Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 90

5 60

Heating energy
Cooling energy
COP

50.2

50

4

40

34.7

37.7

3

30

2

20

1

10

0 0

2001 2002 2003

Figure 4.3 Measured cooling and heating energy and COP of earth heat exchanger

The required inputs for the model are the geometrical boundary conditions and
the thermal underground parameters, which are considered to be constant. Further-
more, the calculation requires the ambient temperature, undisturbed ground tem-
perature, volume flow and pressure loss for every time step. It is difficult to quan-
tify the ground temperature and even more difficult to simulate it (Dibowski and
Rittenhofer, 2000). The ground temperature depends on the history of the ambient
temperature and also on the thermal properties of the subsoil. The latter are not ex-
actly known since they strongly depend on the moisture content, which may vary con-
siderably. Pfafferott therefore considers the undisturbed ground temperature a value,
which can only be approximated (Pfafferott, 2003). However, Tzaferis et al . (1992)
state that it must be regarded as one of the key parameters for determining the out-
let air temperature. In this work the ground temperature is calculated according to
the German guideline VDI 4640, which uses a sinusoidal wave function. All other
inputs to the model are measured and are read directly from a data file. A time step
of one hour was chosen, which can be regarded as the minimum time step for this
kind of model. Outputs of the calculation are the exit air temperature and the thermal
power.
The assumption of a sinusoidal soil temperature is obviously greatly simplified.
Deviations up to 3 K have been measured over the three years from 1 January 2001 to
31 December 2003 (Figure 4.4). The discontinuity in calculated temperature between
the years 2002 and 2003 is due to the fact that the sinusoid is calculated for each year
separately using the average temperature and amplitude of that year.
However, despite the simple calculation of the undisturbed ground temperature and
the constant thermal underground parameters, the agreement between measured and
simulated air exit temperatures for the two moderate years of 2001 and 2002 and the