Advanced Metal Wall Systems The New Generation of Solar Collection By John Hollick, P. Eng., P.E. 1 Robert Goodhart, 2 1 President, Conserval Engineering Inc, 200 Wildcat Road, Toronto Ont. M3J 2N5 Phone: 1 416 661-7057 Ext. 228. Fax: 1 416 661-7146. E-mail: jhollick@solarwall.com 2 Product & Process Development, ATAS International Inc., 6612 Snowdrift Rd., Allentown, PA 18106-9511 Phone: 1 610 395-8446 Ext. 261. Fax 1 610 395-9342. Email: bgoodhart@atas.com ABSTRACT Transpired solar collectors that use metal walls to heat outdoor ventilation air have been used in numerous retrofits of factories and are now gaining acceptance in commercial buildings. Projects have been installed in twenty countries. Both the concept and the commercial application of the system are straightforward. Collector panels with small, precision perforations are mounted a prescribed distance from the outer wall of the building, forming an air cavity. Solar-heated, fresh air at the surface of the collector is drawn through the panel and into the cavity for distribution inside the building. Solar collection efficiencies of over 60% translate into significant energy savings, along with reductions in greenhouse gas emissions from displaced fossil fuel. The United States Department of Energy has rated this transpired collector in the top two percent of energy inventions and has stated, "Transpired collectors provide the most reliable, best performing, and lowest cost solar heating for commercial and industrial buildings available on the market today." Natural Resources Canada now offers software for designers to evaluate these savings. Application of the technology in "green buildings" can result in significant LEED TM credits from saving energy, using renewable energy and improving indoor air quality to using recycled materials. This paper will present the background on the technology, project examples, design considerations, software tools, building integration methods, colors and typical savings. Project examples will include factories, schools, multi-unit residential, commercial and government buildings, along with process air heating applications. Designs also focus on the ability to improve indoor air quality by heating larger quantities of fresh air. BACKGROUND Over ten years ago the Department of Energy rated the transpired solar collector in the top two percent of new energy inventions. Since then, the system, also referred to as SOLARWALL or InSpire, has been extensively monitored, has steadily gained market 1
acceptance and is now installed in over twenty countries. It has the potential to someday become the standard for south-facing walls when heated ventilation air is required. The concept is simple; capture and utilize the sun s energy that falls on a south facing wall or roof. One of the system s hallmark qualities is its fundamental simplicity; with no moving parts, the historical problems that were often associated with traditional solar systems, such as high costs and maintenance, are now non-issues. If solar energy is to be routinely specified, it must be economical. Cost reductions have been achieved by reducing the material in the collector to a single metal sheet mounted out from a wall or roof forming an air cavity with the building surface. Eliminating the glazing increased the solar efficiency, since glass only transmits 85% of the sunlight whereas the transpired collector receives 100% of the sunlight falling on it. The solar panels must also be aesthetically pleasing for market acceptance. The transpired collector system is one of the first solar heaters that can be supplied as a building material rather than an add-on panel. It can be easily incorporated into virtually all architectural styles, making it one of the most versatile renewable energy systems available. As many architects can attest, the metal cladding fits in with most architectural genres, and can augment the appearance of any building. The metal cladding is available in twenty different decorator colors, which ensures maximum design flexibility. The installed product looks like a conventional metal wall, its solar collection efficiencies are over 60%, it is economical and it can improve the indoor air quality of buildings since more fresh air can be brought into a building and heated for free using solar energy. It is technology that makes your building work for you. To have a wall that does nothing else other than provide a barrier between the inside and outside environment, is outdated. To build the same wall, for similar cost as any other wall, and to have that wall provide space heating, and ventilation air for the building that is progress, and that is one example of how to make your building work for you. DESCRIPTION OF SYSTEM The metal wall panel heats ventilation air required in buildings. Figure 1 illustrates a typical application mounted on a south-facing wall. The solar absorber is metal cladding that looks like any other conventional metal wall and is basically installed as a sun screen. The top of the wall, which collects the solar heated air, can double as a raised parapet, architectural feature or mansard roof. The solar heating system may be used either as a pre-heater to conventional air make-up systems or HVAC units or it can be installed as a "standalone" system that turns on when sufficient heat is available. The solar heating system is then connected to the air intake of the HVAC or fan unit. 2
Another interesting observation has been in summertime when heat is not required. The cladding shades the main wall preventing the solar radiation from reaching the wall and thus reducing the cooling demand of that building. Since the metal facade is perforated, any excess heat on the skin or in the air cavity is automatically vented. Figure 1: Cross section showing wall mounted solar cladding connected to roof top fan. Solar Heating Efficiency The efficiency of a solar collector is highest when the temperature of the air entering the solar panels equals ambient temperature. This occurs when outside air enters the system. Heating fresh air is expensive and designers have routinely cut back on fresh air to save energy. Using solar energy to heat fresh air solves the energy problem and improves indoor air quality since more fresh air can be heated. In space heating designs, recirculated or stale air from inside buildings enters a heater or old style solar panel and the air must be heated above room temperature. On cold, overcast days, there may be insufficient solar energy to achieve this, whereas with heating ventilation air, any heat gain, whether it be a rise of two degrees or twenty degrees, is useful energy. Solar space heating may work on sunny days but solar heating of ventilation air works on cloudy days as well as sunny days. Wall Mounting In northern USA states, Canada and northern Europe, a vertical wall is actually better suited to solar air heating than a sloped surface. There are several advantages to vertically mounted collectors versus sloped collectors for this application. Incident radiation during the summer months is greatly reduced on a vertical surface, thus reducing heat gain during these no-load periods. 3
Vertical surfaces receive more reflected radiation from snow (up to 70% more in winter). The structural costs for wall-mounted systems are much lower. Duct losses for wall-mounted systems are much lower. Snow build-up is not a problem on a vertical surface. Vertical panels rarely add wind loads to the building. By mounting the solar panels onto a south wall, there is also an insulating effect on that wall. Heat losses from a building through the wall are picked up by the air stream and returned to the building when the fans are running. The wall acts as a huge heat exchanger recovering any heat loss through the main wall. The wall area to be considered does not have to be blank. Wall surfaces around doors and windows may be suitable if they can be connected together or to fans that deliver air inside the building. Temperature Gain The air temperature rise in a transpired solar collector depends on several factors. There is a direct relationship between the temperature rise and the absorptivity of the collector. Collectors with black, dark brown, or other dark colored surfaces have the highest heat gain. In addition, both geographical location and building orientation affect the amount of solar radiation on the collector. Typical radiation intensities are 800 to 1000 watts/m 2 on sunny days and 200 watts/m 2 on cloudy days. The heat gain also depends on the air flow rate through a unit area of the collector, typically measured in cubic feet per minute per square foot of collector (cfm/ft 2 ). The solar cladding designs have been subjected to several series of exhaustive tests and typical temperature rise data is shown in Figure 2. Figure 2: Heat gain for transpired solar collector at various air flow rates and solar radiation levels. 4
Curve A represents flow rates for applications where higher temperatures are needed and the panels can provide space heating needs for the building as well as ventilation air heating. Curve B is typical of most ventilation heating designs. Curve C and higher air flows are used in industrial applications where large volumes of air must be heated. The solar panels act as a pre-heater, and additional heat either from stratification or auxiliary heaters may be necessary. Annual Solar Energy Savings Annual solar energy produced by a solar heating system will vary based on location and a number of factors such as hours of operation, air flow rate and level of insulation in walls. For estimating purposes, one can assume that each square foot of solar collector will produce between 1.5 to 3 therms (1 therm = 100,000 Btu/hr) of heat energy each year. The actual solar energy produced can be simulated by a number of computer programs. The Canadian government, which has monitored several large installations, has written an easy to use program called RETScreen to simulate the energy savings, greenhouse gas reductions, and financial information. The program module called SAH 2000 (for Solar Air Heating) is available free-of-charge from their web site at www.retscreen.net. The savings from destratification in industrial buildings is additional and can be equal to the solar heat collected so the total energy generated can be significant and enough to heat most of the fresh air required in many manufacturing plants. Costs The question of initial costs and payback analysis can be quickly determined with the RETScreen program. In new construction, the capital cost of a transpired solar cladding system is similar to conventional wall surfaces. In retrofit situations, the perforated solar cladding can be applied over most existing walls of block, metal, glazing, or precast concrete. A few obstructions on a wall do not present a problem. For estimating purposes, the payback of a transpired collector system is generally one to three years in new construction after deducting costs for eliminating the other wall materials. If renewable energy grants are available in the State, including solar heating may cost less than a non solar heated building. Building Integration An excellent example of architectural integration is found with the stylish rocky grey solar collectors that adorn the walls of the Bombardier buildings in Quebec. Figure 3 shows a section of 100,000 square feet of solar wall heating surface on the Canadair building in Montreal. The white canopy facia along the top is actually the collection duct for the huge volumes of fresh air being heated in the grey solar metal panels below the canopy. 5
Figure 3: Photo of one wall of the Canadair complex in Montreal with grey solar collector panels and white canopy and vertical dividers With proper design, solar air heating can be incorporated into any building at minimal additional costs. Since all buildings need walls and ventilation air, the perforated solar cladding can be seamlessly integrated into any design to provide a perfect aesthetic and functional addition. The benefits are substantial; improved indoor air quality, attractive appearance, environmentally friendly technology, and most important, free heating for decades. Solar air heating can assist architects with the new energy codes that allow tradeoffs to achieve the desired energy performance. Specifying solar heating may allow for other less energy efficient areas such as more windows. Each ten square feet of solar wall panel area is equivalent to a 500 watt heater on a sunny day. Figure 4: Solar panels on a curved wall on one of three solar heated bus garages in Calgary Alberta 6
Figure 5: Bronze cladding on all walls of new community centers in Rapid City, SD Industrial and Maintenance Buildings Industrial applications are ideally suited for solar heating. Factories have special ventilation needs depending on the goods produced. Any operation involving painting, welding, chemicals, automotive, or the production of fumes or odors require ventilation to ensure a healthy working environment. Indoor air quality is an issue of great importance today and proper ventilation is the key to preventing many problems. Wideopen plant areas with large clear walls allow for maximum heat collection and low cost air distribution. Solar air heating is best applied when outside air is drawn through the solar collector picking up the solar heat available on the wall. The solar heater can be used as a heater or pre-heater to any air make-up unit. Additional free heating is possible for buildings with high ceilings. In this case, separate inexpensive solar fans can be installed on the inside of the south wall with distribution ducting running a long distance into the building. The warm stratified air that accumulates under the ceiling is dispersed to provide the additional heat required on days when there is insufficient solar heat. The system brings in colder outside air at ceiling level where the hottest inside air is found. The outside air is distributed through a perforated duct system at ceiling level. Outside air mixes with warm air as it falls towards floor level. This results in destratification of building air and a more even air temperature from floor to ceiling. Mixing dampers between the fan and solar absorber can provide even better control by mixing plant air with cold air when there is no solar heat such as during night operation. The solar heated air can be distributed throughout the entire building at ceiling level, providing fresh air to the entire building and also allowing for air which may not have been heated to room temperature to still be utilized and dispersed without any drafts or discomfort. 7
The supply of outside air from the solar wall distribution system solves negative pressure problems typically found in industrial buildings. The unit heaters or space heaters in such buildings are normally designed to account for the building heat loss and infiltration of half an air change per hour. The effective introduction of solar heated ventilation air can counteract the negative pressure, minimizing air infiltration and reducing the requirements for conventional gas-fired air make-up systems. Schools One of the best applications for solar air heating is in schools. The transpired collector heats fresh air which can be the most expensive energy item for some schools. Each student needs 15 cfm of fresh air or 450 cfm for a class of 30 students. This should be considered the minimum requirement not the design target or maximum amount. Anything less may be illegal. Schools are daytime load institutions; when solar energy is at its peak and there is no need to consider heat storage. They also operate primarily during the three seasons that require heating energy. In addition to augmenting the appearance of the facility, and providing free heating and ventilation, the all-metal solar system also represents an opportunity for students to observe firsthand the benefits of environmentally responsible, energy saving technology. Figure 6: Blue Solarwall panels on two walls of Thetford Elementary School, Thetford, Vermont Advantages of Transpired Collector Low cost High solar collection efficiency Handles large volumes of air No additional maintenance Integrates into wall or roof Choice of color Attractive appearance Improves indoor air quality Free heating of ventilation air Greenhouse gas reductions of 40 pounds per square foot of collector 8
Designing Transpired Collector System - Reference Guide 1. Decide on solar panel size and location. Is south wall suitable? If not, consider east or west walls. Note that a south wall may actually be south-west, and the east wall would then be south-east. In this case, both walls could be utilized effectively. 2. Determine volume of outside air required in building. Heat as much fresh air as possible. This will improve indoor air quality while minimizing fuel costs. 3. Calculate volume of air per area of solar heater, then refer to temperature chart to determine expected temperature rise. 4. Select color, profile and type of canopy plenum. 5. Industrial buildings can save more energy from destratification. Determine the amount of ventilation or make-up air required, and then locate the ducting to distribute the air throughout the building. The distribution ducting should be located in the areas where the ceiling temperature is the hottest to disperse the heat and save energy. 6. The solar panels act as a heater or pre-heater for outside air only. It is still necessary to provide a space heating system to take care of heat losses through the walls and roof. Grants One of the best web sites for determining grants for solar energy projects can be found at www.dsireusa.org. Just click on the state where the project is to be located in order to see a list of all the various grants, tax credits or incentives available for using solar energy. Some states such as New York will offer up to 70% for solar projects. There is also a Federal tax credit of 10% for companies installing solar collectors. LEED Points For architects and engineers alike, one of the most exciting aspects of the solar heating system is that it qualifies for LEED points. The recognizable value in obtaining LEED certification is that it acknowledges qualifying buildings as being environmentally responsible, profitable, and healthy places to live and work. There are also additional benefits that accrue to building owners in the form of an aesthetically unique building, lower operating costs, prestige within the industry and community, and added value to the building that can translate into a higher selling price. With a solar air heating and ventilation system, points may be available in the following categories: Energy & Atmosphere, Material & Resources, and Indoor Environmental Quality. 9
Conclusions The all metal design has received numerous honors and awards from the U.S. Department of Energy, Natural Resources Canada, ASHRAE, the Toronto Construction Association, Popular Science Magazine, R&D Magazine, and many more. Unglazed solar air heating systems have been successfully used for over a decade on numerous commercial and industrial buildings. In new construction, renovations, or whenever new makeup air is required, consider the advantages of free heating from the sun. The transpired solar system addresses many issues; the need for improved indoor air quality, the cost of heating fresh air, and the problems of global warming and greenhouse gas emissions. As well, the solar technology fosters the notion that it is possible to keep company money within company coffers, instead of spending needless amounts on unnecessary energy consumption. That is why building owners and architects now have a choice: a conventional wall with no payback, or a high performance metal wall with a payback, and a green image. The solution should be obvious put those metal walls to work and get green back! 10