Power plants with combined cycle gas turbines and supercritical carbon dioxide

13 January 2016

When renewable energies are rendered inert by nature, traditional measures have to take their place and perform as optimally as possible, keeping efficiency in mind. James Lawson talks to Professor Piero Colonna, chair of propulsion and power at Delft University of Technology's aerospace engineering faculty about embracing prompt power plants with combined cycle gas turbines and supercritical carbon dioxide as the future of the sector.

Europe leads the world with its high and still increasing proportion of renewable generation. But when the wind drops and the sun goes in, operators need to find tens of gigawatts quickly. Most employ single cycle gas turbines yet combined cycle gas turbines (CCGTs) would be far more desirable.

A CCGT has high thermal efficiency - up to 60% or more - compared with the 40% from a single cycle gas turbine. It achieves this by recovering waste heat from the gas turbine exhaust to fire a boiler (heat-recovery steam generator - HRSG) that in turn powers a steam turbine generator (STG). Unfortunately, most CCGTs were built as a baseload plant.

"Nobody really anticipated the very fast growth of renewables," says Professor Piero Colonna, chair of propulsion and power at Delft University of Technology's aerospace engineering faculty. "CCGTs were designed to operate steady state all the time. Given the issues with slow start-up and load following, the single cycle peak-shaving gas turbine is still the only option, though inherently inefficient."

A specialist in dynamic modelling of power systems like the Organic Rankine Cycle turbogenerator, Colonna and his research team at Delft have long researched 'prompt power plants', able to respond quickly to rapid changes in demand. Their current projects with utility companies, turbine manufacturers, research institutes and other universities involve modifications and new technologies to improve ramp speeds in existing CCGTs as well as completely fresh, much more flexible designs.

Put the work in
Simply building a CCGT that generates efficiently and within emissions limits at part load is hard enough. Steady OEM innovation means some CCGTs can now operate down to 20% of their maximum rating - perfect as spinning or supplemental reserve. However, gas turbines are much more amenable to this than the STG.

There, very careful tuning is required to cope with increased gas turbine exhaust temperatures and lower mass flows seen at part load without dangerously high variations in steam temperatures. Techniques like steam attemperation plus modifying HRSGs and other components like desuperheater spray valves helps keep steam and so component metal temperatures under control.

Fast-starting and increased cyclic operation add greatly to the challenge. Using higher ramp rates than manufacturers recommend means, rather like rechipping a car's ECU for more power, many operators are pushing their plant's boundaries.

"OEMs are extremely prudent when it comes to programming changes in power output," says Colonna. "Some utility operators take the initiative to modify these conservative control settings in order to operate the power plant more flexibly. That means they assume a higher level risk of course."

The major flexibility hurdle is again the STG, mostly due to its components' sheer thermal mass. "The real CCGT bottleneck is the steam turbine and HRSG," says Colonna. "They have much more inertia and adapt to the load change much more slowly than the gas turbine."

There are many modifications, major and minor, aimed at improving flexibility. Techniques like supplementary firing of the HRSG or pre-heating steam turbines and associated components like the condenser are particularly useful in speeding start up or ramping output.

Similar to part-load running, rapid ramping involves using steam bypass or 'thermal decoupling' to control the amount of high-temperature gas turbine exhaust fed to the HRSG to manage steam production rate and steam temperature. This allows the gas turbine to be ramped without delay and the generator quickly placed on load, well before the time-consuming heat-up and roll-off of the STG is complete. A bypass system also offers a way to pre-heat the STG.

Strategy guide
Operators need a clear and detailed control strategy to make sure this ramp up or down is as rapid as possible while not straining or overheating components. Plant-specific and system-wide automatic optimisation or model-based control (MBC) tools are essential here.

MBC tools simulate the complex interactions within plant components and predict how they will respond. Using constraints like maximum permissible loads of critical thick-walled components or minimum flow rates in steam tubes, using real-time automatic instead of manual control minimises process variations, improves stability and reliability, and reduces thermal cycle stress. With MBC, operators can optimally balance ramp time versus the increased fuel and component lifetime costs.

"You need to study the process with a different eye," says Colonna. "In the past, everyone was looking for maximum efficiency. Now you have to look for maximum flexibility without sacrificing too much efficiency or causing too much wear."
That last one is the most vital metric. Regular, rapid thermal cycling means increased wear and reduced fatigue life, with materials like high chromium steel requiring more frequent, costly assessment and maintenance to detect cracking due to thermal stress.
The steam turbine's high pressure blades, HRSG, heat exchangers and associated piping suffer even more than gas turbine components here. Unfortunately, OEMs still far from fully understand the impact of cycling, affecting MBC accuracy and risking part failure.

"You need to avoid unexpected part failures but one of the struggles for OEMs is to accurately predict the lifespan of the most critical parts," says Colonna. "No one has really studied what happens to components when you cycle plants as frequently as everyone is doing now."

With historical operating data limited, a better understanding of the wear mechanisms involved looks like a fertile area for research. Colonna puts forth the case for simulation and practical experimentation.
"We need models that encompass high-level turbine operation [that are] right down at the molecular level," he says. "You can combine fluid flow simulations with thermal stress analysis and mechanical stress analysis to look at wear in parts like turbine blades."

Very sophisticated experimental material science techniques are now available to look deeper into material behaviour; for example, using a synchrotron to examine alloys at molecular level while they are exposed to loading and thermal stress at 1,300°C.

"This data will help us look even deeper into the very complex mechanisms of fracture initiation and propagation at microscopic level and how this is also affected by manufacturing defects," says Colonna. "Currently, this sort of modelling uses largely empirical models that are affected by very large uncertainties. We need to go more into the physics to better understand what really happens."
But evolving machines originally intended for steady-state operation is always going to be a subpar compromise. The more costly but optimal solution is a CCGT designed to be flexible and efficient from the start.

The drive forward
Colonna thinks this redesign should start by employing supercritical carbon dioxide (sCO2) to drive the CCGT's second turbine. Doing away with steam dramatically lowers the inertia and increases the efficiency of the whole plant.
"The whole cycle is highly pressurised: the minimum pressure is around 80bar and the maximum is 250bar," explains Colonna. "The power density is orders of magnitude larger than a steam power plant so the turbine and heat exchangers are much smaller. You immediately get very good dynamic performance."

Explaining further, he notes that conventional steam turbines condense steam at a very low pressure - around 5mb. "That means the condenser is huge. The boiler, the deaerator, the heat exchangers - a lot of the equipment is very large. With sCO2, there are fewer components and they are much smaller." An sCO2 design might end up sacrificing a little efficiency compared with conventional CCGTs but will be much more flexible. With far less metal and equipment required for the same power output, it will also be cheaper to build.

"It's actually an old concept," Colonna relates. "My PhD supervisor had the original idea in 1967 but his paper was forgotten for a long time until work resumed ten years ago at MIT."

In the past, everyone was looking for maximum efficiency. Now you have to look for maximum flexibility without sacrificing too much efficiency or causing too much wear.

The potential of sCO2 is now making serious waves in many different types of power systems with the US DOE funding R&D at Sandia National Labs among other initiatives. Delft's aerospace engineering faculty members have already worked closely with Sandia, contributing their deep expertise in the study of flows of substances in transcritical and supercritical thermodynamic states.
"Sandia built a small prototype of an sCO2 power system and we simulated their compressor to try to understand the gas dynamics which are quite peculiar and unconventional," says Colonna.

Commercial sCO2 equipment is already on the market. For example, US firm Echogen has been building and selling its EPS1000 supercritical sCO2 power modules since 2014. Generating up to 2MW using waste industrial heat, Echogen's engine employs only a handful of major components: heat exchangers and recuperators, an innovative pump, condenser, turbine, gearbox and generator.Because sCO2 is a non-flammable, non-toxic, non-corrosive, thermally stable fluid, it can interact directly with the waste-heat source, eliminating the need for a secondary thermal loop in the design and so increasing efficiency. As Colonna predicts, sCO2's high density makes Echogen's engines far more compact than steam turbines of equivalent ouput and also much more flexible, able to cope with a wide range of input heat temperatures.

NET Power is now scaling-up sCO2 technology, building a 50MW demonstration plant in Texas due to open in 2016. The plant will employ zero-emission oxy-combustion, using pure oxygen instead of air to burn natural gas and incorporating carbon capture without efficiency penalties. Toshiba will supply the combustor and the sCO2 turbines.

Replacing Rankine Cycle steam equipment with Brayton Cycle sCO2 systems could help lower the cost of electricity globally by approximately 15%. But major challenges remain, including identifying the best materials to handle the elevated temperatures and pressures involved as well as manufacturing the turbo machinery, valves, seals and so on. Many more trials and pilot plants are required.

"The industry is inherently conservative so nobody wants to talk about it very much yet," concludes Colonna. "sCO2 technology will need a great deal of development and that will cost a substantial amount of money. But from a conceptual point of view, it makes a great deal of sense."



An Alstom GT24 gas turbine.

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