The choice to develop a direct air-cooled chiller was ambitious. Air has a poor heat transfer coefficient that raises the required driving temperature, increases the air heat exchanger area and requires a scrupulous thermodynamic design. Nevertheless this choice was based on the careful judgment of the issues discussed above, applied to the Southern European climatic conditions. The vision behind Ao Sol’s development endeavour can be synthesized as follows:

The chiller must be cost-effective in the manufacturing in terms of components used and assembling procedures. The chiller must be direct-cooled, without any dry or wet cooling tower. This saves investment cost, room to place the tower, operation costs for a pump (to bring the heat from the chiller to the tower), a strong fan, water consumption and chemical water treatment, and tower maintenance. The chiller must be developed firstly for fan coil use. The use with radiant ceilings is thermodynamically less demanding and can be achieved without any further development. The other way around requires a rather new development. The control system must allow a very reduced part-load operation, thus reducing startup/shutdown losses. The chiller must be placed outside regardless of the climate, in order to save valuable space indoor.

To confirm the economic potential of the novel chiller an analysis was performed using Excel software to compare the novel ammonia chiller with a standard compression chiller. The comparison was set up for a 200 m2 residential building in Lisbon, Portugal. The data concerning heating, cooling and (DHW) demand were calculated by means of commercial software [7]. Ao Sol’s CPC collectors were simulated with internal software [8] for the calculation of the solar input leading to the solar fraction. Both solar energy yield and house energy demand were averaged on a monthly base. Efficiencies were taken into consideration for the chillers, the back-up gas burner and the electric grid.

The price for the ammonia chiller is calculated on the base of a company’s internal cost breakdown for the full production phase; the compression chiller price was scaled up from a market product of 5 kW [9]. Installation costs were assumed equal for both chillers, as well as the cost for hot water storage tank and gas boiler. Finally, the cost for solar collectors was set according to Ao Sol’s selling price for their new CPC MAXI collector.

Operation parameters, such as gas [10] and power [11] prices and their respective escalations have been set for the dynamic economic calculation. The prices are averages for Portugal and the price escalation formulated by official sites. Table 1 gives the overview.

The economics of the two systems have been compared by means of a differential present net value (Table 2). The solar fraction was set at 67 % by choosing the appropriate amount of solar collectors according to the chillers COP and collector efficiency. This value for the solar coverage was chosen in order to achieve energy savings compared to the reference system connected to the grid [12]. The total investment includes all parameters listed in Table 1. The gas consumption relates to heating and DHW supply by the gas boiler, as well as driving heat for the ammonia chiller.

Several items summed up to the parasitic power consumption, which turned out to be not negligible for the absorption chiller: solar, hot water, chilled water and solution pump, gas burner and chiller control, and cooling fan were taken into account for the absorption chiller. Gas burner control was added to the compressor consumption for the compression chiller. The parasitic consumption has been weighted with the operation hours of each item.

Investment and cumulated yearly operation cost were then calculated for each chiller and summed up in a net present value for a period of 20 years, which represents the technical lifetime of a compression chiller. It must be noted that an absorption chiller has a rather longer lifetime of 25 years, which was not taken into account here. The difference of the cumulated investment and yearly operation cost for both systems tells which system provides the best economic performance over its operational lifetime. As it can be seen, the ammonia chiller causes during its lifetime roughly half of the expenses caused by the compression chiller. The dynamic payback time of the ammonia chiller, i. e. the time span before the higher investment is recovered, occurs during the 8th year (Figure 6).

4. Conclusion

Ao Sol’s chiller development is based on a strong product displaying superiorly in terms of manufacturing effort, maintenance-free reliability, low overall operating cost and compact overall

size. The combination with Ao Sol’s own CPC-MAXI collectors proves to be economically appealing.

The experimental tests already gave an impression of the capacity of the prototype reaching a cooling capacity of 7.8 kW and a COP of 0.53. So, it can be said that most components seem to have the right size or even to be oversized, as e. g. the air heat exchangers and the rectifier. The solution pump worked well and performed the full flow rate requested. The cooling fan — run in part load — provided the necessary cooling for condenser and absorber, even if the laboratory room is very narrow and a certain air backflow could not be excluded. As referred the prototype has shown some limitations, which are now fully identified and can be easily corrected in the next prototype, to be built ant tested until the end of 2008. It is the expectation of the authors that the nominal values will be easily reached then, namely the design COP of 0.6.

The comparison for all-year supply of heating, cooling and domestic hot water of the solar-assisted absorption chiller with a compression chiller system shows the economic viability of the novel technology. With an overall solar fraction of ca. 70 % the novel chiller requires investment and operation expenses roughly half of them of a compression-based system after 20 years of lifetime. The break-even point between the two systems is reached during the 8th year (dynamic payback time).

References

[1] European Commission (2008), EUROPA, http://europa. eu.

[2] ESTIF (2007), ESTTP (European Solar Thermal Technology Platform), Intersolar, June 2007.

[3] Eurostat (2008) — http://epp. eurostat. ec. europa. eu.

[4] The Potential for Solar Assisted Cooling in Southern European Countries. Commission of the European Communities. Contract number: RENA-CT94-0017 (1995).

[5] Mendes, L. F., Collares-Pereira, M., (1999), A Solar Assisted and Air Cooled Absorption Machine to Provide Small Power Heating and Cooling, International Sorption Heat Pump Conference, Munich, pp 129-136

[6] comunication “Collector Testing” AO SOL

[7] W. Feist, (2007). Passive House Planning Package.

[8] Solpro (2008), v.16.5, Ao Sol R&D.

[9] Daikin EWAQ-ACV3P.

[10] ERSE (2007). http://www. erse. pt.

[11] Eurostat (2007).

[12] H. M. Henning, (2005). Solar Assisted Air-Conditioning in Buildings, Springer, Wien, New York.