Page 63 - Low Temperature Energy Systems with Applications of Renewable Energy

52 Low-Temperature Energy Systems with Applications of Renewable Energy

The same situation using the heat of ventilating exhaust streams occurs in coal
mines. A system that decreases energy costs for heat treating air that goes into the
mine during almost the whole year (both for heating in winter and cooling in summer)
was proposed in Ref. [7]. Due to higher heat potential of the mine outlet ventilation air
relative to the ambient air, the energy efficiency of using off-the-shelf heat pumps un-
der the conditions in question considerably improves upon their certified
characteristics.
Therefore it is possible to obtain 100% of prepared air heat energy with electric en-
ergy costs for the compressor drive that does not exceed 20e22% in winter and
30e35% in summer. Thus even if there are boilers in the mines that operate using their
own solid fuel, the use of heat pump systems can provide not only energy saving but
also economic benefits.

2.3 Natural water as a source of energy

2.3.1 Well water

Well water or moist soil can be an ideal heat source for the heat pump, but the decision
concerning its use can be made only after hydrogeological research that should answer
the following questions:

1) How many wells are necessary for meeting the requirements of the heat pump?
2) What pressure does the pump need to develop to force the water into the water bearing
horizon?
3) What are the chemical characteristics of the soil water?
To answer the first question one should have information concerning required water
flow rate for the heat pump. The water flow rate G (in kg/h) can be determined by the
equation:

1:1 3600 Q HP
G ¼ (2.1)
c p Dt w

where Q HP is the heat pump thermal power, kW; c p is water specific heat, kJ/(kg. K);
1.1 is a factor that takes into account a possible decrease in well output; Dt w is water

temperature difference at the evaporator inlet and outlet, generally it is 5 C.
The required pump pressure depends on soil porosity of the water-bearing bed
(see Table 2.2). When soil water penetration is insufficient, owing to low porosity
or permeability, the well pump electric power consumption will be comparable
with that of the heat pump compressor, rendering the system unfeasible for use as
a heat source.
It is important to compare the chemical composition of artesian water with the man-
ufacturer’s specification limits for the heat pump components. Unless the water quality
meets those criteria, it will not be possible to employ that water source for the heat
pump. Most commonly, deep (>700 m) water wells are used according to the method