Solar energy incident on a field of highly efficient plane solar collectors, with orientation towards south, is transferred across the primary circuit into an isolated tank located in the basament of the building with a constant air temperature of 20°C.

The control system actives the flow rate in the primary circuit each time the solar radiation is sufficient to increase tanking water temperature, under the condition that it does not reached the maximum value of 90°C. Storage system has made with an isolated steel tank, whose transmittance value is equal to 0.833 W/m2 K.

Three different types of solar collectors have been considered, each of them being present on Italian market: the first representing the selective plane type (a), the second is a evacuated model (b) and the third is a heat pipe evuacuated model (c).

Fig. 3 shows the expression of the tendency of each collector efficiency,

T — T ^ (T — T )2

m ae I___________ a У m ae /

G ) 2 G

whit G (W/m2) the irradiance, Tm (°C) collectors mean temperature and Tae representing (°C) outside air temperature. Table IV shows the three coefficients a0, a1, a2 appeared in the expression of efficiency of collector (2) and its cost per surface unit.

In order to choose the collector employing in simulations, the system has been simulated, equipped with collector field, the accumulator, radiant floor heating system, the building, with the three collectors to the variation of important parameters such as collectors’ surface and tank’s storage volume. For every configuration the ratio of the collectors’ cost to annual solar energy has been determined. Fig. 3 shows data relative to Milan. The comparison has showed that plane collectors have resulted more convenient, because a lower efficiency is compensated by a lower cost, owing to Italian climatic conditions. Thus the plane selective collectors has been used.

In the period in which building heating is expected, the control system of the secondary circuit, knowing outside air temperature, verifies if thermal level in the tank can provide the energy requirement. In this case it will activate the secondary circuit, that will supply the inlet flow rate to the building at the wished temperature, extracting it from the tank. Flow rate will be otherwise heated by an auxiliary boiler. Regulation of heat transfer fluid temperature in radiant floor is provided by a mixing valve being located on the entrance of each room. By a regulator, the valve is running as above described as to the combined control. A thermostat interrupts the flow rate when the inside air temperature goes above 21 °C. It will be activate again when temperature falls under 19 °C.