Fig. 10 shows the statistics of the driving temperatures. It can be seen, that in the heating mode driving temperatures below 68°C were also used. Unlike in the cooling mode, where the temperature in the buffer storage is used in order to decide whether high enough temperatures are available for operation, in the heating mode the machine is switched on whenever heating is required, assuming that the temperature in the heating net is always high enough. From the resulting operation conditions it can be seen that the available temperatures in the heating net present a high variation and are not constant at 72°C as the design values may suggest.

The temperatures fed into the heating coil (Fig. 11) were moderate (24°C to 30°C) and those from the boreholes where quite high (13-15°C). In Fig. 12 the temperature lift is shown. As in the cooling mode, only low temperature lifts were required, with a mean at about 13K. The power extracted from the borehole system was between 2.5 and 3.7kW (Fig. 13).

Fig. 10. Frequency diagram of driving temperatures. Fig. 11. Frequency diagram of temperatures to the

heating coil (output temperature from the heat pump)

In Erro! A origem da referenda nao foi encontrada. the operation temperatures on November 15th, 2007 as a function of time are shown. A strong variation of the driving temperature (T_HT_in) is observed. This variation is inherent to the operation of the heating net and out of the control of the present system. Nevertheless it shows that even though the heating net is designed for a constant temperature of 72°C reality shows also much lower temperatures. Again, the periodic temperature peaks characteristic of adsorption systems can be seen clearly in all return temperatures (T_HT_out, T_MT_out, T_NT_out).

Fig 14. Temperatures in the heating mode for the 15th of November, 2007.

5. Conclusion and discussion

As a conclusion it can be said that the installed adsorption chiller worked reliably and the rated COP and chilling powers were within the expected range. Certainly the operation conditions were very favourable as the required temperature lifts were around 10K in the cooling as well as in the heating mode. These low temperature lifts favour a high performance of the machine.

The application required cooling only during the day, thus the control switched the machine on only in this period. With this background, the solar fraction for cooling was around 60%. This may be different and may result in a different system design (e. g. larger solar field or larger storage) if cooling is also required beyond the hours with solar radiation. The overall thermal COP of the chiller for cooling operation was 0.574. In this period 1230kWh of cooling was produced with an electricity requirement to operate the system including cold distribution of 125.8kWh.

In the heating operation the overall thermal COP was 1.43, with a total of 3274kWh of useful heat produced and an electricity requirement of 180.5kWh. As it is difficult to obtain high temperatures from the solar collectors in the winter season, only 5% of the driving heat was provided from this source.

The electric power requirement of the system, excluding the pump in the solar loop, is about 500W, independently of the operation mode, cooling or heating power. This power requirement includes cold and heat distribution, therefore any comparison has to be done taking into account the whole system as a basis. However, using the boreholes as heat rejection system favours the low electricity consumption as no additional cooling tower was required for the cooling mode.

The temperature diagrams show strong fluctuations in the driving temperatures, especially when operated in combination with the heating net. The fluctuations were less when heat from the solar system was used. This shows the attenuating effect of the solar buffer storage. Strong fluctuations in the return lines from the machine are inherent of the periodic operation of adsorption units and have to be taken into account in the design and evaluation of the system. As a conclusion, it has to be analysed if some kind of small mixing storage or any other thermal capacity device to attenuate the temperature peaks is required in order to operate such an adsorption system in a stable way.

The reliable operation of the installed unit opened up the path to an improved pilot product, called ACS08, produced and offered by the company SorTech [3].

References

[1] T. NMez, M. Hau, H-M. Henning, »A Small Capacity Adsorption System in a Heating and Cooling Application: The German Field-test in the MODESTORE Projects, Proceedings of the 1st International Conference Solar Air-Conditioning, Bad Staffelstein, Germany, (2005), pp. 335-340.

[2] T. NMez, W. Mittelbach, H-M. Henning, »Development of a Small Capacity Adsorption System for Heating and Cooling Applications^, in: International Journal of HVAC&R Research, USA, (2006) Vol. 12, no. 3b, pp. 749-765.

[3] SorTech Adsorption Chiller ACS 08; www. sortech. de