August 13th, 2007
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EmailRenewable energy has become an amazingly hot topic in the past ten years as fears of global warming, oil shortages/peak oil, and international instability have heightened. As a result, greater levels of political and financial focus have been placed on these technologies. One of the most touted alternative energy solutions is solar power.
On average, the lower 48 states receive a tremendous amount of solar energy each day. To put this into perspective, the entire U.S. requires only 0.2% of all this energy on any given day. In fact, more solar energy falls in one day than the U.S. needs in one year [1].
So there is a ton of solar of solar energy available, but how do we collect all of this energy?
How does solar work?
All solar harvesting devices operate under a few core tenants. First, the device must ‘harvest’ a packet of light known as a photon. This photon has energy and when harvested, this energy allows electrons to flow through the solar device. Electricity is a flow of electrons; when these harvesting devices are wired to electrical circuitry, they can power a wide range of electrical appliances.
Silicon Solar Cells
The most common type of solar cell is made out of highly purified silicon (the same material in computer processor chips). These cells are actually made of two layers of silicon each with trace and precisely controlled levels of additional materials. Simple sunlight is the source of energy that allows the electron to flow through the material and generate electricity. Although this process is relatively simple in words, it’s incredibly challenging in reality.
The silicon material is extremely expensive, difficult to fabricate, and requires tremendous amounts of energy and water. As a result, the cost of solar cells is high and the power they produce is relatively expensive [2]. In 1980, the price of solar power was $1.00 per kilowatt-hour but due to advances in processing and economies of scale, prices have fallen to less than $0.20 per kilowatt hour [for reference, in Seattle the average cost of power is $0.042 per kilowatt hour]. Also, since solar cells require so much energy to manufacture, only after 1.5-3 years of operation have the cells produced more power than it took to manufacture them in the first place [3].
Historical Cost of Solar Power
How do we know when we have a "good" solar cell?
The most basic and core performance metric of a solar cell is its efficiency. The efficiency of a solar cell is straightforward to calculate and only requires two numbers. The first number is how much solar power is hitting the space where the solar cell is to be installed. We can measure it with a photometer " the same tool used in photography to measure light intensity.
Next, we obtain the second number by placing the solar cell in the same spot and measuring how much power is produced. Then we divide the second number by the first number. Last, we multiply by 100 and we then have a percentage. This percentage represents how much energy is converted from sunlight into electricity. Typical silicon solar cells have an efficiency around 15% [4].
What kinds of solar cells can we expect in the future?
Silicon based solar cells are great, but their high costs of production and mid-range efficiency rating have encouraged many scientists to research new and exciting solar cell technologies. One of the newest technologies uses special organic (plastic/polymer) materials that can harvest photons. At the moment, the highest efficiency for this style of cell is 3-4% [5].
Compared to silicon, this number seems pretty unimpressive, but polymers have many advantages. They can be cheap to manufacture and are hard to break (compared to glass-like silicon). On the other hand, like most polymers, these cells are slowly destroyed by UV-light and are fabricated from chemicals that are from the petrol-chemical industry (i.e. chemicals from oil).
Nanotech is the future?
Another promising technology uses nanotechnology. Very small particles of materials (called quantum dots) can be fabricated to absorb a large spectrum of sunlight and result in a more efficient system (with theoretical efficiency guesses ranging between 30-60%) [6]. Like polymer solar cells, this style of solar cell could be cheap (due to cost effective uses of materials) and flexible. However, this style of solar cell is still in the early stages of research and not near commercial viability.
The last style of solar cell to make major inroads is known as dye-sensitized solar cells (DSSCs). These cells use a thin layer of titanium oxide (the same white stuff in sun screen) and an organic dye to harvest and transport electrons. Much like the other alternative solar cells, this technology offers the potential to make cheaper and environmentally friendlier solar cells. The efficiencies for these cells are in the range of 12-15% (making them a more cost effective energy generator than silicon cells). Their shortcoming is their fragility and sensitivity. This style of cell is just approaching commercial viability within a few years.
So there you have it: solar cells 101 " how they work, how to rate them and a handful of different technologies used to harvest the energy of the sun. Ultimately there is no clear ‘winner’ between future solar cell technologies and most likely, a combination of all the above technologies will help us break our non-renewable energy base.
References
[1] National Renewable Energy Laboratory Website: Energy Tidbits http://rredc.nrel.gov/tidbits.html Accessed August 2007
[2] Renewable Energy Cost Trends, National Renewable Energy Laboratory Website http://www.nrel.gov/analysis/docs/cost_curves_2002.ppt Accessed August 2007
[3] Corksih, Richard. “Can Solar Cells Ever Recapture the Energy Invested in their Manufacture?” Solar Progress V18, pg 16, 1997. http://www.csudh.edu/oliver/smt310-handouts/solarpan/pvpayback.htm
[4] National Renewable Energy Laboratory Website: Photovoltaics http://www.nrel.gov/learning/re_photovoltaics.html Accessed August 2007
[5] http://spie.org/x8868.xml?highlight=x2358 Accessed August 2007
[6] National Renewable Energy Laboratory Website: Quantum Dots Solar Cells http://www.nrel.gov/docs/fy02osti/31011.pdf Accessed August 2007
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