How Do We Stabilize the Grid with Higher Penetration of Renewables?
October 23, 2017 | Chris James, Sr. Firmware Engineer
The energy industry is in the process of understanding the full scope of renewable energy on the grid.
As more renewables are
added onto the grid, the stability of the grid is generally decreasing. This is because the continuously rotating mass connected to the grid (turbines and generators on the production end) inherently stabilizes grid frequency. When those systems are taken offline and replaced by renewable energy systems, frequency stabilization becomes an increasing challenge.
Coal-fired power plants and gas turbines are examples. These systems have a lot of mass, and when they are rotating, they store energy. In the past, these systems have been beneficial for the grid because they rotate continuously and are difficult to slow down. If a large load makes a demand on the grid, say an industrial plant turns on a large device that pulls a lot of power, it still takes time to slow down these big machines so they may be able to, at least for short periods of time, source extra power into the grid.
This presents a challenge with clean alternatives. Normally, a solar panel system can’t generate more than what it’s already producing; the system is designed to always run at its maximum capacity. Wind turbines are similar. It would seem that there’s a lot of rotating mass in a wind turbine, but compared to a fast, massive traditional turbine, the wind turbine rotates slowly and doesn’t actually have that much energy in its rotating mass. Also, the clean energy systems being interconnected to the grid must synchronize with the existing grid frequency rather than drive the grid frequency. If you draw a lot of power for a short period
time, or overload the grid, the grid frequency starts lowering, and current clean energy systems can’t compensate for that. This is where ultracapacitors, also called supercapacitors, can be implemented to help compensate for high power transient loads.
The majority of events which destabilize the grid are fairly short. Studies have shown that a majority of grid disruptions are less than a few seconds long. That’s an indicator that destabilization events that are happening on the grid can be stabilized with ultracapacitors, which specialize in short-term, very high power, lower energy content storage.
If one measures the grid frequency very precisely, an ultracapacitor paired with a very large power inverter could push power back into the grid or pull power depending on the grid frequency swings, creating a "virtual rotating mass.” It also may be that a centralized approach will be used where operation centers for the grid dispatch energy storage as needed for stabilization.
The grid is made up of different segments, and there are some that locally have an abundance of power and some need power to be sourced from afar, as power has to be provided where the loads are. In some cases, centralized operation centers may best be able to deal with a power deficit or overabundance by commanding storage systems to come online to compensate for a grid event. On the other hand, since some control decisions have to happen very quickly to be effective, some storage systems may run themselves by self-monitoring a grid segment
and reacting to changes. It’s likely that ultracapacitor-based stabilization systems will need to be autonomous
this, because they must react very fast to be effective. I imagine we will need to employ a variety of energy storage systems to meet our needs. This is a new area for the industry, so different approaches are still under exploration.
The traditional grid is self-stabilizing to a high degree. As clean energy sources that are variable continue to be added to the grid, it will be necessary to provide additional stabilization such as adding large-scale energy storage. It’s general industry knowledge that the lowest cost energy storage available is pumped hydroelectric storage. One problem with pumped hydroelectric storage is it can’t be turned on and off immediately. Time is required for spinning up/down these systems, and it seems that they also will need to be coupled with some sort of rapid stabilization.
Let’s say you’re using energy flowing directly from the wind and sun, and the turbines are off. What happens when you have another load? You will have to spin your turbines up. You need a short-term energy storage to ride through the increase in demand while you bring up the sources. It may be that you have battery systems that can achieve that. I think that ultracapacitors are poised to serve this application best in the long-term: If your lowest cost energy storage system doesn’t always source energy immediately, then you need something to bridge the gap, and ultracapacitors are in a good position to do just that.
The grid stability problem is going to stick around. It’s possible that the grid will need large ultracapacitor farms or other means to stabilize it. Maxwell Technologies has industry-leading energy throughput over life. If stabilizing a grid fed by renewables is the goal, microcycling batteries may prove inefficient. Ultracapacitors, on the other hand, are designed for high cycle applications that require long life and are a viable option for stabilizing a renewables grid. I believe ultracapacitors will provide a very effective buffering solution as we increase the amount of clean energy technology that we employ.
READ NEXT: Smoothing The Sun: Ultracapacitors On UC San Diego’s Microgrid
Sr. Firmware Engineer
About this author
Chris James is senior firmware engineer at Maxwell Technologies. He holds a B.S. in electrical engineering from Michigan Technological University.
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