Environmental Engineering Reference
In-Depth Information
Table 4.4
Heat capacity and specific density of different heat accumulation media at a
temperature of 20 °C (see /4-5/)
Heat capacity
Density
in kJ/(m
3
K)
in kWh/(m
3
K)
in kg/m
3
in kJ/(kg K)
Water
Pebbles, sand
Granite
Brick
Iron
Oil
Pebbles-water
a
4.18
0.71
0.75
0.84
0.47
1.6 - 1.8
1.32
4,175
1,278 - 1,420
2,063
1,176 - 1,596
3,655
1,360 - 1,620
2,895
1.16
0.36 - 0.39
0.57
0.33 - 0.44
1.02
0.38 - 0.45
0.80
998
1,800 - 2,000
2,750
1,400 - 1,900
7,860
850 - 900
2,200
37 vol.-% water
Such storages can be charged directly or indirectly. The forced circulation sys-
tems mainly used in Central and Northern Europe normally have a pressurised
heat store with a heat carrier for the collector circuit plus a cold water inlet and a
hot water outlet. Often, the heat store has a second heat exchanger or an electric
immersed heater for auxiliary heating of storage. The storage is normally divided
into zones. The solar installation feeds the heat into the system at the lowest and
thus coldest point in order to be able to drive the collector with the highest possi-
ble efficiency. The volume for the auxiliary heating is at the top end of the stor-
age. Its size is determined by the efficiency and the required minimum running
time of auxiliary heater (Fig. 4.8).
Provision
volumen of el.
auxiliary heater
Provision
volume
of auxiliary
heater
Domestic hot
water outlet
Heat exchanger
auxiliary heater
Volumen
for solar plant
only
Insulation
Temperature
sensor
Heat exchanger
solar plant
Not useable
volumen
("Dead volume")
Cold water
inlet
Fig. 4.8
Zonal sectioning of water storage for solar plants (el. Electrical; see e.g. /4-6/)
High-grade or enamelled steel, or steel with a temperature resistant coating (ap-
proximately 120 °C) are used as corrosion resistant and long-life tank material. In
individual cases also temperature resistant, glass-fibre enhanced synthetic material
can be used. The tank is insulated with mineral wool, soft foam and special
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