Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 142

Duration of storage. Heat stores can be distinguished as short-term, daily and
seasonal heat stores.
Short-term heat stores only store heat for a number of hours. One typical ex-
ample is the water tank of storage collectors integrated into the collector.
Daily heat stores can store heat for one to several days. This is the classic case
of solar thermal domestic hot water installations and solar combined systems (for
domestic hot water and space heating) with solar fractional savings of up to ap-
proximately 60 %.
Seasonal heat stores are mainly used if a solar thermal installation is supposed
to cover almost the entire heating demand. Large storage volumes are required in
that case. Water, aquifer and vertically ground coupled storages can be used.
− Water storages can be built above or below ground with a heat-insulated steel
or concrete cover or in sealed rock caverns.
− Heat accumulation in aquifers (i.e. water permeable, separated rock formations)
takes place by inflow of hot and removal of cold water through a specific ar-
rangement of wells. The heat store is discharged exactly the other way round
(see Chapter 9).
− Pebble bed-water storages consist of a sealed tank that is filled with pebbles
and water. These storages are self-supporting and can thus be produced
cheaply. Heat capacity is lower than in water storages. It is however possible to
achieve a similar layering as in water storages. Heat is added to or removed
from the different layers of the storage via heat exchangers.
− Probe storages use soil or rocks as the storage media. Vertical probes are bored
or rammed into the earth (see Chapter 9). Heat produced by solar energy is
added or extracted through corresponding pipes serving as underground heat
exchangers. Storage media are mainly rock, loam or clay. It has to be consid-
ered that the store cannot be located in an area with a groundwater stream as it
would divert the heat.
Sensors and control systems. The number and the type of sensors and control
instruments are largely dependent on the concept of the system. Natural circula-
tion systems normally do not require any active regulation instruments. In forced
circulation systems, mainly used in Central and Northern Europe, the collector
circuit is generally actively controlled by a temperature difference control device.
Temperature sensors on or inside the collector, and on or inside the storage,
measure the temperature and convert it into electronic signals. The temperature in
the storage is measured at the level of the heat carrier releasing the heat from the
collector circuit to the storage in the case of internal heat exchangers. If external
heat exchangers are used, the temperature is measured slightly above the store
outlet position to the heat exchanger. Within the collector, the measurement
should be taken at the hottest point near the outlet to the storage. Both temperature
measurement signals are compared in the control instrument. If the target collector
temperature exceeds the storage temperature, the collector circuit pump is
switched on. If the temperature difference sinks below a second set value, the