| Measuring Solar Energy Properties |
Author
Teo Wei-Boon, PhD.
PerkinElmer, Inc. 940 Winter Street Waltham, MA 02451 USA
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Measuring Solar Energy Properties of Architectural Glass, Mirrors and Photovoltaic Materials and Thermal Emissivity of Materials
The true potential of solar energy has yet to be harnessed. Free, clean and renewable the power of the sun has unlimited potential to be put in the far reaches of the world. At the same time, energy conservation will greatly help to reduce the consumption of resources reducing the negative impact on the environment and saving costs. Solar energy applications in the areas of energy conservation and energy production with solar concentrators, photovoltaics and solar thermal heaters can help to meet this goal. The characterization of the solar energy properties of architectural glass, mirrors and photovoltaic materials and thermal emissivity of materials used in solar energy applications are of great technological importance.The technology of solar energy applications are rapidly evolving and PerkinElmer has the tools and software to support the technological advancement.
There has been a phenomenal growth in the solar energy industry in the last few years mainly due to the negative impact of global warming, the high price of oil and the need for energy conservation. To improve the performance of solar devices, the solar energy properties of architectural glass, mirrors and photovoltaic (solar) materials have to be characterized and controlled to improve the design and efficiency of these devices.
Architectural glass with coatings allow builders to use glass as a material which participates actively in heating and cooling control. Efficient window design incorporates coated glass in a double or triple pane configuration. The transmission of visible light, total solar energy and longer wave thermal radiation can be controlled within wide limits to reduce energy consumption for the regulation of temperature in a building. Thermal emissivity of coatings on glazing has a huge influence on the thermal insulation (U-value) of windows. Measuring the performance of glass to determine the thermal and optical performance of window systems as well as to develop improved energy performance of building glass products is crucial to improving building’s energy efficiency.
Capturing sunlight which is an inexhaustible and nonpolluting energy to provide power and heating is especially desirable. However, the power density of sunlight after filtering through the atmosphere is less than 1000 W/m2 and is subject to great variations over different times of the day and during different seasons. To improve the efficiency of the devices to capture solar energy and to lower cost, a host of technical improvements are required in these devices. Measurements are required to find out the properties of the materials and designs so that quality can be maintained in production and further improvements can be made.
There are broad categories of these solar energy applications; energy conservation and energy production with solar concentrators, photovoltaics and solar thermal heaters.
1) Energy conservation can be implemented by use of architectural glass in buildings. Measuring the properties of architectural glass and understanding their use can dramatically improve energy efficiency by controlling the flow of energy in glass windows and walls in different environmental conditions.
2) In solar concentrators highly reflective parabolic mirrors focus sunlight onto an absorber tube. Fluid circulates through the tube, carrying heat to an electrical generator.
3) In a solar cell, sunlight that shines on a semiconductor device made of two semiconductor layers induces a voltage and produces a current to an external circuit connected to the solar cell.
4) A thermal solar collector collects heat from sunlight and the heat is carried away by tubes filled with a liquid.
PerkinElmer delivers solutions to the solar market with our high end Lambda UV-VIS-NIR spectrophotometers equipped with both the 150mm integrating spheres as well as absolute reflectance accessories such as the Universal Reflectance Accessory (URA) to do both transmittance and reflectance work. For architectural glass samples, a software helps to calculate the visible and solar related values. The Lambda instruments also play a key role in coating development, glass and polymer substrate design, long term performance and weather resistance. They are applicable to silicon and new generation thin film technologies and are used for both research as well as manufacturing. PerkinElmer offers an Infrared Emissivity Measurement System to aid in the measurement of low-E glazing in determining the thermal and optical performance of window systems as well as to develop improved energy performance of building glass products.
Figure 1. Lambda series spectrophotometer. Models 950/1050.
Figure 2. 150 mm integrating sphere.
The accurate transmittance or reflectance measurements of solid samples, such as glass are difficult when measured on an ordinary spectrophotometer as solid samples can refract or distort the beam, often drawing the beam off the proper spot on the instrument detector resulting in low reproducibility of measured values. The broad angle collection of light provided by an integrating sphere accessory allows accurate measurement of a wide range of both planar and non-planar samples for both transmittance and reflectance measurements by placing the sample at the transmission and reflection entrance ports of the sphere respectively. Often the sphere used has a diameter of 150mm, as it is often recommended in various ASTM, CIE, ISO, and DIN Protocols (Ref.1). The sphere is highly versatile and can perform many different measurements including %T, Diffuse %T, Variable Angle %T, Absorptance, %R , Diffuse %R, Specular %R, Haze, Solar Direct Transmittance, Solar Direct Reflectance, Solar Direct Absorptance, Solar Light Transmittance and also Solar Light Reflectance.
Figure 3. Top view of integrating sphere and rotation angle (Top left). Centre mount accessory (Top right). The graphic shows the change in absorption (AP=1-T-R) of a solar cell at 10°, 20°, 30°, 40°, 50°, and 60° incidence angles with hemispherical collection using the center mount for the 150 mm Integrating Sphere. The measurement is significant in that it measures how much solar energy is absorbed by the solar cell at various angles which is similar to stimulating the absorption of the solar cell with the movement of the sun across the sky during different times of the day.
Plain glass has high reflectivity as it becomes more mirror-like at high incident angles. The reflection losses due to the high reflectivity of glass together with weaker sunlight intensity due to atmospheric absorptions results in poor performance of the solar cell at high incident angles. The performance can be improved by using glass with a sandy smooth texture and some are even prismatically patterned to promote adhesion as well. This allows higher transmittance at acute incident angles to improve the performance of the solar cell in the morning and evening. Transmittance and reflectance measurements of solar glass are important measurements.
Figure 4. At high angles of incidence plain glass behaves like a mirror.
Figure 5. Patterned solar glass.
Figure 6. Diffuse reflection in patterned solar glass.
Figure 7. Measuring diffuse reflectance (top) and total reflectance(bottom) with the integrating sphere.
The specular reflectance of a silicon wafer can be obtained by measuring the diffuse reflectance and total reflectance with different configurations with the integrating sphere and then subtracting the diffuse reflectance from the total reflectance. The specular reflectance of silicon wafer is high as silicon has a high refractive index of 3.42. This means that a large portion of incident light is reflected away and does not enter the silicon wafer. This large loss must be minimized to improve the efficiency of the solar cell. Ways to increase the transmittance include texturing the surface of the silicon wafer and applying an anti-reflection coating.
Figure 8. Specular reflectance calculation with the integrating sphere of a silicon wafer. The measured total reflectance spectrum minus the diffuse spectrum yields the specular reflectance of the sample.
Solar cells can also be coated with an anti-reflection coating to trap light. Two reflected wavefronts can interfere with each other, depending on the ratio of the optical thickness of the material and the wavelength of the incident light. If a phase shift exists between the reflections, the reflected wavefronts interfere destructively and the overall reflected intensity is a minimum. This is the principle of an anti-reflection coating which is sometimes applied to solar glass and wafer to increase the transmission of incident light. The thickness of the anti-reflection coating will determine the transmittance characteristics of the coated material. The transmittance curve varies with the angle of incidence of the incoming light.
Figure 9. Anti reflection coating on glass.
The transmittance characteristics or haze measurement is important for polysilicon coated solar glass. Haze calculations are especially useful for quality control and specification purposes. The procedure used is based on the ASTM D1003-00 method “Standard Method for Haze and Luminous Transmittance of Transparent Plastics” (ref.2).
Figure 10. T1 Haze Measurement Arrangement.
In this haze measurement, the sample and white standard are placed in different arrangements and measurements made. The four measurements are described in the table below. T1 is the 100%T baseline.
|
| Measurement |
Position A |
Position B |
| T1 |
No Specimen |
White Standard |
| T2 |
Specimen |
White Standard |
| T3 |
No Specimen |
Light Trap |
| T4 |
Specimen |
Light Trap |
The formula to calculate Haze Value = [(T4/T2) - (T3/T1)] x 100%
Figure 11. Haze Measurement.
The calculated haze value of the polysilicon coated solar glass sample above = [(T4/T2) - (T3/T1)] x 100% = 9.10%.
A common application of UV-VIS-NIR spectrophotometers with integrating spheres is the measurement of solar absorptance, reflectance, and transmittance of materials (ref.1). Measurements of spectral near-normal hemispherical transmittance or reflectance are made over the spectral range from approximately 300 to 2500 nm. The solar transmittance, reflectance, or absorptance is determined by calculating a weighted average with a standard solar irradiance. Fig 14 shows the solar spectral curve.
Figure 12. Solar Spectral Curve.
The demand for functional glass with higher energy performance and improved durability has increased significantly. Better energy performance reduces the consumption of fossil fuel sourced electricity and therefore reduces greenhouse gas emissions. Buildings are the largest sector of energy use in the EU, accounting for about half of delivered energy use. The dramatic increases in energy efficiency achieved by low-E and solar-control glass (a nearly twofold increase in the R value of a dual-pane window over uncoated glass) are due entirely to sophisticated application of multiple coatings. The R value (reciprocal of U value) is a measure of thermal resistance or building insulation’s effectiveness. Window performance is crucial to a building’s energy efficiency. Approximately 30 to 40% of building energy consumption is associated with heating, ventilation and air-conditioning (HVAC) and lighting systems. Determining the thermal and optical performance of window systems are essential to researchers striving to develop improved products and to window manufacturers who need to demonstrate the energy performance of their products to architects, engineers, builders, and the general public.
PerkinElmer has an architectural glass software where the transmittance and reflectance spectra of the different layers of glass are used to calculate the light transmittance, light reflectance and light absorptance. With the use of a table derived from the solar spectral curve, the solar direct transmittance, solar direct reflectance, solar direct absorptance under various norms can also calculated.
Figure 13. Screenshot of architectural software to input transmission and reflection spectra.
Figure 14. Screenshot of architectural software showing radio buttons to select glass value calculations.
Low e-glass example
Figure 15. Spectra of low e-glass in transmission and reflectance of both glass and coating side.
Figure 16. Screenshot of results for low e-glass sample is shown above.
Two key window energy performance numbers are the U-factor and the Solar Heat Gain Coefficient (SHGC). The U-factor of a window accounts for conduction, radiation, and convective heat transfer from the warm side to the cold side of the window. A lower U-value means a better-insulated window.
The Solar Heat Gain Coefficient accounts for radiant heat transfer from the sun through the window. A lower SHGC means less solar heat passes through the window. An older term also used is the Shading Coefficient. Visible Light Transmittance - the amount of light that is passed through the window into the building is another important parameter for architectural glass.
For comparison
- Aluminium Foil: No Visible Light Transmittance, Low U-factor
- Normal Glass: High Visible Light Transmittance, High U-factor, High Solar Heat Gain Coefficient
- Low E-Glass: High Visible Light Transmittance, Low U-factor, Low Solar Heat Gain Coefficient
- Solar Glass: Very High Visible Light Transmittance, High U-factor, High Solar Heat Gain Coefficient
Figure 17. Screenshot of U-factor, Shading Coefficient and Specific Heat Gain Coefficient calculations of a double pane glass window using the Lawrence Berkeley National Laboratory’s Windows5 software is shown above.
In solar concentrators highly reflective parabolic mirrors focus sunlight onto an absorber tube and a fluid that circulates through the tube, carries heat to an electrical generator. The device is efficient at many solar angles during much of the day, so it can be mounted in a fixed position. For this device, the reflectance of the mirrors are of importance. Mirrors made of different materials have different reflectance profile. Besides the coating material, the coating process affects the reflectance as well. Mirrors can be coated on the front or second surface. A second surface mirror offers better weather protection and ease of cleaning, but the sunlight must pass through the glass. The optical quality of the glass becomes important.
Figure 18. Reflection spectra of aluminium and silver coated mirrors.
Figure 19. Front surface or second surface?
There are different accessories to measure the specular reflectance of a flat mirror. Some designs measure the relative reflectance while others measure the absolute reflectance of the surface. In a relative reflectance measurement, the values are dependent on the spectral characteristics of a reference material. Absolute reflectance measurements are independent of any other reference material or standard and provide the true ”absolute“ amount of reflected radiation. Amongst the various accessories offered by PerkinElmer, one that stands out above the rest is the Universal Reflectance Accessory (URA). The URA is not only able to measure the absolute reflectance and relative reflectance, it also measures the sample automatically and reproducibly for many angles (range of angles 8 – 68° at 0.5 degrees intervals) without manual adjustments as well.
Figure 20. Sample on Universal Reflectance Accessory.
The URA software has a sample table that allows sampling angles to be pre-set and the URA makes all the adjustments automatically. With a single accessory it can perform multi-angle measurements which previously require several accessories. In addition, the sample is placed on a horizontal sampling plate that is very easy to use without the need for clamping.
Figure 21. URA Software – sample table control.
The graph below shows the reflectance spectra of aluminium mirror at various angles. The spectra vary depending on the angle of reflection. This analysis provides important information on the reflectance efficiency of the mirrors at various angles which simulate the positions of the sun at various times of the day. A depolarizer which can be mounted on the UV-VIS spectrometer provides accurate reflectance measurements.
Figure 22. Reflection spectra of aluminium mirror at various angles.
In a solar cell, sunlight that shines on a semiconductor device made of two semiconductor layers can be converted instantaneously into direct-current electricity. The incident photons must have energy equal to or slightly greater than the band gap energy for the absorbing material. The band gap is the energy needed to move an electron from its ground state in the valence band to the conduction band.
Many technical limitations that affect the efficiency of these devices must be overcame to make these devices more readily acceptable.
Figure 23. Light absorbed near the p-n junction creates voltage potential.
When a light beam is incident on the surface of a sample, reflection, absorption, scattering and transmission can occur. For a homogeneous sample with smooth parallel surfaces the relationship is simply given by
Reflection + Absorption + Transmission = 1
Figure 24. Reflection, Absorption, Transmission and Scattering from a sample.
Reflectance has to be kept as small as possible so that the maximum amount of light can enter the solar cell. The surface texture of the surface has considerable influence on the reflectance of the wafer. Fig 27 shows the reflectance spectra of a smooth and rough side of a silicon wafer. The smooth side shows higher reflectance compared to the rough side which means greater light reflection losses. The reflection loss can be monitored at specific wavelengths like the peaks at around 273 and 366 nm or at certain wavelengths on the baseline e.g 500 or 600 nm. An anti-reflection coating can help to lower the reflectance considerably.
Figure 25. Reflection curves from a smooth and rough surface of a silicon wafer.
Not all solar cells are made from silicon. Thin film solar cell made from depositing copper, indium, gallium and selenium onto a glass substrate to form a complex heterojunction system commonly called CIGS thin film solar cells which are also measured for their transmittance and reflectance characteristics. See spectra below.
Figure 26. Transmission and reflection spectra of CIGS thin film on glass.
One of the most common measurements made is the determination of band gap properties of a material. This relates to the ease in which a semiconducting material will transfer electrons to its conducting band.
Figure 27. The band gap refers to the energy needed to move an electron from its ground state in the valence band to the conduction band.
Figure 28. Transmission spectra of silicon and CdTe thin films and band gap calculations.
The infrared spectrum of undoped silicon has many absorption bands due to vibrations of the silicon lattice called phonon bands. In highly doped n or p silicon free carrier absorption can be observed. The free carrier absorption starts at the long wavelengths and the absorption edge moves to shorter wavelengths with increasing concentrations of the free carriers. Similarly a thin film of CdTe shows the absorption edge. From the cut off wavelength the band gap energy can be calculated. This is fundamental to research on new materials. The onset of the absorption spectrum is the positive inflection point from the tangent line defining the baseline of the spectrum at low energy.
Band gap energy is
Eg = hc/λg = (6.626 X 10-34 Joule Sec) (2.998 X 1017 nm s-1)( 1 eV/ 1.602 X 10-19 Joule)/λg = 1240/λg eV
where λg is in nm and Eg is in eV.
h is Planks constant, c is speed of light and λg is cut off wavelength.
Using the equation above we calculate the band gap energy for silicon and CdTe from the spectra above to be 1.1 eV and 1.5 eV respectively.
Fringes can be observed in both the thin crystalline film of silicon and CdTe which are interference patterns caused by reflection within the film layer.
A solar cell consists of many thin film layers and a complex spectrum is formed on reflection.
Figure 29. Softwares such as FilmStar can help to do design and analysis of optical thin films of these multilayers.
Softwares such as FilmStar can help to do design and perform analysis of optical thin films of these multilayers.
A single solar cell with one junction can only convert a portion of the incident sunlight into useful energy. To improve the solar cell conversion efficiency, several solar cells with different band gaps can be stacked in tandem to increase the wavelength range of absorption.
Figure 30. Triple Junction Solar Cell Design.
Emissivity is the ability of a surface to reflect long-wave radiation. Low emissivity glass coatings are designed to reflect long-wave radiation, thereby improving the thermal performance of the window as measured by the U-factor. The lower the emissivity, the greater the resistance to heat loss through the window which provides better winter performance.
For comparison
- Aluminium Foil: No Visible Light Transmittance, Low U-factor, Emissivity around 0.05
- Normal Glass: High Visible Light Transmittance, High U-factor, High Solar Heat Gain Coefficient, Emissivity around 0.84
- Low E-Glass: High Visible Light Transmittance, Low U-factor, Low Solar Heat Gain Coefficient, Emissivity range from 0.35 to 0.04
Thermal emissivity of coatings on glazing has a big influence on the thermal insulation (U-value). There are increasing national and international regulations for use of low-E glass to lower energy consumption .There are international standards for measuring IR emissivity such as EN673 (ref.3). To determine the emissivity according to EN673 the normal reflectance in the wavelength range 5.5 µm - 50 µm (ca 1820 to 200 cm-1) is first measured. The total normal emissivity using the relation e = 1 – R is calculated and then the corrected emissivity calculated. The accuracy and scope of the EN673 has been questioned – largely due to reproducibility of FT-IR systems. However detailed studies have been performed to understand the accuracy of the IR method (e.g THERMES project in Europe). As a result an improved protocol has emerged which is published in reference 4. A specific hardware design has emerged to deliver improved measurement.
Figure 31. The IR Reflection Set.
The IR reflection set from PerkinElmer is a complete solution for IR emissivity measurements. It is the only hardware system designed specifically for this application. The system can accommodate a huge range of sample sizes. It comes with a unique builtin easy alignment system and comes with specially calibrated gold standards set. A software worksheet for calculation of emissivity values is shown below.
Figure 32. Emissivity Calculation Program Included.
Figure 33. THERMES Round Robin Test Sets – High to Low-E.
The THERMES round robin test sets are shown above. The study established that the variation between samples are all below 1% for high to Low-E samples and are highly reliable.
Attic foils reflect and emit the sun’s energy as light and heat back to the sky instead of allowing it to enter the building as heat. This cools the roof and conserves electricity used for air conditioning. The two basic characteristics that determine the properties of a cool roof are the solar reflectance and thermal emittance. The Cool Roof Rating Council in the US measures these properties for roofing products (ref.5). Cool roof requirements can be part of building energy codes or green building programmes. The IR emissivity set can help determine the emittance value of the foil (see Notes).
A thermal solar collector collects heat from sunlight and the heat is carried away by tubes filled with a liquid. Thermal properties are important: thermal emissivity to trap heat and thermal conductivity to absorb heat. Solar radiation comes in as visible light, passes through the glass and heats up the absorber. It then radiates at a long wavelength to the glass. There is also convection from the plate to the glass. The glass heats up and radiates back to the sky at long wavelengths. There is also convection to the ambient. Below the absorber (back side), there is conduction through the insulator and then convection to the atmosphere, ignoring radiation from the back side.
Figure 34. Solar thermal collector to collect heat from sunlight.
There are many factors involved in the design of a good solar thermal collector; the glass transmittance and reflectance and also absorber plate conductance and insulation. One of the important parameters to consider is the emissivity of the glass and absorber plate materials used. The emissivity set from PerkinElmer can help to measure the characteristics of the glass and absorber plate.
The characterization of the solar energy properties of architectural glass, mirrors and photovoltaic materials and thermal emissivity of glass coatings used in solar energy applications are of great technological importance. Solar properties of materials can be better controlled, production costs can be lowered and technical improvements can be made. The technology of solar energy applications are rapidly evolving and PerkinElmer has the tools and software to support the technological advancement.
References
|
| 1 |
ASTM E903-96 Standard Test Method for Solar Absorptance, Refl ectance and Transmittance of Materials Using Integrating Sphere. |
| 2 |
ASTM D1003-00 Standard Test Method for Haze and Luminous Transmittance of Transparent Plastic. |
| 3 |
EN673: 1998 – Glass in Building. Determining the thermal transmittance (U value). Calculation Method. |
| 4 |
Thin Solid Films, 502(2006) 164-169. |
| 5 |
Cool Roof Rating Council – www.coolroofs.org |
| |
Notes: The terms emittance and emissivity are often used interchangeably, however, we use emissivity to refer to the properties of a material but emittance to the properties of a particular object (attic foil) including oxidation and surface fi nish in this article. |