The basic scheme in this case is shown in the following figure:

Fig.4. Configuration with cold storage

In this case, to have cold storage allows having an intermediate case between the two previous, so that they can be stored excess production and face bigger loads of instantaneous production of the chiller.

The cold storage has been determined to have an autonomy of one hour. And so that, from the flow of the evaporator (that coincides with the one of cold load) it’s determined the volume necessary.

3. Execution of the simulations

As it was mentioned before, for each one of those three schemes they have been tested different control strategies: three for the solar camp (C1, C2 y C3) and another three for the chiller (C4, C5 У C6).

With regard to the solar part:

C1: Constant flow control on the solar part and constant flow on the absorption.

In the case of the solar pump, it has been used the output temperature of the collectors as variable to control, as well as the reference variable is the one at the bottom of the tank. It was used hysteresis, and the values adopted were: upper for the start 6 °C, and for the stop 1 °C. To start up the absorption the storage temperature and the room temperature of the zone to be conditioned are taken into account.

C2: Control with variable flow on the solar part starting by differential temperature and constant flow on the absorption.

The system is similar to the one on the previous case, being the start up in function of a differential temperature but using flow regulation for the solar pump. In this way can be compared the improvement using constant flow on the solar flow.

C3: Control with variable flow on the solar part starting by critical radiation and constant flow on the absorption

In this case the start up of the solar pump is made in function of the critical radiation and the flow is regulated to maximize the output temperature of the collectors.

Regarding to the absorption chiller: For regulation on the following schemes, it was chosen for the solar part the C3 strategy and some changes have been applied to the control of the chiller in order to compare their effect using a constant system for the solar control. For the control of the chiller, once there were the necessary conditions for its start up, was established the control of the corresponding variable, in order to obtain a power produced by the evaporator, equal to the one demanded by the system. Thus facing the real demand in the system taking into account the higher delay produced by the effect of the exterior conditions on the load.

C4: Control with variable flow on the solar part starting by critical radiation and control by temperature on the generator for the absorption

In this case there is a three way valve on the suction of the generator pump, that allows controlling the temperature on the input, and with it the output power for the evaporator.

C5: Control with variable flow on the solar part starting by critical radiation and control with variable flow on the generator using a diversion valve.

Other possibility to regulate the power of the generator is the variation of the flow on the chiller. In this case all the water of the solar part is driven, and a three-way valve is used to let pass more or less water through the generator in function of the demand.

C6: Control with variable flow on the solar part starting by critical radiation and control with variable flow on the generator by means of a pump.

In this case, the flow regulation on the generator has been performed by means of pump with variable flow.

4. Results

Three tables are shown next with the comparative of the different data obtained:

As can be seen, two of the configurations are better than the rest: on the configuration of the solar part the one with cold storage, and with regard to the performance of the performance of the absorption chiller, the one with infinite storage. The fact of not having to fulfill the condition of demand of the load, allows a better use of the available energy. On the other hand, the cold storage allows having a bigger impact over the temperature of the solar installation, what increase the solar collection, but in return, penalizes the chiller COP.

Table 1. Results for the simulations with direct use C1 C2 C3 C4 C5 C6 Energy produced on the collectors [kWh] 7.400 8.650 8.640 8.590 8.650 9.100 Performance of the collectors [p. u.] 0,35 0,32 0,41 0,40 0,41 0,43 Energy given to the generator [kWh] 5.770 6.710 6.670 6.700 6.650 7.660 Energy produced on the evaporator [kWh] 2.575 3.050 3.040 3.180 3.030 3.880 Energy given to the load [kWh] 2.575 3.050 3.040 3.180 3.030 3.880 Medium COP of the chiller [p. u.] 0,45 0,45 0,46 0,47 0,46 0,51 Load Fraction [p. u.] 0,50 0,59 0,59 0,61 0,58 0,75 Solar COP [p. u.] 0,16 0,14 0,19 0,19 0,19 0,22 Table 2. Results for the simulations with total use C1 C2 C3 C4 C5 C6 Energy produced on the collectors [kWh] 9.100 8.650 8.800 8.800 8.740 8.740 Performance of the collectors [p. u.] 0,43 0,41 0,41 0,41 0,41 0,41 Energy given to the generator [kWh] 8.100 7.850 7.950 7.650 7,650 7.950 Energy produced on the evaporator [kWh] 3.550 4.600 4.600 4.450 4.520 4.770 Energy given to the load [kWh] 3.550 4.600 4.600 4.450 4.520 4.770 Medium COP of the chiller [p. u.] 0,44 0,59 0,58 0,58 0,59 0,60 Load Fraction [p. u.] 0,69 0,89 0,89 0,86 0,87 0,92 Solar COP [p. u.] 0,19 0,24 0,24 0,24 0,24 0,25 Table 3. Results for the simulations with finite storage C1 C2 C3 C4 C5 C6 Energy produced on the collectors [kWh] 8.850 8.850 9.050 9.800 9.000 9.500 Performance of the collectors [p. u.] 0,42 0,42 0,43 0,46 0,42 0,45 Energy given to the generator [kWh] 7.210 7.300 7.350 8.270 7.280 8.250

5.

Conclusions

Different control actions on the solar part have repercussion on the chiller performance and vice versa, different control systems for the chiller, change the working conditions of the solar installation.

From the previous results, we can deduce that the configuration with total use is the one that better performance shows, as it has more hours of use than the rest of configurations.

As well, it can be seen how the most adequate control for the solar installation is the one with variable flow, combined whit start based on critical radiation. The control for the chiller that better results offers is the one based on variable flow by means of the pump. The cold storage, reduces the performance of the installation, but from the point of view of design, allows the use of a smaller absorption chiller.

References

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