Objective analytical investment tools, such as Net Present Value (NPV) and Internal Rate of Return (IRR), were used to process the data and set them out to be compared.

Values of NPV and IRR at the end of the estimated service life of the system, 20 years, allow to deciding the economic viability of the installation. As soon as NPV becomes positive, and IRR higher than the banking interest rate, the system starts to be economically viable.

To measure the degree of viability of the installation, the period of time required to reach a positive NPV and an IRR higher than 5% was calculated.

Absolute values of cost and maintenance were also taken into account, showing the influence that each of the savings have, depending on the housing typology considered.

1.5. Results

As shown in figure 2, the cost of installation depends on the typology of housing and the configuration of the system, being the centralized compression system (which is identical for all the cases) the

cheapest one, and the centralized absorption system (also identical for the three housing typology) the most expensive one. Decentralized compression system is more expensive as far as the size of the individual house is smaller, because of the high number of splits required.

6-apartment-building decentralized system is the one which has the highest energy consumption, because of the high simultaneity of use. In absolute terms, the energy consumption cost of the absorption system is the cheapest, followed by the centralized system and the decentralized systems in decreasing surface order respectively.

Once shown the absolute initial and operation costs of all the systems, the results of the comparison between using an absorption cooling system and the rest of the considered are described.

The objective analysis of costs and savings data from the different technologies threw the results shown in the graphics below:

Fig. 4.a Period for NPV>=0 Fig. 4.b Period for IRR>=5%

As expected, the time period required to reach a balance between incomes and expenses of the absorption system, without external aids, exceeds its service life-time. Only considering environmental criteria, its installation could result profitable. The most probably scenario, SBS_CHT_SHW, is the one which gives the best results, with a minimum period of less than five years to get a positive NPV.

It is followed by the second scenario, which only considers the public subsidy. The complementary energy savings obtained by using the solar field to pre-heat sanitary and central heating water are not enough to justify the installation of such a big-surfaced solar field.

Related to the energy consumption of each housing typology, the time required to get a positive NPV grows with the size of the house, being proportionally inverse to the simultaneity of use.

Looking at Fig. XXX, the values of NPV and IRR in the second and third scenarios make the system economically attractive to be installed. It is clear that the solar cooling system will be more viable as far as the consumption of the electric system is higher. Maximum consumption occurs in the building of 6 apartments, where the bigger simultaneity factor increases energy needs.

Taking into account the second and fourth scenarios, the fact that the subsidy was granted or not, could be the angular key_that makes viable the project.

Fig. 5.a NPV in the 20th year Fig. 5.b IRR in the 20th year

advantage of the additional uses of the solar field. Despite the fact that the operation costs are very low, its high initial cost hinders a reasonable period of return.

Due to the small number of low-powered absorption chillers, it would be profitable to centralize them, refrigerating the air of a group of small houses. The smaller the house, the bigger the simultaneity of the use factor, and consequently, the bigger the energy consumption avoided.

By considering a public subsidy to the solar field, profitability could turn positive. That is not enough; profitability rate should be higher to counteract the big initial cost. By adding the effects of sanitary and central heating hot water pre-heating, positive results are obtained.

The most profitable housing typology to use the solar cooling systems is the 6 apartments building, because of its high simultaneity of use coefficient and the habitual splits overpowering.

To conclude, it is remarkable that the results of this analysis are very close to getting a real economic viability, but it requires public support and a large campaign to make people aware of environmental care. This would boost the number of installations, increasing the investment in R&D, which will cause the development of the solar cooling sector, promoting its economic profitability..

References

[1] ANAGNOSTOU, J., PRITCHARD, C., TSOUTSOS, T. et al. (2003). “Solar cooling technologies in Greece. An economic viability analysis”. Applied Thermal Engineering, 23, 1427-2439.

[2] FLORIEDS, G. A., KLGIROU, S. A., TASSOU, S. A. et al. (2002). “Review of solar low energy cooling technologies for buildings”. Renewable and Sustainable Energy Reviews, 6, 557-572.

[3] GARCIA CASALS, X. (2006). “Solar absorption cooling in Spain. Perspectives and outcomes from the simulation of recent installations”. Renewable Energy, 31, 1371-1389.