water in a preheat vessel. In this application it is important that the condensate return passage of the element receives little or no heat from the waste fluids so that circulation of the condensate in the element is achieved.

In other heat recovery applications, the waste fluids could be at relatively high or relatively low, e.g. cryogenic, temperatures. In such applications, the liquid in the system would be chosen to suit the expected temperature range. Examples of liquids for use at extremes oftemperature are liquid nitrogen and mercury.

In a preferred embodiment of the invention, intended for use as a solar heating system, the collector element is a three-walled panel, having a rigid corrugated or dimpled centre plate sandwiched between a pair of relatively thin outer plates. The two cavities so formed both communicate with a vapour collection space at the top of the panel, and also with each other via a space adjacent the bottom edge. One cavity acts as a evaporation space and the other as the condensate return passage. By constructing the panel in this way, use is made of the negative pressure within the system to allow a light non-rigid material having a low heat capacity and good heat transfer characteristics to be used for the other plates. Details of this construction and other advantages it confers will be described below.

At the heat exchanger end of the apparatus, when it is required to heat a body of water in for example a preheat vessel, the preferred embodiment has a single coiled tube immersed in the water to be heated. Vapour fed via a connecting pipe from the collector condenses inside the tube, chiefly at the coldest part of the tube. This has the advantageous effect of creating an even temperature distribution in the vessel and transferring heat energy relatively efficiently since the release of latent heat energy is concentrated at the coldest part of the preheat vessel.

The heat exchanger tube can be single-ended and connected to the collector by a single pipe, the pipe performing the dual function of feeding vapour from the vapour collection space of the collector to the tube and at the same time allowing condensate to trickle back to the collector. The volume of returning condensate in the pipe is much smaller than the volume of vapour travelling towards the heat exchanger. The use of a single pipe connection means that the apparatus is relatively cheap to instal compared with most known solar energy systems which have two pipes between the collector and the heat exchanger, since fewer holes have to be bored and less lagging material is required.

The invention will now be described by way of example with reference to the drawings in which Figure lisa perspective view of a collector panel; Figure 2 is a diagram of a solar energy conversion system in accordance with the invention; Figure 3 is a detail section of the collector panel of Figure 1; and Figure 4 is a diagram of the system of Figure 1 together with equipment required for charging the system.

Referring to Figures 1 and 2 and 3, a solar energy conversion system has a solar energy collector in the form of a hollow panel 1 with a rigid inner support plate 2 and two relatively thin and lightweight outer plates 3 and 4, the plate 3 constituting a solar radiation absorbing wall. The panel 1 has a single opening to a flexible pipe 5 which connects the interior of the panel 1 to a heat exchanger in the form of a coiled tube 6 inside a preheat vessel 7 situated above the panel 1. The tube 6, the connecting pipe 5 and the panel 1 together constitute a sealed container which is charged with liquid and vapour in a manner described below.

The panel construction is more clearly seen in Figure 3 which is a transverse section of the panel shown in Figure 1. The inner support plate 2 is corrugated and acts as the primary constructional member of the plate, in that it supports the outer plates 3 and 4, since under normal conditions the pressure inside the panel and the rest of the sealed container is less than atmospheric pressure. All three plates 2,3 and 4 are, in this preferred embodiment, formed from 0.2 mm sheet metal. The outer plates, being relatively flexible, are drawn inwardly towards the inner support plate such that the spacing between the plates corresponds to the height of the corrugations. The three plates define a vapourisation cavity 8 and a condensate return passage 9, each having an average thickness of approximately 0.75 mm. The total thickness of the panel is only 2.0 mm.

The two cavities 8 and 9 are sealed at the sides 10 and 11 of the panel by soldering or seam welding, and communicate at the upper and lower edges of the panel with a vapour collection drum 12 and a circulation drum 13. The drum 12 defines a vapour collection space which communicates with the connecting pipe 5 as shown in Figure 1.

The radiation absorbing wall 3 is provided with a black body finish for maximum net radiation absorption and the whole panel is mounted in an unsealed galzed box 14 which may be mounted at a variable angle, depending on the expected elevation of the sun. Heat insulation material fills the space behind the panel 1 adjacent the condensate return passage to prevent heat loss by convection, conduction and radiation from the rear of the panel.

The panel 1 is filled with de-ionized water to a level 15 approximately corresponding to the top of the cavities 8 and 9 as shown in Figure 2.

The system is charged as foilows. Referring to Figure 4, a predetermined volume of water is poured into a pressure vessel 16 which has an isolating valve 17. With the valve 17 open and disconnected from the rest of the system (1, 5 and 6), the water in the pressure vessel 16 is heated to boiling point at atmospheric pressure and boiled for a time sufficient to dispel dissolved gases from the water, and air from the space 18 above the water in the vessel 16.

The isolating valve 17 is then closed and the heat source 19 remved from the vessel 16. The valve 17 is coupled to the rest of the system by the pipe 20, the isolating valve 21 and the heat exchanger sealing valve 22 are opened, and a vacuum pump 23 connected to the system is started. At this point the pressure vessel isolating valve 17 is still closed. The vacuum pump 23 evacuates the heat exchanger tube 6, the pipe connection 5 and the collector panel 1, and when this is completed the valve 21 is closed and the pump is switched off. The pressure vessel 16 is then inverted and the valve 17 opened to allow the distilled water in the vessel to flow into the panel 1 via the heat exchanger. The volume of liquid in the vessel is such that the panel 1 is filled to the level 15 as shown in Figure 2. The heat exchanger sealing valve 22 is closed and the charging equipment, i.e.

the pressure vessel 16, the valves 17 and 21,the pipe 20 and the pump 23, are removed from the now sealed apparatus.

Operation of the apparatus as a solar energy heating system will now be described with reference to Figure 2.

When solar radiation 24 is absorbed by the wall 3 of the panel 1, vapourisation of the liquid in contact with the wall occurs, and vapour rises to the vapour collection drum 12 as shown. The formation of vapour in the cavity 8 creates a density difference between the liquid/vapour mixture in the cavity 8 and the liquid in the condensate return passage 9, resulting in the advantageous circulation of liquid condensate in the panel 1. Upward movement of the condensate in the vapoursiation cavity 8 causes the condensate to overflow into the return passage 9 so that condensate flows from the passage 9 via the circulation drum 13 to the lower region of the cavity 8.

Vapour in the vapour collection drum 12 migrates upwards in the pipe 5 and into the heat exchanger tube 6, where it condenses due to the relative coldness of the water in the preheat vessel 7, and the latent heat of vapourisation is transferred. The condensate flows back down the inner surfaces of the pipe 5 under the influence of gravity to the vapour collection drum 12 and thence to the passage 9.

Since the panel 1 is relatively thin (the three plates are be only 0.2 mm in thickness) and the volume of liquid in the panel is relatively low compared with some known systems, the charged panel has a relatively low heat capacity, and is therefore able to take advantage of short periods of radiation energy input. Heat transfer to the preheat vessel can occur with as little as 2 degrees centrigrade difference between the panel temperature and the temperature of the water in the preheat vessel. This brings advantages in efficiency due to the relatively low heat loss by convection and re-radiation when the sun is masked.

The construction of the panel has further advantages in that, as the vapour temperature rises, the temperature of the condensate rises and the pressure inside the panel increases until the vapour The construction of the panel has further advantages in that, as the vapour temperature rises, the temperature of the condensate rises and the pressure inside the panel increases until the vapour temperature reaches 100 C. At this point the pressure inside the panel equals atomspheric pressure and the plates 3 and 4 begin to move outwardly away from the support plate 2. This increases the volume ofthe panel so that the liquid level 15 falls in the cavities 8 and 9 with a consequent reduction in the surface area of the plate 3 in contact with the liquid. This means that a greater area is available to re-radiate the heat in the wall 3.These factors result in a lower rate of vapour generation and subsequent heat transfer. The apparatus is thus self-regulating and a pressure safety valve is not required.

The ability of the plates 3 and 4 to move outwards also minimisesthe possibility oaf freeze damage to the panel.

The thinness of the plate 3, and the consequent good heat transfer between the surface of the plate 3 and the condensate, allows a relatively cheap and strong material such as tinned steel (tin plate) to be used in the construction of the panel. Tin plate also has the advantage that it has a relatively low electrochemical activity.

The method of charging the apparatus ensures that virtually all air is excluded from the apparatus, resulting in an oxygen free environment which minimises the possibility of corrosion and obviates the need for inhibitors in the condensate. De-ionized water is the preferred fluid.

Since condensation in the heat exchanger occurs chiefly at the coldest part of the tube 6, the formation of hot spots in the heat exchanger and a corresponding unwanted rise in the temperature of the condensate in the panel 1 is largely avoided.

Claims

1. Energy transfer apparatus including a sealed container which comprises an energy collector coupled to a heat exchanger element, wherein the collector comprises a hollow element containing a liquid condensate and having an evaporation space in which the condensate is in contact with an energy absorbing wall, a vapour collection space coupled to the heat exchanger element, and a return passage for returning condensate from the vapour collection space to the evaporation space.

2. Energy transfer apparatus according to claim 1, wherein the collector comprises a panel having a first outerwall, a second outer wall parallel with the first outer wall, and an internal dividing wall parallel with the first and second outer walls, the evaporation space being defined by the first outer wall and the dividing wall, and the return passage being defined by the dividing wall and the second outer wall.

3. Energy transfer apparatus according to claim 2, wherein the dividing plate has raised portions for contacting an inner surface of the first outer wall thereby to define the thickness of the evaporation space.

4. Energy transfer apparatus according to claim 2, wherein the dividing plate is corrugated, and wherein the outer walls are formed from sheet metal and are relatively flexible compared with the dividing wall so that when the pressure inside the collector is less than that outside, the outer walls engage the corrugated dividing plate.

5. Energy transfer apparatus according to claim 1, wherein the collector comprises a parallel-sided panel having first and second parallel plates of sheet metal defining therebetween the evaporation space, the panel further having a hollow upper manifold portion extending along one edge of the panel and defining the vapour collection space therein.

6. Energy transfer apparatus according to claim 5, wherein the collector is housed in a protective box, at least one wall of which is glazed.

7. Energy transfer apparatus according to claim 1, wherein the vapour collection space communicates with the heat exchanger element via a single connecting pipe for carrying both vapour and condensate.

8. Energy transfer apparatus according to claim 7, wherein the heat exchanger element comprises a coiled tube mounted inside a tank, the tube having an upper end which is sealable and a lower end attached to the connecting pipe.

9. A solar energy collector comprising a parallel sided panel of laminar construction, the panel comprising:- a first outer wall formed of flexible sheet metal for absorbing radiation energy.

a second outer wall formed of flexible sheet metal parallel to the first outer wall, and a rigid supporting wall arranged between the first and second outer walls, the first outer wall and the supporting wall defin ing a condensate evaporation space, and the second outer wall and the supporting wall defining a condensate return passage, the panel further comprising a vapour collection vessel forming an extension of the first and second outer walls and defining a vapour collection space extending along one edge of the panel and communicating with the evaporation space and the return passage.

10. A collector according to claim 9, wherein the supporting plate is corrugated or dimpled to support the outer plates at a predetermined clearance relative to the supporting plate, the outer plates being arranged to move away from the supporting plate when the pressure inside the panel exceeds the pressure outside the panel.

11. A collector according to claim 9, wherein the panel contains a liquid condensate, the level of which is substantially at the junction of the vapour collection vessel and the first outer wail when the panel is placed in an inclined position with the vapour collection vessel uppermost and when the pressure inside the panel is less than that outside the panel.