EGEE 101
Energy and the Environment

Solar

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Recall that we have already covered solar energy from the standpoint of home heating and water heating (passive and active solar heating). Solar energy is also responsible for nearly all the other renewable energy sources as well (solar energy drives the water cycle, creates wind, and is the energy source for biomass). Historically it has had a relatively low contribution to electricity generation in the U.S. That however is about to change. On this page we are concerned with solar energy being converted into electricity, whether directly via solar cells (photovoltaic) which is the majority of the contribution, or indirectly via heating some type of medium (thermal solar), which is then used to generate electricity from steam.

Solar Cells (Photovoltaic)

Solar cells generate 2% of the U.S. electricity supply. We can directly turn solar energy into electricity utilizing photovoltaic technology.  Solar cells are also used in remote locations such as space, and in remote locations where there is not a connection to the electric grid, on RT 322 across PA to power some of the temporary signs (otherwise someone would have to come out and either recharge or replace the battery), on satellite wings, and many other locations. There is a wide range of efficiencies but ~20% is common for solar cells. Cheaper solar cells are less efficient. A dozen or so solar cells can either be placed on the roof (or close to home) or in solar farms where 1,000's of larger cells can be present.

Watch

The following 2 minute video does a great job showing how Photovoltaic (PV) panels convert solar energy into renewable electricity.

Solar PV
Click here for a transcript of the Solar PV video.

PRESENTER: All right, we all know that the sun's energy creates heat and light. But it can also be converted to make electricity-- and lots of it. One technology is called Solar Photovoltaics, or PV for short. You've probably seen PV panels around for years but recent advancements have greatly improved their efficiency and electrical output. Enough energy from the sun hits the Earth every hour to power the planet for an entire year.

Here's how it works. You see, sunlight is made up of tiny packets of energy called photons. These photons radiate out from the sun and about 93 million miles later, they collide with a semiconductor on a solar panel here on earth. It all happens at the speed of light. Take a closer look and you can see the panel is made up of several individual cells, each with a positive and a negative layer, which create an electric field. It works something like a battery.

So, the photons strike the cell, and their energy frees some electrons in the semiconductor material. The electrons create an electric current, which is harnessed by wires connected to the positive and negative sides of the cell. The electricity created is multiplied by the number of cells in each panel and the number of panels in each solar array.

Combined, a solar array can make a lot of electricity for your home or business. This rooftop solar array powers this home. And the array on top of this warehouse creates enough electricity for about 1,000 homes.

OK, there are some obvious advantages to solar PV technology. It produces clean energy. It has no emissions, no moving parts. It doesn't make any noise, and it doesn't need water or fossil fuels to produce power. And it can be located right where the power is needed, in the middle of nowhere. Or it can be tied into the power grid. Solar PV is growing fast, and it can play a big role in America's clean energy economy, anywhere the sun shines.

Credit: DOE

Read

Another nice explanation of How Solar Photovoltaic Cells Work can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

 Grapic representation of the process used to turn solar energy into electricity.
At the heart of the solar cell is a sandwich of two semiconductors: the n-type and the p-type. The photons in light (if they have the right amount of energy) enter the positive silicon layer (p-type) and excite an electron to "jump" the bandgap into the negative (n-type) layer. The electron leaves behind a "hole" that allows other electrons in the p-type layer to hop in and out. The n-type has atoms that can hold an extra electron and so the electrons can flow (make a circuit). Any photon with too little or too much energy will not promote the electron into the n-type layer. The Silicon chips here are very similar to the chips in your computer. Both require highly purified and crystalline silicon. One of the reasons for the initial high cost of this technology was the silicon material however, the cost has fallen drastically and expectations are for a much greater contribution to electricity generation in the future.
Credit: DOE

Picture of three solar panels.
Left: Backwoods Solar Electric Systems, outside of Sandpoint, Idaho, is located 2 miles from utility lines and uses their off-grid system for home and business power. The system includes photovoltaics, wind power, and a backup generator. 3000 watts of power total are created in this system that was installed in 1978 and upgraded in 1990 and 1999. Right: Part of Littleton, Colorado's "10,000 Trees" project
Credit: LEft, Backwoods Solar Electric Systems,  Right: NREL

Advantages of Solar (PV) Technology

The advantage of using solar technology is no fuel costs, it is a renewable energy that is clean during operation (no air pollution or greenhouse gases) but obviously, energy is used in the creation of the panels, etc. Solar derived electricity had a limited contribution to the US energy profile until recently when prices became more competitive. Where solar panels had initially been most beneficial was in remote locations, where it would be expensive or impossible to link power lines. Falling production costs mean that we can not have large-scale solar farms using photovoltaic approaches.  Tax credits also lower the overall cost of the systems.

Disadvantages of Solar (PV) Technology

The disadvantages of solar energy in general (for electricity generation) are the large plots of land required, and the inconvenience of those cloudy days (intermittency) and at night. Regional haze also reduces the amount of solar energy that reaches the surface so sunny locations like Florida become less economic because of the haze as other locations (we will see a map a little later on). Electricity storage is also an issue unless the photovoltaic can be hooked into the local electricity grid or large-scale battery storage. Some nations such as Germany, however, have managed to generate significant electricity with the photovoltaic approach often now being coupled to some storage solution such as large-scale batteries (or currently in Germany to coal-plants that can quickly generate electricity). Similar to wind turbines, solar panels also use rare earth elements — that are in limited supply.

Thermal Solar Plants

Thermal solar relies on concentrating the solar resource. There are two options: reflective mirrors on a tower and parabolic collectors (troughs). For the concentrating tower solar sites many in the U.S. are in California (high electricity prices, and a very good solar resource). Spain also has a large number of these types of solar thermal plants.

Watch

The following 2:16 minute video explains how CSP works to produce electricity.

Click here for a transcript of Concentrating Solar Power.

PRESENTER: OK. Take the natural heat from the sun. Reflect IT against a mirror. Focus all of that heat on one area. Send it through a power system. And you've got a renewable way of making electricity. It's called concentrating solar power OR CSP.

Now, there are many types of CSP technologies, towers, dishes, linear mirrors, and troughs. OK, have a look at this parabolic trough system. Parabolic troughs are large mirrors shaped like a giant U. These troughs are connected together in long lines and will track the sun throughout the day. When the sun's heat is reflected off the mirror, the curved shape sends most of that reflected heat onto a receiver.

The receiver tube is filled with the fluid, and it could be oil, molten salt, something that holds the heat well. Basically, this super hot liquid heats water in this thing called a heat exchanger. And the water turns to steam. Now the steam is sent off to a turbine, and from there it's business as usual inside a power plant.

A steam turbine spins a generator and the generator makes electricity. Once the fluid transfers its heat, it's recycled and used over and over. And the steam is also cooled, condensed, and recycled again and again. One big advantage of these trough systems is that the heated fluid can be stored and used later to keep making electricity when the sun isn't shining.

Sunny skies and hot temperatures make the southwest us an ideal place for these kinds of power plants. Many concentrated solar power plants could be built within the next several years. And a single plant can generate 250 megawatts or more, which is enough to power about 90,000 homes. That's a lot of electricity to meet America's power needs.

Credit: DOE

Read

Another nice explanation of Concentrating Solar-Thermal Power can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

 A Solar panel field in Spain.
Left: This solar collector is in Spain. The heliostat field focuses on the top tower. The mirrors track the motion of the sun across the sky and reflect the solar energy onto the receiver. The receiver can be a simple boiler producing steam to generate electricity or it can be a molten salt boiler that is used to generate steam (the molten salt will stay hot longer permitting electricity generation later into the evening).
Right: A large solar plant with five Solar Electric Generating Stations (SEGS), with a combined capacity of 150 megawatts. The mirror is blue because of the reflection of a very blue sky.
Credit: Left: SANDIA National Lab, Right: DOE

The DOE says the following about the capacity for a large-scale plant. "At capacity, there is enough power for 150,000 homes. The facility covers more than 1000 acres, with over 1 million square meters of collector surface. The SEGS utilize parabolic trough collectors to focus the sun's energy on a pipe carrying a flow of heat transfer fluid (synthetic oil). The fluid flows to heat exchangers where the heat turns water into steam to drive conventional steam turbine generators, which produce electrical power."

 Closeup of a parabolic mirror.
Here you see a closer view of a smaller system with the pipe carrying oil at the focal point of the parabolic mirror (you own a couple of these, they are in the headlights of many cars). This one is actually using solar energy to kill bacteria in water. The mirror rotates to ensure maximum exposure to the sun.
Credit: DOE