The engine works by alternately heating and cooling a gas such as helium in different compartments. The expansion of the gas upon heating drives the motion of a piston, mechanical work from which electrical power can be derived. The fact that only external heat is required to expand the gas leads to the engine's unique advantage-burning practically anything powers the engine. Wood, decomposed plants, or even cow dung would provide the world's poor with power. These types of fuels are abundant and inexpensive, freeing rural areas from dependence on fossil fuels. The kilowatt of electricity produced is enough to drive multiple low powered devices, like light bulbs, at the same time.

Electricity from the Stirling Engine can purify water collected from local lakes, rivers, or even puddles using a technique called Vapor Compression Distillation (VCD). The technique makes use of excess heat from the engine. In VCD, a machine heats contaminated water until it evaporates, compresses the steam, and allows it to condense. The hot, purified water passes the outside of the input pipes and heats the incoming water. The process desalts water and removes most disease-causing microorganisms through boiling and vaporization. In DEKA's design, which uses only a fraction of the energy of traditional water purification systems, waste is fired from a tube, a property that gives the machine its name: The Slingshot.

Prototypes of the engine were successfully tested in Bangladesh. When the machines were deployed in villages, individual operators sold electricity to customers and bought cow dung from people who gathered it. The tests impressed Emergence Energy of Cambridge, Massachusetts so much that it is considering purchasing the technology and manufacturing the engine. Yet while these machines may seem like easy solutions to some of the developing world's biggest problems, the current drawback is cost. The prohibitive cost of 100,000 dollars for each machine, even without the water purification system, explains the limited spread of the invention. DEKA's goal is to minimize the cost of production so that prices can be decreased by 99 percent from the prototypes. This ambitious project is currently in the hands of DEKA engineers. Considering the success this group has had in revolutionizing health technology in the past, producing portable dialysis machines, electronic wheelchairs that climb stairs, and intravascular stents, it is not beyond them to construct an affordable device that could improve the lives of a billion people in only the immediate future.

Christopher Belknap is a junior Molecular Biophysics and Biochemistry major at Yale University.

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