Combustion-driven, heat-activated heat pumps used for heating and/or cooling have a large performance advantage in terms of size and weight over battery-powered heat-pumping devices. This is due in part to the respective energy densities of commonly used liquid-hydrocarbon fuels (in the vicinity of 42 kJ/g for JP-8 and diesel fuel) compared to the energy densities of zinc/air batteries (approximately 1.2 kJ/g) and of lead-acid batteries (approximately 0.12 kJ/g). High-performance, heat-activated cooling systems able to exploit this advantage of hydrocarbon fuels (by combusting them) would have many commercial and military applications such as cooling of personnel-protective suits (e.g., chemical- and/or biological-protective suits), cooling of vehicle interiors, and recovering and using waste heat from other processes. Even with a heat-to-work conversion efficiency of 10 to 20%, a combustion-driven heat-activated cooling system would be smaller and lighter, and could operate for longer periods of time (compared to battery-powered units) if component size and weight could be effectively limited.
Heat-activated heat-pumps are similar to conventional vapor-compression heat-pumps in that both utilize a working fluid and both include a compressor. In general, the primary difference between a heat-activated heat-pump and a vapor-compression heat-pump is the manner in which compression of the working fluid is accomplished, or in the manner in which power is supplied to the compressor. For example, a classic heat-activated refrigeration process is utilizes a jet-ejector cycle. Although a jet-ejector cycle is simple in design, generally reliable, and able to utilize waste heat, this cycle has not found wide-spread application because it exhibits poor thermodynamic performance. Also, the efficiency of these systems is poor. For example, the heat-activate coefficient of performance (COP), defined as the amount of cooling provided by the cycle divided by the amount of heat required to drive the cycle, is usually very low, typically less than 0.3. Also, the efficiency of these devices diminishes with decreasing system size. For a portable system, a low COP not only increases the size and weight of the boiler and condenser in the Rankine portion of the cycle, but it also increases the weight and volume of fuel that must be carried.