The European Turbine Network (ETN) has put out a position paper on "Enabling the Increasing Share of Renewable Energy in the Grid." Below are excerpts from the paper:
The European Union (EU) has a goal of achieving 20% power generation from renewables by 2020 and 50% by 2050. The increasing share of solar and wind in electricity production is challenging the reliability of the electric grid and security of supply. In the production of electricity there is limited buffer or storage6, so production has to follow demand instantaneously and be in full balance. However, wind, and to a lesser extent solar power, which will make up a bigger share of primary energy in the future, have an intermittent and to some extent unpredictable nature.
To avoid a gap between demand and production, renewables require a fast-reacting back-up system, to balance their intermittent nature and to prevent an unstable grid. If a grid is unstable it can lead to a black-out in a large area due to a domino effect that can occur when 1 or 2 failing power units decrease the grid frequency to such a level that even more power units stop their production. Grid companies try to avoid this by cutting the electricity demand of larger consumers or areas from the grid, but if this happens more often it can lead to negative economic, social and political effects.
Smart grids could act as a buffer in the future by allowing bi-directional transmission, and can isolate regional black-outs to prevent larger black-outs. However, it is highly unlikely that smart grids alone will be able to accommodate the rapid increase in renewable energy. Moreover, the European Commission wrote in a recent communication that "the EU is still in the early stages of the actual deployment of Smart Grids"
It is generally agreed that in cases where high capacity hydro reserve power is unavailable, gas turbine power plants running on natural gas provide the best means to balance power production8. This is because these plants are fast reacting and have the ability to be turned on and off within minutes. Nuclear and coal power generation can follow electric demand to some extent, however are not flexible enough to accommodate large and fast fluctuations.
Wind energy production is only partly predictable and very intermittent. Periods of stable wind will be followed by periods of no wind or intermittent wind and as a result the wind turbine’s full load capacity is seldom achieved. The yearly production is dependent on its location, but usually reaches only around 25% of the theoretical maximum. During very cold and hot days there is often little wind, resulting in no wind electricity production. In addition, the wind flow can be very unstable, so there are large fluctuations in short periods of time, resulting in similar fluctuations in the production of electricity.
As long as the amount of wind electricity production is less than approximately 25% of the total electricity production at that time, the regular non-fluctuating power units will balance the wind fluctuations by setting a production level with a higher spinning reserve9. However, at very low load demand there might be a problem in balancing the wind power, for example on a Sunday morning 6.00 AM when power demand is low (30%) and wind power production is high and unstable at the same time.
When the share of wind power production comes to 50% or even 75% of the total electricity production, as is foreseen in the National Renewable Action Plans of the EU Member States future scenarios10, this will have a serious impact, because the thermal plants will have to respond to those high load differences. The 50% or 75% is related to the maximum daytime load; during the night and at the weekend it can be even more extreme. The question is: will power stations be flexible enough to allow for this? At what cost? And crucially: who is prepared to invest in new power plants with such large uncertainties?
Today, in Spain, examples of this future scenario are already being witnessed. Power units sometimes run at full-speed-no-load to act as a back-up for wind variations. CO2 is emitted, although there is no electricity production. Consequently reduced emission levels will be more difficult to achieve.
In the UK a modelling study was recently performed on the effect of incorporating large shares of wind energy in the grid. This Pöyry model study showed how the current grid power capacity (natural gas, nuclear and coal) would have to react if wind energy variations had to be balanced. This was done for two scenarios: a relatively minor wind generating capacity of 3.5 GW for 2010 and a major wind generating capacity of 45 GW for 2030.
The results of the study show that in the 2030 scenario high-efficiency gas fired power plants will operate only in cyclic mode and very intermittently to incorporate the wind variation.
Even nuclear plants and conventional coal plants will sometimes have to cut production if wind production is very high. This would require an enormous change to current operating modes.
Additionally, incorporating large amounts of wind energy has an enormous effect on other factors, for example on Carbon Capture and Storage (CCS). Because CCS is nowadays a chemical process which - with the current technology - requires uninterrupted operation, CCS can only be applied on those plants which run continuously. However, the Pöyry study indicates that gas powered plants, and even conventional coal plants, will operate in cyclic mode in the future (incorporating the wind variation), hence CCS will not be easily applicable.
The Pöyry study for wind generation shows that the incorporation of large amounts of wind energy in the future will lead to uncertainty and risks, an investment conundrum and extremes in prices, both positive and negative. To add to that, the risk of power interruptions or even a black-out in a large area will increase significantly.
Expected impact on power plant operation
The scenario, as outlined above, will change the mode of electricity production of Combined Cycle Gas Turbine Power Plants (CCGT) dramatically in the coming years (and in some countries, such as Spain, already today). From a base load of 12 starts per year and 8000 hours of operation, operation will be cyclic, with as many as 250 (or more) starts with only 3000 hours (or less) of operation per year at part load. Furthermore, as a result of the multiple starts and lower efficiency, the power station will face higher emissions of CO2 and NOx per produced kWh.
Power stations will also face an increased risk of failure due to low-cycle fatigue problems12, caused by the components and materials being subjected to repeated loading and unloading. These damages will lead to lower availability and reliability. In addition, maintenance of gas-fired power plants will be more costly and more uncertain, due to the fact that the fatigue limits of gas turbine parts are less well known and flexible than the limits for creep behaviour13.
Not only wind power is causing the problems described above. Other large scale application of intermittent electricity production (such as solar sources) causes the same type of problems, with a similar requirement for fast reacting back-up systems of gas turbine driven power plants.
Policy framework to incentivise further and increased research investments
Power plant technologies will have to change radically in the coming decades to become more flexible and to encompass faster start-up and shut-down. Today’s design technology is not optimized for this, except at very high costs per produced product (kWh) and lower lifetimes.
ETN therefore calls for a policy framework to incentivise further research and development of environmentally sound and reliable power stations, which allow highly flexible cyclic operation, to be made available within the next ten years. It is crucial to address this today for solutions to be in place that meet the challenges affecting the grids when the share of renewables is increased.
To develop skills and knowledge of engineers is also crucial and must be dealt with today. This is key to face the above future technological challenges and in order for Europe to keep its position as a leading innovation society.
The technological challenges, investment needs and research opportunities identified by ETN are summarized on the next page.