University School - Hunting Valley
Instructor: Alan Cate
Illuminating the Dark Problems of Solar Cells
Illuminating the Dark Problems of Solar Cells
Every second, 173,000 terawatts of solar energy hit the Earth. One terawatt is a trillion watts: meaning that 173,000,000,000,000,000 watts of solar energy continuously strike the Earth. This constant influx of electricity is estimated to be more than 10,000 times more than the world's total energy consumption. Solar power is a growing energy source to harness the energy of the sun to power the world. There are two main methods to harness solar power. The first is the solar cell, which uses sunlight to generate a current. The second is solar thermal power which uses the heat generated by sunlight to generate usable electricity. Many solar cells are assembled to make solar panels to increase the energy input and the electrical output. Though solar cells are remarkable devices. they are not a feasible source of sustainable energy for the United States. They are inefficient, expensive, need direct sunlight, and require too much space to generate usable electricity. Many of these issues apply to solar thermal power; they are inefficient, expensive, and consume space that should be used for other purposes.
To understand why solar cells aren't feasible, we must understand how solar technology came about. The first solar cell was built in 1883 by American inventor Charles Fritts. He saw it as an alternative to Thomas Edison's coal-fired power plant. Fritts' cell is what modern opponents of solar believe solar cells to be. It was inefficient; less than 1% of the sunlight was converted into electricity. His cell used selenium coated in gold, which was an expensive and unusual combination at the time.
Finally, Fritts' cell was unattractive and required direct light. It wasn't until 1940 that a significant improvement to solar cell technology was made. Bell Labs researcher Russell Shoemaker Ohl was studying semiconductors when he came across a silicon sample with a crack. He noticed that a current flowed through the sample when it was exposed to light. Ohl had inadvertently created a p-n junction and the solar cell was born.
A p-n junction acts as a one-way valve to generate electricity from electrons and holes. It generates electricity by forcing electrons to generate a current based on the charge difference between the two materials. This charge difference is created by "doping" or changing the number of electrons on one side of the junction using impurities. The "p" side has open spots to accept electrons while the "n" side has excess electrons to fill these holes. The p-n junction is the border between the positively charged "p" side and the negatively charged "n" side. In the junction, there is a depletion zone where there are no holes or extra electrons. As light hits this zone, the energy from the photons jars the electrons loose. The holes these electrons create are filled with electrons from the other side of the junction. The electrons that were popped out travel through the metal grid and to the other side of the junction. This passage of electrons is what generates a current. The current generates a voltage that can be stored or used. The flow of sunlight will create more holes and electrons to keep this process going for as long as the solar panel is exposed to light. The diagram below shows a simplified version of the process. The area designated with a "3" is where the current is harvested for use. The p-n junction isn't shown, but the passage of electrons remains the same.
This discovery did little to promote solar power. Ohl didn't understand his discovery entirely. His patent was titled, A Light-Sensitive Electric Device Including Silicon, and related to the flow of current but not the generation of electricity from sunlight. It was a different group of Bell Labs employees that proved it was possible to harness solar energy. Daryl Chapin was trying to develop a power source for isolated telephones. The batteries of these phones didn't last long enough for the phones to be practical. Chapin decided that a modified solar cell would work. He focused on the flawed selenium cells as he was unaware of Ohl's discovery with silicon. Separately, chemist Calvin Fuller and physicist Gerald Pearson were researching semiconductors with impurities. They discovered that silicon with gallium impurities dipped in lithium created a p-n junction; the same as Ohl's silicon sample 13 years earlier. Pearson was aware of Chapin's work and suggested that he forget about selenium cells and work with silicon instead.
The three men worked for months to fix problems with their cell. A major problem was the "sinking" of the p-n junction away from the light source making it less effective. The group experimented with impurities and discovered that boron and arsenic prevented the p-n junction from falling. They further improved on the electrical contacts to create a "solar battery" which Bell Labs quickly displayed. These cells were 6% efficient and drew national attention. One day after their presentation in New Jersey, the front page of the New York Times proclaimed, "it (the silicon solar cell) may mark the beginning of a new era, leading eventually to the realization of one of mankind's most cherished dreams-the harnessing of the almost limitless energy of the sun for the uses of civilization."
This claim was overzealous; solar panels were expensive, which made them difficult to mass produce. That cost made potential manufacturers unsure if they would be usable for individual homes. The main reason solar cells still exist today is due to the space race. The start of the space race eliminated whatever financial constraints existed and jump-started solar cell production. Solar panels made it possible to have satellites, rovers, and the International Space Station. The space race and solar cell technology couldn't have been more mutually beneficial. Satellites required continuous electricity and refueling onboard generators would've been extremely dangerous and expensive. The spacecraft and satellites don't need huge amounts of electricity, but they do need a consistent influx. The solar industry benefited from the government's money which simplified the design and production of solar cells. The cost was lowered, and solar cells were seen as viable commercial and residential alternatives to standard electricity.
Solar panels are still expensive. Most homes would need $15,000-$25,000 worth of solar panels to supplement existing electricity. The initial price is daunting; solar cells won't fully cover electricity bills and cost half as much as a new car. For this reason, the federal government offers tax credits associated with solar cells. Up to 30% of the initial cost can be recouped after one year. The percentage that can be recouped decreases annually. In terms of state incentives, Ohio, for instance, doesn't have much in the way of cost reduction. In addition to the federal tax credits, Ohio uses a net metering system that allows solar panels to generate excess electricity and offset grid energy used later. Ohio's final incentive is a loan interest rate reduction for up to seven years. The average solar panel cost is $23,040 and $6,912 are easily recouped. The remaining cost is a low-interest loan that can be paid back from money saved from electric bills. Ohio is ranked 25th by states despite the lack of effective incentives according to EcoWatch.
Solar electricity can be stored after generation. Modern battery systems are improving, and this will make solar farms and residential panels more appealing. Opponents of solar cells argue that solar panels only generate electricity inconveniently. That is, solar panels use light to generate electricity and the main draw of electricity is for lighting. Lighting is used primarily when there is no sunlight. Solar panels can't generate electricity in cloudy or other adverse conditions. These same conditions are the times when electricity is used the most. Battery storage would help solve these problems. Solar panels could generate electricity during the day and in good weather and store it for when it is needed. Furthermore, it's unlikely that conventional electrical generation will go away. So electricity could still be provided even if solar panels aren't generating it. Cleveland isn't a prime candidate for solar cells, our unfavorable weather makes solar cells unlikely. Cleveland has roughly 170 rainy or snowy days per year. Over half of our year is spent in conditions that don't promote solar generation. Additionally, the weather can damage solar panels and the repairs are very expensive. Manufacturers refute the claim of severe weather damage, yet owners of solar panels frequently complain about repairs.
Another argument against solar panels revolves around the energy lost in conversion from sunlight. Charles Fritts' solar cell was less than 1% efficient. Today, 140 years later, solar cells are a mere 20% efficient. Solar cell efficiency is improving. Researchers at the National Renewable Energy Laboratory created a solar cell that was 39.5% efficient. The flaw with that cell was the complexity and cost of the system making it impractical for production and sale. In addition to the huge amount of energy lost, the remaining electricity must be converted from direct current (DC) to alternating current (AC). Solar panels generate DC current while the national power grid uses AC current. The change from one form to another is relatively simple; the current is run through transistors that force the current to change direction. Based on the law of conservation of energy, energy can neither be created nor destroyed-only converted from one form of energy to another. The energy lost in the transition from DC to AC is given off as heat or vibration and isn't dangerous. This process is relatively efficient with a 90% retention of energy. While 90% retention is high, nuclear, hydro, and standard power plants all produce AC directly and don't lose 10% of their generated electricity. The 10% loss compounds with the 80% lost already from the photons making solar one of the least efficient forms of electrical generation.
The strongest argument against solar as an option are the huge tracts of land required to make enough energy for anything. A solar cell is placed alongside many others to make a solar panel. Several panels are placed together to make an array. These arrays are known as solar farms, and many have sprung up across the United States in recent years. In Cleveland, there is a small solar farm adjacent to Lake View Cemetery that feeds into Cleveland's power grid. These farms are designed to create maximum electrical output from solar panels. They take up huge swaths of land that could be used for farming or other purposes. Solar panels have a tumultuous relationship with the environment. On the one hand, solar panels cleanly generate electricity to combat the global climate crisis and they don't create harmful byproducts. On the other hand, they take sunlight away from plants and occupy usable land. Many farmers have used solar panels' seemingly worst aspects to benefit themselves. By elevating the solar arrays, farmers use solar panels to generate shade and keep animals cooler. This leads plants under the shaded area to die and more land is required to feed the animals. While space isn't an issue in some places, solar farms aren't able to be employed everywhere. Cities consume the most electricity yet, it's where solar would be the least effective due to buildings blocking sunlight and a lack of unused space available.
The second method of generating electricity from the sun is solar thermal power. Solar thermal power plants are almost identical to existing coal and nuclear power plants: they use steam to rotate a turbine which creates a current that can be stored as usable electricity. The difference between solar thermal power and the others is how this steam is generated. Solar thermal power reflects the sun's light using mirrors that concentrate the heat on a specific point. There are two main designs that have been frequently employed. The parabolic trough is a curved mirror that acts as a bowl and directs heat into a metal pipe. That pipe heats itself and the liquid inside of it. The liquid then flows into pipes around the water to generate steam from the water. This exchange of thermal energy cools the previously heated liquid, and it flows back into the troughs to be reheated. The second type of solar thermal power is the linear concentrating system. It uses many of the same materials in different. Instead of troughs, it uses an array of mirrors in different orientations to direct sunlight at a tower that contains the heat-transferring fluid. From there, the fluid passes through pipes to boil the water and generate steam while cooling and flowing back into the focal point of the array.
The linear concentrating system functions identically to Archimedes' death ray of the 3rd century BCE. Archimedes created an array of mirrors to concentrate sunlight on specific points. This system was employed as a defense mechanism, it would light attacking ships on fire from afar and cripple any attacking force. The efficiency of this system is debatable, but the similarity to the system in use for modern electrical generation is remarkable. Instead of incinerating ships, the light is focused to heat a liquid that is then conducted into a vat of water that boils and generates steam. It is simple and relatively little maintenance is required. However, setting up these precise systems to focus all the sunlight on a small area is expensive. Minor changes in the topography would cause recalibration to keep the system usable. The large area required to generate the heat necessary is damaging to the environment, just as solar cells are when used on a large scale.
Another flaw is the window of operation for these systems. They require prolonged, direct sunlight to heat the liquid. However, cold days require more sunlight, and these systems may take time to get up to speed, but they wind down quickly with reduced light. Storing this energy would help, but solar thermal power has a large amount of energy loss that cannot be recovered. One of water's most notable properties is its resistance to heating. Large amounts of energy are required to heat water into steam which is an argument against both nuclear and coal power plants made by proponents of solar power.
The flaws in solar energy systems are consistent. Solar cells and both methods of solar thermal power are inefficient, expensive, and impractical for most uses. A considerable amount of energy is consumed at night when neither system can generate electricity. Further, these systems require peak sun exposure to function well. On average, the United States has a mere 3-5 hours of peak sunlight per day. This minuscule window requires incredibly efficient systems to generate enough electricity to not have a deficit. Additionally, the cost is prohibitive for almost all uses while their size requirements further reduce the potential users for these systems. They are inefficient by nature, due to the laws of thermodynamics, energy must be lost and both systems lose a considerable amount. Only 20% of energy is converted on average for solar cells while thermal systems use water, which has a high resistance to boiling, making the energy input much greater than the energy output. The main way to make these systems practical is to increase the efficiency of the systems, decrease the necessary space, and improve storage capabilities to combat the limited window of generation. It is unlikely that solar power will be able to improve significantly in the areas where it struggles. Without addressing the problems of solar, solar cells nor neither solar thermal power will be a plausible source of sustainable energy in the future.