Abstract
Solar-control films are used worldwide as an effective means of lowering building energy costs by reducing excessive solar heat gain through windows. Determining energy savings from actual installations is difficult due to the common practice of implementing several energy conservation measures simultaneously and due to annual variations in the many factors that can affect a building’s energy usage.
To isolate and quantify the energy-saving benefits of solar films, an energy analysis study on a conventional office building was undertaken using the U.S. Department of Energy (DOE)’s sophisticated DOE-2 energy-simulation software. The study included several types of solar films applied to various glazing systems. Various cities were included in the study to illustrate the energy savings in different climates and to show the effect of differing electricity costs.
Based on the most typical types of installations and on customary installation costs for medium-size commercial projects, the average return on investment (payback) from solar film application was an impressive 2.65 year. These savings were the result of reducing annual electricity kilowatt-hour (kWh) usage by an average of 6.6% and reducing peak summer month kilowatt demand on average by 6.4%.
INTRODUCTION
Solar-control window films are considered in the building industry to be “retrofit” products—that is, products that are applied to existing buildings after construction as opposed to being used in new construction.1-1,2-1
Solar Film Construction
Solar-control window films typically consist of a thin (0.025-mm, 0.001-in.) polyester film substrate that has a microthin transparent metal coating applied to one side. This metal coating is applied using vacuum-based technologies such as vapor deposition or sputtering. The metal coating may be a single metal, an alloy, a metal-oxide, or a combination of these coatings. A second layer of polyester film is laminated over the metal coating to protect the metal. Onto one side of this laminated composite, an acrylic scratch-resistant (SR) coating is applied to the surface that will face the building interior. This SR coating protects the film during normal window cleaning. On the opposite side of this film laminate, a clear adhesive is applied, which will eventually bond the film to the window glass. This adhesive layer is protected by a removable release liner until just before field application. The film is protected from ultraviolet (UV) degradation by UV absorbers that are added to the polyester film layers, the adhesive layer, or both.
Solar Film Appearance and Properties
The appearance of the film (the color, the level of visible light transmission, and the degree of reflectivity) depends on the metal coatings used. Typical all-metal solar films can be silver-reflective, gray, silver-gray, bronze, or light green in color. Visible light transmissions can vary from very dark (10%) to very light (70%), and visible reflectance can vary from the same reflectance as clear glass (8%) to highly reflective (60%). The ability of a glazing system to reduce solar heat gain is measured by its solar heat-gain coefficient (SHGC). As expected from the variety of films available, the SHGC for solar films can vary significantly, from 0.14 to 0.69, as measured on 6-mm (1/4-in.) clear glass.
Solar Film Benefits
This combination of film properties produces a product that provides several important benefits:
• Reduced cooling energy costs by reducing excessive solar heat gain
• Enhanced reduction in cooling and heating energy costs when low-e type films are used
• Enhanced tenant comfort from improved temperature distribution (less hot and cold spots) and reduced glare
• Uniform building appearance from the exterior— improving tenant retention in leased buildings
• Reduced fading of carpets, drapes, and furnishings due to the UV-blocking ability of films
• Privacy for building occupants when using reflective or dark films
Solar Film Installation Process
The first step in the installation of solar film involves rigorous cleaning of the window glass surface. Next, an application solution is sprayed onto the glass; the release liner is removed from the film; and the adhesive side of the film is carefully placed onto the glass surface. The application solution allows one to move the film on the glass for precise film placement, and it prevents air from becoming trapped between the film and glass. Then the film is carefully trimmed around the perimeter of the window, leaving a 1-3-mm (1/32-in. to 1/8-in.) gap between the film and frame. Water is sprayed onto the film surface, and a rubber squeegee is used to remove the application solution and bond the film to the glass surface.
MOTIVATION FOR SOLAR-CONTROL FILM ENERGY ANALYSIS STUDY
Solar-control films have been used since the early 1960s as an effective means of reducing building energy costs. Unfortunately, it is difficult to determine precisely the cost savings from applying film to a building, due to variations in the many factors that affect a building’s energy consumption from year to year (weather changes such as the amount of sunshine and temperature/wind differences, changes in occupancy, upgrades or other changes in building energy-using equipment, changes in maintenance for key equipment, the addition of energy-consuming equipment such as computers, etc). This situation is usually complicated by the fact that building owners usually perform energy-conservation upgrades such as solar-control film application in conjunction with other upgrades, making it impossible to determine the savings from film application alone. Therefore, a means of accurately estimating the energy savings from solar film application was needed.
One of the most accurate and reliable energy simulation software packages available is the DOE-2 energy analysis program.[3] DOE-2, which uses an hourly calculation method, has been validated many times by comparing its results with thermal and energy-use measurements on actual buildings (see http://gundog.lbl.gov).[4] DOE-2 is used worldwide by energy engineers, architects, government organizations, and utilities as a means of estimating the effect of various measures on a building’s energy consumption and for developing building energy codes. As a result, it was determined that a reasonable course of action was to use DOE-2 modeling to estimate the energy savings from application of solar-control window film.
SCOPE OF ENERGY ANALYSIS STUDY
The DOE-2 energy study was performed on a conventional (1990s) 10-story office building with a total floorspace of 16,257 m2 (175,000 ft2). To gauge the effect of different films, four films were chosen and categorized as “Maximum-Performance,” “Maximum-Performance
Table 1 Solar and thermal performance factors for windows and films in study
| No film | Maximum performance film | Maximum performance low-E film | High performance film | High performance low-E film | |
| Solar heat-gain coefficient (SHGC) | |||||
| Single clear | 0.81 | 0.23 | 0.17 | 0.36 | 0.28 |
| Dual clear | 0.70 | 0.31 | 0.27 | 0.42 | 0.35 |
| Single gray | 0.57 | 0.27 | 0.21 | 0.33 | 0.26 |
| Dual gray
[/-values (W/m2 °C) |
0.45 | 0.24 | 0.21 | 0.30 | 0.25 |
| Single clear | 6.18 | 5.81 | 4.78 | 5.87 | 4.69 |
| Dual clear | 2.74 | 2.65 | 2.35 | 2.66 | 2.32 |
| Single gray | 6.19 | 5.81 | 4.78 | 5.87 | 4.69 |
| Dual gray | 2.74 | 2.65 | 2.35 | 2.66 | 2.32 |
| [/-values (BTU/h/ft2/°F) Single clear | 1.09 | 1.02 | 0.84 | 1.03 | 0.83 |
| Dual clear | 0.48 | 0.47 | 0.41 | 0.47 | 0.41 |
| Single gray | 1.09 | 1.02 | 0.84 | 1.03 | 0.83 |
| Dual gray | 0.48 | 0.47 | 0.41 | 0.47 | 0.41 |
Low-E," "High-Performance," and "High-Performance Low-E," based on the film's SHGC on single-pane, 6-mm (1/4-in.) clear glass.
The study was performed on single-pane clear, dual-pane clear, single-pane gray-tinted, and dual-pane gray-tinted window systems. Each film type was analyzed on each of these glazing systems. All windows consisted of 6-mm-thick (1/4-in. -thick) panes, and for the dual-pane units, the panes were separated by a 12-mm (1/2-in.) air space.
The SHGC and Winter Night-Time {/-values for all glazing systems contained in the study are shown below. The SHGC shown is at normal solar incidence angle, and the Winter Night-Time {/-value is based on American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) standard Winter conditions, using a — 17.8°C (0°F) outdoor air temperature, an indoor air temperature of 21°C (70°F), a 24.1 km/h (15 mph) wind speed outdoors, a 0 km/h (0 mph) wind speed indoors, and 0 W/m2 (0 Btu/h/ft2) solar intensity. In the analyses, the SHGC was calculated (by DOE-2) for the precise sun angle at each hour of the day; and the window {/-value was calculated at each hour based on indoor and outdoor temperatures, outdoor air speed, and solar intensity (Table 1).
The model building was a square building with equal glass area facing north, south, east, and west. The glass area on each of the four building exposures was 557.4 m2 (6,000 ft2). Models with film applied used film on the east, south, and west exposures only.
Other model parameters, typical of modern office buildings, used in the study included
• Indoor lighting 10.76 W/m2 (1.0 W/ft2)
• Office equipment 11.95 W/m2 (1.1 W/ft2)
• Heating setpoint 21.1 °C (70°F)
• Heating setback 18.3°C (65°F)
• Cooling setpoint 23.9°C (75°F)
• Cooling setback 26.7°C (80°F)
• Medium-colored blinds used 25% of the time; SHGC of blinds 0.69.
• Windows recessed from building face 15 cm (6 in.) providing partial shading of all windows
• Variable-air-volume (VAV) air-distribution system and air-side economizer
• Heating plant using gas boilers with an efficiency of 80%
• Chillers with full-load efficiency of 0.69 KW per ton (Coefficient of Performance [COP] of 5.1)
The parameters used (such as the use of blinds, recessed windows, the VAV air distribution system, the air-side economizer, and the high-efficiency chiller system) effectively reduce the savings from solar film application. This was desired to provide reasonable and conservative estimates of energy savings from solar film installation, not to create a “best case” scenario.
Electricity costs for each location were determined from the commercial rate schedules published on the Web sites for electric utilities in each city. Rate schedules that applied to buildings with peak kilowatt demands of approximately 1000 KW were used. Both kilowatt-hour and kilowatt demand charges were used. The rate schedules used typically vary the kilowatt-hour and kilowatt charges by time of year and time of day, and are too complex to provide here; however, to provide the reader a general idea of the costs used, Table 2 shows the average costs for each city based on the total annual kilowatt-hour used and the total annual electricity costs for the four building models without film.
RESULTS OF STUDY
Tables 3-5 show the payback, reduction in annual kilowatt-hour usage, and reduction in summer month peak demand for each location and for each film and window combination. Also shown are the averages for each window type and each location. The overall average for all locations, window types, and films is also given.
Table 2 Average electricity costs by location
| City | Average cost ($/kWh) | Electric utility and rate schedule |
| Boston | 0.1220 | Boston edison, rate G-3 |
| Chicago | 0.0892 | Commonwealth edison, rate 6L |
| Dallas | 0.0907 | Texas-new mexico power, large general service |
| Jacksonville | 0.0697 | Florida power & light, GSLD-1 |
| Los Angeles | 0.1336 | Southern California edison, TOU-8, large general service |
| Memphis | 0.0604 | Memphis light, gas & water, general service GSA |
| Phoenix | 0.0595 | Arizona public service co., general service |
| Toronto (Canada) | 0.0553 | Toronto hydro-electric, business rates |
| Washington, D.C. | 0.0706 | Baltimore gas & electric, schedule GL (option 2) |
| Overall average | 0.0834 |
Natural gas was used as the heating fuel and for domestic hot water production at a cost of 70 cents per therm for all locations.
Table 3 Simple payback by location, window, and film type
| Single clear Max-perf film (%) | Boston
0.89 |
Chicago
1.11 |
Dallas
0.87 |
Jacksonville
1.18 |
Los Angeles
0.69 |
Memphis
1.42 |
Phoenix
1.10 |
Toronto
1.73 |
Washington
1.28 |
Averag
1.14 |
| Max-perf low-E film (%) | 0.81 | 1.00 | 0.85 | 1.15 | 0.69 | 1.33 | 1.07 | 1.43 | 1.16 | 1.06 |
| High-perf film (%) | 1.12 | 1.37 | 1.10 | 1.53 | 0.87 | 1.82 | 1.38 | 2.19 | 1.66 | 1.45 |
| High-perf low-E film (%) | 0.91 | 1.10 | 0.95 | 1.31 | 0.79 | 1.48 | 1.20 | 1.55 | 1.30 | 1.18 |
| Single clear-all film types average (%): Dual clear | 1.21 | |||||||||
| Max-perf film (%) | 1.46 | 1.79 | 1.42 | 2.03 | 1.12 | 2.57 | 1.77 | 2.64 | 2.12 | 1.88 |
| Max-perf low-E film (%) | 1.34 | 1.65 | 1.34 | 1.90 | 1.08 | 2.29 | 1.70 | 2.29 | 1.93 | 1.72 |
| High-perf film (%) | 2.01 | 2.50 | 2.00 | 3.02 | 1.57 | 3.69 | 2.47 | 3.94 | 3.14 | 2.70 |
| High-perf low-E film (%) | 1.55 | 1.91 | 1.60 | 2.31 | 1.25 | 2.68 | 2.01 | 2.69 | 2.30 | 2.03 |
| Dual clear-all film types average (%): Single gray | 2.09 | |||||||||
| Max-perf film (%) | 2.09 | 3.14 | 2.18 | 2.81 | 1.78 | 3.81 | 2.39 | 4.16 | 2.90 | 2.81 |
| Max-perf low-E film (%) | 1.42 | 1.94 | 1.59 | 2.09 | 1.36 | 2.49 | 1.90 | 2.34 | 1.96 | 1.90 |
| High-perf film (%) | 2.62 | 3.95 | 2.81 | 3.70 | 2.26 | 4.96 | 2.99 | 5.31 | 3.84 | 3.61 |
| High-perf low-E film (%) | 1.49 | 1.98 | 1.73 | 2.28 | 1.49 | 2.61 | 2.03 | 2.39 | 2.08 | 2.01 |
| Single gray-all film types average (%): Dual gray | 2.58 | |||||||||
| Max-perf film (%) | 3.92 | 5.57 | 3.71 | 4.63 | 2.99 | 6.90 | 3.86 | 6.53 | 4.96 | 4.79 |
| Max-perf low-E film (%) | 2.90 | 3.56 | 2.81 | 3.57 | 2.32 | 4.65 | 3.18 | 4.26 | 3.57 | 3.42 |
| High-perf film (%) | 5.60 | 8.02 | 5.49 | 6.82 | 4.34 | 10.26 | 5.20 | 9.19 | 7.04 | 6.88 |
| High-perf low-E film (%) Dual gray-all film types average (%): | 3.12 | 3.86 | 3.19 | 4.04 | 2.62 | 5.11 | 3.58 | 4.53 | 3.99 | 3.78 4.72 |
| Location average (%) | 2.08 | 2.78 | 2.10 | 2.77 | 1.70 | 3.63 | 2.36 | 3.57 | 2.83 | |
| All location, all window types, all film types (%) | 2.65 |
Table 4 Reduction in annual kilowatt-hour usage
| \ | Boston | Chicago | Dallas | Jacksonville | Los Angeles | Memphis | Phoenix | Toronto | Washington | Average |
| Single clear | ||||||||||
| Max-perf film (%) | 9.5 | 9.6 | 12.1 | 11.4 | 11.7 | 11.0 | 13.6 | 9.2 | 10.3 | 10.9 |
| Max-perf low-E film (%) | 10.9 | 10.9 | 13.5 | 12.7 | 12.9 | 12.3 | 15.3 | 10.5 | 11.7 | 12.3 |
| High-perf film (%) | 7.5 | 7.8 | 9.6 | 8.9 | 9.3 | 8.7 | 10.7 | 7.2 | 8.1 | 8.6 |
| High-perf low-E film (%) | 9.4 | 9.6 | 11.8 | 11.0 | 11.1 | 10.8 | 13.4 | 9.1 | 10.2 | 10.7 |
| Single clear-all film types average (%): Dual clear | 10.6 | |||||||||
| Max-perf film (%) | 6.2 | 6.4 | 7.9 | 7.1 | 7.6 | 6.8 | 9.1 | 6.2 | 6.7 | 7.1 |
| Max-perf low-E film (%) | 7.3 | 7.4 | 9.2 | 8.4 | 8.8 | 8.0 | 10.5 | 7.3 | 7.9 | 8.3 |
| High-perf film (%) | 4.5 | 4.6 | 5.7 | 4.9 | 5.5 | 4.8 | 6.6 | 4.3 | 4.7 | 5.1 |
| High-perf low-E film (%) | 6.1 | 6.2 | 7.6 | 6.9 | 7.4 | 6.7 | 8.8 | 6.0 | 6.5 | 6.9 |
| Dual clear-all film types average (%): Single gray | 6.8 | |||||||||
| Max-perf film (%) | 4.2 | 3.8 | 5.3 | 5.2 | 5.0 | 4.6 | 6.7 | 4.0 | 4.7 | 4.8 |
| Max-perf low-E film (%) | 5.9 | 5.4 | 7.3 | 7.1 | 6.8 | 6.5 | 9.0 | 5.7 | 6.5 | 6.7 |
| High-perf film (%) | 3.4 | 3.0 | 4.2 | 3.9 | 4.0 | 3.6 | 5.4 | 3.1 | 3.6 | 3.8 |
| High-perf low-E film (%) | 5.4 | 5.0 | 6.6 | 6.3 | 6.1 | 5.9 | 8.2 | 5.1 | 5.8 | 6.1 |
| Single gray-all film types average (%): Dual gray | 5.3 | |||||||||
| Max-perf film (%) | 2.6 | 2.5 | 3.5 | 3.4 | 3.3 | 3.0 | 4.6 | 2.8 | 3.1 | 3.2 |
| Max-perf low-E film (%) | 3.5 | 3.5 | 4.7 | 4.6 | 4.4 | 4.1 | 6.0 | 3.8 | 4.2 | 4.3 |
| High-perf film (%) | 1.8 | 1.8 | 2.4 | 2.4 | 2.3 | 2.1 | 3.5 | 2.0 | 2.2 | 2.3 |
| High-perf low-E film (%) | 3.1 | 3.1 | 4.0 | 3.9 | 3.8 | 3.6 | 5.2 | 3.3 | 3.7 | 3.8 |
| Dual gray-all film types average (%): Location average (%)
All location, all window types, all film types (%) |
5.7 6.6 | 5.7 | 7.2 | 6.8 | 6.9 | 6.4 | 8.5 | 5.6 | 6.2 | 3.4 |
Table 5 Reduction in summer peak kilowatt demand
| Single clear Max-perf film (%) Max-perf low-E film (%) | Boston
11.4 13.0 |
Chicago
10.6 12.2 |
Dallas
11.2 12.9 |
Jacksonville
10.0 11.5 |
Los Angeles
9.7 11.2 |
Memphis
10.1 11.8 |
Phoenix
11.3 13.6 |
Toronto
11.4 13.0 |
Washington
10.4 12.3 |
Averag
10.7 12.4 |
| High-perf film (%) | 8.7 | 8.2 | 8.5 | 7.4 | 7.4 | 7.6 | 8.9 | 8.7 | 7.7 | 8.1 |
| High-perf low-E film (%) | 10.9 | 10.4 | 11.1 | 9.9 | 9.1 | 10.0 | 11.7 | 11.1 | 10.3 | 10.5 |
| Single clear-all film types average (%): Dual clear | 10.4 | |||||||||
| Max-perf film (%) | 6.5 | 6.8 | 6.9 | 5.7 | 5.7 | 5.8 | 7.4 | 7.6 | 6.3 | 6.5 |
| Max-perf low-E film (%) | 7.8 | 8.1 | 8.4 | 7.1 | 6.5 | 7.0 | 8.9 | 9.0 | 7.6 | 7.8 |
| High-perf film (%) | 4.7 | 4.9 | 4.8 | 3.6 | 4.0 | 3.7 | 5.2 | 5.2 | 4.1 | 4.5 |
| High-perf low-E film (%) | 6.6 | 6.6 | 6.9 | 5.5 | 5.8 | 5.8 | 7.4 | 7.3 | 6.1 | 6.4 |
| Dual clear-all film types average (%): Single gray | 6.3 | |||||||||
| Max-perf film (%) | 5.3 | 4.4 | 4.9 | 4.7 | 4.6 | 4.4 | 5.7 | 5.3 | 5.4 | 5.0 |
| Max-perf low-E film (%) | 7.4 | 6.3 | 7.3 | 7.0 | 6.5 | 6.7 | 8.3 | 7.4 | 7.5 | 7.2 |
| High-perf film (%) | 4.2 | 3.4 | 3.8 | 3.6 | 3.4 | 3.3 | 4.6 | 4.0 | 4.2 | 3.8 |
| High-perf low-E film (%) | 6.8 | 5.9 | 6.6 | 6.3 | 5.4 | 6.1 | 7.7 | 6.6 | 6.9 | 6.5 |
| Single gray-all film types average (%): Dual gray | 5.6 | |||||||||
| Max-perf film (%) | 2.9 | 2.8 | 2.9 | 3.0 | 2.3 | 2.6 | 3.3 | 3.8 | 3.2 | 3.0 |
| Max-perf low-E film (%) | 3.7 | 4.0 | 4.3 | 4.2 | 3.2 | 3.8 | 4.7 | 5.2 | 4.4 | 4.2 |
| High-perf film (%) | 2.0 | 1.7 | 1.9 | 1.9 | 1.6 | 1.7 | 2.4 | 2.7 | 2.4 | 2.0 |
| High-perf low-E film (%) | 3.5 | 3.4 | 3.6 | 3.7 | 3.0 | 3.4 | 4.2 | 4.5 | 3.7 | 3.7 |
| Dual gray-all film types average (%): | 3.2 | |||||||||
| Location average (%) | 6.6 | 6.2 | 6.6 | 5.9 | 5.6 | 5.9 | 7.2 | 7.1 | 6.4 | |
| All location, all window types, all film types (%) | 6.4 |
Following are some general observations concerning the results of the study.
• For all window and film types and all locations, the overall average payback for solar film installation was 2.65 year (see Table 3). The average payback by window type: Single Clear, 1.21 year; Dual Clear, 2.09 year; Single Gray, 2.58 year; and Dual Gray, 4.72 year (in almost 50% of the cases involving Dual Gray windows, the payback was less than 4 year).
• As shown in Table 3, it appears that the payback period is affected more by the cost of electricity than by climate effects. The average payback in Boston (2.1 year), for example, is less than the average payback in Jacksonville, Florida (2.8 year), due mainly to the higher average cost of electricity in Boston compared with Jacksonville (12.2 cents per kWh average vs 6.97 cents). Also, the average payback for Memphis (3.6 year) was more than in Washington, D.C. (2.8 year), even though the climate in Memphis is somewhat warmer, solely due to the lower cost of electricity in Memphis.
• The data also show that solar-control film is not a “warm climate only” product. The average payback for cities not considered to be in the Sun Belt was still less than 3 year on average (Boston, 2.1 year; Chicago, 2.8 year; Washington, DC, 2.8 year). The average payback for Toronto (the coolest climate of all cities considered) was still a very respectable 3.6 year, despite the fact that Toronto has the lowest overall electricity prices of cities in the study.
• Solar-control film has a considerable positive effect on reducing annual kilowatt-hour and summer peak demand, on average reducing annual kilowatt-hour usage by 6.6% and summer month peak demand by 6.4% (see Tables 4 and 5).
CONCLUSIONS
This study clearly indicates that solar-control window film can play a useful and viable role in improving the energy efficiency of many buildings and that window films can be effective in reducing energy costs and energy consumption for buildings in many locations. Excellent energy savings can be provided by this technology—typical 5%-10% reductions in peak demand and annual cooling costs with such savings provided within a reasonable payback period (averaging less than 3 year). Although the focus of this entry was locations in the United States, it has been the author’s experience (and it should be apparent) that solar-control window films are applicable to a wide range of locations, climates, and countries.
It is important to note that while providing these important energy-saving benefits, window films are able to provide many other benefits that directly hit the mark of key scoring components for “green building” specification programs, such as the Leadership in Energy and Environmental Design (LEED).[5'6] As such, solar-control window films are able to meet the needs of many different design professionals, from property owner/managers to architects to energy engineers to green-building professionals.
