## 4 Development of Control Strategy
In a cooling operation, if the measured temperature/humidity deviates from the desired temperature/humidity values, the controller changes the heat input/water mass output in proportion to the magnitude of the deviation. For the control technique, proportional (P) control is preferred due to its simplicity, suitability for real-time operation, and easy applicability to industrial operations. However, classical P control is improved by adding different operation modes and automatic Stop/Run properties. So, P control aims to reduce the error value obtained after comparing the desired point with the actual value by multiplying it with a coefficient (\(K_{p}\)). In this control technique, it is the choice of \(K_{p}\) value that is the most effective parameter on the control performance. The ideal \(K_{p}\) value varies from system to system in which the P control is used. P control is expressed by the following formula:  
\[U_{k}=K_{p_{i}}\;e_{i}\;,\;\;\;\;\;\left(i=\left\{ \begin{array}{ll}1\;\;for\;T \\ 2\;for\;RH\end{array} \right)\right. \tag{12}\]  
where \(e\) and U represent the error and the control input at the controller output, respectively. The \(K_{p}\) values in the developed control strategy were determined by the trial and error method and operation conditions to achieve successful performance. During the adjustment of \(K_{p}\) values for both temperature and relative humidity, attention was paid to ensure that the set value is the fastest and most suitable for heat/mass transfer physics without steady-state error and overshoot. For this reason, the time responses of the temperature and relative changes in the room were investigated for different \(K_{p}\) values. The \(K_{p}\) value is determined when the desired time domain analysis criteria, which are zero overshoot and steady-state error, and short settling time, are obtained. As a result of this process, this value was determined to be 7500 for the control of the fans, while it was determined to be 18 for the desiccant wheel, and these values are used for all simulations under the considered case. With the control inputs produced in this active mode and changing the actuator speeds according to the ambient conditions, room comfort is provided in a fast and energy-efficient way. In addition, an automatic Stop/Run mode has been added to the developed control strategy to increase energy efficiency. In this mode, the room conditions are brought to the desired values and kept at these values for a certain period, then the cooler is switched to OFF mode (fans and desiccant wheel are stopped) until the room comfort deteriorates. Then, the cooler is switched back to ON mode and the room conditions can be quickly adjusted to the set values in the active control mode. The room comfort deterioration limits are determined as +2 \({}^{\circ}\)C variation in temperature and +2% variation in relative humidity. Thus, the actuators will operate intermittently depending on the magnitude of the disturbance effects, and energy savings will be achieved by stopping the actuators at certain intervals instead of constantly consuming energy. Like other actuators, a three-mode operation is designed for the water pump. The first mode is the OFF mode, which also stops the water pump when the cooler is off. There is a second mode to feed low-flow water into the channels of the water pump DPCs when the chiller is on. During the entire operation, the water pump operates in the second mode and pumps water at a low flow rate to the channels for cooling. The third mode of the water pump is the cleaning (flashing) mode which creates a high flow rate and intermittent water supply for channel cleaning. This cleaning mode is automatically activated after user instructions or after a certain period of time and ends automatically after cleaning for a certain period of time. In order to operate in this mode, the cooling part of the cooler must not work.