Solar Energy

Virginia Renewables - Solar Energy

The world is illuminated by the sun's energy, and each of us uses solar energy for many purposes. Although most US residents no longer hang laundry out to dry in the sun, we do depend on the sun's warming rays to heat the day and to keep our lawns, gardens, and forests growing. Although solar energy has a profound influence on our world, the prevalence and impacts of non-commercial (or non-marketed) solar-energy uses are not tracked by government or industry statistics.

Energy from the sun is widely available, but its properties create barriers to its potential for widespread use as replacement for those energy sources that are bought and sold in today's commercial markets such as fossil fuels. For example:

• Solar energy is diffuse, meaning that the solar energy falling over large areas must be collected over long time periods to make a significant contribution to energy use. For example: On an annual basis, the average amount of energy from the sun that strikes a square foot of ground in Lynchburg (located close to Virginia's geographical center) is 1386 Btus per day, while a single gallon of gasoline contains about 125,000 Btus.
• Solar energy's availability varies with time solar energy is not available after the sun goes down, and only minimal amounts of solar energy can be harvested on cloudy days. Average availability of solar energy during summer days in climates such as Virginia's far exceeds daily availability during the winter months.

Despite these disadvantages, solar energy also has advantages relative to other energy sources. Unlike fossil fuels, the sun's radiant energy does not have to be "burned"to be useful, and it does not produce air pollutants. Although the equipment for "collecting" solar energy can be costly, the energy itself is available in the natural environment at no cost. As a result of its advantages, use of solar energy to displace non-renewable energy sources has been expanding rapidly and this expansion is expected to continue.

The following are the primary uses for solar energy in today's market economy:

Electricity generation:

Devices known as photovoltaic cells convert solar energy into electrical energy directly. Photovoltaic technology is well developed, reliable, and widely used in small scale applications such as hand-held calculators and in areas remote from conventional power sources for uses such as water pumping and electrification of livestock fencing. However, the capital cost of purchasing and mounting photovoltaic cells, when amortized over expected useful life, results in costs for solar-generated electricity that are in excess of rates charged by commercial suppliers for electricity generated from more conventional sources. Also, photovoltaic cells convert only a fraction of the solar radiation that they receive into usable electricity. The energy-conversion efficiency of commercially-available photovoltaic systems is on the order of 10 percent, although conversion efficiencies in excess of 30 percent have been achieved by research installations.

For home uses, other factors unique to photovoltaic systems must be considered. Photovoltaic systems generate electricity most efficiently as direct current (DC) at relatively low voltages (12 v. is common), as opposed to the US standard for grid-connected electricity supplies (120 volts, alternating current). Home users of photovoltaic systems seeking to "disconnect" from the national electrical-supply grid commonly install battery systems to store electrical for use during sunless periods. Some home users of photovoltaic systems, however, have chosen to remain connected to the grid, selling excess photovoltaic electricity to the connected utility during times when generation exceeds the home's demand and drawing power from the grid when needed.

A small portion of the electrical energy generated by commercial suppliers in Virginia is produced at a photoelectric facility maintained by Dominion Energy at its North Anna facility (http://www.energy.vt.edu/vept/electric/table_generation.asp)

Additional information on photovoltaic energy systems can be obtained from the U.S. Department of Energy at http://www1.eere.energy.gov/solar/photovoltaics.html

Active Solar Collection Devices

"Active Solar" systems circulate a fluid (typically, air or a liquid) through transparent "solar collectors" that may be mounted on rooftops or any other location exposed to the sun's rays. During times when the sun is warming the collector, the fluid is circulated through the collector to gather the sun's energy; the fluid's temperature increases as it passes through the solar collection system. The warmed fluid is then circulated to either a direct use or a heat storage device. Typical uses for active solar collection systems are water heating and space heating (see http://www1.eere.energy.gov/solar/solar_heating.html).

The principles of operation for active solar systems are easy to understand, and the systems can be operated through use of pumps, valves, piping or ductwork, and other "low tech" mechanical equipment. The operating efficiency of such systems can be improved by adding sophistication to system controls, such as mechanisms that allow collectors to "track" the sun's daily journeys through the sky. Complications include the need to protect fluid-based systems against freezing in climates such as Virginia's, and maintaining the transparency of collector-cover materials (many of which are plastic-based, so as to limit weight and cost) over the long periods of time that these systems are expected to operate.

Passive Solar Systems

The term "passive solar" refers to buildings with components designed to harvest and store solar energy as heat, so as to displace use of conventional heating systems. A common type of passive solar systems emphasizes south-facing windows for space heat in residential and commercial structures.

An advantage of passive solar designs is that they can provide heat energy for buildings at relatively low cost. The ability of such systems to retain heat during for use during sunless periods (such as the night) is enhanced when designers are able to place "thermal mass" building elements capable of storing large amounts of heat energy and releasing it slowly, often comprised of masonry or containing water within the structure. Creative design can integrate thermal mass into building components such as floors, walls, support columns, chimneys, decorative planting beds, or other building elements in an aesthetically pleasing manner.

One disadvantage of passive solar buildings is the difficulty of temperature control. Again, creative design can help to minimize this problem. The tendency of buildings to overheat during warmer months can be reduced by placing horizontal shading devices over south facing windows, so as to allow penetration by the low-angle winter sun while screening out the summer sun which is typically higher in the sky. Where the building lot configuration allows, use of deciduous vegetation on the building's south and west sides can also help to limit unwanted summertime solar heating while encouraging winter solar gain. Strategic placement of thermal mass within the building structure can help to dampen interior temperature change. Even with these strategies, however, passive solar buildings are likely to experience greater fluctuations of temperature than is common in today's homes and commercial buildings that use advanced temperature controls.

For further information on passive solar building design, see http://www1.eere.energy.gov/solar/sh_basics_space.html.

Solar Energy Maps

Solar radiation - no link available

Data Tables