Alternative Energy Part 3: Solar Power

This is my last section on alternative energy. I know this has taken forever to get up – I had a lot of background research to do, and it make much more sense to do this all in one full post rather than a few smaller ones. I might elaborate on some of these issues in later posts, since this topic such a large one. There are so many cool things being done in the area of solar energy, it’s hard to keep up sometimes!

Projections for energy use estimate that by 2030, the entire globe will be consuming 16.9 trillion watts of power, compared to our current amount of approximately 12.5 TW (trillion watts). If energy production were acquired solely through non-fossil fuel sources, the amount of energy needed would actually decrease to approximately 11.5 TW. This is because renewable energy is used to create electricity, which is a more efficient source of power. An example of this can be seen in cars, where only 17-20% of the energy is actually used, where as the rest is expelled as waste energy in the form of heat. An electric car would utilize 75-86% of that electricity to power the vehicle.  Similarly, generating electricity through burning coal is inefficient.  If energy demand continues to rise while we are using fossil fuels only, thousands of new plants will be needed to meet this demand, and could cost upwards of $10 trillion. While creating an infrastructure for new solar and wind energy production could be costly, the benefit lies in the fact that we would be investing in a source that would not become outdated. If we invest trillions of dollars in coal plants, those plants are useless once coal is gone. If we stay reliant upon fossil fuels, then as energy demand increases, we will need to build more plants. Solar power plants and wind turbines constitute an infrastructure that needs no expansion once established. Many scientists believe that in the switch to alternative energy, combining both solar and wind power will create a sufficient supply. The two are complementary sources, since solar power is not generated at night, but strong winds can power turbines during the night instead. The article in Scientific American that I referred to in the first part of this section on alternative energy outlines the various facilities to supply the globe with enough energy without using fossil fuels. Using a combination of WWS sources, solar would constitute 40% of the global power supply through solar power plants and solar panels on rooftops of homes and commercial buildings. 90,000 solar power plants producing 300 megawatts of electricity each would be needed. The solar panels not located on buildings would take up only .33% of the planet’s land. If we continued to use coal, then 13,000 new plants would be needed. These take up considerably more space than solar panels and solar power plants. The additional space-saving benefit to solar energy also comes from onsite energy production, which is the ability for individual houses to get electricity directly from the solar panels that are on the roof or surrounding property. More and more city and state governments are offering incentives and tax cuts to residents who install renewable energy sources like solar panels.

Personally, I feel strongly that there is great potential in solar energy as prices continue to drop, efficiencies increase, and number of materials that can be used to create solar panels expands. Once built, solar panels need very little maintenance and have very little downtime. The average US coal plant is actually shut down for about 12.5% of the year due to planned and emergency maintenance procedures. In comparison, photovoltaic systems are down less than 2% of the year. Combined with other alternative energy sources, this would mean that we could have power generated by systems hardly ever shut down for maintenance. There are also hopes that in 10 or so years, the cost of photovoltaic energy will be comparable to conventional sources. In 2007, the average cost of power was 7¢ per kWh (kilowatt hour), and it is estimated it will increase to 8¢. By this same, photovoltaic power will hopefully by about 10¢ per kWh.

There are several issues of concern expressed by the public, environmentalists, and scientists. While I accept these concerns as valid, I do not think they should be reasons to not invest in solar technology and further research in alternative energy. The idea of disrupting the status quo is very scary to many people, but I feel confident this is largely due to a lack of education about the environmental issues and the real solutions. Much of the public does not understand the real crisis of continuing our reliance upon fossil fuels, and they think that environmental issues are incredibly blown out of proportion. People go about their lives not thinking about where their power comes from, not thinking about the global consequences of their actions. Things like solar technology are actually viewed by many people still as being some futuristic science fiction technology still in experimentation. Solar power has been in use since around 1970s, and while experiments to increase efficiency and find safer or more abundant materials to use is still being explored, the basic solar power technology is long past the experimental stage. Some of the concerns I’ve seen people express about solar power are these:

  • How the hell does it work?
  • How efficient is solar technology?
  • It’s expensive and unreliable
  • The materials used are still possibly dangerous and environmentally hazardous
  • Solar panels only work in consistently bright and sunny areas like the Southwest desert of the US

How does solar power work?

Solar energy works by taking light from the sun and converting into electricity.

Solar panels

Solar panels are made up of things called photovoltaic cells. Multiple cells are connected through wires upon a substrate of some kind, and a number of these modules are placed on a larger panel to create an array. The most common associate people have are the large, reflective flat solar panels arranged in rows in the middle of the desert.

Photovoltaic cells are made of semiconductor materials that exhibit the photovoltaic effect. The photovoltaic effect happens when an electrical current is created in a material when it is exposed to sunlight. A related term is photoelectric, which refers to when electrons are simply emitted from a material that is exposed to sunlight with high enough energy.

The most common material used in PV cells is silicon, which is classified as a metalloid on the periodic table of elements.

Crystalline structure of Si atom

It is the most common metalloid on the earth. (For reference to those not versed in chemistry, one form of silicon is silicon dioxide, which is sand). Three forms used in different types of photovoltaic cells are monocrystalline silicon, polycrystalline silicon, and amorphous silicon. Crystalline silicon is named as such because of its structure.

Diagram of orbitals around an Si atom

A silicon atom has 14 electrons and 14 protons.  The electrons are constants moving around the nucleus of the atom at different distances from the nucleus based upon their energy level. The four outermost electrons, called valence electrons, have the highest energy level. The crystalline structure of silicon is formed when multiple silicon atoms form covalent bonds.

Covalent bods formed between Si atoms

When light hits a photovoltaic cell made up of crystalline silicon, what we see occur is called the Compton scattering Effect. When high-energy photons from sunlight collide with a material, this releases electrons from the outer shell of the atom. These outer shell electrons are the valence electrons and have the loosest bonds. These freed electrons move, creating a charge, which can be directed a certain direction using an electric field on the PV cells. This flow of electrons in a certain direction creates a current, which can be drawn from the cell using metal contacts on either side of the cell. This electric current can then be directed to power various things.

Often times, silicon is “doped” with other chemicals to make it easier to free those valence electrons. This doping process creates impurities, and while impurities are usually a bad thing, in this sense it creates a substance that requires less energy to free electrons. Less energy needed means it is more efficient and the photovoltaic effect occurs during a wider range of conditions. Silicon is commonly doped with Phosphorous, which bonds with silicon and has one unbounded electron around it. Silicon doped with phosphorous has more free carriers, which are those electrons freed due to the Compton scattering effect. This is a basic explanation of how PV cells work, but there are variations of this system that are being explored today. These variations include different materials from silicon, or using nano-scale silicon particles. Different materials for substrates of panels are being researched along with flexible thin-film photovoltaics. That brings us to the issue of….

How efficient is solar technology? And isn’t it expensive and inefficient?

This depends upon many factors, but the general assessment of photovoltaics is that they have a conversion efficiency somewhere between 12-18%. Most universities are investing in solar technology to create PV cells that have efficiencies of 40%. A company in San José has developed PV cells with a conversion efficiency of about 19.5%, which is relatively efficient. If you think about it from the standpoint of how many gathering sources are needed, even if solar technology is less “efficient” than burning coal, it is more efficient in the sense that for the amount of energy gathered, there is less energy loss in transmission to the end user. Let’s say you get less energy from the sun due to how much that panel can gather; more of that energy gathered is going to be given to you, the consumer. Coal on the other hand, can be produced only in large processing plants, so electricity will take longer to get to you and the energy loss between that plant and your home will be greater. You also have to think of the impacts of mining and transportation of a physical energy source that needs to be moved from its original locations to a plant. As solar technology advances, we can have almost every surface turned into a generating station. There can be small solar energy stations and large ones. With coal, it’s incredibly inflexible. The efficiency of coal plants sits at about 30%, with the most efficient being around 45%. As I stated earlier, in my mind, it’s important to look at the big picture in the fact that, although they are more efficient, there are many factors that must be taken into account when assessing the impact of energy. These include environmental and human impacts of mining, transporting, and pollution from coal combustion.

Currently, the method of producing silicon wafers for solar panels is labor intensive, wasteful of materials, and expensive to do. Wafers are made by sawing a rod of silicon, similar to cutting a slice of cheese for a sandwich from a larger block. Thin film solar panels are made by coating a substrate with amorphous silicon, but these are still difficult to cost-effectively mass produce. What many solar energy proponents are pushing is the idea of basic economics – that as consumer demand increases, and more people are buying solar panels made from cheaper material, production costs and cost per kWh will decrease. The efficiency of thin film solar panels is about 8% on average, which is lower than the crystalline wafer ones. The benefit of thin-film is that they can be applied to a wider range of surfaces, and opens up the possibility of flexible substrates. This means that one could potentially have curtains that can gather solar energy.

Thin-film solar device

Experiments in new materials will potentially help in lowering costs and increasing efficiencies. Thin film PV cells made from Copper Indium Diselenide (CIS), and these have an average efficiency of about 11%. Solar panels made from CdTe (Cadmium Tellurium) have an efficiency of 11% also, but have a lower cost of manufacturing. In 2000, this technology was successfully tests in the US on a large scale. The safety of these new materials is another issue being explored, since dealing with certain chemicals can be potentially dangerous. The health issues of handling Cadmium is under scrutiny right now, and this makes me think about the fact that what is bad for humans is usually bad for the environment. If the concentrations of this chemical are high enough to produce health risks, could the environmental impact be of concern also once these panels came to the end of their lifespan?

Materials: expensive and possibly dangerous?

As addressed above, there have been some questions over the materials. This is mostly a speculative issue right now for me, because there are so many different technologies being produced. In general, though, I wonder how much of the materials needed can be made, or must be mined, and if they were made, would this be a dangerous chemical intensive process? Silicon is a very safe material, but the costs of it keep rising. There are more promises in polymer based solar technologies, and if those polymers are made from naturally occurring and abundant materials, rather than being petroleum-based (like many plastics now are), then I think that could be a very good alternative.

Solar panels only work in very sunny areas:

This is the biggest myth in solar technology, while unfortunately having a tiny hint of truth. Solar panels DO work in cloudy weather, but they do not work as efficiently. Solar panels work when photons hit them and break those electrons free. The electromagnetic spectrum is made up of various radiation types and energy waves.

Electromagnetic spectrum

The light that we see is only one part of the spectrum, classified as visible light waves. There are many different types besides visible light, such as radio waves, infared, and ultraviolet. Many photovoltaic cells have decreased efficiency in cloud cover because they are made to absorb energy from only visible light. Efficiency can be increased by making solar cells that can take other wavelengths and produce electricity.

Solar-thermal device from Wake University

Another form of solar energy I didn’t mention above is thermal, which takes heat energy to produce electricity. Photovoltaic cells are actually impaired by heat, since they gather energy from light, and not heat. PV cells are not as efficient in high temperature locations. Wake Forest University is experimenting with a new polymer based hybrid solar device to capture both heat and light.  This device captures solar infared radiation to generate heat. Testing has shown a conversion efficiency of about 30%, with the additional benefit that this device is designed to capture light energy at angles, which is one problem solar panels have.

So, this, despite its length, is a very basic and brief summary of how photovoltaic systems work. It seems when I talk to many people, they don’t realize the potential of solar technology, and the fact that this technology has been around since the 1970s and is not some new and mysterious method of producing energy. The roadblocks are in price, efficiency and materials.  A combination of consumer support, and advances in technology and manufacturing processes will allow us to overcome these roadblocks.

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