Scanning for ideas: Three stages boost microturbine efficiency

Jan. 24, 2008
Most microturbines are 25 to 35% efficient, which means they need some sort of waste-heat recovery system to be economical.

So only sites that need continuous hot water or hot air can really use them. A new approach from Wilson Turbopower Inc., Woburn, Mass. (wilsonturbopower.com), uses ceramic components and a three-stage turbine to get efficiencies of 50% or greater. And if there is a need for on-site hot air or water, total system efficiency can approach 90%. Bottom line: This microturbine generates electricity from natural gas for 5 to 12 cents/kW-hr.

The 300-kW turbine uses three stages, unlike typical single-stage versions, which improves performance and helps with efficiency. The device’s 13-in. ceramic blades let the turbine handle higher inlet temperatures than metallic blades. The turbine also spins relatively slower than conventional microturbines, so stress in the rotor is reduced and its life extended.

Another key to the system is its rotating ceramic heat exchanger. Hot exhaust air goes through a portion of a slowly turning ceramic disc honeycombed with 200 to 1,100 holes/sq in. The hot gas warms the disc and, as it turns, hot gases pass through all portions of the disc. Relatively cooler air flowing toward the turbine passes through another portion of the disc, but in the opposite direction, and absorbs that warmth. And heat has practically no effect on the ceramics. The ceramic heat exchanger, therefore, transfers more heat and permits higher operating temperatures than metal-core heat exchangers, so it contributes more thermal energy to preheating inlet air. Heat-transfer can be 97% efficient in the ceramic heat exchanger, according to the company.

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