The variabilities of wind and solar provide a challenge for grid companies.
Not all that long ago renewable energy was thought of as an alternative technology. That is no longer the case. Renewables have gone mainstream in every sense of the word. Globally, nearly 80 GW of renewable power capacity was added in 2009 (the last year for which figures were available). Wind generation accounted for about 38 GW of that total. According to a recent report by GTM Research, utility photovoltaic (PV) contracts exceeded 5 GW in 2010. GTM also reported that the utility PV market for 2010 was about $1 billion in the United States and is expected to reach $8 billion by 2015.
A couple of things are happening to make this gigawatt phenomenon become a reality. Firstly, technology has improved all aspects of renewable energy — from generation efficiency (larger turbines) to increased capacity factors (improved forecasting).
Secondly, mega projects improve the economies of scale by reducing the installed costs. Developers have long realized larger projects reduce costs and speed up the timeline. This makes sense, as 10 100-MW projects require 10 sets of system studies, 10 collections of permits, 10 clusters of public meetings and 10 series of other assorted paperwork, but one 1-GW project reduces those numbers significantly.
Additionally, many government regulators worldwide have set ambitious goals for their utilities of 20% renewable power generation by 2020, with some having upped the ante to 30% or more.
The Key Word is Big
Yesterday's wind farms were considered big if they produced 20 MW, and solar farms were huge at 5 kW. Today a wind farm has to be above 600 MW to get much attention, and solar farms above 50 MW are becoming commonplace. Tomorrow's wind farms will have capacities of several gigawatts.
A proposed solar farm in the Sahara Desert will cover thousands of square miles. It is projected to be able to generate enough power to supply the needs of the European Union (EU).
The North American Electric Reliability Corp. (NERC) published a special report in 2009 titled “Accommodating High Levels of Variable Generation.” According to the report, more than 145 GW of variable generation is projected to be added to the North American grid in the next 10 years.
NERC believes if only half of that variable generation comes into service, it will be a 350% increase over what was connected to the grid in 2008, and that is going to be the industry's challenge.
Not Just a North American Occurrence
The European Commission Joint Research Center's “Renewable Energy Snapshots 2010” report confirms the trend is worldwide. It reports that variable energy sources supplied 62% of all the new generation capacity installed in the 27 member states of the EU (EU27) for 2009. It also reports that renewables provided 19.9% of the electricity consumed in Europe in 2009.
The World Wind Energy Association reports wind generation in Asia accounted for the largest share of new wind generation installations, with 40.4%, in 2009. The association expects that when the global wind capacity figures are available, they will show installed capacity will have exceeded 200 GW by the end of 2010.
Unfortunately, variable generation is not controllable; it is unpredictable and can be erratic. Unlike conventional generation, the fuel source (wind and sunlight) is capricious. It is characterized by steep up and down ramps as opposed to the traditional gradually controlled ramps of conventional generation resources.
The grid, however, needs a steady and reliable supply of electricity. It has to be able to respond to predictable changes in demand over the day. It also needs to be able to respond to the random minute-to-minute fluctuations so characteristic of the electric system.
This was not a problem when wind and solar farms were much smaller than the load. Now, large penetrations of variable and intermittent renewable resources have been added to the system. The large swings so typical of variable generation can significantly impact the system.
Deal With It
Integrating today's high levels of renewables into the power grid requires significant changes in the way utilities think about the grid and the standards used to operate it. One bit of pending legislation is the Federal Energy Regulatory Commission's proposed new rule for intra-hour scheduling. If adopted, transmission providers will have to allow customers the ability to schedule transmission services at 15-minute intervals instead of the present hourly schedules. This would restructure open access transmission tariffs and large generator interconnection agreements, making for more efficient integration of variable generation resources.
Regulation is not the only change needed. Utilities have to be consistent on power-quality issues (voltage and frequency levels and harmonics) for all forms of generation.
Fortunately, the application of smart grid technologies has had utilities thinking outside the box for many years. It also has provided the means (tools and techniques) to mitigate the impact of the unique behavior characteristics of renewable generation. Renewable resources not only require a smart grid, but the grid has to be interactive as well.
Smart, Interactive and Flexible
A system such as the U.S. Department of Energy's (DOE's) pilot program VERDE (Visualizing Energy Resource Dynamically on Earth) is a good example of this type of approach. VERDE is an interesting concept. It is a software application that uses Google Earth and integrates synchrophasors for real-time power grid monitoring.
VERDE also incorporates real-time weather data overlays to create predictive transmission grid modeling, which includes situational awareness by data analysis and other geographical information. The DOE says it has the potential to monitor the grid at a national level, but can be drilled down to the local level in the event of a problem.
In addition to enhanced monitoring and forecasting, large-scale storage is one of the most controversial issues facing renewables. Under the best circumstances, wind and solar are only available 30% and 20% of the time (capacity factor), respectively, and that is in the best of locations.
Without large-scale storage, utilities have a large amount of capacity that is either sitting dormant when the load is up or being rejected when there is too much wind or solar and the load is down.
Local, Bulk, Dynamic Storage
Fortunately, there are several mature energy storage technologies (multi-megawatt capacity) commercially available to be applied to the grid. The traditional lead-acid battery technology is being updated to offer increased capabilities.
Detroit Edison announced a solar project using Xtreme Power's battery technology improvements. The Tres Amigas project also selected Xtreme Power as its energy storage provider for its venture to move renewable generated electricity between the Eastern, Western and Electric Reliability Council of Texas grids.
NGK Insulators has teamed up its sodium-sulfur batteries with S&C Electric Co.'s IntelliTEAM technology to provide power that is intelligent and interactive on American Electric Power's feeders.
ABB and Saft Group collaborated to combine a lithium-ion battery and voltage source converter to produce a static VAR compensator (SVC) device that provides voltage control, active power flow control and dynamic energy storage.
Power electronics are being used to address the problems associated with renewables in the areas of voltage sag, frequency excursions and reactive power control (VAR consumption).
Beacon Power is offering kinetic energy storage technology. It is building a 20-MW flywheel energy storage system as part of a DOE-funded project on the New York Independent System Operator grid.
The system uses Beacon's Smart Energy 25 flywheel. The Beacon flywheel is state of the art. It stores energy in the form of inertia by a spinning disk on a metal shaft levitated with magnetics, using an electromagnetic bearing in a vacuum chamber.
Wire in the Air
Experts have gone on record saying storage is not needed once the renewable generation facilities cover the landscape and sufficient transmission facilities connect them to the grid. Their argument is the wind is blowing and the sun is shining somewhere at any given time.
“Many of the low-hanging fruit in wind has been taken advantage of,” said Jack Hand, president of POWER Engineers. “In 2006 to 2008, we had a substantial growth in the wind farm business. In 2009, it pretty much died. In 2010 and 2011, we're seeing some increased interest but it's still very limited for several reasons. Many of the builders are developers and funding is difficult, and many are waiting on backbone transmission lines to get approved or permitted, funded and built.
“I see wind as fairly stable but not very strong in the next few years,” Hand continued. “But after some of the mega western transmission projects start getting built, then maybe in 2012 we should see another big boom in wind.”
Generation needs will be met if transmission resources are available to move the electricity around. They argue that the renewables will back themselves up.
It seems like an unrealistically optimistic approach considering the state of most transmission systems today, but with a judicious application of advanced technology, there could be some merits to it.
Consider high-voltage direct-current (HVDC) transmission technology. The Electric Power Research Institute hosted its annual HVDC conference in late 2010. EPRI brought together some of the world's foremost HVDC experts to discuss the latest developments in the technology and how it is being applied.
ABB, Alstom Grid and Siemens reported about growing interest in combining HVDC transmission technology with renewable energy projects. Offshore projects have been using HVDC for collector systems with marine cables for quite some time.
Onshore, interests have been growing, as developers realize HVDC offers solutions to some of the problems with which they are dealing. HVDC is more efficient for moving large blocks of power at great distances. It also offers better power control and has fewer line losses than alternating-current technology.
Advancements in voltage source converter technology make onshore projects very cost effective. HVDC gives them the power to bypass grid congestion and deliver power where it is needed.
HVDC Transmission Technology
Wayne Galli, vice president, transmission and technical services for Clean Line Energy Partners, made a presentation on four HVDC transmission projects proposed by his company. Galli reported the projects will move wind-generated electricity from the Great Plains to the load centers on the East Coast and West Coast.
The Plains & Eastern Clean Line is the largest of the projects. It will be constructed in two phases. Each phase will be rated 3.5 GW with an 800-mile (1,288-km) ±500/600 bipole transmission line to the southeastern United States.
If projects such as this are successful, then the utilities on the other end are going to be the recipients of large amounts of variable power. They are going to require some very sophisticated generator and load modeling combined with statistical and probabilistic forecasting tools to be able to handle the fluctuations inherent with variable generation resources.
Of course, installing different types of variable generation over a large geographical area provides a more diverse mix of that generation, but the application of advanced smart grid technologies accommodates the intermittency of these resources.
Technologies such as demand-response programs and synchrophasor monitoring make the grid more flexible, transmission reinforcements make it more robust, and enhanced forecasting makes it more reliable.
Combining Smart Grid and Renewables
The Pacific Northwest Smart Grid Demonstration Project addresses many of these issues with smart grid enhancements. The demonstration project is intended to cover five states — Idaho, Montana, Oregon, Washington and Wyoming — with about 60,000 metered customers and more than 112 MW of load. It will combine advanced analytical tools, software and smart grid devices from 3TIER Inc., AREVA USA, IBM, Netezza Corp., QualityLogic Inc. and Drummond Group Inc.
One very interesting feature of this project is the use of wind and solar forecasting tools provided by 3TIER, which will monitor individual wind and solar farms and the region as a whole with hour-, day- and week-ahead forecasts. The forecasting tools will predict how the weather affects the power generation of the renewable facilities to optimally integrate the energy into the grid and dispatch other assets when production decreases.
Real-time management of the output from a wind farm can provide a utility with supply-side control, including increasing or decreasing output from other system generation as needed or decreasing wind generation output by curtailing the wind generator.
There are advantages and disadvantages to all generation technologies. Fossil fuels and nuclear energy are reliable and produce huge quantities of electricity, but fossil fuels produce greenhouse gases and nuclear energy has waste disposal issues. Wind and solar energy are pollution free and use free fuel, but they are intermittent and require large amounts of land.
There are always tradeoffs, and diversity is the only approach that reduces pollution while improving reliability. A wide variety of renewable generation resources combined with the advanced technologies available from the smart grid provides plenty of opportunities to stretch the electric utility industry's creative muscles.