The TRNSYS® interface interacts with the user as a graphic programming tool. It permits to build a virtual facility and easily change from different types of configurations. Using an already validated simulation of solar cooling facilities benchmarked with the UC3M’s experimental solar cooling facility, the model of the wet cooling tower has been substituted by the model of a Ground Heat Exchanger (GHE). This model is the Type 557a from the TESS libraries for TRNSYS 16 and simulates a U-tube GHE. For more information about the TRNSYS simulation program and TESS libraries please refer to [6]. The simulation has been conducted in a trial an error way in order to size the GHE. Different numbers of boreholes connected in parallel have been simulated at different depths until the heat rejected to the ground equals the heat generated in the absorber and condenser.

During this simulation the soil thermal properties are going to be estimated because lack of information about the soil in the Madrid region. Normally, to estimate the thermal properties of the soil

a Thermal Response Test has to be performed first in order to be accurate. In the Thermal Response Test a probe is introduced into the ground and a defined heat load is circulated. The temperature difference is recorded and the properties of the soil can be calculated easily. This technique allows the sizing of the GHE to be accurate [7].

Typical market dimensions have been used in order to simulate the GHE. The borehole radius is of 11 cm and the single U-tube in each borehole has an outer radius of 2 cm and an inner radius of 1.6 cm. Different number of boreholes corresponding to 4, 6 and 8 connected in parallel has been simulated at different depths. A summary of the input values for the GHE model is presented in Table 3.

1.3. Simulation Results

Simulation of July 9 is conducted in order to compare the behaviour of the simulated facility incorporating the GHE with the current experimental facility. Figure 5 shows the simulation results for July 9.

The simulated heat rejection demanded by the absorption chiller reached 135.339 kWh for the day.

Making a trial and error analysis it is found that to dissipate the heat generated in the absorption chiller with 4 boreholes, a depth of 160 meters is necessary. With 6 boreholes, the depth needed for the heat rejection is of 100 meters and with 8 boreholes, a depth of 80 meters is necessary. Estimating a price per borehole of 45-65€ (depending of type of soil) per meter, it is found that the best design should be 6 boreholes of 100 meters deep.

In the Figure it is shown a time delay between the experimental and the simulated heat rejected. This is motivated by the temperature control of the recooling loop. Not circulating water through the condenser when is not needed lowers the thermal inertia of the chiller. Nevertheless, almost the same value of heat rejected is achieved but there is a slight increment in the value of the outlet water temperature from the condenser. Experimental values for July 9, 2008 reached maximum values of 30 °C while the simulation reached 34 °С. Nevertheless, the cooling energy produced does not experience major changes. Simulated cooling energy produced reached 36,547, with simulated weather conditions, a slight difference from experimental.

2. Conclusions

Although the wet cooling towers behave well in lowering the water temperature, the GHE seems to be the way to promote the use of solar cooling facilities in the sector.

The simulation conducted shows that a GHE formed by 6 boreholes of 100 meters deep and each containing single U-tubes, connected in parallel could be sufficient to supply the heat rejection rate to cool down the absorption chiller in this kind of facilities.

The construction of this kind of heat sink is more complicated than the installation of a wet cooling tower, but once installed its maintenance cost is low.

Another good characteristic of coupling a GHE to an absorption chiller is that in winter time the facility could operate to supply low temperature heat using the absorption chiller as a heat pump and the cold produced in the evaporator sent to the GHE supplying the load.

A more extensive simulation should be conducted in order to evaluate the thermal depletion of the soil. This could be counteracted by operating the facility during the whole year.

References

[1] G. Grossman, A. Johannsen, Prog. Energy Combust. Sci., 7 (1981) 185-228.

[2] G. A. Florides, S. A. Kalogirou, S. A. Tassou, L. C. Wrobel, Energy Conversion and Management, 44 (2003) 2483-2508.

[3] M. Izquierdo, R. Lizarte, J. D. Marcos, G. Gutierrez, Applied Thermal Engineering, 28 (2008) 1074-1081.

[4] M. C. Rodriguez, P. Rodriguez, M. Izquierdo, A. Lecuona, R. Salgado, Applied Thermal Engineering, 28 (2008) 1734-1744.

[5] R. Salgado, P. Rodriguez, M. Venegas, A. Lecuona, M. C. Rodriguez, 5th European Thermal-Sciences Conference (Eurotherm 2008), ISBN 978-90-386-1274-4, 123-124.

[6] TRNSYS 16 User’s Manual, Solar Energy Laboratory, University of Wisconsin-Madison.

[7] B. Sanner, C. Karytsas, D. Mendrinos, L. Rybach, Geothermics, 32 (2003) 579-588.