Gas-fired power plants

Author: Dr. Klaus Hassmann, Energy Technology Cluster (April 2017)

This article is based on information taken from /1/ and /2/. /1/  refers to a block of presentations on "Gas engine versus gas turbine" at the Euroforum conference "CHP growth market" in Düsseldorf on August 20, 2014; there, a high-ranking representative each from MAN Diesel & Turbo SE based in Augsburg, GE Energy from Jenbach, Austria, and Wärtsilä Deutschland GmbH from Hamburg exchanged their assessment of the engine/turbine technologies for the tightened boundary conditions of the energy transition.

/2/ summarizes the studies of the University of Duisburg-Essen and MAN Diesel &Turbo on the same topic; various power plant combinations were compared. In this context, the large-scale implementation of a highly efficient gas engine power plant by the Kiel municipal utility is an important indication that the gas engine has technical and economic advantages over the gas turbine even as a GUD in energy transition applications /3/.   

Features of a modular 200 MW power plant /1/

Plants with ten 20 MW gas engines or four 50 MW gas turbines were compared. For a load requirement of 160 MW, eight gas engines would run at full load, 2 would be at standstill; for the turbine version, 4 engines would run at full load and one at part load. Due to the efficiency losses of the part-load gas turbine, the engine version has an advantage under this assumption.

Construction times: According to the manufacturers, a 200 MW gas engine power plant can be built in one year. This time specification is based on the experience in the context of the  approval of small plants; since experience for large power plants is still missing, it is an expected value of the manufacturers. With total investment costs of 650 €/kW, the engine version is more expensive than the turbine version, but cheaper than the GUD version.

Representatives of the gas engine manufacturers rightly point out that modular power plants supplemented with heat storage on the electricity exchange enable price-optimized operation. Author's note: Theoretically, this also applies to the gas turbine version despite the disadvantage of longer start-up times and the shortcoming of lower partial load efficiency. Some specific construction projects are being implemented to bring uneconomic GUD power plants back into or closer to the money.

Operational characteristics gas engine versus gas turbine power plant /1/

Quick start-up from warm condition to reaching rated power (approximate values): gas engines 2 to 5 minutes. Gas turbines (solo operation): 10 minutes. Gas and steam turbine (GUD) power plants: 30 minutes.

Electric efficiency in solo operation: MAN: 47% (10 MW gas engine)/ 34% (7 MW gas turbine); GE 48.7% (9.5 MW gas engine)/ 45% (100 MW gas turbine); Wärtsilä 44 - 45% gas engine; downstream connection of a steam turbine increases efficiency to 50%. GUD power plants in the above power range are at 55%.

Efficiency losses at part load: no concrete figures, but the engine performs significantly better compared to the gas turbine.

Influence of frequent starts and load changes on maintenance frequency/on efficiency: gas engine: influence small, efficiency remains constant. Gas turbines: shorter service intervals; consequence: higher maintenance costs - no details are given in relation to efficiency.

Operating conditions: Gas engine: full power up to 35 °C and 1,000 m geographic altitude. Gas turbine: performance decreases with increasing temperature/altitude.

Operating gas pressure: gas engine: 6 bar. Gas turbine: 40 bar (additional compressor necessary).

Secondary fuel: a plus for the turbine, since oil can also be used, which is not possible with the engine.

Study of the University of Duisburg-Essen/MAN Diesel & Turbo

The 3 variants GuD, engines BHKW and the compound power plant engine with steam cycle and steam turbine were examined. All 3 power plant types are designed for the production of an annual heat quantity of about 590 GWh and feed into the district heating network of a small city. The highest fuel utilization rate of 86 % is achieved by the engine CHP, and the highest electricity ratio by the combined cycle power plant. Economically, the combined cycle power plant is ahead. This result is a confirmation of the statements at the Euroforum conference - no wonder, at both sources of information experts of MAN Diesel&Turbo went to work. 

Note of the author: It could be defined also load requirements, with which the turbine power station is in the advantage. Of course, the following is true: 10 machines with a small output can be better adapted to a specific demand than 4 more powerful turbine modules for the same nominal power plant output. It is also possible to build a 200 MW power plant with turbines of smaller capacity. The decisive factor under the difficult framework conditions of the energy transition for fossil-fired power plants is the application, the resulting load profile over time, the environmental compatibility and the fixed and variable costs - this results in advantages and disadvantages that the interested user must evaluate and make the purchase decision.

Whether the new power plant construction of Stadtwerke Kiel (SWK) with engines confirms the high expectations?

SWK is building a 200 MW gas engine power plant; the municipal utility has been testing different variants since 2007 to succeed a coal-fired power plant to be shut down in 2018. One of the arguments in favor of the 20 gas engines combined in 4 units was, in addition to the short start-up time to full load, above all the individual controllability of each engine in order to be able to react to changing demand at any time. The primary energy utilization is 90% (45% thermal and 45% electrical). The power plant will be equipped with a 60 m high heat storage tank. Also, when there is too much electricity in the grid, e.g. due to wind, additional hot water can be generated in an electric boiler for heat supply if required. The general contractor is Kraftanlagen München; the engines will be supplied by GE's Jenbacher Motorenwerk. The total investment volume is €280 million. The power plant is scheduled to supply Kiel with electricity and district heating in fall 2018. The innovation content and international attention of the project is high. Award ceremonies confirm this.

We will follow up on the question posed in the headline. 

Literature

/1/ Jan Mühlstein "Speed competition" Journal Energy & Management (e&m) issue of 3. November 2014 /2/ Armin Müller "Engines for district heating" e&m issue dated July 15, 2016 /3/ Press release of Stadtwerke Kiel dated Nov 11, 2016. Supervisory board gives green light: new power plant construction decided.