Negative Net Load

With more solar capacity coming online the California Independent System Operator (CAISO) is routinely experiencing a dramatic drop in net load, defined as the demand remaining after subtracting variable renewable generation, during the mid-day hours when solar generation is at its peak. The situation is especially pronounced during spring and the fall when temperatures are cool to mild but solar insolation high. Graph below shows a typical spring day with the midday dip in the net load curve followed by a steep rise in the evenings as solar generation drops off. With more solar capacity, both utility-scale and distributed added each year the midday dip in net load has become more of a challenge to handle, especially when it goes negative, as it already has. 

Since neither solar nor wind resources can be adjusted at will, this leaves CAISO with several alternatives:

• Reduce generation from all other resources including thermal units with flexibility and imports;
• Export the excess renewable generation, mostly solar, to neighboring regions if adequate transmission capacity is available and there are willing buyers;
• Store; or as a last resort
• Curtail.

The first option is already utilized to its limit, but some thermal units cannot be turned off in-mid-day if they are needed after the sun goes down, the neck of the “duck curve”.

The attractiveness of the second option is enhanced as prices drop or occasionally go negative – which means importers are paid to take the excess generation out of California. But as further explained in Transmission Impossible? Prospects for Decarbonizing the US Grid, a June 2023 Working Paper by Davis, Hausman & Rose, capacity limitations on the transmission network does not allow excess generation in one area or state to be exported elsewhere even when the price differentials would warrant such an exchange.

As for the third option, California’s installed battery storage capacity has climbed to 5,600 MW from 500 MW in 2020 and the California Public Utilities Commission has directed the state’s utilities to install at least 10,000 MW of battery storage by 2026.

Curtailment is the last resort – it is not anybody’s favorite option, the equivalent of spilling water from a full dam during heavy rains.

The deepening of the net load clearly presents at least 2 challenges for CAISO – and the same applies to other grid operators around the world who are experiencing “duck curves” of their own:

• First, the extreme swings in the net load exert excessive wear and tear on conventional thermal generators such as natural gas-fired plants because they must ramp down in the morning and quickly ramp up production to meet the peak demand typically between 4-9 pm; and
• Second, the resulting low (or negative) mid-day prices eat into the revenues of the thermal power plants because they are utilized for far fewer hours on many days and hence experience significant loss of revenues. If such conditions persist – as expected – some flexible plants may become uneconomic to maintain, pushing them towards early retirement. 

The big beneficiary of the “duck curve” is energy storage, which explains why California is rushing to build more battery storage in record time. But the scale of the problem is such that the industry needs to look for out-of-the-box solutions, some obvious and some less so.

Among the obvious are efforts to store the excess mid-day solar in electric vehicle batteries by encouraging daytime – rather than evening – charging. It makes a lot of sense and is already being promoted by variable tariffs as in the accompanying rates (above). The high rates during the peak demand hours, especially during the summer months, incentivize EV owners to avoid charging during the 4-9 pm period while encouraging mid-day charging until 3 pm.

The obvious next step will be to export some of the stored energy in the car batteries back into the grid using vehicle-to-home (V2H) and vehicle-to-grid (V2G) technology as further explained in article on page 17.

But storage does not have to be chemical or in batteries. The humble electric water heater can store the excess solar in mid-day in an insulated water tank for later use. Likewise, energy can be stored in ice or chilled water and buildings can be pre-cooled to reduce consumption at peak demand hours.  

This article originally appeared in the August 2023 issue of EEnergy Informer, a monthly newsletter edited by Fereidoon Sioshansi who may be reached at [email protected]"

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https://haas.berkeley.edu/wp-content/uploads/WP338.pdf