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3.21.2010
Illinois' Green Energy Finance Initiative
March 2, 2010 Information Provided By, Scott's Contracting Green Builder, St Louis "Renewable Energy" Missouri
A recent initiative from Illinois may serve as a model for other states that hope to attract green energy projects and stimulate economic development.
by Marnin Lebovits, Illinois Finance Authority
There are plenty of reasons to encourage and support the development of renewable energy projects. But in this down economy, the credit markets are typically discouraging. Here in Illinois, we've found a way to open new doors to financing renewable energy projects because we know they're good business–and good for our state.
Like other states, Illinois recognizes the climate and environmental benefits from generating power from renewable energy sources. We know that developing these projects will reduce our region's dependence on foreign oil.
Renewable energy will also help to meet state renewable portfolio standard (RPS) targets. Illinois established its targets in August 2009 and requires that by 2025 one-quarter of all of the power used annually in Illinois must be generated from renewable energy sources. Of this amount, 75 percent of the renewable power used must be generated from wind projects. This requirement has created a significant interest in renewable energy projects, especially wind projects. Also, like other states our state wants, to the extent possible, to provide incentives to help reduce the cost of power generation from these projects so that savings can be passed on to ratepayers.
Finally, renewable energy projects will spur economic development and provide a secure revenue stream for many farmers and other property owners struggling during this recession. The projects will create jobs–many in depressed, rural regions of the U.S., where municipalities will also benefit from the increased tax revenue sparked by such development.
Despite these benefits, developers typically face serious obstacles when they attempt to finance renewable energy projects. Capital markets continue to face severe challenges, and the ability for developers to obtain traditional project financing is much more limited than in the past. Only a handful of financial institutions will provide loans for renewable energy projects, including wind farms. Even then, the terms and conditions for the loans are quite arduous.
Most recent transactions include loan terms of only 5 or 7 years, with a 20-year amortization period, generating significant refinance risk in most cases. Developers with the ability to raise capital in other forms, either on a portfolio basis or through a rated parent entity, are forced to prioritize projects in different stages of development across the U.S. market and even around the world.
To better compete, the Illinois legislature created new financing tools to aggressively attract renewable energy projects to our state. The state's approach might serve as a model for others hoping to attract similar types of development.
The Illinois legislature recently passed a bill adding renewable energy projects to the portfolio of developments eligible for assistance through the Illinois Finance Authority (IFA), which provides expert, hands-on support to help businesses get the capital they need for growth.
The IFA's assistance for renewable energy will come in the form of up to $3 billion of loan guarantees for project debt. This project finance can contain long-term tenors to fully repay the project debt, thereby eliminating the risk of refinancing. The loan guarantees will be secured by the state's moral obligation. While moral obligation is not a full faith and credit guarantee, it is a model that has been used extensively in the municipal finance markets, and it's used often in Illinois. As of September 2009, the State has outstanding debt (unrelated to this renewable energy finance initiative) of over $100 million using this model. Eight state agencies have the ability to issue moral obligation-supported debt totaling around $1.5 billion for local governments and economic development purposes. Clearly, this is an important funding tool.
These incentives will reduce a project's financing costs by an estimated 100 to 175 basis points. Combined with other incentives offered by the state, such as grant funding available from the Department of Commerce and Economic Opportunity for renewable energy projects, Illinois' incentive package is drawing attention from developers. In fact, the IFA late in 2009 was already reviewing a number of renewable energy projects for inclusion in this program in anticipation of the legislation's effective date, January 1, 2010.
Under the first of three IFA funding models, a developer can work with its traditional project finance lenders and add the IFA as a partner, providing a "loan guarantee" to private sector lenders. The private sector lender would also have the support of Illinois' moral obligation pledge.
Lenders will need to first look to the renewable energy project revenues to cover the debt service. If the project doesn't generate enough revenue, the lender (or lead arranger bank for a syndicated loan transaction) may call in the IFA. The addition of the state's moral obligation may allow the private sector lenders to extend the term of their project debt, possibly even to fully amortize the debt (based upon the tenor of the power purchase agreement) and should help to reduce the cost of the private sector financing.
In a second financing model, the IFA would issue bonds secured by both project revenues and the state's moral obligation support. The IFA would then loan the bond proceeds to the project developer to pay for project construction. Again, the first repayment source for the debt service on the bonds is project revenues. Illinois will be called upon by the Bond Trustee to fund any debt service deficiency on a moral obligation basis. In this instance, the tenor of the bonds could be set to correspond to a final term that will be near the PPA maturity, fully amortizing the project debt. The bond investors will assume the project risk. However, investors will also benefit from the security of the guarantee of the State of Illinois on a moral obligation basis. This additional security will reduce the project's interest rate.
These two models can be combined with the private sector providing a loan for a shorter-term piece and bonds issued for a longer-term piece of the debt financing. For example, the IFA can provide a loan guarantee to private sector lenders on their shorter-term financing (also known as "Series A") and the IFA can be the lender, on a pari-passu basis (in other words, without partiality) for a "Series B" financing that will represent the debt's longer-term portion. The combination of the proceeds from the Series A and Series B financings will provide the total debt funding for the project, thereby reducing total debt service costs and eliminating the refinance risk of traditional private sector funding.
The U.S. Department of Energy (DOE) loan guarantee program for renewable energy projects requires a participating lender (either a financial institution or an economic development authority) to share risk with the DOE. Although the IFA intends to work with projects participating in the DOE program, it is not required.
To back its push for renewable energy projects the state created the Illinois Energy Team (IET) to help review environmental and technical aspects of renewable energy projects and help expedite project development. The IET includes specialists from the state university system, Argonne National Laboratory and state agencies such as the Illinois Finance Authority, the Illinois Power Agency, the Illinois EPA and the state Department of Commerce and Economic Opportunity. This panel reviews feasibility studies and reports, evaluates technology, and considers project siting, grid interconnection and environmental impact issues. The IET will also provide a forum for developers to work with various state agencies to help projects come to fruition.
The IFA has accepted program applications for three wind projects. Inquiries have come in from developers involved in virtually all renewable energy sectors, including wind, solar, clean coal, geothermal, biodiesel and biomass.
Marnin Lebovits joined the Illinois Finance Authority as a senior funding manager in August 2009 and helped create program guidelines and credit criteria. For the last 20 years prior to joining the IFA, he has been active in municipal and project finance, managing and actively participating in the municipal and project finance groups for both Sumitomo Bank and DEPFA BANK. Mr. Lebovits received his MBA from the Wharton School of the University of Pennsylvania and is a CPA.
For more information on this Illinois financing initiative, contact Mr. Lebovits at 312-651-1344 or mlebovits@il-fa.com. Contact: scotty@stLouisrenewableenergy.com for all your green building needs
Illinois' Green Energy Finance Initiative
A recent initiative from Illinois may serve as a model for other states that hope to attract green energy projects and stimulate economic development.
by Marnin Lebovits, Illinois Finance Authority
There are plenty of reasons to encourage and support the development of renewable energy projects. But in this down economy, the credit markets are typically discouraging. Here in Illinois, we've found a way to open new doors to financing renewable energy projects because we know they're good business–and good for our state.
Like other states, Illinois recognizes the climate and environmental benefits from generating power from renewable energy sources. We know that developing these projects will reduce our region's dependence on foreign oil.
Renewable energy will also help to meet state renewable portfolio standard (RPS) targets. Illinois established its targets in August 2009 and requires that by 2025 one-quarter of all of the power used annually in Illinois must be generated from renewable energy sources. Of this amount, 75 percent of the renewable power used must be generated from wind projects. This requirement has created a significant interest in renewable energy projects, especially wind projects. Also, like other states our state wants, to the extent possible, to provide incentives to help reduce the cost of power generation from these projects so that savings can be passed on to ratepayers.
Finally, renewable energy projects will spur economic development and provide a secure revenue stream for many farmers and other property owners struggling during this recession. The projects will create jobs–many in depressed, rural regions of the U.S., where municipalities will also benefit from the increased tax revenue sparked by such development.
Challenges for Project Finance
Despite these benefits, developers typically face serious obstacles when they attempt to finance renewable energy projects. Capital markets continue to face severe challenges, and the ability for developers to obtain traditional project financing is much more limited than in the past. Only a handful of financial institutions will provide loans for renewable energy projects, including wind farms. Even then, the terms and conditions for the loans are quite arduous.
Most recent transactions include loan terms of only 5 or 7 years, with a 20-year amortization period, generating significant refinance risk in most cases. Developers with the ability to raise capital in other forms, either on a portfolio basis or through a rated parent entity, are forced to prioritize projects in different stages of development across the U.S. market and even around the world.
To better compete, the Illinois legislature created new financing tools to aggressively attract renewable energy projects to our state. The state's approach might serve as a model for others hoping to attract similar types of development.
The Illinois Solution
The Illinois legislature recently passed a bill adding renewable energy projects to the portfolio of developments eligible for assistance through the Illinois Finance Authority (IFA), which provides expert, hands-on support to help businesses get the capital they need for growth.
The IFA's assistance for renewable energy will come in the form of up to $3 billion of loan guarantees for project debt. This project finance can contain long-term tenors to fully repay the project debt, thereby eliminating the risk of refinancing. The loan guarantees will be secured by the state's moral obligation. While moral obligation is not a full faith and credit guarantee, it is a model that has been used extensively in the municipal finance markets, and it's used often in Illinois. As of September 2009, the State has outstanding debt (unrelated to this renewable energy finance initiative) of over $100 million using this model. Eight state agencies have the ability to issue moral obligation-supported debt totaling around $1.5 billion for local governments and economic development purposes. Clearly, this is an important funding tool.
These incentives will reduce a project's financing costs by an estimated 100 to 175 basis points. Combined with other incentives offered by the state, such as grant funding available from the Department of Commerce and Economic Opportunity for renewable energy projects, Illinois' incentive package is drawing attention from developers. In fact, the IFA late in 2009 was already reviewing a number of renewable energy projects for inclusion in this program in anticipation of the legislation's effective date, January 1, 2010.
Private Sector Debt Loan Guarantees
Under the first of three IFA funding models, a developer can work with its traditional project finance lenders and add the IFA as a partner, providing a "loan guarantee" to private sector lenders. The private sector lender would also have the support of Illinois' moral obligation pledge.
Lenders will need to first look to the renewable energy project revenues to cover the debt service. If the project doesn't generate enough revenue, the lender (or lead arranger bank for a syndicated loan transaction) may call in the IFA. The addition of the state's moral obligation may allow the private sector lenders to extend the term of their project debt, possibly even to fully amortize the debt (based upon the tenor of the power purchase agreement) and should help to reduce the cost of the private sector financing.
In a second financing model, the IFA would issue bonds secured by both project revenues and the state's moral obligation support. The IFA would then loan the bond proceeds to the project developer to pay for project construction. Again, the first repayment source for the debt service on the bonds is project revenues. Illinois will be called upon by the Bond Trustee to fund any debt service deficiency on a moral obligation basis. In this instance, the tenor of the bonds could be set to correspond to a final term that will be near the PPA maturity, fully amortizing the project debt. The bond investors will assume the project risk. However, investors will also benefit from the security of the guarantee of the State of Illinois on a moral obligation basis. This additional security will reduce the project's interest rate.
A Third Option
These two models can be combined with the private sector providing a loan for a shorter-term piece and bonds issued for a longer-term piece of the debt financing. For example, the IFA can provide a loan guarantee to private sector lenders on their shorter-term financing (also known as "Series A") and the IFA can be the lender, on a pari-passu basis (in other words, without partiality) for a "Series B" financing that will represent the debt's longer-term portion. The combination of the proceeds from the Series A and Series B financings will provide the total debt funding for the project, thereby reducing total debt service costs and eliminating the refinance risk of traditional private sector funding.
The U.S. Department of Energy (DOE) loan guarantee program for renewable energy projects requires a participating lender (either a financial institution or an economic development authority) to share risk with the DOE. Although the IFA intends to work with projects participating in the DOE program, it is not required.
To back its push for renewable energy projects the state created the Illinois Energy Team (IET) to help review environmental and technical aspects of renewable energy projects and help expedite project development. The IET includes specialists from the state university system, Argonne National Laboratory and state agencies such as the Illinois Finance Authority, the Illinois Power Agency, the Illinois EPA and the state Department of Commerce and Economic Opportunity. This panel reviews feasibility studies and reports, evaluates technology, and considers project siting, grid interconnection and environmental impact issues. The IET will also provide a forum for developers to work with various state agencies to help projects come to fruition.
The IFA has accepted program applications for three wind projects. Inquiries have come in from developers involved in virtually all renewable energy sectors, including wind, solar, clean coal, geothermal, biodiesel and biomass.
Marnin Lebovits joined the Illinois Finance Authority as a senior funding manager in August 2009 and helped create program guidelines and credit criteria. For the last 20 years prior to joining the IFA, he has been active in municipal and project finance, managing and actively participating in the municipal and project finance groups for both Sumitomo Bank and DEPFA BANK. Mr. Lebovits received his MBA from the Wharton School of the University of Pennsylvania and is a CPA.
For more information on this Illinois financing initiative, contact Mr. Lebovits at 312-651-1344 or mlebovits@il-fa.com. Contact: scotty@stLouisrenewableenergy.com for all your green building needs
3.20.2010
Solar Water Heater, green ideas, eco conscious
Solar Water Heater
$1,000 to $5,000
While solar-heating costs are still high for most whole-house applications, heating water with the sun has become practical. For $2,500 to $3,500, installed, an active, flat-plate solar collector system will produce 80 to 100 gallons of hot water per day. The payoff: Your water-heating bills will drop by 50 percent to 80 percent. Plus, you'll be shielded from future energy price increases.
Green Space:
http://www.bhg.com/home-improvement/remodeling/eco-friendly/real-life-eco-friendly-make-over/
1.When Tom and Sheryl Stone set out to remodel their 900-square-foot condo, their goal was to make it as green as possible using materials that were recycled, renewable, or sourced from environmentally responsible companies.
2.The first renovation they made was removing the wall that separated the living room and kitchen. Removing the wall opened up the condo without changing its footprint.
3.Cork flooring was used throughout the space. Cork is a popular option for flooring especially in high-traffic areas because it's water- and mold-resistant. Cork is roughly the same price as wood, but it's a sustainable material. Cork trees regenerate every nine years, while trees such as oak or maple can take 30 years or more.
4.Storage was an important factor in this small space. Kristin Lomauro-Boom, the designer, carved out storage wherever she could. Among the more intriguing solutions were drawers that hide within the toe-kicks, and a tall tambour door. She also designed a niche for the TV on the living room side of the island. The table on wheels can also serve as additional counter space.
5.The Stones used compact fluorescent lamps in the green glass pendant lights above the island and the track lights throughout the kitchen. The CFLs uses less electricity than standard incandescent bulbs. Although they cost a little more, they last up to 10 times longer and can be installed in most light fixtures.
6. See Remaining Eco Friendly Ideas: http://www.bhg.com/home-improvement/remodeling/eco-friendly/real-life-eco-friendly-make-over/?page=6
Info Provided by: Scotty, Green Builder St Louis "Renewable Energy" Missouri
$1,000 to $5,000
While solar-heating costs are still high for most whole-house applications, heating water with the sun has become practical. For $2,500 to $3,500, installed, an active, flat-plate solar collector system will produce 80 to 100 gallons of hot water per day. The payoff: Your water-heating bills will drop by 50 percent to 80 percent. Plus, you'll be shielded from future energy price increases.
Green Space:
http://www.bhg.com/home-improvement/remodeling/eco-friendly/real-life-eco-friendly-make-over/
1.When Tom and Sheryl Stone set out to remodel their 900-square-foot condo, their goal was to make it as green as possible using materials that were recycled, renewable, or sourced from environmentally responsible companies.
2.The first renovation they made was removing the wall that separated the living room and kitchen. Removing the wall opened up the condo without changing its footprint.
3.Cork flooring was used throughout the space. Cork is a popular option for flooring especially in high-traffic areas because it's water- and mold-resistant. Cork is roughly the same price as wood, but it's a sustainable material. Cork trees regenerate every nine years, while trees such as oak or maple can take 30 years or more.
4.Storage was an important factor in this small space. Kristin Lomauro-Boom, the designer, carved out storage wherever she could. Among the more intriguing solutions were drawers that hide within the toe-kicks, and a tall tambour door. She also designed a niche for the TV on the living room side of the island. The table on wheels can also serve as additional counter space.
5.The Stones used compact fluorescent lamps in the green glass pendant lights above the island and the track lights throughout the kitchen. The CFLs uses less electricity than standard incandescent bulbs. Although they cost a little more, they last up to 10 times longer and can be installed in most light fixtures.
6. See Remaining Eco Friendly Ideas: http://www.bhg.com/home-improvement/remodeling/eco-friendly/real-life-eco-friendly-make-over/?page=6
Info Provided by: Scotty, Green Builder St Louis "Renewable Energy" Missouri
3.15.2010
Highly Absorbing, Flexible Solar Cells
Highly Absorbing, Flexible Solar Cells
Supplied by: Scotty,Scott's Contracting, Green Builder, St Louis Renewable Energy.
02/16/1002/17/10
Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays
PASADENA, Calif.—Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional solar cells.
"These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials," says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech's Resnick Institute, which focuses on sustainability research.
This is a photomicrograph of a silicon wire array embedded within a transparent, flexible polymer film.
[Credit: Caltech/Michael Kelzenberg]The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. "We've surpassed previous optical microstructures developed to trap light," he says.
Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.
Atwater notes that the solar cells' enhanced absorption is "useful absorption."
"Many materials can absorb light quite well but not generate electricity—like, for instance, black paint," he explains. "What's most important in a solar cell is whether that absorption leads to the creation of charge carriers."
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."
The key to the success of these solar cells is their silicon wires, each of which, says Atwater, "is independently a high-efficiency, high-quality solar cell." When brought together in an array, however, they're even more effective, because they interact to increase the cell's ability to absorb light.
"Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires make the array very absorbing," he says.
This is a schematic diagram of the light-trapping elements used to optimize absorption within a polymer-embedded silicon wire array.
[Credit: Caltech/Michael Kelzenberg]This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell's surface area.
"When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires," explains Kelzenberg. "So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells."
Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”
In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.
Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.
The composite nature of these solar cells, Atwater adds, means that they are also flexible. "Having these be complete flexible sheets of material ends up being important," he says, "because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells."
Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. "They were visually striking," says Atwater. "But it wasn't until now that we could show that they are both highly efficient at carrier collection and highly absorbing."
The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. "The structures we've made are square centimeters in size," he explains. "We're now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell."
Atwater says that the team is already "on its way" to showing that large-area cells work just as well as these smaller versions.
In addition to Atwater, Lewis, and Kelzenberg, the all-Caltech coauthors on the Nature Materials paper, "Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications," are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.
Their research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Boettcher received fellowship support from the Kavli Nanoscience Institute at Caltech.
Contact:
Lori Oliwenstein
Supplied by: Scotty,Scott's Contracting, Green Builder, St Louis Renewable Energy.
02/16/1002/17/10
Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays
PASADENA, Calif.—Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional solar cells.
"These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials," says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech's Resnick Institute, which focuses on sustainability research.
This is a photomicrograph of a silicon wire array embedded within a transparent, flexible polymer film.
[Credit: Caltech/Michael Kelzenberg]The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. "We've surpassed previous optical microstructures developed to trap light," he says.
Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.
Atwater notes that the solar cells' enhanced absorption is "useful absorption."
"Many materials can absorb light quite well but not generate electricity—like, for instance, black paint," he explains. "What's most important in a solar cell is whether that absorption leads to the creation of charge carriers."
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."
The key to the success of these solar cells is their silicon wires, each of which, says Atwater, "is independently a high-efficiency, high-quality solar cell." When brought together in an array, however, they're even more effective, because they interact to increase the cell's ability to absorb light.
"Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires make the array very absorbing," he says.
This is a schematic diagram of the light-trapping elements used to optimize absorption within a polymer-embedded silicon wire array.
[Credit: Caltech/Michael Kelzenberg]This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell's surface area.
"When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires," explains Kelzenberg. "So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells."
Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”
In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.
Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.
The composite nature of these solar cells, Atwater adds, means that they are also flexible. "Having these be complete flexible sheets of material ends up being important," he says, "because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells."
Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. "They were visually striking," says Atwater. "But it wasn't until now that we could show that they are both highly efficient at carrier collection and highly absorbing."
The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. "The structures we've made are square centimeters in size," he explains. "We're now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell."
Atwater says that the team is already "on its way" to showing that large-area cells work just as well as these smaller versions.
In addition to Atwater, Lewis, and Kelzenberg, the all-Caltech coauthors on the Nature Materials paper, "Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications," are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.
Their research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Boettcher received fellowship support from the Kavli Nanoscience Institute at Caltech.
Contact:
Lori Oliwenstein
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