Page 76 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 65

When the refrigerator compressor 1 operates, cold is produced that is used in a cold
supply system. At the same time the refrigerator working fluid condensation heat is
partly drawn by an intermediate heat carrier that circulates between the water
condenser 3 built into the refrigerator and a heat exchanger 5 in which water of a
hot water system is warmed up. The working fluid condensation process finishes in
the air condenser. Heated water is accumulated in the tanks 9 into which water from
the water pipe WP is fed. Water finishes heating up in the water heater 7 by the
heat carrier from the heat network HN before it is fed into the hot water supply system.
The system requires minimal heat supply from an external source when the tempera-
ture of water in the tanks 9 is quite high; when the weather is rather hot, the stored wa-
ter may provide water supply when it is switched off from the heat network for
maintenance procedures.

2.5 Use of soil heat

Unlike soil water that is not always available in necessary amounts for a heat pump,
soil itself is everywhere, and its stored thermal energy can almost always be used.
Thus, a large number of systems with soil heat collectors have been installed recently,
with great capital investments being made for the sake of increasing the COP in the
coldest month, January. However, it is not easy to capture the soil heat and the
main part of the capital expense goes to constructing soil heat exchangers and connect-
ing them to the HPHS system. Either horizontal or vertical soil heat exchangers may be
used to draw the heat from soil.

2.5.1 Horizontal in-ground heat exchangers
Horizontal soil heat exchangers can be put into the foundation ditch or a trench around
a building. To determine the ditch or trench dimensions and general pipe length one
must know the heat pump thermal power Q HP and its calculated COP, 4. [Note: 4
will be used in equations for simplicity.] Then the general pipe length L h can be
defined by the equation:

3
10 Q HP 4 1

L h ¼ (2.3)
q s 4
where q s is the specific heat flow through 1 m of the pipe put into the soil, W/m. The q s
parameter depends on a number of factors and its precise value can only be found
experimentally. For preliminary calculations q s is taken to be 25 W/m, but for soils
with different heat conductivity this value can vary widely. In addition, the q s value is
characterized by an unsteady heat extraction process and it decreases toward the end of
the heating period. Heat transfer with the soil can worsen from year to year especially if
there is an unbalance in heat requirements from summer to winter. However, heat