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U.S. Army Soldier & Biological Chemical Command
U.S. Army Soldier Systems Center-Natick
Public Affairs Office
Kansas Street
Natick, MA 01760-5012

Contact: Chief, Public Affairs Office
(508) 233-5340

Date: May 18, 2001
No: 01-35

Extra electricity with liquid-injected cogeneration

Natick, Mass. --- Utility customers want inexpensive and plentiful electricity, military leaders desire leaner and lighter yet still capable equipment, and environmentalists seek cleaner air. A new technology called liquid-injected cogeneration could satisfy all of these wishes.

Cogeneration is the combined production of electric power and heat. Through a Dual Use Science and Technology project awarded in 1998, the Department of Defense Combat Feeding Program at the U.S. Army Soldier Systems Center (Natick) and Yankee Scientific, Inc., in Medfield, Mass., are creating an effective way to heat and generate electricity for military and commercial equipment.

The research and development has already spun-off a new company called Climate Energy Inc. The company will be spending $5 million during the next three years to develop a residential cogeneration system with plans for initial sales of 5,000 units per year growing to 20,000 units per year or more. Although initially designed for home and water heating, the cogeneration system would also work for heat-driven air conditioning.

Until now, homeowners had no choice but to pay their public utility for electric power, but the deregulation of the electric power industry provided incentive for alternative sources and approaches to powering homes.

Deregulation enables homeowners to buy electric power from any supplier, or generate their own electricity to help cut energy costs and provide a measure of independence from local utilities.

"It's great timing. The public utilities are reeling from consumer energy demands, particularly in California, and environmentalists are reporting continued environmental damage by the power plants," said Don Pickard, the Combat Feeding Program's Equipment and Technology Team leader. "From a technology standpoint, I think it's a sure bet."

Homeowners understand their dependence on electricity every time their power fails. Engine-driven generators are adequate for occasional back-up power, but noise and low reliability make this type of power source a poor choice for home use.

Liquid-injected cogeneration is a process where water is pumped to high pressure, heated with a conventional oil or gas burner and then injected into a scroll expander that turns a conventional generator to produce electric power.

As the water is expanded, it changes to steam. The steam is then condensed for space and water heating.

It requires few extra components to a residential heating system and solves the shortcomings of conventional generators.

Pickard said the average time between failures for a generator is 450 hours, while a liquid-injected cogeneration system should run tens of thousands of hours before maintenance is required.

The criterion for electric efficiency of a cogeneration system is that it be high enough to provide sufficient electric power to be practical.

For a residence, this may mean that it can offset enough of the monthly electric bill to pay for the incremental hardware costs.

"Liquid-injected cogeneration is so simple and inexpensive, it could pay for itself in a few years," Pickard said.

These same advantages could be realized in military field services that are predominantly heat-driven. Generators are the weakness of logistics systems, which fail if the supporting generator fails. An efficient integrated cogenerator would provide logistical reductions by eliminating the need for a separate generator set.

This cogenerator is not intended to be the homeowner's sole supply of electric power.

It needs to be connected to the power grid though a conventional meter and switch box. When the burner cycles on to heat the home or water, electricity is generated to power the home, and any surplus causes the meter to spin backwards. At the end of the month, the meter's backward motion cancels some of the forward motion, and the consumer's electric bill will be much lower, Pickard said.

An application of liquid-injected cogeneration is under development for the Army's Battlefield Sustainment Center, a field kitchen for the future Objective Force.

Water heated to 450 degrees F is used to heat the ovens and griddle, and steam at 240 degrees F is used to heat kettles and water. Cogenerated electricity powers the burner, lighting, ventilation, refrigeration and controls.

Pickard said he expects cogeneration to yield an efficiency of about 75 percent.

With a field kitchen, eliminating the generator cuts fuel use in half. The cogenerator also reduces noise levels from 80-90 decibels to a less jarring 60-70 decibels.

Besides kitchens, liquid-injected cogeneration could be used for field laundries, showers and shelter heating.

Reducing energy costs, and providing quiet and reliable power are all major benefits, but Pickard said the primary argument for cogeneration is environmental.

Cogeneration presents a major opportunity to reduce the emissions of greenhouse gases and acid rain from coal and oil burned to produce electricity for millions of homes and buildings in the industrial world. Instead of burning coal or oil as most public utilities do, the homeowner burns natural gas that will have already been used to heat the house.

Since the electricity is generated at very high efficiency, the overall fuel consumption will be only 5 or 10 percent higher, which means a lower amount of cleaner fuel is used.

Furthermore, fuel is burned in low concentrations and causes negligible damage as compared to the environmental damage downstream or downwind of the power plant.

"In the short-term you can save the consumer money, and in the long-term you can reduce pollution and global warming," Pickard said.

Variety of methods for cogeneration
For heat-driven military field systems and homeowners, liquid-injected cogeneration appears to provide the best balance of heat and electric power in terms of quality and temperature of the heat produced, simplicity, reliability, and cost. However, several other types of cogenerators have been researched.

Among them are thermoelectric, fuel cell, stirling cycle and thermophotovoltaics.

Thermoelectric generators based on the peltier effect produce electricity when one side of the solid state material is heated to 450 degrees F with a burner, and the opposite side is cooled with air or 212 degrees F water. Their 3-5 percent efficiency means plenty of hot water must be used to make them cost-effective.

Many fuel cell systems are under development and their technology is relatively mature. With a hydrogen fuel source they are very efficient and reliable.

The problem is that neither homeowners or the military want to store hydrogen. Fuel cell users want to be able to reform standard fuels such as methane, propane, natural gas, gasoline or JP8. Methane reformers are available, and natural gas has worked for some high temperature fuel cells. However, gasoline remains a challenge, and JP8 use may not be accomplished in the near future.

Stirling engines, driven by external combustion, were developed before internal combustion engines but have had historical problems with reliability primarily because of seal failure. Recent breakthroughs have reportedly overcome these problems, and NASA has chosen a stirling generator for their deep space probe, a significant vote of confidence.

Thermophotovoltaic (TPV) generators are being developed by U.S. Army Communication-Electronics Command (CECOM) at Fort Monmouth, N.J., for portable power in the 500 watt range. TPV is a process where fuel is burned and heats a black body that radiates energy, which is filtered to emit 1 micron photons. These photons are absorbed by a special photocell that converts their energy to electricity.

The Combat Feeding Program is funding technology base work that leverages both NASA and CECOM's efforts by configuring their respective air-cooled generators as water-cooled cogenerators that can be used to replace the 2 kilowatt generator on the Mobile Kitchen Trailer.

Both cogenerators will be configured with atmospheric boilers that will provide heat for heating tray packs, sanitation and hot beverages.

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