Solar Beyond PV – the Transpired Air Collector Story
Red Transpired Air Collector
30% of the energy use in the U.S. is used for thermal applications. This includes considering all the energy used for transportation. Thermal applications can be as simple as the need for air that is 10 degrees warmer than the outside air or the other extreme of high temperatures used for industrial processing.
Commercial, institutional, and industrial buildings often have high ventilation rates. Think about the need for ventilation in the typical gymnasium or poultry farm! Although all that fresh air may be required for good indoor air quality, it can be very expensive to heat all that air. A simple solar technology has been developed to preheat ventilation air that can dramatically reduce utility bills.
Transpired air collector systems consist of a dark-colored, perforated façade installed on a building’s south facing wall. An added fan or the building’s existing ventilation system draws ventilation air into the building through the perforated absorber plate on the façade and up the plenum air space between the absorber and the south wall. Solar energy absorbed by the dark absorber and transferred to the air flowing through it can preheat the intake air by as much as 40oF. The US Department of Energy’s National Renewable Energy Laboratory (NREL) was the co-developer of the concept and installed them on their new Platinum LEED rated Research Support Facility in Golden, Co in 2010. NREL claims typical reduced heating costs will pay for the systems in 3-12 years.
This R&D Award-winning technology is the product of the practical initiative of private industry and the scientific expertise of NREL in the 1990’s. Unlike previous technologies for space heating, the transpired collector requires no expensive glazing with associated energy loss to reflection. Design refinements boosted the amount of available solar energy (diffuse as well as direct sunlight) that the transpired air collector can capture up to a record-breaking 80%. Using transpired collectors provides numerous other advantages:
- The collectors are virtually maintenance free, with no liquids and no moving parts other than the ventilation system fans.
- At night the collector assists heating by recapturing the heat lost through the building.
- By providing better ventilation, they help improve indoor air quality.
- Can be integrated into a new building design or added to an existing building’s south wall.
- In hot climates the collector provides shading for the south wall and lowers cooling costs.
- More cost efficient than previous glazed flat-plate collectors especially when replacing a building’s south façade finish.
Transpired air collectors are ideal for buildings with large ventilation requirements but also have been used for:
- Pre-warming and supplying heated air for space heat
- Preheating combustion air for central heating plants or industrial furnaces
- Hybrid heating and electricity. Capturing the waste heat from behind PV systems and air-c collectors are ideal for buildings with large ventilation requirements but have also beeooling the PVs to increase their efficiency. The heat produced by PV systems can be up to 4 time more than the electrical energy produced.
- Ventilation stratification reduction.
- Providing higher temperatures when combined with glazed sections in a hybrid system.
- Roof mounted versions for preheating.
- Night time air cooling.
- Crop and process drying.
Since their further development in the 1990’s, thousands of systems have been installed in a variety of commercial, industrial, agricultural and process applications in over 35 countries around the world. The Ford Motor Company installed their system in 1987 and now claims over $10 million in solar energy savings. Transpired air collectors are showing up in the northwest with several projects in the Spokane Community College District. Case studies from the 2 manufacturers of transpired air collectors show they are being used in our region at:
- Schools and gymnasiums: Hood River Middle School, OR: Hayes Freedom High School, Camas, WA: Spokane Community College, WA, Spokane Falls Community College, WA; Western Oregon University, Monmouth, OR.
- Warehouses / Industrial: Bonneville Power Administration (Bonneville, OR)
- Military Aircraft Hangers and Maintenance Buildings: Joint Base Lewis-McCord , Tacoma, WA
My own experience with the transpired air collector system is in a recently finished Portland Parks and Recreation Flavel Maintenance building. This 7,200 sf building is about ½ offices and the other ½ is shop and storage space for the park maintenance crews. Our first sustainable design thoughts were that this typically industrial type building with low heating needs and higher ventilation requirements might be a good fit for the transpired air collector. And that thermal heating would be more cost effective than a PV system. The PV system area was also limited by the budget and the requirement for a green roof.
This would be the first experience with a transpired air collector for Merryman Barnes Architects and for the engineering firm Environmental Engineering Services Inc. For the owner, Susan Meamber of Portland Parks and Recreation, the no maintenance - free heat combination was encouraging.
We were all anticipating a south facing wall in a steel building that would be mostly wall. As architects, we were glad to see it could be easily integrated with the building design and allow for a few small windows high in the wall for daylighting. The final design incorporated 650 sf of collector area (50 % of the south wall area) and uses the system to preheat air for the lightly heated shop/storage areas and for the office because ventilation needs ended up being very low. Separate heating systems provide backup heat and are on separate controls.
What we learned is, although the transpired air collector is simple, the coordination of controls and set points take some management. This seems to be a common complaint for many heating and cooling systems too. Complete commissioning and balancing is required to fine-tune the efficiency and operation to optimally fit the needs of the client.
The transpired air systems have been extensively modeled by several government agencies in the U.S. and Canada. Natural Resources of Canada developed the feasibility tool RETScreen to model the energy savings from transpired air collectors. This free program is available at www.retscreen.net. RETScreen calculations provided by Dave Baasch of WesCor Portland (a Solar Walltranspired air collector distributor), for a 1,000 sf system in eastern Oregon with 9 months of heating resulted in 65,000 kWh production annually. This is about as much as a 5 KW PV system.
Construction costs for the transpired air collector are usually discussed in terms of incremental costs - how much is added since the buildings typically have ventilation systems, walls, and exterior finishes. In a very efficient design, the air collector adds about $3/sf installed according to the manufacturer. This is assuming new construction where it can replace the new building finishes. Our own experience with a very small project showed an added cost of about $15/sf. ODOE’s analysis used $11/sf. Economy of scale on a larger project would bring the costs down. As a retrofit, one manufacturer estimates $14-$17/sf making it comparable to the cost of a brick veneer wall. Using those figures and comparing the cost effectiveness of a PV system with that of a transpired air collector on a new building, the 1,000 sf, 65,000 kWh system would add about $3,000 - $15,000 extra to the construction cost for the transpired air collector and $25,000 for the PV system (at $5/watt) to produce the same energy. That means about ½ the cost for the same energy produced! If that summer heat produced by the transpired air collector could also be put to work this comparison would look even better for the collector system.
This comparison does not take into effect the variety of tax credits and incentives available for each of these systems. ODOE’s website pamphlet estimated a 4 year payback for a transpired air collector system in eastern Oregon after tax credits.
DON’T FORGET THE WALL!
Space heating is such a universal need - 13% of U.S. energy is used for heating residential and commercial buildings alone. Maintenance free, cost effective transpired air collectors have major potential for energy savings especially in warehouses, schools, gymnasiums, airplane hangars, agricultural buildings, and vehicle maintenance facilities requiring ventilation. With the increasing drive to install renewable energy systems on buildings, transpired solar collectors are now used across the entire building stock because of high energy production (up to 50 peak thermal watts/square foot), high solar conversion (up to 80%) and lower capital costs when compared to solar photovoltaic and solar water heating. They pay for themselves quickly, have a long life span, and produce substantial environmental and economic benefits with virtually no negative side effects. Transpired air collectors provide a highly effective means for substituting renewable energy for fossil fuel consumption. The roof is not the only place to collect solar energy effectively.
“Transpired collectors provide the most reliable, best performing, and lowest cost solar heating for commercial and industrial buildings available on the market today.” (U.S. Department of Energy)