By Ralf Grosshauser
Gas engines show advantages in their single cycle efficiency value (figure 2) and a very fast startup performance. Photo courtesy: MAN Diesel & Turbo
The transforming energy market shifts focus to reducing power plant environmental impacts, where financial and technical benefits improve competitiveness. This leads to an increased share of renewable power generation and also a focus on highly efficient, flexible and cleaner conventional power plants. Consumer perception and recent regulations have led to some coal and oil fired power plants to be shut down, depending on changing weather conditions, are not consistent and require very fast power generation capacity response to ensure a stable grid.
Power plant operators and investors looking to operate on natural gas have the choice between gas turbines and pure gas or pilot oil fueled engines, the latter technologies enjoying a recent and significant development. Engine power outputs now exceed 20 MW and benefit from increasing efficiency. Combined cycle engine based power plants emerge in the market place. Exceeding 200 MW becomes more common.
This article presents specific decision criteria that highlight key differences between applications and site performance of both technologies in gas-fired power plants.
Some obvious criteria that follow allude to the paper’s content: Single cycle efficiency, an expedient fats startup performance (within 3 minutes), and reduces load operation (below 25 percent) benefit the support of fluctuating renewable power generation. Low gas pressure requirements benefit distributed power projects. A project’s heat energy and electricity balance will favor a specific technology and site specific conditions and will also influence the decision process.
Power plant projects below 400MW require modernized decision criteria when it comes to selecting engines and/or turbines. This article offers guidance in a more objective choice between both technologies.
Technical Parameters Comparison
In any power plant technology comparison the list of parameters needing appropriate consideration includes at a minimum:
- Power plant load profile/start up time
- Start-up time
- Plant life cycle cost
- Project site ambient air temperature
- Plant altitude
- Agine/maintenance over operating time
- Power to heat ratio
- Dual Fuel requirements/capabilities
- Overall plant footprint
Most of these parameters impose severe impact when considering technical concepts or commercial feasibility and therefore are discussed in more detail.
Startup Time Comparison – 1
Electrical Efficiency Comparison at MCR for Single Units – 2
Power Plant Load Profile/Start-Up Time
The traditional load scenarios are:
- Base Load – with dominant constant load phases and basically continuous operation
- Intermediate – with more fluctuating load phases needed across a significant amount of operating hours
- Peak Load – with quick need of extra power at fast ramp-up rates
With an increased amount of fluctuating renewable power generation being fed to the grids sometimes during the day the demand may possibly be generated from renewable sources. However, depending on weather conditions on other days or at other times during the day, generation from such sources often remains insufficient. Sub sequentially required back-up, or reserve-means of power generation, therefore are a factual necessity. Many existing thermal plants, however, were designed for more or less continuous high loads. Presuming renewable power generation being prioritized when feeding the grid, the existing thermal plants can no longer do what they were intended to, but have to consider a stand-by position with decreasing annual operating hours at highly fluctuating load requirements. As a consequence, intermediate and peak load scenarios with the need of frequent fast equipment start for limited operating times of few hours only become a common requirement.
Gas engines show advantages in their single cycle efficiency value (figure 2) and a very fast startup performance. Multiple equipment starts per day are possible and reduced load operation at 25 percent or even lower are common features of modern engines. One hundred percent of output can be achieved under five minutes, starting from warm standby condition, compared to 30 minutes for a turbine under the same conditions. Such technological features are tentatively better suited to match the modern industry and energy market demands as described above.
Figure 1 shows a typical comparison of a gas engine plant start-up versus gas turbine combined cycle, both from warm conditions i.e. prior shut down of more than eight hours.
Gas Turbines, however, demonstrate superior performance under a relatively continuous stable load regime.
Tentative Plant Life Cycle Cost
While life cycle costs of any thermal power plant are vastly dependent on fuel cost, appropriate reflection of the expected load profile need to be incorporated into any comparison of various technological concepts. The number of full load hours and especially the increasing amount of part load hours need to be forecasted as precise as possible, however strictly individually. Conversion to full load equivalent hours tentatively includes the risk of ignoring the efficiency losses factually occurring under part load operation. Whenever limited overall operating hours and part load phases or even multiple starts and stops dominate the load profile, a GT and/or combined cycle option may disqualify. Gas engine maintenance costs often turn out to be lower than those for turbines, depending on actual project parameters.
Project Site Ambient Air Temperature
For gas turbines, maximum power is often defined by maximum component temperature in the turbine, permissible forces to the shaft, or the generator frame size. For gas engines, maximum cooling water temperature often is the limiting factor. The gas engine output is hardly affected by increases in ambient air temperature and stays at 100 percent up to around 38OC. When running a gas turbine, however, power output continuously decreases.
Plant Altitude over Sea Level
Figure 3 compares the plant altitude effects on the performance of gas engines versus gas turbines. Again, the diagram duly takes into account the different “regular” ISO conditions for gas engines as shown in the diagrams legend. The equipment behavior differs dramatically. While engines offer full load output at any altitude up to 1,000 meter above sea level, the industrial gas turbine’s output decreases by 10 percent.
Effect of Altitude Comparison – 3
Aging/Maintenance over Operating Time
The aging behavior of the different technologies can be seen by examining the “heat rate” evolution as a continuously increasing factor in between maintenance periods as compared in Figure 4. Furthermore, and “peaking” demand vs. a regular baseload operation has additional effects on gas turbines because every gas turbine start accounts for some extra operating hours being added to the counter. Operating hours counting of gas engines is not affected by multiple starts. Subsequently peaking operation with gas turbines will exaggerate gas turbine maintenance costs with overhaul activities appearing earlier.
Comparing both technologies under the same plant load, in single or combined cycle, helps to understand the superior efficiency of the gas engines over operating time.
If we add the particular consideration of part load efficiencies for a single machine, we can clearly see the efficiency difference between the competing technologies where the gas engines are significantly less affected by reduced load demands.
Power Plant Footprint & Civil Works
Gas Engines are now available in up to 20.2MWe where a power plant of 100MW requires an area of around 60mx60m. A gas turbine power plant can achieve ~100MW output by installing 2x50MW units, which will install with a more compact foot print at subsequently reduced civil works cost.
Aging Effect Comparison – 4
In general, with gas turbines, the total installed masses are smaller. This is an advantage for transportation into remote areas and installation. A gas turbine power plant requires fewer auxiliary systems, as well as no, or fewer, additional exhaust devices. Pure machine weight-related issues should be considered as well where gas turbines benefit from much lower equipment weight than gas engines.
Many technical and commercial parameters need due consideration when selecting the proper gas power plant technology in accordane with the actual project parameters. Such parameters and other required data will be presented and further discussed as part of the convention’s presentation.
In general, the reciprocating four-stroke gas engines show advantages in single cycle efficiency, high efficient part load operation and a very fast startup performance. Reduced load operation at 25 percent or lower is also possible if needed. This makes gas engines ideally suited to compensate for the fluctuating renewable power generation.
Low gas admission pressure requirements for engines (6 bars comparing to around 21 – 40 bar for turbines) reduces infrastructure costs and risks and allows placing of such generators close to the consumers. Therefore engine based power generation also supports the decentralized power generation concepts, as well as reducing CAPEX and OPEX by eliminating the need of fuel gas compression.
In case thermal energy can be utilized, an overall plant efficiency beyond 90% can be achieved.
The engine technology is furthermore less sensitive to hot ambient temperatures and altitude in comparison to gas turbines. Base load gas turbine combined cycle power plants of >400 MW can provide full load efficiency of >60 percent. When running many thousand full load hours annually, such big plants clearly outperform any gas engine configuration as a function of reduced fuel spending. Gas turbine plants typically also benefit from a smaller footprint compared to engine based power plants.
Finally, gas turbine combined cycle plants may also take advantage from any location being incorporated in industrial areas by selling steam to neighbor industries. However, the same logic applies to a potential gas engine plant in CHP configuration. Thermal energy provision to neighbor industries or any district heating provider with heat being provided by means of hot water efficiently generated from advanced heat recovery systems can create extra profitability.
In the power range up to ~200-300 MW we see an interesting field in which both technologies can be fairly considered.
At the time this article was written, Ralf Grosshauser was the senior vice president of MAN Diesel & Turbo SE. Grosshauser now serves as CEO of Thermamax GmbH