Environmental Engineering Reference
In-Depth Information
Figure 3.16
Storage Temperature
ϑ
S
for a 300-litre Storage Tank without
Loading or Unloading
environment. After more than 1 week, the storage temperature is only half the
initial value. Such a storage tank can therefore only maintain the temperature
for a few days. The ratio of volume to surface area increases for larger storage
tanks, so the relative heat losses decrease. Storage systems of 1000 m
3
or more
can achieve time constants of about 6 months. Such systems can be used for
seasonal storage, i.e. heat storage from summer to winter. Good insulation is
also very important for seasonal storage systems to keep the storage losses low.
However, realistic integrated solar thermal system hot water storage is
usually not operated under stationary conditions. The solar collector feeds
heat continuously into the storage tank and consumers take hot water from
the tank. Then, only detailed computer programs can estimate the variation of
the storage temperature (see CD-ROM).
The storage temperatures calculated above are all average temperatures;
however, most storage tanks have desirable
temperature stratifications
. The
temperature at the top of the storage tank, i.e. near the water outlet, is higher
than at the bottom, near the water inlet. This stratification can be considered
if the storage is subdivided into several layers. In this case, the heat flow
between all layers must be calculated separately.
Large systems for higher demands, such as for family houses, often use
two storage tanks
. Here, the tanks are connected in series. For a system with
solar preheating, the first storage tank contains the solar heat exchanger and
the second storage tank contains the heat exchanger for the auxiliary heater.
The solar fraction of these systems is relatively low and the profitability of this
system compared to one with low conventional fuel prices is relatively high.
Another concept with two storage tanks contains a solar heat exchanger in
both tanks. The solar collector can heat up both tanks separately. This can
