Image: Rkraft via Wikimedia CommonsThe twin coil cylinder system is a cylinder which combines solar preheated storage with a heat source in a single container. Other solar water heating units rely on a two cylinder system, in which on the cylinder is used to hold the heating element, while the other is dedicated to storage.
Cooler Water at the bottom, hot water on top
The twin coil cylinder contains two electric immersion elements which can be found at the top half and at the bottom half of the cylinder. The top coil is attached to a boiler and functions as an emergency back up, while the bottom coil heats is attached to a solar panel which is used to heat the water. The twin coil cylinder system operates by taking advantage of the Laws of Thermodynamics: Cooler water settles at the bottom of the tank while hot water floats on top.
A boiler heats the top
On cloudy days or during the night, the boiler is used to heat the cylinder. The hot water near the top coils expand and becomes lighter than the nearby cooler water, and thus rises. The cold water near the top of the cylinder is displaced by the rising hot water which pushes it towards the bottom. This process of water displacement is known as a convection current, and it is what is responsible for circulating the hot water around the top of the cylinder. This is a fairly effective method of maintaining the temperature of the water; however, it is not an efficient way of heating the contents of the whole cylinder.
The solar installation heats the bottom
The same process occurs when the solar collectors heat the bottom coils. Except, instead of the water only circulating near the top of the tank, bottom coil heated water will heat the entire height of the cylinder. A Twin coil cylinder are much larger and pose some difficulty fitting into an existing airing cupboard than conventional water cylinders. However, the twin coil cylinder system benefits from solar panels throughout most of the year; and, if properly insulated, it can be an immensely cost efficient method for heating water.
The thermostat is located near the top of the tank, and it is responsible for regulating the temperature of the tank. The thermostat controls the boiler and responds to the solar heat when water is pulled from the tank. Optimizing the efficiency of a twin coil cylinder can be done by using a timer programmer to activate the boiler only in the evenings, after the solar panels have successfully heated the cylinder throughout the day. The twin coil cylinder's thermostat will ensure that the boiler will only be activated if the water inside drops below a certain temperature.