GB2090963A

GB2090963A - Solar powered heating apparatus

Info

Publication number: GB2090963A
Application number: GB8134850A
Authority: GB
Original assignee: SUNSENSE Ltd
Current assignee: SUNSENSE Ltd
Priority date: 1980-12-17
Filing date: 1981-11-19
Publication date: 1982-07-21

Abstract

An indirect solar energy heating apparatus comprises a solar panel (10), a heat exchanger (14), a pump (18) and a drain-down tank (24). In normal operation liquid circulates through the system with the drain-down tank mainly containing air but with a layer of liquid in the bottom of the tank. When the pump in the system is switched off, e.g. when the temperature of liquid in the first system approaches boiling point, liquid in the system drains down into the drain-down tank, passing to the drain-down tank via a back pressure valve (34) and expelling air from the tank into the system via a vent pipe (32). <IMAGE>

Description

SPECIFICATION Improvements in and relating to solar powered heating apparatus Field of invention This invention concerns solar energy powered heating apparatus of the type comprising one or more panels facing towards the sun having ducts through which a liquid flows and is heated by thermal radiation impinging on the panel.

Background to the invention With the development of solar panels having improved radiation absorbing surfaces such as black chrome selective surfaces, very high temperatures can be attained on such panels.

Further increases in panel temperature can be expected with improved panel surfaces which will no doubt be developed in the future.

Water (or a water-based solution) is popularly used as the liquid medium. However, even in the United Kingdom, and certainly in hotter climates, boiling of the liquid can occur, which presents serious problems.

It is known to use a direct solar circuit, i.e. a liquid circulatory circuit through the solar panel and from which liquid can be drawn off for use, and simply to switch the pump off when the system becomes too hot so as to stop the liquid circulating. However, using a direct circuit has a number of design objections, the principal one being that as the circuit is always using fresh water, precipitation in the pipework of the solar panels becomes a major problem and efficiency very quickly falls off.

In an alternative attempt to overcome the problem of boiling to overcome the problem of boiling we have proposed the provision of a socalled drain-down tank in the circulatory liquid path of a first system in an indirect solar heating apparatus, i.e. apparatus wherein the liquid passing through the solar panel transfers heat by energy exchange to fluid in a second system which provides the heated fluid for use. Thus, the specification of our Patent Application No.

5595/78 describes and claims, inter alia, solar energy heating apparatus comprising a pumped circulatory first system in which liquid circulates through a heating panel to be heated by solar radiation impinging on said panel and through a heat exchanger wherein liquid in said first system transfers heat to liquid in a second system from which liquid can be drawn off for use, said first system including a drain tank into which the liquid contained in the first system is caused to drain when the pump in said first system is switched off.

See also the specification of our Belgian Patent No. 878156.

Solar energy heating apparatus in accordance with our proposals has been constructed and found to function well, and the present invention is concerned with an improved version of such apparatus.

The invention Accordingly, the present invention provides solar energy heating apparatus comprising a pumped circulatory first system in which liquid circulates through a heating panel to be heated by solar radiation impinging on said panel and through a heat exchanger wherein liquid in said first system transfers heat to fluid in a second system from which heated fluid can be drawn off for use, said first system having associated therewith a drain-down tank into which the liquid contained in said first system is caused to drain when the pump in said first system is switched off, first fluid communication means incorporating a restricting vent extending between the first system and the upper portion of the drain-down tank, second fluid communication means extending between the first system and the lower portion of the drain-down tank, a back pressure valve being provided in the first system between said first and second fluid communication means.

Apparatus in accordance with the present invention functions in a similar manner to our earlier apparatus as discussed above. Thus, in normal operation the liquid circulates through the first system with the drain-down tank mainly containing air but with a layer of liquid in the bottom of the tank. The restricting vent prevents any liquid from passing from the first system to the drain-down tank via the first fluid communication means and simiiarly prevents any air from the drain-down tank from passing to the first system.

Depending on the construction of the second fluid communication means, to be discussed below, there is either no liquid flow between the first system and the drain-down tank via these means, resulting in a static body of liquid being maintained in the drain-down tank, or alternatively, liquid from the first system passes through the lower portion of the drain-down tank via the second fluid communication means.

When the pump in the first system is switched off, e.g. when the temperature of liquid in the first system approaches boiling point, liquid from the first system drains down into the drain-down tank, passing from the first system to the drain-down tank via the second fluid communication means and expelling air from the tank into the first system via the first fluid communication means.

On restarting the pump, e.g. when the temperature of the liquid has fallen, the liquid is pumped from the drain-down tank to the first system via the second fluid communications means, with the air returning to the drain-down tank via the first fluid communication means.

As compared with our earlier apparatus discussed above, with the present apparatus the risk of air entrainment and the formation of air locks is reduced, leading to quieter operation.

Further, with embodiments in which there is no liquid flow between the first system and the draindown tank in normal use, heat losses are reduced, eliminating any need to lag the drain-down tank.

Preferably means is provided for causing the drain-down, i.e. switching off the pump, which means is sensitive to the operating temperature of the liquid in the first system in the panel, e.g. a suitable thermostat.

Preferably the drain-down is effected if the temperature of the liquid in the first system exceeds a predetermined maximum temperature and/or falls below a predetermined minimum temperature.

The size of the tank can vary according to the capacity of the system with the proviso that the tank itself is of sufficient capacity to allow the first system to fully drain down into the tank and still leave a space for air at the top of the tank, and, when the first system is circulating, to still have a layer of liquid at the bottom of the tank. A suitable typical working volume of the tank is found to be about twice that of the solar collectors -- i.e. the volume of the solar collectors plus a reasonable allowance for pipework of the first system.

Means is preferably provided for venting off any steam which may be generated, e.g. a pressure relief valve.

The tank does not necessarily have to be mounted vertically and it may be inclined to the vertical with the design and capacity adjusted to suit.

Since both high and low temperature extremes can be catered for the liquid in the first system need not be anything other than plain water and as the same drain-down principles can apply under cold conditions, anti-freeze need not be incorporated.

The restricting vent in the first fluid communication means may be of any suitable form. For example, the first fluid communication means may simply comprise an inverted "U" tube constituting a vent pipe extending between the first system and the top of the drain-down tank.

The back pressure valve in the first system, which is located downstream of the inverted "U" tube in normal running, acts to create a slight back pressure in the pipework sufficient to cause the liquid in the first system to rise up the vent pipe in normal running.

Aiternatively, in place of the inverted "U" tube a coil of pipe may be utilised. A coil functions in a similar way to an inverted "U" tube, with liquid hanging in the lower portions of the loops of the coil, providing a cumulative head effect. For example, if a coil having 10 loops is used with each loop having a head of 50 mm, then the total cumulative head will be equivalent to 500 mm. A coil has the advantage of compactness, facilitating handling.

The second fluid communication means may be of any form which permits the required fluid flow.

For example, these means may comprise a single pipe extending between the first system and the lower portion of the drain-down tank, opening into the drain-down tank below the level of liquid therein in normal running. Such a pipe acts as either an inlet or outlet to the drain-down tank as appropriate in start-up and drain-down operations, and in normal running no flow will occur through the pipe, resulting in a static body of water being maintained in the drain-down tank as discussed above.

Preferably, however, the second fluid communication means comprises separate inlet and outlet pipes, with the pipework of the first system leading directly to the lower portion of the drain-down tank below the level of liquid therein in normal running to constitute an inlet to the tank in normal running, with the pipework of the first system further extending from the base of the tank, to constitute an outlet from the tank in normal running. With such an arrangement, during normal running liquid circulating through the first system will also circulate through the lower portion of the drain-down tank. This arrangement ensures that there is a proper break in the flow sufficient to discharge air and hence reduce likelihood of noise problems, and for this reason this alternative arrangement is preferred.

With such an alternative arrangement the pipe which constitutes an inlet to the tank during normal running is preferably turned so that liquid entering the tank is deflected by the tank wall and caused to flow in a generally circular path thus creating a vortex whereby any air entrained in the liquid is removed. Further, the outlet pipework is preferably at least one size larger, most preferably two sizes larger, than the inlet pipework. For example, if the inlet pipework is, say, 1 5 mm in diameter, the outlet pipework is preferably 22 mm, more preferably 28 mm, in diameter.

Such an arrangement reduces the exit velocity of liquid from the tank and therefore reduces the risk of entraining air.

A similar effect is preferably also achieved in embodiments in which the second fluid communications means comprises a single pipe which constitutes either an inlet or an outlet; in this case the pipework of the first system downstream (in normal running) of the junction with the pipe extending therefrom to the draindown tank and also the pipe itself are preferably at least one size larger than the pipework upstream of the back pressure valve.

The centre line of the pump in the first system is preferably about 250 to 300 mm below the working liquid level in the drain-down tank. This relationship is found to give very good results in practice, providing a good pressure at the inlet side of the pump (in normal running) for satisfactory operation thereof. Preferably the minimum depth of liquid in the drain-down tank during normal running is also sufficiently great to eliminate any risk of the pump generating a vortex on emptying the tank and thus entraining air: a working liquid level of 100 to 200 mm is found to satisfy this requirement.

The drain-down tank may be of any suitable construction, and preferably comprises a copper cylinder.

In preferred embodiments the drain-down tank is fitted within a hot-water cylinder (which constitutes the heat exchanger) which itself is preferably lagged, so as to reduce heat losses and eliminate any need to lag the drain-down tank.

The restricting vent may also with advantage be located within the hot-water cylinder, particularly in embodiments in which this is in the form of a coil of pipe.

The pipework diameters in the first system are normally chosen so that the liquid velocity in the first system is about 1 m/sec when the apparatus is in its normal running condition. This typically means that for installations up to about 7 m2 the return pipework from the heating panel is of 1 5 mm diameter and 22 mm for circuits of 7/12 m2.

The invention will now be described, by way of example, with reference to the accompanying drawings.

In the drawings Figure 1 is a schematic illustration of solar energy heating apparatus in accordance with the present invention; Figure 2 is a schematic view of the drain-down tank illustrated in Figure 1, shown on an enlarged scale; Figure 3 is a vertical section of an alternative drain-down tank suitable for use in apparatus in accordance with the present invention; and Figure 4 is a horizontal section of the tank of Figure 3 taken along line IV--IV in Figure 3.

Detailed description of the drawings Referring to the drawings, in the apparatus illustrated in Figure 1 a solar panel 10 is supplied with water in a pipe 1 2 from the upper end of a heat exchange coil 14 in a hot-water cylinder 1 6.

The lower end of the coil 14 is connected to the output of a circulating pump 1 8 via a pipe 20. The input of the pump is supplied from a low level outlet 22 of a drain-down tank 24. Water is returned to the drain-down tank 24 from the upper end of the panel 10 via a pipe 26 which terminates within the tank 24 at a level which is close to the level of the outlet 22 so that only a small depth of water is required to remain at the bottom of the tank 24 to cover both the outlet 22 and the open end of the return pipe 26 and allow the water to circulate around the system by means of the circulating pump without air-locks occurring.

Cold water is supplied to the lower end of the cylinder 1 6 by a feed 28 and hot water (heated by the heat transferred from the exchanger 14) can be drawn off through hot water outlet 30.

A vent pipe 32 in the form of an inverted "U" tube extends between the pipe 26 and the upper end of the drain-down tank 24 and a back pressure valve 34 is provided in the pipe 26 between the location of the vent pipe 32 and the inlet to the tank.

Figure 2 shows the drain-down tank 24 in more detail. The tank 24 comprises a copper cylinder, the internal volume of which is selected to be greater than the internal volume of the remainder of the system (i.e. the internal capacity of the pipes, pump, heat exchanger coil and panel etc.) so that all the water required to fill the system can be stored in the tank 24 which thus serves as a preliminary reservoir. Typically the tank 24 is filled to the upper level A during initial commissioning.

A hole 36 is provided to allow the tank to be filled in this way and a cap 38 is provided for closing the hole after filling. A pressure relief valve (not shown) is incorporated into the cap 38 so that the system can operate at a positive (i.e. above atmosphric) pressure.

As shown, the inlet of pipe 26 to tank 24 is turned to promote creation of a vortex.

In the illustrated embodiment the pipe 26 typically has a diameter of 15 mm and, for the reasons explained above, the portion of the pipe downstream of the back pressure valve and also the pipe leading from the outlet 22 is typically at least one size larger, eg 22 mm diameter pipe.

The vent pipe may comprise any small diameter pipe, e.g. in the range 5 to 1 5 mm, with the height of the take-off point thereof from pipe 26 typically being about 50 to 100 mm above the top of the tank 24. The maximum height of the inverted "U" of the vent pipe 32 is typically about m.

Operation of the illustrated apparatus is as described above, and upon first operating the pump 1 8 water is sucked from the tank 24 and passed through the coil 1 4 and panel 10, displacing air from the pipes and panel etc., with the air passing via vent pipe 32 into the tank 24 allowing the water level in the tank 24 to drop until all the air has been displaced from the system into the space above the water remaining in the tank 24. This operating water level is denoted by reference B in Figure 2 and, for the reasons given above, is preferably about 250 to 300 mm above the centre line of the pump 18. This level B is maintained in the tank 24 while the pump is operating and water is circulating in the system.

During such normal circulation, the effect of the back pressure valve 34 is to cause water to rise up the vent pipe 32 to the level B' shown in Figure 2.

A static column of water is maintained in vent pipe 32 during normal running, ensuring that water is not easily forced into the top of the tank nor is air easily drawn from the top of the tank during normal running. The back pressure valve 34 is appropriately adjusted during commissioning to achieve an appropriate balance with the desired stationary column of water in the vent pipe.

In the event of an electrical failure or in the event that a thermostat (not shown) sensitive to the temperature of the water leaving the panel 10, operates (because the water temperature is approaching boiling point) so that electrical current is no longer supplied to the pump 18, the latter will stop which causes water to drain back down into the tank 24 via the pump 18 from the pipes, panel etc. which are above the tank. This displaces the air in the tank back into the system via the vent pipe 32 so that the water in the tank rises to level A with the water in the pipe 26 settling at corresponding level A'.

An electronic monitoring devioce or a simple thermostat may be used to determine the operating temperature of the solar panel and at a signal from the monitoring device or thermostat the pump 18 can be stopped. This can be arranged to occur when the primary water temperature from the solar panel has reached a temperature at or just below boiling point.

Additionally further temperature monitoring equipment or a thermostat may be incorporated to sense the temperature of the water in the cylinder 16 to shut off the pump 1 8 in the event that this water in the cylinder 1 6 attains a sufficiently high temperature. This temperature will of course be lower than the maximum operating temperature permitted within the primary system and will typically be set to a temperature in the range 60 to 800C (this temperature range being found most satisfactory for domestic hot water).

When the pump 18 is stopped air in the tank 24 returns to the upper part of the system via the vent pipe 32 and water in the upper part of the system flows down into the drain-down tank to displace the air therefrom as described above.

If the pump in the event of an electrical failure, automatic drain-down will again occur so that the system will fail safe. The risk of boiling is thus very much reduced but it is inevitable that if the operating temperature of the solar panel is set at a high level very close to boiling, some steam will inevitably be generated and in any case changes in air volume due to changes in temperature will result in variations of pressure within the circuit. It is thus very desirable that the air/stream expelled from the circuit is kept to a minimum and for this reason the system is preferably pressurised as has hitherto been described so as to operate with a relief pressure of perhaps 0.7 kg/cm2 (10 Ib/in2).

Figures 3 and 4 illustrate an alternative apparatus in which the vent pipe 32 is replaced by a vent coil 40 which comprises 5 coils of 8 mm diameter soft tube 200 mm high and 100 mm wide. As before the pipe 26 is of 15 mm diameter and is provided with a back pressure valve 34, e.g.

a 1 5 mm diameter ballofix or similar valve.

The outlet 22 in the base of the tank 24 is connected to 8 coils of 28 mm diameter tube, 350 mm in diameter, which leads to the pump and other components (not shown).

The tank 24, vent coil 40 and outlet coil are fitted within a lagged hot-water cylinder 42 as shown in order to minimise heat losses. The cylinder 42 functions in the usual way receiving cold water from an inlet (not shown) in the base thereof and providing hot water from outlet 44, the water being heated in its passage through the cylinder by contacting components housed therewithin, including heat exchange coil 46.

In order to permit access from the exterior of the cylinder 42, the hole 36 of tank 24 is provided with a bent 28 mm diameter stub pipe 48 which extends outwardly through the cylinder wall, with the cap 38 fitted on the end thereof, external of the cylinder 42.

A sensor pocket 50, 8 mm in diameter is provided in the wall of cylinder 42 for monitoring purposes.

The illustrated tank 24 has a capacity of approximately 210 1 and is suitable for use with solar collectors having an area of 4.5 m2 with a primary circuit having a length of 20 m or less of 1 5 mm diameter tube.

Claims

1. Solar energy heating apparatus comprising a pumped circulatory first system in which liquid circulates through a heating panel to be heated by solar radiation impinging on said panel and through a heat exchanger wherein liquid in said first system transfers heat to fluid in a second system from which heated fluid can be drawn off for use, said first system having associated therewith a drain-down tank into which the liquid contained in said first system is caused to drain when the pump in said first system is switched off, first fluid communication means incorporating a restricting vent extending between the first system and the upper portion of the drain-down tank, second fluid communication means extending between the first system and the lower portion of the drain-down tank, a back pressure valve being provided in the first system between said first and second fluid communication means.

2. Apparatus according to claim 1 , further comprising means for switching off the pump, which means is sensitive to the operating temperature of the liquid in the first system in the panel.

3. Apparatus according to claim 1 or 2, wherein drain-down is effected if the temperature of the liquid in the first system exceeds a predetermined maximum temperature and/or falls below a predetermined minimum temperature.

4. Apparatus according to claim 1,2 or 3, wherein the tank is of sufficient capacity to allow the first system to fully drain down into the tank and still leave a space for air at the top of the tank, and, when the first system is circulating, to still have a layer of liquid at the bottom of the tank.

5. Apparatus according to any one of the preceding claims, wherein the working volume of the tank is about twice that of the solar collectors.

6. Apparatus according to any one of the preceding claims, further comprising means for venting off any steam which may be generated.

7. Apparatus according to any one of the preceding claims, wherein the liquid in the first system comprises water.

8. Apparatus according to any one of the preceding claims, wherein the first fluid communication means comprise an inverted "U" tube constituting a vent pipe extending between the first system and the top of the drain-down tank.

9. Apparatus according to any one of claims 1 to 7, wherein the first fluid communication means comprise a coil of pipe extending between the first system and the top of the drain-down tank.

1 0. Apparatus according to any one of the preceding claims, wherein the second fluid communication means comprises a single pipe extending between the first system and the lower portion of the drain-down tank, opening into the drain-down tank below the level of liquid therein in normal running.

11. Apparatus according to claim 10, wherein the pipework of the first system downstream (in normal running) of the junction with the pipe extending therefrom to the drain-down tank and also the pipe itself are at least one size larger than the pipework upstream of the back pressure valve.

12. Apparatus according to any one of claims 1 to 9, wherein the second fluid communication means comprises separate inlet and outlet pipes, with the pipework of the first system leading directly to the lower portion of the drain-down tank below the level of liquid therein in normal running to constitute an inlet to the tank in normal running, with the pipework of the first system further extending from the base of the tank, to constitute an outlet from the tank in normal running.

13. Apparatus according to claim 12, wherein the pipe constitutes an inlet to the tank during normal running is turned so that liquid entering the tank is deflected by the tank wall and caused to flow in a generally circular path thus creating a vortex whereby any air entrained in the liquid is removed.

14. Apparatus according to claim 12 or 13, wherein the outlet pipework is at least one size larger than the inlet pipework.

1 5. Apparatus according to any one of the preceding claims, wherein the centre line of the pump in the first system is preferably about 250 to 300 mm below the working liquid level in the drain-down tank.

1 6. Apparatus according to anyone of the preceding claims, wherein the minimum depth of liquid in the drain-down tank during normal running is sufficiently great to eliminate any risk of the pump generating a vortex on emptying the tank and thus entraining air.

1 7. Apparatus according to any one of the preceding claims, wherein the drain down tank comprises copper cylinder.

1 8. Apparatus according to any one of the preceding claims, wherein the drain-down tank is fitted within a hot-water cylinder (which constitutes the heat exchanger).

19. Apparatus according to claim 18, wherein the hot water cylinder is lagged.

20. Apparatus according to claim 1 8 or 19, wherein the restricting vent is located within the hot-water cylinder.

21. Apparatus according to any one of the preceding claims, wherein the pipework diameters in the first system are chosen so that the liquid velocity in the first system is about 1 m/sec when the apparatus is in its normal running condition.

22. Solar energy heating apparatus substantially as herein described with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.

23. Solar energy heating apparatus substantially as herein described with reference to, and as shown in, Figures 3 and 4 of the accompanying drawings.