The new control strategy was developed and tested for a 10 kW water/LiBr absorption chiller [2, 3, 4] in combination with an appropriate tailor-made wet cooling tower [5]. Chiller and cooling tower have been both modelled in the simulation environment TRNSYS. To model the chiller, TRNSYS type 158 [6], which is based on the model of the characteristic equation, has been modified using adapted parameters instead of the Duhring parameter [3]. To model the wet cooling tower, TRNSYS type 51a [7] was chosen. This model uses an external performance file based on laboratory measurements to calculate heat and mass transfer coefficients. The type has been modified in terms of the calculation of the fan power. The equation proposed by the model has been substituted by an exponential equation that fits better to measured values of the cooling tower applied. Figure 1 shows the basic simulation scheme of the solar cooling system.

Chiller driving temperatures are taken from a simulation of a solar collector field depending on location, size, orientation and inclination. Collector field and chiller are separated by a heat exchanger only. System operation without a storage tank offers the possibility of a fast and nearly complete supply of the solar offer to the chiller but it also causes transient load operation and thus unstable chilled water temperatures. A save operation of the chilled water distribution system requires the chilled water temperature to be within certain limits. Therefore a chilled water temperature control is necessary. The simulation furthermore provides environmental conditions as input for the cooling tower type. Chiller water inlet temperature was set to 18°C and includes small oscillations as they occur at real users. Mass flows of the external water circuits are kept constant.