TMY - typical meteorological year - What should it represent and what means P50, P70 and P90

Technical Top Tip courtesy of Suntrace Solar Resource Assessment

1. What is a TMY?

A Typical Meteorological Year (TMY) is a data set giving values in high time resolution. It shall well represent the characteristic meteorological conditions of average years at a site.
Energy performance calculations to predict the potential annual energy production of a CSP plant require a TMY as input. Therefore, TMYs need to cover a full year and shall provide the most relevant meteorological parameters for the technology to be evaluated. For Concentrating Solar Power (CSP) first of all this is Direct Normal Irradiance (DNI). Also ambient air temperature, humidity and wind are relevant for CSP. A 60 min time resolution for TMY and performance model is currently the standard, but higher time resolution can lead to more realistic simulation results.

2. What is the goal of a TMY?
The goal of a TMY is to represent the long-term average meteorological conditions at a site. It shall represent the P50 case, which means that it will be exceeded in 50 % of all years. The TMY shall also have realistic frequency distributions especially for DNI. This is prerequisite for reliable simulation of plant performance.
The TMY shall be based on as many years as possible - ideally using up to 30 years of site-specific observations. Almost no CSP project site will have historic data from a meteorological station available for such a period. Satellite data can provide the long-term base, while ground-measurements deliver the more accurate data with highest time-resolution (e.g. 1 min data) with realistic frequency distribution.
Extreme weather conditions are filtered out for the TMY. Hence, a TMY is not suitable as the only base for system or component design.

3. Uncertainty of the long-term average of solar irradiance …
One of the greatest uncertainties for CSP energy production estimates is the reliability of DNI. For each project the DNI potential should be assessed as accurate as possible. However, the accuracy of DNI data can be improved step by step during the project development process, as each improvement involves additional cost.

4. … and its impact on economics
TMYs are the "fuel" input for calculating the energy produced by the plant, and thus it is highly influences the predicted income of a plant through power sales. Lenders and banks usually take the most conservative approach to the commercial base case for financing of a project.

5. The P50, P70 and P90 values and their meaning
P-measures represent a value that is exceeded by XX % of the population of a data set. P70 or P90 values are a common practice in the financial world, and widely applied for PV and wind energy projects. Thus, P-values of solar radiation (DNI) are provided to judge the reliability of the solar resource of a project.

P50 refers to 50% of the years exceeding the value. Accordingly, the P70 (P90) value defines the DNI, which is exceeded in 70% (90%) of the years.
Accordingly, the DNI value related to P90 / P70 for a given site are lower compared to P50, which represents the long-term average. Such values are derived based on the uncertainty of the data basis.
Example P50 1940 kW/m˛/a,
P70 (5% uncertainty) 1839 kW/m˛/a
P70 (15% uncertainty) 1787 kW/m˛/a
The more accurate the assessment, the higher are the corresponding P70/P90 values, assuming that the related P50 remains constant. Usually lenders/banks apply either P70 or, the even more conservative P90 values to determine the required DSCR (Debt Service Coverage Ratio), which in turn will define the applicable volume of debt for the project.

6. Conclusion:
Project sponsors and banks typically hire independent experts for giving their opinions on the projected solar resource at the site of interest. Based on the experts experience he can also recommend measures to reduce the uncertainty of DNI. Reducing the uncertainty of DNI at a site does improve the P70 and P90 values and hence leads to more reliable energy production estimates and also to more favorable financing conditions for the project.

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