A stirling solution for combined heat and power

Diesel Progress North American Edition, August, 2004 by Dawn M. Geske

Already ridding an extensive line of distributed generation products under its energy|now brand, DTE Energy Technologies (DTE Tech) has expanded the renewable side of its lineup with the introduction of the ENX 55 energy system. The 55 kW cogeneration package is powered by a Stifling engine fueled by natural gas, propane, flare gas, methane, wood gas, biogas or, in its heat-fired version, heat produced from industrial processes.

"The way it (the ENX 55) is being employed today is in both CHP (combined heat and power) and power only packages," said Mark Fallek, vice president and chief marketing officer for the Farmington, Mich., company. "The applications that we think make sense are employing it in places where there is free gas--things like landfills or digesters at wastewater treatment plants or agricultural operations that produce methane, which can then be burned to produce power.

"In the agriculture arena, more and more digesters are being used to breakdown manure, making methane and carbon dioxide. We can take that gas and burn it in a Stirling engine for power as well as use the waste heat to keep The digesters warm. Digesters for these biogas applications need to be kept at 100[degrees]F to keep the anaerobic bacteria alive using the hot water. So it's a good marriage."

In heat applications, the ENX 55 module works off any high-temperature process that will operate "as long as it's clean" Fallek explained. "It's important to note that the Stirling engine requires a very high temperature between 1472[degrees] and 1832[degrees]F," he added.

Using a Stirling engine from STM Power, Ann Arbor Mich., the ENX 55 provides enough rotational power to produce 55 kW of electricity and a heat output of 310,000 btu/hr. According to DTE Tech, the external combustion engine employs a swashplate that takes the linear motion of the pistons and converts it into rotary motion to drive a generator. "You've got rotary" motion driving the generator to produce power, and waste heat from the unit is employed for making hot water," said Fallek. "It's all external to the cylinders and it's a continuous process of providing heat, which can be used for various means such as heating, hot water or whatever you need it for."

Heat is removed from the engine's combustion process using a water cooling system. Water is directed through an internal cooling loop at a temperature of 140[degrees] F and then runs through a heat exchanger. The heat from the engine jacket water is used to heat water for building loads or processes. If the heat recovery is not required, a radiator is available to cool the engine. Electrical efficiency of the ENX 55 unit is 31% with 82% efficiency in the total CHP system, the company said.

Working with STM Power since 1999, DTE Tech develops and sells the external combustion modules worldwide, excluding Asia, under the energy|now brand. The company has 16 locations throughout North America and distributors covering Switzerland, Austria and Germany, Northern Italy; Turkey and South Korea. "Today our business is solely working in the distributed generation business which includes CHP and standby generator sets of" all sizes," said Fallek. "We offer a broad range of technologies that run from Stifling engines to internal combustion engines to a turbine product that we're developing to fuel cells."

DFE Tech is a wholly owned subsidiary of DTE Energy, a Detroit-based developer of merchant power and industrial energy projects. The largest operating subsidiaries of DTE Energy are Detroit Edison, an electric utility serving 2.1 million customers in south eastern Michigan, and MichCon, a natural gas utility setting 1.2 million customers in Michigan.

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