It is variable and hard to predict, but wind energy is increasingly being employed in European countries in order to hit their renewable energy targets. The EU is currently aiming for 22% renewable energy production by 2010. Portugal has the far more ambitious target of 39%. Therefore, Portugal is planning a large increase in its wind energy capacity over the next couple of years, in order to achieve an installed capacity of 5300 MW by 2012.
Assuming a scenario where large-scale penetration of electricity generation from wind and other intermittent renewable energy resources is achieved, it is of fundamental importance that the electricity system into which these new power generators are being integrated is able to compensate for the variability of production.
Hydropower and solar power can be used to boost production capacity – Portuguese hydropower capacity is large and significant photovoltaic development is planned – but energy storage, demand side management (DSM) and demand side-response (DR) could also play a major role in optimizing capacity for wind power production. As wind energy is stochastic in nature and essentially ruled by random meteorological changes, its ability to reach peak load requirements is the biggest problem for producers of wind energy. Therefore, wind energy should be considered an energy resource but not a peak capacity resource, as only a small fraction of wind capacity has a high probability of running consistently. Wind is, and should be, used when available and if capacity exceeds demand then this should be viewed as a bonus.
Wind intermittence can also affect the economic viability of projects, as it leads to lower value energy. Most wind generation occurs in hours when energy use is low, making it less valuable. Studies have shown that as wind penetration increases, three factors lower the economic value of wind power:
- increasing wind generation usually displaces capacity from increasingly lower cost plants
- operational losses due to repeated plant starts or partial plant loading
- unnecessary wind energy, which cannot be absorbed, due to operational constraints or excess production
Intermittency is a long standing and recognized problem for many forms of renewable energy. Efforts to overcome the effects of variability and randomness of wind power availability has been traditionally addressed by the promotion of wind power resource studies in the industry, and the identification of solutions based on reversible hydroelectric dams, or pumped storage.
The intermittency of wind energy can also be reduced by some other estalished techniques:
- improved forecasting techniques
- grid integration
- technical distribution of the generators
- geographic distribution of the generators
The last three of these techniques can be grouped as aggregation and distribution methods. These techniques aim to increase the predictability of the production of wind power and therefore achieve a substantial reduction in system variations.
However, although those improvements bring benefits, as Table 1 (below) shows, periods of low wind production and substantial variations will remain. Thus tools to respond to short-to medium-term and long-term variability of power production are necessary in order to manage the operational and capacity reserve. For large-scale integration of wind power the provision of flexible capacity reserve is of crucial importance. To achieve this aim several options are available [as follow].
Wind Power Forecasting
In addition to being variable, it is also challenging to accurately predict wind power production within the time scales which are necessary for long-term planning. It is easy enough to predict energy production from a large wind generation facility over a long period of time – even over the life time of the plant – but over shorter time periods, production is less easy to predict. Large divergences can occur in the timing of and the wind’s amplitude.
Aggregation and Distribution
Generally, the more wind turbines which are operating in a given period, the lower the production variability is. Similarly, the more turbines which are installed across a geographical area, the more predictable production becomes, as shown in Table 2 (below).
The number of hours with zero output will also decrease when using turbines based over a large area. A smoothing effect in the system wind power production can be achieved over large areas, as the correlation between the number of wind generators and the energy produced will be lower.
Interconnection With Other Grid Systems
This will enable the export of energy in times of wind power production in excess of demand; and imports when production is reduced.
Power Plants Providing Reserve
The use of alternative power plants to provide operational capacity and reserve is the most traditional method of integration of intermittent power. Plants which can provide such system must be flexible and have short response times, in order to make up the lost capacity from wind production quickly. Hydropower is the technology which presents the most advantages. Fossil-fuel supported power plants, can be also be used. However, the biggest disadvantage associated with this method is that it is not cost effective to run extra capacity which will only operate when wind capacity is suddenly reduced. Also, the greenhouse gas emissions released by this kind of installation negates the positive benefits of using renewable wind energy.
Curtailment of Intermittent Technology
To ensure system stability and control, a minimum level of conventional generation must be maintained, even in periods of low demand. Other situations which can limit wind power may occur when the transmission and distribution capacity is congested near the wind farm. In such situations the curtailment of wind power generation can be used to reduce the overall system integration costs.
The curtailment is made by constraining the output of a group of wind generators, shutting down some or all the turbines. This will result in a loss of energy production and in economic losses. The costs also include the time taken for the wind farm to become fully operational following grid curtailment.
The use of other types of distributed generation can provide several benefits in the network, such as: alleviating congestion, reducing transmission losses and providing ancillary services. Distributed generation can also help in wind power integration, providing reserve capacity as a substitute for conventional power plants. However, wind power is normally a form of distributed generation and has the same requirements for grid connection. Several other types of distributed generation technology can also have intermittency problems, like solar power for example.
Synergy Between Renewable Sources
Hydropower and solar power are also intermittent resources, due to their dependence meteorological conditions. However, the variables affecting these three different forms of renewable resource are independent of each other and do not necessarily occur at the same time. They can therefore be partially mutually compensated.
Analyzing 50 years of data on wind velocity in Portugal shows high variation relative to the average year, with a consequent impact on the yearly variation curve.
The wind velocity and the water inflow have average variations through the year which follow a similar pattern and their two curves have a high correlation (0.98). The solar radiation varies almost inversely, relatively to the wind velocity and the water flow (correlations of −0.7 and −0.66, respectively). That observation indicates that the complementary relationship between solar energy and wind/hydro is favourable. Solar energy can therefore be used to smooth seasonal variations of wind power. Hydropower is not complementary to wind power, but due to similar variations it is the ideal means to store excess wind energy in order to cope with the intermittence, using the storage, dispatchable power and dynamic response capacities. Also, other dispatchable energy technologies, such as biomass, can have a positive contribution, reducing the intermittent power requirements.
Most critical situations occur in periods with high energy consumption. Thus, demand side management technologies could play a major role in avoiding critical situations due to intermittent power generation resulting from the increasing use of wind in the national generation portfolio. One of the most serious peak load problems is the need for elevated electricity production due to the increased use of air conditioning on summer days with high temperatures and reduced wind velocities. Therefore DSM technologies in space conditioning are important.
The European Union Energy Services Directive in Portugal aims to achieve a consumption reduction of 9% between 2008 and 2016. A variety of DSM technologies were considered in order to execute the plan. The aggregated impact in the load diagrams of the selected technologies in the residential (lighting, appliances and space conditioning), services (lighting, office equipment and space conditioning) and industrial (lighting, power factor, energy efficient motor systems and drives) sectors was determined. It was found that the application of DSM measures will reduce the amount of investment needed to integrate intermittent power and will lead to a large reduction in peak power demand.
Demand Side Response
DR is another technology which can play a major role in the integration of renewable intermittent power. With these technologies it is possible to direct or indirectly force a consumption reduction in critical situations, over a short period of time.
In the past, the grid has been planned and operated under the presumption that the supply system must meet all customer’s energy needs, and that it is not possible to control the demand. However, that is starting to change due to the creation of opportunities for customers to manage their energy use in response to signals, such as those coming from prices or load contracts.
If the marginal peak load price is higher than the value that a consumer gets out of the services derived from the electricity, they may be willing to modify the demand, if paid the peak price or slightly less. A grid operator can obtain a greater economic benefit by providing incentives for a customer to reduce their consumption rather than paying a power producer to supply more output. Traditionally the DR technologies were typically used to attend to economic concerns. However, they can now be used to improve the system reliability, instantaneously reducing the energy consumption to prevent the most unbalanced situations, like the problems that result from the large space conditioning consumption on days with reduced wind velocity. As more customers practice automated price-responsive demand or automatically receive and respond to directions to increase or decrease their electricity use, system loads will be able to respond to, or manage, variability from wind power production.
Energy storage has crucial importance in the electricity sector, because the energy demand has relatively large hourly, daily and seasonal variations. Additionally, as energy generation from renewable energy sources also has significant variations, either in the short term (periods of a few seconds) or in the long term (hourly, daily and seasonal), storage is becoming increasingly important.
Energy storage is an appropriated option for allowing the large-scale integration of intermittent renewable sources. Energy storage in electric energy generation systems enables the adjustment between energy production and demand. The energy produced by intermittent renewable sources can be transferred to be released in low production or high consumption times. Storage technology has the advantage of generally not using fossil fuel generation, so storage facilities do not directly contribute to greenhouse gas production. One disadvantage of energy storage are the inherent losses due to the efficiency of energy conversion (about 75%–80% typically), as Table 3 (below) shows.
The storage devices do not need to be located in the wind farms and can be installed at any point on the grid. Several energy storage technologies can mitigate over-generation problems, absorbing the surplus energy in a few seconds. Each technology has its response rate, varying from a few seconds to some minutes, but all can quickly connect to the system and ramp up to add load to the system. Hydro storage facilities, whether in the form of pumped-hydro or hydro reservoirs, have played a key role in providing several grid balancing services. Large pumped storage hydropower plants can be switched from generation mode to pumping mode within a few minutes, storing the excess energy produced by the installed base of wind power capacity and releasing the energy for use meeting demand when wind production decreases.
This kind of storage has potential for large-scale electricity storage, fast response times and reduced operating costs.
Some storage devices can provide regulation services and frequency control. Hydropower can ensure such a requirement due to having fast ramp rates and can maintain maximum power for several hours. However, other technologies can be used, such as NaS (sodium-sulphur) batteries or flywheels. Storage systems that incorporate an inverter can also deliver reactive power, supporting voltage regulation.
Of the storage energy technologies available (as shown in Table 3 above), only hydropower has been used for many years and is well established in the market. The other storage technologies present non-competitive costs and reduced commercial availability. The major barrier for construction of new storage facilities is not the technology but the absence of market mechanisms which recognize the value of the storage facilities and financially compensate them for the services and benefits they can provide. Certain storage systems such as flywheels, flow cells and certain battery types could become viable. Another viable technology is compressed air depending on available locations, which is stored in geologic structures under the ground and released when necessary.
All of these options have the same aim: to balance supply and demand continuously. The first course of action when ensuring the effective integration of variable energy sources into an electricity grid should be various project techniques, including grid integration, technical distribution of the generators, geographic distribution of the generators and improved forecasting techniques. However, with large-scale integration of wind power, periods of large intermittence will remain. Therefore it is important to integrate other technologies alongside these project techniques. The complimentary relationship between renewable sources, demand side management and demand side response are all important, and energy storage technologies are rapidly improving, becoming more available. The diversity of these options means there is great potential for wind energy to be successfully integrated into domestic electricity grids, if proper planning is implemented.
Sidebar: Extreme Variations in Wind Power Ramp
Several extreme ramp rates have been recorded during storms:
- Denmark – 2000 MW (83% of capacity) decrease in 6 hours or 12 MW (0.5% of capacity) in a minute on 8 January, 2005.
- North Germany – over 4000 MW (58% of capacity) decrease within 10 hours, extreme negative ramp rate of 16 MW/min (0.2% of capacity) on 24 December, 2004.
- Ireland – 63 MW in 15 mins (some 12% of capacity at the time), 144 MW in 1 hour (29% of capacity) and 338 MW in 12 hours (68% of capacity).
- Portugal – 700 MW (60% of capacity) decrease in 8 hours on 1 June, 2006.
- Spain – 800 MW (7%) increase in 45 minutes (ramp rate of 1067 MW/h, 9% of capacity), and 1000 MW (9%) decrease in 1 hour and 45 minutes (ramp rate -570 MW/h, 5% of capacity). A generated wind power of between 25 MW and 8375 MW has occurred (0.2% and 72% of capacity) in a single year.
- Texas, USA – loss of 1550 MW of wind capacity at the rate of approximately 600 MW/hr over a 2.5 hour period on 24 February, 2007.
Pedro S. Moura is a researcher at the Institute for Systems and Robotics (ISR), at the University of Coimbra in Portugal. Anibal T. de Almeida is professor of Electrical Engineering and Computers, and a director of the ISR.
This article was adapted from a paper which was first presented at the Renewable Energy World Europe conference in Cologne, May 2009.