Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 151

sealed off and a frost-resistant liquid flows through. This concept of closed forced
circulation is the most practical solution for applications in Central and Northern
Europe. If used within buildings, the collector is normally installed on the roof.
The heat from the collector circuit is normally transferred to a heat store in the
basement. As for closed natural circulation systems, in addition an expansion tank
and a pressure control valve are required. Additionally, a check valve, as in the
open forced circulation system, also has to be installed.

4.2.5 Applications

Solar heating of open-air swimming pools. One of the best ways of using solar
thermal energy is heating open-air swimming pools; where the timing of demand
for heat and the available solar radiation more or less correlate. Additionally, an
external heat store is not required as the open-air swimming pool filled with water
can function as the storage. As the water in the pool only has to be heated up to
comparatively low temperatures (a maximum of approximately 28 °C), the use of
simple and inexpensive non-covered absorber mats, either installed on the roof of
the open-air swimming pool or an adjacent free space, generates high energy out-
put.
Fig. 4.12 shows the diagram and the flows of a solar-heated open-air swim-
ming pool. Whether an additional heating source based on conventional energy
carriers is necessary, largely depends on site-specific requirements. Thus, the heat

gains of the open-air swimming pool consist of the following: energy Q . abs released
into the pool by the absorbers, heat gains created by radiation impinging on the

pool Q . G , and the release of heat by the pool users Q . human . Convective thermal

losses Q . conv , radiation losses Q . rad and evaporation losses at the water surface Q . evap

as well as the transmission losses into the soil Q . cond are the inherent losses. Due to
the water circulation ( m . in or m . out ), a small amount of heat is also lost in the circu-
lation pipes to the ambient.

In an approximation, the total of radiation and convection losses ( Q . rad and

Q . conv ) is linearly dependent on the difference between the water temperature in the
pool and the mean temperature of the air. If the ambient temperature is above the
pool water temperature, the convective thermal flow is reversed; the pool water
then convectively absorbs ambient heat. The thermal losses caused by evapora-
tion, which are normally the highest ones, depend on the pool area, the wind
speed, the atmospheric humidity and the difference between water temperature
and ambient temperature. Transmission losses to the ground are low, at around
3 % of the entire heat losses.
If the pool is covered over night, the convection, radiation and evaporation
losses can be reduced significantly. Covering the pool for ten hours with the stan-
dard absorber materials reduces evaporation losses by approximately 30 % and
the radiation and convection losses by approximately 16 %.