Due to the high electrical coefficient of performance of conventional compression chillers, the primary energy consumption is despite the use of electricity still reasonable. In order to save primary energy with replacing these systems with a solar thermal driven air conditioning system and a thermal back-up system using fossil fuels, high solar fractions and high coefficients of performance of the cold production are necessary. The coefficient of performance depends mainly on the chiller technology and on the temperatures in the heating, cooling and re-cooling circuits.

For temperatures delivered by typical solar collectors and for typical absorption chillers, this value does not vary much and is in the range of 0.6-0.8. Based on such a thermal coefficient of performance and compared to typical compression chillers with electrical coefficients of performance of about 3.5, only solar air conditioning systems with solar fractions of more than 60% are leading to primary energy savings [2].

However, the solar fraction of a solar air conditioning system depends on many factors of which the most important are:

■ Time dependent matching of available heat from the solar thermal system and cooling demand

■ size of the solar collector field

■ efficiency of all system components

■ storage capacity

■ control strategy

For the reference case in the simulation studies, a flat plate collector field with an area of 150 m2, which would be appropriate for typical solar air conditioning applications using absorption chillers with a cooling capacity of 50 kW, and a hot water store volume of about 16 m3 was chosen. With this configuration, a significant contribution of the solar thermal system to the cold production was determined. However, the solar fraction for cooling of the office rooms, seminar rooms and the server room determined for the the building described was less than 50 %. Regarding this, Fig. 5 shows the back-up heater operation depending on ambient temperature and actual irradiation for a period in August. It can be seen that back-up heating is mainly required during times of high ambient temperatures but comparable low irradiance.

Thus, it can be expected that a change in the system configuration of the solar thermal system can increase the solar fraction. Regarding this, three different approaches have been investigated:

a) Collector area:

Increasing the collector has a significant effect on the solar fraction. It could be shown that doubling the collector area can increase the solar fraction up to 70 %

b) Collector type:

Changing the collector type to 150 m2 vacuum tube collectors can increase the solar fraction up to 75 %

c) Storage volume:

Doubling the storage tank volume can increase the solar fraction only up to around 65 %

Based on these investigations, it could be shown that with an adjusted system design, high solar fractions for cooling are possible. However, all these adjustments are leading to high investments. Due to this, alternative concepts need to be investigated. Some of these are discussed as follows:

Alternative storage concepts

Because of the small temperature step of supply and return flows during the operating of absorption chillers, the effective storage capacity of water tanks is limited, especially since temperatures of more than 80°C are required to drive the chillers. Thus, due to a high storage density at small temperature steps, a replacement of water storage tanks by storage tanks using phase change materials seems to be promising. Herewith, a reduction of the storage volume of down to 1/5 seems possible.

Alternative cooling technologies

Since mostly summer days with low radiation are responsible for the relatively low solar fraction, alternative solar cooling technologies which require lower driving temperatures could overcome

this problem. Since server rooms are usually cooled with ventilated air, open cycle systems using liquid dessicants, e. g. [7], can lead to efficient system designs. These systems require temperatures of only 60-70°C, which can be supplied even on days with lower radiation.

Different server room temperatures

One approach to further decrease the energy demand for cooling of server rooms is the increase of the room temperature in the server room itself. Investigations in [8] on the tolerable temperature of data processing equipment are recommending temperatures of up to 26°C. With such an increase, the proportion of free cooling would be, especially for Germany, increased by a large extend. However, the solar fraction of the remaining cooling demand would not necessarily increase, too.

Integrated system for heating and cooling

To increase the overall efficiency of the system for heating and cooling, the use of waste heat from the server room for heating the building in winter times should be applied. For this, the amount of energy reduction depends on the efficiency of the ventilation strategy and its heat recovery system as well as on the temperature in the server room. In addition, heating can be supplied by the solar thermal system at times when the energy is not used for the cooling process. In that case, the yearly gain of the collector field would be increased. However, the amount of primary energy reduction depends on the building design, too.

Bivalent cooling strategy

If the solar fraction cannot be increased with one of the measures mentioned above, a bivalent cooling strategy would still lead to significant primary energy reductions. In that case, both thermal driven chillers and vapour compression chillers would be used for the cooling of the building and the server room. As long as thermal energy would be available from the solar thermal system, the thermal chiller would supply the cold. As back-up during times of low radiation, the conventional electrical chiller would be operated. A draw-back of this approach is that the investment would increase due to the requirement of two different cooling technologies. Furthermore, the complexity of the design and operation of the system itself would increase, too. However, both issues would be eliminated if an emergency back-up system would be required anyway and if such a system could be used as a normal operational back-up, too.

3. Conclusion

In the present paper, investigations on using solar thermal energy to cool a passive office building with seminar rooms, office rooms and especially a server room are presented. It could be shown that even if the cooling demand for the seminar and office rooms of the investigated building is quite small, the peak cooling load can be significant even if appropriate shading devices are used. Due to the good coincidence of solar radiation and cooling demand, solar air conditioning systems can lead to significant improvements of the user comfort. Due to the relatively constant cooling demand of a server room, special measures have to be implemented in order to use the solar thermal systems to cool the server room, too. It was shown that the use of ambient air during cold or mild periods can reduce the cooling demand of the server room by almost a factor of 3. Thus, ambient cold represents the most efficient way to reduce the energy demand for cooling of server rooms. In addition, it was shown that with this reduced cooling demand, which almost matches the solar radiation available, the use of solar air conditioning system can reduce the primary energy demand for cooling even more. However, for a significant primary energy reduction compared to the use of conventional compression chillers, high solar fractions are necessary. Thus, for a special

application, it needs to be proven whether additional measures like the use of compression chillers as back-up, or the use of the solar thermal system and the server waste heat for heating the building during winter time, can be applied.

5. Acknowledgment

The authors wish to thank the company Wagner& Co Solartechnik for supporting the investigations presented in this paper.

References

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