Legislation to foment the use of renewable energies and solar thermal energy in building construction: The case of Spain - Solar Energy Sciences and Engineering Applications - page 655

Figure 20.4.2 Hot water system with external heat exchangers (courtesy of Solaris S.L.).

The collectors can be installed on the rooftop of the building or in an obstacle-
free area where the building is located. The collectors are mounted on a supporting
structure that should be sufficiently sturdy to withstand the action of external forces
(e.g. snow, wind). The system should allow thermal expansion without transmitting
loads that could affect the integrity of the collectors of the hydraulic circuit.
The system consists of one or various tanks that store hot water until it is needed.
The connections of the heat exchangers with the main installation should permit their
individual disconnection by means of cut-off valves without interrupting the system's
operation. In Europe, solar storage tanks should be certified according to Directive
97/23/EC 20 . The installation has a heat exchange system that transfers thermal energy
captured by the thermal collectors or the primary circuit to the hot water for user
consumption. In certain cases, the heat exchanger is a coil of tubing directly inside the
storage tank (see Figure 20.4.2). In this case, the heat exchanger is installed on the
inner wall of the lower section of the tank.
When the heat exchanger is located outside the storage tank (see Figure 20.4.2), it
is made of copper or stainless steel plates and should be able to withstand maximum
operation pressures and temperatures. The minimum design power of an external heat
exchanger, P , in watts, depending on the solar collector area A in square metres, should
fulfil the following condition: P 500 A .

This installation generally has a supplementary back-up system running on an
alternate more conventional energy source to ensure that there is no interruption in
the heating supply. For a better energy performance, it is not advisable to use an electric
back-up system that generates heat by the Joule effect. The back-up system generally
consists of a boiler that runs on gas, biomass or diesel fuel.

≥

20 Directive 97/23/EC of the European Parliament and of the Council of 29 May 1997 on the
approximation of the laws of the Member States concerning pressure equipment.