We humans have put more planet-warming carbon into the atmosphere in the past 30 years than we had in all of human history up until that point. Many believe recent climate events demonstrate that if we do not preserve the delicate balances in the natural world that make life here possible, we are risking our very existence.

All images courtesy of ATAS International, Inc.

Transpired solar-collector panels attached over any exterior, noncombustible, weathertight wall provide a building with preheated ventilation air.

Of all the greenhouse gas emissions released into the atmosphere, some 30–40 percent of them comes from the built environment. The energy needed to ventilate, heat, and cool the air inside of buildings is responsible for a significant portion of these emissions. Experts in the design, engineering, and construction professions believe with growing intensity that creating a built environment in harmony with nature is critical for our survival.

Such is the power of metal features in building envelopes that exploit the natural forces of physics, of air and energy flow, to heat and cool buildings. This course illustrates the intelligence of specifying transpired metal walls that preheat fresh air in winter prior to intake, cool roofing in cooling climates, and roofing that is solar ready. Not to be underestimated is the tremendous potential for LEED v4 points when designing a smart building envelope with metal features. We will begin with the surprisingly simple yet profoundly effective energy-saving strategy of installing transpired solar collectors on a building wall.

Transpired Solar Collectors: Low Tech, High Impact

The phrase “transpired solar collector” may sound complex, but it is an ingeniously uncomplicated and straightforward energy-saving building system. A U.S. Department of Energy Federal Technology Alert titled “Transpired Collectors (Solar Preheaters for Outdoor Ventilation Air),” states that “transpired collectors are a renewable energy technology that is well proven” plus is “readily available and has considerable potential” for many applications.¹ A transpired solar-collector system is used to preheat outside air before it enters the building to provide fresh-air changes and natural dehumidification. As a result, it offers opportunities to reduce energy consumption in the built environment.

The outside air is heated by the sun and then drawn through the perforations into the plenum. The air is further heated as it passes by the edges of the openings. It is warmed even more inside of the plenum, and then the warmed air is drawn into the building.

Here is the concept: A transpired solar collector is a perforated metal wall panel (on the market, aluminum, zinc, steel, and polycarbonate are found) that is usually mounted onto the south-facing exterior of a building (though mounts on southeast and southwest walls are also adequate to provide preheated air). The precision perforations in the wall panels allow for outside air to travel through the face of the panel. When the metal panel is exposed to sunlight, it heats up, which creates a layer of warm air on the surface of the panel. This solar-heated air is then drawn through the perforations, where it rises between the two walls and enters the building’s central ventilation system or supply fan, where it is then distributed through the building’s duct work.

Precision-lanced perforations allow heated outdoor air to enter the gap between the building’s outer wall and metal panels.

Imagine a brisk winter day with an ambient temperature of 20 degrees Fahrenheit. Now imagine that the air entering the building’s air-handling system is not 20 degrees but instead has been preheated by the transpired solar collectors and is a relatively toasty 40, 50, or even 60 degrees depending on various factors, including instantaneous solar radiation, airflow through the collector, and outside wind conditions. The preheated ventilation air entering the building will result in a major reduction of energy use.

Shown are the types of buildings that can benefit from transpired solar systems.

In summer, the same transpired panels perform as insulation, preventing the full force of solar heat from reaching the wall. However, in this season, the heated air created in the plenum bypasses the building’s air-handling system and is released into the atmosphere, keeping the building cooler.

A Technology Developed by NREL

Transpired solar collectors were developed in 1989 by the National Renewable Energy Lab (NREL), as well as being developed concurrently by private researchers.

Prior to 1989, the Federal Technology Alert on transpired collectors explains, “Solar air-heating systems resembled flat-plate water-heating systems. These early systems contained a dark metal absorber, but they also had glazing—a transparent cover—that prevented heat loss to the atmosphere. Rather than pull air through the absorber, these systems heated the air flowing parallel to the absorber. Compared to transpired collector technology, these systems were less efficient because solar radiation was reflected off of the glazing, and they had comparatively poor heat transfer from the absorber to the air. They were also more expensive because of the added material cost as well as installation.”²

Once the glazing was removed and perforations added, the system to heat intake air took on new efficacy, with higher efficiency and lower costs.

Installation of the transpired solar system does not require special skills or tools.

Almost Unlimited Application Potential

Transpired solar collectors are used for a variety of applications, including:

• Schools K–12
• Higher-education facilities
• Office buildings
• Medical facilities
• Government buildings
• Military facilities
• High-rise apartments
• Mixed-use facilities
• Shopping centers
• Industrial/manufacturing facilities
• Warehouses
• Agricultural buildings
• Residential (small-space heating)

How the Systems Are Installed

Transpired solar-collector panels can be installed over any noncombustible wall material and over or around existing wall openings. These systems offer ease of installation, as they do not require any special tools or skills.

First, light-gauge framing members (vertical Z-sections and horizontal hats) are attached to the wall. The perforated aluminum or zinc panels are then attached to the framing, creating an offset or a plenum. Panels are typically installed 4–8 inches from the wall, as determined through an engineering study.

This is a typical collector panel installation showing the horizontal hat sections, vertical Z-sections, and horizontal chamber.