Storage and the duck

The California duck curve is now infamous and is very often features in discussions around storage. The duck phenomenon is a result of several factors coming together at once to create a scenario in which there is significant strain on the electricity generation system.

The infamous California duck

The now infamous California duck.

Typically the output from solar panels is well-aligned with times of high electrical demand, especially in systems which have large cooling dominated loads. This is because it often gets hot when the sun is shining and people tend to be most active during the daylight hours.

Solar generation typically occurs when demand for electricity is high - during the middle of the day.

Solar generation typically occurs when demand for electricity is high – during the middle of the day.

However when there is a cool sunny day in systems which have a lot of solar panels that are typically used to meet cooling-driven loads, then the situation can arise in which the net demand for electricity which must be generated by conventional powerplants (i.e. coal, nuclear, gas) becomes depressed, as most of the demand can be met by the solar. This is a problem for utilities in itself as turning down the output on some of these plants (especially nuclear, to a lesser extent coal) is difficult and costly, so instead they sometimes opt to sell their electricity very cheaply (or even pay for it to be used when prices go negative). For utility-scale renewables this is also a problem, as they can end up in the situation where they simply have to stop producing electricity. On top of this, the power output from all the solar panels in a local region is very well correlated. Therefore they all start and stop producing power at close to the same time (there is some spread due to orientation and location). This leads to a sharp increase in the net demand leading up to the evening peak which typically occurs after the sun goes down. There are only certain types of plant which can react to changes in demand quickly (they have high ramp rates), for example gas and hydro and only hydro can do it cheaply, as conventional gas plants must already be running for some time at their Minimum Stable Generation levels before ‘ramping up’, which is often less economic and more polluting per unit of electrical output.

Solar Panel outputs from the Pecan Street project (

Solar Panel outputs from the Pecan Street project ( all producing electricity at the same time. Red line is the average

The concern about the duck is a prime driver for energy storage development. This storage can come in several forms – i.e. not just batteries coupled with the solar panels. Some of these are highlighted in this NPR discussion which includes fuelless Compressed Air Energy Storage, Concentrated Solar Power with thermal storage in Molten Salts and Ice Storage for cooling.

Ultimately it is all down to the economics. If the costs of storage are less than the increased costs of utilities as a result of having to provide the additional flexibility the duck requires, or if storage can increase the value of renewable energy sufficiently then it will become a viable option. At present the costs of curtailment are likely to be less than storage, but as the amount of curtailment increases and storage costs fall then this could rapidly change.

One thought on “Storage and the duck

Leave a Reply

Your email address will not be published. Required fields are marked *