Annett Kuhn*, Jose Luis Corrales Ciganda, Felix Ziegler

Technische Universitat Berlin, KT2, Marchstrasse 18, 10587 Berlin, Germany
* Corresponding Author, annett. kuehn@tu-berlin. de

Abstract

The choice of the control strategy of a solar cooling system is essential for the system performance. Control strategies should not only have the focus on increase of cooling capacity but also on reduction of electric energy consumption of the overall system. Normally, electric power consumption of the cooling tower fan is the major fraction of power requirement in such a system. Therefore, a new control strategy which uses the cooling water temperature as a control parameter has been developed. This strategy is based on the method of the characteristic equation which describes a linear relationship between driving, cooling and chilled water temperature and cooling capacity. Chilled water temperature can be kept constant even at such unsteady driving temperatures as in case of solar cooling. Simulations of a cloudy day in Berlin showed that full cooling capacity can be provided nearly an hour earlier compared to the conventional control strategy. This offers the possibility to save a short-time storage tank. If irradiation is high as in the case of a sunny day in Iran up to 50% electricity consumption of the cooling tower fan can be saved applying the new control strategy.

Keywords: solar cooling, absorption chiller, control strategy, energy saving

1. Introduction

Cold water distribution systems often require a constant chilled water temperature or chilled water temperatures within a certain range. Chilled water entering chilled ceilings must not be to cold to prevent condensation of water on the panels. But chilled water entering fan-coils has to be cold enough to ensure air dehumidification. So, the chilled water temperature must be controlled. The control of a solar cooling system must be carefully chosen and designed in order to maximise efficiency and minimise electric energy consumption. This is especially important as experiences with solar cooling applications revealed several mistakes in control and hydraulic scheme design.

There are different strategies to control the chilled water temperature of an absorption chiller. The most common strategy is to control the hot water inlet temperature using a three-way valve to mix the return flow from generator with the supply flow from the hot water source [1]. The result is a decrease of the hot water inlet temperature and a decrease in cooling capacity. Consequently, the chilled water temperature rises. Nevertheless, this widespread control strategy is not the optimal solution, as it can only be used to prevent the chilled water temperature from too low values. An increase of the cooling capacity at low irradiation is not possible. Another disadvantage is the high power and water consumption of the cooling tower as the cooling water temperature is kept constantly low.

A newly developed control strategy uses the cooling water temperature as a control parameter.

This strategy shows special advantages for solar cooling. If the hot water temperature is not high enough for a given cooling load, the cooling water temperature can be lowered in order to reach the desired chilled water temperature. On the other hand, if solar radiation is high, cooling water temperature can be increased to save power and water consumption of the cooling tower.