A typical solar hot water system consists of a solar collector to heat the water, an insulated hot water storage tank, and pipework to connect the two together. Most systems will also incorporate a pump, and a temperature differential controller, an electronic device that turns on the pump only when the collector is hotter than the water in the storage tank.
The flat plate solar collector consists of a blackened plate, over which or through which a fluid, usually water, can be passed (other fluids used include air and special oils). The black plate has a clear or translucent cover which allows solar radiation to fall on the plate. Because the plate is black it absorbs radiation very effectively. If you doubt this try touching the bodywork of a dark coloured car and a white car that have both been parked in the sun: the dark car will be hotter because dark colours absorb more solar radiation than light colours, and matt black absorbs the most.
The cover, usually of glass or clear plastic, reduces the loss of heat from the absorbing surface while allowing the solar radiation to reach the surface. Without the cover the collector would lose so much heat that it would never become hot enough to be of any use. An increase in the number of covers, through double-glazing, increases the temperature of the collector and therefore of the hot water, but adds to the cost and the complexity of the construction. Double-glazing will also reduce the amount of radiation reaching the collector surface because some radiation is absorbed in each sheet of glass and some is reflected off each glass surface. Most solar collectors for domestic hot water supply in the UK are single-glazed. Behind the black plate a thick layer of insulation is used to reduce heat losses from the plate and the fluid passing through it.
The hot water storage tank can be either an open tank of glass reinforced plastic or a conventional copper hot water cylinder. It should be an indirect system: in other words the solar-heated water should flow through a coil of pipe (or heat exchanger) inside the hot water tank so that the heat from the solar water passes into the water in the tank through the walls of the pipe. The disadvantage of this system is that there is a loss of energy in the exchange of heat between the water in the coil and the water in the tank.
This can be overcome by using a direct system where the water from the tank passes through the solar collector and back to the tank without going through a heat exchanger. In such a system the water in the tank is constantly being topped up with fresh water from the mains. In an indirect system the same lot of water goes round and round the collector and is not replaced by fresh water. The air that is dissolved in water when it comes out of the mains accelerates the corrosion of solar collectors, pipes and tanks, even if these are made of copper. In a direct system fresh water is constantly being introduced, encouraging corrosion; but in an indirect system the dissolved air and any other impurities in the water are not continuously reinforced by fresh supplies so they do less damage.
In the simplest set-up, the hot water flows from the collector to the tank and back again by ‘thermosyphoning’: hot water is less dense than cold water so it rises and creates a flow round the circuit. When the sun stops shining the water cools and stops flowing. There are two main problems with a thermosyphoning solar installation.
The first is that the bottom of the hot water tank must be at least 900mm above the top of the collector for the flow to work properly. This often makes it difficult to find a suitable location for the collector, which usually cannot go on a roof because there would not be room above it for the storage tank. One answer is to put the collector on the south-facing roof of a porch or garage so that it is lower than the main roof and the tank can then go in the attic.
The other problem is that the water in the collector can freeze on cold nights, because the black surface which is so good at collecting solar radiation in the daytime will lose heat quickly to the sky on a cold clear night. You can overcome this problem by using anti-freeze in the solar collector circuit, which must then operate on an indirect system to avoid contaminating the hot water supply to the house.
Most solar hot water installations use a small pump of the type used for central heating to push the water round the circuit. This gives considerably more freedom in locating tanks and collectors, and allows smaller diameter and therefore cheaper pipe to be used for connecting them. The pump is often switched on and off by a temperature differential controller which turns on the pump when the temperature of the collector is higher than that of the water in the hot water storage tank. This ensures that the collector is not operated unless there is some useful energy to be collected.
Getting the most out of the system
The best way to use a solar water heater is as a ore-heater for a conventional supply. This requires the use of a solar hot water storage tank, the water from which forms the ‘cold’ feed to the existing hot water tank. When there is plenty of sunshine the water coming into the existing tank will be hot enough for direct use; when there is only a little sunshine the water coming into the tank will be at least a bit warmer than mains temperature, so the boiler or other water heating system will use less fuel to heat the water as required. The most efficient systems have a valve worked from the pump controller: this allows water from the solar tank to be used directly if it is hot enough before it passes through the existing tank.