  Design and operation of a utility scale photovoltaic (PV) power plant depends
on accurate modeling of the power generated, which is highly correlated with
aggregate solar irradiance on the plant's PV modules. At present, aggregate
solar irradiance over the area of a typical PV power plant cannot be measured
directly. Rather, irradiance measurements are typically available from a few,
relatively small sensors and thus aggregate solar irradiance must be estimated
from these data. As a step towards finding more accurate methods for estimating
aggregate irradiance from avaialble measurements, we evaluate semiparametric
spatio-temporal models for global horizontal irradiance. Using data from a 1.2
MW PV plant located in Lanai, Hawaii, we show that a semiparametric model can
be more accurate than simple intepolation between sensor locations. We
investigate spatio-temporal models with separable and nonseparable covariance
structures and find no evidence to support assuming a separable covariance
structure.
