Page 82 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 71

Thus, it is desirable to use other characteristics to find the efficiency of the entire sys-
tem. Therefore, in the following analysis the value of specific losses of external energy
are determined for the heat supply system, which, in case of energy losses for only the
heat pump, is a value different from the heat pump COP.
In general the total specific losses of external energy l h for the heat pump and
ambient coolant driver of a low-temperature heat carrier can be presented in the
following way:

L tot L c þ L m$e
s
l h ¼ ¼ ; (2.9)
Q h Q c
where L c , L m$e are energy losses for a heat pump compressor and ambient coolant
s
driver, respectively, kW; Q c is the heat flow extracted from the heat pump condenser,
kW.
Energy losses for the heat pump L c are defined by the equation:

Q ev
L c ¼ : (2.10)
4 1
Heat flow in the heat pump evaporator Q ev is given by:

in out
Q ev ¼ V s r c p t s t s ; (2.11)
s
where V s , r s , and c p are volume loss, density and isobaric specific heat of the sur-
in
roundings, respectively; t , t out , are surroundings temperatures at the evaporator inlet
s s
and outlet, respectively.
Energy losses for ambient coolant driver are:

V c Dp
m:e
L ¼ ; (2.12)
s m:e
h h
s dr
where Dp is aerodynamic and hydraulic pressure losses at the heat pump evaporator
that depend on the chosen energy source; in the case of using soil heat,
Dp ¼ Dp ev þ Dp s.h , namely, total pressure drop in the lower loop (in the heat pump
evaporator and soil heat exchanger); h m:e and h are the efficiency of ambient coolant
s dr
driver and its motor, respectively. One can assume that at the optimum operating point
h m:e ¼ 0:8 for a fan and pump, and the driver motor efficiency, h dr ¼ 0.95.
s
Heat flow Q c in Eq. (2.9) is determined from the HP heat balance equation:

Q c ¼ Q ev þ L c (2.13)
The actual COP of the heat pump 4 can be presented in the following way:

4 ¼ 4 h ; (2.14)
t HP