Physical Properties of Sensible Energy Storage Media [7, 8]
Storage medium |
Specific heat (kJ/kg·K) |
Density (kg/m3) |
Temperature (°C) |
Average heat conductivity (W/m·K) |
Gravimetric storage density (kJ/kg) |
Volumetric storage density (MJ/m3) |
|
Cold |
Hot |
||||||
Concrete |
0.9 |
2200 |
200 |
400 |
1.5 |
315 |
693 |
Sintered bauxite particles |
1.1 |
2000 |
400 |
1000 |
- |
385 |
770 |
NaCl (solid) |
0.9 |
2160 |
200 |
500 |
7.0 |
315 |
680 |
Cast iron |
0.6 |
7200 |
200 |
400 |
37.0 |
210 |
1512 |
Cast steel |
0.6 |
7800 |
200 |
700 |
40.0 |
210 |
1638 |
Silica fire bricks |
1 |
1820 |
200 |
700 |
1.5 |
350 |
637 |
Magnesia fire bricks |
1.2 |
3000 |
200 |
1200 |
5.0 |
420 |
1260 |
Graphite |
1.9 |
1700 |
500 |
850 |
- |
665 |
1131 |
Aluminum oxide |
1.3 |
4000 |
200 |
700 |
- |
455 |
1820 |
Slag |
0.84 |
2700 |
200 |
700 |
- |
294 |
794 |
Sand-rock mineral oil |
1.3 |
1700 |
200 |
300 |
1.0 |
127 |
216 |
Nitrate salts (KNO3-0.46NaNO3) |
1.6 |
1815 |
300 |
600 |
0.52 |
560 |
1016 |
Nitrite salts |
1.5 |
1825 |
250 |
450 |
0.57 |
600 |
250 |
Therminol VP-1 |
2.5 |
750 |
300 |
400 |
- |
875 |
656 |
Silicone oil |
2.1 |
900 |
300 |
400 |
0.10 |
735 |
662 |
Mineral oil |
2.6 |
770 |
200 |
300 |
0.12 |
257 |
198 |
Synthetic oil |
2.3 |
900 |
250 |
350 |
0.11 |
228 |
205 |
Carbonate salts |
1.8 |
2100 |
450 |
850 |
2.0 |
630 |
1323 |
Caloria HT-43 |
2.8 |
690 |
150 |
316 |
- |
980 |
676 |
Sodium liquid metal |
1.3 |
960 |
316 |
700 |
71.0 |
455 |
437 |
Na-0.79K metal eutectic |
1.1 |
900 |
300 |
700 |
- |
385 |
347 |
Hydroxide salts (NaOH) |
2.1 |
1700 |
350 |
1100 |
- |
735 |
1250 |
HITEC solar salts |
- |
- |
120 |
133 |
- |
- |
- |
The quantity of heat stored in a latent heat storage process, Qlatent, depends on the mass of material, m, and the enthalpy of fusion or vaporization, Δhfusion, and also on the sensible energy stored in the solid, liquid and vapour phases.
The advantages of heat storage systems with phase change materials are the operation with small temperature differences between charging and discharging (Fig. 13) and high energy densities compared to sensible heat storages. The main disadvantages of many phase change materials are the low thermal conductivity, and that leads to slow charging and discharging rates [8].
Figure 13. Capacity of heat storage.
The most useful phase change materials in thermal storage systems are the materials with solid–liquid transition. A classification of heat storage materials is shown in Fig. 14. There are many applications of phase change materials such as thermal storage for heating/cooling of buildings, electronic goods, automobile engines, and space craft, food industries, medical sector, waste heat recovery, heat pump systems and cold storage systems [9]. The phase change temperature of PCM used for thermal energy storage should fit the application.
T
he
cold storage systems are classified according to the storage medium
as shown in Fig. 15 [9].
Figure 14. Classification of materials with phase change solid-liquid [8].
Fig. 15. Classification of cooling thermal energy storage systems [9].
The main characteristics of phase change materials used for heat storage in buildings are given in Table XIX. Tables XX, XXI and XXII contain the thermophysical properties of phase change materials available commercial.