Page 89 - Low Temperature Energy Systems with Applications of Renewable Energy

78 Low-Temperature Energy Systems with Applications of Renewable Energy

Fig. 2.23 Temperature variations of the heat collection system from a soil array during a heating
period: 1, working fluid temperature at the HEO; 2, soil temperature close to HEO at 3 m depth;
3, far-field soil temperature at 3 m depth.

t w.f. ¼ 30, 40 and 50 C. The calculations were performed at an ambient temperature

of 20 C and a working fluid temperature at the evaporator inlet t in ¼ 3 C.

ev
Substituting the corresponding values into Eq. (2.17) and taking into account Eqs.
(2.14) and (2.15), we obtain the results graphically presented in Fig. 2.24.
It is seen that for each value of the factor A there is an optimal value of working fluid
temperature at the evaporator outlet at which the specific external energy losses are
minimized.
The qualitative results for the soil heat source are very similar to the previous cases
of air and water heat sources. Figure 2.25, for a working fluid temperature t c ¼
hc

40 C, displays the same trends as Figs. 2.20 and 2.22 for air and water heat sources,
respectively. Approximation of the curves shown in Fig. 2.25 can be obtained using
logarithmic coordinates, and yields the following correlation:
Dt opt ¼ 13:5A 0:5 (2.21)
cp

2.7 Summary

This chapter reviews the main systems using heat pumps in heat supply systems for
buildings using various heat sources. Analytical and practical methods together with
numerical examples of typical calculations are given.
It is possible to use heat pumps effectively in heating and air conditioning systems
in residential, administrative and industrial buildings, in shopping centers, sporting