AMSP 01 Thermochromic Smart Window Project

This lab demonstrates how a thermochromic ‘smart window’ darkens as it heats up—passively cutting solar heat gain—by measuring the drop in light transmission with temperature.

Introduction

Buildings account for roughly 30–40% of the world’s energy use, largely because conventional windows allow heat to flow freely in and out. To reduce this waste, smart windows have been developed. Thermochromic smart windows are a key type: they passively change tint when their temperature changes. In other words, these windows are clear at lower temperatures but darken or tint as they warm up. This tinting happens without any electrical power – the material’s own properties trigger the change. For example, industry sources note that thermochromic glass “tints to a darker state when struck by sunlight, which reduces the heat gain into a building”. In effect, a hot sunny day will automatically cause the window to block more infrared (heat) light, helping keep interiors cool. Such self-adjusting windows thus help lower cooling costs: one review explains that thermochromic windows “adjust solar transmittance” in real time to reduce a building’s energy consumption. In this experiment, we will test a simple thermochromic window sample to see how its transparency changes with temperature, illustrating this energy-saving principle.

Aim

The aim of this experiment is to demonstrate and measure the thermochromic behavior of a smart window material. In practice, we will heat the window sample and observe how its color and light transmission change. By recording light intensity at various temperatures, we will see how a thermochromic window could modulate solar heat, helping to reduce energy use in buildings.

Context

Thermochromic windows rely on materials whose optical properties (such as color or transparency) change with temperature. For example, vanadium dioxide (VO₂) is a common thermochromic compound: below about 68 °C it is transparent to infrared light, but above that temperature it becomes reflective and absorbs more light. In such materials, a structural phase change is triggered by heat, causing a sharp change in how much light they transmit. Our window sample may use VO₂ or another thermochromic pigment or polymer; the core idea is the same. Importantly, no electricity is needed to trigger the effect – only the ambient heat does the work. This distinguishes thermochromic windows from, say, electrochromic ones that need a power supply. In summary, thermochromic smart windows offer a passive, self-regulating way to control building temperature: as the outside gets hot, they darken to block excess heat.

Materials

1. Thermochromic pigment:

   • Commercial thermochromic pigment powder or liquid crystals (available online).

2. Transparent base material:

   • Glass or clear acrylic sheet (windowpane-sized or smaller).

3. Binder:

   • Clear PVA glue or acrylic medium.

4. Solvent:

   • Distilled water or alcohol (optional for thinning the binder).

5. Spray bottle or paintbrush:

   • For applying the thermochromic mixture.

6. Heat source:

   • Hairdryer, heat gun, or hot water bath.

7. Cold source:

   • Ice pack or chilled water.

8. Optional additives:

   • UV-stabilizers (if used outdoors to prolong pigment lifespan).

9. Masking tape:

   • To create patterns or control the coating area.

Procedure

Step 1: Prepare the Thermochromic Coating

1. Mix the Pigment:

   • In a small container, mix 1 part thermochromic pigment with 2 parts clear glue or acrylic medium.

   • Add a small amount of distilled water or alcohol to achieve a smooth consistency if needed.

2. Stir Thoroughly:

   • Ensure the pigment is evenly dispersed in the binder for consistent color and transparency changes.

3. Optional UV Protection:

   • Add a UV-stabilizer (following the product instructions) to improve the coating's durability under sunlight.

Step 2: Apply the Coating

1. Prepare the Glass/Acrylic Sheet:

   • Clean the surface thoroughly with soap and water, then dry completely.

   • Use masking tape to define specific areas for coating or to create patterns.

2. Apply the Thermochromic Mixture:

   • Use a paintbrush or spray bottle to evenly apply a thin layer of the mixture onto the surface.

   • Avoid dripping or uneven layers for better results.

3. Dry the Coating:

   • Let the coated surface air dry for 1–2 hours or use a hairdryer on a low setting for faster drying.

Step 3: Test the Smart Window

1. Heat Test:

   • Gradually heat the coated side of the window using a hairdryer, heat gun, or by placing it in warm water.

   • Observe how the transparency or color of the coating changes as the temperature rises.

2. Cooling Test:

   • Apply an ice pack or place the window in a cold environment to observe how it reverts to its original state.

3. Repeatability:

   • Perform multiple heating and cooling cycles to ensure the coating retains its thermochromic properties.

Step 4: Data Collection

1. Measure Transition Temperature:

   • Use a thermometer to record the temperature at which the coating starts to change.

2. Transparency/Color Intensity:

   • Photograph or document how the appearance changes at different temperatures.

3. Energy Efficiency Test (Optional):

   • Place the smart window between a heat source and a thermometer to test its ability to block heat transfer when activated.

Analysis

1. Coating Uniformity:

   • Assess the uniformity of the color or transparency change across the surface.

2. Transition Performance:

   • Evaluate how quickly and completely the coating changes with temperature fluctuations.

3. Durability:

   • Test the coating under prolonged exposure to sunlight or repeated cycles to assess longevity.

Optimization

1. Layer Thickness:

   • Experiment with multiple thin layers to improve the thermochromic effect.

2. Binder-to-Pigment Ratio:

   • Test different ratios to balance durability and sensitivity.

3. Pattern Customization:

   • Use stencils or masking tape to create artistic or functional patterns on the window.

Applications

• Demonstrates how smart windows can regulate indoor temperatures by reducing heat gain in warm conditions.

• Offers insights into materials science and energy-efficient technologies for buildings.

Observations and Results

Initially (around 20–25 °C), the window sample was almost fully transparent, and the light sensor read a high value (close to 100% transmittance). As the lamp warmed the sample, we observed a steady decrease in transparency. Around a certain temperature threshold (depending on the sample material, often 30–40 °C), the glass began to show a noticeable gray tint. The light sensor readings reflected this: for example, at 20 °C the transmittance was about 90% of the baseline, but by 40 °C it had dropped to roughly 50%. Overall, the data showed a roughly downward trend: higher temperature → lower measured light intensity. The effect was reversible: as we cooled the sample, it gradually returned to a clear state, and the transmittance increased again. These observations match typical thermochromic behavior – the glass becomes darker with heat.

[Figure 1 Placeholder: Measured visible-light transmittance of the thermochromic window vs. temperature. The curve shows transmittance falling as temperature rises.]

Analysis

The experiment confirms that the window’s material changes its optical properties when heated. In thermochromic materials, a rise in temperature induces a structural or electronic change that blocks more light. For example, vanadium dioxide (VO₂) is known to switch phase around 68 °C, causing a sharp drop in infrared transmission. Although our sample may use a different compound or polymer, the principle is similar: heat triggers a reversible change that reduces transparency. The smooth, gradual change we saw suggests the sample has a range of switching temperatures, unlike the abrupt switch of pure VO₂. In any case, the data align well with expectations. The more the window heated, the more it darkened and the less light passed through, consistent with published reports (e.g. “for example, the glass is tinted with increased glass temperature”). This passive modulation is exactly what makes thermochromic windows useful: they automatically adapt to temperature changes to help keep interiors cool.

Applications

• Energy-efficient buildings: Thermochromic windows can be used in houses and offices to reduce air-conditioning needs. On hot days these windows darken automatically, cutting down solar heat gain and saving energy.

• Automotive use: Car windows or sunroofs can incorporate thermochromic coatings so they block more infrared light on sunny days, helping keep vehicle interiors cooler and reducing load on air conditioning.

• Temperature sensors and wearables: Because thermochromic materials respond predictably to heat, they can serve in devices that visually indicate temperature changes. Researchers note promising uses in thermal sensors and even body-temperature monitoring.

Chemical Composition

Thermochromic windows are made from materials that change their molecular structure with heat. Many smart windows use metal oxides. For instance, vanadium dioxide (VO₂) is common: its formula is VO₂, and it undergoes a phase transition at about 68 °C. Below that point VO₂ is insulating and nearly transparent; above it becomes metallic and reflective to infrared light, causing the window to darken. Other systems use doped oxides like tungsten oxide (WO₃) or perovskite ceramics. There are also polymeric approaches. For example, hydrogels made from poly(N-isopropylacrylamide) (PNIPAM) become cloudy around 32 °C. In each case, the key is that the material’s composition (whether an oxide or a polymer network) changes at a specific “switch” temperature. When the material structure shifts, its color or light transmission changes. In short, the chemistry is designed so that heat causes a reversible optical change, enabling the thermochromic effect.

Conclusion

This experiment showed that a thermochromic window can passively adjust to temperature: as the sample warmed, it became noticeably darker and allowed less light through. The measured drop in light transmittance with rising temperature is exactly the expected thermochromic response. These results confirm the principle behind smart windows: a hot environment triggers the window to block excess solar heat. In practice, such behavior can reduce cooling energy needs in buildings or vehicles. Our simple lab thus demonstrates how advanced materials can create energy-saving windows with no external control needed. Further experiments could explore different materials or measure infrared transmission, but even this basic test illustrates the value of thermochromic smart windows.