◊ This is part of the ‘solar energy’ series of articles ◊
As I described in my wind energy capacity article, there are two different ways to describe generation capacity. The capacity represents the rated output of a generator for a prescribed set of conditions which is typically the maximum power available continuously. Capacity factor (CF) represents how much output is produced over a time period. Solar PV energy typically has a low CF since it only produces energy under adequate light conditions. The CF is critical for the economic assessment of any generation project and must be considered in the conceptual stage to determine viability. A low CF implies a need for a higher unit cost for energy than a high CF since the capital investment must be recovered by energy production over the same time frame.
Capacity factor is not the same as efficiency.
Efficiency determines the PV system rating.
Capacity factor is determined from the system’s operation based on that rating.
Efficiency should never be confused with CF. They are independent measures. Efficiency is a measure of the ability to convert solar radiation to electrical energy. CF is a measure of how much energy is actually produced over a period of time compared to how much could have been produced if the facility ran at full output all the time. Another way to think of it is that efficiency is determined by the choice of equipment and CF is determined by how it is installed and operated.
What impacts solar capacity factor?
- Radiation loss due to atmospheric conditions (weather and altitude)
- The incident angle of the sun’s rays on the PV module
- The duration of solar radiation striking the PV module with sufficient intensity to produce electricity
There are two common implementations for solar energy installations with distinct CFs. These are rooftop and ground-mounted implementations. The capacity factor for each can be significantly different as a result of the 3 fundamental CF principles. A rooftop installation will not be optimized for the PV module orientation but will have it determined by the existing structure. The advantages of rooftop mounting include reduced structural costs and the shared use of an existing space. The orientation limitation of the existing rooftop means a lower CF and a higher unit cost or longer period of time required to recover the investment. Ground-mounted solar will have optimum module orientation to maximize CF. The disadvantages of ground mounting are the area required and the cost associated with occupying it. Most new utility-scale, ground-mounted solar farms are equipped with tracking mechanisms to follow the sun’s path and increase daily energy output.
PV Solar farm CF examples – the measured values
Based on 2018 data from the Independent Electricity System Operator (IESO), the 100MW Sol-Luce solar farm in Kingston Ontario the capacity factor was 17% in 2018. Using the U.S. Energy Information Administration (EIA) data from 2015 to 2018 for the 550MW Topaz solar farm in southern California, it had an average capacity factor of 26.7%. According to the EIA, the average solar CF for the US in 2018 was 26.1%. These numbers represent utility-scale installations. Smaller-scale and residential rooftop PV would be lower. It is possible to estimate this factor for any location using The National Renewable Energy Laboratory (NREL) online tool.
Looking at the anecdotal relationship between latitude and CF the following 3 examples illustrate how output seems to decline as you move farther away from the equator.
One explanation for the relationship is that light passes through more of the atmosphere to reach the ground at higher latitudes. The lower the sun is in the sky, the greater attenuation from atmospheric effects.
In reality, there are many factors that impact CF. The most critical ones are atmospheric attenuation (i.e. weather) and PV panel orientation. PV installations with a fixed tilt will see a seasonal variation in the sun’s incident angle which affects their power output.
The takeaway
The capacity factor (CF) of solar generators
- determines the economic viability of a solar farm
- tends to be higher for ground-mounted vs rooftop installations because of mounting optimization
- anecdotal data indicates it decreases the farther you move from the equator
- decreases with cloud cover and weather disturbances
- can be estimated by online resources such as The National Renewable Energy Laboratory (NREL)
Derek
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