On Electricity Grids, Output Must Match Consumption At All Times

Decarbonizing electricity grids is critical to rapidly reducing carbon dioxide emissions and fossil fuel use. Globally, fossil fuels are the dominant form of electricity generation. Policies are being created to electrify transportation and heating, therefore, all the more reason to decarbonize electricity grids.

A challenge for electricity grid operations is the fact that supply must match demand at all times. I think of this somewhat like a bucket of water with holes in it, in which the water level must remain filled to the top constantly, and never overflow. However, the “holes” vary in size throughout any given day, smallest at times of least electricity demand (approx. 3AM) and largest at times of most electricity demand (approx. 6PM and also when air conditioning is at peak use). The electricity generation sources being analogous to water flowing into the bucket.

This is why flexible, or controllable, electricity generation sources are so critical, also referred to as dispatchability (load following is another similar term). Not enough electricity supply to match demand can cause brownouts or blackouts, too much and the electricity grid operator may be forced to “dump” the excess electricity generation to a neighboring electricity grid at a financial loss. Curtailment (reduce, or possibly completely eliminate, electricity generation for a time) is another option when there is excess electricity generation, but this generally costs the electricity operator to pay the specific generation source operator to do so.

The electricity grid operator in Ontario, Canada is experiencing such financial challenges as it deploys more non-dispatchable, intermittent, asynchronous, weather-dependent wind turbines.

Germany has regularly experienced similar financial challenges when weather-dependent renewables over produce in relation to electricity demand, for example early May 2016.

Another metric that requires serious consideration is capacity factor. For example, wind turbines may have a capacity factor of 35% (eg, a 100 megawatt (MW) wind farm, over an annual basis, may have an average output of 35MW). It seems high pressure weather systems are getting larger in size and lasting longer in duration (high pressure systems generally mean minimal wind). Today, if the wind is calm, that electricity generation gap is generally filled with natural gas (it is currently the most flexible and inexpensive dispatchable source, specifically in USA). What happens if a wind farm operates at 100 percent of its capacity? It depends on the demand at that time, its output could help meet demand or if demand is low, curtailment may be required. In the future, nuclear could be a zero-carbon source of dispatchable electricity generation. Nuclear is occasionally operated for load following in some parts of the world, for example, France and Germany.

Other factors to consider for electricity generation and grids is Levelized Cost of Electricity and System Value of variable renewables, ie wind and solar (“it is determined by the interplay of positives and negatives”), as described by the International Energy Agency.

California captures detailed data of their electricity grid operations. We see that the least amount of electricity consumed at any given time is about 18 gigawatts (GW). Therefore, they could likely support almost this amount in baseload generation capacity (the linked post to baseload also contains a brief discussion regarding storage).

Ben Heard describes “Ancillary Services“, the criticality of synchronous generation on electricity grids for voltage and frequency control. Bellingham Technical College has created a four-part (five minutes each) youtube series entitled “Synchronising AC (alternating current) generators“. Enron didn’t seem to account for these complexities, amongst others, when it deregulated electricity grids as outlined in “What’s Wrong with the Electrical Grid“, from the article “for an AC power grid to remain stable, the frequency and phase of all power generation units must remain synchronous within narrow limits. A generator that drops 2 Hz below 60 Hz will rapidly build up enough heat in its bearings to destroy itself. So circuit breakers trip a generator out of the system when the frequency varies too much. But much smaller frequency changes can indicate instability in the grid. In the Eastern Interconnect, a 30-mHz drop in frequency reduces power delivered by 1 GW”. With regards to electric grid stability and reliability, the Energy Policy Institute of Australia wrote a short, to the point paper entitled “The ‘Pressure Cooker’ Effect of Intermittent Renewable Generation in Power Systems“.

An expert workshop was conducted in July 2015 and it outlined the challenges of decarbonizing an electricity grid, detailed here.

Armond Cohen, Executive Director of Clean Air Task Force, presented on the findings of this workshop (28 minute+ presentation), titled “Solving Climate: The Need for Zero Carbon On-Demand Power“.

As for handling changes (and increases throughout the day) in demand, perhaps a 40%renewables/40%nuclear/20%gas (and here) could be a solution.

Feel free to add to the conversation on twitter @tder2012

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One thought on “On Electricity Grids, Output Must Match Consumption At All Times

  1. Pingback: International Energy Agency Electricity Production Statistics for OECD Countries 2014 – 2016 | Blogging about energy and transitioning to clean energy

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