Capacity Factor in Practice


What are Typical Capacity Factors for Real Power Plants?

This is a common question – and one with a common “it depends” answer!

For “weather-dependent” power plants such as solar and wind farms, the actual energy output (and hence the capacity factor) depends on how often the sun is shining and the wind is blowing. Plus, in both cases, how strongly. In addition, plant design and technology choices can make a difference.

So Solar farms in localities which are sunny more often and where the sun is stronger when it’s out (because of clearer skies or higher altitude, for example) will have higher capacity factors than those in gloomier climates. Solar farms which deploy technology which tracks the sun as it moves across the sky will capture the available resource more effectively too, thus raising the capacity factor.

Modern solar farms in excellent resource areas might expect capacity factors of 25-30% (1), but poorly optimised rooftop arrays in countries such as Germany or the UK can be down at 10% (or less).

AN IMPORTANT ASIDE: If someone states the capacity factor of a solar PV (photovoltaic) power plant, a point to be careful of is knowing which figure they have used for the capacity of the plant. PV is different to other types of power generation in that the primary output of an individual solar module/panel is direct current (DC). Other power sources, utilising spinning generators, output alternating current (AC). It’s not uncommon to see the capacity of a solar farm described in terms of its DC output (basically the output of each panel multiplied by the number of panels). However before its output is fed into the grid, somewhere within the solar array there is a conversion from DC to AC. Since any conversion of energy involves losses, plus there are a variety of other “array losses” (in the cabling connecting individual modules for example), the AC capacity of a PV power plant is less than its capacity stated in DC terms. To compare like-for-like with the capacity factor of other types of (AC) plant, you therefore need to use the rated AC capacity.

Wind capacity factors vary widely, according to the windiness of a location. A typical range onshore might be 25-40% (but note that both worse and better numbers can be seen in individual cases). Offshore wind is typically stronger and more consistent, so investors would be expecting capacity factors more like 40% and above in order to make a viable business case (given the extra cost and complexity). (2)

Unlike its “variable” renewable cousins, Geothermal power is able to operate at a steady output close to its design capacity, at capacity factors up to 85-90% in some cases. After all, unlike the variability of wind and sun, if you drill a deep hole in the ground, it doesn’t produce hot water one minute and cold water the next!

For fuel-based power plants such as gas and coal, the capacity factor depends on how often the plant operator chooses to burn the fuel; which in turn depends on the price they can get for the electricity they’ll produce. If the price of electricity is low, it may be more economic to save the fuel (if it was expensive), choosing to burn only at times when you can make more money from the electricity produced. Another consideration is whether the plant can be turned on and off (or up and down) efficiently. We’ll consider this aspect again in future lessons.

Generally-speaking, nuclear plants have high capacity factors, which means they operate on a steady, constant basis. Reductions tend to be due to time spent not generating because of maintenance shut-downs, rather than because output has been regularly turned up and down (nuclear plants are generally not designed to do that and have low fuel costs, so prefer to keep running).

On the other hand gas is a flexible fuel (and, in some markets, still a relatively expensive one). So gas plants often turn on and off to meet changes in electricity demand. Some only operate for a relatively few hours at times of peak demand: so-called “peaking plants”. Since they operate for few hours, their capacity factor is very low. This is by economic choice though, in contrast to something like solar, where low capacity factor is a product of the natural resource available. Diesel generators are another example of peaking plants in some markets: inefficient and expensive compared to other options, but only used at very, very low capacity factor; a last resort when demand is higher than the other supplies can meet.

Different hydropower plants in different markets can be hugely different in terms of capacity factor: from a few tens of percent to well over 50%. Plant operators can choose when and when not to turn their plant on (“dispatch” it) – as long as sufficient water resource is available; which is generally a question of seasonal climate. In the wet season, rivers may be at full flow or dams at full capacity and regularly topped up, so maximum power may be available for lengthy periods. In the dry season though, the opposite may be true.

Finally, it’s important to bear in mind that the capacity factors of different sources in a market – particularly of those sources which can operate (or not) through choice – will be highly affected by the mix available in that market.

Whether or not certain plants operate at particular times and levels of demand will be decided by their economics relative to competing sources in the mix – which can be quite complicated to work out! Fuel prices, efficiencies and a variety of policy mechanisms (such as carbon pricing) are just some of the variables at play.

The bottom line?

Making assumptions for one project based on the results of others is a very dangerous approach!

It’s extremely important to recognise that the capacity factors of different power plants can vary widely; both between different countries or regions and even within the same markets. A browse through the example references below should help illustrate this.

References:

(1) This report (pdf) from the Lawrence Berkeley National Laboratory in the US is a good example of a study looking at capacity factors in solar farms: https://emp.lbl.gov/sites/all/files/lbnl-1004374.p…

(2) For real data on offshore wind from Denmark, where capacity factors have in some cases risen to >45%, see this excellent blog post: http://energynumbers.info/capacity-factors-at-dani…

(3) Some interesting charts from a study of capacity factors for different plant types across the world by the US Energy Information Administration are available here: https://www.eia.gov/todayinenergy/detail.php?id=22…

(3) The Grey Cells Energy Virtual Library is another resource where you’ll find a range of reports which include data on capacity factors of various sources in various markets: https://greycellsenergy.com/reading_database/