A twin coil cylinder is one of the most popular ways to provide hot water with a solar thermal system. The reason for this is that it is both efficient on space and cost-effective when compared with using two cylinders. It works by taking advantage of the fact that hotter water floats on cooler water.
The cylinder has two heat exchanger coils arranged one above the other. The boiler (or other auxiliary heater) is connected to the upper coil, with the solar circuit connected to the lower one.
The twin coil cylinder
The boiler heats only the top part of the cylinder
When the boiler is providing heat to the cylinder a heating fluid is pumped from the boiler to the cylinder, where it flows inside the coil. The coil has thin metal walls which conduct the heat into the surrounding water. The heated water near the coil expands and becomes less dense than the surrounding cooler water, and so rises. Cooler water at the top of the cylinder falls to replace the rising hot water. This so called "convection current" means that the boiler heats the top part of the cylinder above the boiler coil.
The solar panels heat the whole height of the cylinder
The Solar Dedicated Volume is the volume below the boiler coil
The solar coil works in the same way, but because it is at the bottom of the cylinder, it can heat the whole height of the cylinder.
If the zone above the solar coil is already hot from the boiler, then the convection currents from the solar coil only heat the volume of water below the boiler coil. This volume is termed the "Solar Dedicated Volume". UK Building regulation Domestic Heating Compliance Guide require that this volume is at least 25 litres per square metre of solar panel area, or 80% of the hot water demand of the household (whichever is the lower). The reason for setting a minimum solar dedicated volume is to ensure that the solar panels have somewhere to put the energy they collect, even if the residents run the boiler during the day.
The way to get the best out of a twin coil solar cylinder is to use a timer programmer to control the boiler to come on only in the evening after the solar panels have had all day to heat the cylinder. The cylinder thermostat will ensure that the boiler will only switch on if the cylinder is not hot enough from the solar heating.
As hot water is drawn off, the volume available to the solar increases
As hot water is drawn out of the cylinder for bathing in the evening and the following morning, cold water is introduced at the bottom, and the hot water layer floats on top. The next day, the solar panels will have a good volume of cold water to get to work on.
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