SOL-Perpetua™ Solar Water Heating System Installation Instructions and Operating Manual For Series "A" Pumps
HOW A SOL PERPETUA™ BUBBLE PUMP WORKS
The bubble pump depends upon the boiling of the working fluid for its operation. The working fluid is basically water; enough glycol is added to keep it from freezing in cold climates. Because the glycol has little effect on the bubbling action, in this discussion we shall refer to the working fluid simply as "water".
The desired temperature of domestic hot water in the storage tank is usually 50 to 66 C (122 to 149 F). To obtain this temperature in the tank we require the working fluid in a solar collector to be about 65 to 75 C (149 F to 167 F) in order that the heat exchanger size is reasonable. If we are using a flat plate collector with a selective absorber we can easily reach this temperature. Evacuated tube collectors can do the same even on cloudy days.
If we were to allow the bubble pump to operate at atmospheric pressure, water would have to be about 100 C (212 F). Operating a flat plate collector at this temperature, however, would lower its efficiency considerably due to re-radiative and convective losses. We achieve a lower boiling temperature of the water in our system by sealing the system against leakage and evacuating all air from it. This causes the bubble pump and therefore the solar collector to work efficiently at a lower temperature, generally between 60 and 80 C (140 and 176 F) depending mainly on the temperature of the water in the bottom of the storage tank and the strength of the sun.
When heat is added to water it will boil at a temperature that varies with the pressure exerted on the surface of the water by the gas above it. The lower the pressure of the gas, in this case, water vapour, the lower the boiling temperature will be. (See Fig. 1 below).
Figure 1 : Boiling Point of Water
When heat is added to water in a closed system until it boils, the pressure rises because steam is generated. If the walls of the system are strong enough to contain the pressure, active boiling stops. In order to maintain boiling action at about 60 to 80 C in the bubble pump, the pressure is kept low by condensing the vapour in a chamber containing the cooler water coming back up from the heat exchanger at the storage tank. The upcoming water is heated slightly by the vapour before it flows into the bottom inlet of the solar collector panel.
When the sun begins to shine on the collector panels the water in them becomes warm. Because there is very little weight of water and very little pressure of vapour on the warm water in the collector panels, it begins to boil at a reasonably low temperature, perhaps 60 C. Vapour bubbles rise toward the bubble tube and at first collapse because they condense into the cooler water around them. As more and more bubble rise and condense in the bubble tube they heat the fluid in it until, instead of condensing and disappearing, they grow larger as they rise. They grow larger because the more they rise, the less pressure there is on them. As the vapour bubbles rise, they lift slugs of water up into Chamber A of the bubble pump called the Separator (Refer to Figure 2).
Raising the water level in Chamber A causes it to drain down the pipe leading to the heat exchanger. The warm vapour gathering in Chamber A travels through a tube leading to chamber B where it condenses in the cooler water returning from the heat exchanger at the storage tank. The cooler water is warmed somewhat by the condensing vapour before it drains out of the bottom of Chamber B on its way to the lower inlet of the solar collector panel to start the cycle again.
Now you are probably saying "But what makes the water go around like that?" It's really quite simple if you examine the two pipes leading water into the pump and the two pipes leading water out of it. Three of these pipes are full of water while the fourth pipe, the bubble tube, is partly water and partly water vapour. Water vapour at this low pressure weighs practically nothing so we can say that part of the bubble tube is empty. The spaces above these four pipes are interconnected so that the vapour pressures on their upper surfaces are practically the same. The lower ends of these tubes are also interconnected so that the presures from below are practically the same. As a result, water rises in the partly empty bubble tube; the weight of water in the other three pipes assures that this will happen.
As heated water rises in the bubble tube it boils violently throwing up slugs of water into chamber A. Because the outlet of Chamber A is higher than the inlet of Chamber B and the two are directly interconnected through the Heat Exchanger, cooler water is forced up into Chamber B. So long as heat comes into the collector panels and is withdrawn at the heat exchanger the pumping action will continue.