By Jochen Kreusel
Head of Smart Grids, Global | ABB, Inc.
A couple decades ago, wind power slowly began to make its way into the mainstream electric power-supply system. At the time, it was assumed this source of renewable energy could easily connect to the existing grid without fundamental changes. As wind power has grown over the years, this assumption hasn’t proven exactly true.
Today, wind is one of the least expensive energy sources worldwide and has become one of the largest generation subsectors in some regions. Further growth and acceleration is expected, especially in light of its significant cost reductions over the past few years. To meet demand, electric power-supply systems must find ways to integrate renewable energy on a larger scale.
How exactly this is going to happen is still open for debate, particularly in the United States where an ageing grid and differing state regulations are a challenge. Ideally, what’s needed is a comprehensive analysis of grid codes and utility practices in use globally to provide an in-depth evaluation and understanding of renewable power generation and its effect on transmission systems.
Since the end of the 20th century, an increasing number of countries have promoted and incorporated the use of wind and solar energy. Denmark is a pioneer in this field. By 2011, it was supplying more than 40% of its electric demand with renewable sources – three quarters of which came from wind power.
Germany is also watched closely as the first large industrial country attempting to transform its electricity supply with a strict focus on new renewable sources. Renewables accounted for nearly 28% of Germany’s power consumption in 2014. For a comparison, the U.S. gets about 13% of its energy from renewable sources.
But times are changing. California, for example, recently passed legislation that increases the state’s Renewable Portfolio Standard to 50% renewable-generated electricity by 2030. The power plants of tomorrow will need to keep up with demands. They will have to operate seamlessly and economically even at low loads and in fast-changing situations.
Renewable energies have three main characteristics that fundamentally change the electric power-supply system:
1. Remote generation. This characteristic is mainly driven by location. Winds are often strongest in more remote regions. This is an issue for power plants that tend to reside closer to the urban areas they serve.
2. Distributed generation. On a unit basis, distributed wind installations (those 10-kW or less) account for more than 67% of all turbines installed in the U.S. since 2003 and more than 33% of all turbines installed in 2014, according to the Department of Energy. Distributed wind systems can offer reliable electricity generation and back-up energy in a wide variety of settings, including for schools, farms, towns and communities, and more.
3. Volatility. This characteristic is introduced to the electric power-supply system because wind flows vary in intensity and over time.
Changing power needs
The rising share of wind and renewables are influencing the operation of conventional power plants.
The increased use of plants for steep power-output gradients (and originally only intended for base loads) is taking its toll and poses a significant technical challenge.
Cost is also an issue. Not just in terms of plant maintenance but also in terms of the market. Wind and solar power have no variable costs so they will always rank at the lower end of the merit order in an energy-only market. This means that when renewables reduce or displace conventional generation, it becomes more difficult to provide a fixed cost coverage for energy.
These economic effects mean that building and operating conventional power plants is no longer as attractive as it once was in the past. While this poses a challenge, it also provokes change. As it stands today, conventional generating capacity is still indispensable as backup during periods of low renewable power output and for power system control. Therefore, new plant designs are up for discussion and these designs are helping shape the electric power-supply systems and the energy market of the future.
Here’s at a few potential changes.
• Transmission. The transmission systems required for upgraded power plants that handle multiple sources of energy generation will look different from those of the past. In transmission networks, remote generation leads to increased capacity requirements. Also, the volatility of the generation – particularly in combination with the low number of full-load hours of the renewable energies – increases transmission requirements.
Expanding the interconnected power system represents the most cost-efficient option to match volatile generation and consumption. One likely scenario is the addition of a super-imposed transmission level or an overlay grid that’s based on high-voltage direct-current (HVDC) transmission lines. An overlay refers to a grid (most often envisioned as HVDC) that essentially “sits on top” of the existing one to bring power from renewable sources over long distances. HDVC serves as a highly efficient alternative for transmitting large amounts of electricity over long distances and for variable loads.
• Distribution. Resolving challenges isn’t a new a task for electric power suppliers. Currently, when the grid is unable to offer sufficient capacity, it’s imperative that congestion or other problems are proactively detected and resolved. In many cases, an increase in distributed generation requires a reinforcement of the grids. Mitigating problems and finding solutions are already a common practice in the coordination between large-scale power plants and system operators. But these solutions have yet to be largely standardized and automated – an important goal for the future.
• Consumption. Because of the volatile power output associated with renewables, the short-term demand response is a potential issue and one garnering attention in the domain of energy storage. Storage can make a difference, especially in the first 15 minutes of a cycle. This is an important period because it’s sufficiently long enough to ramp-up power plants with fast startup capabilities.
Storage capabilities could serve as a less costly alternative in the long run. Depending on the application, their capacity for demand response can help in short timeframes and provide a stabilizing effect for power plants. Besides the proven, but landscape profile-dependent pumped storage plants, battery storage facilities can contribute in the short term for frequency stabilization and peak shaving. In the long term, and for the compensation of seasonal variations, extending grid-interconnected systems or interconnecting hybrid systems (such as natural gas supply) might be the answer.
The road ahead
Future conventional generation will require plants that operate economically even at low loads and in frequently and fast-changing load situations. The transmission networks will have to take over long-distance transmission tasks with varying load flows.
To compensate for the volatility of the new renewable sources, wide-area interconnected systems are a possibility. With a fundamental redesign of power systems, a significant change in system management will have to include the integration of many distributed units on the generation and consumption sides.
Frequency control will prove a challenge, especially with a decreasing number of rotating masses acting as stabilizing elements. Balancing loads and generation will become more difficult in systems with varying primary energy supplies and without power storage. But change is possible and necessary if renewable energy is to reach its full potential. With its determination and a commitment to innovation, the North American wind industry can help pave the way for a new and improved electric power-supply system for the 21st century.
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Filed Under: Energy storage