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Running a Marine Unit on Solar and Diesel

Navy research program eschews solar PV for hybrid solar-liquid fuel system to dramatically cut fuel use.
October 19, 2012

Military researchers are exploring whether the combination of solar power and conventional fuels can reduce warfighters’ vulnerability in the field.

A Stirling engine can run on heat from concentrated sunlight or by burning fuel, but for military use can’t be nearly as large as this dish for utility-scale power generation. Credit: Infinia

An Office of Naval Research project called the Renewable Sustainable Expeditionary Power (RSEP) program has set a target of reducing fuel consumption by 40 percent for mobile military units by relying more on solar power. (See video description here.) The research will put new concepts into practice and could lead to commercial products, particularly for off-grid power generation.

Raytheon yesterday announced it was awarded a contract to develop one phase of the program. None of the three companies participating in the research program will be using conventional solar photovoltaic panels in an effort to bring boost system performance.  

There’s a strong desire in the U.S. military to reduce fuel use in battle zones, such as Afghanistan, because fuel convoys are vulnerable to attack. The Marines have been using traditional solar PV panels in Afghanistan, but the RSEP program is geared at more dramatic fuel savings and combining solar with other power generators for round-the-clock operation.

The design goal is to reduce the fuel needed for a “forward operating base,” or a small unit of marines away from a larger base for about 15 days, according to David Altman, the senior principal mechanical engineer for advanced technology at Raytheon’s Integrated Defense Systems business. The hardware has to be small enough so that it can be towed behind a Humvee, the same way that mobile diesel generators are now.

Raytheon and research company Battelle are designing their systems around a sun-powered Stirling engine. A dish will concentrate light onto a helium-filled cylinder to produce heat. The difference between the heat and the cooler incoming outdoor air causes the helium in the engine cylinder to contract and move the piston to generate electricity.

Stirling engine’s are relatively efficient, quieter than diesel generators, and take up less space than solar PV panels. Another key advantage is that they can operate with any heat source. Raytheon’s Altman didn’t provide many details on the company’s proposal but said that the system will burn fuel to run the Stirling engine at night, for instance, and have some energy storage, such as a battery. 

One of the toughest challenges is fitting a parabolic dish and burner onto a trailer, Altman said. Stirling engine maker Infinia makes a solar dish for power generation but it’s more than 20 feet high.

“Dish sizing and construction of the concentrator configuration are central to having this a viable and successful concept,” he said.

Emcore, another company involved in the project, will design a Marine-ready system around its concentrating photovoltaic technology, where lenses focus light onto very efficient solar cells, combined with fuel cells and batteries for on-demand power. Concentrating PV technology, normally used by utilities, uses less space than flat solar panels.

The program calls for participants to study how the component pieces can meet the 40 percent efficiency goal and have a reasonable cost. A next phase, expected next year, will lead to prototype devices. 

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