It’s all about efficiency

By Steve Hodgson, Contributing Editor

For the CHP/cogeneration industry, the key messages within the latest report from the Intergovernmental Panel on Climate Change (IPCC) were the explicit endorsement of CHP, and the new acceptability of gas as an alternative to coal in electricity generation, particularly when power is generated in CHP mode.

Previously the IPCC was keen to stress the need to exit the fossil fuel economy completely in favour of renewables, nuclear power and demand reduction. It now suggests that switching from coal-fired electricity generation to lower-carbon gas, particularly in a high efficiency CHP plant, makes sense.

But the cogeneration industry is also keen to position the technology as the perfect balance to intermittent renewables, as larger amounts of wind and solar energy are connected to power grids, particularly in Europe.

As we know, the economics of power-only combined-cycle gas turbine (CCGT) generation are not very healthy in Europe at the moment, due to high gas prices, lower coal prices, the inevitable march of renewables onto the system and, perhaps most fundamental, flattening or even falling electricity demand. Happily, cogeneration plants are in a better position than CCGTs, in that they operate at higher efficiencies and benefit from another major income stream from heat sales.

Siemens is making much of the 600 MWe cogeneration plant it is building at Lausward, in the harbour area of Dusseldorf, Germany. Based on a gas turbine, a steam turbine and a waste heat recovery boiler, the proposed plant will operate at an electrical efficiency of over 61%, says Siemens, beating the previous record of just under 61% achieved by the Irsching CCGT, also in Germany. But the plant will also supply 300 MW of thermal energy to feed the local district heating scheme, so that the total operating efficiency will be a highly respectable 85%.

The key to the scheme’s viability, says Siemens, will be its high utilisation factor. Many gas-fired power plants in Germany only get to run for one or two thousand hours per year, but Siemens expects the Lausward plant to run for 5000 hours a year ” due to the guaranteed purchase of large quantities of heat. The ability of gas turbine-based plant to respond quickly to load changes ” both CCGT and cogeneration plants need to respond to fluctuations in generation from wind and solar plants ” will also help. The Lausward plant will be able to reach full generating capacity within 30 minutes from startup.

But gas turbines are not the only prime mover for medium and large-scale cogeneration schemes, and Finland’s Wärtsilä makes a case for the use of several reciprocating engines. The company has built large power-only plants using multiple engines in many areas of the world, and many more small CHP plants featuring an engine or two; so why not larger, multi-engine cogeneration plants? Wärtsilä suggests that the modular (multiple engines) approach has several advantages, two being lower capital costs and the ability to run only some of the engines when the plant operates at part load.

Clearly, Europe and North America are going to be using gas-fired power generation for many decades to come, partly in the role of balancing more intermittent generation elsewhere on the grid. While market conditions are making the viability of mainstream CCGT plants difficult, cogeneration’s unique efficiency advantages are going to be increasingly important.

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