Edinburgh Napier University

  This thesis deals with- various aspects of broadband horizontal solar
irradiance. Quality control of measured datasets are identified and analysed. It
was found that solar irradiance datasets may contain significant errors. These
sources of errors were divided in two categories, the inherent instrument
errors and operation related errors. Methods of assessing the quality of the
datasets were evaluated and found to be unsatisfactory. A new method was
hence developed to quality control the solar irradiance data.
The quality control procedure consists of two tiers of tests. The first tests are
physical tests that identify and remove data points that are physical
impossibilities. The second tier tests consist of the creation of a mathematical
envelope of acceptance in a sky clarity index domain. This envelope of
acceptance is based on multiples of standard deviations of the weighted
mean of clearness index to diffuse ratio. The available datasets in this study
were thus quality controlled to remove any obvious outliers.
Modelling the solar resource is an important tool for engineers and scientists.
Such models have been developed since the second half of the 20th century.
Some models rely on one or two meteorological parameters to estimate the
solar irradiance, while other models are more complex and require a far
greater number of points. Two of these models have been analysed and
evaluated. The two models are all-sky, broadband solar irradiance models.
The first model analysed is the Meteorological Radiation Model, or MRM. This
model is in fact a sunshine based model, with atmospheric turbidity taken into
account as well. The beam irradiance component was found to be acceptable
given the number of inputs required by the model. Any extra parameters
would increase the complexity of the model, without noticeable improvements.
The regressions were modified to take into account sunshine fraction banding.
However the diffuse irradiance was identified as one which had the potential
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for improvement. Thus, in the present work an attempt has been made to
develop improved models. The new model was found to be far superior to the
older, original model, thus the name Improved Meteorological Radiation
Model, IMRM.
The second type of model investigated is the cloud based radiation model.
This type of model is simple to use and rely on regressions between
irradiation, solar altitude angle and the cloud cover. Careful analysis of the
cloud distribution reveals certain flaws in the current regressions. New
regressions were formulated and the result was a model superior to all its
predecessors.
Clear-sky modelling is important for maximum load calculations; however,
there is no method of extracting with accuracy clear-sky broadband data.
Clear-sky identification techniques were evaluated and a new method was
devised. These new datasets were used on four clear-sky models, MRM,
Page's Radiation Model, PRM, Yang's radiation model and Gueymard's
REST2 model. It was found that using this new method of extracting extremeclear-
sky data, the models performed better than when using quasi-clear-sky
data.
Solar radiation modelling is not an end by-itself, it must serve a purpose for
engineers in their applications. Napier University has installed a 160m2
photovoltaic facility in 2003. A 27 -year solar radiation dataset was available
for Edinburgh, to do feasibility calculations for the project; however this
dataset contained gaps in the data. The cloud radiation model developed in
this study was utilised to this end. In addition a complete life cycle analysis
was performed on the project, and it was found that with an average efficiency
at around 12%, the facility will payback its embodied energy in eight years,
and based on a relatively conservative forecast of energy prices, the financial
payback is set at under 100 years.