Environmental Engineering Reference
In-Depth Information
Table 3.3
Thermal Conductivity of Various Materials
λ
Material
Heat conductivity
Material
Heat conductivity
λ
in W/(m K)
in W/(m K)
Steel
60
Cork
0.045
Iron
81
Sheep's wool
0.040
Ice (0°C)
2.23
Glass wool
0.040
Reinforced concrete
1.80
Rock wool
0.040
Glass
0.81
Polystyrene (PS)
0.034
Water (20°C)
0.60
Polyurethane (PUR)
0.030
Gypsum
0.45
Air (dry, 20°C)
0.026
Wood (spruce)
0.14
Glass fibre vacuum
0.003
Swimming pools in moderate climatic zones usually need heating systems;
otherwise they are usable for only a few weeks per year. For instance, about
500,000 swimming pools have been built in Germany. Since average ambient
temperatures are below 20°C even in summer, there is a huge potential for
solar pool heating. In many cases simple solar swimming pool heating systems
have already become competitive with conventional heating systems.
The heat demand of outdoor swimming pools corresponds very well to the
solar irradiation. In winter, when the solar irradiation is low, outdoor
swimming pools are generally not in use. During the pool season in summer
and transitional periods, solar heating is a good option. Today, huge amounts
of fossil fuels are wasted in outdoor pool heating, although solar pool heating
as shown in Figure 3.2 could replace most fossil fuel-based heating systems.
Water temperatures of exposed areas in central European climates are
usually between 16°C and 19°C during the pool season. A temperature rise of
a few degrees is normally sufficient for comfort. For such a low-temperature
heat demand, simple absorbers can be used. These absorbers convert solar
radiation to available heat for the swimming pool. A pump moves the pool
water through the absorber. The absorber heats up the water and returns it to
the pool. A hot water storage tank such as is common in domestic water
heating systems is not needed - the pool water itself serves as heat storage.
If a dark-coloured hose is laid onto the solar irradiated ground in summer,
the water inside the hose becomes hot in a relatively short time. A swimming
pool absorber is not much more complicated. It can be made of black tubes
that are installed permanently on large surfaces such as roofs.
The
absorber material
is usually plastic. However, the material must be
resistant to degradation caused by ultraviolet sunlight and chlorinated pool
water. Some suitable materials are polyethylene (PE), polypropylene (PP) and
ethylene propylene diene monomer (EPDM). EPDM has a longer lifetime but
also costs more. PVC should not be used for ecological reasons - it can emit
highly toxic dioxins if it burns.
The
pump
of the system should only operate if the absorber can achieve a
temperature rise of the pool water. If the pump operates under very cloudy
