Renewables

Solar

Sunlight - solar energy - can be used to generate electricity, provide hot water, and to heat, cool, and light buildings.

Producing electricity

Photovoltaic (solar cell) systems convert sunlight directly into electricity. A solar or PV cell consists of semiconducting material that absorbs the sunlight. The solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. PV cells are typically combined into modules that hold about 40 cells. About 10 of these modules are mounted in PV arrays. PV arrays can be used to generate electricity for a single building or, in large numbers, for a power plant. A power plant can also use a concentrating solar power system, which uses the Sun’s heat to generate electricity. The sunlight is collected and focused with mirrors to create a high-intensity heat source. This heat source produces steam or mechanical power to run a generator that creates electricity. Concentrating solar power is the least expensive solar electricity for large-scale power generation, and has the potential to make solar power available at a very competitive rate.

Hot water

Solar water heating systems have two main parts: a solar collector and a storage tank. Typically, a collector - a thin, flat, rectangular box with a transparent cover - is mounted on the roof, facing the Sun. The Sun heats an absorber plate in the collector, which, in turn, heats the fluid running through tubes within the collector. To move the heated fluid between the collector and the storage tank, a system uses either a pump or gravity, as water has a tendency to naturally circulate as it is heated. Systems that use fluids other than water in the collector’s tubes usually heat the water by passing it through a coil of tubing in the tank.

A typical system will reduce the need for conventional water heating by about two-thirds. Sometimes the plumbing from a solar heater connects to a house’s existing water heater, which stays inactive as long as the water coming in is hot or hotter than the temperature setting on the indoor water heater. When it falls below this temperature, the home’s water heater can kick in to make up the difference. High-temperature solar water heaters can provide energy-efficient hot water and hot water heat for large commercial and industrial facilities.

In solar heated swimming pools, the pool’s filter pump pumps water through a solar collector, and the pool itself stores the hot water.

Passive solar heating, cooling and daylighting

Many large commercial buildings can use solar collectors to provide more than just hot water. Solar process heating systems can be used to heat these buildings. A solar ventilation system can be used in cold climates to preheat air as it enters a building. And the heat from a solar collector can even be used to provide energy for cooling a building.

A solar collector is not always needed when using sunlight to heat a building. Some buildings can be designed for passive solar heating. These buildings usually have large, south-facing windows. Materials that absorb and store the Sun’s heat can be built into the sunlit floors and walls. The floors and walls will then heat up during the day and slowly release heat at night - a process called direct gain. Incorporating passive solar designs can reduce heating bills as much as 50 percent. Proper building orientation so the longest walls run from east to west, allows solar heat to enter the home in winter, while allowing in as little sun as possible during summer. Shading and overhangs also reduce excessive summer heat, while still permitting winter sun. In passive solar designs, the optimal window-to-wall area ratio is 25-35 percent. In cold climates, south-facing windows designed to let the Sun’s heat in while insulating against the cold are ideal. In hot and moderate climates, the strategy is to admit light while rejecting heat. Interior spaces requiring the most light, heat, and cooling are located along the south face of the building, with less used space to the north. Open floor plans allow more sun inside.

Passive solar designs can also include natural ventilation for cooling. Installing casement or other operable windows for passive solar gain and adding vertical panels, called wing walls, perpendicular to the wall on the windward side of the house, can accelerate the natural breeze in the interior. Another passive solar cooling device is the thermal chimney, which can be designed like a smoke chimney to vent hot air from the house out through the roof.

Many of the passive solar heating design features also provide daylighting. Daylighting is simply the use of natural sunlight to brighten up a building’s interior. In addition to south-facing windows and skylights, clerestory windows - a row of windows near the peak of the roof - can let light into north-facing rooms and upper levels. An open floor plan allows the light to reach throughout the building. This can result in substantial savings on electric bills, and not only provides a higher quality of light, but improves productivity and health.

South-facing sunrooms are often added on as a way to retrofit a home to take advantage of the Sun’s heat and light. It is also possible to use a sunroom to help ventilate the rest of the house. Lower vents from the sunroom to the interior rooms draw air through the living space to be expelled out the upper vents to the outside along the top of the sunroom.

A Trombe wall consists an 20-40 cm (8-16 inch) thick masonry wall coated with a dark, heat-absorbing material and covered by a single or double layer of glass, placed from about 2-15 cm (3/4 - 6 inches) away from the masonry wall. Heat from the Sun is stored in the air space between the glass and dark material, and conducted slowly to the interior of the building through the masonry. Adding a Trombe wall and south-facing windows is an easy way for a home to take advantage of solar heat.

Passive solar heat is sometimes captured to cook food. A basic cooker consists of an insulated box with a glass top. Heat from concentrated sunlight gets trapped in the box and can be used to heat food enclosed in the box.