High-wire act - making conventional power plants more flexible

1 January 2015

With renewable sources playing an increasingly prominent position in the energy mix, conventional power plants need to be more flexible than ever. Modern Power Systems – BRICS Edition speaks to Anna Stoppato, industrial engineer at the University of Padova, about the operational challenges ahead.

The liberalisation of the electricity markets in the 1900s changed everything for modern power plants. The old baseload stations, designed for a time when production was continuous and predictable, suddenly looked out of place in a market that needed fast load ramps and quicker start-ups. The demand never went away. The increasing importance of renewables, and the accompanying emphasis on discontinuous, irregular energy production, has made flexibility of paramount importance.

At the turn of the century, just 2% of electricity production stemmed from renewable sources. Today, the vast majority of new installations are for renewables - usually solar or wind. By 2030, their share in the energy mix is expected to rise to 30% in Europe.

Weather forecast
This is good news for the environment, but it poses challenges for plant operators whose stations are expected to perform a purely supplementary service. While some sources, such as hydropower, don't cause the electricity grid any real issues, others, such as wind and solar, which vary according to the time of day and weather, are notoriously unpredictable and that's the crux of the problem.

"The more renewable energy methods are used, the greater the load fluctuations will be on the grid and the conventional power plants that back it up."

The more these kinds of energy production methods are used, the greater the load fluctuations will be on the grid and the conventional power plants that back it up.

Take wind. If a breeze drops from 10m/s to 7m/s, a turbine's output can decrease by up to 60%. This is especially problematic for older coal plants that were designed to operate at a consistent baseload. During periods of strong wind and clear skies, the plants have to shut down completely, but when the air is still and the sun goes down, the response needs to be rapid.

Feeling the strain
This process puts a serious strain on the equipment of conventional power plants, causing creep, corrosion, thermo-mechanical fatigue, slowly eating away at the lifespan of a station's critical components.

Of course, the magnitude of the problem will to a large extent depend on the way each individual plant is designed. Plants built in the 1990s are generally less tolerant of cycling than modern combined facilities.

For good reason: it wasn't until the liberalisation of the energy markets and the accompanying swings in gas prices that plants needed to adopt more flexible approaches with quicker start-up times.

Deregulation meant competition could flourish, but with wholesale gas prices no longer decided according to the cost of production, older, less flexible power plants became dependent on supply and demand.

Load variation is still a cause of stress for plants that can vary their output; the speed and severity of changes in temperature caused by the sudden stops and starts causes severe fatigue to metal components, and to the welds and pipes holding them together.

Knowledge is power
Anna Stoppato, an assistant professor and engineer at the University of Padova's department of industrial engineering, has been researching and exploring these challenges.

"Traditional power plants will have to change their load frequently and much more quickly than before," she says. "The components that have the most problems are those that work at higher temperatures, like the boilers or the heat recovery steam generators, and those that work at high velocities. The other at-risk parts are those with greater thickness, especially the steam drums, which have weak points that are affected by thermo-mechanical fatigue, which becomes severe when heating gradients are present."

Maintenance is also a problem in this new operational environment. Not only are replacement parts and repairs required more frequently, but it also becomes more difficult for plant managers to schedule times in which upkeep can take place.

"This is related to the unpredictability of the renewable sources," Stoppato says. "At certain times, the price of electricity can be extremely high, which means it's really important that the plant is able to operate at short notice. It's very difficult to know in advance which hours that will be, however, and it is therefore hard to schedule with proper warning.

"Two or three years ago, we knew when it would be convenient to service plants: now we can't be sure when it's appropriate to stop the station functioning, and this has had a major impact on the life cycle of components."

Stop/start process
So what can be done? Much of Stoppato's work at Padova involves exploring the stop/start period, in order to produce a dynamic model that can study how the different components of the plants are likely to behave during transient operating conditions.

"The idea is to help plant managers recognise their most critical operations and components," she says. "Many studies are being done that explore the procedures of start-up and shutdown and how the stations can maintain readiness.
"The goal is to study simulation models that can predict how power plants and the components therein will work during these periods. This can play a big role in improving the wider cycle of the plant."

"What is important is that plant managers have the right tools with which to evaluate the consequences of their operation strategies."

As well as simulating and predicting the residual life of plants' different components, various devices and changes can be implemented to optimise overall operation.
"Constructors are working on designs that can do all these things," Stoppato says.
"Components are made to be as light as possible to reduce the risk of cycling. The use of particular shapes on components, like tubes and heat exchanges, can also help reduce the effects of thermo-mechanical fatigue.

Different, less-expensive materials can be developed, and when plants are built we can avoid the presence of the welds and control how they are fitted so that problems don't arise".

New designs must also bear in mind the emissions caused by cycling plants. It's usually assumed that any shift to wind or solar power will cause an overall reduction in CO2 levels, but some sources suggest that emission levels can be even greater coal and natural gas plants are forced to fill the gaps with start/stop cycles.

"Plants perform best when they work at their design point," says Stoppato. "At very low loads, carbon monoxide and nitrogen oxide emissions can be too high. Of course, it's possible to work on this aspect, but it can be very expensive. Some plants have new systems for pollution removal that work only under certain loads (about 50% of the design load), but studies will be done on this area for as long as it remains a problem."

Opportunities, not problems
As Stoppato indicates, various options exist. Air pre-heaters can improve the efficiency of boilers and bring down aggregate fuel consumption. For more modern plants, variable-pitch guide vanes can be installed on gas turbine compressors to optimise minimum and part-load efficiency.

In a way, it represents an opportunity. Making modern plants more ecologically friendly is something that has always concerned the industry, but there are significant costs to this process. Add in the price of new components and maintenance, and the commercial benefits can look increasingly uncertain.

"Short-term income obviously increases if the plant is able to produce only when electricity has high prices," Stoppato says, "but we have to look to the long-term expenses that arise because of extra maintenance and the substitution of parts.
"We can't really give an answer to this before a very deep analysis on plants, their performance and the life of the components and the market has been done. What is important, though, is that plant managers have the right tools with which to evaluate the consequences of their operation strategies."

The commercial costs may be undecided, but these are interesting times for modern power plants. As energy sources like wind and solar continue to grow, those plants that want to succeed will have to change with them.

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