What does a solar controller do?
The so-called differential controller is connected to two (or more) thermostats and a pump. The thermostats measure the temperature of the water in the solar panel and in the tank. The pump is switched on when the solar panel temperature exceeds the tank temperature by an appropriate margin, about 6°. This temperature difference is set in the controller; some controllers can also display the water temperatures. See Resol Controllers.
What about freeze protection?
Freeze protection can be provided by two means; either the water can be treated with (Propylene) Glycol antifreeze or the controller can be configured to circulate the water just enough to prevent freezing; this requires
- A controller that supports this function
- An evacuated tube system, not flat plates
- A modest probability of hard frost – as is the case in most of Ireland, especially the West
- A reasonable security of electricity supply – normally the case
- A pipe run that is both not too long and that falls nearly continuously from the panel to the cylinder
Glycol degrades over time, especially if the system goes into stagnation; if this rarely or never happens, then the system is undersized; Glycol should be replaced every five years – standard recommendation
Additionally, the water should be drained out of the panel if the house is to be left empty for an extended period, especially during the winter; this is a straightforward process
What about overheating; is a heat dump needed?
Overheating is not a problem as such; the system is designed to accommodate it. If the hot water cylinder has reached a pre-set maximum temperature (typically 80°), the system will shut off the pump, even if the solar panel is still collecting heat; the system may then go into “stagnation”, which means that the water in the panel boils and the steam drives the water out of the pipework in and adjacent to the panel; the pressure vessel absorbs the increase in volume due to the steam; when the panel cools the steam condenses and the system returns to normal. By this means possible high temperatures are confined to the panel area, which is designed to cater for it. As a fail-safe, the system also includes a pressure release valve.
However, if the pipe run is not optimal, especially if there are “sumps” (low points in the pipe run) near the panel, the glycol can be continuously boiled, in fine weather. If this goes on long enough, the glycol can break down and even start to form solid matter. This problem is exacerbated if the pump is off for any reason e.g. another fault.
If the pipe entry through the roof (often through specially designed roof tiles / slates) is higher than the panel, which is common, the panel pipe run becomes a sump.
A well designed system should not need a heat dump; however there may be circumstances when it is advisable, particularly with flat plate panels and glycol.
Ultimately, it is a risk trade-off: the risk of damage due to frost against the risk of damage due to degraded glycol. There is also the recurring cost of glycol to consider.
Thermostatic control valve
In fine weather, solar panels can heat water to very high temperatures (up to 80°, see above); for safety, it is recommend that a thermostatic control valve is fitted, at the cylinder; this limits the temperature of hot water delivered to the taps, to a lower temperature, say 60°. A similar argument applies to a stove system.
How do solar panels affect the conventional heating system?
There is no direct link between the systems. Ideally, the conventional system (i.e. oil, gas) should have the heating and water systems decoupled and separately controlled; in this case the conventional hot water system can be timed to come on in the evening, or used on demand. This will then top-up the hot water to the required temperature; the hotter the water heated by the solar panel, the less energy the conventional system will consume.
Similar comments apply if you have a stove and a triple coil cylinder (see also Hot Water Cylinders); a stove complements solar water heating well, since you naturally use a stove when the solar is least effective – in winter, after dark; if you use your stove regularly, you may well find that you rarely need to use the conventional boiler for water heating.
It is recommended that all water heating systems include an electric immersion heater; although the most expensive form of water heating to use, the installation cost is small and when a modest boost to the water temperature is required, an immersion heater can be the most cost-effective, since it is 100% efficient; typically, this occurs with solar or a stove, which sometimes heats water to a lower temperature than required for use. See also Facts & Figures.
Is a controller always required?
If the solar panel is lower than the tank, and not far off vertically below it, the water could circulate by convection, similar to a stove i.e. without any controller or pump, which significantly simplifies the system, although antifreeze must be used. In practice, the solar panels are usually mounted on the roof, above the hot water tank, so a pump and a controller are needed.
Other control issues
There a few circumstances, in more complicated systems, when settings of various sub-systems may need to be considered as a whole; a couple of examples, not an exhaustive list:
- Most controllers have setting to control the maximum temperature that the cylinder should reach, above which the solar pump is switched off; a suitable value for stainless steel is 80° and for copper 60°; copper degrades more quickly at higher temperatures
- This temperature should be less than the safety cut-out of an immersion heater, typically factory set at 90° or 95°
- This temperature should be more than the trigger temperature for a heat dump – or perhaps a second cylinder, which might be a 75° – for a stainless steel cylinder
- Similarly, the water thermostat for any oil or gas heating (perhaps 65°) should less than the trigger temperature for a heat dump