Page 86 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 75

Thus, the optimal air cooling level in the evaporator depends on the factor A, the
ambient temperature t 0 and the calculated temperature of the working fluid in the heat-

ing system, t c , i.e., t opt ¼ f A; t 0 ; t c
hc a hc
The relationship between the optimal level of air cooling in the heat pump evapo-
rator and the ambient temperature is shown in Fig. 2.19, for different values of A and
c
t . It follows from Fig. 2.19 that the optimal air cooling level in the heat pump evap-
hc
orator increases with an increase in factor A (which basically depends on the pressure
loss in the convective heat exchanger) and very weakly depends on the working fluid
temperature for heating in the range of 30e50 C.

Considering that the effect of the working fluid temperature is minimal, a cross-plot
c
of Fig. 2.19 is presented in Fig. 2.20 for the average value of t .
hc
It is seen from Fig. 2.20 that the optimal air cooling level in the heat pump evapo-
rator depends strongly on the factor A that characterizes the air flow pressure losses in
the evaporator and depends weakly on the ambient temperature over the range of t 0
from 20 to þ15 C.

2.6.2.2 Natural or waste water as heat source

To find the best operating point for a heat source in a heating system, we will examine
the variation of the specific external energy losses l h as a function of the water temper-
ature at the evaporator outlet for a working fluid temperature of t w.f. ¼ 30 C. The

value of the factor A is taken as 0.15, 0.05, and 0.01 C, according to the range of
values for Dp. The results are presented at an assumed ambient temperature

of 20 C. By substitution of the corresponding terms into Eq. (2.17), and taking
into account Eqs. (2.14) and (2.16), we obtain results given in Fig. 2.21.

Fig. 2.19 Optimal air cooling level in the evaporator as a function of ambient temperature: I, II,
III (at A ¼ 0.1, 0.5, 1 C); 1, 2, 3 (at t c ¼ 30, 40, 50 C).

hc