Page 81 - Low Temperature Energy Systems with Applications of Renewable Energy

70 Low-Temperature Energy Systems with Applications of Renewable Energy

valid if the space inside the well between the pipes and the soil is filled with a
specially-prepared heat conducting material, namely, bentonite.
A weighted-average thermal conductivity l should be used in Eq. (2.7) when the
well passes through soil layers with different values of thermal conductivity, and it
is estimated by the equation:

P
l i d i
: (2.8)
l ¼ P
d i
where l i is the thermal conductivity of i-th soil layer and d i is thickness of the i-th soil
layer.
Generally for a large project several wells of 100 m depth are used. In order to avoid
interference of temperature fields around separate wells, the distance between them is
taken to be no less than 6 m. The required number of wells is determined by the total
soil heat exchanger length divided by the well depth.
Polyethylene pipes with a diameter in the range of 20e40 mm are used in wells. It is
recommended that the pipe diameter be chosen so that the hydraulic head of a well
should not exceed 50 kPa. If the hydraulic head of a well is less than 10 kPa, it is rec-
ommended that two or more wells be connected in series.

2.6 Optimal usage of low-temperature heat sources

When using low temperature heat sources in HPHS, there is some ambiguity in
choosing operating conditions for the heat pump evaporator. In some references one
finds recommendations relating to the temperature difference at the heat pump evap-
orator inlet and outlet, depending on the nature of the lower heat source, but these sug-
gestions are given without justification. We present here a technique for estimating the
temperature difference mentioned above that characterizes the nature of various types
of low-temperature heat source.

2.6.1 Specific external energy losses for HPHS using different
energy sources

The temperature level of the heat-transfer fluid that cools the low temperature energy
source (e.g., ambient or ventilation air, water, soil) in the evaporator affects the operating
conditions of both the heat pump compressor and the mover (or driver) of the surround-
ings (e.g., a fan or a pump). The problem arises in estimating the optimal level of
ambient cooling which minimizes the total specificelectric energy loss for theHPHS
system. The situation is complicated by the fact that the energy losses change in opposite
directions depending on the heat-carrier temperature at the heat pump evaporator outlet.
The efficiency of heat pump systems is usually assumed to be measured by the heat
pump COP. But for complex systems, the overall operating efficiency depends not
only on the efficiency of the heat pump itself, but also on other system elements.