By Mattie DeDoes
A new standard in solar performance is being set with the recent developments of multi-junction solar cells. With world records in efficiency being set every few months, this rapidly-improving technology is primed to make a strong impact on the world of solar energy. As is true with many new developments in renewable energy science, cost-effectiveness is still an issue to be reckoned with. However, in specific applications, multi-junction (MJ) cells have already drastically outperformed their single-material counterparts.
While cost is still a limiting factor, MJ cells have found uses in some extremely exciting applications. From the far reaches of space exploration, to some of the most efficient power stations on Earth, MJ cells have already made their mark on the world of solar energy. Like many previous solar technologies, one can expect multi-junction cells to continue to expand their impact in our daily lives. As this technology becomes more affordable, it's expected that it will find its way into broader use.
Read on for a brief explanation of the operation and application of these industry-leading devices.
Solar Basics
The basic principle of solar cell function is based upon electrons being excited by the influence of energy (radiation) in a semiconductor material. With no incident radiation, all of the electrons in the semiconducting material will be at their lowest possible energy levels. If a photon (the basic unit of electromagnetic radiation/light) strikes the semiconductor with enough energy, its energy is transferred to an electron, thus exciting the electron to a higher energy level. The energy required for the the electron to jump up a level in a certain material is known as the bandgap energy. By creating a circuit loop through which the excited electrons can flow, the energy carried by the electrons can be used to supply power to a building or external device.
However, if a photon with energy less than the required bandgap energy hits the material, there will be no interaction with the electrons and it will pass right through. Also, if the incoming photon has more energy than the bandgap energy, the excess is lost as heat after the excitation of the electron. The most effective materials for solar cells are those with a bandgap energy near the energy at which the sun emits the most radiation (about 500 nm, or yellow-orange visible light) [1].
MJ Cells
Multi-junction solar cells were first introduced in the late 1970's by the Research Triangle Institute [2]. The idea behind MJ cells is to layer multiple materials on top of one another so that higher-energy photons will be absorbed in the upper layers, while photons with lower energies will pass through and be absorbed by lower layers (see image at left). Using materials with bandgap energies that respond to radiation emitted across the range of the solar spectrum will drastically reduce heat losses and increase efficiency.
The current world record for efficiency was set, using an MJ cell, at 46% last December by a French-German collaboration team [3]. While 46% may not seem impressive in everyday contexts (a failing grade or an atrocious free-throw percentage), compared to Panasonic's current world record for crystalline silicon of 25.6% [4], this is outstanding productivity from a solar cell. There is every reason to expect this figure to continue to improve in the future, as the world record has increased by almost 10% efficiency from the previous 37.8% mark set in the spring of 2013.
The main drawback to MJ cells is the same as that of many exciting new solar technologies: cost. The cost per square centimeter of a MJ cell is a whopping $25, compared to $1-$2 for crystalline silicon. Measures are being taken to combat this issue. One popular method for lowering costs is to create alloys that combine cheaper and more readily available materials with more expensive semiconductors. This technique will often diminish the efficiency of the cell; if a minor sacrifice in performance could be made in exchange for a substantial economic gain, the possibility of mass production could be much more feasible. An example of this trade-off occurs in the use of monocrystalline silicon for the majority of the solar applications we see every day. Silicon does not have an ideal bandgap energy for absorbing solar radiation, but is adequate enough and far cheaper than other, slightly more effective materials.
While MJ cells are not yet cost-effective for residential or commercial rooftop use, concentrated photovoltaic (CPV) power stations have been developed which take advantage of the higher efficiency of the technology. These CPV stations use optical lenses to focus the sunlight onto the solar cell, making the amount of radiation that gets absorbed equivalent to that of hundreds of suns. Because the cost of producing these lenses is much less than the cost of a new cell, these power stations can be made economically viable.
MJ cells have also found a home in space applications, because the solar spectrum is quite different without the presence of an atmosphere. Satellite power stations and attachments to the Mars rover missions have been able to employ MJ cells effectively. Companies like SpectroLab manufacture devices that are specifically designed for these purposes with efficiencies that reach above 30% [5].
The record-setting performance of multi-junction cells has already been demonstrated in limited, yet exciting, applications. As this technology continues to be made more viable, look for this improved performance to make a much broader mark on the world of renewable energy.
Believe It, or Not! It costs less to go solar in New York than it does in Arizona.
No doubt, there are several states, primarily in the southern and western United States that get considerably more sunshine than does western upstate New York. Unfortunately for the residents in many of those states partisan politics has fought hard to minimize the incentives their states provide for investing in alternatives like solar in favor of maintaining the status quo of power control by the coal and natural gas industries. So, while there is more sunshine in places like Arizona and there are large numbers of the population who are ready and willing to join the move to clean energy, many are hindered by the still significant up-front financial investment required to go solar.
Indeed, thanks to subsidies enjoyed by the powers that be within the previously mentioned status quo and the fact that there is currently little to no accountability for the environmental or health consequences involved with energy production, it is still cheaper for consumers, even in many of those sun-rich states, to use fossil fuels to power their energy needs. For many, despite how much sun their area gets, the unequal distribution of subsidies and favoritism toward fossil fuel energy sources makes the decision to go solar, more of a financial one than one of true preference.
This is not the case in New York. Those who live in western New York have access to some of the best incentives for going solar in the country thanks to the New York State Energy Research and Development Authority (NYSERDA), NY state personal tax credits, U.S. Investment Tax Credits (ITC) and to local property and sales tax exemptions. It may not be as sunny in western New York as in the south and west but according to the National Renewal Energy Laboratory analysis there is more than enough solar falling on the western New York region to power all its electricity requirements.
The U.S. ITC covers up to 30 percent of the cost for installing a PV solar system is available to all eligible American home and commercial business property owners, through 2016. Additionally, there are several states that offer a state tax credit. Both New York and Arizona are among those states. However, New York offers a maximum tax credit of $5,000 while Arizona’s maximum is only $1,000. When considering the tax credits alone, a New York consumer’s investment in a 5 kW solar PV system is about $8,100. In Arizona, the same system costs $11,600.
It doesn’t stop there though; New York's leadership team has helped create a vision and has had foresight enough to understand the importance of building a sustainable clean energy industry. They are accomplishing this by proactively motivating its citizens to opt-in to a safer, cleaner and more reliable energy future.
The NY-Sun is a rebate program launched by Governor Andrew Cuomo in 2012 as part of New York’s commitment to protect the environment and lower energy costs for all New Yorkers. In April 2014, New York increased its commitment to expand the deployment of solar throughout the state and to transform New York’s solar industry to a sustainable and soon-to-be subsidy-free and self-sufficient industry to almost $1 Billion. Thanks to NY-Sun, NYSERDA is currently offering upstate New Yorkers an additional rebate of 90 cents per watt, upfront. Arizona’s biggest utility, APS, is offering zero.
In our 5 kW PV solar system cost comparison; NYSERDA provides New Yorkers with an additional rebate incentive of $4,500 (5,000 watts x $.90 per watt = $4,500); APS customers get zilch, (5,000 watts X zero = zilch). The bottom line is that concerned citizens in Jamestown, New York who decide to take responsibility for their part in providing a healthier future for their families and friends by going solar will have a final investment of only $3,600 for that 5 KW PV solar system. Citizens in Phoenix that have the same values will have to pay almost $12,000.
That $8,000 savings for New Yorkers makes up for a lot of sunshine. Believe It!
Stay tuned to YellowLite for all your solar information and installation needs!
References
[1] Cherucheril, George, Steven March, and Avinav Verma. "Multi-junction Solar Cells." Iowa State Department of Electrical Engineering. N.p., n.d. Web. 13 Jan. 2015.
[2] Cotal, Hector, Chris Fetzer, Joseph Boisvert, Geoffrey Kinsey, Richard King, Peter Hebert, Hojun Yoon, and Nasser Karam. "III–V Multijunction Solar Cells for Concentrating Photovoltaics." Energy & Environmental Science 2.2 (2009): 174. 10 Dec. 2008. Web. 13 Jan. 2015.
[3] New World Record for Solar Cell Efficiency at 46% French-German Cooperation ConfirmsCompetitive Advantage of European Photovoltaic Industry." New World Record for Solar Cell Efficiency at 46% — Fraunhofer ISE. Fraunhofer ISE, 1 Dec. 2014. Web. 13 Jan. 2015.
[4] Panasonic HIT® Solar Cell Achieves World's Highest Energy Conversion Efficiency of 25.6% at Research Level." Headquarters News. Panasonic, 10 Apr. 2014. Web. 13 Jan. 2015.
[5] Multijunction Solar Cell Could Exceed 50% Efficiency Goal." Phys.org, 20 Feb. 2013. Web. 13 Jan. 2015.
