Performance Ratio (PR)

Continuing from last week, where we mentioned another critical parameter for the analysis of large utility-scale solar power plants: Performance Ratio (PR).

It is defined as the ratio of specific generation to average POA (plane of array) irradiation, of a solar plant. It is calculated in percentage and it signifies the performance of a PV system taking into account environmental factors like irradiation and temperature

We know that,

and, DC Capacity can be represented as a product of active area of PV modules and PV module efficiency. If we substitute these values, we get:

Theoretically, PR can be defined at STC i.e. 25 ℃. In practical conditions, the temperature is never 25 ℃ and it affects this value considerably. To solve this problem, we need to consider some more terms:

1. γ (gamma): temperature coefficient of power, which corresponds to % change in power per ℃, i.e. γ = %Δpower / ℃
2. ΔT: this denotes the difference in temperature from STC, i.e. ΔT = 25-T
3. Aggregate POA radiation = ∑Gi*ti, where Gi is the instantaneous radiation
4. The temperature corrected power can now be evaluated as, P(25) = P(T)*[1+γ*ΔT]
5. So now, Total Energy corrected = ∑Pi*[1+γ*ΔTi]*ti

So the temperature corrected PR is: