Page 117 - Low Temperature Energy Systems with Applications of Renewable Energy

106 Low-Temperature Energy Systems with Applications of Renewable Energy

The exhaust air at the exit (ventilator) of the room has a high enough temperature to
be efficiently used for heating. The exhaust air flow is divided into two streams. One
part is sent to a filter, and then to the mixing chamber (recirculation process). The sec-
ond part is fed to the heat exchanger (heat recovery heat exchanger) in which ambient
air is heated by the exhaust air. This reduces the energy costs of heating the ambient air
in the heat pump.
The analysis of such a system showed that the use of air recirculation in the heating
and ventilation system led to a reduction in the air flow through the heat pump, caused
the necessity to reduce the air temperature at the outlet of the evaporator and increase
the temperature at the outlet from the HP condenser to obtain air with a given temper-
ature at the entrance of a room being heated. As a result, the thermal conditions of the
heat pump become worse, which manifests itself in a significant reduction in the HP
COP with an increase in the coefficient of recirculation C rec . Nevertheless, the specific
external energy consumption in the air recirculation circuit is somewhat reduced due to
the dominant effect of utilizing the heat of exhaust air. The use of a recovery unit in a
heat pump heating and ventilation circuit is a more effective means of improving the
thermodynamic efficiency of the energy supply system than that of using exhaust air
recirculation. At the same time, it is interesting to use both means of heat usage, at the
expense of heat recovery and at the expense of recirculation of exhaust air.
On the basis of an analysis of the thermodynamic efficiency of the heat pump air
heating and ventilation systems with a heat recovery unit and recirculation of exhaust
air, the following conclusions were obtained: (1) the recirculation of air in a heat
pump air heating and ventilation cycle has its limitations and can only be applied
with relatively small coefficients of recirculation (C rec 0.5e0.6) due to the sharp
increase in air temperature at the outlet of the HP condenser; (2) the application of
a heat recovery unit of exhaust air leads to a more significant increase in the COP
of the heat pump circuit of heating and ventilation compared with that of exhaust
air recirculation alone; (3) the additional use of air recirculation in the heat pump
and recuperator circuit of air heating and ventilation gives a positive effect only at
relatively low values of the recuperation factor and at high values (h r ¼ 0.8), and
(4) the circuit with recirculation (Fig. 3.19) has a lower efficiency than a simpler
one having only the heat recovery unit.

3.3.3 Hot water heating systems, hot water, and air-
conditioning facilities
Heat pump systems for water heating and hot water supply for various objectives can
be carried out using various low-temperature sources of energy. The advantages of
such systems are illustrated by the example of a heat pump system using earth (or
ground) heat for the heat supply of a railway station. The system utilizes the natural
heat of ground and provides water heating for a passenger waiting room and offices
in the cold season, air conditioning in office rooms in summer, and hot water supply
throughout the year. The schematic diagram of the heat supply system is shown in
Fig. 3.22.