ABSTRACT
A combination of a resorption and a compression heat pump cycle, known as the hybrid cycle, using a water/ammonia mixture enables a high efficient operation in a wide temperature range. The gliding evaporation temperature of the mixture leads to good temperature matching between the working fluid and the heat flow transporting media. The pressure in the cycle is lower than in a cycle with pure ammonia as working fluid.
This cycle suffers from two major drawbacks. The cycle is somewhat more complex than a simple adsorption or compression cycle, thus the design needs sophisticated programming for layout and optimization. In addition the desorption and resorption of the ammonia rich vapour is a coupled two phase two component heat and mass transfer problem, so the design of the heat exchangers is not standardized yet, even though several of these hybrid heat pumps have been operated already in different laboratories.
These problems have been addressed in a research project which is reported in this presentation. The project includes a 40 kW setup, which uses plate heat exchangers as an evaporator (degaser) and as a condenser (resorber). A theoretical model of the cycle was created using the software EES with the external property routine AWMIX. This model allows insight into the thermodynamic condition of the cycle. Some parameters like over all ammonia mass fraction in the system and steam quality after the evaporator have been varied to find the optimal operating point for the heat pump. The model was validated against literature data.
Based on these calculations an experimental rig is being constructed, using the results from the simulation for dimensioning. The experimental results will be used for the improvement of the simulation as well as for the gain in experience with heat exchanger design.
The theoretical work as well as first results are presented.
