Development of an ammoniawater absorption-compression heat pump at high temperature operation

Abstract

Decarbonization of the industrial sector is one of the most important measures to tackling global warming. Energy demand and associated greenhouse gas (GHG) emissions are continuously increasing in various industrial processes. The increasing demand in the industry for energy efficient, cost-effective, and environmentally friendly energy systems results in a growing interest in using heat pumps. Integrating high temperature heat pumps (HTHPs) for waste heat recovery and supply temperatures of more than 100 °C is a sustainable solution for many industrial high temperature applications. This study investigates the absorption-compression heat pump (ACHP) using the zeotropic ammonia-water mixture as working fluid. The ACHP system combines the technologies of an absorption and vapor compression heat pump with the ability of achieving high supply temperatures above 120 °C with large temperature lifts (> 60 𝐾) and non-isothermal heat transfer (𝑇𝑔𝑙𝑖𝑑𝑒 > 30 𝐾). The working principle and characteristics of different ACHP cycles were discussed and an overview of the current state-of-the-art was elaborated with respect to experimental investigations available in the literature. The existing solutions and challenges for the realization of ACHP systems with focus on the application at high temperature operation were identified: the compressor design with respect to discharge temperature and lubrication; the design and operation of the absorber and desorber; the establishment of efficient liquid-vapor mixing and distribution; and the selection and cavitation protection of the solution pump. It was concluded that the oil-free operation of the ACHP system can lead to improved efficiency and reduced costs by saving on the required oil infrastructure. This can further improve the competitiveness of ACHP systems for the usage in industrial high temperature applications compared to conventional energy supply systems. The analysis of an existing dairy plant, which uses heat pumps for heating and cooling at multiple temperature levels, revealed that the process efficiency can be significantly improved with a waste heat recovery rate of more than 95%. This reduced the primary energy demand by 37.9% and emitted GHG emissions by 91.7%. The ACHP system supplied a temperature of 95 °C and a temperature lift of 33.5 K. An average COP of 5.8 was achieved with a Carnot efficiency of more than 50%. The results demonstrated that ACHP systems are reliable and efficient in commercial high temperature applications up to 100 °C. The potential application area can be further expanded by increasing the achievable heat sink supply temperature above 120 °C. Based on the identified challenges and demands, the task description of the experimental ACHP prototype was specified, and a simulation model of the ACHP cycle with single-stage solution circuit was developed using Engineering Equation Solver. This model was used to investigate the system behaviour of the ACHP system and to determine the design parameters of the ACHP prototype. The energy and exergy analysis revealed that the use of liquid injection during the vapor compression process is a sufficient measure to decrease the discharge temperature and to provide lubrication. The determined design parameters were the basis for the design of applicable and feasible component section solutions. The main component sections for the defined research areas were designed to meet the specifications, considering existing limitations for available components on the market and their potential for modifications. A numerical simulation model of an oil-free liquid-injected twin-screw compressor was developed and used to reduce the limiting compressor discharge temperature and achieve the desired oil-free system operation. The investigation revealed that it is preferable to inject the lean solution with a low NH3 mass fraction (40%), and an injection ratio of 10% of the compressor’s suction mass flow rate is required. The distribution of the lean solution over multiple injection ports located at the beginning (360°) and in the middle of the compression phase (450° or 495°) can ensure a continuous liquid film. The liquid film is utilized for sealing, lubrication, and decreasing of the discharge temperature. The obtained findings are considered as a kind of best-case analysis and were used to determine modifications to an existing compressor and to design the required injection line. The outcome of the conducted research work is the ACHP prototype with a maximum heat capacity of up to 200 kW and a maximum operating pressure and temperature of 40 bar and 190 °C, respectively. Together with the pressurized water heat source and sink circuits, the test facility allows the investigation of different component sections and application cases with heat sink outlet temperatures up to 140 °C. The ACHP prototype is an important tool for the validation of numerical models, testing new elements and optimization of the control strategy. This thesis has successfully completed the aim of the development of an ammonia-water absorption-compression heat pump at high temperature operation. The results were disseminated in the context of three published journal articles, several peer-reviewed conference papers, and other presentations. This project was done in collaboration with industrial partners and generated spin-off projects.