How do Solar Thermal systems work?

The components of a solar thermal system are:


There are 2 main types of solar collectors –


These contain a dark-coloured absorber component enclosed within an evacuated glass tube, shaped to reflect and concentrate radiation into a central receiver. The collector comprises a series of glass tubes mounted in rows and plugged into a manifold box at the top. The vacuum insulates the system and prevents almost all heat loss. This is especially ideal in winter when wind-chill would otherwise cause heat loss, and also in conditions of extreme cold or high humidity.  We utilise either of two evacuated tube technologies:

  • Heat pipe
    The heat is transferred to a heat pipe, which has a partial vacuum. Here an evaporating-condensing cycle operates using a volatile fluid, which has a low boiling point. The resultant vapour rises up the heat pipe to the condenser, which fits into the manifold. Here the latent heat is released by reverse transformation (condensation), and is transferred in the solar fluid to the cylinder. This cycle repeats continuously.
  • Direct flow
    In this version the solar fluid is circulated in a coaxial manner through the manifold and tubes. The heat exchange fluid passes through the manifold and transports the heat away through pipe work to the solar cylinder.


These collectors are usually constructed using flat sheets of absorber material and flat glazing. The absorber contains pipes, which in turn contain the fluid that transports the heat away. The surface of the collector resembles a Velux window and can be mounted in-roof or on-roof.


The twin-coil solar cylinder acts as a heat store for the solar gains, and is sized to hold 30% - 50% extra water than your current cylinder. It therefore acts as a ‘thermal battery’ and storing the energy generated.  There are 2 main types of cylinders –

  • Vented
    Vented solar cylinders utilize the existing cold water storage cistern above. They will be bigger than the existing hot water cylinder to ensure there is a sufficient solar volume. We can have them made to measure to better fit within the allocated space available.
  • Unvented
    Unvented solar cylinders will deliver mains pressure hot water around the property. Therefore shower booster pumps are not required. Hot and cold supplies will be balanced. The cold water storage cistern can be removed so making more space available in the loft. The cylinder can be located anywhere in the property. However, it is important to ensure that mains pressure is high enough with a good flow rate. Some extra safety controls are required and some annual checks need to be performed.

In a twin-coil cylinder there are two heat exchangers; the upper heat exchanger is connected to the boiler circuit, while the lower heat exchanger is connected to the solar circuit. The cylinder is tall and narrow to encourage stratification (temperature increases towards the top), and is well insulated (~50mm thickness) to prevent heat loss.


The heat transfer fluid transports the heat produced in the collector to the solar cylinder. It is 40% propylene glycol with water and inhibitor. This ensures freeze protection down to –28°C and ensures thermal stability at high temperatures with an increase in the boiling point to 150°C depending on the pressure. This solar liquid is also non-corrosive, non-toxic, non-irritant and biologically degradable.

The solar circuit is pressurised (unless a drain-back system) to approximately 1.5 bar and comprises insulated copper pipework in which are various components such as pump, expansion vessel, flow meter, pressure relief valve, air vent, non-return valve, fill-and-flush valves, pressure gauge. The automatic air vent releases any air trapped in the system.

The expansion vessel (if installed) is designed to incorporate expansion of the heat transfer fluid due to heating and increase in pressure. It is an enclosed metal container in which there is a flexible membrane. On one side is nitrogen under pressure and on the other side is the solar fluid.

The pump circulates the heat transfer fluid round the system when activated by the controller. A low speed setting will normally optimise efficient heat transfer.

The flow meter displays and permits the control of the volumetric flow of the heat transfer fluid. A flow rate of 2-3 litres per minute is recommended, i.e. around 1 litre per minute per square meter of collector.

The pressure relief valve is for safety and is designed to operate should there be a high-pressure build-up. However, the system pressure will rarely exceed 3 bar.

The pressure gauge (if installed) indicates the pressure within the system. It will vary slightly depending on the temperature of the fluid. Normally it will be between 1 – 3 bar. The minimum operating pressure that should be maintained is 1 bar.

The non-return valve prevents reverse circulation in the solar circuit.

The fill-and-flush valves are for filling the system with heat transfer fluid.

It is possible to include a PV (photo-voltaic) solar panel to generate enough power (DC) to drive the pump so that the entire solar system is independent of mains electricity.

Sometimes we install drainback solar systems in which the solar fluid drains back from the collector(s). In these systems some of the above components are not necessary. Additionally the fluid is not pressurised. We often specify these systems for situations where there may be a reduced hot water demand (e.g. holiday home or school). Our engineers will discuss which systems may be best for you depending on various factors such as annual hot water demand and building configuration. 



The controller must control the pump so that the sun’s energy is harvested in the best way. This is achieved by temperature difference regulation. Sensors are located on the collector and at the top and bottom of the solar cylinder.  The pump switch-on temperature difference is normally 6-8°C, (i.e. the collector is at least 6-8°C greater than the water temperature at the bottom of the cylinder).

The controller will display all sensor temperatures and produce data on kWh produced as well as cope with potential freezing and overheating situations.  Data can be stored in data loggers or remotely displayed on computer.