The waste heat driven vapor jet ejector cooling cycle is a very promising approach to produce 'free' cooling by utilizing low-grade energy sources. The mechanism behind ejector-based waste heat cooling is very different from absorption or adsorption cooling technologies that are also aimed at producing heat driven cooling. The ejector cooling system is actually more closely related to vapor compression technology, in which an ejector, a waste heat source, and a liquid pump are used to replace the vapor compressor. Rising energy costs and the desire to utilize otherwise unused low-grade energy that becomes available as a byproduct in many processes, such as power generation, justify increased research efforts on this promising approach. This paper presents both numerical and experimental research carried out with vapor jet ejector cooling cycles. A military-style, trailer-mounted technology demonstrator was designed, built, and evaluated. The concept consists of a diesel-electric generator with a nominal electric power output of 15 kW. A conventional, transcritical R744 vapor compression Environmental Control Unit (ECU) is powered by the generator, thereby loading the generator's combustion engine. Waste heat from the generator is extracted at two different temperature levels, namely from the generator's exhaust and engine coolant streams. The extracted heat is transferred to the R134a working fluid inside the vapor jet ejector ECU where it ultimately produces the desired cooling effect. Measurements show that a cooling effect of 1.54 kW can be produced with electrical input of approximately 0.16 kW. It is demonstrated that the total cooling output per liter fuel spent is improved by up to 11 % by operating the ejector system in addition to the conventional vapor compression system.