Bachelor's thesis Matthias Mersch


Design and Implementation of a Programmable Logic Controller for Heat Pump Heating Systems and Development of a Test Bench to Evaluate Different Control Strategies

Residential heating systems are a major contributor to fossil fuel consumption and greenhouse gas emissions. The reduction of both is regarded as important target by politicians and scientists in order to tackle climate change. Heat pumps are an energy efficient alternative to conventional heating systems. However, most heat pumps do not use their full potential due to inefficient and outdated control strategies.

In commercially available heat pump systems, the control logic is implemented by a built-in controller. It also serves as safety system to prevent damage to the heat pump and its surroundings. Common controllers use conservative control strategies that do not account for the dynamics of realistic operating conditions. Therefore, the efficiency of commercial heat pumps can be significantly improved by using more sophisticated control systems.

Only few data are known from built-in controllers and their operation cannot be modified, since it is not possible to access program code. Hence, a test bench for the development and evaluation of advanced control strategies for heat pump heating systems is designed in this thesis. The central component of the test bench is a programmable logic controller, which replaces the built-in control system. It uses a modular Beckhoff platform enhancing flexibility. The system is easily adaptable, while it ensures safe operation of the heat pump. The test bench is equipped with additional sensors, so that it is capable of delivering sufficient data for comprehensive analyses of heat pump control strategies.

As a first step, the hardware components as well as the control software of an existing heat pump system are analysed. The hardware analysis focusses on the input and output signals of the components as well as their operating limits and other characteristics important for the control system. The software analysis is based on the description of the heat pump operation in the installation manual. Special attention is paid to safety measures.

Based on the analysis, a programmable logic controller is developed. Hardware components, such as input and output terminals as well as the processing unit, are selected and the control software is implemented. The main heat pump controller retrieves process data from the sensors and calculates control signals for all components. It also includes the software safety systems. The main controller provides an interface which needs to be implemented by an independent temperature control system. For this thesis, a simple temperature controller that copies the operation of the built-in controller is developed. In future, advanced control systems can easily be connected to the interface. At last, a validation of the control software is performed. All tests are successful.