21. Properties of Some Phase Change Materials Produced by teap Energy, Australia [11].

    material	Melting point (°C)	Heat of fusion (kJ/kg)	Sensible heat solid/liquid (Wh/l·K)	Heat conductivity (W/m·K)	Density (kg/l)	Volume expansion (%)	Heat storage capacity (kJ/ball) (diameter 75mm)	Heat storage capacity (kWh/m3)
    TH 24	24	70.2	2.7/2.2	0.8	1.6/1.5	8	≈ 45.5	≈ 32.2
    TH 29	29	79.0	0.67/0.99	1.09 (23°C)/ 0.54 (38.7°C)	1.71(25°C)/ 1.56(32°C)	8-9	≈ 51.2	≈ 36.2
    TH 58	58	80.7	1.12/1.63		1.45/1.28	9-10	≈ 52.3	≈ 37.0
    H 89	89	63.6	0.84/1.29	0.69 (110°C)/ 0.60 (140°C)	1.64 (25°C)	8-9	≈ 41.2	≈ 29.2

22. Properties of some phase change materials (paraffins) produced by the Rubitherm GmbH Germany [11].

material	Melting point (°C)	Heat of fusion (kJ/kg)	Sensible heat solid/liquid (Wh/kg·K)	Heat conductivity (W/m·K)	Density solid/liquid (kg/l)	Volume expansion (%)	recommended operating temperature range (°C)	Heat storage capacity in the recommended temperature range (kJ/kg)
RT 6	≈ 8	≈ 140	1.8/2.4	0.2	0.86 (-15°C)/0.77(15°C)	10	(-3) +12	174
RT 27	≈ 28	≈ 146	1.8/2.4	0.2	0.87 (-15°C)/0.75(70°C)	10	19-35	179
RT 41	≈ 45	≈ 125	1.8/2.4	0.2	0.88 (-15°C)/0.76(70°C)	10	35-50	155
RT 42	≈ 43	≈ 150	1.8/2.4	0.2	0.88 (-15°C)/0.76(70°C)	10	36-51	174
RT 52	≈ 52	≈ 138	1.8/2.4	0.2	0.90 (-15°C)/0.76(70°C)	10	40-55	167
RT 54	≈ 55	≈ 148	1.8/2.4	0.2	0.90 (-15°C)/0.77(70°C)	10	46-61	179
RT 58	≈ 59	≈ 154	1.8/2.4	0.2	0.90 (-15°C)/0.76(70°C)	10	48-62	181
RT 65	≈ 64	≈ 154	1.8/2.4	0.2	0.91 (-15°C)/0.79(70°C)	10	56-71	173
RT 80	≈ 81	≈ 140	1.8/2.4	0.2	0.92 (-15°C)/0.77(100°C)	10	71-86	175
RT 90	≈ 90	≈ 163	1.8/2.4	0.2	0.93 (-15°C)/0.77(100°C)	10	82-97	194
RT 100	≈ 99	≈ 137	1.8/2.4	0.2	0.94 (-15°C)/0.77(130°C)	10	91-106	168
PK 80 A6	≈ 81	≈ 119	2.0	0.2	0.90 (20°C)	8	71-86	149

Typical commercial phase change products are shown in Fig. 16.

The energy stored in a thermochemical process, Q_{thermochemical}, depends on the mass of the thermochemical material, m, the reaction enthalpy, Δh_reaction, and although on the sensible heat may be stored above and below the reaction temperature.

The chemical thermal energy storage systems include sorption (adsorption and absorption) and thermochemical reactions [13]. In thermochemical energy storage systems the energy is stored through a reversible endothermic dissociation reaction and then recovered through the chemically reverse reaction.

The thermochemical storage reaction can be expressed as follows:

(8)

where: C - thermochemical material that absorbs energy and is converted chemically into the components A and B.

Fig. 16. Commercially heat storage products with phase change materials [11].

The components A and B are stored separately. The stored heat can be released when materials A and B are combined together and C is formed (exothermic reaction):

(9)

The quantity of stored heat is the heat of reaction, enthalpy of formation of material C.

A thermochemical energy storage process is schematicaly presented in Fig. 17 [13]. It comprises charging, storing and discharging phases.

The concept of thermochemical energy storage is illustrated in Fig. 18.

Adsorption reaction consists in accumulation of an adsorptive on the surface of an adsorbent resulting a molecular or atomic layer. The adsorptive can be a liquid or gas while the adsorbent can be a solid or liquid. Absorption reaction consists in distribution of a substance into a liquid or solid resulting a solution.

Fig. 17. Thermochemical energy storage process: charging, storing and discharging [13].

Figure 18. A schematic of a thermochemical storage system [7].

Thermochemical energy storage systems have a higher storage density than the other types of heat thermal energy storage systems and are appropriate for long-term storage applications, e.g., seasonal storage of solar heat.

the main thermochemical storage materials are listed in table XXiii.