Results and Conclusions from the Glazing Model
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Below is the final results and conclusions determined for the various types of glazing tested. A complete breakdown and set of results for each glazing system can be found in the excel file attached.
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The first technology we tested was thermochromic glazing. This varies its optical properties depending on temperature and so our first task was to analyse the impact of the switching temperature range on energy consumption. The results of this investigation are presented in the graph below which shows the annual energy consumption for TC glazing with the same optical properties but different switching temperature ranges.
Diagram 1: Impact of the switching temperature range
From the above graph it can be seen that the lower the switching temperature range, the better the energy performance as the glazing is in a dark state longer and so cooling delivery is reduced. It is clear that it is important to focus on having as low a switching T as possible. Interestingly, a typically commercialised TC glass has a switching temperature range of [24°C-76°C].
The way that the optical properties vary within the switching temperature range also impacts the energy consumption. This is represented by the variation of the solar transmittance from the clear state to the dark state. The relationship between the transmittance of the glazing and the energy consumption was investigated to determine how it impacts building energy performance. Diagram 2 shows the results analysis for TC glazing with different transmittance values but with the same switching temperature range.
Diagram 2: Impact of the visible transmittance
From the graph it can be stated that the lower the solar transmittance, the lower the energy consumption as less radiation entered the room and so there was less solar heat gain. Thus, there was less cooling delivered and so this decreased the energy consumption. This observation demonstrates that it may be necessary to focus research on how to get bigger changes in glass transmittance between the clear and dark states to improve glazing energy efficiency even more.
The following graph shows the comparison between different types of electrochromic glazing and the resulting reduction in annual energy consumption in each case.
The following graph shows the comparison between different types of electrochromic glazing and the resulting reduction in annual energy consumption in each case.
Diagram 3: Energy performance of EC glazings
For the electrochromic glazing appropriate control was achieved by adapting the optical properties according to the incident solar radiation reaching the window. The best performance was achieved for the glazing with clear, intermediate and dark states having the typical optical properties of commercial EC glazing. The optical control used involved 4 different periods with 3 different sets of optical properties that were changed according to the level of exterior solar radiation. Thus, the considered glass changed its color 4 times a day and was in the darkest state from 10am to 4pm as the incident radiation was over the set point of 500 W/m2. The darker the glass during bright periods, the better the reduction in energy delivered, as peak cooling was reduced due to less solar radiation entering the room. Refining even more the optical control could potentially lead to better results.
Diagrams 4 presents the reduction of the annual energy consumption for the glazing of each type that gave the best results. As the base case model was our baseline for the project, the results are presented in the form of a percentage of energy savings, given the annual energy consumption of the base case model.
Diagrams 4 presents the reduction of the annual energy consumption for the glazing of each type that gave the best results. As the base case model was our baseline for the project, the results are presented in the form of a percentage of energy savings, given the annual energy consumption of the base case model.
Diagram 4: Energy savings for each type of glazing
From the results of the modelling simulation outlined in the graph above, it can be seen that the percentage reduction in the annual energy consumption resulting from each type of glazing compared to the base case model was as follows:
•Commercialised Thermochromic Material: 1.4 %
•Low-e glass: 4%
•Electrochromic Material: 4.2%
•Triple Glazing: 4.4%
•Thermochromic Material: 4.9%
From the results summary above, it can be found that the thermochromic material with the lowest solar transmittance and lowest switching temperature range has the biggest energy saving out of the glazing types tested, reducing energy consumption by almost 5% annually. Though there was a significant energy saving for this type of thermochromic material, the commercially available thermochromic glazing material does not result in significant energy savings, with an annual energy saving of just over 1%. As the cooling and heating delivered were calculated separately, it was possible to notice than for all glazing systems, the heating consumption was slightly increased. In the case of improved static glasses, this might be explained by better insulation properties which resulted in less heat gain through the glazing. In the case of dynamic glazing, this might be explained by changes in solar transmittance in response to changing glass temperature which is directly linked to the outside temperature. As the glass became hotter, the light transmittance was decreased which resulted in less heat gain in the room. However, this observation has little impact given that there was only a little heating required in February and that the heating delivered represents less than 0.01% of the annual energy delivered in each case.
Also, peak cooling was reduced during the hottest periods for all the aforementioned reasons.
From the present study, it can be concluded that the performance of these smart glazing systems will be maximized when used in optimal conditions. It was found that they will perform best when used on large glazed surfaces in a hot and sunny climate, and hence it will be in the cooling load of the building where energy consumption will be significantly reduced. Also, a general rule is that building energy consumption for fully glazed façades
is higher than for an identical building where the façade is only 25% glazed. As a consequence, dynamic glazing would be even more efficient if they were incorporated into a fully glazed façade.
•Commercialised Thermochromic Material: 1.4 %
•Low-e glass: 4%
•Electrochromic Material: 4.2%
•Triple Glazing: 4.4%
•Thermochromic Material: 4.9%
From the results summary above, it can be found that the thermochromic material with the lowest solar transmittance and lowest switching temperature range has the biggest energy saving out of the glazing types tested, reducing energy consumption by almost 5% annually. Though there was a significant energy saving for this type of thermochromic material, the commercially available thermochromic glazing material does not result in significant energy savings, with an annual energy saving of just over 1%. As the cooling and heating delivered were calculated separately, it was possible to notice than for all glazing systems, the heating consumption was slightly increased. In the case of improved static glasses, this might be explained by better insulation properties which resulted in less heat gain through the glazing. In the case of dynamic glazing, this might be explained by changes in solar transmittance in response to changing glass temperature which is directly linked to the outside temperature. As the glass became hotter, the light transmittance was decreased which resulted in less heat gain in the room. However, this observation has little impact given that there was only a little heating required in February and that the heating delivered represents less than 0.01% of the annual energy delivered in each case.
Also, peak cooling was reduced during the hottest periods for all the aforementioned reasons.
From the present study, it can be concluded that the performance of these smart glazing systems will be maximized when used in optimal conditions. It was found that they will perform best when used on large glazed surfaces in a hot and sunny climate, and hence it will be in the cooling load of the building where energy consumption will be significantly reduced. Also, a general rule is that building energy consumption for fully glazed façades
is higher than for an identical building where the façade is only 25% glazed. As a consequence, dynamic glazing would be even more efficient if they were incorporated into a fully glazed façade.
Optimal Conditions for Smart Glazed Facades
Diagram 5: Optimal environment for smart glass
The optimal conditions in which to implement dynamic glazing can be summarized as follows:
Potential Benefits of Smart Glazed Facades
- Extremely hot and sunny environment
- Long day light period over the year
- Main
load
is
cooling that is expected to decrease significantly
- Less lighting required in cities in low latitudes
- Large glazed surface in hot cities
Potential Benefits of Smart Glazed Facades
Diagram 6: Potential benefits of smart glass
If correctly implemented, a great number of incentives can be attributed to the use of dynamic glazing technologies, as shown in the diagram above:
Future Investigation
Though dynamic glazings have shown potential for reducing energy consumption they still have a long way to go before they will reach their full potential and reduce their cost to a price that is considered affordable. According to Scottmaden, EC glazing is expected to be cost competitive within the next decade. Although the cost of EC windows can be from two to three times that of a standard window , this cost is expected to decrease significantly when manufacturing techniques are improved. The 2006 Berkeley Study recommended that “EC manufacturers develop an accurate intermediate-state controller and work toward faster switching speeds, less color in the tinted state, lower minimum transmittance, and reduced manufacturing costs”.
Also, to make the most of electrochromic glazing, work is needed to develop integrated window-lighting control algorithms that accommodate different visual tasks, building occupant behaviour and climate/HVAC conditions. A great number of measurements would be required in order to accommodate all these multiple parameters such as room luminescence and room temperature, air conditioning and heating status, EC window control status and lighting control system status.
In terms of thermochromic glazing, improvements have to be made to achieve a dynamic glass with a much higher reflectance when temperature is above the switching temperature. Thus, less solar energy would reach the inside of the building which would potentially result in higher energy savings. Glazing with switching temperatures close to room temperature (namely between 20 and 25 °C) could also be tested to minimise temperature variation within the room and increase thermal comfort.
- Greater architectural design freedom: due to higher efficiency, dynamic glass enables designers to use more glass while still meeting the performance objectives of building energy codes and standards. This is especially valuable given that there is a sudden interest for fully-glazed façades in commercial buildings.
- Increased occupant comfort and productivity: in addition to improved daylighting and thermal comfort, dynamic glass allows for the visual comfort of occupants and links them to the exterior by means of unobstructed views even in the tinted state. This benefit is enhanced by user control abilities in the case of electrochromic glazing and could potentially result in increased productivity. Moreover, as dynamic glazings have a lower U-value than low-e glazing and triple glazing, they result in less airtight houses and consequently are a better option to provide fresh air and get rid of indoor air pollutants. Thus, indoor air quality could be improved and the need for ventilation systems reduced.
- Reduction of shading device use: which usually result in excessive interior lighting usage as well as loss of passive solar heat in the winter. Smart glass allows for solar hear gain control while light levels and outdoor views are still maintained. Also, this reduces the maintenance costs of exterior shading devices which are quite high.
- Reduction of cooling requirements: Dynamic Glass can tint during peak cooling demand periods corresponding to the hottest occupied periods. To do so, they can block more than 90 percent of solar radiation which results in huge savings in peak load cooling energy use and allows for a smooth management of peak load in the building.
- Downsize HVAC equipment: This results in reduced HVAC equipment sizing as well as system simplicity in comparison with traditional glazing solutions.
- Reduction of annual energy bills: Due to its dynamic nature, smart glass reduces overall HVAC energy consumption and costs by reducing unwanted heat gain in summer but allowing the benefits of passive solar in the winter. Also, given that dynamic glazings have intermediate states, this allows for additional incentives by saving lighting energy as the glazing can be controlled to make the most of daylighting.
Future Investigation
Though dynamic glazings have shown potential for reducing energy consumption they still have a long way to go before they will reach their full potential and reduce their cost to a price that is considered affordable. According to Scottmaden, EC glazing is expected to be cost competitive within the next decade. Although the cost of EC windows can be from two to three times that of a standard window , this cost is expected to decrease significantly when manufacturing techniques are improved. The 2006 Berkeley Study recommended that “EC manufacturers develop an accurate intermediate-state controller and work toward faster switching speeds, less color in the tinted state, lower minimum transmittance, and reduced manufacturing costs”.
Also, to make the most of electrochromic glazing, work is needed to develop integrated window-lighting control algorithms that accommodate different visual tasks, building occupant behaviour and climate/HVAC conditions. A great number of measurements would be required in order to accommodate all these multiple parameters such as room luminescence and room temperature, air conditioning and heating status, EC window control status and lighting control system status.
In terms of thermochromic glazing, improvements have to be made to achieve a dynamic glass with a much higher reflectance when temperature is above the switching temperature. Thus, less solar energy would reach the inside of the building which would potentially result in higher energy savings. Glazing with switching temperatures close to room temperature (namely between 20 and 25 °C) could also be tested to minimise temperature variation within the room and increase thermal comfort.