Community Renewable Energy

Solar Technology

Solar Energy can be harnessed to generate electricity, heat water, and building space. The different solar technologies are discussed below.

Solar Electricity

Solar Electricity Photovoltaics (PV)
Photovoltaics = Photo (Light) + Voltaics (Electricity)

When the sun's rays fall on the PV cell it increases the energy stored in the atoms of the wafer. That increased energy leaves the atoms’ electrons free to move, creating electricity. This electricity is captured by wires running through the PV cells. The electric current generated by PV cells is direct current (DC), while the electric grid and most appliances in the United States use alternating current (AC). PV produces electricity in a process that is completely self-contained, where no materials are consumed or released.

Solar cells usually need to be interconnected with other system components that ultimately serve a specific electrical demand, or 'load'. PV systems can either be stand-alone, or grid-connected. The main difference between these two basic types of systems is that in grid-connected systems, the PV system produces power in parallel with the electrical utility, and can feed power back into the utility grid if the on-site load does not use all of the PV system's output. As the name implies, stand-alone systems are not connected to the electric grid.

PV cells are wired together to make modules, and modules are connected into larger PV arrays. The following PV system types, including technology and components are discussed in greater detail:

• Grid-tied
• Grid-tied with battery back-up
• Off-grid/Stand-alone with batteries
• Off-grid/Stand-alone without batteries

Solar Water Heating

Solar water heaters can supply a substantial portion of home or building’s domestic hot water usage (meaning water used for cooking, washing, bathing, but generally not for heating systems that use steam or forced hot water). The amount of hot water the system produces depends on the type, size and location of the system. Solar hot water is best used for homes and businesses that have high hot water usage.

The systems are set up so that a fluid flows through a rooftop collector where it is heated by the sun's energy. Heated water is stored in an insulated storage tank until used. On cloudy days and at night when there is not enough solar energy, a conventional backup heater provides the necessary additional energy to heat your water to the desired temperature, usually around 120°F.

Pool heating is another effective application of solar water heating, as the systems are incredibly simple. While the pool serves as its own storage tank, the solar heat can lengthen the swimming season, without adding to utility costs.

Common Types of Solar Water Heaters:

Passive vs. Active Systems: The terms passive and active in solar water heating systems refer to whether the systems rely on pumps (active) or only thermodynamics (passive) to circulate water through the systems.

Anti-Freeze vs. Drainback: In the climate of the Pacific Northwest, solar water heaters are designed to withstand freezing temperatures. The most common choices are closed-loop anti-freeze systems and “drain back” systems. In a closed loop (or indirect) system using antifreeze, the anti-freeze solution is pumped through the solar collector and then transfers it’s heat to the domestic hot water via a heat exchanger. Drain back systems are indirect as well, but do not use anti-freeze. Instead, water passes through the collectors, and to prevent freezing, “drains back” to a tank within a heated area when not collecting energy.

Solar Space Heating

Solar space heating takes place when sunlight heats air in one part of a building, and that heat circulates throughout the building to help offset the building’s heating load. The circulation of solar-heated air can take place either passively (when no ventilation systems are used), or actively (when a ventilation system is used).

Passive solar space heating is the most popular and cost-effective way to use solar energy in the Northwest. In this and many other US climates, passive solar design techniques can significantly reduce heating requirements for residential and small commercial buildings. New construction offers the greatest opportunity for incorporating passive solar design features. Buildings designed for passive solar and daylighting incorporate design features such as large south-facing windows and building materials that absorb and slowly release the sun's heat. Passive solar designs can also include natural ventilation for cooling.

Active solar space heating systems can offset building heating loads by either heating recirculated building air, or by preheating outside or ventilation air. Recirculated air is heated by direct sunlight when it passes through solar panels, then circulated back to the building. Outside ventilation air can be heated effectively with a simple mechanism called a transpired solar collector, which consists of a dark collector surface with perforated metal plates. The collector is mounted on a south-facing wall, leaving space between it and the building wall. Fans pull fresh air in through the holes in the collector, the air is heated just behind the dark collector surface, and then it is circulated throughout the building. Transpired solar collectors work for buildings with large ventilation requirements. For more information on solar space heating visit the Department of Energy’s Solar Energy Technologies Program.