Fossil power plants with a highly versatile operating response are the key to integrating renewables into the power grid and are an essential prerequisite if the intended rapid growth of these energy resources is to be accommodated.
Operational flexibility of conventional power plants is becoming increasingly important due to ongoing shifts in Europe's power generation market driven by the need to reduce carbon dioxide (CO2) emissions over the long term and the rapidly growing share of renewable energy resources. At the turn of the millennium, only about two per cent of the electric power generated in Europe came from renewables, but this figure has already risen to over 12 per cent and, by the year 2030, is expected to top 30 per cent.
Because power from renewables is today given priority over electricity from other sources, it serves as a kind of variable baseload in the power supply network. Unlike the conventional providers of baseload power however, the feed-in of renewables-based power to the grid depends strongly on the time of day and the weather. The availability of wind and solar power at any given time is not entirely predictable, so as a result renewables do not lend themselves to grid control and stabilisation. This means that the inevitable shortfalls that occur when there is no wind or heavy cloud cover have to be made up by power generated using conventional means.
One of the most important questions is how this integration of renewable energy into the European power generation market is going to develop and how this will influence the design and construction of new power plants - when the primary aim is to build a power plant capable of running profitably over its 20 year lifecycle.[Page Break]
Shoring-up the grid
At this relatively early stage in the expansion of renewables technologies in Europe, it is important to understand how further growth in adoption is going to affect the reliability of the power supply as a whole. A long-term prognosis of anticipated renewables-based power generation within typical power demand scenarios is necessary to identify potential over-supply or shortfalls, which will need to be balanced or backed up by conventional power generation.
Based on studies in Germany, which predict that 40 per cent of the power supply will come from renewables by 2020, the main conclusion is that the majority of conventional power generating capacity will not be required continuously.
The critical challenge for the European power industry is to find the right type of conventional power generation technology that can compensate for the widely fluctuating residual loads from renewable sources. For example, combined-cycle power plants (comprising a gas turbine and a water/steam cycle with steam turbine) are among the power plant designs with the best dynamic features in the field of fossil power generation, offering much greater flexibility than nuclear and other steam power plants and helping to shore-up the grid in times of shortage.
A further challenge will be how to meet the unpredictable demand surges caused by the sudden loss of renewable power. What counts in this context is being able to start-up idle power plants as quickly as possible to bridge the gap in supply. Again, combined-cycle power plants are particularly suited to this.[Page Break]
Versatile operating response
One of the primary motives for increasing the share of renewable power is the desire to minimise the CO2 emissions associated with generating electric power from fossil fuels. The power plants intended to provide the standby power need to be based on a technology that emits as low CO2 emissions as possible. In this context, nuclear power plants are prime candidates as they emit almost no CO2 at all. But since the catastrophe in Fukushima, public opinion in many countries, especially Germany, has turned against this form of power generation.
Aside from the associated risks, power production in nuclear power plants cannot be readily ramped up and down. As a result, they are not suitable as standby power plants for backing up renewables. When comparing other types of fossil power plants, combined-cycle facilities appear to offer the best solution. Not only is the 60 per cent efficiency of combined-cycle power plants far superior to the 47 per cent efficiency achievable by modern steam power plants, but the ratio of carbon to hydrogen in natural gas fuel is much better than in coal. As a result, a modern combined-cycle power plant emits more steam but significantly less CO2 than a steam power plant of the same rating.
Fossil power plants with a highly versatile operating response are the key to integrating renewables into the power grid and an essential prerequisite for the intended rapid growth of these energy resources. An analysis conducted in Germany of the forecast feed-in and consumption for the year 2020 found that combined-cycle power plants will in future be operated across the entire load range and not only, as in the past, limited to just a few operating points (full load, peak load, etc). This makes it essential to design the plants for the widest possible duty range. In particular, the plants should be able to operate at the lowest output possible.[Page Break]
In the context of stabilisation of the grid, the aim is to respond to changes in demand as quickly as possible. Power plants with short start-up times can feed extra power into the grid at short notice. It is already apparent in some power generation markets that load dispatchers are giving preference to power plants with short start-up times. Playing the spot market (tertiary reserve) is particularly attractive for power plant operators as the grid pays high prices for last minute power.
Players on this market have to guarantee that they are able to provide the offered power within 15minutes of it being requested. Shorter start-up times also reduce the amount of fuel consumed during the start-up event, so that shortening the start-up sequence helps improve the start-up efficiency. This results directly in significant savings in fuel and possibly CO2 emissions for the power plant operator.
Start-up reliability of 100 per cent is desirable for plants that operate mainly on a start/stop basis to be sure that they can be put online as soon as there is a demand for power that cannot be met from renewable sources. With this level of reliability, the remaining risk of unsuccessful start-up can be handled by appropriate fleet management.[Page Break]
Combined-cycle power plants are going to play a major role in the future as a load reserve in the power generation market. While high efficiency continues to be important as a means of achieving targets on reducing CO2 emissions, full-load efficiency will not remain the only assessment criterion. Minimising fuel consumption during start-ups and better part-load efficiencies are becoming more and more important too. Fossil fired power plants will be called upon to meet demand peaks and to compensate for output reductions and non-availability of the rapidly growing installed capacity based on renewable energy sources.
Forecasts indicate a mid-term power generation market scenario for Germany in which renewables will account for an average of 30 per cent of all power generated. There will be phases during which renewables can meet the entire power demand, while in other phases back-up power generation by fossil power plants will be necessary. The situation is complicated by the fact that the different phases frequently give way to each other within a short time period and the transitions are not entirely predictable.
Under these conditions, a smaller number of fossil power plants will have to cope with ever steeper load ramps. If a power plant is seen as a system to be optimised as a whole, there are solutions for meeting the future challenges of the power generation market. Modern combined-cycle power plants are particularly suitable for this, and show that the versatility required for the market of the future is already available. An optimised duty cycle with (overnight) shutdowns and rapid start-ups coupled with high start-up reliability and avoidance of excessive service life expenditure of the components is the economically and ecologically superior solution, compared with running plants continuously in parked load mode.
Andreas Pickard is Head of Product Line Marketing with Siemens AG Energy and winner of POWER-GEN Europe 2012 Best Paper Awards. He is based in Munich, Germany. www.energy.siemens.com