As was detailed in an earlier lesson, solar energy is electromagnetic (aka radiant) energy that is generated by the (nuclear) fusion of hydrogen atoms into helium atoms in the sun. The amount of radiant energy that is released by an object is related to its temperature, and since the sun is so hot (~10,000º F!), it is able to reach the ~94,000,000 miles (the distance depends on the time of year) to the earth. It is a massive amount of energy! A commonly cited statistic is that enough solar energy reaches the earth each hour to provide all of humanity's energy for an entire year. There is no shortage of solar energy.
Types of Solar Energy Technologies
Without the sun, life on earth would not be possible. It provides energy for vegetation to grow and provides sufficient heat to allow water to exist in liquid form, among other things. But there are many ways that humans can use this radiant energy more deliberately. The following is an overview of the major types of solar technologies. We could spend weeks analyzing each of these - keep in mind that this is just an overview.
- Solar photovoltaics (solar PV): Certain materials have the natural property of converting energy from the sun into electricity. When the sun hits these materials, electrons start to flow, creating a direct current (DC). This is the photovoltaic effect. This is described in more detail below.
- Solar thermal: This is a broad term for systems that use energy from the sun to heat water (or other material) for a variety of purposes. One common application is heating water for domestic hot water or swimming pools.
- Concentrated solar: There are a variety of ways to concentrate solar power for use. All of them gather solar energy over a wide area (usually by using mirrors) and concentrating it into a smaller location. This very high level of power is then often used to generate electricity. The DOE has a good explanation of some technologies here.
- Passive solar: Passive solar is a type of solar thermal that uses passive system design (e.g. south-facing windows, strategically-placed overhangs) to passively heat interiors of buildings. This is a great low-tech way to use solar energy! (Click here for more information about passive solar from the DOE.)
Solar Photovoltaics and Availability of Solar
The rest of the solar lesson will focus on solar photovoltaics or solar PV. As noted above, photovoltaic technology (aka the photovoltaic effect) converts radiant solar energy into electricity. View the short video below from the U.S. Department of Energy for a brief explanation. Note that the narrator of the video indicates that photons provide the energy that is converted into electricity. NASA describes the relationship between photons and electromagnetic energy thusly: "Electromagnetic radiation can be described in terms of a stream of mass-less particles, called photons, each traveling in a wave-like pattern at the speed of light." So photons are generally considered to be what carries the energy that is emitted in waves.
Okay, so a solar panel converts radiant to electrical energy by using the unique properties of a semiconductor, usually, silicon combined (doped) with other elements (usually boron and phosphorous). But how much energy and power does a panel generate? As you might guess, it depends on a lot of factors. The following is an overview of some of these factors.
- First, a quick primer on power vs. energy.
- Energy is the ability to do work. It is a discrete amount of "something," and that "something" makes things happen (makes things move, generates sound, generates heat, etc.). If one thing has more energy than another thing, then it is hypothetically capable of doing more "stuff." In the U.S., energy is usually measured in Btus, or if it's electrical energy, kilowatt hours (kWh). One kWh is 1,000 Wh, which is also a unit of energy. The international unit of energy is the Joule (J). Since there are 1,000 Wh in one kWh, to convert Wh to kWh you must divide by 1,000. To convert kWh to Wh, you multiply by 1,000.
- Power is the rate at which energy is converted. Practically speaking, it indicates how quickly you are "using" energy or the rate at which energy is being provided. Power is usually measured in watts (W) or horsepower (HP). A light bulb that uses 100 W of power is converting 100 Joules of electricity into heat and light each second. A 200 W light bulb is converting energy twice as quickly. A 1,000,000 W power plant (1 MW) is providing energy twice as quickly as a 500,000 W (500 kW) power plant. (Note that the light bulbs and power plants are all just converting one form to another, but from our perspective, one is "using" energy and one is "generating" energy. It's a matter of perspective.)
- Energy = power x time. For example:
- If you use a 100 W light bulb for 1 hour, you use (100 W x 1 h = ) 100 Wh.
- If you use a 100 W light bulb for 1 hour each day for a year, you use (100 W x 1 hr/day x 365 days/yr = ) 36,500 Wh = 36.5 kWh.
- If you use 10 light bulbs, each 100 W, for 1 hour you use (10 bulbs x 100 W/bulb x 1 hr = ) 1,000 Wh or 1 kWh.
- If a 5,000 W (5 kW) solar array operates at full capacity for 1 hr, it generates (5,000 W x 1 hr = ) 5,000 Wh or 5 kWh of electricity. This is the energy that is generated by the panels, but some of that energy will be lost by the time it is used.
- If a 1,000,000,000 W (1 GW) power plant operates at full capacity for 24 hours, it would generate (1 GW x 24 hr = ) 24 GWh of electricity.
- If a furnace with a 100,000 Btu/hr output operates at full capacity for 2 hours, it would use (100,000 Btu/hr x 2 hr = ) 200,000 Btu of energy.
- Irradiance is the amount of solar power (not energy) incident on a given surface area at any given time. This is usually measured in Watts per square meter (W/m2). All else being equal, more irradiance results in more output from a panel. Irradiance levels change throughout the day, as you can see in the image below. These charts illustrate the average hourly irradiance in State College, PA in July and December. As you can see, the peak irradiance in July averages about 750 W/m2. This is the irradiance on a horizontal surface. If you could tilt the surface (say, a solar panel) so it is perpendicular to the sun, the noon irradiance would be more than 1,000 W/m2. So if you want to imagine 1,000 W/m2, think of how your skin feels on a really hot summer day at Penn State. Hopefully, this gives you a "feel" for irradiance.
- Irradiation is the amount of solar energy that hits a surface over a given period of time. This is usually measured in kWh/day/m2 or kWh/yr/m2. See the chart below for a map of irradiation in the U.S., which illustrates the average daily irradiation levels throughout the year in the U.S. Of course the irradiation will change throughout the year (more in the summer and less in the winter), but this chart provides a clear idea of the overall amount of solar energy that is available all year. (Note that you could easily find the average annual irradiation by multiplying the daily irradiation by 365.) Keep in mind that this is the average daily irradiation on a surface that is "latitude tilt," which will be explained in more detail below. The chart provides an indication of solar potential. (Note that due to panel inefficiency and a few other factors, a solar panel will only convert a fraction of this solar energy into electricity.)