EME 444
Global Energy Enterprise

Solar Energy


Every single hour, the Earth’s surface receives more energy from the sun than the entire world's human population uses in a year. And, as far as fuel prices go, the price is right!

It is only natural that we have learned to work with the sun--to use it for our convenience and well being. We use energy from the sun in all sorts of ways, to heat water, dry clothes, warm spaces and generate electricity. Be they simple or complex, these designs and technologies all use “solar energy” for useful purposes.

Uses of Solar Energy, Other than Electricity Generation

Passive Solar Energy

This is the art and science of designing systems (typically buildings) to work in cooperation with the sun, without any mechanization. There are no motors, no fans or blower or switches, for example. Instead there are simple features, such as deep overhangs that provide shading in the summer, when the sun is high and temperatures are warm, but let the sunlight in in the winter, when the sun is low and the warmth is welcomed. If you would like more information, a good starting place is the Department of Energy's Passive Solar Design page. (Clothes lines are another example of passive solar, and wind. A "renewable dryer" investment has a terrific return financially and environmentally!)

Solar Thermal Systems

This is a broad term for systems that use energy from the sun to heat water (or other material) for a variety of purposes.

  • Sometimes the water is used for “domestic” purposes, such as drinking, cooking, bath and showers, laundry, and so forth. These are called solar domestic hot water (DHW) systems.
  • Solar-heated water may also be used for other purposes, such as swimming pool heating, to warm or cool (really!) buildings, and industrial process heating.
  • Solar concentrating systems are another form of solar thermal. (See below)

For clear understanding and communication, it is useful to keep in mind the broad meaning of “solar thermal” and to be specific regarding the technology of a given application.

Solar Energy for Electricity Generation

These are systems that use energy from the sun to generate electricity. There are two general categories: photovoltaics (PV) and concentrating solar power (CSP).

Photovoltaics (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. Photovoltaic materials (semiconductors) are packaged into solar cells, which are appropriately wired and connected together into modules (also called panels) to collect the flow of electrons into a current and make it available for our use. If you have a solar-powered calculator, the little window is a small solar cell. The solar arrays that you may see on a roof top are an installed group of solar modules wired together. Systems that use photovoltaic components to generate electricity are photovoltaic (PV) systems.

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Solar PV Output

The output of an array is primarily dictated by the amount of solar energy (insolation) hitting the panel over a given time period. Insolation is highest when the panel is directly facing the sun, when the sun is at its peak in the sky (at solar noon, which is usually not the same as local noon), and when it is unshaded. Insolation is synonymous with irradiation, noted earlier in this lesson. Irradiance, on the other hand, is the amount of solar power (not energy) hitting a surface at any given moment, or the average power over a given period of time. This is typically measured in W/m2.

Average daily insulation in the U.S. Greatest irradiation occurs in the south west and south with the least occurring in Washington and new england
Figure 9.7: Average daily irradiation in kWh/m2/day in the U.S.  Note that this image indicates the output at a perfect tilt, ideal azimuth, and without derating. Larger map image available here
Credit: National Renewable Energy Laboratory Dynamic Maps, GIS Data, & Analysis Tools.

Like wind, a solar array's capacity is rated in power (usually kW, but larger ones can be rated in MW). Also like wind, solar panels only generate full capacity under optimal conditions, mostly having to do with panel temperature and irradiance level. Further, the capacity is what is directly generated by the panels, and does not include other losses. After generated by a panel, the electricity must travel through wires and (usually) an inverter. There are other factors that impact output, such as panel imperfections, loss of efficiency over time, and mismatch of panels in an array. All of this adds up to losses, usually in the range of 10% - 20%. All of these losses together are called the derating factor (sometimes called a "derate factor"). A derating factor of 80% means that 20% of the energy generated by the panel is lost (to heat) before it leaves the PV system. Note that derating does not include losses associated with shading or imperfect panel placement! Finally, the hotter a panel gets, the less energy it generates, and the colder it gets, the more it generates (all else being equal). Because of this, it is not uncommon for a solar array to generate nearly as much electricity on a very cold, clear winter day as a hot summer day, despite the fact that irradiance is significantly higher in the summer.

When all is said and done, it is not unusual for an array to generate 20% - 30% less than its rated capacity, especially if the panels are not tilted at a perfect angle and facing an ideal direction (the compass direction a panel is faced is called its azimuth), and/or is partially shaded during certain times of the year/day.

A 1 kW array will generate 1 kWh of electricity over the course of one hour if it is operating at full capacity, but if it has a derating factor of 85%, it will only generate 0.85 kWh. If there is a 10% additional loss due to shading and other losses, the output would be 0.765 kWh (0.85 kWh x 0.9 = 0.765 kWh).

Concentrating Solar Power (CSP)

Systems that use mirrors (heliostats) to reflect (focus) the sun's energy onto a single point or area are called concentrating solar power or CSP systems. They use mirrors to focus energy from the sun to heat synthetic oil, molten salt, gasses, or other materials to high temperatures for purposes of generating electricity (by generating steam to turn a turbine or with a Sterling Engine.) The focused energy may be used to create very high temperatures for generating electricity (with a Sterling Engine or by creating steam to drive a turbine).

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Concentrating Photovoltaic (CPV)

These systems use highly concentrated (focused) sunlight to generate electricity directly from photovoltaics. According to a December 2013 report (Concentrated PV (CPV) Report, from IHS), "After years of slow progress, the global market for concentrated photovoltaic (CPV) systems is entering a phase of explosive growth, with worldwide installations set to boom by 750 percent from 2013 to the end of 2020. CPV installations are projected to rise to 1,362 megawatts in 2020, up from 160 megawatts in 2013." For better or worse (despite promising research like this at Penn State), the market for concentrated solar PV has yet to materialize, due in large part to the rapid drop in PV module prices.