|
Options for Reducing Thermal Heat Loads Windows Persist as Least Efficient Building Component
April, 2008
| |
|
|
|
STUDY RESULTS QUANTIFY SAVINGS ATTRIBUTABLE TO WINDOW FILM
By Steve DeBusk
Energy-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 use in new construction.
Determining energy savings derived from energy-control films has been difficult due to the common practice of implementing several energy conservation measures simultaneously, and due to annual variations in many factors that can affect a building's energy usage. To isolate and quantify the energy-saving benefits of solar films for a typical commercial office building in the Greater Toronto Area, an energy analysis study was undertaken using the U.S. Department of Energy's sophisticated DOE-2 energy-simulation software. The study included several types of window films applied to various glazing systems.
Based on the most typical types of installations and on customary installation costs for medium-sized commercial projects, the average return on investment (payback) from energy-control film application was a 3.57 years. These savings were the result of reducing annual electricity kilowatt-hour (kWh) usage by an average of 5.6% and reducing peak summer month KW demand on average by 7.1%.
The DOE-2 energy study was performed on a conventional 1990s era 10-storey office building with approximately 175,000 square feet of floor space. To gauge the effect of different films, four films were chosen and categorized as: Maximum-Performance; Maximum-Performance Low-E; High-Performance; and High-Performance Low-E. These categories were based on the film's solar heat gain coefficient (SHGC) on single-pane, 6 mm (1/4 inch) clear glass, and whether the film was a low-e film.
The study was performed on four different types of building models, changing the window type between each model: single-pane clear; dual-pane clear; single-pane grey-tinted; and dual-pane grey-tinted window systems. Each film type was analyzed on each of these glazing systems. All windows consisted of 6 mm (1/4 inch) thick panes and for the dual-pane units the panes were separated by a 12 mm (1/2 inch) air space.
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 6,000 square feet.
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 21.1oC (70oF)
* Heating setback 18.3oC (65oF)
* Cooling setpoint 23.9oC (75oF)
* Cooling setback 26.7oC (80oF)
* Medium-coloured blinds used 25% of the time. SHGC of blinds, 0.69
* Windows recessed from building face 15 cm (6 inches) 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 0.70 KW per ton (COP of 5.1)
Below is a summary of the resulting energy savings from window film application for each type of window, along with the overall averages. For simplicity, each figure is an average of the four film types used. The Maximum Performance Films and Low-e films have better payback and savings than the averages shown.
| |
Single Payback, yrs |
Annual kWh Savings |
Summer Month Peak KW Savings |
| single clear |
1.73 |
9.0% |
11.0% |
| dual clear |
2.89 |
5.9% |
7.3% |
| single tinted |
3.55 |
4.5% |
5.8% |
| dual tinted |
6.13 |
3.0% |
4.1% |
| Average |
3.57 |
5.6% |
7.1% |
Energy-control window film can play a useful and viable role in improving the energy efficiency of many buildings. Excellent energy savings can be provided by this technology - typically 5-10% reductions in peak demand and annual cooling costs - with such savings provided within a reasonable payback period, in many cases three to four years. Although the focus of this study was for Toronto, energy-control window films are applicable to a wide range of locations.
Steve DeBusk, CEM, CMVP is with CPFilms Inc. For more information, see the web site at www.cpfilms.com.
|
| |
|
By Bob Alsip
Excess solar heat gain on south, east and west exposures can wreak havoc with the HVAC system even in the winter. To make matters worse, internal heat loads have increased within buildings due to the increase in the number of occupants in a given area and the increase in the use of equipment such as computers, printers, copiers, etc.
Building operators have to remove the majority of the excess heat out of the building in order to provide a productive environment for tenants. Balancing the temperature to address cold and hot spots becomes even more of a challenge if a building does not have an effective zone control system.
The majority of the newer commercial buildings have installed double pane reflective glazing as part of their construction. Other buildings may have clear or tinted glass and even single pane windows. Replacing the lower performing glass is an option available to building owners but the disruption to occupants and the cost can be prohibitive. Many buildings cannot pursue the option of window replacement due to additional weight loads associated with the double pane versus single pane windows.
Energy-control window films provide various degrees of filtering using micro thin layers of metals/oxides coatings on thin transparent polyester-based materials. They have a pressure sensitive adhesive layer with a removable protective liner to attach the film to any glass. Higher quality films also incorporate abrasion-resistant coatings that resist scratching and allow standard window cleaning procedures and products to be used.
Energy savings calculations suggest that energy-control window film saves up to one ton of air-conditioning for every 10 square metres of filmed sun-exposed glass. It has been shown that the higher-quality films block up to 79% of the excess solar heat, reduce heat loss in winter by up to 30% (low E films), reduce glare up to 83% and block 99% of UV rays. (See study results.)
Even so, some building owners and managers have expressed reservations. The following are responses to some common perceptions.
Film applied to glass will cause thermal glass breakage
* Films applied to glass according to manufacturer's specifications can never be the sole cause of thermal breakage. Only if the glass is not manufactured and/or installed to established industry standards can window film be a contributing factor. To reinforce this point, reputable film manufacturers offer a 5-year thermal breakage warranty that includes glass and film replacement including labour.
Films applied to Insulated Units will cause seal failure
* Window film solutions have been applied to millions of square feet of insulated glass units throughout the world for more than 40 years and have never been shown to be the cause of seal failure. Major window manufacturers will honour their warranty if recognized quality window films are installed.
Films will discolour over a short period of time
* Most higher quality commercial window films are protected by strong ultraviolet inhibitors in the adhesive guaranteeing that the films will not discolour over time like most automotive tint films. 10-year material and labour warranties back up these claims.
Films are a short-term band-aid solution
* Although films have been around for some time - in fact, 3M invented window film in 1966 - the technology has improved dramatically during the last 40 years. Life expectancy of higher quality films range from 15 to 20+ years. When necessary, films can be removed and replaced without damage to existing glass. Abrasive-resistant films are compatible with standard cleaning solutions and procedures.
Bob Alsip is with Convenience Group, a certified dealer of energy-control films for windows. For more information, see the web site at www.windowfilmcanada.ca.
|
|