Page 94 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 83

Example: sizing a European residential ground-source
heat pump

Case A. Determine the length of the ground collector of a heat pump for heating a new
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residential building in Europe with dimensions: 20 m 20 m ¼ 400 m . You may as-
sume that a radiant floor heating system is employed. The soil at a depth of 5 m is wet
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clay (r ¼ 600 kg/m , moisture content is 30%); coefficient of performance, 4 ¼ 3.5;

W c /W w ¼ 0.4; W ec ¼ 4.9 kW; soil temperature is 8 S; and brine temperature is

0 S, soil coefficient of thermal conductivity l ¼ 3.56 W/mK (Table 2.3 or standards
VDI 4640).
Solution. Let us determine the required heat output of the heat pump. According
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to EU-Norm EN 12831, the heat consumption for new buildings is 30e50 W/m .
So the required capacity of the heat pump is 400 50 ¼ 20,000 W ¼ 20 kW.
Horizontal collector
The specific heat flux from the ground to the horizontal pipe, from Eq. (2.4),is

q 0 ¼½ð1:4 3:56Þ 0:5ފð0:4 þ 1Þð8 0Þ¼ 50:2W=m

The thermal capacity of the reservoir is Q 0 ¼ Q hp W ec , where Q hp is heat pump
power, and W ec is electric power of the compressor.
The required collector power is Q 0 ¼ 20e4.9 ¼ 15.1 kW.
The length of the collector pipes, from Eq. (2.3),is

15100 3:5 1
L ¼ ¼ 214:9m.
50:2 3:5
Thus, for the construction of a ground collector, two loops of 108 m length are
required.
The necessary area of the site to accommodate the collector is

n 1
A ¼ L a
n
where a is the distance between the pipes (assuming a ¼ 0.75e1 m), and n is the
number of loops.

2 1 2
A ¼ 215 0:75 ¼ 81 m
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If three contours of length of 40 m are used, the area of the plot will be

3 1 2
A ¼ 215 0:75 ¼ 108 m
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