An investments analysis for the different configurations of the system and for all the localities we have considered has been made by using the economic index NPW to estimate the better investment first of all and then the value of the other indices.

The economic indices taken in consideration are:

whereas REc is the amount of annual energy supplied by the plant. It has been assumed an economic lifesoan of the plant of 20 years, and Italian economic market be characterized by the following rates: g=2.5 %, e=8%, d=4%. In Italy methane costs 0.0187 €/MJ, diesel oil does 0.0244 €/MJ and the LPG 0.0369 €/MJ.

and system efficiency for the heating period. Supplied energy for domestic hot water

SHAPE * MERGEFORMAT

Fig 6 — Milan. Solar fraction and system efficiency in relation to the variation of collectors surface for the different volumes in the period of heating building.

Our analysis does not consider at least at first government financial supports and only methane, which is the cheapest fuel oil has been considered for feeding an integration boiler. Afterwards advantages coming from other fuel have been evidenced, so as government financial supports. In order to estimate the better investment on the considered period, the best NPW value of the three localities has been considered (fig. 8 Cosenza and Rome have quite the same values while for Milan lower values have been obtained). The system configuration with collectors’ surface of 12 m2 and a tank’s storage volume of 1 m3, provides the greatest value of NPW. For Cosenza a value is obtained, which is next to NPW, even with 8 m2. Figure 9 shows profit index (PI) for the towns here considered; As a result PI value is always above zero tending to fall with the increasing of collectors’ surface. While Cosenza and Rome both share almost the same values, those ones concerning Milan are lower. Profit index also show that volumes of 0.5 m3for low surfaces are cheaper, while for surfaces grater than 8 m2 the index of 1 m3 provides best results. It values 1.8 for Cosenza and Rome for a surface of 12 m2 and a volume of 3 m3, it assumes the value of 1.5 for Milan. The growth of payback time is directly proportional to increasing of collectors’ surface and of tank’s storage volume (figure 10). For the optimal configuration obtained it amounts to 12 years for Cosenza and Rome and 14 years for Milan. The cost of energy produced by solar plant "cep" (figure 11) is kept below the cost of methane (0.0187 € / MJ) for those surfaces until 20 m2; it sometimes excedees in such a value for 28 m2. In particular for a surface of 12 m2 and a volume of 1 m3, the "cep” was equivalent to 0.0124 € / MJ for Cosenza, 0.0121 € / MJ for Rome and of 0.0139 € / MJ for

Milan. All values being below the cost of methane with a reduction between 20% and 33%. If integrating feeding with diesel oil or the LPG is taken in consideration the best values produced from the economic indices can be seen in table VI. Payback time are reduced of approximately 2 years with the diesel oil and of approximately 5 with the LPG. The NPW value improves of approximately 70% with the diesel oil while with the LPG it is more tripled.

Fig. 8 — NPW: Net Present Worth Figura 9 — PI: Profit Index for the three

for the three localities. localities.

Table VI — Economic indices for a collector’s surface of 12 m2 and a storage volume of 1 m3 in absence of financial supports.

Figura 11: Cost of energy produced for the three localities.

In case a capital account financing of 30% is considered, as provided for by Italian public bands, an improvement of all the economic indices and the cep takes place as shown in the table VII.

Table VII — Economic indices for a collector’s surface of 12 m2 and a storage volume of 1 m3 with financial supports.

2. CONCLUSIONS

In this work the possibility has been analyzed to heat residential buildings using solar energy. Instead of traditional heaters radiant floor has been chosen, for its bigger suitability, for its thermic capacity and because it can be supplied even at low temperatures. That allows solar collectors to work more efficiently: It also provides

thermic energy in the tank to be used to very low temperatures. Simulation code has made possible to determine for three localities of Italian territory thermic and economic performances of the system, to the variation of collectors’ surface and tank’s storage volume. The fraction of thermic requirements both for building heating and domestic hot water, supplied by solar energy, as a result is deeply dependent on collectors’ surface. For a surface of 4 m2 a solar fraction of approximately 30% for Cosenza has been obtained, being instead of 25% for Rome and 7% for Milan; eventually to get a fraction of 72% for Cosenza, 68 % for Rome and 41% for Milan for a surface 28 m2 and a volume of 3 m3. System efficiency decreases with increasing of collector’s surface and with the falling of tank’s storage volume, above all because of a raising in temperature in the tank providing low collection performances. Among the three localities considered there is not a great difference in seasonal energy supplying for building heating, because a great amount of monthly incident solar energy, which characterizes harsh areas, is balanced with a longer period of heating. With a collectors’ surface of 28 m2 and a tank’s storage volume of 2 m3, seasonal energy supplied for building heating, was of 16.7 GJ for Rome, 15.5 GJ for Cosenza and 13.8 GJ for Milan. The economic analysis has shown that the system with collectors’ surface of 12 m2 and a tank’s storage volume of 1 m3 is the more suitable one. For such a configuration economic indices obtained for three different types of fuel, being financed or not, have shown the advantage of such an investment. In particular, in such bad conditions, as lack of government supports and using methane as integration fuel, the cost of the energy produced by the plant in a 20 years long lasting lifespan resulted below a percentage between 20% and 33%.

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