Usually, 300 litres (80 gallons) of hot water storage is sufficient for four people. The volume of the storage tank can also be calculated from the solar collector area. Typically, for every square metre of solar panel about 50–100l (15–25 gallons) of storage volume is appropriate, depending on the solar resource at the location. So, for example, for a system with 5m² of solar panel you might need a tank capable of storing around 300–375l.
Figure 3.34 Flat-plate collectors on a house in the UK. Source: © Energy Saving Trust.
Table 3.2 Collector area, assuming a requirement of
75 litres of hot water per person per day.
latitude collector area per person (m2/ft2)
0° 0.5/5.5 20° 0.75/8 30° 1/11 35° 1.3/14 40° 2/22 50° 2.5/27.5 60° 3/33
at which it does so depends on the temperature difference between the hot water at the top of the tank and what is adjacent to it: the inside surface of the tank and the cooler water below. It also depends upon the degree of convection occurring within the tank. We can limit these variables. The first is simple: by wrapping as much insulation around the tank as possible. The second is, in practice, dependent on the available space, the budget and the climate, as discussed below. The third variable – convection – is affected by the positions of the coils, any electrical heating element present, the location of the outlet and inlet pipes, and the temperature of the water at these points. Systems to control this are called stratification devices.
Hot climates
If only one tank were used, under sunny conditions it would rapidly reach a maximum temperature throughout, close to that of the collector fluid temperature. After this it could not get much hotter. If the collector fluid temperature were 82°C (180°F) and the absorber plate temperature 93°C (200°F) the rate of heat transfer would be negligible. This means we would be losing the possibility of capturing more heat than the approximately 4kWh this may contain. Might we compensate by having a larger tank? If so, the tank might not be fully heated at other times. In this case, stratification occurs: the water in the tank will contain levels, with the warmest at the top and the coldest at the bottom. Heat will always want to leak from the top level to the bottom. Therefore, if the tank is too big for the size of the collector then the collector will not operate at maximum efficiency. If the tank is too small, some of the heat collected will be wasted. The storage volume needs to be matched to the collector area for as much of the year as possible with minimal heat wastage. In both cases, to collect more heat we would need more tanks.
Suppose we had two, or even three, well- insulated tanks, in a sequence. The closed loop from the collector passes through these tanks. As it does so, it transfers its heat successively to the water in each tank. The first tank will start off being the warmest, since it is first in line to receive the heat. Tank two will receive the heat in the loop left over from tank one. Tank three will collect the last bit of heat. The heat transfer fluid is then recirculated back into the collector to gather more heat and start the process again. This way, the water in tank one reaches the maximum temperature. Any further heat from the collector is then absorbed by the second tank. And so on. But crucially, the heat in the first Figure 3.35 The use of two, or, in this illustration,
three, well-insulated tanks in sequence, maximizes the output of the collector.
Solar collector Expansion tank Water pump
Tank 1 Tank 2 Tank 3
145.9
Hot water out
degrees of sophistication to maintain stratification and maximize efficiency. Near the bottom of the tank in Figure 3.36 is the heat exchanger coil in the circuit that returns to the collector. This pre-heats the water, causing it to rise. In the top half is another coil. The fuel source for this coil may be electricity, gas or a biomass boiler. This source is controlled by a thermostat, and it is
Figure 3.36 A basic twin-coil
sealed, indirect single tank system.
Source: © Wagner Solar UK Ltd.
Figure 3.37 Another sealed,
indirect single tank system utilizing a twin coil, but with stratification devices. Notice how the hot water is taken off at a higher point in the stratified water levels, the space heating is taken from a lower level, and returned to a lower level still. Sensors detect the temperature at different outputs and different levels within the tank, and apply the heating, remove the water, or return the water, at different levels accordingly, to maintain the stratification and maximize efficiency.
brought into play at times when there is insufficient heat from the sun to bring the temperature of the water to the required level. Figure 3.37 shows a more sophisticated tank employing stratification devices that regulate the input and output of heat and water. Stratification devices help to curtail water movement by convection within the tank, and include baffles and compartments. These are the most efficient type of tank.
If there is sufficient space in the building, two or more tanks (Figure 3.39) can be employed. The first is a buffer tank to store more solar-heated water. This water is then drawn into the primary tank when it is needed, where its temperature can be topped up from the auxiliary heating source. This means Figure 3.38 A simpler layout for stratification,
without sensors.
Figure 3.39 A twin tank,
sealed, indirect system. Solar heated water is stored in a buffer tank, and fed into a second tank, which can top up the temperature using a secondary heat source if required.
Source: © Wagner Solar UK Ltd.
that this secondary source has to expend less energy heating up the water. There are single tanks available which combine a domestic hot water tank situated inside a heat store – in other words, a tank-within-a-tank. This can be used if space is at a premium.
If the property already utilizes a combi boiler, multipoint, single point or electric point-of-use heating appliance, then the auxiliary heater or boiler can be supplied by the water from the solar heater tank, as if it were coming from the cold mains. When domestic hot water is drawn off the system, preheated water is fed into the boiler. However, specialist advice is needed with such a system, because only some boilers are designed to take preheated water. The thermostat must be set so that the incoming water temperature is not so hot that it damages the boiler.