Active Systems

Transpired Air Collectors

Fairly simple in concept and construction, transpired air collectors preheat ventilation air. A dark-colored, corrugated-metal facade covers the south-facing wall(s) of a building. The metal is perforated with thousands of regularly spaced and specifically sized holes. The sun shines on the dark metal (which can be blue, red, black or other dark colors) and heats the metal, which warms the air next to it.

The heated air flows upward between the metal sheeting and the building wall and is captured in a chamber at the roof. A fan pulls this preheated air into the heating system. Since the air has been preheated, less fossil fuel is needed to provide the total heat for the building. These systems even work at night to capture heat that is conducted out through the building wall after the sun goes down.

Well-suited to gymnasiums, warehouses, garages, manufacturing plants and other such buildings, the technology is most cost-effective when there is sufficient south-, southwest- or southeast-facing wall surface; when relatively high ventilation is required; and when the heating season is long. Researchers are studying whether these systems can dry grain effectively.

For more information on this technology, visit this U.S. Department of Energy Web page.


Photovoltaic (PV) cells are solid-state semiconductor devices (similar to computer chips) that convert light energy directly into electric current. Small PV cells are found on calculators and wrist watches, and larger ones are used to power such devices as lights on buoys in ocean waterways, yard lights, whole-house electric systems, and camel-back refrigerator systems that carry precious vaccines into remote desert communities.

Several different technologies exist for producing PV cells, and continuing research is developing more. Currently, three types - single crystalline, multi-crystalline and thin-film amorphous - all use silicon as the main component in the cell material. Single crystalline cells are round because of the way the crystals are manufactured. Multi-crystalline cells are usually square, while thin-film amorphous cells are made in sheets because, rather than forming solid crystals, the silicon is deposited as a film on backing materials.

Because costs and efficiencies for solar cells are constantly changing, getting current information about them requires you to contact manufacturers and vendors. In general, crystalline and multi-crystalline cells have higher efficiencies and higher prices. Many of these companies offer web sites, including the American Solar Energy Society, the Interstate Renewable Energy Council and Solar Energy Industries Association.

While PV cells used for small appliances like calculators are sometimes less than one square inch in size, solar cells used for larger scale applications are usually about 4 inches wide and either square or circular in shape. Groups of these cells can be wired together, attached to a backing material, and placed into frames with a tempered glass cover. These framed groups of cells are called "modules," and vary in size depending on the amount of power needed. A group of modules can be wired together to form an array. Depending on the voltage and amperage desired for a given application, cells and modules can be wired in a variety of combinations.

PV System Components

PV cells produce direct current (DC) electricity, which is the type of current in automobiles and flashlights. Our homes, offices, and factories use alternating current, or AC, electricity. Using DC electricity requires different wiring, plugs and operation than AC. Also, PV cells obviously produce no electricity unless light is shining on them. To deal with these issues, as well as some safety issues, a complete solar electric system must have, in addition to the PV modules, what are called balance of system (BOS) components:

  • Inverters convert DC electricity to AC and ‘condition’ that electricity to make sure it is compatible with the needs of the devices being served.
  • Batteries store electricity for use at times when system load exceeds the amount of electricity being provided from the array.
  • Battery Charge Regulators or Controllers make sure that batteries are not overcharged (overcharging can lead to explosions in extreme cases) and are not excessively discharged. Charge regulators come in varying degrees of complexity; some models simply display a warning light when discharge is too great, while other models can be programmed to drop certain non-critical loads when necessary.
  • Auxiliary Battery Chargers are backup electric generators such as a gas or diesel engine, windmill, or small hydro-powered generator. They are used to keep battery packs fully charged to provide direct electricity to users during periods of low solar delivery or if electric load demands beyond those the system was designed to address. Sometimes electricity directly from the utility distribution system serves these purposes.
  • Distribution and Safety Hardware includes wiring, outlets (DC outlets are different than AC outlets), conduits, connectors, switches, fuses, breakers, and surge protectors.

These devices are all needed to complete the installation of a PV system. Even if you are installing the system on a building that is already wired for AC electricity, you will need some amount of these components.

Types of PV Systems

PV systems fall into three categories:

  • Stand Alone systems are those in which the PV system is the only source of electricity. They can vary in size, depending on whether they power a wristwatch, a fence charger, a water pump, or something as large as a whole house electrical system.
  • Hybrid Systems usually use PV for a majority of the electric generation, but also have a backup generator, such as an internal combustion engine, to supply power in periods of extended low-sun conditions.
  • Grid or Utility Intertied Systems use PV for a portion of their electric generation but remain hooked up to the local electric utility lines for backup electricity. In many cases, utility intertied systems do not have any battery backup. Instead, they send any excess electricity generated by their PV system to the utility for use by other customers. Grid intertied systems require close cooperation with the utility to ensure both the safety of utility-line workers and the PV system components, as well as compliance with local building ordinances. Such systems are much more economic and popular in states that allow what is called net metering.

Read more about determining the type of system you will need.

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