Environmental Engineering Reference
In-Depth Information
1.5-2 times the daily demand. Besides the storage volume for the daily
demand, a standby volume of 50 per cent and a preheating volume of 20 litres
per square metre of collector surface should be considered. Commercial
pressurized hot water tanks are available in sizes from less than 100 litres to
more than 1000 litres. The recommended storage size for a one-family house
with four to six persons is between 300 litres and 500 litres.
Most solar storage tanks have two heat exchangers (see also Figure 3.4).
The heat exchanger of the solar cycle is in the lower part of the storage tank
and the heat exchanger for the auxiliary heater is in the upper part. The tank
has an opening near the middle of the heat exchangers for integrated
temperature sensors for the control system. The cold water inlet is at the
bottom and the hot water outlet at the top of the storage tank to achieve good
heat stratification.
Figure 3.15 shows a horizontal, cylindrical hot water storage tank with
spherical ends. Storage losses are calculated for this tank, as an example. Heat
storage tanks always suffer
heat losses
due to heat transition through the
insulation. Good insulation should have a thickness of at least 100 mm for a
heat conductivity of
= 0.04 W/(m K). Some new materials have very low
heat conductivities; for instance, a super-insulation glass fibre vacuum
insulation can reach heat conductivities of
λ
λ
= 0.005 W/(m K) at pressures
below 10-3 mbar.
The heat storage capacity of a hot water tank is:
(3.37)
This heat capacity depends on the temperature difference between the average
storage temperature
ϑ
A
as well as on the heat
capacity
c
and mass
m
of the storage medium. The heat capacity of water at a
temperature of 50°C and with a density of
ϑ
S
and the ambient temperature
ρ
H
2
O
= 0.9881 kg/litre is
c
H
2
O
=
4.181 kJ/(kg K) = 1.161 Wh/(kg K). Hence, the heat storage capacity of a
300-litre hot water storage tank with a temperature difference of 70°C is 24
kWh.
The
storage losses Q
•
S
of a cylindrical and spherical hot water storage tank
are the sum of the losses
Q
•
S,cyl
of the cylindrical part and the losses
Q
•
S,sphere
of
both spherical caps:
(3.38)
The losses in the cylindrical part
(3.39)
can be calculated similarly to the losses of the pipes in the previous sections
with the heat transition coefficient
k'
, the length
l
cyl
and the difference between
the average storage temperature
ϑ
S
and the ambient temperature
ϑ
A
.
The heat conductivity of the insulation
λ
, the surface coefficient of heat
transfer
α
between insulation and air as well as the outer diameter
d
o
and the
