Why fast regulating power must now take centre stage

Thomas Hàƒ¤gglund à‚ 
The growing share of grid power being supplied from fluctuating renewable sources is rapidly enhancing the need to stabilise systems with highly-flexible back-up power. In most cases conventional power generation solutions are not capable of providing this fast response availability, at least not in a profitable way, says Thomas Hàƒ¤gglund, vice president, Technology, Power Plants at Wärtsilä.
Thomas Hàƒ¤gglund: “The business of power production is profoundly changing.”

The share of wind and solar power in many grids has increased rapidly during the last few years, especially in Europe and the US. This is sometimes causing severe problems in stabilising the power systems. As a result, it can be foreseen that in the future all electricity generation, except nuclear and power from renewable energy sources, will be run on a completely flexible basis, responding rapidly to the prevailing needs of the grid. Undoubtedly, the business of power production is profoundly changing.

Thus, developing superior flexibility in power generation, combined with highly competitive cost efficiency, has for several years been a major focus area for technology development at Wärtsilä. The key areas of research and development (R&D) are total power solutions for customers and the use of new fuels. The work has resulted in power solutions with market-leading fuel flexibility, making them extremely competitive in terms of both economic and environmental efficiency.

Our gas-fired combustion technology has the flexibility to enable maximum use of fluctuating renewables, while ensuring optimal power production with the highest total efficiency. While other thermal power generating systems typically need to be started an hour or more beforehand to reach full load, advanced power plant solutions can reach full load for power generation in just a few minutes.

We aim to continue to improve and to find the most cost-effective solutions for power generation. Our R&D team, consisting of hundreds of experts in different disciplines, aims to constantly improve the technology, and actively looks into various ways by which to develop our offering as the ‘Smart Power Generation’ concept provider. Furthermore, an important area of research and development is the use of new fuels. This work has resulted in power solutions with market leading fuel flexibility, making them extremely competitive.

Operational flexibility

When rapid up and down ramps of power generation are necessary to stabilise the system load, our gas-fired combustion technology offers one of the highest total efficiencies on the market. The essence of this fast-access efficiency is that the power generation can be started and stopped so rapidly that one can speak in terms of generating power in ‘pulses’. The length of such a pulse containing one cycle of power generation, could typically be from half an hour to a few hours.

Our system reaches full load for power generation in just a few minutes, while other thermal power generating systems typically need to be started an hour or more beforehand to reach full load. Our technology makes it possible to achieve fast ramps up and down that take just one minute, while providing very high total efficiency for the cycle of power generation.

The shorter a single cycle of power generation is, the greater the importance of a short starting time to reach full load and total efficiency during the cycle. For example, when running a 60-minute cycle, a gas power plant can operate well at full load for more than 50 minutes. With a net electrical efficiency of over 45 per cent at full load, the efficiency for the whole cycle thus exceeds 44 per cent.

A short ramp up time, together with high total efficiency, further implies that the exhaust emissions are minimised. Emissions from power generation are generally measured at full load, since they vary and are difficult to measure during the start-up phase. For instance, the nitrogen oxides catalyst takes some time before it is on line and performing optimally. With short ramp ups and ramp downs, you save in fuel costs and reduce emissions

wind power

The need for speed

Coal combustion is not suitable for use as fast regulating power. To get the full effect from combined cycle gas turbines (CCGT), the system has to be started at least one hour before power is needed. As a consequence, when used as back-up, turbines in these power plants are continuously spinning and thus lose a lot in total efficiency.

In addition, conventional plants have to run for many hours at a time to make power generation profitable. In order to generate a positive cash flow, today’s high-efficiency gas fuelled power plants cannot only be run for a couple of hours at the time.

On the spot market electricity is normally traded by the hour, but in fact the fluctuations in demand and price are a lot faster than that – and accelerating. Thus the business of power production has changed altogether and is getting ever harsher.

For a power plant that is not fast enough, and is working as back-up to the growing load from renewable energy like wind and solar, the situation is becoming more and more difficult.

It is not just about levelling out the peaks and valleys in the fluctuating power output from renewables. The real challenge is that constantly, and at any point in time, the decision must be made: Is now the right time to start up or shut down different power units? At the same time, one has to take into account the fact that the power plant is slow in starting, and once having started it, one must be able to sell the electricity over a period of several hours, and not to produce it at a loss.

It is not only about forecasting the daily load pattern any more. It is also about forecasting the production of wind and solar power.

My view is that in the future all power generation, with the exception of nuclear and power from renewable energy, must be run completely flexibly, according to the prevailing grid needs. Such a power generation system has to be extremely fast and flexible.

Predicting problems

As I mentioned previously, the share of wind and solar power in the grid has increased rapidly over the last few years, especially in Europe and the US. International energy policies are changing, and the European Union, for instance, continues to persuade its member countries to increase their share of energy production from renewable energy sources. I foresee severe problems in stabilizing these power systems.

For example, in just a couple of years Germany has installed a large amount of photovoltaic systems feeding solar power into the grid.

Additionally, Germany had previously invested in extensive wind power installations, which now produce some 8 per cent of the country’s electricity. There are indications of increasing instability in the German grid and, as a consequence, a need for fast regulating power generation.

Production of electricity by wind turbines is not stable; it fluctuates all the time with occasional rapid changes. Adding electricity produced with solar panels makes the electricity generated even more unstable – even though the daily and annual rhythm of solar conditions should be easier to forecast than wind conditions. To this should be added the fact that the storage of enough electricity to level out these fluctuations is very difficult, if not impossible, to achieve.

When peaks and lows in energy production from renewable energy sources are local, a grid covering an extensive geographical area can level out the largest fluctuations. However, occasionally not even an all-European grid is large enough to balance the fluctuations. The study of some historical data shows that the fluctuations in the combined wind power output from three leading European wind power countries – Spain, Germany and Denmark – are not necessarily levelled out, but are in fact somewhat reinforced.

Fast and flexible balancing power is needed, not only to enable more renewable energy, but also to enable adequate use of today’s fossil fuel power plants. Without flexibility, the benefits from renewable energy are lost in lower efficiencies and higher emissions from the existing plants.

Full speed in 30 seconds

There are several features that contribute towards achieving this fast start-up, and the technical solutions are continuously being fine-tuned. An example of what can be accomplished is two plants that Wärtsilä has supplied to Estonian transmission operator Elering. The plants are under construction at Kiisa, near Tallinn, and will have a combined output of 250 MW.

They are intended to secure the availability of Estonia’s electricity supply in case of sudden drops and are capable of compensating for a system failure within 10 minutes. The 27 Wärtsilä 20V34DF engines involved will operate individually on average for 200 hours per year.

“In Kiisa we use heat pumps and efficient insulation to keep the temperature inside the engine hall relatively high during stand-by. In that way we can reduce the loss of heat in the engines during stand-by and, consequently, also the need for pre-heating prior to starting,” Hàƒ¤gglund explains.

“The actual starting of the engine takes place by letting in air under pressure from a tank, followed by ignition. Using compressed air makes the procedure fast, and no electricity is needed to start the system. When the engine is warm at start, it reaches full speed in 30 seconds, after which it is individually synchronised and connected to the grid.”

“Putting on a load and ramping up the engine to full load takes about two minutes with a warm engine,” Hàƒ¤gglund says.

Also ramping down is a fast procedure, normally taking about one minute. While compressed air is used to get a rapid start, letting off the pressure from an engine by opening a valve is an efficient way to instantly stop the engine, if needed.

“During the ramp down, preparations are made at the same time for the next cycle of generation with, for instance, residual gas being vented out. Within five minutes the engine is ready to start again and the heat remains in the engine for quite a while. We don’t count the number of starts and cycles of operation, since the engines are not affected by how often they are started,” he concludes.

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