Estimation of Global Solar Radiation for Four Selected Sites in Nepal Using Sunshine Hours,
Temperature and Relative Humidity
2
topographical conditions [1] to be established and run.
All the above facts demand a shift of the emphasis to the
new and clean alternative energy sources to replace
costly and unrealistic sources and ensure sustainable de-
velopment of the country.
In the year of 2008/2009, total energy consumption in
Nepal was 401 million GJ, out of 87% of total energy
consumption was derived from traditional resources (also
called biomass energy resources), 12% from commercial
sources (coal, grid electricity, and petroleum products)
and less than 1% from the alternative sources (biogas,
solar power, wind and micro/pico level hydropower) [1].
The average global solar radiation in Nepal varies from
3.6 - 6.2 kWh/m2·day; the sun shines for about 300 days
a year, the national average sunshine duration is 6.8
hours/day and average insolation intensity about 4.7
kWhm−2·day−1 (=16.92 MJ/m2·day) [1], it is greater than
the 15.8 MJ/m2·day measured by Solar Energy Research
Laboratory, Department of Physics, Silpakorn University,
Thailand for Lao PDR [7]. The data is based on one
year’s several sites of Nepal. With the consideration of
12% efficiency of PV module and 4.7 kWh/m2·day1 of
insolation intensity, the total energy generation potential
of the country will be 83,000 GWh/day = 18.36 TW.
This is more than present energy demand (13 TW) of the
world [1]. Under this circumstance, the development of
solar energy technology in many parts of the country and
especially in rural sectors is desirable and favorable
where there is no viable alternative to the solar electricity
[1].
Choice of solar energy, in countries like Nepal, is the
best and ultimate option among the different energy in-
cluding alternative energy sources. If we think of com-
plete solution of rural electrification in Nepal, we have to
plan to link up micro-hydro/pico-hydro with solar energy
exploitation. Thus, an accurate knowledge and database
of solar radiation at a particular place and selected sites
are important for the development of many solar devices,
the establishment of solar plant at the proposed site and
for estimation of their performance [8].
The radiation reaching the earth surface is modified
significantly by clouds [9], water vapor, ice, aerosols,
and atmospheric constituents in its intensity and the sun-
shine duration. The beam radiation (radiation coming
directly from the solar disk) is attenuated by the presence
of cloud in its path, as well as by the various atmospheric
elements. The depletion of the direct beam by the cloud
depends on the type of clouds, their thickness and the
number of layers [10]. The radiation scattered by the
atmospheric constituents is called diffuse radiation where
a portion of this radiation goes back by about 6% of the
incident radiation to space, and a portion, about 20% of
the incident radiation, reaches the earth surface [11]. The
sum of direct and diffuse radiation on the earth surface is
known as global/total radiation which is very important
for the design of certain solar energy applications [10].
In developing countries like Nepal, the facility of
ground-based measurement of solar radiation is available
only at selected sites whereas meteorological and hydro-
logical data are available at different parts of the country.
Obviously the best way of knowing the amount of global
solar radiation at the site of consideration is to install
pyranometer at many locations in the given region and
look after their day-to-day maintenance and recording,
which is a very expensive venture. The alternative ap-
proach is to correlate the global radiation with the mete-
orological parameters where the data can be collected.
The resultant correlation may then be used for locations
of similar meteorological characteristics [12]. Thus, de-
veloping the empirical model to estimate the global solar
radiation using easily available parameters such as sun-
shine duration, maximum and minimum temperature,
relative humidity, rainfall and geographical location, etc.,
is an essential assignment for countries like Nepal, which
will be a vigorous scientific research. So far, various
models have been developed by a number of researchers
with different regression coefficients using linear regres-
sion techniques [13] for various countries and for differ-
ent locations to estimate solar radiation. The most and
commonly used model in most of the countries including
Nepal is Angstrom-Prescott model which is based on
correlation of global solar radiation with sunshine hours.
Available literatures show that there is a very few and
limited study done in Nepal to develop the model and to
calculate the regression coefficients. This may be due to
inadequacy of existing solar energy data and lacking sense
of necessity to develop solar energy techniques. Either the
researchers may have been satisfied with the available data
or our research culture may be such that the research
works are not well tied up with our ground reality.
Empirical models which have been used to calculate
solar radiation are usually based on astronomical factors,
geographical factors, geometric factors, physical factors
and meteorological factors [14].
In the present study, annual radiation, meteorological/
hydrological data have been used to derive the regression
coefficients b, c, d and intersection constant a to develop
a model based on linear regression technique to estimate
the monthly average daily global solar radiation for four
sites of Nepal, and to compare the values with the esti-
mations derived from sunshine-based Angstrom-Prescott
model. The linear regression relation of the model is
ln
mav
OM
d
HnT
abcd RH
HNT
(1)
where m
is measured monthly mean daily solar ra-
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