Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 154

DHW system with closed natural circulation. Most solar thermal DHW plants in
Southern Europe are built with closed natural circulation systems. Fig. 4.14 shows
such a system consisting of a metal frame which holds the collector, the water
storage (with either horizontal or vertical axis) and all other necessary parts. It is
prefabricated and can be easily installed on the flat roof of a house. Only the cold
and hot water tubes have to be connected to the house installation. Electricity for
pumps or control is not needed. The store is mostly built as mantle tank with the
domestic hot water in the inner tank and the collector fluid floating in a mantle
around this tank.
One disadvantage is, that the collector loop can not be shut down, when the
water store is already at its maximum temperature. If the sun keeps shining, the
temperature in the water store rises until it starts boiling. As long as the sun is
shining, the boiling carries on and there has to be a pressure-controlled steam
release valve incorporated into the water circuit. One simple solution to the prob-
lem is to use cheap collectors with a low stagnation temperature (i.e. collectors
with non-selective surface).

Deventer

Insulation

Mantle
tank

DHW
tank

Heat carrier

Warm upward
flowing
collector
fluid

Frame

Cold
collector
fluid

Collector

Insulation

Expansion device

Emptying valve (winter)

Fig. 4.14 Example of a closed natural circulation system to cover domestic hot water
(DHW domestic hot water; see /4-6/)

Solar combined systems. If solar thermal systems have to cover larger portions of
the entire heat demand, also space heating has to be partially supplied by solar
energy. These plants are called solar combined systems. In general, improving the
heat insulation of the building is more efficient than integrating a solar system into
the heating system - and less cost-intensive.
There are various ways of integrating solar systems into heating systems. The
following parameters have the most significant impact:
− Type of heating boiler (rolling or automatic boiler with on/off operation, solid
fuel boiler);
− Type and characteristics of the heating system (high level storage mass, e.g.
floor heating) or low level of storage mass (e.g. radiators); high and low-
temperature system);