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How to configure GHI and DNI weights within Solcast when extracting TMY data

The Solcast API allows customers to vary the relative weights of GHI and DNI used to construct a TMY from TGY to TDY or anything in between by using parameters <ghi_weight> and <dni_weight>. These weights are applied to the the long-term historical time series average to create the target values used to construct the TMY: 

AVERAGE(ghi_weight*GHI + dni_weight*DNI) 

For example

• TGY (Typical GHI Year):
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ghi_weight=1.0, dni_weight=0.0

• TDY (Typical DNI Year):
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ghi_weight=0.0, dni_weight=0.0

The DNI GHI ratio can be customised when a request is made to better match your system.  Generally, systems that track the sun (dual-axis solar PV, single-axis solar PV, concentrated solar power) are more sensitive to DNI (direct normal irradiance); whereas fixed-tilt solar PV systems are more sensitive to GHI (global horizontal irradiance).

Why would I customise the GHI:DNI ratio?Main reasons why engineers and analysts may want to tune these weights are:

• To match an existing workflow e.g. you prefer to use Typical GHI Years (TGY, 100% GHI weight) for all projects
• To better align with the relative importance of direct and global irradiance on power yield e.g. single axis tracking vs fixed tilt systems have decreasing importance of direct irradiance.

On the last point, the optimal way to model the local solar climate for a PV system potential yield, accounting for the fact that the conversion of irradiance to power is not 1-to-1, would be to generate a long-term irradiance historical time series, do the power modelling on that, and then extract the "typical power year". Varying the GHI:DNI weight is a way to get closer to this ideal without breaking the TMY format and workflow.

Solcast's defaults and recommended valuesThe default weights in the Solcast are ghi_weight=0.2 and dni_weight=0.8 but the Solcast recommendation varies based on the type of PV system:

• for fixed-tilt systems:
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ghi_weight=0.8, dni_weight=0.2

• for tracking systems:
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ghi_weight=0.2, dni_weight=0.8

These weights have been obtained by a small optimization performed on measurements comparing TMY and long term historical time series based estimates.