Is it smart to use smart windows?
Investigating the use of Electrochromic glass system in a cold climate countries AR0531 Innovation & Sustainability AR1B025-D3 BT Research Methodology
Ankur Gupta 4732960 a.gupta-13@student.tudelft.nl 26.10.2017 Number of words: 1666 (max. 2500, excluding figures tables and appendices on a maximum of 8 pages A4, font Cambria 11 pt)
Focus and restrictions – Effect of electrochromic coating on glass in office buildings.
Abstract – Due to increased carbon emission rate through production of excessive
architectural materials, it has become essential to rationalise the use of upcoming technologies based on their application in different climatic conditions. This paper describes and evaluates the performance of Electrochromic windows. At a first glance it can be assumed that EC windows are useful for any climatic or exterior façade condition. But, this paper shows why it might not be ideal to use electrochromic glass windows in a cold climate country. To justify this several current research articles and review papers were collected and studied. It was then concluded that based on current state-of-the-art, use of electrochromic windows in a hot climatic condition would result in better energy conservation compared to its use in cold climate countries.
Key words – electrochromic, liquid crystal, adaptive facade, climate responsive, thermal comfort, smart material, glass façade, AR0531
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1 Introduction In today’s time heating and cooling of a building is the most controversial thing with respect to the amount of energy goes into it. We can also see that the use of glass on external façade in cold countries (where you need sun to heat the buildings) is extensive than warmer countries (where the cooling load is extensive). It is also proven that energy required for cooling a building is more than heating (Khandelwal et al., 2015). To tackle this increased demand for energy, innovation architectural material is necessary for a sustainable future. Till now, there have been many options in market to deal with the cooling/heating loads of the building by the use of double-glazed glass windows, glazing incorporated blinds, curtains, external shading devices etc. These types of alternatives has a lower heat transfer rate (hence, cooling/heating loads of the building are also lower) as compared to the single-glazed glass windows. But adding all these material on top of a regular façade does not justify the energy that goes into producing (embodied energy) and maintaining these material/equipment. Researchers are now working on a system that will replace the need for blind/curtains or external shading devices from the façade and incorporate the same function by regulating the opacity of the glass itself. We are now able to change the opacity of the glass by passing current through it as shown in Figure 1. This behaviour of the glass is known as electrochromism. So, for example, on a bright sunny day we can actually reduce the glare effect and avoid solar radiation to enter our building by making the glass windows (almost) opaque(Ghosh, Norton, & Mallick, 2018).
This paper investigates the performance of EC windows in a cold climate country. It is essential to rationalise (scientifically prove) the use of upcoming technologies based on their application and performance in different climatic conditions. References and studies has been mentioned to analyse the visual and thermal comfort of the space with an EC window. Comparison between a single-glazed, doubleglazed and EC window have been drawn from experiment conducted by (Tavares, Bernardo, Gaspar, & Nio Martins, 2016), researchers.
2 Methodology In order to analyse the use of electrochromic windows various research articles and papers were referred. Most of the literature was selected based on their date of publish vs the number of citations. It ensured that the information is current as well as authentic. The literature is structured in three sections. First section elaborates the working principle of the technology. It describes the working mechanism of the EC windows and the different types of material/compounds that are being used for its production. The second and third section analyses the visual and thermal performance of the product. Small-scale test results from research articles and papers have been referred here which justifies the conclusions made at the end of the paper. These tests are compared between the performance of a single-glazed and doubleglazed window types. Section three also shows the energy saved in maintaining the thermal comfort in the building throughout the year. There are several terms that were used to express Electrochromic glass. The following keywords mostly useful; electrochromic façade, adaptive façade, smart materials, liquid crystal.
3 Understanding Electrochromism: basics
Figure 1: Electromchromic windows. (a) shows the transparent state when the solar gain is required, (b) shows the opaque state when the solar gain is not required (By Author).
Electrochromism is defined as the property that changes the opacity of a glass from transparent to opaque when an electric current passed through it; and this is reversible when the direction of the current is reversed. This is an 2
arrangement of a standard glass or plastic and several thin transparent layers (Baetens, Jelle, & Gustavsen, 2009). There are five such thin layers; the middle layer is usually a polymer electrolyte that acts as an insulator but conducts ions. It is then covered with an electrochromic coating (with both ionic and electrical conductivity) on both the sides. This material is usually Tungsten Oxide (WO3) and is most widely used in EC windows(Granqvist, 2014). Then a layer of transparent electrodes is added on both the sides of the EC coating. This arrangement can either be pasted on top of a glass window or between two regular glass panes (refer Figure 2) by a process called sputtering.
Figure 2: Five layer EC device (Granqvist, 2014).
3.1 Working When an electric current is passed, the ions migrate from the EC film to the ion storage film through the electrolyte. These ions are replaced by the electrons added or removed from the EC or ion storage coating. The optical property of the EC coating is changed due to the adding of these electrons. This effect can be reversed by reversing the voltage. (Granqvist, Pehlivan, & Niklasson, 2017) “No power is needed to maintain electrochromic windows in their clear or dark state only to change them from one state to the other(Costanzo, Evola, & Marletta, 2016).”
4 Visual comfort and energy benefits Piccolo, Pennisi, & Simone, (2009) conducted and documented the daylight performance of an EC window in a small scale test cell. They tested the
EC window under real weather conditions and found that the EC window was able to maintain the daylight indoor illuminance levels constant to the design value except when direct sun penetrates the building. They also mentioned that EC windows cannot fully avoid the glare from the direct sun but can be effective in reducing the discomfort glare. “Tavares et al., (2016) also found that the glare problem isn’t solved by EC windows on the East and West façade and these façade needs to be accompanied by vertical shading devices/louvers to tackle the glare problem.” In South façade regular double glazing window should be used accompanied by exterior horizontal shading device to achieve the thermal comfort. But in refurbishment projects this could not be possible always for example, in historical or heritage buildings. This would however compromise the indoor daylighting requirement. Piccolo & Simone (2009) also mentioned that glare effect was difficult to control from the east and west oriented windows. Even though EC windows are not able to fully block the direct sun, it is high beneficial to reduce the uncomfortable glare that is caused by the diffused daylight without the use of blinds, curtains or external shading device. The users can still have a view to the outside at all times during the day. Also it helps achieving the necessary daylight inside the buildings without any increase in the artificial lighting (Piccolo et al., 2009).
5 Thermal comfort and energy savings Ajaji & André, (2015) in there test for thermal comfort using EC windows showed that solar heat gain coefficient and solar transmittance decreased gradually when the EC window was switched from transparent to dark. They also mentioned that energy efficiency can increase in the refurbishment of existing building with the use of EC glass in East and West orientation (where it is difficult to control heat gain). Although, in climates where heating load is more than the cooling load, use of EC window could increase the heating loads (Tavares et al., 2016). A paper presented by Warner et al., (1992) compared annual and peak energy demands in a commercial building in the hot inland climate of Southern California, for five 3
different types of glazing; Tinted, reflective, selective laminated, broad-band EC, narrowband EC windows . It was found that use of narrow-band EC windows with controlled dimming lighting controls can effectively reduce annual cooling energy demands as compared to static windows as shown in Figure 3.
Figure 4: Heating Degree days (HDD) and Cooling Degree Days (CDD) in ten different locations in US and Canada (Dussault & Gosselin, 2017).
Figure 3: Annual cooling energy demands of five types of windows; Tinted, reflective, selective tinted, broad-band EC and narrow-band EC window in the hot inland climate of Southern California. The graph shows that narrow-band EC window has the lowest cooling energy demands even if the window-to-wall ratio increases (Warner et al., 1992).
A conference room in an office building in Washington DC was tested with EC windows by Lee, Claybaugh, & Lafrance ( 2011). It was found that there was a drop of 91% in lighting, 39-48% annual, 22-35% peak energy demands. It should be noted that the experiment was done in a subtropical climate region. In a recently published paper by Dussault et al, (2017), ten US and Canadian locations were tested for EC smart windows. The Heating Degree Day (HDD) and Cooling Degree Day (CDD) can be referred from Figure 4. Figure 3 shows that the total peak energy demand has increased in countries like Calgary, Montreal, Toronto, Washington DC, and Chicago where HDD is significantly more than CDD. Whereas, there is a huge drop in total peak energy demands in countries like Miami, San Francisco and Vancouver where the CDD is more than HDD.
Figure 5: Total peak energy demand reductions in ten different regions of US and Canada (Dussault & Gosselin, 2017).
6 Economic benefit Cost-effectiveness of use of EC windows can be determined by examining all the costs saved in with their use. For example- building operation cost reduced due to the reduction in annual electricity demands. The cost saved by eliminating the use of blinds, curtains and external shading device. In some cases a smaller HVAC system would be required due to decreased cooling loads (Warner et al., 1992).
7 Conclusions This paper was written to analyse the visual and thermal performance of EC windows and whether it is effective to use it in a cold climate country. Through the referenced information in previous sections it can be inferred that it is not effective to use EC windows in cold climate regions. The average peak energy demands increases with the use of EC window in a cold climate country. Although there is drop in annual energy demands, it is significantly less in a cold climate region compared to a hot climate region. 4
For hot climate regions it can effectively reduce annual energy demands by reducing the cooling and lighting loads. The costs saved by eliminating the use of blinds, curtains and external shading device would also be accounted as a reduced carbon footprint. In addition the reduced size of HAVC system will also reduce initial investment cost. This paper does not analyse the following: o The embodied energy and production cost of EC windows, which if computed will change the results of the energy performance of the building. o It will also affect the carbon footprint of the building which hasn’t been taken into account in this paper. o The current production challenges hasn’t been taken into account. o Energy required for switching the EC window from transparent to opaque state. o User regulated customised control for privacy and its impact on energy demands. With the exploration in this topic, it is highly subjective to say whether the use of EC window is suitable or not because it depends on factors like transmittance, number of switchable states, U-value etc. which might change with the development in the state-of-the-art.
8 Reflection The approach of the research was aimed to first understand what Electrochromism is and which the most used technology is. Then suitable literature was referenced to analyses its performance in different climatic conditions. The found literature helped reference the answer to the research question. It was necessary to conduct this conclusion because addition of new architectural materials adds up to more carbon footprint. It is important to rationalizing the use new technologies based on their performance in different context because, if the total energy reductions is not sufficient then, why should it be used? It is sustainable to use EC windows in hot climate regions because there is a significant drop in energy consumption for cooling needs. It would affect the annual energy needs, which is profitable for the developers and environment friendly for the planet.
I hope that architects and clients would become aware of the realities of the current state of the EC windows and make a convincig choice for their design.
References Ajaji, Y., & André, P. (2015). Thermal comfort and visual comfort in an office building equipped with smart electrochromic glazing: an experimental study. Energy Procedia, 78, 2464–2469. https://doi.org/10.1016/j.egypro.2015.11. 230 Baetens, R., Jelle, B. P., & Gustavsen, A. (2009). Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: A state-ofthe-art review. Solar Energy Materials and Solar Cells, 94, 87–105. https://doi.org/10.1016/j.solmat.2009.08. 021 Costanzo, V., Evola, G., & Marletta, L. (2016). Thermal and visual performance of real and theoretical thermochromic glazing solutions for office buildings. Solar Energy Materials & Solar Cells. https://doi.org/10.1016/j.solmat.2016.01. 008 Dussault, J.-M., & Gosselin, L. (2017). Office buildings with electrochromic windows: A sensitivity analysis of design parameters on energy performance, and thermal and visual comfort. Energy and Buildings, 153, 50–62. https://doi.org/10.1016/j.enbuild.2017.07 .046 Ghosh, A., Norton, B., & Mallick, T. K. (2018). Daylight characteristics of a polymer dispersed liquid crystal switchable glazing. Solar Energy Materials and Solar Cells, 174. https://doi.org/10.1016/j.solmat.2017.09. 047 Granqvist, C. G. (2014). Electrochromics for smart windows: Oxide-based thin films and devices. Thin Solid Films, 564, 1–38. https://doi.org/10.1016/j.tsf.2014.02.002 Granqvist, C. G., Pehlivan, İ. B., & Niklasson, G. A. (2017). Electrochromics on a roll: Webcoating and lamination for smart windows. Surface & Coatings Technology. https://doi.org/10.1016/j.surfcoat.2017.0 8.006 5
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