Solar Air-heating Systems 101: Ensuring efficient and economical renewable energy

by Lee Ann M. Slattery, CSI, CCPR, LEED AP
The cost of heating a building is a major expense for most building owners, and it is also a cost to the health and well-being of the general population in carbon pollution. By incorporating renewable energy technologies into the design and construction of a building, both these costs can be substantially reduced.

Building owners are seeking ways in which to reduce their energy consumption. According to the U.S. Environmental Protection Agency (EPA), the country’s buildings account for 36 percent of total energy use, and 30 percent of greenhouse gas (GHG) emissions.¹ Existing and emerging renewable energy technologies are offering several methods in which to reduce both energy consumption and emissions. One example is a transpired solar collector—an air-preheating system that augments the building’s standard heating system.

The technology is quite simple and effective. Perforated metal wall panels in a dark color are installed several inches from a generally south-facing wall, creating an air space or plenum between the non-combustible watertight wall and the metal panel. Sunlight heats the air at the surface of the collector, and fans at the top of the plenum or in the air handling units (AHUs) draw the warmed air through the perforations into the plenum.

This heated air is distributed into the building, often through the existing HVAC system (Figure 1), or, in the case of a warehouse or manufacturing facility, distributed directly into the workspace (Figure 2). This simple and efficient system can be used on various building types, and is suitable for both new construction and retrofit projects.

A transpired solar collector system can also create healthy, productive indoor environments. In recent years, EPA’s Science Advisory Board has consistently ranked indoor air pollution among the top five environmental risks to public health, and there is mounting evidence that inadequate ventilation affects human performance.² This finding applies to institutional buildings, such as schools, as well as offices, laboratories, and other commercial buildings. Requirements for outside air are established by various associations, code bodies, and government agencies. It is important to deliver enough fresh air, while maintaining a comfortable temperature within the building.

Installation of a transpired solar collector
There are two primary requirements for the outer covering of exterior walls to which transpired solar collector panels are attached: an approved water-resistive barrier and an approved material for the plenum’s fire safety.

In other words, the covering must provide a drainage plane to prevent intrusion of liquid water to the interior of the wall assembly. The International Building Code (IBC) requires a water-resistive barrier be provided behind the exterior veneer for this purpose. For fire safety, the cavity between the transpired solar collector panels and the exterior of the main wall serves as a plenum that distributes ventilation air to the occupied space in the building. In terms of minimizing the spread of flames and the creation of smoke, this is of paramount importance. Requirements of various fire safety codes dominate this consideration.

Common building materials—such as metal panels, brick, or concrete masonry—meet both of these requirements. The most direct way to meet these requirements is by using a watertight, non-combustible material as the outer covering. The use of any alternative materials must be considered very carefully so neither assembly integrity nor occupant safety is compromised. Any alternative materials must be approved water-resistive barriers and also meet the minimum fire safety requirements for Underwriters Laboratories (UL) 181, Factory-made Air Ducts and Air Connectors, Class 1 materials.

Once it has been determined there is a proper watertight, non-combustible wall over which to install the transpired solar collector panels, the contractor is ready to begin panel installation. The panels are typically installed 100 to 200 mm (4 to 8 in.) from the existing wall, creating the air space (plenum). A series of vertical zees and horizontal hat channels are used to create the air space (Figure 3).

The panels can be installed over or around existing wall openings, and no special skills or tools are needed. Any excess moisture collected in the plenum drains out the bottom flashing assembly, and/or is naturally vented at the top of the system in the cooling season. The plenum is heated, which lowers relative humidity, and there are high ventilation rates in the heating season.

Direct-to-space heating applications
Hangars, light manufacturing, maintenance garages, industrial buildings, and other large, open spaces often employ direct-to-space heating and ventilation systems. Some characteristics of these types of buildings include:

• high ceilings;
• heaters suspended from ceilings;
• air exhaust fans in ceilings;
• negative pressure (which draws in cold outside air); and
• temperature stratification (hot air collects at ceilings).

A transpired solar collector system will heat make-up air for these buildings and provide a simple, economical approach to meet indoor air quality (IAQ) standards. This renewable energy technology benefits these facilities in three ways:

1. Ventilation air is actively heated by solar energy.
2. Heat loss through the wall is recaptured.
3. Stratified heat at the ceiling is utilized at the working level.

Fan units are located at regular intervals along the wall near the roof to draw the preheated fresh air through the tiny perforations in the metal wall cladding. Each fan has modulating outside air and return dampers, discharge air temperature sensors and controls, and a flame-retardant duct that distributes the solar-heated air along the ceiling through numerous precision openings. When air is no longer required and the fan system is shut down, the outside air dampers close automatically.

During the summer, when heating is not required, outside air can be brought directly into the distribution ducts through bypass louvers or through a separate inlet. As well, during the warmer months, cool evening air can be brought into the building using the system at night, so it is a much more comfortable environment for the employees in the building the following morning.

Ventilation air, destratification, and balanced ventilation
American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 62, Ventilation for Acceptable Indoor Air Quality, is recognized as a foundation standard for IAQ. It states:

Indoor air should not contain contaminants that exceed concentrations known to impair health or cause discomfort to occupants.

Ventilation requirements for industrial facilities vary widely according to the type of process. Air change rates of one-half to four per hour are common. Heating these large volumes of air during cooler months can be expensive.

In a typical building, the temperature of the hot stratified air at the ceiling can rise to over 27 C (80 F) in winter. The air distribution fans and duct reclaim the stratified heat trapped at the ceiling by creating natural convection currents that carry the heat down to the working level for employee comfort.

With a lower ceiling temperature, heat losses through ceiling-mounted exhaust fans are reduced. At the same time, the constant supply of make-up air pressurizes the building, stopping infiltration of cold air around openings and reducing uncomfortable drafts along the floor. The controlled intake of fresh air also purges contaminants efficiently, eliminates high-velocity cross-drafts through windows and doors, and prevents down-draft in combustion flues. When system fans are running, a transpired solar collector can also have an insulating effect and recapture heat loss through the walls.

Pre-heated replenishment air for institutional and commercial facilities
A transpired solar collector system is an excellent renewable energy source for schools, as well as office buildings and laboratories, for numerous reasons. For example, these types of buildings are occupied primarily during daylight hours when the sun is available for heating. Schools, in particular, operate during the coldest seasons when heating requirements predominate.

Fresh air helps students and employees learn and work better. Further, these buildings are long-term investments offering energy savings over many years. The installed cost of a system is comparable to that of a brick wall, with a typical payback period of three to eight years. Grants and other incentives for renewable energy projects may also offset costs.³

Each square foot of collector panel can heat 3.4 to 13.6 m³/hour (2 to 8 cfm) of air and provide 1 to 2 therms (100,000 to 200,000 Btus) of heating energy annually. Air temperature increases of 16.7 to 27.8 C (30 to 50 F) are common, and heating cost savings generally range from $16.15 to $59.20 per m² ($1.50 to $5.50 per sf) of collector, depending on the fuel displaced. Annual energy savings are typically 20 to 40 percent or more. The panels have an approximate 30-year life and are virtually maintenance-free. Transpired solar collector panels with a polyvinylidene fluoride (PVDF) paint finish offer long-term color integrity and heat absorption values.

Perforated solar cladding can be seamlessly integrated into many architectural designs. Collector walls are most advantageously installed on south-facing walls, but east and west walls are also acceptable. Non-perforated panels can be used to unify design elements. Darker colors are better absorbers than lighter hues. As shown in Figure 4, different profiles of panels, with both horizontal and vertical installations, can be employed, with exposed or concealed fasteners (depending on the profile chosen) and mitered corners for the system are also available.

A solar air-heating wall assembly is an energy system that is custom-engineered per project. Building orientation, climate, and ventilation requirements are just some of the factors that must be considered. In most cases, a feasibility study is recommended to predict energy savings and define the construction details.

The Canadian government wrote a free, easy-to-use software program called RETScreen to determine the energy, environmental, and financial impact of these systems.⁴ Proprietary software is also used to fine-tune installations.

The technology for perforated solar air-heating systems was developed through extensive testing at the National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy (DOE), and in Canada at the CANMET Energy Diversification Research Laboratory, an agency of Natural Resources of Canada (NRCan).

Project suitability
When designing a new building (or renovating an existing one), a transpired solar collector may assist in the solution of design challenges such as:

• reducing energy consumption and air pollution;
• minimizing maintenance;
• heating with a cost-effective supplemental system;
• maximizing use of renewable, non-polluting fuel; and
• ventilating with preheated fresh air.

More specifically, modular transpired solar collector units can be used for drying agricultural produce such as herbs, spices, and coffee beans. Modular units can be mounted on a roof, wall, or the ground. They can assist in obtaining fuel savings in the drying process, extending the life of drying equipment and improving control of moisture content reduction.

Another use for a transpired solar collector is to heat barns. Young pigs, turkeys, and broiler chicks require a substantial amount of supplemental heat because they cannot produce enough body heat themselves. A transpired solar collector system serves this purpose, efficiently and economically.

There is the possibility for other unique applications. This author knows of a commercial dive shop in Southern California that installed the solar collectors on their building to accelerate the drying of wetsuits hung inside of that south-facing wall.

Of course, these assemblies are also often employed in office buildings and educational settings. The use of a solar air-heating wall system improves the building environment for occupants by providing solar pre-heated ventilation air. The system reduces heating energy costs and carbon footprints, and therefore may contribute to credits under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) program:

• Energy and Atmosphere (EA) Credit 1, Optimize Energy Performance;
• EAC Credit 2, Onsite Renewable Energy;
• Materials and Resources (MR) Credit 4, Recycled Content;
• Indoor Environmental Quality (EQ) Credit 1, Outdoor Air Delivery Monitoring;
• EQ Credit 2, Increased Ventilation;
• EQ Credit 7.1, Thermal Comfort?Design; and
• EQ Credit 7.2, Thermal Comfort?Verification.

All these benefits, based on free energy from the sun, can translate to financial savings and an improved working environment.

Notes
¹ Visit www.epa.gov/oaintrnt/projects. (back to top)
² Visit www.epa.gov/smokefree/pubs/etsfs.html. (back to top)
³ Funded by the U.S. Department of Energy (DOE), the Database of State Incentives for Renewable Energy Visit (DSIRE) is a comprehensive source of information on incentives and policies that support renewables and energy efficiency in the United States. Established in 1995, it is currently operated by the N.C. Solar Center at North Carolina State University, with support from the Interstate Renewable Energy Council. Visit www.dsireusa.org. (back to top)
⁴ Visit www.retscreen.net. (back to top)

Lee Ann M. Slattery, CSI, CCPR, LEED AP, is the sales support manager for ATAS International, and president of the CSI Allentown Chapter. Specializing in the architectural building materials industry for the past 22 years, she has worked with architects, specifiers, engineers, contractors, facility managers, property owners, and distributors. Slattery also assists in the development and maintenance of ATAS’ continuing education programs. She can be reached at lslattery@atas.com.

To read the case study on the Cigas Machine Shop, click here.