Solar heat-reflective coatings for buildings and other infrastructure can save energy and reduce the carbon footprint, AS Khanna explains the energy- and money-saving coatings and where they can be used.

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Today, India is trying to reduce its carbon footprint by producing electricity through alternative energy, especially wind and solar power. However, it is also possible to use a simple paint coating on a rooftop to bring down the structure’s temperature, thereby consuming less power for air-conditioners or coolers.

Technology at work

Initially, the main purpose of paint coatings was aesthetics and corrosion protection for steel and concrete structures. But their scope has widened substantially. Today, the market offers high-performance, functional and eco-friendly coatings. Several kinds of coatings are now function-specific—for acid-resistance, high temperature-resistance, automotive use, concrete and plastics. In addition, many coatings are characterised as ‘smart’ as they do more than just protect the surface. These include hydrophobic, self-cleaning and self-healing, conductive, anti-bacterial and anti-dust coatings.

Recently, a new kind of smart coating that can cool roofs has been developed that saves electricity and reduces the carbon footprint by reflecting about 89 per cent of the infrared (IR) heat absorbed from the sun. In addition to heat reflection, the coating also has an insulation effect. Thus, such a combination can bring down the ambient temperature in a room, say from 45oC to 37oC.

This coating can find application in various sectors in addition to housing and other structures, such as the Railways (a market of about Rs 300 crore); transportation (~ Rs 100 crore); defence, which includes warehouses, hangers and remote barracks (about Rs 400 crore) and cold storage. There are basically two types of coatings, one for concrete and the other for steel surfaces. These are cost-effective, easy to apply and durable, with a longevity of five to eight years.

The evolution of energy-saving coatings

‘Cool roof’ coatings have existed for over a decade and were made using rudimentary methods. One such method was to add ceramics and hollow-sphere particles in the paint to act as insulation and reduce heat passing from the roof to the ceiling (see Figure 1c). Another way was to apply white colour coatings on the roof to reflect part of the heat from the visible section of solar radiation. However, a more effective cooling effect can be obtained by absorbing the entire IR radiation and then reflecting it.

Figure 1 (a) Sunlight falling on the roof; (b) breakup of solar radiation; (c) conventional cool roof coatings; and (d) principle of solar heat-reflective coatings

As seen in Figure 1b, solar radiation consists of ultraviolet (UV), visible and infrared in the ratio 5 per cent, 43 per cent and 52 per cent. By using suitable additives and pigments in normal architectural paints, it is possible to absorb all IR radiation. When this is followed by reflection of this absorbed radiation, it results in extensive cooling on the roof surface as shown in Figure 1d. In addition, it is possible to add natural fillers in the coating to provide sufficient insulation to the coating as well. Based upon this, a patented solar heat-reflective (SHR) coating was developed at IIT-Bombay. This is now available under the Thermacool coatings brand in a wide range of products for concrete and steel roofing as well as any other kind of synthetic roofing like asbestos, bitumen or ceramic tiles (see Table 1).

Table 1: Range of Thermacool solar-heat reflective coatings (www.thermacool.in)

Electricity savings

In a usual Indian summer, peak temperatures are around 44-47oC in many cities. With such ambient temperatures, roof temperatures would be about 65oC. Under these conditions, there is only a small drop in temperatures ~10oC and one witnesses ceiling temperatures of ~55oC. Thus, one has to reduce the temperature from 55oC to 25oC using an AC or cooler for comfortable living, which requires a large amount of electricity. However, after applying the SHR coating on the roof, the ceiling temperature drops to 37oC ( a drop of 18oC), requiring much less electricity to reach 25oC.

Figure 2: Electricity saved using solar heat-reflective coating and reduction in carbon footprint

Let us calculate the savings. Figure 2 summarises how the application of the coating on a rooftop saves seven units of electricity per day for a 1-tonne AC in a 1,000 sq ft room, when run for eight hours a day. This would save Rs 314 per month. If converted to carbon units, it amounts to 4.2 kg coal per day—1,073 kg coal equivalent or 1,788.5 units of power per year. This is a tremendous saving.

Proof of concept with various industrial applications

The most important part of the technology is job execution. The first step is surface preparation, either through simple washing to remove dust or a soap solution to remove grease and oily contamination. Following this, if the surface is sound with no cracks or blisters, it is suitable for the final cool roof coating application. If the surface is not sound, a surface tolerant primer is recommended. In case of a new concrete roof, the primer can be a simple primer to adjust the moisture. But if the concrete roof is old, it is important to mend it by filling cracks and applying a waterproofing primer.

Here are some case studies of application.

TVS scooter showroom and store, Vijayawada: There was drop of almost 12oC on the roof and 20oC on the ceiling after application of Thermacool SHR coating 0.3M. No primer was necessary as the surface after pressure washing was sound.

Figure 3: TVS showroom in Vijayawada

Shed in Air Force Station, Race Course Road, New Delhi: The Station has a closed shed that stores hydrogen cylinders and gets very hot in the summer. In October 2018, when the ambient temperature was 34oC, the temperature on the roof of the shed was 53.5oC. After application of SHR coating 0.3M, it dropped to 33.6oC. The corresponding ceiling temperature was 46.5oC before coating and 32.7oC afterwards; the total drop in room temperature was 9.3oC.

The results are summarised in Table 2 and shown in Figure 4.

Figure 4: Air Force Station, Race Course Road, New Delhi

Indian Railways: In summers, travelling in non-AC compartments is a great challenge, while AC compartments use excess electricity. Use of SHR coatings helped increase passenger comfort in non-AC compartments, while AC compartments consumed a much lower amount of electricity (see Figure 5).

Figure 5: (left) Drop in roof temperature of Shatabdi Express (AC coach) by 24oC; (right) drop in roof temperature of Flying Rani Express (non-AC coach) by 18oC

Civil buildings with RCC roof: A drop of 18oC was reported in a normal house made with RCC concrete (see Figure 6).

Figure 6: Drop in roof temperature with SRH coating 0.3C

Water tanks: A drop of 16oC was noted for Sintex water tanks on house roofs with the use of SHRs (see Figure 7).

Figure 7: Temperature drop in water tank on rooftop

Buses: SHR coatings are very useful for transport buses, both AC and non-AC. The results are shown in Figure 8.

Figure 8: Example of temperature drop of 20oC on the roof of an AC bus with 14oC drop in ceiling temperature

Indeed, SHR coatings are a simple way to save electricity and, in turn, protect the environment. These products are eco-friendly and GRIHA-certified. Thermogreen Cool Coat Pvt Ltd is the only company with patented technology for cool roof coating and has established itself with several demonstrations across industries.

About the author: Prof AS Khanna retired from IIT-Bombay and is now Chairman, SSPC India, a professional body focusing on surface engineering. In his 27 years at IIT-Bombay, he focused his research on corrosion-related topics and surface engineering. His last PhD work was on SHR coatings, which got two patents. The technology was sold to Thermogreen Cool Coat Pvt Ltd, where is a Technical Director.