New control by cooling water adjustment

Figure 3 shows a scheme of the new developed control strategy. In this case, driving temperature can rise as high as the chiller control allows. Thus, high irradiation can be used and no driving energy or exergy is wasted. Cooling water temperature is not kept constant anymore but it is used as a control parameter. Base of the control strategy is the method of the characteristic equation — a simple model which yields the cooling capacity of an absorption chiller as function of the so-called characteristic temperature function AAt, a function of the temperatures of the external water circuits (hot, cooling, chilled water). According to this equation, cooling capacity does not change, if the characteristic temperature function remains constant. From the characteristic temperature function follows immediately how changes of one external temperature can be compensated by control of the other external temperatures. That means for example a higher driving temperature can be compensated by increasing the cooling temperature without a change in the chilled water temperature.

Подпись: temperature tAin yields tAin image504 Подпись: Q E - 0,9 0,42 t tCout Подпись: If a constant

In [2] the method of the characteristic equation was presented. The adapted characteristic equation for the 10 kW absorption chiller is Q E = 0,42 • AAt + 0,9 with the adapted characteristic temperature function AAt’ = tG — 2.5 • tAC +1.8 • tE . Temperatures tG, tA, tC, tE are the arithmetic mean values of inlet and outlet of the external water loops at generator, absorber, condenser and evaporator, tAC is the arithmetic mean value of tA and tC. Solving of AAt’ for the cooling water inlet

chilled water outlet temperature tEout is required, QE is substituted by VE • p • cp • (Ein — tEout) and tEout is set to the required value, e. g. 15°C if a chilled ceiling is applied. The measured values of the other temperatures and the chilled water flow rate are continuously inserted and tAin recalculated hereupon. I. e., the characteristic equation model defines the necessary cooling water temperature. A standard PID-controller in a closed loop is used to control the fan frequency to provide this temperature. Via a frequency converter the speed of the cooling tower fan is set according to the output of the controller.

Summing up, this control strategy can manage unsteady driving temperatures. The cooling water inlet temperature is adapted to hold chilled water temperature constant. First successful laboratory tests of this control are presented in [8]. This paper focuses on comparison of the two control strategies by simulation.