Interconnecting different modules offers flexibility of design to meet a variety of project needs. Because generating units are incrementally sized, a wide range of plant capacities and fuel options – including multi-fuel use – can be designed. The control systems and interfaces between modules are designed to accommodate fuel selection, expected operations and emissions permit limits. Combustion engines are ideally suited to modular use, as sets of 4 – 30 MW engine units can provide a range of incremental part load power without sacrificing efficiency. For example, a Wärtsilä power plant that has 20 modular 34SG combustion engine units, each sized at approximately 10 MW, can deliver a range of output from just a few MW to close to 200 MW. By operating only a subset of the engines at full load to produce the desired output, high efficiency is maintained. Further, because the startup time for combustion engines is within minutes, the power plant can quickly adjust load by bringing additional engine sets online to meet changes in electric demand.
Wärtsilä modular power plant comprised of multiple combustion engine generating units provides output versatility.
Modularity in architecture provides limited operational modularity for gas turbines, however. This is particularly true of gas turbine power plants due to the size of units, limited number of units, and efficiency tradeoffs for simple cycle versus combined cycle. Industrial gas turbines for power generation may be 100 – 350 MW apiece and have limits on the lower range of output at which they can operate. This minimal load, or “turndown” percentage, is bounded by emissions limits. When the turbine operates at low load, the compressor airflow may not be enough to support conversion of carbon monoxide (CO) into carbon dioxide (CO2) in the combustion chamber. Although manufacturers offer enhanced design features such as inlet guide vane control, gas turbines are generally constrained to a turndown of 30 to 40 percent of full load to meet emissions regulations. A simple cycle power plant with two gas turbines can adjust plant output down to about 15 to 20 percent of full load by operating only one turbine.
Large gas turbine power plants (1x1 configuration shown) are limited in their turndown capability.
Combined cycle operation introduces more complexity into the operating parameters of the plant. Modular architecture for combined cycle gas turbine (CCGT) power plants consists of one to four gas turbines, HRSGs for each gas turbine, and a common proportionally-sized steam turbine. Modularization is intended to provide turnkey power plant solutions with reduced on-site assembly and higher part load efficiencies because of the steam turbine output. The steam turbine accounts for about one third of the total plant output and is inflexible capacity because of the time required to achieve necessary steam conditions and for the steam turbine to warm up. The lower load limit is affected by the turbine exhaust temperature, which must be high enough to generate sufficient steam pressure in the HRSG to power the steam turbine. The typical configuration of a 2x1 CCGT plant which has two gas turbine/HRSG units supplying one steam turbine may have the ability to operate one of the gas turbines independently of the other, depending on emissions criteria. Emissions compliant turndown for CCGT plants is usually 40 to 50 percent of full load. For example, a combined cycle power plant design based on 200 MW gas turbines (in the typical 2x1 configuration) has a rated output of over 600 MW, limiting turndown ability to about 300 MW.
A Flexicycle power plant based on modular combustion engine units does not have similar restrictions on load turndown because sufficient steam pressure can be developed by operating only 25 percent of the generating units. Modularity using combustion engine units has other operational benefits over gas turbine power plants. The “economies of numbers” provides combustion engine power plants built-in redundancy in case of unit outages or maintenance without significantly affecting overall full plant output.
Modular engine technology also allows siting to complement distributed renewable energy sources in areas that lack transmission infrastructure to support large power stations, providing better matching to changing grid needs. Expanding power needs in the future can be met with the addition of more engine units and ancillary modules, rather than the construction of a new power plant. Combustion engine technology provides the needed versatility and flexibility of load to compensate for variable smaller, distributed scale of renewable sources which may only be a few MW. And importantly, using small modular combustion engines to provide flexible load allows larger combined cycle plants to operate at full output, taking advantage of their high efficiencies at full load and reducing electric system costs.