Photovoltaic Cells

Picture: DOE/EERE

Photovoltaic cells (PVs) (also known as "solar cells") work by transforming light that comes from the sun directly into electricity without an intermediate mechanical device or thermal process. The term photovoltaic is derived by combining the Greek word for light, "phos", with the word "voltaic". The term "volt" is a measure of electricity named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. Photovoltaics literally means light-electricity.

The basic building unit of PV technology is the photovoltaic cell (PV cell). PV cells are made of a semiconductor material, typically silicon, which is treated chemically. When light hits the cell, a field of electricity is created within the layers causing the electricity to flow. This "photovoltaic effect" results in direct current (DC) electricity which is the same type of current produced by batteries.

In order to use this energy in most homes, an inverter is used to change the DC electricity to AC. Once electricity is generated, it can go to power anything in your house or be stored in batteries for later use. The greater the intensity of the light, the greater the flow across the layers and so the more electricity generated. But such a system does not necessary require direct sunlight to work.

Single PV cells are connected electrically to form PV modules, which are the building blocks of PV systems. Depending upon the application, the solar modules are typically wired together to form an array. Individual PV cells – averaging about 4 inches per side – typically converts 15% of the available solar radiation into about 1 or 2 watts of electrical power. Larger modules or arrays of modules are used to generate power for the grid.

Passive Solar Energy Designs

Here are a few video clips which show how to take maximum advantage of the sun's light and heat using passive solar energy designs:

Active Solar Heating

Diagram of a house with a combined solar domestic hot water and space heating system ("combisystem") (www.estif.org)

Active solar energy is more complex and requires mechanical devices to capture, store and convert the solar power into other useful forms of energy.

Active solar heating generates much more heat than passive systems do. Active solar heating relies strongly on three components: a solar collector to absorb the solar energy, a solar storage system, and a heat transfer system to disperse the heat to the appropriate places in your home. Liquid-based heating systems use a liquid to collect the energy in the solar collector; whereas air-based heating systems absorb the energy through the air.

Active solar technologies are used to convert solar energy into useable heat and cause air movement for ventilation or cooling.

Passive Solar Space Heating

Picture: westernwashingtonsolar.com

Space heating means heating the space inside a building. Passive solar space heating relies on incorporating building features that absorb heat and then release it slowly to maintain the temperature within the home. These building features, known as thermal mass, may include large windows, brick walls, and stone flooring. Passive solar heating techniques generally fall into one of three categories: direct gain, indirect gain, and isolated gain.

Direct gain is solar radiation that directly passes through the home's windows and is traped in the living space. Direct gain uses classic passive solar design strategy - the sunlight falls directly into the space and is absorbed by an abundance of thermal mass materials.

Indirect gain collects, stores, and distributes solar radiation using some thermal storage material (e.g., Trombé wall or a thermal storage wall). Conduction, radiation, or convection then transfers the energy indoors. Sunlight is absorbed by the wall, which heats up slowly during the day. Then as it cools gradually during the night, it releases its stored heat over a relatively long period of time indirectly into the space.

Isolated gain (e.g., sunspace) collect solar radiation in an area that can be selectively closed off or opened to the rest of the building. That heat than can be distributed into the living area in a variety of ways. The sunspace has the same characteristics as a direct-gain system - extensive south-facing glazing and thermal mass, and it should be well constructed, with low infiltration and high insulation levels.

In the case of passive solar space heating the whole house operates as a solar collector (passive solar home). A passive solar home is designed to let in as much sunlight as possible. It is like a big solar collector.

Sometimes for passive solar energy to be utilized effectively there must also be a means for the heated air to circulate throughout the home. Usually, the natural circulation of air is enough as long as doors are left open throughout the home, however, sometimes fans are also incorporated into the design to facilitate this.

Passive solar heating features can reduce heating bills by almost 50 percent and it requires little or no investment of external equipment. Building a passive solar home may even cost the same as building a conventional home, especially if you're working with a builder who is familiar with the processes of passive solar heating systems.

Solar Heating Systems

Solar heating can be used to heat the space in homes and buildings or to heat the water. There are two basic types of solar heating systems: passive solar heating and active solar heating.

The appropriate use of windows along with building design is called passive solar heating. The buildings can be designed to make the best use of the sun in winter while keeping the heat out in summer. South-facing, large windows, building materials that absorb and retain heat (such as stones and bricks), and efficient airflow are among the design features of a home that takes advantage of passive solar.

Active solar heating systems use mechanical equipment, such as pumps and fans, to increase the usable heat in a system. The heat is primarily used for heating water in homes, commercial buildings and industrial facilities. Active solar heating can be further divaded into liquid-based and air-based systems according to the kind of energy transfer fluid that is used.