Combined Cycles – A Brief History and Evolution of Cycles

Combined power cycles are a common source of energy, since they offer higher energy efficiency while also making use of common technology. The idea of combining two different heat-engine cycles, however, has been around longer than you think. Today’s blog is going to cover the basics of combined cycle power plants, and their history of how they went from experiments to one of the most common sources of energy in the United States, for example. But how did this come to be, and what really is a combined cycle?

An animated exterior of a combined cycle power plant, image courtesy of General Electric
An animated exterior of a combined cycle power plant, image courtesy of General Electric

Basics

At its most basic form, a combined cycle is the synthesis of two independent cycles into one, which allows them to transfer thermal energy into mechanical energy, or work. On land, this is typically seen in power-generation, so the heat of these two cycles makes electricity. At sea, many ships operate using combined power cycles, but instead of just electricity, the mechanical energy is put to work by propelling the ship as well as providing onboard power.

The most popular combination of cycles are the Brayton and Rankine cycles, which have different names depending on the application. In terrestrial power generation, it’s simply known as a combined cycle gas turbine (CCGT) plant. Onboard ships at sea, it may be referred to as a combined gas and steam (COGAS) plant.

Steam and gas aren’t the only cycles that can be combined together to great effect, however. A relatively new cycle that has shown promise in simulations and testing supercritical carbon dioxide combined cycles.

History

The first power plant to utilize a combined cycle configuration was the Korneuburg-A plant in Austria in 1961. This plant made use of 2 Brown Boveri & Cie 25MW gas turbines and a 25MW steam turbine, with an overall efficiency around 32.5%.

In this day and age, that’s hardly a groundbreaking efficiency percentage, as most coal-fired and natural-gas fueled power cycles have efficiencies in the 30% range. However, Korneuburg-A was proof that the concept could work.

From then on, combined cycle power plants became more common, especially as higher efficiencies became more feasible. Fast forward to the modern day, and combined cycle power plants are abundant, as gas turbine technology paired with reliable and well-designed steam turbines generate more power with less fuel, and in turn, less emissions.

Modern Day and Looking Ahead

Combined cycle power plants are now responsible for generating almost one third of the electricity consumed in the United States. Worldwide, combined cycle power plants are among the largest-scale power generation projects being undertaken. General Electric, a power generation magnate, has been designing the steam and gas turbines used in these projects, with some of their products being record-holders.

GE Gas Power has an area of their website where they cover combined cycles, and their latest advances. Thanks to the latest advances in flowpath design, analysis, and optimization as well as combustion advances and fuel diversity, GE’s H class of gas turbines can reach the low 40’s in simple cycle efficiency and around 64% efficiency in a combined cycle.

A look at a combined Brayton and Rankine cycle in our AxCYCLE thermodynamic solver
A look at a combined Brayton and Rankine cycle in SoftInWay’s AxCYCLE thermodynamic solver

While the most popular configuration for a combined cycle plant is with the Brayton cycle as the “topping” cycle, and then the Rankine cycle using the waste heat from this topping cycle to produce steam in the bottoming cycle, there are other emergent cycles.

One such emergent cycle that SoftInWay has extensive experience with is a combined Brayton and supercritical carbon dioxide cycle, or sCO₂ for short. With advantages like even higher efficiencies (depending on the topping cycle) and lower emissions when compared to a “traditional” combined cycle power plant, sCO₂ combined cycles have a lot to offer on the path forward to cleaner energy and lessened reliance on fossil fuels. So how does it work?

On a macro level, sCO₂ takes the place of the Rankine stage of a combined cycle, being the bottoming cycle for the Brayton topping cycle. Once the fuel is combusted and passes through the gas turbine, the exhaust gasses pass through a heat exchanger which then enters a supercritical state, and under high pressure passes through a turbine, which creates electricity when coupled to a generator.

Without getting too detailed, as we have other cases and papers that cover this, the density of sCO₂ as a working fluid means that it requires a much smaller physical footprint as opposed to conventional steam turbines, and lower emissions mean an overall smaller carbon footprint, since this system uses less fuel and less material to generate electricity.

Conclusion – Looking to the Future

This has simply scratched the surface on the ins and outs of combined cycles, as well as their potential applications. As our global society continues to move more towards green and carbon neutral alternatives for energy and propulsion, combined cycles are likely to stick around in one form or another. For more information on sCO₂ and combined cycles, be sure to browse our other blog posts, and to have a look at some of our technical papers on this next-generation topic.

Can’t get enough of combined cycles? Lucky you! We have a webinar later this week on optimizing combined cycle power plant performance during part load operation. Register for free here.

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