GB2172100A - Self-circulating solar water heater - Google Patents

Abstract

In a water heating system using solar energy heat collecting surfaces the water pump is powered by solar energy which heats and thereby raises the pressure of a gas in a chamber 3. This pumps water through T-pieces 5 and non-return valve 8 into reservoir 7. The temperature of this gas is lowered by intermittent discharge of water over the surface of the chamber (e.g. via syphing). This alternating heating and cooling is used to give a water flow over surfaces exposed to solar radiation. <IMAGE>

Description

SPECIFICATION Self-circulating system for solar-energy water heating In cases where the solar collectors have to be higher than the water storage system (often the case in domestic premises) it is not possible to use the the simple self-circulating effect of natural convection and some form of pump has to be introduced so that the water warmed in the solar collector can be returned to the tank at a lower level and replaced by colder water from the tank.

The system described here will perform this circulation by the use of solar energy and without the need for other external power sources or sensing instrumentatioin.

The circulating pump uses the expansion of heated air to create pressure in a closed vessel. This pressure is applied to one leg of a 'U' tube containing water and raises the level of the water in the other leg. This 'open' leg has its outlet situated above the closed air vessel so that when the water has risen sufficiently, it flows out and over the vessel sur- face. This flow of water cools the vessel surface and the air inside it contracts. The 'U' tube is fitted with non-return valves.One is positioned so that, during the cooling phase when the air in the chamber reaches subatmospheric pressure, it will allow water to be drawn up from a lower resevoir into the 'U' tube and the other, which is situated in the 'discharge' leg of the 'U' tube prevents atmospheric air entering this leg during the suction phase but allows water to pass during the expansion phase.

When applied to a solar-powered water heater the air vessel will have a blackened surface facing the incident solar energy and will be shielded by a transparent substance such as glass to prevent loss of heat and improve the solar energy collection properties.

The shaded surface of the air vessel will be suitably insulated to minimise heat loss. When the water discharges over the vessel, the water flow will be directed so that it runs over the surface directly exposed to the sun.

This will have the effect of transferring heat to the water from the warmest part of the system.

As described so far, the system is an application of a well-known principle (similar to the one used in the Newcomen steam engine) with the distinction that, as water heating is the main objective, the loss in pumping efficiency due to temperature cycling of the pump structure is of secondary importance so long as sufficient pumping effort is obtained.

For sufficient pumping effort and reliable operation it is necessary to arrange for the air in the closed vessel to cycle in temperature over a reasonable range. This temperature cycling is enhanced to the extent which is necessary for reliable pumping when the water discharging over the air chamber surface is made to flow in a 'spurt' at the end of the expansion phase to give a sharp cooling effect rather than allowing the same quantity of water to be discharged as a succession of droplets throught the expansion phase. (The actual minimum temperature cycle needed for reliable pumping will be different for different installations as the layout will determine the 'water head' to be pumped to raise the water from a storage tank to the discharge level.) Of the alternative ways to achieve the desired water hold-up and release, two are selected as likely methods for this application.

The first relies on the resilient expansion of the warm air to cause a water flow when there is an abrupt reduction in pressure. As an example of this. If the discharge valve has the characteristic that, when a rise in pressure across it causes it to open, it requires a lower pressure to keep it open, the air in the chamber will expand from the higher to the lower pressure and displace a volume of water equal to this expansion. Various means of achieving this valve pressure characteristic are available.

The simplest is probably a valve in which magnetic attraction is used to close the valve on to the seat.

The second is to arrange for the trickle discharge of water during the expansion phase to be collected in an intermediate vessel and then, when sufficient water has collected, to empty this accumulated water over the hot surface of the air chamber. Again there are various ways of achieving this but mechanical movement can be avoided if the collection vessel is designed so that it is self-emptied by a water siphon once a the water reaches a certain height in it.

By way of example, Fig. 1 shows an arrangement illustrating one arrangement of the water heater.

In this example, it is implied that the complete water heater system will consist of a multiplicity of small modules such as illustrated. Each of these draws water from the colder lower zone of a storage tank (not shown) and returns the warmed water to the upper levels of the same tank. The water from the warm upper level is used as the output from the solar water heater.

item 1 is an enclosing glass vessel (such as an inverted jam jar with a close fitting lid) which is positioned so that the sun's rays fall on it. The side (2) of the glass away from the sun is coated with a reflective paint such as alumium metallic paint to minimise the heat loss from this surface and reflect the solar energy back into the inside of the jar. In this glass vessel is a thin-walled air chamber (3) the surface of this is blackened to aid heat absorbtion. A pipe (4) leads from the foot of this chamber to a 'T' connection (5). The lower leg of the connection draws from a water storage tank (not shown) via a nonreturn valve (6). The upper leg of the 'T' connection leads through a water-tight gland in the lower end of vessel (1) to a collecting vessel (7) via a second non-return valve (8).

There are some advantages to be had in operation if the connecting pipe (4) between the air chamber (3) and the 'T' connection (5) is of a somewhat larger diameter than the diameter of the tube between the 'T' and the collecting vessel (7). Vessel (7) has a discharge pipe (9) of fairly small diameter. One end of this pipe protrudes through the bottom of the vessel and turns in an inverted 'U' shape with one open end near the bottom of vessel (7). This arrangement results in the pipe (9) acting as a water siphon when the level in the collecting vessel reaches the upper part of the inverted 'U' and flow is initiated in pipe (9) till the collecting vessel is empty. The other end of pipe- (9) opens out to a shaped nozzle which distributes the water from the collecting vessel over the surface of the air chamber (3).After the water has flowed over his heated surface (and been warmed in the process) it is drained from the bottom of the glass vessel (1) via pipe (10) and returned by gravity to the upper part of the storage tank (not shown).

Once the system had been installed, no particular action need be taken to start the pump operating as it will prime itself in due course.

If air at normal ambient temperature is in chamber (3) initially this will become warm and expand when the sun shines. This air will escape through the non return valve (8) and into the vessel (1) and thence to the atmosphere through a small vent (11). When a cloud passes over the sun or a similar event causes a decrease in solar energy, the pressure in the chamber (3) will fall to sub-atmospheric when the vessel again reaches its initial ambient temperature. This fall in pressure will cause valve (6) to open and a cetrtain amount of water to be drawn into suction tube. Successive cycles of sun and shade will prime the system till the 'U' tube formed by the tube 4 and the part of the delivery tube between non-return valves (6) and (8) fills. Suceeding sun-and-shade cycles will force water up past valve (8) and into the collecting vessel (7). When this becomes full to the point of operating the siphon, the system will operate in steady solar-energy conditions to warm and circulate water (as well as continuing pumping during the natural variations in incident solar energy).

Claims (5)

1. A means of using solar energy to heat water by the promption of water flow over a surface exposed to solar radiation. The energy required for this circulation or pumping effect to be provided by solar energy.

2. A pump or circulator which will operate when conditions are appropriate for the collection of energy from the sun and will not oper ate when conditions are such as to dissapate water heat when incident solar energy is insufficient to add energy to a heated store of water.

3. Pumping or circulating water by using solar energy to impart a pressure variation to a gas (normally air saturated with water va pour).

4. A pump or circulator as above which provides a means of alternately heating and cooling a gas in a cyclic fashion during perids of relatively constant solar energy input.

5. A pump or circulator which will self prime due to changes in incident solar energy.