Glass in energy Glasses for solar energy I: low-e and solar control glass MAT 498 Lehigh University Rui M. Almeida Glass in energy Spring 2012 1
The use of glass in solar energy involves two general types of applications: - bulk glass applications, requiring specific optical, thermal and chemical glass properties, such as glass tubing in solar thermal concentrators (in concentrated solar power, CSP); - applications where glass is essentially a substrate for functional coatings (generally not glassy), which include again CSP (glass mirror substrates), but also low emissivity and solar control glass windows, solar panel glass windows, photovoltaic (PV) panels and photocatalytic (photochemical) self-cleaning glasses. The scale of solar systems ranges from power plants to individual power units. The four main applications which will be considered are, therefore: - solar control glass (namely low emissivity) - today s lecture 4 - thermal: including solar concentration (parabolic trough and flat heliostat mirror technologies) and solar hot water panels; - lecture 5 - photovoltaic (glass containing) panels, including solar panel glass windows; - lecture 6 - photochemical (namely photocatalytic, self-cleaning glass windows) Rui M. Almeida Glass in energy Spring 2012 2
The solar spectrum Rui M. Almeida Glass in energy Spring 2012 3
Adapted from: http://www.google.co.uk/imgres?q=solar+radiation+spectrum&hl=pt- (20 Jan 2012) Rui M. Almeida Glass in energy Spring 2012 4
From the second lecture, Kirchoff s law; its more general form is: T + A + R + S + K = 1 (Transmitted + Absorbed + Reflected + Scattered + emitted = 100 %) Sun light can be: transmitted, absorbed, reflected, scattered and re-emitted by a glass window. Scattering by architectural glass is negligible. Absorção por cromóforos The emissivity of a glass is the ability of a glass surface to radiate energy when its temperature rises above that of its surroundings. ssasw (Adapted from: The science and design of engineering materials, McGraw-Hill, 1999) Características ópticas dum vidro de silicato azul Rui M. Almeida Glass in energy Spring 2012 5 a absorção da luz no azul é devida a uma impureza, e.g., Co 2+,
Solar power density (W/m 2 m) Blackbody power density (W/m 2 m) Solar and thermal spectral distributions 1600 1200 Solar 80 60 800 Thermal 40 400 Visual 20 0 0 1 10 50 Wavelentgh (m) Blackbody @ 300 K Rui M. Almeida Glass in energy Spring 2012 6
Solar and thermal spectral distributions, and two idealized window transmission spectra Rui M. Almeida Glass in energy Spring 2012 7
Glass for a sustainable society Window coatings for energy efficiency Active coatings smart Thermochromic Photochromic Electrochromic Passive coatings non-switchable Anti-reflection Up/down converting Low-E Solar control Rui M. Almeida Glass in energy Spring 2012 8
Adapted from: Windows of the Future, Andre Anders (Lawrence Berkeley Laboratory), International Workshop on Glass for Harvesting, Storage and Efficient Usage of Solar Energy, Nov. 16-18, 2008, Pittsburgh, PA Rui M. Almeida Glass in energy Spring 2012 9
Adapted from: Windows of the Future, Andre Anders (Lawrence Berkeley Laboratory), International Workshop on Glass for Harvesting, Storage and Efficient Usage of Solar Energy, Nov. 16-18, 2008, Pittsburgh, PA Rui M. Almeida Glass in energy Spring 2012 10
Flat (window) glass is currently coated with passive coatings in order to reduce heat transfer. To improve the energy efficiency of windows, more and more commercial architectural glass is being coated with films which allow solar radiation to pass through, but reduce heat transfer (particularly the emissivity). Coating processes include spray pyrolysis, CVD, sputtering and microwave heating; the latter two can be applied online. With microwave heating, only the glass is heated and not the furnace atmosphere. Sol-gel is also a possible alternative. Rui M. Almeida Glass in energy Spring 2012 11
- Low emissivity glass - Solar control glass Rui M. Almeida Glass in energy Spring 2012 12
Radiations % Radiations % Two functions to reduce heat transfer Solar Control Low Emissivity Sun s IR rejected 100 75 50 25 Sun Radiations Visible light transmitted Coating Reflection Solar Control (Cool-Lite SKN155) 100 75 50 25 Sun Radiations Keep the heat inside Coating reflection Low-emissivity (Planitherm UN) Black Body at 300K 0 0,3 0,8 1,3 1,8 2,3 Wavelength (µm) 0 0,1 1 10 100 1000 Wavelength (µm) From: Chuck Anderson, Saint-Gobain Recherche, Montpellier EFONGA Workshop, 2009 Rui M. Almeida Glass in energy Spring 2012 13
Low-emissivity glass coatings for energy savings in buildings Substantial energy savings in buildings can be achieved by the use of double-glazed insulating glass windows (or IGU insulating glass units) which reduce heat transfer through the air/gas sandwiched layer and particularly by double glass pane windows coated on the inside with a low emissivity (low-e) coating. Such coating is usually deposited by spray pyrolysis (hard pyrolitic metal oxide coating), by CVD or by sputtering (soft multilayer coating formed by different types of alternating metal and metal oxide layers). Often, such coatings are made of F-doped tin oxide (SnO 2 :F), but they may also be made of other oxides like ZnO, In 2 O 3 or TiO 2, whereas the metal layers may consist of Ag, Al, Ti, Zn, Sn, Ni, Cr, or stainless steel. Rui M. Almeida Glass in energy Spring 2012 14
Radiations % Radiations % Solar Control Low Emissivity Sun s IR rejected 100 75 50 25 A low-e coating transmits shortwave visible light into the building (while absorbing UV and short-wave IR), but reflects long-wave thermal Sun Radiations Visible light transmitted radiation back inside the building (inside pane of glass). Coating Reflection Solar Control (Cool-Lite SKN155) 100 75 50 25 Sun Radiations Keep the heat inside Coating reflection Low-emissivity (Planitherm UN) Black Body at 300K 0 0,3 0,8 1,3 1,8 2,3 Wavelength (µm) 0 0,1 1 10 100 1000 Wavelength (µm) Adapted from: Chuck Anderson, Saint-Gobain Recherche, Montpellier EFONGA Workshop, 2009 Rui M. Almeida Glass in energy Spring 2012 15
glass In 2 O 3 :Sn Adapted from: Windows of the Future, Andre Anders (Lawrence Berkeley Laboratory), International Workshop on Glass for Harvesting, Storage and Efficient Usage of Solar Energy, Nov. 16-18, 2008, Pittsburgh, PA Rui M. Almeida Glass in energy Spring 2012 16
Reflectance / Transmittance / Normalised Solar Irradiance Sun ~ 0.3 3.0 μm Optical properties of Ag-based low-e coated glass 1 0.9 0.8 0.7 0.6 0.5 Solar T R 0.4 0.3 0.2 0.1 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 Wavelength (m) From: Chuck Anderson, Saint-Gobain Recherche, Montpellier EFONGA Workshop, 2009 Rui M. Almeida Glass in energy Spring 2012 17
IGU keeps the heat inside Low-E glass Low-E coatings are usually metal or metal oxides which reduce the amount of heat which enters or escapes the home through the windows. They also block UV rays, reducing fading to furniture, etc.. In cold climates, to keep the heat inside, the coating should be applied to the interior pane of the glass (as shown). In warmer climates, to keep heat from entering the home, the coating should be applied to the exterior pane (like in solar control glass). Rui M. Almeida Glass in energy Spring 2012 18
incorporating low-e glass coating Adapted from: Windows of the Future, Andre Anders (Lawrence Berkeley Laboratory), International Workshop on Glass for Harvesting, Storage and Efficient Usage of Solar Energy, Nov. 16-18, 2008, Pittsburgh, PA Rui M. Almeida Glass in energy Spring 2012 19 Q.
Radiations % Radiations % Solar control Solar Control Low Emissivity Sun s IR rejected 100 75 50 25 Sun Radiations Visible light transmitted Coating Reflection Solar Control (Cool-Lite SKN155) Keep the heat inside A solar control coating reflects near-uv and near-ir radiation from the sun, while transmitting the visible portion. 100 75 50 25 Sun Radiations Coating reflection Low-emissivity (Planitherm UN) Black Body at 300K 0 0,3 0,8 1,3 1,8 2,3 Wavelength (µm) 0 0,1 1 10 100 1000 Wavelength (µm) Adapted from: Chuck Anderson, Saint-Gobain Recherche, Montpellier EFONGA Workshop, 2009 Rui M. Almeida Glass in energy Spring 2012 20
IGU Sun s IR rejected Rui M. Almeida Glass in energy Spring 2012 21
While low emissivity often relies on coatings prepared by deposition in vacuum (namely by magnetron sputtering), sol-gel is a less expensive alternative technique to prepare solar control glass coatings for energy efficient windows in architectural, automotive and microwave applications. This method enables the preparation of multilayer coatings which have selective high reflectivity properties in the NUV and NIR domains. Rui M. Almeida Glass in energy Spring 2012 22
Objectives - To develop sol-gel glass coatings for solar control: only ~ half of the solar energy (the visible part) is transmitted, while the near-uv (NUV) and NIR are reflected as much as possible. - Using Bragg mirrors (BMs), photonic bandgap-type coatings with high reflectivity in: NUV and NIR. - These Bragg mirrors are multi-layered alternating silicate glass/titania coatings, prepared by sol-gel, with a single layer thickness x = λ c / 4n, λ c being the peak wavelength of the stop band of high reflectivity and n its refractive index. - Aim: to cover a glass window with two different BMs: (1) a NUV-reflecting BM and (2) a NIR-reflecting BM. In real life, the films will be prepared by sol-gel and deposited by roller- or spray-coating, e.g.. Rui M. Almeida Glass in energy Spring 2012 23
Using Bragg mirrors (BMs), which are 1-D photonic bandgap-type coatings with high reflectivity in: near-uv and near-ir. outside inside AS Aluminosilicate T - Titania Prototype (single pane) These BMs are multi-layered alternating aluminosilicate glass/titania coatings, prepared by sol-gel, with a single layer thickness x = λ c / 4n, with λ c being the peak wavelength of the stop band of high reflectivity. Rui M. Almeida Glass in energy Spring 2012 24
Interference filter (Distributed Bragg Reflector, DBR, or simply a Bragg Mirror, BM) a c n 0 a k a l= 2a Interference filter as 1-D PC stop band dielectric band Reduced zone scheme for optical dispersion relation d c n 0 d k Typical filter characteristics with stop band Rui M. Almeida Glass in energy Spring 2012 25
R % Distributed Bragg Reflector (Bragg mirror) n 1 n 2 1.0 l n 0 n s 0.8 d 1 d 2 1 2 N n d 1 1 n2d2 l / 4 0.6 0.4 0.2 Stop band R n ( n ) n ( n 2N 2N ( 0 2 s 1 2 ) 2N 2 n0 ( n2) n ( 1) N s n ) 0.0 1000 1500 2000 2500 Wavelength (nm) The higher the refractive index contrast and the more DBR periods, the higher the reflectivity. The individual (high and low index) layers are transparent. Rui M. Almeida Glass in energy Spring 2012 26
Simulations of BM spectral responses (Transfer Matrix Method) Rui M. Almeida Glass in energy Spring 2012 27
Rui M. Almeida Glass in energy Spring 2012 28 28 * S 11 S 11 n Si cav Si Si N k k k k k k k k k k k o o n n d n d n in d n n i d n n n S 1 1 2 cos 2 sin 2 sin 2 cos 1 1 1 1 l l l l The reflection spectrum of a BM structure, cav, can be calculated by the transfer matrix method (TMM). In the present case, only light at normal incidence was considered: where n Si is the refractive index of the (Si) substrate and S 11 is the (1, 1) element of the transfer matrix S, defined as: here n o = 1 is the refractive index of air, N is the total number of alternating layers, λis the wavelength of light and n k and d k are the refractive index and thickness of each layer. Transfer matrix method (TMM)
Si / (TS) 4 TMM simulation n S = 1.44 n T = 2.0 1 0.9 x S = 60 nm x T = 45 nm 0.8 % R 0.7 0.6 0.5 0.4 0.3 Short-λ BM normal incidence 0.2 0.1 0 300 350 400 450 Wavelength / nm Rui M. Almeida Glass in energy Spring 2012 29
Si / (TS) 4 TMM simulation n S = 1.44 n T = 2.0 1 0.9 x S = 175 nm x T = 125 nm 0.8 % R 0.7 0.6 0.5 0.4 0.3 Long-λ BM normal incidence 0.2 0.1 0 700 800 900 1000 1100 1200 1300 1400 1500 Wavelength / nm Rui M. Almeida Glass in energy Spring 2012 30
Sol-gel film processing 3 ml TEOS 6 ml isopropanol 0.48 ml 1% HCl stirring 3h at 70ºC stirring 1h aluminosilicate sol (Si:Al = 10:1) 2.5 ml Ti-isopropoxide 3 ml acetic acid 0.5 g Al(NO) 3 stirring 1h 12 ml ethanol stirring 1h ageing 24 h for spin-coating Substrates: SiO 2, Si or BS glass Heat treat @ 450 ºC titania sol Rui M. Almeida Glass in energy Spring 2012 31
% T Optical transmission of substrates 100 80 60 40 Fe 2+ silica (x = 1 mm) Glass A (x = 1mm) Borosilicate (x = 1 mm) Window glass (x=2 mm) 20 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) Rui M. Almeida Glass in energy Spring 2012 32
% T Optical transmission of BMs (NUV and NIR) 100 80 60 Glass A Glass A 3 NUV + 0 NIR 0 NUV + 3 NIR glass/t/s/t/s/t/s 40 20 stop band stop band 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) Rui M. Almeida Glass in energy Spring 2012 33
T Simulations of BMs spectral responses (NUV+NIR) 1,0 0,8 0,6 1 0,4 0,2 3 NIR 0 NUV + 3 NIR 1 NUV + 3 NIR 2 NUV + 3 NIR 3 NUV + 3 NIR 0,0 200 400 600 800 1000 l(nm) Rui M. Almeida Glass in energy Spring 2012 34 2 0 3
T Simulations of BMs spectral responses (NIR) targeted to 1 µm 1,0 0,8 0,6 0,4 0,2 0 NUV 0 NUV + 3 NIR 0 NUV + 4 NIR 0 NUV + 5 NIR 0 NUV + 6 NIR 0,0 200 400 600 800 1000 1200 1400 l(nm) Rui M. Almeida Glass in energy Spring 2012 35
% T Optical transmission of BMs (NIR) 3-5 pairs 100 80 N19-0UV+3IR N19-0UV+4IR N19-0UV+5IR glass/t/s/t/s/t/s 60 3 40 20 0 200 400 600 800 1000 1200 1400 Wavelength (nm) stop band (designed to peak @ 1 m) Rui M. Almeida Glass in energy Spring 2012 36 4 5
T Simulation 1,0 0,9 0,8 0,7 0,6 TMM 6 IV (ST)6 6 IV (ST)6+S 6 IV (TS)6 0,5 0,4 0,3 0,2 0,1 0,0 200 300 400 500 600 700 800 900 1000 1100 l (nm) Rui M. Almeida Glass in energy Spring 2012 37
% T 100 Glass substrate 80 60 40 glass/s/t/s/t/s/t... (2S+3T) 3 38 20 0 n23 (3iv) n24 (6iv) n25 (6iv+1s) Glass substrate 400 600 800 1000 l (nm) Nanostructured glass coatings for solar control (2S+3T) 6 (2S+3T) 6 +1S Rui M. Almeida Glass in energy Spring 2012 38
R % Reflection spectroscopy results 110 100 90 80 70 60 50 40 30 20 (2S+3T) 6 (2S+3T)6 +1S (2S+3T) 3 R10-N23 R10-N24 R10-N25 700 800 900 1000 1100 1200 1300 l (nm) Rui M. Almeida Glass in energy Spring 2012 39
Conclusions A solar control coating reflects near-uv and near-ir radiation from the sun, while transmitting the visible portion. The sol-gel method is a low-cost coating technology for this purpose. Good quality Bragg Mirrors may be prepared by solgel processing. The combination of multiple types of coatings on the same glass offers a possibility to obtain multi-functional window glass. Rui M. Almeida Glass in energy Spring 2012 40 Q.
References: Rui M. Almeida, Luís M. Fortes and M. Clara Gonçalves, Sol-Gel derived photonic bandgap coatings for solar control, Opt. Mater. 33 (2011) 1867 1871. Commercial glasses, Advances in Ceramics, Vol. 18, ed. D.C. Boyd and J.F. MacDowell, The American Ceramic Society (Columbus, Ohio, 1986). Rui M. Almeida Glass in energy Spring 2012 41