For the single-family house, in Rome it is possible to achieve economic viability for a solar system with gas or electric backup when the solar fraction is between 40-80% — see Figure 3. Comparing with a conventional electric air-conditioning, this system leads to a reduction of 3.8 c€/kWh of produced energy. In situations where an electric backup is not possible (only gas backup), solar systems installed in Rome and Lisbon with solar fractions between 20-80% are economically interesting. For Berlin, profitability occurs just with the use of a flat-plate collector and solar fractions between 20-40%. A gas boiler as backup solution, instead of an electric compression chiller, allows a reduction in solar collector area between 0-20% for the same solar fraction. In the Mediterranean cities, the flat-plate collector compared to the vacuum tube collector, allows a reduction in the total cost of produced energy between 0-2.2 c€/kWh — see Figure 3. In Berlin, for an electric backup both collector technologies show similar results, but for gas backup the flat — plate collector presents better results. Vacuum tube technology has the advantage of allowing a reduction of collector area between 5-45% — see Figure 4.

Fig. 3. Evolution of the total energy cost with solar fraction for electric (left) and gas (right) backup solution.

Fig. 4. Collector area needed according to annual solar fraction for the different building types.

5 Conclusions

By using an integrated solar system for combined heating and cooling, it is possible to save in total costs and CO2 emissions. This is particularly true for South-European locations, and economical results are more interesting when natural gas is used as system backup energy. Minimum costs depend on building type and location, but usually happen with annual solar fractions between 20 and 60%.

The single-family house and the hotel are the cases where the solar integrated system has a higher economic feasibility. Considering the present energy costs, Rome is the only city where it is possible to achieve a break-even situation. Compared to flat-plate collectors, vacuum tube collectors allow a reduction in collector area between 15 and 50%, although, due to their initial cost, flat-plate collectors lead to a higher economic viability. The best system in combination with solar is the electrical chiller; nevertheless, it is a gas boiler backup that leads to the best solar system efficiency.

An annual solar fraction of 60% can only represent a reduction between 35 and 45% of exploitation costs, because of significant maintenance and water consumption costs.

Although the exploitation cost of a solar assisted air-conditioning system is considerably lower when compared to a conventional system, the total cost (including investment, operation and maintenance costs) is actually high, even when extending the operation period as much as possible throughout the year. For solar cooling (and heating) to become more competitive, it is necessary that initial costs for absorption chillers and solar collectors are further reduced, considering the present costs of energy sources (gas, electricity).

References

[1] ESTIF, Solar Assisted Cooling — State of the Art. Report of project “Key Issues for Renewable Heat in Europe (K4RES-H)”, 2006.

[2] Solar Energy Laboratory, TRNSYS 16: A Transient System Simulation Program — Program Manual. University of Wisconsin-Madison, USA, 2004.

[4] IEA-SHC, TASK 38 — IEA Solar Heating and Cooling, 2007

[5] European Comission, Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Evaluation Report, 2003

[6] European Comission, Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Guidelines, 2003

[7] Henning H. S., Wiemken E., Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Economic Study Report, 2003

[8] Henning H. S., Solar-assisted air-conditioning in buildings — a handbook for planners, second edition, Springer, 2007