A post-doctoral researcher at the Norwegian University of Science and Technology (NTNU) has found that an ammonia-water absorption compression heat pump (ACHP) is able to provide a high temperature of up to 105°C (221°F) at low discharge levels at a COP up to 2.85, offering a feasible solution for steam production in industries such as food processing, where fossil fuel currently dominates.
The researcher, Shuai Ren, presented the research paper “Performance modeling of an ammonia-water absorption compression heat pump for steam generation in food processing” at the 10th International Institute of Refrigeration (IIR) conference, held in Ohrid, North Macedonia, April 27–29.
The study has received funding from the European Union’s Horizon 2020 research and innovation program under a project named ENOUGH.
Ammonia-water absorption compression heat pumps offer a way to achieve climate neutrality in the food sector by decreasing energy consumption and reducing carbon footprints. This type of high-temperature heat pump produces steam of up to 150°C (302°F) to perform various applications like drying, scalding, pasteurization, sterilization, steaming and cooking.
The absorption process normally does not employ a compressor, but in this application it does. An ACHP uses a cycle that begins with the introduction of low-grade heat from a source on the desorber side, where ammonia separates from water. Upon leaving the desorber, the working fluid exists as a two-phase mixture – a liquid and a vapor – where the liquid phase has a low concentration of ammonia. The liquid vapor is then passed to the separator. The vapor phase is directed to the compressor, while the lean solution (liquid phase) is pressurized to a high pressure using a solution pump.
The vapor and lean solution are combined in a mixer to create an ammonia-water mixture prior to entering the absorber. The ammonia is absorbed into a lean solution inside the absorber, and heat is released.
Intercooling improves COP
Ren and the team from NTNU studied the ACHP prototype dubbed the “Osenbrück 4.0 Heat Pump,” using an ammonia-water mixture as its working fluid. The team investigated the ACHP’s performance under various operational parameters. One significant outcome involved a case of 150kW (42.65TR) heating capacity with a high pressure of 23.65bar (343psi) and a low pressure of 4bar (58psi). The system’s COP improved from 2.14 to 2.85 when intercooling was introduced in the compressor. The discharge temperature also decreased from 276 to 160.3°C (528.8 to 320.5°F).
Another result was the resilience of the ACHP’s performance even under high sink outlet temperature and temperature lifts. A COP of 1.83 was still achievable at a high sink outlet temperature of 120°C (248°F) and a temperature lift of 50K. However, the study cautions against further increasing these parameters as they significantly hamper the heat pump’s performance.
Part load and overload conditions also proved to be significant factors in the ACHP’s performance. The study observed that a reduction in the heat pump’s load from 110% to 60% of design point led to a 19.3% increase in COP, along with a 7% decrease in discharge pressure.
These results were obtained using the simulation in the Dymola-Modelica modeling language with the same setup as in the NTNU lab.
Advanced system
The NTNU lab also set up an advanced ACHP system, designed to handle a maximum heat capacity of 200kW (56.86TR) and a maximum operating pressure of 40bar (580psi) with an operating temperature range of -10 to 190°C (14 to 374°F). In this case, the system features an oil-free twin-screw compressor that uses liquid injection technology, avoiding the risk of contaminating the working fluid. An auxiliary system provides heat source and heat sink water circuits to model the industrial operational environment.
In this scenario, thermodynamic modeling gives a sink outlet temperature of 105°C (221°F) with a COP of 2.85.
An ammonia-water absorption compression heat pump (ACHP) is able to provide a high temperature of up to 105°C (221°F) at low discharge levels at a COP up to 2.85, offering a feasible solution for steam production in industries such as food processing, where fossil fuel currently dominates.
Shuai Ren, Post-doctoral researcher, Norwegian University of Science and Technology
