Feasibility study of innovative absorption heat pump processes
In recent years climate change and increasing energy costs have been extensive discussed in public. Thermally driven heat pumps can contribute substantially to energy conservation and the reduction of CO2-emmisions. This is due to the reduction of the primary energy demand, the increased utilization of renewable energy sources or the utilization of otherwise not usable waste heat.
The main objective of the project “InnovAP” was to investigate alternative absorption heat pumping processes concerning different applications, e.g. “solar cooling. These new concepts and/or working mixtures should make a contribution to increase the application of absorption heat pumps in order to tap the whole potential of this environment-friendly technology.
In the project “InnovAP” the following innovative absorption heat pumping concepts have been identified, detailed investigated and compared to each other and to the state-of-the-art:
- Condensable „auxiliary fluids“ in the refrigerant circle
- Processes with partially miscible working mixtures
- NH3/H2O AHP-process with additives.
By means of computer models in ASPEN Plus the thermodynamic correlations within different absorption heat pumping processes have been detailed investigated. As a result of that two processes have been identified as the most promising: the Rojey cycle and the conventional NH3/H2O AHP-process with natrium hydroxide (NaOH) as additive.
The Rojey cycle, which operates with a condensable auxiliary fluid between the evaporator and absorber, shows promising simulation results. Using a 2-stage absorber and the working mixture NH3-H2O-C4H10the COP of the Rojey cycle is ca. 20% higher in comparison to a conventional NH3/H2O AHP. However, the necessary temperature level of the driving heat is higher, which might be a disadvantage of this cycle. A possible area of application for this cycle could be found, where low temperature lifts and/or high generator temperatures are applicable.
The simulation results of the NH3/H2O AHP-process with NaOH as an additive have shown, that the COP of an NH3/H2O AHP can be increased by ca. 3-20% and at the same time the necessary temperature level of the driving heat is decreased. In that way this process is of great interest for applications, in which the generator temperature is limited, e.g. for “solar cooling applications”.
In order to verify the results of the thermodynamic simulation the process has been also experimentally investigated. A test rig has been built and measurements have been conducted without NaOH and with 5% of NaOH in the working pair (NH3/H2O). The expected increase of the process efficiency has not been demonstrated yet. A possible reason for that could be reduced absorber efficiency when using NaOH.
It can be concluded, that within the project “InnovAP” promising innovative AHP-processes have been found. In order to evaluate the potential of these processes more in detail further research work is necessary.
Ao. Univ.-Prof. DI Dr.techn. René Rieberer
Technische Universität Graz, Institut für Wärmetechnik
- Mag. Oleksandr Kotenko
- DI Dr.techn. Harald Moser
- DI Dr.techn. Andreas Heinz
- DI Joachim Kalkgruber (Diplomand)
- DI Stefan Ganster (Diplomand)