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Alex Toombs

Electrical Engineering Student

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I am a senior electrical engineering at the University of Notre Dame, concentrating in semiconductor devices and nanotechnology. My academic, professional and...

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Solar Power Breakthroughs vs. Exponential Energy Concerns

Among the more important contributions that electrical engineers can make to society is the pursuit of attaining global energy sustainability. Current projections estimate that, if we are not already at it, that peak oil production will come about within the next 50 years. That implies that we are producing the maximum amount of oil possibly extracted from the ground. Additionally, public concern over nuclear safety and proliferation have hindered efforts to develop and install safe, modern nuclear plants. With annual global energy consumption now over 15 TW, solar power is becoming an ever more economical option for providing the electrical energy demanded world over. As an energy source, solar is democratic, plentiful, clean, and fully renewable; the only issue is that the technology for effectively capturing light and storing electrical energy is not yet entirely there. However, recent technological innovations have made more progress toward that goal.

Solar panels have become much cheaper to fabricate over the past decade, largely due to improvements in manufacturing processes, especially for generation one solar cells. Generation one solar cells are those that consist of single semiconductor PN junctions, like a typical silicon diode. These cells, largely created from silicon, comprise the majority of those that are used in home installations, campground solar chargers, and other simple devices. They are subject to a fundamental limit known as the Shockley-Queisser limit, confining the maximum power extraction efficiency from a single-junction solar cell to around 34%. This limit is due mostly to losses accounting to the recombination of holes and electrons in the cell, emission of energy by the cell due to blackbody radiation, and from the ability to absorb only a certain fraction of the solar spectrum of light incident upon the cell.

With the cost of these solar cells coming down, more people are considering home installations of cells to reduce their monthly electricity bills. Often, many do not consider the fixed costs required for a home installation of solar cells. Among these are the costs paid to install the cells, the power electronics required to convert generated electricity to AC, and the costs associated with financing a solar installation. Installation of the cells is a cost that will always come into consideration, but can vary depending upon roof size and power demands. For example, some homeowners install solar trackers, which are mechanized devices that move the solar panel throughout the course of the day as a way of maximizing the light incident on the solar cell. Typically, these higher cost items only make sense for large scale and high efficiency applications.

Power electronics, such as inverters, are required to convert the power generated by a solar cell, which is DC, and generate three-form AC power that can be used around the house or sold back to a utility company. Inverters and AC power generators are among the most expensive of costs associated with solar installations. These cost around as much as the cost of the solar cell— or even more. Fortunately, research in the area of power electronics has been lowering costs. High speed switching devices, such as gigahertz and terahertz transistors, need to become more economical in order for home installations of solar cells to become more viable. Solar installations for home use can range in the tens of thousands of dollars, which may require a loan from a bank in order to purchase. Despite government tax breaks, it is simply not economical for some to purchase solar cells. Assuming a fixed cost per kilowatt-hour from the power company over the lifetime of the solar cell, the cost of a solar installation, in addition to tax breaks and eventual writeoffs, it may simply make more sense for all of a household’s electricity to come from the utilities. At the moment, solar power is still more expensive in the long run.

In addition to decreasing costs of power electronics, there are two different approaches, both with different applications, dedicated to solving the economic issues associated with solar power. The first possible solution lies with generation two solar cells, also known as organic solar cells. Unlike generation one solar cells, which are comprised of semiconductors like silicon, organic solar cells are made up of other materials that accept light and donate electrons, converting sunlight to electricity much in the same way. There are several formulations currently used, much as there are multiple semiconductors used in solar today. The main difference is that organic solar cells are cheap and available by design. An example cell is shown below:

Organic Solar Cell Diagram (courtesy of Wikimedia user S.Babar)

Organic Solar Cell Diagram (courtesy of Wikimedia user S.Babar)

As opposed to semiconductor solar cells, which require expensive fabrication labs, highly pure materials, and complicated assembly lines, the goal of organic solar is that the “cells” may be mixed anywhere, making a cheap and available solar cell. Organic solar cells are usually amorphous or liquid in addition to being low cost, meaning that any area exposed to sunlight can be coated and turned into a solar cell. Generation one cells are generally rigid and fixed in size, which can be a disadvantage in some installations. However, generation two cells are much less efficient than other comparable cells, generally limited to under ten percent conversion efficiency. Additionally, they still require inverters in order to utilize the electricity generated in any household connected to the electrical grid.

To overcome the Shockley-Queisser, solar researchers have developed a variety of techniques. Generation three solar is the umbrella term for those cells that are designed in order to defeat that fundamental limit of power extraction. The foremost of these are multi-layer, or tandem, solar cells. Multi-layer cells are designed to defeat one aspect of the Shockley-Quiesser limit, that of the solar spectrum divisions. Each semiconductor has a band gap, measured in electron-volts, that corresponds to the amount of energy required to move an electron from the valence band to the conduction band— effectively freeing that electron to be used as electricity. These band gaps are inversely proportional to the maximum wavelength of light that may be absorbed by the cell, meaning that each cell can only absorb one part of the solar spectrum. To ensure that the rest of the solar spectrum may be absorbed, multiple solar cells are layered. The largest band gap cells come first, absorbing whatever light it can. The top cell is transparent to the remaining wavelengths of light, allowing it to pass through and get absorbed by the next layer, or the one thereafter. A typical application is shown in the picture below:

Generation Three Multi-Layer Solar Cell (courtesy of NASA Science News)

Generation Three Multi-Layer Solar Cell (courtesy of NASA Science News)

These multi-junction cells are not without their problems, however. Even solar cell stacks are subject to Ohm’s law, meaning that each cell in series is limited to the current of the lowest current-producing cell in the stack; this is known as the current-matching problem. Additionally, each cell is much more expensive than a similar generation one cell, meaning that it is much more difficult to pay to cover a roof with them. As research continues toward increasing the efficiencies associated with multi-junction cells, costs are also driven down with better manufacturing techniques. The problem of lattice matching remains a compelling one. Each semiconductor has a crystal lattice size that determines the spacing between atoms in a crystal. Growing different semiconductors on top of each other is difficult because each atom does not line up nicely. As of last year, the best lab-grown multi-junction cells were around 42% efficient.

Despite the difficulties associated with it, solar technology offers the best chance at providing the electricity the world demands. Other challenges remain, such as storing generated electricity for use during hours when the sun has set. But considering current year over year increases in electricity consumption, current sources of electricity will not last, and solar power hurdles will need to be overcome. Oil and natural gas are fossil resources that will soon run out, and nuclear is currently tied up in safety, security, and health concerns. As long as the sun shines, solar power will make sense as a peak supply electricity source.

Tags: solar cells, solar energy, solar, energy, efficiency,

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