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Published on June 15th, 2019 | by Barry A.F.

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Curtailment Is The Easy Answer

June 15th, 2019 by  


In a recent article, we found that curtailment of renewables may be cheaper than grid scale energy storage. Sometimes, the simplest solution is the best solution. However, given the complexity of electrical grids around the world, we should also look at all of the available solutions to make sure we are making the best choice for each application.

Renewable energy production not only varies from day to day, but season to season. Spring conditions often provide excellent solar generation as the sun is bright, yet AC use is low. Hot summer days, on the other hand, track well with AC usage but the afternoon duck curve comes into play, making it challenging to pair up generation directly with consumption. Wind, while forecastable with reasonable accuracy, also has great variations from day to day and season to season.

The right already weaponizes the intermittency of renewables and this will only escalate as curtailment of renewable generation enters the discussion. They will use this as yet another rallying cry against all renewables, regardless of its merit. It will be convincing to the masses because while curtailment is already in widespread use, knowledge about the cost and effectiveness of curtailment is mostly confined to industry players and very learned members of the public.

Interestingly, California is already curtailing solar generation, though it seems to be a case of solar being easier to curtail than fossil fuels or hydropower.

A Primer On Curtailment

For those who don’t know how curtailment is used today, fossil-based electricity generation must be used as soon as its generated because most grids have very little storage capacity beyond pumped hydro (which is only available in some locations). In general, natural gas can be throttled down when demand is low (older plants are the least likely to have this ability), coal is relatively difficult to throttle and nuclear is generally not throttled. In theory it is possible to throttle all energy sources, but in practice, throttling coal and nuclear are often more of a headache than they are worth so it is not commonly done.

In Australia, instead of throttling coal, they use it to heat hot water at night to help stabilize demand by creating artificial demand when generation would otherwise be curtailed. Doing this also reduces daytime demand when grid usage is highest and they struggle to meet peak power. The practice is essentially time shifting daytime loads into the night. The practice is lossy because the hot water is being generated and stored when its not really needed and it loses some heat as it waits to be used. More on this later.

Alternatives To Avoid Curtailment

So what are our other options to avoid curtailment? Battery storage is the most obvious solution. Just install enough grid scale battery storage capacity to meet demand, then size solar and wind generation to fill it. This will inevitably not track perfectly, leading to some curtailment and during the shoulder seasons, the curtailment will be higher than average. Grid scale batteries are reasonably good solutions to match up supply and demand as the prices of batteries are expected to continue falling.

Expect technological advances and that grid storage batteries will not be limited exclusively to lithium-ion chemistries, as they largely have been to date. New chemistries and energy storage techniques from flow batteries to solar thermal to molten salt to nickel iron are already meeting these needs and that is not even touching on the technological advancements we can expect from future innovations.


Stationary residential battery systems will likely become commonplace as prices drop. Tesla has done an amazing job designing the software to make their Powerwall battery work seamlessly to optimize efficiency in reducing grid loads while maximizing its utility. Home batteries also add resiliency in case of natural disasters which can be maximized by the addition smart software, like Tesla Powerwall’s Storm Watch, which actively looks for external threats to the grid and proactively stores up power if a threat is identified. These smaller batteries can also be linked together to form virtual power plants. Finally, community batteries are a very cost effective concept if they become widespread.

Adjusting the direction that of solar panels face is another viable option to tune production timing. At one point California had a $500 incentive to install solar on the west side of buildings to fight the duck curve. However, these incentives or even the addition of solar panel trackers add cost to a system, and for roof installations they are impractical. A few extra panels may cost less but only add to the curtailment problem.

Interestingly, California and other jurisdictions have existing electricity contracts to import/export power that are incompatible with the new renewable landscape. These will need to be renegotiated. On the flip side, selling extra power to neighboring states or provinces is a reasonable alternative. In fact, with some well negotiated contracts, one can virtually send power across a continent.

While it would be inefficient to directly transmit power over thousands of kilometers, if California, for example, sells its excess power for cheap to a neighboring state, that state can send its power to its neighbor and daisy chain the technique as far as needed. Since weather patterns are regional, there will be places with less renewable generation at any given time. This requires forethought and contracts built to support the new logic as well as a well-designed grid. Both are fundamental in the modeling work that has already been done to map out 100% renewable grids.

Pumped hydro storage is an option for places that have geographically-favorable terrain for hydropower generators. Not all of these locations already have the pumps required to move water back up to the top of the reservoir to store any excess power though these do exist in many locations already.

As mentioned earlier, Australia uses inflexible coal to heat hot water at night. This technique can be used elsewhere to absorb excess generation from renewables as well. If there is excess power generation during the day (from solar) or night (from wind), smart controllers can be used in homes/commercial/industrial locations to soak it up in the form of heating up water for storage in a water tank, extra cold weather heating of buildings beyond the thermostat set point (within reason), extra cooling in hot climates and so forth.

Interestingly, systems have already been developed such as the Ice Bear which freezes water at night to provide air conditioning during the day to work around peak power rates. Similar systems can be developed to store heat as well. Utilities can support the electrification of transportation and build up a new market for themselves by offering excess power for electric vehicle charging at a reduced rate. This can be automated through smart chargers or through manual triggers to let owners know they can plug in and soak it up. Utilities around the world are already starting to leverage EV chargers to absorb or shed loads to keep the grid balanced, so this is much more than just blue sky thinking.

There are also less conventional methods for storing energy that may become viable for large scale deployment in the future such as:

Climate Change Challenges

Batteries are excellent for expected demand but one challenge of designing the renewable grid of the future is meeting exceptional unexpected demand. While models typically use historical demand and weather data in designing required amounts of renewable and storage assets needed, climate change has already screwed with our weather patterns significantly. A severe heat wave or very extreme winter storm could deplete our batteries. Coupled with unexpected low generator output could lead to energy shortages as we won’t have long-term seasonal level storage if we only use batteries.

Some of the suggested options and others not yet discussed can be leveraged to handle long-term seasonal level storage of months or years.

Excess energy can be used for carbon capture. Burning natural gas or diverting power from other productive uses to sequester CO2 is ludicrous, but if you have huge amounts of excess energy being produced and the only option is to waste it, you can use it to take CO2 out of the air. Of course, it is easier to not produce that CO2 in the first place, but our emissions to date have already locked in a good amount of changed climate, so there is still a benefit to leveraging excess electricity to pull some of that carbon back out of the atmosphere.

Conclusion

In conclusion, curtailment is an inefficient use of resources and will result in us not reducing fossil fuel usage as quickly as we potentially could if renewable generation and energy storage are installed in places where they will cut the most carbon by not being curtailed. That said, some curtailment is acceptable and may even be unavoidable, but large scale curtailment should be minimized with and ultimate goal being zero curtailed electricity.

This article is a concept that can benefit from even more good energy storage ideas, so please feel free to add more ideas in the comments. 
 





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About the Author

I've had an interest in renewable energy and EVs since the days of deep cycle lead acid conversions and repurposed drive motors (and $10/watt solar panels). How things have changed. Also I have an interest in systems thinking (or first principles as some call it), digging into how things work from the ground up. Did you know that 97% of all Wikipedia articles link to Philosophy? A very small percentage link to Pragmatism.



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