Page 79 - Low Temperature Energy Systems with Applications of Renewable Energy

68 Low-Temperature Energy Systems with Applications of Renewable Energy

Fig. 2.15 Nomogram for estimating circulation loop volumetric discharge capacity.

Considering that soil can have temperatures lower than 0 C and the evaporator will
always have negative Celsius temperatures in winter, an intermediate heat transfer
fluid is used for heat extraction from the soil: anti-freezing liquids, e.g., ethylene glycol
and propylene glycol. The first is used more often, and for temperatures of 5, 10

and 15 C, its required water concentrations are 12, 20 and 27.4%, respectively.
Thermal and physical properties of ethylene glycol and propylene glycol water solu-
tions are given in Table 2.7.

Table 2.7 Properties of aqueous ethylene glycol and propylene glycol solutions.
Density, Freezing temperature, Heat capacity, c,
Concentration, z,% kg/m 3 t fr , 8C kJ/(kg$8C)
Ethylene glycol
12.2 1015 5 3.98
19.8 1020 10 3.85
27.4 1035 15 3.73
35.0 1045 21 3.56
42.6 1055 29 3.43
Propylene glycol
20 1021 7.5 4.06
30 1036 12.3 3.92
40 1043 20.5 3.74