With reference to Figure 1, in winter functioning if the internal air temperature is lower than 19°C, the control system activates the pump (b) and determines the inlet temperature by means of the Equation (4): if the water temperature within the tank is lower than that required, the three way valve system is activated (3) and the inlet flow rate is completely supplied by the auxiliary system at the required temperature. On the contrary, if the temperature within the tank is higher, the three way valve system (1) is activated and the water flow rate from the tank is mixed with a fraction of the recirculating flow rate returning from the radiant ceilings in such a way as to obtain the required temperature.

During summer, the generator inlet temperature is regulated based on the thermal power required by the radiant ceiling and of the temperature in the condenser of the water flow rate provided by the evaporative tower, hypothesised as 8°C higher than the wet bulb temperature of the external air. The dependence of these two temperatures on the power supplied by the absorption machine considered is shown in Figure 2, which highlights the operative limits of the chiller, for example the minimum functioning temperature of the generator is equal to 76.7°C. Moreover, in the hypothesis that the condenser temperature is 29°C, the minimum distributable power at the minimum functioning temperature of the generator results as being equal to 21.4 kW. For condenser temperatures of 30°C and 31°C, the minimum distributable powers at the minimum functioning temperature of the generator become 12.9 kW and 2.4 kW respectively. From these considerations it is possible to deduce that, on the basis of the temperature supplied to the condenser, acting upon the inlet temperature of the generator it is not always possible to regulate the absorption chiller in such a way as to provide the required refrigerating power. It is in fact possible to observe that powers greater than 12.9 kW can be distributed at the minimum functioning temperature of the generator only for condenser temperatures lower than 30°C. The summer control system is therefore capable of determining the required generator temperature based upon the water temperature in the condenser and upon the thermal power required by the building [7].

In order to guarantee the complete coverage of the refrigerant load required by the building, it is necessary to use an electrical heat pump which intervenes in situations in which the chiller is not capable of supplying the required refrigerating power. The auxiliary chiller intervenes each time that within the tank there is a lower temperature compared to that determined by the control system for the generator. The auxiliary heating system used for the winter period could increase the water temperature of the generator, yet is it not used as this solution does not permit the attainment of primary energy saving. The control strategy used for the summer period regulates the generator inlet temperature according to the following logic:

♦ If the water temperature in the tank is greater than that required by the generator, the three way valve system (1) is activated which mixes the load extracted from the tank with that exiting the generator, and the absorption machine distributes precisely the required load. If the inlet temperature in the tank is lower than that required by the generator, the remaining quota is supplied by the traditional auxiliary chiller.

♦ If the conditions are such that it is not possible to guarantee the effectiveness of the absorption chiller (temperature in the tank of less than 76.7 °C or required powers that are incompatible with the condenser inlet temperature), the refrigerator power is totally supplied by the auxiliary chiller.

The refrigerator flow rate supplies the radiant ceiling system only when the internal air temperatures exceeds 27°C, with the control system that activates pumps (c) and (d) of figure 1.

The refrigerant power requested by the environment is estimated with the relation successive based on the difference in temperature between the internal air and the reference value, set at 26°C:

Qcool = K2 ‘ (tIA — 26) (6)

A preliminary simulation campaign permitted the determination of the value of constant K2 which resulted as being equal to 47.35 kW°C-1: