Glazing Model Methodology
The concept of dynamic glazing is based on the idea of windows being a multifunctional
"appliance-in-the-wall" rather than simply a static piece
of coated glass. Façade systems include switchable windows and
switchable glazing that have variable optical and thermal properties that can
be changed in response to climate, occupant preferences and building system
requirements.
There are many different variations of dynamic glazing emerging at the moment and so for our investigation we had to make
a choice of what types of glazing we
wanted to feature in our façade. We decided to test one active
technology and one passive technology capable of controlling solar heat gain in
the room. Firstly, we chose electrochromic
glazing (EC) as it is the most commercialized
dynamic glazing and it allows for a customized control to meet the occupant’s requirements. Secondly, we chose thermochromic
glazing (TC) which is a self-regulated technology whose operation does not required
any additional energy source.
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As well as testing these dynamic systems, we also decided to run simulations for the base case model with standard double glazing to use this as a benchmark in order to compare results and determine whether or not dynamic glazing could in fact reduce energy consumption. As an additional investigation, we also modeled static glazing systems with improved insulation properties, namely low-e glazing and triple glazing. This study was conducted to see if dynamic glazing could achieve higher energy savings than energy efficient glazing.
There are several different properties which have an impact on solar gain and heat gain through the dynamic glazing and these are transmittance, U-value and the solar heat gain coefficient (SHGC). The visible transmittance (Tvis) is the amount of light in the visible portion of the spectrum that passes through a glazing material. A higher transmittance means there is more daylight in a space which, if glazing is properly designed, electric lighting can be offset along with its associated cooling loads. However, reducing solar gain often also reduces visible transmittance as dynamic glazing types do not have the ability to reduce their solar transmittance only. Regarding the U-value, it quantifies overall heat flow. A higher U-value means the material has higher thermal insulating properties and so there will be less heat loss through glazing. The last critical parameter for glazing is the SHGC, which is the fraction of incident solar radiation that enters a building through the window as heat gain. Consequently, a higher coefficient signifies higher heat gain. The table below outlines the typical values for each of these properties for the simulated glazing systems. They were acquired using the Window software which is an advanced daylight simulation software.
Diagram 3 highlights the progressive variation of the optical properties of thermochromic glazing according to its temperature. The visible consequence of these changes is the variation of the glass color which develops from a clear state to a darker state as the glass temperature increases.
Diagram 4 highlights how the solar heat gain of each type of glazing varies with different angles of incidence.
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This graph (diagram 4) shows that the various glazing types have different thermal properties which also vary according to the angle of incidence of light on the window. Electrochromic and thermochromic glasses are of particular interest when trying to avoid heat gain in a hot climate especially for high angles of incidence as shown in the graph above. However, thermochromic is not controllable and may darken on a cold sunny day when heat gain is required while electrochromic is more efficient but significantly affects the amount of daylight in the room and may induce the need for artificial lighting.
In order to simulate thermochromic materials in ESP-r the user must define a special component file with the optical properties required, and then select the layer and at which node they wish the thermochromic material to be located within the glass. The switching temperature range must also be defined i.e. the lowest temperature at maximum transmission and the highest temperature at minimum transmission. Once these temperatures have been defined, each of them must be allocated a solar transmittance value. To vary the optical properties of glazing throughout the switching temperature range, ESP-r uses a linear function which modifies the properties at run time based on the calculated node temperature of the glass. An example of solar transmission variation within the temperature range is given in Diagram 5.
During our thermochromic glazing investigation, we ran simulations for the climate all year round analysing interior temperature and solar radiation and it was found that the 2 periods with the most extreme conditions were 20-26 January (winter week) which was the coolest, and 4-10 August (summer week) which was the hottest. This was conducted to investigate the cases when glazing performance is particularly critical and dynamic glasses are expected to be the most beneficial. Various different TC glazings were investigated through different ranges of switching temperatures and different changes of transmittance according to the glass temperature. These results could then be used as a comparison with static improved glazing such as low-e glazing and triple glazing and against the base case model to evaluate whether this technology can reduce energy consumption within the building.
Secondly, electrochromic glazing was tested. This type of technology changes its optical properties in response to an electrical control (low voltage). The process is illustrated in Diagram 6.
Secondly, electrochromic glazing was tested. This type of technology changes its optical properties in response to an electrical control (low voltage). The process is illustrated in Diagram 6.
In order to model electrochromic materials in ESP-r an optical control loop must then be defined and was set to be active during working hours. Alternative optical properties are based on time variation. The time periods for the control loop were determined according to the level of solar radiation at different times of the day. Three different types of EC glazing were defined (EC1, EC2, EC3) by means of three different optical controls, which are described in the excel file attached to the glazing results section.