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Current Optics and Photonics
Vol. 1, No. 4, August 2017, pp. 263-270
- 263 -
I. INTRODUCTION
To evaluate the performance of radiometric devices, both
in a laboratory setting and in the field, it is necessary to
have accurate knowledge of the luminous radiation that
stimulates the equipment [1]. Pyrheliometers, pyranometers,
and pyrgeometers are used to obtain solar and infrared
radiation data from observatories worldwide [2]. Typical
solar radiometers measure the entire optical range of the
electromagnetic spectrum emitted by the sun, including
certain parts of the ultraviolet (UV - A, B; 0.285-0.4 µm),
visible (VIS; 0.4-0.7 µm), and near-infrared regions (NIR;
0.7-2.8 µm). The pyrheliometer was invented by Pouillet
in 1837, and he observed a TSI (Total Solar Irradiance) of
1,227 W m-2, which is within 10% of the current standard
TSI of 1,366 W m-2 [3]. In addition, the Smithsonian
Institution began surface solar radiation observation in
1890, while the US National Weather Service contributed
to the improvement of meteorological observation capability
and the development of agrometeorology by beginning
observations in 1901 [4]. The Kimball-Hobbs pyranometer,
developed in 1923, became the prototype of the Eppley
pyranometer. Meanwhile, the Moll-Gorczynski radiometer,
which was based on the first model developed by Kipp
Analysis of the Thermal Dome Effect from Global Solar Radiation
Observed with a Modified Pyranometer
Ilsung Zo1, Joonbum Jee2, Buyo Kim1,3, and Kyutae Lee1,3*
1Research Institute for Radiation-Satellite, Gangneung-Wonju National University, Gangneung 25457, Korea2Weather Information Service Engine, Hankuk University of Foreign Studies, Yongin 17035, Korea
3Department of Atmospheric & Environmental Sciences, Gangneung-Wonju National University,
Gangneung 25457, Korea
(Received March 21, 2017 : revised June 5, 2017 : accepted June 19, 2017)
Solar radiation data measured by pyranometers is of fundamental use in various fields. In the field of
atmospheric optics, the measurement of solar energy must be precise, and the equipment needs to be
maintained frequently. However, there seem to be many errors with the existing type of pyranometer, which
is an element of the solar-energy observation apparatus. In particular, the error caused by the thermal dome
effect occurs because of the thermal offset generated from a temperature difference between outer dome
and inner casing. To resolve the thermal dome effect, intensive observation was conducted using the method
and instrument designed by Ji and Tsay. The characteristics of the observed global solar radiation were
analyzed by classifying the observation period into clear, cloudy, and rainy cases. For the clear-weather
case, the temperature difference between the pyranometer’s case and dome was highest, and the thermal
dome effect was 0.88 MJ m-2 day-1. Meanwhile, the thermal dome effect in the cloudy case was 0.69
MJ m-2 day-1, because the reduced global solar radiation thus reduced the temperature difference between
case and dome. In addition, the rainy case had the smallest temperature difference of 0.21 MJ m-2 day-1.
The quantification of this thermal dome effect with respect to the daily accumulated global solar radiation
gives calculated errors in the cloudy, rainy, and clear cases of 6.53%, 6.38%, and 5.41% respectively.
Keywords : Modified pyranometer, Thermal offset, Global solar radiation, TDE (Thermal dome effect),
Dome temperature
OCIS codes : (010.1290) Atmospheric optics; (010.1615) Cloud; (010.3920) Meteorology; (010.5630)
Radiometry
*Corresponding author: [email protected]
Color versions of one or more of the figures in this paper are available online.
*
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
*Copyright 2017 Current Optics and Photonics
ISSN: 2508-7266(Print) / ISSN: 2508-7274(Online)
DOI: https://doi.org/10.3807/COPP.2017.1.4.263
Current Optics and Photonics, Vol. 1, No. 4, August 2017264
and Zonen, was developed in 1924 [5]. In 1930, the Eppley
Laboratory began production of commercialized pyrano-
meters, leading to their worldwide installation and operation.
Nowadays the significant growth of the photovoltaic
industry has led to an increase in the use of pyranometers,
which are often used to select a development site, and
help to develop models for predicting solar radiation [6, 7].
The amount of generated solar power increases as the
solar radiation reaching the surface of the earth becomes
stronger, and thus accurate solar radiation observations are
considered to be a very important tool for solar power
generation [8].
Solar radiation is observed according to WMO (World
Meteorological Organization) and WRC (World Radiation
Center) regulations [9, 10]. The pyranometers used are
classified based on performance, using the WMO and ISO
9060 pyranometer classification standards. The WMO standard
classifies pyranometers as High Quality, Good Quality, and
Moderate Quality, while the ISO classifies them as Secondary
Standard, First Class, and Second Class. Such an instrument
must be carefully managed and handled because pyrano-
meters are affected by uncertainties resulting from their
sensitivity function, thermal offset, other spectral effects, and
geometrical, environmental, and instrumental factors [11].
Although most of the uncertainty factors have already been
studied, the problem related to thermal offset has not been
solved [12-14].
Thermal offset refers to errors generated because the
thermopile in the pyranometer is affected not only by solar
energy, but also by the ambient temperature [15]. This
effect generates a larger error when the sky is clear during
the night, compared to cloudy conditions [16]. One study
attempted to decrease the error by reducing the increase of
case temperature using a fan [17]. Analysis has also been
performed of the correlation of thermal effect with the
difference between long-term observation data (BSRN obser-
vations) and model results from the GCM (Global Circu-
lation Model), and the MODTRAN (Moderate resolution
atmospheric Transmission) radiative transfer model [18, 19].
Ji and Tsay [20] calculated the temperature difference
between outer and inner domes of a pyranometer using
pressure observations and the ideal gas equation. The thermal
offset was calculated using the TDE (Thermal Dome Effect),
and the change in solar radiance was determined using the
calculated temperature and the temperature observed at the
inner pyranometer case. In this study the thermal offset
difference was calculated and the change in solar radiation
due to the TDE was analyzed, using solar radiation data
classified into clear, cloudy, and rainy days, from intensive
observations at Gangneung-Wonju National University.
II. RESEARCH METHODS AND MATERIALS
2.1. Research Methods
For a general pyranometer, the global solar radiation (I, in
units of W m-2) is determined by multiplying the calibration
constant (c, in units of W m-2 mV-1) of each pyranometer
by the voltage measurement from the pyranometer (V, in
units of mV), as shown in Eq. (1). In the night case without
solar radiation, the observation is 0 W m-2. However, most
pyranometers actually show a negative (-) value, such as the
result of Carnicero [21]. This is because the temperature of
the case (Tc) was higher than the temperature of the dome
(Td), opposite the daytime situation. Thus the TDE can be
found using Eq. (2) as the energy difference generated by
the temperature difference between dome and case.
I cV (1)
Thermal of fset ∝
(2)
where σ denotes the Stefan-Boltzmann constant (5.6697 ×
10-8 W m-2 K-4).
Similarly, Ji and Tsay [20] built a modified pyranometer
to study the thermal offset, as illustrated in Fig. 1. Unlike
for a general pyranometer, the Tc and Pd (dome pressure)
between outer and inner dome were also observed. Smith
[11] directly attached a temperature sensor to the dome to
observe Td, but because this can cause an error in the
pyranometer’s measurements, the Ji and Tsay method was
used in this study.
Using Eqs. (1) and (2), the equation for global solar
radiation of Ji et al. [22] can be deduced as:
I cV f
(3)
Here the dome factor f accounts for the optical and
radiative properties of the domes and detector surface [23],
and Ts is the thermopile temperature calculated from Tc
using Hickey’s method [24], while Td is calculated using
Pd and the ideal gas equation. Thus, to observe an accurate
value from the pyranometer, the energy generated by the
first general term and the temperature difference between
the pyranometer case and dome must be considered.
Table 1 shows the TDE calculation procedure of Ji and
Tsay, consisting of 4 steps. In step 1, Tdnight is estimated
FIG. 1. Schematic diagram of a modified pyranometer.
Analysis of the Thermal Dome Effect from Global Solar Radiation Observed with … - Ilsung Zo et al. 265
during the night (2000 LST to 0400 LST) when the solar
radiation becomes 0 W m-2, by setting the left side of
Eq. (3) to zero. To compensate for the small pressure loss
generated around the pressure observation hose, Pd is
defined by 2 terms, as in Eq. (4), and each term can be
expressed as in Eqs. (5) and (6).
(4)
(5)
(6)
Here P0 is the dome pressure without pressure loss using
the state equation in Eq. (5), δPd is the pressure difference
between inner and outer domes with pressure loss, and k
is the dome factor. Combining these gives
(7)
where Pd and Pa are the measurements. r0 (y-intercept) and
k (slope) are calculated with the first equation form in
Step 2, by dividing both sides of Eq. (7) by the Td calculated
in Step 1. In Step 2, the dome temperature during daytime
can be calculated by substituting the calculated r0 and k
into Eq. (7) and arranging the Pd-related equation into the
Td-related equation, as given in Step 3. The daytime Tdnight
is calculated with the obtained r0 and k, using the daytime
dome pressure (Pd), atmospheric pressure (Pa), and night
data. The Td (Tdnight + Td
daytime) calculated in Steps 1 and 3
is substituted into Eq. (3), and the irradiance including the
TDE can be calculated.
2.2. Research Materials
This study used a modified Eppley PSP (Precision Spectral
Pyranometer) device, as illustrated in Fig. 1. The c and f
values of the modified pyranometer required for Eq. (3)
were determined by experiment [25]. In addition to the usual
observation data, Tc and Pd were observed. In addition,
measurements were made of the solar radiation (direct, global,
and diffuse), temperature, relative humidity, and pressure
on the rooftop of Life Science Building 2nd, Gangneung-
Wonju National University, Gangneung, Republic of Korea
(Lat.: 37.77°E, Lon.: 128.87°N, Alt.: 63.50 m). This obser-
vation site was chosen because there was no impinging
shade from other buildings. However, shading occurred
between 1600 LST and 1700 LST due to a hill to the
west, and varied depending on the altitude and azimuth of
the sun. Thus the observation data from these times were
omitted from the study. Table 2 shows the three classifi-
cations for sky and weather conditions (clear, cloudy, and
rainy) during the intensive observation period from 9 October
until 7 November 2011, which was in collaboration with
NASA. The observation period consisted of 5 clear days,
16 cloudy days, and 9 rainy days [26]. In addition, Fig. 2
presents the time series for solar radiation (in units of MJ
m-2) with the pyranometer and the total cloud amount (in
units of tenths) per day [27], as observed with a sky viewer.
The maximum irradiance was found to be higher than 2.0
MJ m-2 at noon on a clear day, but less than 1.0 MJ m-2
on a cloudy or rainy day. The rainfall accumulated during
the corresponding day is summarized in Table 2.
TABLE 1. Procedure for obtaining the solar radiation
corrected thermal dome effect
Step Action Equation
1Calculation of Td
night
during night timeTd
night = (Ts4 + cV/fσ)1/4
2Calculation of r0 and k
using Tdnight
Pd/Tdnight =
r0 + k(Pd-Pa)/ Tdnight
3 Calculation of Tddaytime Td
daytime =
[(1-k)Pd + kPa)]/ r0
4calculation of solar radiation
corrected thermal effectI = cV + fσ(Ts
4-Td4)
TABLE 2. The cases were classified as clear, cloudy, and
rainy according to sky and weather conditions during an
intensive observation period in 2011 (from 9 October to 7
November, 2011)
Case Date (Month/Day)
Clear
(5 days)10/13, 10/17, 10/18, 10/19, 10/31
Cloudy
(16 days)
10/09, 10/10, 10/11, 10/12, 10/16, 10/20,
10/23, 10/24, 10/25, 10/26, 10/27, 10/28,
11/01, 11/02, 11/03, 11/04
Rainy
(in units of mm)
(9 days)
10/14(4.0), 10/15(4.5), 10/21(4.5),
10/22(37.5), 10/29(9.0), 10/30(0.5),
11/05(6.0), 11/06(28.0), 11/07(2.0)
FIG. 2. Time series of hourly solar radiation and daily mean
total cloud amount at Gangneung-Wonju National University
during an intensive observation period in 2011 (from 9
October to 7 November, 2011).
Current Optics and Photonics, Vol. 1, No. 4, August 2017266
III. RESULTS
3.1. Clear Case
The global solar radiation data from 18 October 2011,
which is among the selected clear days, is shown in Fig. 3.
The global solar radiation was observed using the modified
pyranometer (PSP, EPPLEY), while the direct and diffuse
solar radiation were observed at 1 minute intervals using
the Kipp & Zonen instrument (direct: CHP1, diffuse: CMP
21, sun-tracker: 2AP). The direct (red dots) and diffuse
(blue dots) solar radiation observations could be considered
accurate because the effect of atmospheric components
such as aerosols, including clouds in the optical path, was
insignificant [28, 29]. From about 1620 LST, the majority
of direct solar radiation was shaded by a mountain, and a
value close to 0 W m-2 was observed. Accordingly the total
solar radiation and diffuse solar radiation were observed to
have a very similar. Figure 3(b) shows the magnification
of the solar radiation change at night. As stated before, the
observation during the night should be 0 W m-2; however,
a negative (-) energy was observed, due to the TDE error.
The TDE in the clear case was determined according to
the procedure in Table 2. Figure 4 presents the graphs of
Tdnight during the night as a function of time from the
observed Tc and Step 1 in Table 2. As shown in Fig. 4(b),
Td was lower than Tc during the night. The difference
between these two values resulted in a small current being
generated, and an error in the observation data.
The calculated dome factor (k = 0.064) and r0 (3.457 g
m-2 K-1) were obtained using the equation and the night
observation data from Fig. 5(a) for the calculation in Step
2 of Table 1. The R² and standard error were 0.889 and
FIG. 3. Time series of hourly solar radiation at Gangneung-
Wonju National University on 18 October 2011: (a) solar
radiation (global, direct, and diffuse), and (b) global radiation
captured at night.
FIG. 4. Same as Fig. 3, except for case temperature (Tc, black
dots) and dome temperature at night (Tdnight, blue dots).
(a)
(b)
FIG. 5. (a) Estimation of r0 and k from nighttime mea-
surement in Step 2 from Table 1. (b) Time series of case
temperature and dome temperature at daytime in Step 3
from Table 1.
Analysis of the Thermal Dome Effect from Global Solar Radiation Observed with … - Ilsung Zo et al. 267
0.0008 MJ m-2 respectively. The dome factor and r0
calculated in Step 2 were implemented in Step 3 and are
presented in Fig. 5(b), which shows both Tc and Tddaytime.
It can be seen that the case temperature was higher than
the dome temperature, in a similar situation to that at
night. Hence, because of the difference between the two
temperatures, the actual daytime irradiance was lower than
the observation from the pyranometer. It can be concluded
that the reduction of global solar radiation during daytime
was larger than the reduction that occurred at night,
because the temperature difference was higher in the
daytime than the nighttime.
After the daytime temperature of the inner dome was
calculated per Step 3, the global solar radiation in one day
could be calculated using Eq. (3) to compensate for the
pyranometer temperature. Figure 6 shows the change of
pyranometer thermal offset for 18 October 2011, while
Fig. 6(a) shows the global solar radiation before (red dots)
and after (blue dots) correction with the pyranometer thermal
offset. Additionally, Fig. 6(b) presents the global solar
radiation change at nighttime after sunset, without global
solar radiation. The global solar radiation at night without
sunlight must be 0 W m-2, but in Fig. 6(b), the pyrano-
meter showed a negative (-) global solar radiation at night.
Thus, as has been mentioned previously, the compensation
for pyranometer temperature is needed since a negative (-)
global solar radiation was found during the night due to
the difference between the dome temperature and case
temperature. The corrected irradiances by pyranometer (blue
dots) give a value much closer to 0 W m-2.
3.2. Cloudy Case
The cloudy cases in this study were the case with upper
and middle clouds (15 October 2011; Fig. 7(a)), and the
case with the rainfall until morning and clouds in the
afternoon (30 October 2011; Fig. 7(b)). The case on October
2011, which same season as the clear case, was selected.
The temperature difference between Tdnight and Tc before
sunrise is shown in Fig. 8 using the analysis method in
Table 1, in a similar manner as for the clear case. The red
color indicates data from the clear case on 18 October
2011, while blue and black indicate data from the cloudy
and rainy cases on 15 and 30 October 2011 respectively.
The clear case data exhibited the highest temperature
difference of 0.3 K, while the temperature difference for
the rainy day was less than 0.05 K. Hence, the temperature
of the inner pyranometer during the night was higher than
0900
LST
1200
LST
1500
LST
(a) (b)
FIG. 7. Total sky images for (a) cloudy (15 October 2011) and
(b) rainy (30 October 2011) cases, observed from Gangneung-
Wonju National University.
FIG. 8. Temperature difference between Tc and Tdnight during
the night (from 2000 LST to 0400 LST) for all cases.
FIG. 6. Same as Fig. 3, except for observed global solar
radiation (black dots) and modified global solar radiation
(blue dots) corrected for the thermal effect.
Current Optics and Photonics, Vol. 1, No. 4, August 2017268
the dome’s temperature, whereas the largest difference was
found for a day with a higher temperature difference.
The calculated Tdnight from Fig. 8 was used to calculate
Tddaytime, and then inserted into Eq. (3) to produce the results
in Fig. 9. First, the maximum value of the cloudy case on
15 October 2011 was consistent with the clear day (18
October 2011). However, the global solar radiation change
was significant, due to the effect of clouds. Especially at
1400 LST and 1500 LST, the global solar radiation that
reached the surface of the earth was significantly reduced,
owing to the clouds. The global solar radiation for the
rainy day (30 October 2011) was also highly reduced. In
addition, negative (-) global solar radiation was found for
the cloudy case before sunrise and after sunset, as shown
by the black line. Correction of the offset reduced the
negative (-) global solar radiation, producing a measured
global solar radiation close to 0 W m-2, as shown in blue.
As the negative (-) global solar radiation before sunrise
was very small owing to the rain, the difference between
Tc and Tdnight was also very small (Fig. 8, black points; 30
October 2011). Moreover, because the difference between
Tc and Tdnight becomes larger after sunset for the rainy
case, the negative (-) error was decreased by the offset
correction.
The daily accumulated TDE during the observation
period is quantitatively presented in Fig. 10. The gray bar
represents the daily accumulated global solar radiation, the
red bar shows the daily accumulated TDE, and the line
indicates the total cloud amount. The TDE was proportionally
larger for days with high daily accumulated global solar
radiation. The maximum daily accumulated TDE of 1.16
MJ m-2 was calculated for 31 October 2011. Moreover, the
lowest TDE of 0.02 MJ m-2 was found on 6 November
2011, when clouds and rain occurred all day. When the
FIG. 10. Same as Fig. 2, except for daily accumulated solar
radiation (gray bars), daily accumulated thermal dome effect
(red bars), and daily mean total cloud amount (blue dots).
TABLE 3. Daily accumulated solar radiation (DSR, in units
of MJ m-2), thermal dome effect (TDE, in units of MJ m-2) and
ratio (%) of TDE to daily accumulated solar radiation
CaseDSR
(MJ m-2)
TDE
(MJ m-2)
Ratio of TDE
(%)
Clear 16.29 0.88 5.41
Cloudy 10.56 0.69 6.53
Rainy 3.35 0.21 6.38
(a) (b)
FIG. 9. Same as Fig. 6, except for the cases with (a) cloudy (15 October 2011) and (b) rainy (30 October 2011).
Analysis of the Thermal Dome Effect from Global Solar Radiation Observed with … - Ilsung Zo et al. 269
surface-reaching global solar radiation became stronger, the
temperature increased. Thus, the temperature difference of
the inner and outer pyranometer cases became larger,
resulting in the TDE. Table 3 shows the average TDE for
the clear, cloudy, and rain cases, with the highest value
found for the clear case. These maximum differences can
also be stated for the cloudy, rainy, and clear cases as
percentages: 6.53%, 6.38%, and 5.41%.
IV. CONCLUSION
Pyranometers for the observation of global solar radiation
have been developing, into their present form since the
19th century. However, studies are still being conducted to
solve the problems related to thermal offset, owing to the
temperature difference between case and dome. In this
study, intensive observations were performed and the TDE
was analyzed using the Ji and Tsay (2010) method [20]
and a modified pyranometer at Gangneung-Wonju National
University, in collaboration with NASA. The analysis
involved classifying the observation period into clear,
cloudy, and rainy cases using observations of the total
cloud amount. The difference between Tc and Td was large
for clear days, leading to an increase of the TDE, and a
value of 0.88 MJ m-2 day-1 was found. For the cloudy
case, the temperature difference between case and dome
was reduced and the TDE increased to a value of 0.69 MJ
m-2 day-1, while the rainy case showed only 0.21 MJ m-2
day-1. However, when these were expressed as a daily
accumulated global solar radiation percentage ratio, the
cloudy case day exhibited the highest percentage difference
(6.53%), followed by the rainy case (6.38%), and clear case
(5.41%).
As a result, even though the TDE strongly affected the
measured global solar radiation on a clear day, the TDE
had a relatively higher effect on cloudy or rainy day. Thus,
the TDE must be considered in all weather conditions.
Correcting for the TDE in a solar observation instrument
should be considered necessary for global solar radiation
observations and the study of its application. In particular,
the observation of the TDE in the field of atmospheric
optics could provide an error range for the utilization of
global solar radiation data. However, the data used in this
study resulted from the analysis of intensive observation
data over only one month, and thus a generalization for all
global solar radiation instruments is problematic. Hence the
quantification of TDE should be considered as a prerequisite
for long-term observations and analysis.
ACKNOWLEDGMENT
This work was funded by the Korea Meteorological
Administration Research and Development Program under
Grant KMIPA 2014-21080.
This research was supported by Basic Science Research
Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education (NRF-
2017R1D1A3B03034467).
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