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It surprises me that burning gas in a power station to provide and distribute electricity to power electric trams is more energy efficient than gas-powered internal combustion trams would be. Do you have a source for that?

This shouldn't be surprising. The internal combustion engine has losses a gas turbine doesn't: air compression, piston movements. In addition, a diesel engine on a public transport vehicle is constantly revved up and down, while a power plant gas turbine is operated closer to the ideal range, thus practical average thermal efficiency differs from the maximum more. Electricity transmission losses are only a few percent.

Here is one source with a concise summary:

Trolleybus UK

The thermal efficiency (electricity out/fuel in) of modern combined cycle thermal power stations is essentially constant at around 60% (i.e. 40% gets lost to heat, not the 70% mentioned elsewhere), very large (marine) diesels do almost as well at around 55%, automotive diesels (bus engines) do about 40% at best but less at part load. In a diesel bus operating an urban duty cycle and unlike a trolleybus there are significant losses in fluid element automatic transmissions, while idling and moving at low speeds and operating at well below engine maximum efficiency levels, while burning fuel to regenerate particulate traps, etc. The efficiency of a diesel bus at the road wheels is a lot less than engine peak efficiency might suggest.

...Including all the grid, substation, overhead wiring, on-vehicle etc, losses, and regeneration gains for a trolley and all the transmission, cooling, idling and part load running losses, etc, for diesel, a diesel bus is somewhere around 25% efficient at the wheels based on typical MPG and a trolley around 40%.

For the "around 60%" figure for (combined-cycle) gas plants, here is the state-of-the-art in 2010:

CCGT: Breaking the 60 per cent efficiency barrier - Power Engineering International

GE, of course, announced the achievement of that lofty goal several years ago. But that turned out to be a little premature. These days, GE is a lot more conservative - by far the most conservative of the major OEMs - at least with regard to efficiency announcements.

Siemens and Mitsubishi Heavy Industries Limited (MHI), on the other hand, are claiming an imminent victory. According to Carlos Koeneke, technical director at Mitsubishi Power Systems Americas, the company's J-class machine will provide in excess of 61 per cent by 2011. In the meantime, the Japanese company leads the pack with 59.1 per cent verified on an M701G2 gas turbine at the 1500 MW Tokyo Electric Kawasaki power station in Japan.

Siemens claims that it can match this figure. Fischer states the Siemens F class in combined cycle operation scores in the 58 per cent to 58.7 per cent range at ISO conditions 1-on-1 configuration. Recently they reached well over 59 per cent in Irsching 5 which is a combined-cycle gas turbine (CCGT) power plant in a 2-on-1 configuration based on two SGT5-4000F with a special optimized cycle design. In future, the coming H-class will offer above 60 per cent," says Willibald Fischer, programme manager for the Siemens SGT5-8000H. After undergoing heavy testing at the Irsching prototype plant in Germany the machine will be released on the market. The company is so confident of its numbers that it is virtually guaranteeing 60 per cent for current orders.

Note that the figure can be even higher if the plant supplies distance heating, too, and heating use is included in the efficiency number:

Austria's most efficient thermal power plant goes on line - Hand-over ceremony for the Mellach combined-cycle power plant - Siemens Global Website

In the opening ceremony held on June 22, 2012, Siemens officially handed the Mellach combined-cycle power plant (CCPP) over to the Austrian power provider VERBUND Thermal Power GmbH & Co. KG. The heat-and power cogeneration plant had already commenced operation in May 2012. With an electrical rating of 838 MW and an efficiency of 59.2 percent in straight power generation, the new plant surpasses the contractually agreed performance values. Thanks to the district heating output of 400 MWth, more than 80 percent of the energy in the fuel is put to effective use.

For the "about 40%" figure for bus diesel maximum thermal efficiency, here is a state-of-the-art number:

Scania Delivering 85 Ethanol Buses to Stockholm Suburbs; E95 in a Diesel Engine | OnGreen

The third-generation ethanol engine is an adaptation of Scania's 9-liter diesel engine with charge-cooling and exhaust gas recirculation (EGR). The engine delivers 270 hp (201 kW) of power and torque of 1,200 N·m (885 lb-ft), and offers a thermal efficiency of up to 43%, compared to thermal efficiency of up to 44% for diesel, according to Scania.

For transmission losses, here is an overall number which is a national average for the USA:

How much electricity is lost in transmission and distribution in the United States? - FAQ - U.S. Energy Information Administration (EIA)

According to EIA data, national, annual electricity transmission and distribution losses average about 7% of the electricity that is transmitted in the United States.


*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Thu Oct 18th, 2012 at 08:32:29 AM EST
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