3. Medium temperature systems

Two basic types of systems are common:

1. Close coupled (thermosiphon) natural circulation systems where the storage tank is physically connected to and above the collector (as seen on many roofs).

2. Remote tank (forced circulation) systems where only the collector is exposed to the sun, and the storage tank is in another (remote from the collector) location, often in the roof space or even at ground level in some cases.

A. Close coupled system

The advantage of the close coupled system is:

• high efficiency
  

• lower heat losses between the collector and the tank
  

• more rapid installation
  

• and generally a lower price.
  

The tank must always be above the collectors in a given distance range.

These systems can provide gravity feed to the hot water outlets, or can provide mains pressure hot water. Some of the low pressure (not mains pressure) systems have an electric booster pump on the outlet to increase the final outlet pressure.

The system controls of the close coupled units are minimal.

B. Remote tank (forced circulation) systems

Depending on the final location of the storage tank in the remote or forced circulation type, there may be a pressure pump in the system.

Particularly in the low level tank systems, an electronic control system is required to sense the temperature of the water in the collector then start a pump to ensure that the water in the tank remains as hot as possible.

The disadvantages of the close coupled type are:

• the full weight of water is supported by the roof
  

• the poor aesthetics of the tank on the roof
  

• the need to bring the cold and hot water piping to the roof location.
  

In times of low solar activity, it may be necessary to boost the water temperature in the tank with the aid of an electric element, or gas booster. Some systems allow for boosting from a fuel (wood) heater in the winter, using a wetback coil arrangement.

The type of collector material is of great importance. Ideally this material (or design) must absorb as much heat energy as possible, whilst reflecting or radiating as little heat as possible to maintain high efficiency. Many different materials, often called selective surfaces, are in common use. The materials include:

• copper plate with finned piping
  

• plastic moulded collectors
  

or the most efficient (but costly) type being a matt black piping collector in a vacuum (evacuated tube). These evacuated tube types are almost 100% efficient in the conversion process.

Insulation of both the tank and collector is critical for
performance to reduce all heat losses