Maryland Energy Administration

Solar Energy

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The Sun

Our sun is a giant nuclear fusion reactor running on hydrogen. Each second it converts 564 million tons of hydrogen to 560 tons of helium through the fusion process. The loss of four million tons of mass equates to 91,000,000,000,000 gigawatt hours of energy (a gigawatt is equal to 1,000,000 kilowatts). This is more energy in one second than six billion nuclear power plants would produce in a year.

The sun's energy is an enormous and constant energy resource, but because of the earth's protective atmosphere only a small amount of the total energy produced by the sun reaches earth. Astronomers have determined that the sun's energy has remained relatively constant over the last century and this "solar constant" will continue to be 1.35 kilowatts per square meter (+-3.5%) for about the next four billion years.

On average, the State of Maryland receives 5.3 kilowatt hours per day per square meter of solar energy. Naturally, the amount is higher in the summer and lower in the winter.

On a sunny summer day Maryland receives about 196,000 Gigawatt hours of solar energy. This is more than all the electric power plants in the State would produce in a year. With all of this potential energy, it is no wonder many people find solar energy a useful, environmentally friendly and economical energy source to heat their homes and businesses, as well as water.

We are able to use this vast resource primarily in three ways: passive solar heat, active solar heat, and photovoltaics. The first two energy sources involve collecting the heat produced by the sun for use in heating living or working space, or hot water. Photovoltaics uses the light produced by the sun (or any light source) to generate electricity directly. Sunlight striking a photovoltaic or solar cell causes a voltage and current to be created in a semiconductor that can be used just like the electrical energy from a battery or DC generator.

Passive Solar Thermal Energy

Passive solar buildings are designed with large areas of glass facing south. Because our region is north of the Tropic of Cancer, the sun is never directly overhead. Instead, it is always south of overhead making the southern face of a building the one receiving the most sun.

By placing large areas of glass on the south side of a building, the warmth of the sun passes through the glass and warms the interior of the building. To enhance the solar gain, thermal storage is often added in conjunction with the southern glass exposure. Thermal storage devices can be as simple as dark colored floor tile which helps to store the heat, or as complex as dark colored hollow walls filled with water. As a general rule, the darker the color of a material and the greater (heavier) the density, the better it is at storing heat. For example, a dark tile covered concrete floor is better at storing heat than a white carpeted floor.

Active Solar Thermal

Active solar thermal systems employ solar collectors that concentrate the sun's heat and usually transfer the heat to circulating water. Because the solar collectors are very good at collecting the sun's heat, they can be used not only for keeping a living space warm, but also for providing domestic hot water.

Active solar systems are more complex than passive solar designs. A typical active system has one or more solar collectors mounted on a tilted roof facing south. Water is pumped through the collector, transferring the heat from the collector to the water. The water is then stored in a large insulated tank inside the building. It can then be used to provide heat to the building or provide hot water.

Electricity Generation from the Sun

Photovoltaics (PV) use semiconductor material to convert sunlight directly into electricity. Originally designed for space missions, photovoltaics now produce energy for many cost effective terrestrial applications.

Solar calculators and watches have small photovoltaic cells built into them providing power for the electronics or to charge a small battery in the device. On a larger scale, many PV cells can be combined to make solar modules which can provide enough power for a walk light or a street light. Several modules can be combined into a solar array can provide electricity for house or even commercial buildings.

In some cases it is cheaper to install PV to serve small isolated electrical loads than it is to run wires from the electric company. Street lights and roadway sign lighting are two typical applications that are sometimes cheaper than wired service. Some residential customers in remote areas, because they are far away from the electric company's wires, can be served at less cost than running wires to the house.

PV installations that are independent of utility lines are also called off-grid systems. For a typical residence to be off-grid it would require approximately 4 kilowatts (kW) of photovoltaics. Because PV produces power only when the sun is shining, an independent residential system must include batteries to store energy when the sun is not shining. Often these systems include a backup generator (gasoline, diesel or propane) to provide additional assurance that power will always be available. The cost of a complete off-grid residential PV system is about $40,000 to $60,000.

More common uses of PV are utility interconnected, or grid-tied, systems. These systems use electricity generated from the sun and electricity provided by an electric company. There is more flexibility in designing these systems since whatever power requirements are not met by the PV system will be provided by the electric company. Typical residential installations are from under 1 kW to over 3 kW with a cost range from under $9000 to over $30,000. Grid-tied systems do not provide electricity during power outages or at night unless they are designed with an optional battery storage system. Grid-tied systems can also take advantage of the Maryland Net Energy Metering law and feed any excess generation onto the utility grid.

Local solar consultants, designers and installers can be found on the Maryland, DC, Virginia Solar Energy Industry Association (MDV-SEIA) Web site www.mdv-seia.org.

Solar Thermal Generation

Electricity also can be produced by concentrating the sun's rays onto a small location. By using reflectors (mirrors), the concentrated rays magnify the sun's heat to the point where steam can be produced. The steam is used to turn a steam turbine much like an ordinary power plant produces electricity.

A new technology called the "Sterling Dish Engine" uses concentrated solar energy to run an external combustion engine. The engine drives a small generator typically producing from 5 to 25 kilowatts of electricity at peak. Sterling engines, together with their solar concentrators, are small and portable and can be sited near electrical loads. The sterling engine also can be powered with natural gas or propane so energy can be produced even when the sun isn't shining.

Programs

• Solar Grant Program
• Solar Roofs

Additional Information

• A Consumer's Guide: Heat Your Water with the Sun
  This publication introduces consumers to solar heating technologies, and guides them through the basics of the technology and how to purchase it for the home.
• Mid-Atlantic Region Consumer's Guide to Buying a Solar Electric System
• Moving Toward Zero Energy Homes
  This fact sheet describes the different projects and partners in the Zero Energy Homes initiative. A Zero Energy Home combines state-of-the-art, energy-efficient construction and appliances with commercially available renewable energy systems such as solar water heating and solar electricity. This combination can result in net zero energy consumption from the utility provider.