Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 89

The contribution of the earth heat exchanger during daytime operation was inves-
tigated both experimentally and theoretically. Inside the earth to air heat exchanger,
tubes of temperature sensors are placed at a mutual distance of 9m. Additionally the
humidity of the air is measured at the inlet and the outlet of the tubes. The soil tem-
perature is measured at the front and at the back of the building at various depths and
at a range of distances from the tubes of the heat exchanger.
At inlet ambient air temperatures for the earth to air heat exchanger between
9 and 16 ◦ C, overlaps occur between heating defined for outside temperature below
15 ◦ C and cooling operation fixed at outside temperatures above 15 ◦ C. This means
that sometimes the air is heated during the cooling period (summer) and vice versa -
a control strategy which could certainly be improved. To calculate the coefficient
of performance of the earth heat exchanger, it is necessary to determine the addi-
tional electrical energy of the fan to overcome the pressure loss of the earth to air
heat exchanger. By measuring the pressure loss p (Pa) for different volume flows
V (m 3 h − 1 ), the following empirical equation was obtained for the two tubes of 90 m
length:

10 − 5 V 1.9728 p = 6 · (4.6)

With an efficiency of the fan η f of 57%, the electrical energy P el can then be
determined by

Vp
η f

P el = (4.7)

The annual coefficients of performance were calculated from the sum of heating
and cooling energy divided by the electrical energy consumed. They achieve excellent
values of 50, 35 and 38 in the years 2001 to 2003 (Figure 4.3). However, the earth heat
exchanger cannot fully remove the daily cooling load: the hygienically required fresh
air volume flow limits the cooling power which can be supplied by such systems. From
the average internal load of 131Wh m − 2 d − 1 in the south office the earth heat exchanger
efficiently provided 24Wh m − 2 d − 1 in the summer period of 2003, that is 18%.
An extensive set of experimental data was used to validate a theoretical model
of the earth heat exchanger. A numerical simulation model of the earth to air heat
exchanger was implemented, which enables the user to check the performance of the
system as well as to test different control strategies (Albers, 1991; Tzaferis et al ., 1992;
Henne, 1999). It was implemented as a block for the INSEL simulation environment
and features a graphical user interface (Schumacher, 2004). In the model the heat
exchanger is divided into a number of elements and the analytical steady-state solution
of the differential equation of heat transfer is found for each element using a conform
transformation. It is possible to account for multiple tubes as well as for the influence
of groundwater and nearby buildings.