An overview of the use of statistical probability to understand the results of solar production.
If you work in solar energy, you are destined to hear or encounter the term P50 or P90 or P99. This is initially a very confusing concept. The ‘P’ is for probability and the number of backlog is the probability level (eg 50%, 90%, 99%).
However, the probability is inherently quite difficult to understand for us people. We love determinism; cause and effect. Do not do homework, I get a bad grade on my test. Train for a sport and increase in construction. Stop doing cardio and I’m out of breath after a staircase. Cause: Effect. But let’s drive to the gym, am I coming to a car accident? The average American has a car accident every ± 700,000 miles. That is why we would say that I will probably not go to the gym in a car accident Today. But if I could guarantee that I would drive 700,000 miles to the gym for the next 40 years or so probably That I will have an accident somewhere during those 40 years. That is the essence of statistical probability, it can see how likely something is given a domain.
In the world of solar energy, P50, P90 and P99 represent the chance that a solar project will generate at least a specific amount of electricity in a certain year. Such as the use of probability to determine whether you will crash to drive to the gym, I try to lead you through solar wounds; Why they are needed, how they are calculated and how they can use them.
Image: Solesca
Solar is very different from other generators of electricity, with the exception of wind. If you build a nuclear power plant, as long as the fitting material arrives and the human operators continue to appear, you can guarantee the power of the facility for an indefinite period of time. It is approximately deterministic. With solar energy, however, we are literally dependent on the weather. Ask every farmer, national lab researcher, outdoor man or Punxsutawney Phil to know that the weather is not easy to predict.
Because the weather is so unpredictable, how should we determine the amount of sunlight that happens to be the modules that we have placed at this location for a year or lifelong of the solar system? And it is here that we get part of our answer. Do you know what the weather will be tomorrow? You can use every weather app and know with a high degree of certainty what the weather will be for tomorrow. What about this weekend? Well, for me it is five days away, so I’ll keep an eye on it. What about 10 days from now on? A month? What about the wedding planned for September? The further we are removed from a date, the harder it is to predict it again.
However, there is something funny about the weather that works in the reverse. If you live in the northern hemisphere, you know that June will be warmer than December, that April will probably have showers and the leaves will change in the fall. We can have more certainty about the latest statements than a prediction on a certain date for months.
With that knowledge in hand, how do we use it to our advantage for solar energy? Fortunately for us, the government financed research stations and private satellite time companies to follow the weather for each year and every location (at least in the US). We can decrease the 5 -minute data that decreases decades. This is available via NREL (National Renewable Energy Lab) Noaa on the ground stations and satellite interpratators such as Solaranywhere, Solcast and Solargis. Temperature, wind, radiation, cloud cover, precipitation and much more are followed and then used to develop a TMY (typical meteorological year) file using a scientific process such as the Sandia method.
Although math is a bit complex, we need to know that 15 to 30 years of data is used to generate a typical year. The TMY file is then uploaded to your software for solar energy modeling and is used to calculate how much energy will be produced by a system. This energy output is considered a P50 energy value and is usually in an hour format (8760). Fifty percent of the time the energy will be over, 50% below. But during the life of the system, the average of the energy for all years (counting relegation and non-availability differences year to year) will be very close to the output of the model.
So you have designed your system, have used a good solar source (or more to compare) to get a TMY file and the first year generated an MWH (Megawatt hour). You can take that and build a project, right? That is possible, but if you want to finance a project, you cannot do that with a simple P50 number.
When financing a project, having a bank or external entity give capital to cover project costs, the organization expect a return with a high degree of certainty. A coin peel in a certain year does not inspire trust. Fortunately for us, we can use probability to determine a higher probability number that goes to the banks. Many wish P90 and/or P99 numbers. A P90 means that 90% probability up or exceeds energy, by 10% less than the given number. P99 is 99% higher, 1% below. It is easiest to show the differences in a Bell Curve graph.
There are two ways to reach a P99 number of a P50. The first is to take the historical weather information and to perform multiple models to get a different TMY file. The second is to take the weather data and P50 data and to enlarge it to P90 and P99 using a model developed in Geneva and is generally accepted by the industry. Both work and are proven that they are bank label. The Geneva model takes the GHI out of the system to get the variability, takes a Gaussian probability distribution and uses the P50 data and variability to calculate the P values based on standard deviation.
Although we cannot use these models to predict a certain day or year with a high degree of accuracy, we can have a high degree of accuracy over the years. P50 numbers are used for asset managers the goal of touching system production and can be adjusted for the weather. P90 and P99 numbers are used by the financial team and banks for projects because they have a higher degree of certainty.
Rocco Fucetola is Chief Operating Officer Solesca. Solesca offers pre-cad software for C&I and Ground Mount Solar. The company has helped in evaluating more than 100 GW of projects.
The views and opinions expressed in this article are the author, and do not necessarily reflect it by PV -Magazine.
The views and opinions expressed in this article are the author, and do not necessarily reflect it by PV -Magazine.
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