Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 133

To determine if this approach is valid the direct solar energy water heating system per-
formance thus needs to be compared with that of an indirect system. The latter should
contain the optimum aqueous propylene-glycol solution concentration that yields the
maximum total output during all the months where it can survive the minimum ambient
temperature.

4.4.4 Sensible and latent heat storage
Energy storage is employed in solar thermal energy systems to enable excess energy
produced during high insolation to be available when insolation is low or non-existent
(at night). Energy storage may be needed; for where some of the solar thermal energy
produced during the day is stored for use later during the night or to provide energy over
sequences of cloudy days. There is a broad range of heat storage media for solar thermal
energy systems. However, practical design considerations limit thermal energy storage
options to sensible and latent heat energy storage in liquids/solids and phase change
materials respectively. Thermochemical energy storage has considerable promise but
as yet to realise practical deployment.
In sensible heat thermal energy storage, cold fluid in an insulated store is heated
to a higher temperature by the hot fluid from the solar energy collectors. Colder fluid
is withdrawn from the bottom of the store and is heated in the collector. The hot fluid
from the collector returns to top of the hot water store. The less dense hot storage
fluid will form a stratified layer delineated by a thermocline on top of the cold fluid
(Hollands and Lightstone, 1989). Water heated by the solar energy collector can enter
the store at a temperature lower than that of the local stored heated water. This can
ensure of, for example, high insolation has been succeeded by cloudier skies. Fluid inlet
arrangements have been devised to reduce destratification by water solution of colder
water to the warmer layers of a store (Lavan and Thompson, 1977; Davidson and
Adams, 1994). When the hot fluid is withdrawn from the store, the latter is usually
replenished by cold mains water. The introduction of such cold water is best accom-
plished via a low velocity flow that does not disrupt the stratification of the hot water
store (Eames and Norton, 1993; Furbo and Fan, 2008). This can cause an unacceptable
rate of hot water delivery in both industrial and domestic (e.g. showers) applications
that will require intermediate stores to accomplish end-use effectiveness. Large-scale
sensible heat storage systems (Lund, 1986) have been built to supply district heating
(Dalenback, 2010). The majority of large-scale interseasonal storage systems serve
housing via distinct heating networks. Systems with output of 7.0 MW are currently
most prevalent in Denmark as can be seen from Table 4.4.1 which summarises such
systems with an output > 4MW th in operation in 2010 (Dalenback, 2010).
The cost of thermal energy storage systems is dominated by the initial cost of the
storage medium. The use of water or steam (assuming a low cost pressurized storage
tank) as a storage medium reduces storage fluid costs. In addition, the use of water or
steam as a storage fluid in a solar thermal electric system using a steam-driven power
generation unit obviates the need for an oil/water steam generator.
In a latent heat energy storage system, the

• latent heat storage materials must be of low cost and available readily

• latent heat storage material, if a mixture, must not separate into component

materials after repeated phase change cycles.