Figure 1. Desorber/Condenser Test Loop
Michele Friedrich
This project's research is focused on assembly, demonstration and performance characterization of a complete mesoscopic heat-actuated heat pump. This project will demonstrate our ability to integrate microtechnology-based components into complete systems. The first system to be demonstrated is a single-effect lithium bromide (LiBr)-water absorption cycle heat pump.
Heat and mass transfer are immensely improved in micro-structures. Heat-actuated heat pumps rely primarily on heat and mass transfer rather than mechanical means to provide the vapor compression effect. This makes heat-actuated heat pumps an attractive option for energy systems miniaturization.
Previously, researchers at PNNL have successfully demonstrated all of the microtechnology-based components of a mesoscopic absorption cycle heat pump except for the miniature liquid pump. Microtechnology-based evaporators and condensers were shown to have heat transfer coefficients exceeding 3.0 W/cm2-K. In addition, microchannel absorbers and desorbers have been tested and shown to absorb and desorb refrigerant at a rate that is 20 times the rate of the falling film design. The results confirm analysis prediction of high absorption rates due to microstructure mass transfer enhancement. And using desorption of water from lithium bromide, we have demonstrated a heat transfer rate of 0.1 W/cm2-K. We believe we can achieve 1 W/cm2-K with the right design. Last year, the absorber/evaporator pair were demonstrated; the absorber absorbed up to 0.022 kg/s-m2, which is approximately 17 times better than a smooth tube with a falling film; the heat transfer coefficient was 0.67 W/cm2-K. The evaporator obtained a heat transfer coefficient of 0.24 W/cm2-K, which was lower than normal operation because the water entering evaporator was subcooled rather than the normal two-phase cooling.
The mesoscopic heat-actuated heat pump demonstration focused on three activities. The results of the research in these areas follows.
Figure 1. Desorber/Condenser Test Loop |
The condenser was tested by evaporating dionized water in the desorber and allowing the water vapor to flow through the path designed for refrigerant vapor. The condenser performance was excellent, with a heat transfer coefficient of 2.9 W/cm2-K. Startup and operational characteristics of the condenser were noted.
Test of the Condenser - The condenser was tested independently to confirm performance specifications
Test of the Condenser/Desorber Pair - The desorber and condenser pair was tested to make sure it would perform to system design specifications
The desorber/condenser pair was tested several times and the desorber design modified until we were able to run a successful test. The desorber had a refrigerant mass flux rate of 0.0021 kg/s-m2 and a heat transfer coefficient of 0.4 W/cm2-K.
The desorber design requires further modification to increase the refrigerant mass flux rate to a sufficient level for the condenser and evaporator to operate. Once the mass flux is increased, the heat pump loop can be reassembled and the micro heat pump demonstrated.
|