Competing with recip engines

Competing with recip engines

Kalyanaraman, Kalyan


Though microturbine models have been available in the market for over a decade, they have been unable to make significant inroads into the distributed generation market. Reciprocating engines, whose selling price is typically less than half that of microturbines, continue to dominate the market.

But two inventors Jon and Michael Teets – who have over 35 years of experience in turbomachinery and worked in OEMs such as Pratt & Whitney, Honeywell and Elliott Energy Systems – have developed a new microturbine TMA-70SC that they hope can compete with the recips. The Teets brothers plan to sell the microturbine at around S350/KW, with mark ups for the turbine enclosure and project engineering.

A rankine-microturbine

The TMA-70SC incorporates features from three other gas turbines the Teets brothers have invented since they developed an early microturbine model in 1994 (see TMI, Jan./Feb. 2001, p. 28). The TMA-70SC is a hybrid microturbine that consists of two non-concentric rotor spools (Photo). Each spool has one turbine stage and one compressor stage.

One rotor produces power and is integral to the alternator rotor, and the other is a turbocharger. Keeping just one compressor stage and one turbine stage on a rotor simplifies the rotordynamics, yet the two compressor stages together produce a pressure ratio of 11:1, says Michael Teets.

The TMA-70SC delivers 70 KW and can be scaled up to 500 KW. Scaling up will reduce sale price by $80/KW, says Jon Teets.

Target applications for the microturbine are stationary, as well as marine and aircraft power generation. Besides low cost, the TMA-70SC’s light weight is said to be a key distinguishing feature. At around 100 lbs, it is lighter than other microturbines that weigh over 1,200 lbs and marine diesel engines that weigh over 1,800 lbs, says Jon Teets. The total weight of the TMA-70SC package is around 270 lbs. The package volume (34,272 in3) is said to be 25% of that of the typical diesel engine generator set.

Microturbines usually incorporate a recuperator, which recovers heat and almost doubles efficiency. But the TMA70SC does not have a recuperator. This reduces cost and footprint, and also improves starting capabilities, says Jon Teets. He explains that recuperators have thermal inertia and could therefore delay starting. “If sufficient time is not allowed for starting and stopping, thermal stresses could build up in the heat exchanger and reduce its life.”

The turbine is said to burn both liquid and gaseous fuels at over 22% electrical efficiency at 70 KW, and produce less than 25 ppm NO^sub x^ with a turndown of 65%. The lack of a recuperator contributes to the lower efficiency of the TMA-70SC’s. Conventional microturbine models have around 28% efficiency. “Less weight, compactness, and fast-start capability compensate for the lower efficiency,” says Jon Teets.

“Also, the hybrid microturbine’s offdesign efficiency does not drop as much as the typical microtubine unit,” Teets says and adds that at 55% load the hybrid microturbine has the same efficiency as other microturbines. At lower loads its efficiency is said to be higher.

The TMA-70SC can achieve maximum power in less than 10 seconds of starting. An oil system lubricates the four bearings. Air bearings that are often used in microturbines increase ramp up time.

Addition of a steam bottoming cycle could increase the efficiency to nearly 60%. In the combined cycle, heat from the outlet steam of the steam turbine is recovered in an external counter-flow regenerator. The steam turbine is integral to the microturbine rotor shaft and drives a common alternator rotor. Patents are pending for both the hybrid and the rankine models.

While conventional microturbines are started by electric motors, the TMA-70SC is started by an air impingement system. Michael Teets explains that air starting needs a simpler control system. A rully charged air cylinder is necessary tor starting the microturbine, which produces its own 24 V dc power for the control system. When it runs, the turbine charges the air cylinder.

The TMA-70SC has a two-stage combustion system. In this “Rich-burn, Quick-quench, Lean-burn” (RQL) design – invented by the Teets brothers – the fuel-air ratio is rich in the leading area of the combustor, but quickly becomes lean downstream to control emissions. The rich primary stage has an equivalence ratio range of 1.1 to 2.2 (the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio) and the lean secondary stage has an equivalence ratio range of 0.25 to 0.50. The brothers claim that the microturbine can run 40,000 hours (four and a half years) without major maintenance.

Copyright Turbomachinery International Mar/Apr 2006

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