The novel chiller unit is developed for the year-round thermal energy supply of residential buildings under Mediterranean conditions. Envisaged are detached houses with a heated area from 150 to 250 m2 or more. This building segment was chosen for the high relevance of air­conditioning in buildings with high living comfort.

The present development is based on previous work accomplished at the University of Lisbon, Instituto Superior Tecnico, by Prof. Mendes [5], who developed several water-cooled ammonia/water absorption chillers. The thermodynamic cycle of the present solar absorption chiller has been consequently re-calculated and fine-tuned for direct air-cooled operation. The absorption unit is designed to work using solar thermal energy from CPC collectors or waste heat from a cogeneration engine fuelled by biomass. For the use linked to solar collectors a back-up gas burner is foreseen. The device is designed to produce 8 kW of chilled water in a range of temperatures between 5 °C and 18 °C. The chilled water is normally used in buildings either for air-conditioning via fan coil elements or directly for space cooling via radiant ceilings. The chiller developed can be operated for both concepts. Moreover, using ammonia as the refrigerant, ice production or even deep refrigeration could be achieved via changes in the control strategy. The solar chiller is driven by hot water at temperatures at around 95 °C, stored in a solar buffer. The device directly dissipates the own waste heat produced at around 40 °C without any need for an external wet, hybrid or dry cooling tower.

The detailed technical distinctive features of the AO SOL solar chiller are summarized below:

The chiller is air-cooled. Overall sizes of the chiller and operation cost are minimized. No water is needed; this ensures market compatibility even in the extremely hot and dry continental regions of the peninsula’s interior. The absence of a cooling tower decreases considerably the overall dimensions compared to concurrent products.

Plate heat exchangers are used wherever possible in the thermodynamic cycle and take advantage of the high efficiency typical of this technology (up to more than 95 %!) coupled with very compact dimensions and low specific weight (kg/kW).

The remote monitoring system acting via mobile phone technology ensures complete safety. A timely alarm transmission with the possibility of intervening remotely on the chiller control adds to the on-site safety features and it minimizes the service needs.

2.2. Control strategy

The concept envisaged for the chiller is completely self-sufficient. Automatic procedures for start­up, load changes, shutdown, and safety issues have been implemented. The control acts on two 3­way valves, which stabilize the water temperatures in the hot and chilled water loops. Further, it triggers the adjustment of fan and solution pump speed, and the actuation of the refrigerant throttle valve. The chiller is controlled through the adjustment of temperatures in the hot and chilled water loops. The adjustment is activated by means of PID controllers. The machine reaches stable operation within 15 minutes. The automatic shutdown procedure is immediate; the machine reacts quickly and without swings. A new start without any manual intervention in between is secured.

The chiller is the core component of a harmonised chain of components representing a turn-key solution: an absorption cooling machine driven by hot water and producing chilled water for fan coil or radiant ceiling use, and Ao Sol’s unique solar CPC MAXI collector, which will deliver on the order of 25% more energy than a very good flat plate collector of today [6].

The ideally combined system will provide hot water all year around, heating in winter and cooling for half of the year. The control unit developed combines and actuates all components of the solar

heating, cooling and DHW system, from the solar collector, to solar buffer and backup, up to the chiller and the space to be conditioned. Figure 3 shows the flow scheme.

3. Experimental results

The prototype has been taken successfully into operation. The novel machine showed a satisfactory behaviour in the start-up phase and was from the very beginning stable and predictable in its reactions. The generator capacity ranged roughly between 7 and 14 kW, whereas the evaporator has been regularly producing cooling between some 3 and a peak power of 7.8 kW. The COP, defined as the ratio between evaporator and generator power, showed a maximum of 0.53. The results of a test rig at nominal conditions are shown in Fig. 4 and 5. The unit was run with 96 °C hot water and produced chilled water at 12.5 °C. The cooling air left the machine at around 36 °C. Under these conditions more than 7 kW of chilled water can be already steadily produced with a COP above 0.5.

Fluctuations due to oscillation in air temperature can be clearly noticed in the evolvement of thermal capacities and relative COP. Nevertheless, these fluctuations did not show any trend and did not compromise the reliability of the operations.

Fig. 4. Experimental working conditions for hot water, cooling air and chilled water.

The experimental tests already gave an impression of the capacity of the prototype. 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 refiner. The air 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

However 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