Simulation and Optimal Control Strategies of Hvac Systems

An optimal combination of the characteristics of an air conditioning system and the control strategy is necessary to minimize costs and the energy demand. Therefore a sequential control for such a system was developed, tested by simulation and implemented in an existing plant. The simulation runs have been done with TRNSYS [1],[2]. For the sake of comparison, an optimized control was calculated using a nonlinear optimization method [3]. For this purpose, TRNSYS had to be combined with MATLAB [4],[5] which includes some optimization tools. INTRODUCTION Both control strategies, sequential and optimized reference control, are tested with the help of a DEC (Desiccative and Evaporative Cooling) air conditioning system. The basic configuration of the system is shown in Fig.1. Figure 1: DEC device The device consists of the following components: sorption regenerator, heat regenerator, humidifier and air heater for supply air and humidifier and air heater for return air. The supply air fan as well as the return air fan transport the air masses. The heaters are loaded by hot water. A cooling device is not required. In winter the DEC system works as a conventional air conditioning system. With the aid of the two regenerators a high level of recovery of heat and humidity is possible. In the summer the heater for supply air is out of action. The dehumidification is done by the sorption regenerator. The cooling can be achieved by an adiabatic humidification. Figure 2 illustrates the principle of the classical sequential control. There are the two control loops temperature (2a) and humidity (2b) of the supply air. Each component of the air handling system is controlled in dependence of the supply air conditions and on the control status of all the other components. Figure 2: Classical sequential control scheme METHODOLOGY The optimization problem is the determination of the control signal of each component (rotational speeds of the sorption regenerator ( 1 u ), the heat regenerator ( 2 u ) and the fans ( 5 u ), valve lifts of supply air heater ( 3 u ), supply air humidifier ( 4 u ), return air humidifier ( 6 u ) and return air heater ( 7 u )). On the assumption that the air mass flow is constant there are only six control variables which have to be determined. They can be integrated into the control vector u, Eq.(1). T AB , ERH AB , BEF ZU , BEF ZU , ERH RW RS ] H H H H n n [ = u (1) The rotational speed of the regenerators and the valve lifts of the humidifiers and air heaters, respectively are restricted, Eq.(2). % 100 H 0 min 10 n 0 1