Study of Electrochromic (EC) and Gasochromic (GC) Glazing for Buildings in Aspect of Energy Efficiency

Thermal protection of the glazing technologies can be defined by:

3.1.2.

U-value

Single EC and GC panes have unsatisfactory thermal insulation properties to be applied within facades [11]. Furthermore, in both EC and GC glass, switching occurs through absorption rather than through reflection. This means that such windows are susceptible to heating, which heat is transmitted into room interiors and, consequently, may cause their overheating.

For the above reasons, EC and GC glazing is produced as combined glazing: double glazed units (DGU) and triple-glazed units (TGU).

In DGU systems, EC and GC coatings are applied on the inside to the outer pane, usually of tempered glass. In EC glazing, the coating may be located between two layers of laminated glass [12].

The same applies to TGU systems, but adding a third pane causes additional thermal insulation layer to be created by the gap between the 2^nd and 3^rd pane.

In GC glazing, the space (8 mm as standard) between the outer and inner (DGU) or middle (TGU) panes creates a void for the flow of active gas.

Also, a low-emission (low-e) coating is often added to the outer surface of the pane that faces the room [3]. Manufactured glazing units with the use of EC glazing are currently characterized by a greater range of U coefficients than GC glazing systems (Fig. 2). They also reach lower values of this coefficient: 0.5–1.7W/(m²K) [13], compared to 0.9–1.3W/(m²K) for GC glazing [6].

Figure 2.

In terms of energy efficiency, the optical characteristics of EC and GC glazing is determined by the visual (visible) light transmittance factor (Tv) as well as the relationship between Tv and g-value.

3.2.1.

Tv-value

Both technologies noticeably change their Tv values according to the mode they are in.

The visible light transmittance largely depends on the glazing structure. For EC DGU and TGU insulation systems, it is possible to achieve practically impermeable surfaces (Tv = 0.01) in the dimmed mode. On the other hand, in the bleached state, the threshold is currently Tv = 0.7, with the most common value being in the range of 0.5-0.6. The detailed optical properties of the Tv value of EC glazing are given in the graph (Fig. 3).

Figure 3.

In the case of GC glazing, visible light transmittance values are respectively slightly higher and usually reach the range of 0,1 in the darkened and 0.75 in the neutral mode [7, 9]. Baetens et al. [8] provides a greater range of 0.06-0.77, whereas these values depend, albeit to a small extent so, on the thickness of the gasochromic coating and on gas concentration in the glazing unit [8]. As in EC glazing, the number of panes matters as well. DGU systems are characterized by a higher Tv value compared to comparable TGUs, that is up to approx. 10% [3, 13].

3.2.2.

Tv to g-value

When analyzing Tv to g-value in the bleached and tinted states, this relationship is proportional for EC and GC. This means that as the g-value decreases, the Tv-value goes down proportionally, and vice versa. For EC glazing, the curve that defines the discussed relation is marked lower on the graph than the curve for GC glazing, i.e. EC glazing is generally a stronger barrier in terms of both heat and sunlight transmittance. The shape of both charts is similar, so the change in the characteristics of the thermal and optical parameters takes place in a similar proportion for both technologies.

The comparison of the Tv to g-value relationship for exemplary EC (DGU) and GC (DGU) glazing is shown in diagram 1 (Fig. 4). The characteristics of traditional (static) glazing were also marked on it, thus highlighting the advantages of the discussed switchable technologies as solar control glazing.

Figure 4.

In graph 2 (Fig. 5) an analogous comparison is presented between EC glazing and conventional glazing technologies with a larger Tv and g-vlaue span compared to the technology illustrated in graph 1 (Fig. 4). The comparison was made, for cases such as glazing with high solar protection properties (reflective glazing) and - for contrast - highly transparent low-e glazing. The graph for EC glazing shows that this technology may be seen as a bridge between the compared technologies of static glass, as it combines their mutually exclusive advantages.

Figure 5.

In the case of switchable glazing, apart from the thermal and optical parameters, switching time is another important element of assessment of glazing in terms of energy efficiency. This aspect has a potential impact on the effectiveness of solar protection. Therefore, switching time may ultimately translate into the energy balance of the building related to shaping of the thermal and lighting environment.

In this aspect, there are fundamental differences between the discussed technologies. In the case of GC, switching speeds are 20 seconds to color and less than a minute to bleach the glazing [5].

EC technology speed is inversely proportional to the glazing area - for façade glazing, full coloration is achieved typically in 5-10 minutes [5], although it may require up to 20 minutes [13].

EC glazing is the slowest switchable glazing technology available today. The graph (Fig. 6) shows the transition period of EC and GC glazing, as well as of other switchable glass technologies: liquid crystal devices (LCD), micro blinds (MB) and suspended particle devices (SPD).

Figure 6.

The EC and GC technologies typically require small amount or no power, respectively.

The GC glazing is not powered by electricity. The switching process is stimulated by gas (hydrogen/oxygen) that can be generated at the window wall with an electrolyzer and a distribution system integrated into the façade [5].

EC glazing requires low-voltage power, up to 10 V DC, to change the mode they are in. For some EC types (polymer laminate), the device is switched to its desired mode and then no power is needed to maintain the state. This type of device has a long memory once switched (power is not required for three to five days to maintain a given switched mode) [16].

When no voltage is applied, EC glass remains in the clear (off) mode.

It is also possible to completely eliminate electricity consumption by integrating the glazing with photovoltaic (PV) technology, which generates electric current for, among other purposes, the supply of EC glazing.

PV-EC technologies, including photoelectrochromic (PEC) glazing, are passive devices that do not require external power for operation. They can work both as free electricity generators, as well as a semitransparent solar window [17]. When integrated in building application, they provide enormous energy savings. As reported by NREL [18], 1 kW_p of PV power can remove approximately 3 W_p of heat from a building envelope, while the same 1 kW_p of PV used to activate a PV-EC window can avert 110 kW_p. To color the window in 5 minutes requires only about 0.1 mA/cm² current density from the PV device [19].

Simulation studies were conducted under Task 27 of the Solar Heating & Cooling Programme at the Fraunhofer Institute of Solar Energy in Freiburg [20]. As the subject matter of the study, typical cell offices with a northern and southern orientation, covering the same area and with the façade glazing share of 30% were adopted. The tests were conducted for windows equipped with EC and GC glazing, as well as traditional (static) solar control glazing with a low-emission (low-e) and reflective “mirror” coating (the parameters of the tested glazing are presented in Table 1). The research was conducted for three locations: Stockholm, Brussels and Rome, each representing the zones of cool, moderate and warm Mediterranean climate in Europe, respectively.

Research has shown that switchable windows are energy-efficient compared to traditional windows with low-e and solar control glazing (Tab. 1).

The main energy savings result from a significant reduction in the need for cooling and air-conditioning of rooms with such glazing. This applies to all climatic zones. In this aspect, the GC glazing is slightly inferior to the EC glazing tested in the simulation, which fact results from its higher g-value. This fact, however, translates into a more favorable result in terms of savings on demand for heating. GC glazing is more successful at fulfills its role of a passive solar heating element.

Low demand for cooling caused by the application of switchable glazing enables maintaining thermal comfort with the use of passive cooling methods only. The night-time ventilation strategy applied in the case of Brussels and Stockholm significantly reduced the number of overheat hours (only a few hours above 27°C, while for low-e glazing, approximately 700 hours).

However, it should be emphasized that passive cooling, if implemented as the only strategy to maintain a comfortable indoor microclimate, is in practice possible in cold climates only. It is unsuitable for locations such as Rome, where the air temperature is high in extreme periods, even at night. According to the present study, in conditions of long-lasting high air temperature outside the building, greater usefulness should be attributed to the GC glazing as it is characterized by a lower U-value. Such glazing may prove more effective in protecting the building not only against thermal energy loses in winter, but also against overheating in summer.

The above mentioned studies present average results for the north and south oriented windows. More detailed results were presented in the research [21] conducted for the same office rooms and locations, but with slightly different glazing parameters. Two types of switchable glazing and static glazing with improved thermal insulation properties with a low-e coating were analyzed. Even though the study did not distinguish between the EC and GC technologies, it can be assumed that the parameters of glazing A correspond more to EC, and glazing B - GC (the parameters of the tested glazing are discussed below the graphs in Fig. 7).

Figure 7.

The results confirmed the usefulness of switchable glazing as solar protection, i.e. their major contribution to protection against room overheating and, consequently, to relieving cooling and air conditioning systems. In the tinted mode, both A and B glazing types show a significant advantage over static glazing in all discussed locations, both for the northern and southern orientations. In terms of impact on heating needs, the simulations point to advantages of switchable glazing only in the case of high thermal insulation properties and high solar energy transmittance in the neutral mode (g-vale). However, the studies confirm that the main importance of switchable glazing for the energy efficiency of a building stems from its influence as a protection against overheating, regardless of the windows orientation.

Similar simulative studies were conducted for different climates in five locations across eastern China [14]. Harbin and Chonguing represent cool climate zones characterized by severe winters and relatively mild summers. Beijing is located in moderate climate. Shanghai is a city with mild winters and hot summers, while Guangzhou is set in a hot climate. Contrary to the presented studies based in Europe, all cities are located in a zone characterized with similar annual irradiation amounting to approx. 1.300 kWh /m² [22] (the equivalent of central France and Hungary). Detailed data on average temperatures are presented in Table 2.

The research was conducted for open space offices located around the perimeter of the building. In offices with a northern and southern exposure, the share of windows in the elevation was 45%, and in the east-west exposure - 30%. The height of the rooms was set at 3 m. The HVAC system was a split heat pump with a DX coil system. The heating set point was established at 21°C and cooling set point at 26°C. Comparative studies were conducted for windows with EC and GC DGU systems and for eight types of static glass (Table 3).

The research proved the advantage of EC and GC glazing in terms of reducing energy consumption by HVAC systems in all tested locations. Compared to the reference single clear float glass, EC and GC glazing can reduce energy requirements of the building by 25–35% and 27–31%, respectively.

The advantage of switchable glazing over static glazing, however, applies to the annual balance sheet. Certain types of traditional glazing offer an advantage in individual months. For example, in Shanghai, the most energy-efficient type of glazing in winter is low –e DGU (#7).

According to the simulation data, the most energy efficient type of static window varies in different regions. Solar control DGU, colored absorbing DGU, low-e DGU, solar control DGU and colored absorbing DGU seem the best choice for Beijing, Guangzhou, Harbin, Changchun and Shanghai respectively.

This derives from the fact that heating contributes the largest proportion of HVAC loads in cold regions, while the opposite is true in Guangzhou. Thus, in this region, a lower U-value is desired to offset major heat loss through direct conduction due to extreme cold temperature, whereas a high SHGC is desired to help passively heat the building. Nevertheless, a low SHGC is required to attenuate the intense sun and limit solar gain in warm climates.

In terms of the annual energy balance, a comparison of switchable glazing with most energy-efficient static glazing technologies for a given location shows the greatest advantage, of EC glazing in warm climates (Shanghai and Guangzhou), and the lowest benefit in cold climates (Harbin). In the case of GC glazing, this advantage is opposite - i.e. the greatest advantages of GC glazing in terms of energy saving are revealed in the cold (Harbin), while the least beneficial situation was observed in the hot climate (Fig. 8 right).

Figure 8.

This is confirmed by simulation calculations of annual energy outlay on HVAC for the tested locations. Compared to both switchable technologies, GC glazing proves a more energy-efficient technology per year only for the Harbin conditions. In turn, the greatest advantage of EC glazing is noted in hot climates (Guangzhou) (Fig. 8 left). This may suggest that high transparency of the glazing during the heating period is similar to the low transparency during hot periods. It may also be predicted that if the proportion between cooling needs and heating load of buildings is closer to 50%, smart windows will provide more energy saving potential.

In the simulation study [24], EC and GC windows together with termochromic (TR) and static tinted type of glazing have been compared in order to define which of the above offers a better balance between reducing energy consumption on cooling and saving daylight. The simulation was conducted for an office building in southern Egypt for all orientations and various window area-to-floor ratios (WFR): 8%, 16%, 24% and 32%.

The research showed the greatest efficiency of switchable glazing in terms of saving energy, which results from the relief of air conditioning systems. The EC glazing has an advantage over the GC glazing for each orientations and for each WFR tested.

With regard to traditional clear glass (Tv = 0.78, g = 0.86), the greatest savings for both technologies concern the southern and western orientations at WFR 32%, while the lowest amount of energy saving was noted for the north and east for WFR 8–16%. As an optically dynamic but non-switchable technology, TR glazing ranks in the discussed aspect between EC/GC glazing and traditional (static tinted) solar protection glazing. This fact proves the effectiveness of dynamic glazing as a solar protection with the superiority of switchable technology (EC and GC).

In terms of lighting aspect, the differences between the tested glazing are insignificant. EC reduced glare in the eastern and western orientations by 64%. Moreover EC reduced consumption of lighting energy by 60%, 61% and 57% in eastern, southern and western orientation, respectively. The performance of GC glass was similar to that of EC glass in providing daylight. Tinted blue glass achieved similar performance as did GC glass in the northern, eastern and western orientations, but GC glass is superior in the southern orientation. For daylighting availability, all types of smart glazing and tinted color glass are used to reduce the glare of daylight in the morning hours in the eastern orientation and the afternoon hours of the day for the western exposure.

Simulation proved that static glazing systems are unable to adapt to the changing environment in the hot dry climate of Egypt. EC glass proves the most favorable solution and its significance increases along with the rise of WFR value. It can save cooling energy by 40%, and light energy by 43% at 8% WFR in southern orientation, whilst at 32% WFR by 46%, and 61% respectively. In both cases it is better than other glass types.