Thermal storage

Gerhard Faninger

13.1.1 The basics of heat storage in heating systems

A heating system may need thermal storage when:

• there is a mismatch between thermal energy supply and energy demand;

• intermittent energy sources are utilized; or

• solar fluctuations in solar heating systems must be evened out.

In highly efficient houses, the need for thermal storage is often short term. In such instances, water is a very efficient storage medium for both space heating and hot water production. Heat can be stored in three ways:

1 Sensible heat storage. The storage is based on the temperature change in the material. The unit storage capacity (J/g) is equal to heat capacitance x temperature change. Possible 'sensible heat' storage media are liquids (typically water) and solid materials (typically soil and stone).

2 Phase-change storage. If the material changes its phase at a certain temperature while heating the substance, then heat is stored in the phase change process. Reversing the process, heat is released when, at the phase-change temperature, the material cools back to the original state. The classic material used in phase-change storage is Glauber salt (sodium sulphate).

3 Reversible chemical reaction storage. Heat can be stored through sorption or thermo-chemical reactions. Systems based on this principle have negligible heat losses. The storage capacity is the heat of reaction or free energy of the reaction. Thermo-chemical storage materials have the highest storage capacity of all storage media. Some of the materials may even approach the storage density of biomass. Solid silica gel has a storage capacity that is up to about four times that of water. Prototypes of sorption storage are in the design and testing phase. One approach is to use metal hydrides.

Table 13.2.1 illustrates some of the materials that can be used for thermal storage. The storage capacity and desired temperature range are the two key parameters to select and size the storage.

Table 13.1.1 A sample of storage materials and key properties

Medium

Temperature

Capacity

(°C)

(kWh/m3)

Water

Temperature difference = 50°C

60

Rock

40

Na2SO4x10H20

24

70

CaCl2x6H20

30

47

Paraffin

20-60

56

Lauric acid

46

50

Stearic acid

58

45

Pentaglycerine

81

59

Butyl stearate

19

39

Propyl palmiate

19

52

Silica gel N+H20

60-80

250

Zeolite 13 X + H20

100-180

180

Zeolite + methanol

100

300

CaCl2 + ammonia

100

1000

MeHx + H2

50-400

200-1500

Na2S + H20

50-100

500

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