July 13 2015

Capacity Factors and Load Factors 

Often used interchangeably, Capacity Factors and Load Factors are subtly different measures of the performance of electricity generation. Richard Ford, Principal at Sageline Consulting explains...

Load Factor

Thermal generation: Nuclear; Coal; and, to an extent, Natural Gas power stations are expensive to build and maintain. To be economic, they would prefer to operate continuously at full load to maximise their operating income. Load Factor is a measure of their performance and is calculated as actual output over a year divided by the theoretical maximum output over a year. A generator that operates for an entire year with no breakdowns or maintenance outages could achieve a Load Factor of 100%. 

Breakdowns and maintenance are a fact of life for mechanical equipment, and so Load Factors are essentially a measure of plant reliability.  Direct comparisons can be made between two different sites or reliability of a single site can be tracked over time using this simple measure.

Electricity Markets

Electricity markets add a complicating factor. Demand is not constant and generation matches demand. Some (baseload) generators may operate continuously 24/7. Other (mid-merit) generators may be required to shut down overnight. More expensive (low-merit) generation may only be required to run seasonally or for daily demand peaks and the most expensive (peaking) generation may only be called to run for a few hours each year.  As such, Load Factor becomes a measure of the running cost of a generator. High 90s are possible for baseload plant whilst a load factor of below 5% is possible for peaking generators.

Capacity Factor

Capacity Factor is used to measure performance of renewable generators notably solarPV and wind. The calculation is essentially the same as for Load Factors. It is actual output over a year divided by the theoretical maximum output over a year. Capacity factor will never approach 100% since to do so would require the sun to remain directly overhead and for wind to blow at gale strength continuously for the entire year. 

Renewable generators are usually not constrained by the electricity market but what exactly do Capacity Factors tell us?

Solar

Capacity factors of Solar PV are typically between 10% and 20%. Regional differences (latitude, average cloud cover) account for most of this variation.  Capacity factors also provide reliability comparisons between sites that are geographically close. An unusually low Capacity Factor could (for example) highlight site specific issues with array components.

Wind

Capacity factors for Wind generation are more complicated. They are directly affected by the design of the wind turbines. For two identical turbines in the same location, capacity factor provides a comparison of reliability. For two identical turbines in different locations, capacity factor essentially provides a comparison of wind conditions. For example, offshore wind farms have (on average) higher capacity factors than their onshore counterparts. 

Design plays the biggest part. The energy available to be captured by the turbine blades is proportional (squared relationship) to the rotor diameter and proportional (cubic relationship) to the windspeed. And windspeed increases with height above ground. So a taller tower increases available energy. To extract the most wind energy from a given location, you need the biggest rotor on the tallest tower. Planning considerations aside, this would also be the most expensive design. Modern windfarms are not designed to extract maximum wind energy at any cost. Their design is a series of compromises designed to deliver electricity at the lowest unit cost. Each of the design decisions impacts on Capacity Factor.  Finally the choice of generator size has a major impact on the final Capacity Factor. 

Remember that Capacity factor is the ratio actual generation to theoretical maximum generation. For a given design of tower and rotor, a very small generator will have a high capacity factor.  It will generate at maximum electrical output at low windspeeds and also at all higher windspeeds. But the amount of electricity generated in the year will be uneconomically low as excess wind energy is left unharvested.  On the other hand an overly large generator will convert the maximum available energy from wind to electricity but, by design, will operate for most of the time below maximum output leading to a low capacity factor. Increasing the generator size to capture the last available scraps of energy is a case of chasing diminishing returns. As with tower and blade design the final generator selection – and, therefore, capacity factor - is made on economic grounds.