Mesoscopic Heat-Actuated Heat Pump

Kevin Drost

Project Description

The purpose of this Defense Advanced Research Projects Agency (DARPA) project is to demonstrate a mesoscopic absorption cycle heat-actuated heat pump for a range of military microclimate control applications. Although currently available cooling systems can be integrated with protective suits to provide some degree of cooling, these systems are based on a vapor compression cycle that requires significant amounts of electricity. An absorption heat pump is similar to a vapor-compression device except that compression is accomplished in the absorption heat pump through the use of a thermochemical compressor. The simple thermochemical compressor consists of an absorber, a solution pump, a heat exchanger, and a desorber (gas generator). While several heat-actuated heat pump cycles have been investigated, current research at Battelle is focused on a single-effect lithium bromide and water (LiBr/H₂O) absorption heat pump.

While the mesoscopic heat-actuated heat pump can be applied to many military cooling applications, one specific application of the device is being demonstrated: manportable cooling. Personnel performing labor intensive tasks in a hot environment are vulnerable to heat stress, especially when using nuclear, biological and chemical (NBC) protective clothing. Supplemental cooling will permit the soldier to perform tasks under NBC conditions in hot climates with enhanced efficiency.

By using a heat-actuated cooling cycle, the Battelle mesoscopic absorption heat pump has radically reduced the requirements for electric power by substituting thermal energy for electric energy. When fuel, batteries for fan and pump power, and an air-cooled heat exchanger are added to the mesoscopic heat-actuated heat pump, the complete system weight is projected to be between 3.7 and 5.0 kg for an 8-hour mission. This is less than one-half of the weight of competing conventional microclimate control systems.

Preliminary estimates predict that the mesoscopic absorption heat pump will have a volume of 420 cm³, weigh approximately 0.72 kg, and will be capable of providing 350 watts thermal (W_t) [W_e refers to a watt of electric energy; W_t refers to a watt of thermal energy] of cooling.

Technical Accomplishments

In completing the design of the mesoscopic heat-actuated heat pump:

Computer simulations of the heat pump were conducted using ABSIM, an absorption cycle system simulator, to find out the impact of the cooling system variables on the heat pump coefficient-of-performance (COP) and weight. Assumptions were made and values held constant for component heat transfer characteristics, combustor performance, solution pump flow rate, and evaporator load. Four system values were varied over an accepted range: 1) cooling water temperature entering the absorber (temperature of water leaving the system radiator), 2) chilled water temperature leaving the evaporator (temperature of water entering vest), 3) thermal capacitance of cooling water loop, and 4) thermal capacitance of chilled water loop. A correlation was developed between the four variables and heat pump COP using a regression analysis for all variables, cross products of the variables and squares of the variables. This information was then used to optimize the manportable cooling system.
Computer simulations were used to optimize the manportable cooling system design. Commercial supplies were identified for all of the components required to support the mesoscopic heat-actuated heat pump. The projected weight of the system is 4.7 kg using thermoelectric cogeneration and 5.33 kg using batteries for heat pump auxiliary power. In addition, we have identified other system improvements that should reduce weight to between 4.0 and 4.5 kg.
Ranges of materials were screened for use as desorber and absorber contactors. The requirements for the contactors at the absorber and desorber are significantly different. The absorber requires a higher permeability than the desorber does because there is less pressure difference available. The temperatures in the desorber are much higher than in the absorber, on the order of 160°C for the desorber compared to approximately 60°C in the absorber. Because of the temperature difference, the preferred contactors for the absorber, are not acceptable for use at the high temperatures in the desorber. Overall, we have identified one class of contactor materials that we expect will work for the absorber and a second class of contactor materials that we expect will work at the desorber.