Benchtop Mesoscopic Heat Pump Demonstration

Michele Friedrich

Project Description

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-water (LiBr) absorption cycle heat pump.

Heat and mass transfer are immensely improved in microstructures. Since heat-actuated heat pumps rely primarily on heat and mass transfer to provide "vapor compression" instead of mechanical work, they are an attractive option for energy system 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 that exceed 3.0 W/cm²-K. In addition, a range of conceptual designs of the microchannel absorber and desorber have been tested. The results confirm analysis prediction of high absorption rates due to microstructure mass transfer enhancement. Using ammonia absorption in water, we have demonstrated the ability to absorb ammonia at a rate that generates 10 W/cm² of heating. We believe that we can ultimately reach a heat generation rate of 30 W/cm². 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 contactor.

Technical Accomplishments

FY 1998 mesoscopic heat-actuated heat pump demonstration focused on two activities:

Design, Assembly, and Commission of the Evaporator/Absorber Test Apparatus - The low pressure side of the heat pump, which consists of the evaporator and absorber, had to be tested independently from the heat pump in order to minimize test variables. A test loop was designed and a test matrix developed for the absorber and evaporator pair. The test loop was commissioned and the evaporator/absorber pair were tested and characterized.
Measuring the pressure of a liquid in a vacuum was a difficult thing to do. We had to measure the vapor pressure in the tanks then use differential pressure (dP) sensors to measure the difference between the tank pressure and the process pressure. It was important to measure very small pressure differences (1 torr), so accuracy was critical. A method to measure pressure accurately was developed.

Commissioning of The Heat Pump Test Loop - The heat pump test loop had to be redesigned from last year to accommodate separation of the high and low pressure sides of the system. The high pressure side of the system was designed with 3-way valves at the exits to either send the fluids to tanks or to the low pressure side of the system. This enabled the high pressure side of the system to be tested independently from the low pressure side. When the conditions of the fluid leaving the high pressure side meet the requirements of the low pressure side, the valves are switched to make the entire heat pump system operational.

Results of the FY 1998 LDRD mesoscopic heat pump project:

1) The absorber absorbed up to .022 kg/s-m² which is approximately 17 times better than a smooth tube with a falling film. The heat transfer coefficient was 6700 W/m²-K.

2) The evaporator obtained a heat transfer coefficient of 2400 W/m²-K. This is lower than previous evaporator tests, so we investigated the reason. We found that because the refrigerant (water) entering the evaporator was subcooled (not the normal operating condition) that a good portion of the evaporator was doing single-phase heat transfer which has a lower heat transfer rate. Further tests of the evaporator with saturated refrigerant entering the evaporator were conducted and the data is being analyzed.

3) The evaporator had to be operated at a higher pressure than anticipated because of the pressure drop of the water vapor through the contactor plate in the absorber.

4) The method of measuring process pressure was developed. Capacitance manometers were used to obtain the tank pressure and very accurate differential pressure sensors were used to obtain the process pressure.

6) Desorber and absorber start-up operation procedures were developed to minimize pressure drop across contactor which avoids liquid break-through contactor.

Future work will focus on the completion of the heat pump demonstration and performance characterization including operation at different cooling loads, with transient loads and for long periods of time.

Publications

Drost, M.K. and M. Friedrich, 1998. "A Microtechnology-Based Chemical Heat Pump for Portable and Distributed Space Conditioning Applications", Proceedings of the Process Miniaturization: 2nd International Conference on Microreaction Technology, New Orleans, Louisiana. pp. 318-322, March.

Drost, M.K. and M. Friedrich, 1998. "A Miniature Heat-Actuated Air Conditioner For Distributed Space Conditioning Applications", Proceedings of the 1998 Oil Heat Technology Conference, Brookhaven National Laboratory, Upton, New York. pp. 15-20, April.