Domestic hot water temperature control system design and its analysis

Air source heat pump domestic hot water heaters uses electricity energy to transfer the heat to where it is needed, which is obtained from surrounding air, instead of producing it therefore they are two or three times more efficient than traditional electricity energy based water heating systems. They need a refrigerant fluid to transfer the heat from a lower temperature medium to a higher tempeature medium and use vapour compression cycle. To obtain a stable hot water temperature from this kind of systems is always considered as an engineering problem. Although coefficient of performance of this systems mainly depends on the ambient temperature and thermodynamical specifications of the refrigerant fluid, variable seasonal effects, hot water usage profile and insufficient control strategies can also affect the performance and make it lower than expected. In this thesis, it is aimed to control water temperature in an evaporative heating system which produces hot water by deriving from air-water energy by using frequency response methods. The system which is being used in a hotel operation, has a PLC based on-off controller. Since the system outdated and the necessary technical support is no longer available, a new and robust controller is needed. The goal of the thesis is to provide an appropriate improvement to the needs of the system. For this purpose, a mathematical model of the system will be created and a controller will be designed according to system. Due to the dynamic and nonlinear nature of the system, seasonal mathematical models will first be achieved by applying system identification techniques with the system identification module of the Matlab® Simulink software. The relevant data will be obtained in a predictive manner and verified to implement the final model. The controllers which designed by frequency response method will be applied to the obtained model. Primarily, phase lag/lead compansators and a PID controller will be designed, thereafter the system's transient and frequency responses will be examined. It will be observed that the PID method turns out best results. Control system will be simulated with hotel's hot water usage profile, seasonal meteorological statistics and disturbances. Once all simulation scenarios ran, it will be observed that the designed PID control system response gives the optimum results and considerable energy savings. Due to having a high thermal inertia, designing a control system for water heating systems is always a challenging duty. In the first place, determining the right control strategy is very important because of the complex and dynamical system characteristics. In a heat pump system, the required thermal energy to heat the water is obtained from the surrounding air and the heated water is stored in storage tanks. In vapour compression heating systems, the usage water to be heated up gains energy when it goes through condenser therefore the temperature of water varies in condenser. If the stored usage water in the tank is cold, it should be heated and the temperature of water to be sent to the tank should be controled to provide temperature demand. Due to the aim of heat pump system is to minimize the heating cost using surrounding air as the energy source, the accuracy of the control system is quite important to maintain this energy saving. When controlling the heat pump water heating system, the hot water user comfort and the energy cost preservation should be considered. There are several control strategies such as conventional or intelligent controllers for heating systems. Heat pump system controllers measure the flow rate of water supplied to the condenser and control it. All heat pump control systems have the control organs either to recognize the tank water level or temperature of outlet water. In this thesis, it is aimed to design controllers such as lead/lag compensators and PID to control water temperature in heat pump system by using frequency response methods. The reasons behind choosing those control techniques are, robustness, simplicity and superior control effect that they have. All heat pump water heater systems use compressed refrigerant in order to heat the water inside the condenser. Due to the variable water temperature inside the system, the aim of the control system design is to control the output hot water temperature by controlling the flow rate of the refrigerant. To obtain the best result from controller design, sytem must be modelled accurately. In order to create a system model, estimation and validation algorithms of system identification module will be used. One of the aims of the system identification process is to create mathematical models of the complex physical systems where detailed experiments cannot be conduct by the researcher. Syste identification procedures use the data samples which are collected from the physical system. System identification procedure consists of many steps such as, model estimation, model validation, error analysis. There are many types of model structures in the literature which provides a sense of comfort to the researchers who use the system identification techniques. Residual and correlation analysis are very important in model validation. Compansators and PID controllers are widely used in the industry and they give fast and effective responses. Eventhough the tuning process of PID controllers are complex, they have very good performance on many process. In the thesis, Kp, KI and KD parameters of PID controller calculated by using frequency response methods. Because discrete time systems are not linear in the frequency domain, bilinear transformation is used to work in the –w domain. During controller parameters analysis, it is shown that the controller gives ideal results on closed loop systems. Ambient air and feed water temperature variations are included to the system as disturbances to test the robustness of the controller. Phase lag, PID and on-off controllers are compared. All the controllers are designed by using MATLAB® software. Comparison simulation of the controllers showed that the designed PID controller achieved to maintain demanded hot water temperature at the output of the condenser. Output water temperature of condenser is basically the out put of the control system design. A model based PID controller is designed to optimize the system output and to get rid of the disadvantages of the conventional on-off controller. Designed controller is provided desired hot water temparature regardless of disturbances i.e. ambient air temperature deviations and gave better transient response characteristics. To demonstrate the daily working cycle of the system, the operation of the hot water distribution system is first described and hot water usage profiles for different seasons of the year are created. The hot water produced by the heat pump system is used only domestic purposes not for industrial purposes such as, laundry, kitchen. Depending on the occupants in the hotel and seasonal meteorological conditions, three different usage signal is created to simulate daily hot water usage and heat pump operations. The results of this thesis work will be a foundation for the researchers who face the technical problems in such hotel establishments when providing necessary comfort conditions and energy consumption reduction.