1

Methods for Measuring Solar Radiation

Eldwin Djajadiwinata1*

Student ID: 434107763

1 Graduate Studies, Mechanical Engineering Department, College of Engineering,

King Saud University, Kingdom of Saudi Arabia.

ABSTRACT

This report is part of Radiative Heat Transfer course, PhD program, at M.E. Dept.,

College of Engineering, King Saud University. It focuses on describing the methods for

measuring the most common radiation-quantity measured on a weather station, i.e., direct

normal irradiation (DNI), diffuse horizontal irradiation (DHI), and global/total horizontal

irradiation (GHI). It starts with giving a review on the benefits of solar energy and some of its

terminologies. Afterwards, general explanation about types of solar radiation measurement

instruments is presented. Finally, techniques to measure the three aforementioned quantities

(DNI, DHI, and GHI) are explained in detail which encompasses the instruments, the

components, and the way these instruments work. Summary is given in the end.

Keyword: Solar, radiation, instrument, measurement

*Corresponding author: Eldwin Djajadiwinata; email: eldwin_dj@yahoo.com

Telp: +966-530823159

1. INTRODUCTION

Energy saving topic gains more and more attention globally. This is a logical

consequence of the increase of people awareness all over the world regarding the limited

fossil energy resources. It keeps depleting and eventually will become extinct.

In order to solve this issue, many researchers have been searching for renewable

energy resources as well as developing new efficient technologies to utilize that energy.

Furthermore, they also struggle to increase the energy efficiency of the existing technology to

slow down the depletion of the non-renewable energy resources.

2

There are several sources of renewable energy, namely, solar energy, biomass, wind,

geothermal, and water. Solar energy, especially in places having high solar intensity and clear

sky most of the day, has been one of the most prospective renewable energy. It becomes so

attracting due to several reasons such as:

1. Solar energy is free in the sense that one does not need to pay to utilize what is

emitted from the sun.

2. It is clean which means it does not result in any pollution.

3. Solar energy is the most abundant energy resource on earth.

4. It can be used not only for power generation purpose but also as heat source for

any processes that require thermal energy.

In order to utilize solar energy, the solar radiation intensity trend should be assessed

throughout the year so that the feasibility for utilizing it is confirmed in the sense that the

amount of solar energy received is relatively high and stable throughout the year. Therefore,

measurement of solar radiation is one of the most important aspects in the solar energy

application. This report focuses on describing the methods for measuring the most common

radiation-quantity measured on a weather station, i.e., direct normal irradiation (DNI), diffuse

horizontal irradiation (DHI), and global/total horizontal irradiation (GHI). These

terminologies will be explained below.

Some of the solar radiation entering the earth's atmosphere is absorbed and scattered.

Solar radiation which comes directly (through a straight line) from the sun onto a surface is

called direct beam irradiation. Here, the word irradiation means radiation incident on a

surface. Solar radiation which is scattered out of the direct beam by molecules, aerosols, and

clouds and is incident on a surface is called diffuse irradiation. The sum of the direct beam,

diffuse, and ground-reflected radiation arriving on the surface is called total or global solar

irradiation (Figure 1). They are presented in terms of power per unit area of the surface, i.e.,

Watts per square meter.

3

Figure 1: Illustration of the solar radiation components traveling

into the earth’s atmosphere [1]

If one considers irradiation on a horizontal surface, the ground-reflected irradiation is

zero. Thus, the global irradiation only consists of the direct beam and diffuse irradiation. The

global irradiation and diffuse irradiation on a horizontal surface are called global horizontal

irradiation (GHI) and diffuse horizontal irradiation (DHI), respectively.

The direct or beam irradiation is usually measured by detectors oriented perpendicular

to the sun’s ray direction. This means that the value obtained corresponds to irradiation on a

surface perpendicular to the sun which is called direct normal irradiation (DNI). In order to

relate DNI value to the amount of direct irradiation on a horizontal surface, it needs to be

multiplied by the angle of incidence ( ), i.e., angle between the sun’s ray direction and the

normal vector of the surface. For horizontal surface, the angle of incidence is equal to the

solar zenith angle and, hence, the DNI can be multiplied by the solar zenith angle instead.

The relation between GHI, DHI, and DNI can be written as follows:

(1)

4

Usually, a complete solar radiation monitoring station has instrumentation for

measuring the following three quantities [2]:

1. The global irradiation on a horizontal surface (GHI)

2. The diffuse irradiation on a horizontal surface (DHI)

3. The direct normal irradiation (DNI)

Having data of these three quantities, one will be able to understand the solar resource and

able to assess the quality of the data rigorously.

2. GENERAL OVERVIEW OF MEASUREMENT INSTRUMENTS

Instruments to measure solar radiation are generally divided into two basic types, i.e.,

pyranometer and pyrheliometer [3]. The former is used to measure the global solar irradiation

on a surface while the latter is used to measure the direct normal solar irradiation on a

surface.

These instruments utilize a certain detector to sense/detect the solar radiation. Among

the available types, thermoelectric and photoelectric detectors are the ones widely used

recently.

The thermoelectric detectors exploit the principle of thermocouples. Basically, it is a

series of thermocouple junctions called thermopile which generates a voltage proportional to

the temperature difference between the hot junctions and the cold junctions. The meaning of

cold and hot junction can be found in thermocouple specific literature. Furthermore, this

temperature difference is proportional to the solar irradiation hitting the detector which,

consequently, leads to voltage vs. solar irradiation relation. In other words, since the relation

is proportional, higher solar irradiation corresponds to higher voltage output [3].

The photoelectric detectors mainly utilize photovoltaic effect of a certain material

(solar cell) which is usually silicon solar cell. The advantage of this detector is that it is cheap

and has much faster response (on the order of microseconds) to solar irradiation change

(small time constant) compared to that of thermoelectric detectors (1 – 5 s). Its spectral

sensitivity, however, generally limited to only the visible and near infrared spectral regions

from about to . It does not respond fraction of solar radiation outside this

wavelength range. On the other hand, thermoelectric detector senses solar radiation of

5

wavelengths between about and which is broader compared to that of

photoelectric detector [1].

3. THE TYPES OF SOLAR INTENSITY MEASUREMENT

It is important to measure solar radiation intensity in order to confirm its feasibility to

be harnessed at a certain location and, if it is found feasible, the measurement result is used to

construct a suitable and more efficient solar energy system at that location.

As mentioned in the introduction section, there are three main solar radiation

quantities that need to be obtained, namely, direct normal irradiance (DNI), global horizontal

irradiance (GHI), and diffuse horizontal irradiance (DHI). Actually, knowing any two of

them, one will be able to get the third data via mathematical relation between them as shown

in Eq. (1).

Despite having such mathematical relation, it is possible and preferable to obtain all

of these solar radiation data, i.e., DNI, GHI, and DHI, independently through measurement

devices which will be presented in the following subsections.

3.1. Measurement of Direct Normal Irradiation (DNI)

Measurement of DNI is done using pyrheliometer. The typical geometry of a

pyrheliometer available in the market is shown in Figure 2. It consists of a long tube

(component no. 3) with the detector located on the base of this tube.

Figure 2: Typical geometry of pyrheliometer [4]

6

Figure 3: Picture (left) and schematic diagram (right) of pyrheliometer mounted on a tracker

with thermoelectric (a series of thermocouples) detector [5]

This long tube, also called collimated tube, limits the view of the detector so that it

captures all the solar irradiation coming from the sun’s disk and only small amount of the

diffuse irradiation. This will be explained in more detail later.

In order to allow direct sun’s rays to be intercepted by the detector, pyrheliometer

must be pointed towards the sun all the time. The front sight and back sight, also called

alignment indicator (component no. 2), must be aligned with the sun’s disk to ensure that the

sun’s rays and the detector are perpendicular. This explanation is also illustrated in Figure 3.

Still in Figure 2, the tube-end facing the sun is covered with quartz window

(component no. 5) to protect the detector and other inner components from dust and any

destructive objects. The output voltage can be measured on the output cable (components no.

6 and 7) and multiplied by the voltage-to-irradiation constant provided by the manufacturer to

get the irradiation value. Humidity indicator as well as desiccant (component no. 1) is

7

provided to keep the cylinder dry which will prevent condensation on the inside of the

aperture window. Rain screen, component no. 4, can be included as an additional feature.

Since the sun is moving, pyrheliometer should be mounted to a two-axis tracker to

keep its direction to the sun [6]. The movement of the first axis is to account for the earth

rotation and the second axis movement is to follow the change in declination angle. Figure 3

shows pyrheliometer mounted on a two-axis tracker.

Previously, it has been explained that the long-tube (collimated tube) limits the view

of the detector so that it captures all the solar irradiation coming from the sun’s disk and only

small amount of the diffuse irradiation. This diffuse irradiation comes from the forward-

scattered radiation near the solar disk (also called circumsolar radiation). The following is

further explanation regarding this which is quoted from NREL’s technical report [1] and is

illustrated as well in the previous figure (Figure 3):

“The World Meteorological Organization (WMO) defines DNI as the amount of

radiation from the sun and a narrow annulus of sky as measured with a pyrheliometer

designed with about a 5-degree field of view (FOV) full angle. In the absence of scattering by

the atmosphere, the sun would appear to have a diameter subtending a 0.5-degree FOV.

Therefore, DNI includes the forward-scattered radiation near the solar disk (also called

circumsolar radiation). The effects of this scattering are as variable as the composition of the

atmosphere at the time of observation.”

The detectors used may be either the thermoelectric type or the photoelectric type

depend on the accuracy and response time required. As have been addressed in section two

that if high accuracy is needed, the thermoelectric type is recommended and, on the other

hand, if low response time (faster response) is more important, photoelectric detector should

be chosen.

3.2. Measurement of Global Horizontal Irradiation (GHI)

Pyranometer is used to measure the GHI. The detector of pyranometer can be either

thermoelectric type or photoelectric type. First, the thermal electric type will be explained.

Figure 4 shows the components (pointed by numbers) of thermoelectric type

pyranometer. It has identical working principle with that of thermoelectric Pyrheliometer except

that the detector is exposed to global radiation. As in thermoelectric-type pyrheliometer, the

thermoelectric detector (component no. 4) is coated by an optically black material which

allows all solar radiation of wavelengths ranging from to to be sensed with

uniform spectral response.

8

Figure 4: Typical geometry of pyranometer where the above components (3, 5, and 6) shown

cut to half [7]

The sun screen or white guard disk (component no. 6) is installed to ensure that the

acceptance angle is (hemisphere solid angle). The inner dome and outer dome,

component no. 3 and 5, respectively, shield the detector surface from wind and rain. They are

also used to reduce the convection heat transfer effect on the reading. Furthermore, these

domes can also be used to filter the required radiation wavelength range. If the GHI is

required, the domes should be designed so that wavelengths ranging from to are

transmitted onto the detector.

The output voltage can be measured via the output cable (components no. 1 and 11)

and multiplied by the voltage-to-irradiation constant provided by the manufacturer to get the

irradiation value. Humidity indicator and desiccant holder (component no. 7 and 8) is

provided to keep the environment inside the dome dry. This is to avoid condensation inside it

which will affect the reading.

To ensure that the detector is laid horizontally, leveling feet (component no. 9) and

bubble level (component no. 10) are provided. Component no. 1 is a tool to fix the white

guard disk on its place (component no. 6).

9

The photoelectric type pyranometer has diffuser above the detector (Figure 5 and

Figure 6) in order to reduce the cosine error which meaning is explained as follows.

Pyranometer’s detector should be able to sense different incidence angle of irradiation. The

incidence angle is angle between sun’s rays and the normal of the detector’s surface. The

response should be maximum when the incidence angle is and minimum when it is .

The change in response from to should follow a cosine function. However, the actual

response of photoelectric pyranometer, unfortunately, is not exactly equal to a cosine function

which discrepancy is called cosine error [8]. Moreover, the diffuser also acts as a protective

layer to the detector.

Figure 5: Photoelectric-type pyranometer mounted on anodized-aluminum mounting and

leveling fixture [9]

10

Figure 6: Schematic diagram of a type of photoelectric pyranometer

(source: [8], with some modification)

3.3. Measurement of Diffuse Horizontal Irradiation (DHI)

The diffuse horizontal irradiation (DHI) is obtained also by means of pyranometer.

However, the direct or beam irradiation component must be excluded from the reading by

blocking the direct irradiation from reaching the detector. In order to do this, one can use

either a small shading disk/ball in conjunction with a sun tracker (Figure 7 and Figure 8) or,

alternatively, a shadow ring aligned with the sun’s path (Figure 9). The first technique

requires sun tracker because the blockage is only using a small disk/ball sufficient only to

shade the pyranometer’s detector. Therefore, a tracker is needed so that this small disk/ball

always follows the sun and, consequently, the pyranometer is shaded all the day. It should be

mentioned that the pyranometer should be mounted horizontally on the same tracker where

the shading disk/ball is attached.

The second technique does not need tracker since the ring itself covers the sun’s path

within a day. However, it needs to be adjusted once a day or once per several days, depends

on what month of the year it is, to take into account the change in solar declination angle.

11

In both techniques there is a small amount of diffuse radiation which is blocked by the

arm of the disk/ball or by the shadow ring. Therefore, a correction must be made to the output

data to take into account for this blockage effect.

Figure 7: Meteorological station which measures the DNI, GHI, dan DHI. Shading disk (hold

by an arm) is used to shade the pyranometer from direct irradiation.

12

Figure 8: Illustration of how to measure the DNI, GHI, and DHI. Here, the DHI is measured

using small shading disk/ball technique.

Figure 9: Pyranometer shaded using a shadow ring to block the direct irradiation

13

3.4. Measurement of GHI and DHI Using Rotating Shadowband Technique

The rotating shadowband technique uses photoelectric (such as silicon photodiode)

pyranometer to measure, both, the GHI and DHI. The pyranometer is momentarily shaded

periodically by a moving shadowband. The data recorded before and after shaded is the GHI

data while during it is shaded is the DHI data. From these two data, the DNI data is calculated

using Eq. (1). This technique utilizes the fast time-response characteristic of the photoelectric

detector to record the DHI during the momentary or short-time shading period. Figure 10

illustrates the concept clearly. An example of rotating shadowband radiometry available in

the market is shown in Figure 11 [10].

Figure 10: Rotating shadowband technique output during shadowband motion [10]

14

Figure 11: Schematic diagram of Rotating shadowband radiometer manufactured by

Irradiance, Inc. [10]

3.5. Effects of Dust/Contamination on Thermoelectric and Photoelectric Pyranometer

Maintenance is one of the aspects that should be considered in choosing the type

instruments. Usually the choices are made based on whether the thermoelectric detector type

is used or the photoelectric ones. A side from the accuracy and fast time-response

considerations, there is another consideration that should not be overlooked, i.e., the ease of

maintenance. This really depends on where the sensor is located (the site) and the availability

of man-power to conduct the periodic maintenance.

Before that, it should be addressed that the thermoelectric type of instruments are

prone to the effect of dust or contamination attached to the quartz window for pyrheliometer

or to the quartz dome for pyranometer. Hence, the measurement uncertainty will increase

significantly by many-folds due to dust/contamination. On the other hand, the photoelectric

type of instrument (the silicon-photodiode pyranometer and rotating shadowband system) has

less susceptible to dust/contamination attached on it. This could be because fine dust on the

surface of a diffuser can become an integral part of the diffuser, and may lessen the impact of

15

the dust on the diffuser transmittance compared to that on optical quartz window or optical

quartz dome [1].

Based on the aforementioned facts, the photoelectric type of instruments is more

preferable if the measurement station is located at a remote area such that it is difficult to

maintain (clean) on a daily basis. It will not be optimal to have high accuracy measurement

instruments while it is not well maintained as the recorded data will not be accurate due to

dust/contamination [1].

4. SUMMARY

The measurement methods for measuring the most common radiation-quantity

measured on a weather station, i.e., DNI, GHI, and DHI have been explained. DNI and GHI

are measured by means of pyrheliometer and pyranometer, respectively. The DHI is also

measured using pyranometer with the difference that the direct irradiation component is

blocked while it is not when measuring the GHI. There are two types of detectors used for

pyrheliometer and pyranometer. One type is thermoelectric type and the other is photoelectric

type. The accuracy of the former is higher than that of the latter. However, time-response

wise, it is the opposite. Thus, the decision on which one to be used is based on what the main

concern of the measurement is; the accuracy or the fast time-response.

A relatively newer technique compared to the aforementioned ones to measure GHI

and DHI is the rotating shadowband technique. The rotating shadowband technique uses

photoelectric (such as silicon photodiode) pyranometer to measure, both, the GHI and DHI.

From these two data, the DNI data is calculated using Eq. (1). This technique utilizes the fast

time-response characteristic of the photoelectric detector to record the DHI during the

momentary or short-time shading period.

Finally, maintenance aspect is considered to choose between the thermoelectric type

of instruments (thermoelectric pyrheliometer and pyranometer) and photoelectric type of

instruments (photoelectric pyranometer and rotating shadowband system). The former is

more susceptible to dust/contamination on its accuracy compared on that of the latter.

Therefore, despite its high accuracy, using the thermoelectric pyrheliometer and pyranometer

may not be beneficial for a remote location where daily maintenance is not possible as the

reading’s accuracy will drop quickly by many-folds.

16

REFERENCES

[1] Stoffel, T., Renne, D., Myers, D., Wilcox, S., Sengupta, M., George, R., and Turchi, C.,

2010, "Concentrating Solar Power, Best Practices Handbook for the Collection and Use of

Solar Resource Data," Technical Report No. NREL/TP-550-47465, U.S.A.

[2] What Parts of Solar Radiation Are Measured?, Last accessed Dec. 31, 2014,

http://rredc.nrel.gov/solar/pubs/shining/chap4.html#fig3

[3] Goswami, D. Y., Kreith, F., and Kreider, J. F., 2000, Principles of Solar Engineering, 2nd

Edition, p. 63-69, Taylor & Francis, U.S.A.

[4] Hukseflux First Class Pyrheliometer, Last accessed Dec. 30, 2014,

http://www.hukseflux.com/sites/default/files/product_brochure/DR01_v1418.pdf

[5] 3rd Energy Engineering Solution, Last accessed Dec. 31, 2014,

www.pesitsouth.pes.edu/pdf/mech/3rd_energy_engineering_solution.pdf

[6] Paulescu, M., Paulescu, E., Gravila, P., Badescu, V., 2013, Springer, Weather Modelling

and Forecasting of P.V. Systems Operation, Ch. 7, Page 20.

[7] Hukseflux Secondary Standard Pyranometer, Last accessed Dec. 31, 2014,

http://www.hukseflux.com/sites/default/files/product_brochure/SR20_v1405.pdf

[8] Martinez, M. A., Andujar, J. M., and Enrique, J. M., 2009, "A New and Inexpensive

Pyranometer for the Visible Spectral Range," Sensors, 9(unknown), pp. 4615-4634.

[9] Pyranometer Light Sensors, Last accessed Dec. 31, 2014,

http://www.licor.com/env/pdf/light/RMB.pdf

[10] Irradiance, Rotating Shadowband Radiometer, Last accessed Dec. 31, 2014,

www.irradiance.com/sites/default/files/RSR_Brochure_080122.pdf