Target: 60% thermal efficiency

By Robert Smock

Editorial Director

Siemens said that 60 percent thermal efficiency is the economic limit for combined-cycle generating plants, but it`s within reach of current technology. At a recent press briefing in Berlin to introduce the firm`s new 3A gas turbine design, Siemens executives said they had targeted 60 percent thermal efficiency and hoped to achieve it soon.

New conventional coal-fired power plants are achieving efficiencies of approximately 45 percent. The best combined-cycle efficiency being achieved right now is near 55 percent using natural gas as a fuel. The next step for Siemens will be a net efficiency of approximately 57 percent (58 percent gross) in combined-cycle plants built around its new, 3A gas turbine design. “The objective of a net … efficiency of 60 percent is no longer purely speculation,” said Hans Bohm, executive management team member, “it can be proven with today`s technology.”

Booming world market

High efficiency is expected to be one of the main design goals of the rapidly growing world market. Siemens expects that electric power consumption will grow by 60 percent between now and the year 2010.

That will drive rapid increases in the rate of orders for new power plants, as shown in Figure 1. Orders for fossil-fired plants averaged 53 GW a year during the past five years and are expected to rise to a rate of 64 GW a year in the period 1995-1999. A leap in the order rate to 75 GW a year is predicted for 2000-2004.

The rising importance of efficiency is shown by the rising share for combined-cycle plants in the figure. Figure 1 shows the gas turbine portion in the top part of each bar and the steam turbine portion in the other two segments. Steam turbines that are part of conventional Rankine cycle plants are shown in the bottom segment of each bar.

Steam turbines that are part of combined cycles are in the middle segment. The combined-cycle share, which is the top and middle segments combined, climbed from 12 percent of the fossil total in the 1985-1989 period to a predicted 37 percent after the turn of the century.

Drive for higher efficiency

Siemens sees two main drivers for the rising importance of thermal efficiency: the desire for lower emissions/KWhr (particularly CO2) and the need for lower costs. At an efficiency of 55 percent a natural gas-fired combined-cycle plant has a CO2 emission rate of 0.37 kilogram per KWhr. A hard coal plant with an efficiency of 44 percent has an emission rate of 0.8 kg/KWhr.

A lignite plant at 41 percent has a rate of 1.01 kg/KWhr. The combined cycle benefits from the efficiency advantage and the relatively low carbon content of the gas fuel.

At a fuel price of (US)$3.75 per gigajoule, a one percentage point gain in efficiency for a 350-MW combined-cycle plant is worth approximately $6.3 million in capital cost, equal to approximately 4.5 percent of the total capital cost. According to Siemens, that means an increase of 4 percent in initial cost spent on improving efficiency could be justified for every one percentage point gain. A larger increase in capital cost can be justified where fuel prices are higher. Siemens said that over a period of 20 years the value of the efficiency gain resulting from its new design (57 percent “reference” efficiency) compared to its previous design is $81 million in reduced fuel cost for a plant with three 350-MW combined-cycle blocks. That`s equal to almost 20 percent of the initial purchase price.

Achieving higher efficiency

The route to even higher efficiency was described by Albert Kruetzer, Siemens` head of fossil plant marketing. The main factors are:

– increased gas turbine inlet temperature

– advanced gas turbine cooling techniques

– reduced auxiliary power consumption

– hydrogen cooled generators

– multiple-pressure steam cycles with reheat

– better heat recovery steam generator design

– fuel preheating

“By intelligently combining these measures in modern combined-cycle plants, efficiency levels are achieved which today can otherwise only be attained using complex, high risk schemes …,” said Kruetzer.

The 30 C boost in gas turbine inlet temperature for the 3A machine to 1,190 C and some other gas turbine design changes yielded a two percentage point gain in efficiency.

The Siemens route to high efficiency is shown in Figure 2. The firm`s engineers feel that 60 percent is probably the economic limit, meaning that the economic value of gains above that level are not high enough to cover the cost of getting there. Current dual-pressure steam cycle designs are achieving 55 percent efficiency. The next step, with triple-pressure reheat steam cycles and the other steps outlined above will achieve close to 60 percent by about the year 2000.

“In addition to further gas turbine development, it appears that just the introduction of relatively straightforward system improvements, such as the use of supercritical steam cycles, will make it possible to achieve a combined-cycle efficiency of 60 percent,” said Kruetzer.

Compared to a “base” combined-cycle design, which has a dual pressure subcritical steam cycle (80 bar at 540 C), a triple pressure subcritical cycle will have an overall efficiency gain of 0.6 percent and a triple pressure subcritical cycle with reheat will have a 1.2 percent gain. A supercritical cycle (250 bar at 540 C), compared to the base design, will have a 1.9 percent gain with dual pressure, 2.3 percent gain with single pressure and reheat, and a 2.4 percent improvement with triple pressure at 570 C, according to Siemens.

Kruetzer said that combined-cycle power stations to be quoted over the next 10 years will typically have a single shaft, single-generator design to keep down costs and will have triple pressure steam cycles with reheat to achieve high pressure.

That was the design concept for the largest order to date for a combined cycle using the new gas turbine technology. That order is for a 1,000 MW combined-cycle plant using three single-shaft blocks based on the 240-MW V94.3A gas turbine. This is the Tapado do Outeiro plant in Portugal scheduled to begin operating in 1998. It will have a triple pressure steam cycle with reheat. The high pressure turbine will operate at 98 bar and 536 C. The “introductory” efficiency level guaranteed for this plant is 55.4 percent and Siemens said that the potential is for 56 percent efficiency.

Another of the first orders for the new technology features a quest for high efficiency. It is a combined-cycle topping plant for a district heating plant for the German utility Neckarwerke. Siemens said the Altbach Deizisau district heating plant will be the world`s first “parallel powered” combined-cycle design.

This will be a repowering project featuring a combined-cycle topping cycle added to an existing coal-fired steam plant. For Unit 2, exhaust gas from the new natural gas-fired V64.3A gas turbine (65 MW) will produce steam in the new heat recovery steam generator. That steam will be used as supplementary, intermediate pressure steam for the steam turbine and will preheat part of the condensate. The overall result will be a gain in efficiency of three to four percentage points at both full and partial load.

Another source of pressure for high efficiency and resulting lower cost is the worldwide trend toward privatization and independent power. Siemens projects that orders for new generating capacity will come from a variety of customer types:

– 23 percent from independent power producers

– 35 percent from utilities in developing and newly industrialized countries

– 23 percent from privatized utilities, and

– 10 percent from utilities in industrialized countries.

In fact, Hans Bohm said Siemens planned to increase its investment role in power projects where it is an equipment supplier. The goal in the future will be to become the majority owner to reduce conflicts of interest with other partners.