Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 176

radiation is inhibited. Such absorber materials are thus characterised by high ab-
sorption coefficients α abs with regard to solar radiation and low emission coeffi-
cients ε abs in terms of long-wave heat radiation: they are sometimes also referred
to as α/ε coatings (see Fig. 5.2).
5.1.3 High-temperature heat storage
Solar radiation is an energy source whose intensity varies deterministically due to
the rotation of the earth (day/night) and stochastically as a result of actual mete-
orological influences (clouds, aerosols, etc.). To compensate for such fluctuations
thermal storages can be applied.
In this respect, heat transfer medium storage, mass storage and storage of
phase-change material are distinguished.
- In case of storage of the heat transfer medium, it is intermediately stored in
thermally insulated containers. However, this implies that the heat transfer me-
dium is inexpensively available and has a high volume-specific heat capacity to
minimise container costs. To date, thermal oil and molten salt containers have
been applied; however, also water/steam accumulators have been planned. The
advantage of this storage mode is the constant temperature of the hot heat
transfer medium, which is only reduced by heat losses of the storage tank (and
thus is a function of the storage period, the container surface and the insula-
tion).
- In case of mass storage, the heat transfer medium thermally charges a second
material of a high heat capacity. For this purpose a good heat transfer (i.e. large
surfaces and high heat transmission coefficients) must be provided between the
heat transfer medium and the storage material, to ensure the required driving
temperature difference and to reduce the ensuing exergy loss of heat transmis-
sion. Mass storages are applied if the heat transfer medium itself is too expen-
sive (e.g. synthetic heat transfer oil) or difficult to store (e.g. depressurised air).
For mass storages the following combinations are applied: thermal oil/concrete,
thermal oil/molten salt, steam/oil-sand and air/ceramics bricks. Mass storage
offers the advantage of very inexpensive storage material. However, it also pre-
sents the disadvantage that in addition to the common heat loss of the storage
tank also an exergy loss occurs during double heat transmission when charging
and discharging the storage material.
- Within storage tanks with phase-change material steam is condensed isother-
mally, so that a storage material (e.g. salts such as NaCl, NaNO 3 , KOH) is so-
lidified/melted isothermally. Also in this case, an exergy loss occurs due to
double heat transmission. Moreover, such phase-change materials are still very
expensive.
Table 5.2 shows the thermodynamic data of selected storage media. One charac-
teristic parameter is the thermal penetration coefficient a th . According to Equation