Updated: Feb 24, 2020
In the first part of this ROOTT Insights series we looked at the mismatch between typical seasonal and daily power demand and renewables supply in temperate and tropical climates. One of the consequences of these mismatches is that the grid operator is forced to “shift load” between different hours of the day in order to match supply and demand over short periods of time. The grid operator has various means at its disposal to do this, which we shall come to later on.
This second part of the series will look at an added complication which the operator faces, namely the difference between “variability” and “uncertainty” in our two main renewables supplies, wind and solar.
The figure below shows the variability of a solar array in the US as the difference between its maximum output and zero (night time production of the array). The variability changes over the year, being higher in the summer and lower in the winter, as the incident solar radiation changes. The variability is also dependent on the latitude, being greater the further you are from the equator.
Similarly, for wind, the variability for an array is typically between zero on totally still days and the maximum output of the wind turbines. The relationship between the power output of a wind array and the wind speed is a function of the type of turbine being used and will typically be non-linear, as shown in an example for a 2 MW turbine below. In the first zone I, the wind is not strong enough to turn the blades of the turbine. In zone II, as wind speed gradually increases, the output of the turbine grows, although not in a linear fashion. When the wind speed reaches a certain threshold value, around 12.5 m/s in this example, the turbine is producing its maximum output of 2 MW. It carries on producing at this level until, when the wind becomes too strong, a safety cut out is activated, the turbine blades are feathered, and output returns to zero. In our example, this is at about 21 m/s. This example shows that the maximum output of a wind turbine is only produced over a limited range of wind speeds:
“Uncertainty”, on the other hand, is the difference between what the wind or solar array was expected to produce during a given hour, estimated from short term weather forecasts, and what is actually produced in real time. An example for a US wind farm over a nine-day period is shown below. The dark blue line was the forecast production, 24 hours ahead, whilst the light blue line was the actual output “on the day”, with the red line being the difference between the two:
Uncertainty in solar arrays is caused typically by variations in the amount of cloud cover from minute to minute, or by the amount of haze covering the array in desert regions, as in the example below:
Source: IEEE Power and Energy Society General Meeting (PESGM) 2016
It’s clear that whilst variability can be planned for, uncertainty is an order of magnitude more complex for the grid operator, particularly as the amount of renewable generation on the grid increases. A plant's variable output can be anticipated some hours in advance, the uncertainty in the system will require a certain amount of generation reserve to be on standby to meet actual minute-by-minute demand.
Balancing the system
What sort of mitigating actions can a typical grid operator take to counter these effects?
The first port of call is typically to look at the renewables output from a number of plants over a much wider area, in the hope that individual weather effects will be averaged out to some extent. This always assumes that there are no transmission constraints which prevent power being moved around the system between the various renewable supplies and the centres of demand.
The next option may well be to call upon peaking capacity from rapid response conventional power plants, which are part of the grid operator’s “spinning reserve”. That is, they can be started first thing in the morning and are ready to produce electricity at short notice. This spinning reserve has up to now typically used fossil fuel.
Finally, or possibly in parallel to the “spinning reserve” option, if the grid operator has access to any form of energy storage, this can be used to supplement the intermittent power source. Traditionally, most of such storage has been in the form of pumped hydro, which can be switched on relatively quickly in the time that it takes to activate the pumps. Increasingly, however, attention is turning to other forms of "grid scale" storage such as underground caverns storing methane or compressed air. These caverns are filled up in times of low demand and depleted at peak periods or when intermittent sources fail.
At low levels of intermittent power on a grid, the management of both variation and uncertainty in renewable power production has been effective without much alteration to existing generation configurations. As the proportion of renewables on a grid increases, typically driven by national policy imperatives, then accommodating renewable energy becomes more problematic.
The point at which significant problems start to appear is not clear cut, but studies in North America, particularly involving large amounts of solar capacity, indicate that having 15% of renewable supply on a grid is something of a turning point.
This is currently the subject of a good deal of research to try to establish guidelines for the most cost-effective means of backup and support for the grid. The technical and commercial aspects of this will be the subject of the next brief in the series.
Joskow, P.L., (2019) “Challenges for wholesale electricity markets with intermittent renewable generation at scale: the US experience”, MIT Center for Energy and Environmental Policy Research, working paper series, CEEPR WP 2019-01
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