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use np.expm1 in singlediode model for exp(x) - 1 #500

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mikofski opened this issue Jun 30, 2018 · 0 comments
Closed

use np.expm1 in singlediode model for exp(x) - 1 #500

mikofski opened this issue Jun 30, 2018 · 0 comments
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easy enhancement good first issue help wanted

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@mikofski
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Problem
it is possible that at extremely small voltages there may be machine precision error of less than 1 part per million because the single diode model has the term exp(x) - 1 which has an increasing error as x → 0 because exp(x) → 1, so 1 - 1 is a very, very small number close to zero.

Solution
Use np.expm1(x) instead of np.exp(x) - 1 as suggested in this comment

Additional context
#409
@mikofski mikofski mentioned this issue Jun 30, 2018
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mikofski added a commit to mikofski/pvlib-python that referenced this issue Jul 10, 2018
@mikofski
MAINT: use np.expm1(x) for exp(x) - 1 in bishop88 single diode method
337d7b4 
* closes pvlib#500
* also add a comment that we are creating some temporary values to make
calculations simpler
* also use more descriptive names instead of a, b, c, use v_star, g_sh,
and g_diode
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