A 2007 McKinsey Global Institute study estimated that more than $50 billion in energy savings were possible by adopting combined heat and power (CHP)technologies. Doing so could even cut U.S.fossil fuel use by nearly 20%, according to a Lawrence Berkeley National Laboratory study. It is no wonder, then, that several emerging U.S. companies and research institutions have launched ventures to recover waste heat and convert it into electricity by means other than CHP. Clean technology firm Electra Therm Inc. in late May, for example, announced that testing of a commercial waste heat power generator at Southern Methodist University in Dallas, Texas, had exceeded expectations, reaching output well beyond its 50-kW rating.

Dubbed the “Green Machine” the device uses a closed-loop organic Rankine cycle (ORC) whose turbo-expanders have been replaced with a patented twin-screw expander. Surplus heat as low as 200F (liquid) is captured by the Green Machine’s evaporator, where it “boils” a chemical working fluid into vapor. Under pressure, the vapor is forced through the company’s twin-screw expander, turning it to spin a generator. The vapor is then condensed, again pumped to higher pressure, and returned to the evaporator to repeat the process.Because the twin-screw expander is simpler to manufacture than turbo-expanders, the Nevada-based company estimates that its machine will cost 30% less than a turbine ORC system and pay for itself, “subsidy-free,” in three years. More important,

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ElectraTherm Inc. successfully tested a commercial waste heat generator at Southern Methodist University in May. The “Green Machine” produced more than 50
kW using heat as low as 200F to run a closed-loop organic Rankine cycle (ORC). To make it cost-effective, the ORC was fitted with a patented twin-screw expander instead of typical turboexpanders.

once the initial investment is paid off, ElectraTherm expects that the Green Machine could produce power at 1 cent per kWh (its current output costs 3 to 4 cents per kWh).

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