Solar heat reflective paint & coatings using 3M Glass Bubbles

Transcription

1 Solar heat reflective paint & coatings using 3M Glass Bubbles 1- Introduction In most countries, energy use in the building sector represents about one third of the total energy consumption. In developing countries the residential building sector accounts for more than half of the electricity consumption. The economic growth is expected to boost residential demand for electricity, such as electric lighting, air conditioning and other appliances. Energy consumption for residential cooling shows an increasing trend worldwide and is of primary concern for countries that are characterized by hot climatic conditions. The extensive use of air-conditioning is an economic concern. Increasing electricity demand for cooling increases peak electricity loads which leads to burn more fossil, increasing energy costs and pollution levels. In addition, problems with indoor air quality related to the use of air-conditioners are of serious concern. To decrease the demand for air-conditioning use, heat reflective coatings have gained a lot of interest during the past few years. Heat reflective coatings are characterized by high solar reflectance (SR) and high emittance values in the thermal infrared region. Those coatings can be roof coating and wall façade paint. 2- How it works Understanding how heat reflective coatings work requires knowing how solar energy heats walls and roofs and how the properties of these materials can contribute to warming. This section explains solar energy, the properties of solar reflectance and thermal emittance, and the combined effect of these two properties working together. 2-1 Solar Energy The spectral energy distribution of solar radiation at sea level is shown below:

2 Solar Irradiance (W/m².nm) 1.6 UV 5% Visible 46% N-IR 49% Wavelength (nm) This energy is usually divided into three bands: The ultraviolet band, between 200 and 400 nm, represents 5% of the sunlight energy, including the type of rays responsible for sunburn. The visible band, between 400 and 700 nm, represents 46% of the sunlight energy, in colors ranging from violet to red. The near-infrared band, between 700 and 2500 nm, represents 49% of the sunlight energy, felt as heat. The total solar irradiance at normal incidence is about1.3 kw/m². 2-2 Solar Reflectance Solar reflectance, or albedo, is the percentage of solar energy reflected by a surface. Researchers have developed methods to determine solar reflectance by measuring how well a material reflects energy at each solar energy wavelength, then calculating the weighted average of these values to obtain the Total Solar Reflectance (TSR). Traditional paint and coatings have low solar reflectance of 5 to 15 percent, which means they absorb 85 to 95 percent of the energy reaching them instead of reflecting the energy back out to the atmosphere. The coolest paint and coatings have a high solar reflectance of more than 80 percent, absorbing and transferring to the building 20 percent or less of the energy that reaches them. These materials reflect radiation across the entire solar spectrum, especially in the visible and infrared (heat) wavelengths. 2-3 Thermal Emittance

3 Any surface exposed to radiant energy will get hotter until it reaches thermal equilibrium (i.e., it gives off as much heat as it receives). A material s thermal emittance determines how much heat it will radiate per unit area at a given temperature, that is, how readily a surface gives up heat. When exposed to sunlight, a surface with high emittance will reach thermal equilibrium at a lower temperature than a surface with low emittance, because the highemittance surface gives off its heat more readily. 2-4 Solar Reflectance Index (SRI) Solar Reflectance Index is the combined value of Reflectance and Emittance. SRI is defined so that a standard black surface is zero (reflectance = 5, emittance = 90) and a standard white is 100 (reflectance 80, emittance = 90). Because the way SRI is defined, very cool materials can have SRI values exceeding Cool pigments Reflectance of solar light is achieved by scattering visible and infrared radiations. The scattering of radiations by a pigment is a function of the difference between the refractive indices of the pigment and the resin in which it is dispersed. The greater difference, the better is the reflectance. Common organic resins have refractive indices between 1.45 and The refractive indices of commercial pigments vary between 1.4 and 2.8. Rutile titanium dioxide TiO 2 is the most effective white pigment to scatter visible light due to its high refractive index (2.8) and its particle size being half the average wavelengths of the visible band. 4-3M Glass Bubbles 3M Glass Bubbles are high-strength, low-density additives made from a water resistant and chemically-stable soda-lime-borosilicate glass. They are used in a variety of applications, including automotive, marine, oil and gas and construction. Because of their hollow shape, 3M Glass Bubbles are used in the paint industry for many years to provide thermal insulation. Those paints are sold as anti-condensation paint for high humidity rooms such as kitchen and bathroom. Resistance to condensation comes from the greater surface temperature of the paint compared with ordinary paint. Due to their hollow structure, 3M Glass Bubbles have a low refractive index, close to 1.0, much lower than the refractive index of common resins, hence to expect visible and nearinfrared scattering efficiency:

4 Volume (%) Grade Composition Target Crush Strength 90% Survivals (bar) True Density (g/cm 3 ) Particle Size (microns, by volume) Calculated Refractive Index 10% 50% 90% Effective Top Size(95%) K K S K XLD K25 Soda-lime S32LD Borosilicate XLD6000 Glass S K S S38HS K S S60HS im30k Evaluation of 3M Glass Bubbles in solar paint and coatings To quantify the effect of 3M Glass Bubbles on reflectance of solar light, a design of experiments is conducted: 3 grades of 3M Glass Bubbles are selected according to their density and particle size distribution, K20, S22 and XLD6000. For comparison purpose, a standard grade of calcium carbonate is selected. For additional comparison, a widely promoted grade of ceramic microspheres in solar paint, also called cenospheres is included in the study. Particle size distribution of each filler is compared: Particle Size Distribution Ceramic d50 = 90µm K20 d50 = 60µm S22 d50 = 30µm 10 XLD6000 d50 = 17 µm Particle Size (µm) 5 paints are prepared having the same Pigment Volume Concentration of 60%. Titanium dioxide is set at 15%, the remaining 45% are calcium carbonate or 3M Glass Bubbles grade K20 or S22 or XLD6000 or ceramic microspheres according to the following starting formulations:

5 With calcium carbonate raw material Density(g/cc) Weight W % Volume V % % Vol Dry Water Wetting agent TiO CaCO Resin Glass Bubbles Thickener Anti-foam Totals With 3M Glass Bubbles raw material Density(g/cc) Weight W % Volume V % % Vol Dry Water Wetting agent TiO CaCO Resin Glass Bubbles Thickener Anti-foam Totals With Ceramic microspheres raw material Density(g/cc) Weight W % Volume V % % Vol Dry Water Wetting agent TiO CaCO Resin Ceramic Bubbles Thickener Anti-foam Totals Solar Reflectance Measurement Total solar reflectance of coating is measured using a UV-Vis-NIR spectrophotometer and an integrated sphere, according to ASTM E Testing equipment: Spectrophotometer CARY Labsphere Integrating Sphere

6 % Reflectance Test results: Solar Reflectance according to size of particles Wavelength (nm) CaCO3 400µ S22 400µ XLD 400µ K20 400µ Ceramic 400µ The solar reflectance is calculated using the ASTM method E and reference table for solar irradiance from ASTM G173-03:

7 % Reflectance Total Solar Reflectance CaCO3 S22 XLD6000 K20 Ceramic Sample TSR UV Visible IR CaCO S XLD K Ceramic Thermal Emittance Measurement Thermal emittance is measured according to NF EN standard using an ABB BOMEM MB-154S FTIR spectrometer and a gold coated integrated sphere available from SphereOptics:

8 % Emissivity 100 Thermal Emittance CaCO3 S22 K20 XLD6000 Ceramic Wavelength (microns) 5-3 Overall test results: Sample Thermal Emittance Total Solar Reflectance Solar Reflectance Index CaCO S XLD K Ceramic Infrared lamp test: An additional in-house test was developed to evaluate the effect of solar heat reflective paint and coatings on inside temperature of buildings:

9 IR lamp Painted aluminum panel Data logger Thermocouple Aluminum painted panels are exposed to a Philips BR W IR lamp light. Distance between bulb and painted panel is 10 centimeters. A thermocouple is attached to the back side of the aluminum panel. A Sefram log 1522 data logger is used to record temperature versus time. The test is conducted for 1 hour. The steady state is reached within a time period of 20 minutes:

10 Temperature ( C) IR Lamp test CaCO3 (ref.) K20 (-8.5 C) S22 (-10.3 C) XLD6000 (-15.1 C) Ceramic Microspheres (-3.2 C) Time (minutes) The paint formulated with calcium carbonate is set as the reference paint. Temperature difference is calculated using the average temperature for each paint at steady state: Sample CaCO3 K20 S22 XLD6000 Ceramic Average T at plateau ( C) T difference versus CaCO3 paint ( C) In steady state, a temperature difference as high as 15 C can be measured for the solar heat reflective paint formulated with 3M Glass Bubbles XLD Energy Saving Evaluation 6-1 European Union Cool Roof Council (EU-CRC) The EU COOL ROOFS COUNCIL has been founded in February The aim of the Cool Roofs Project is to implement an Action Plan for the cool roofs in EU. The specific objectives are: To support policy development by transferring experience and improving understanding of the actual and potential contributions by cool roofs to heating and cooling consumption in the EU To remove market barriers and simplify the procedures for cool roofs integration in construction and building s stock To change the behavior of decision-makers and stakeholders so to improve acceptability of the cool roofs

11 To disseminate and promote the development of innovative legislation, codes, permits and standards, including application procedures, construction and planning permits concerning cool roofs. The work is developed in four axes, technical, market, policy and end-users. 6-2 Calculator of energy savings A specific calculator was developed in the framework of the EU-CRC technical axis to support the evaluation of the energy and cost benefits of the cool roofs technology: The model used for estimating savings is adapted from the paper of Petric et al., "Effect of solar radiation control on energy costs - A radiation control fact sheet for low slope roofs", Proceedings, Performance of the exterior envelopes of whole buildings VIII: Integration of Building envelopes, Atlanta, December 2001, American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc. The present calculator works well for low slope roofs, i.e. roofs with pitch less than 2:12 and the output gives annual savings relative to a black roof. Using the above mentioned test results from the paint made with 3M Glass Bubbles XLD6000, one can calculate the potential energy saving compared to the same roof painted in black:

12 If we consider an average roof surface of 120 m², the potential annual energy saving using this configuration compared to a black roof would be: 120 m² x = ± 5000 If we extrapolate this model by doing the comparison with a roof having a average solar reflectance of 50, the potential annual energy saving could still be ± Conclusions 3M Glass Bubbles do improve solar reflectance properties of paint and coatings. A solar Reflectance Index greater than 110 can be achieved using 3M Glass Bubbles S22 or XLD6000. In the event the paint is applied by airless spray at high pressure, a greater pressure strength 3M Glass Bubbles such as XLD6000 will be needed to survive airless spray conditions. Solar reflectance enhancement appears to be related to microspheres particle size: the smaller, the better. Ceramic microspheres being very coarse compared to 3M Glass Bubbles do not provide solar reflectance improvement compared to ordinary paint and coatings. In addition to provide solar reflectance properties, 3M Glass Bubbles contribute to reduce the thermal conductivity of coatings due to their hollow structure with partial vacuum inside. The use of solar heat reflective coatings using 3M Glass Bubbles is an inexpensive solution that can contribute to the reduction of cooling loads in air-conditioned buildings, so to save energy and money. In non air-conditioned residential buildings, the improvement of indoor thermal comfort conditions is achieved by decreasing the maximum temperatures in summer time. Furthermore, the same technology can be transposed to non-residential applications such as refrigerated storage trucks and warehouses, oil & gas storage tanks and cryogenic tanks and tankers where there is a need to keep the inside cool. Jean-Marie Ruckebusch Senior Technical Service Specialist 3M Energy and Advanced Materials Division - Specialty Additives Laboratory 3M France Route de Sancourt Tilloy lez Cambrai - France Tel: Fax: Mobile: