Category Archives: Energy Storage
The wind and sun can produce great amounts of power, but it can usually only be harnessed when it’s windy and the sun is shining. Researchers at Grand Valley State University and Ann Arbor-based Vinazene are working to change that by creating a new type of flow battery technology that will allow the capture, collection and storage of energy through organic compounds.
The project, funded by a Phase II Small Business Innovation Research grant through the U.S. Department of Energy to Vinazene, includes researchers from Grand Valley’s Michigan Alternative and Renewable Energy Center (MAREC) and Chemistry Department.
Andrew Lantz, associate professor of chemistry at Grand Valley, Bill Schroeder and John Schroeder, research consultants for Grand Valley, and a group of students are developing and testing a prototype device to showcase the redox flow battery technology concept.
Lantz said the flow cell technology is similar to batteries, except that instead of all the chemicals contained in the battery, the chemicals — or electrolytes — are stored in batteries on large external reservoirs and are pumped into the battery as needed during charge or discharge cycles.
“The main roadblock with many renewable energy sources is their lack of consistent power output over time,” said Lantz. “Flow battery technology can help deal with this issue by storing energy reserves during times of peak collection and discharging the energy when it is needed.”
While other companies and universities are conducting similar research, Vinazene founder Paul Rasmussen, professor emeritus of chemistry and macromolecular science and engineering at the University of Michigan, said many rely on expensive, scarce elements to supply the batteries; his team is using organic compounds that are less expensive and more accessible.
Lantz said as the country shifts to renewable energy, this concept will be especially well suited for solar and wind energy sources.
The group will continue to perform research through April with funding through the SBIR grant.
Project lead David Rosewater said Sandia will evaluate the 1 megawatt, lithium-ion grid energy storage system for capacity, power, safety and reliability. The lab also will investigate the system’s frequency regulation, which grid operators need to manage the moment-to-moment differences between electrical supply and demand.
“Independent evaluations provide valuable feedback for industry efforts to standardize metrics for characterizing and reporting reliability, safety and performance. Companies need the standards to develop large procurement goals for grid energy storage because they must be able to compare performance and cost,” said Rosewater.
The data generated from characterizing a large system like GridSaver will improve operational models, identify technology or research gaps and provide feedback to manufacturers to improve system performance, reliability and safety. Additional specific tests will help validate Sandia’s grid energy storage characterization protocols, which have been developed jointly by industry and the national labs, as pre-standards to measure and express energy storage system performance.
“Industry needs these standards and they don’t yet have them. The protocol will give us critical information that can be used to compare flow battery systems, lead-acid battery systems, lithium-ion systems and flywheel systems on an even field, apples to apples,” Rosewater said.
Utilities and other electricity and transmission providers and regulators often require that equipment be proven safe and reliable before it is permitted to operate on the electric grid. However, energy storage manufacturers and integrators are often unable to afford or provide the logistics necessary for this long-term testing and monitoring.
Sandia’s Energy Storage Test Pad and Energy Storage Analysis Laboratory test facilities validate manufacturers’ specifications of energy storage devices through characterization and application-specific cycle testing. They can also help users evaluate system parameters, including storage device efficiency, performance to specifications, reliability and balance of plant operation.
Rosewater said national, state and local policies that push for a cleaner, more secure electric grid are driving significant increases in variable renewable generation, but that makes the job of operators much more difficult. Storage helps to mitigate that variability, when it’s safe, reliable, sustainable and cost-effective.
“Developing an energy storage system involves the complex integration of many components beyond just the battery, including sophisticated power electronics and controls — often communications. Sandia is assessing the entire system,” said Imre Gyuk, energy storage program manager in the Department of Energy’s Office of Electricity Delivery and Energy Reliability. The office has identified four challenges to the widespread deployment of energy storage: the cost of energy storage technologies (including manufacturing and grid integration), validated reliability and safety documentation, an equitable regulatory environment and industry acceptance.
“Third-party evaluation of large systems like TransPower’s GridSaver can help break down the barriers to grid energy storage proliferation,” Rosewater said.
GridSaver was commissioned by the California Energy Commission’s Public Interest Energy Research (PIER) electric program.
Sandia’s work is funded by DOE’s Office of Electricity Delivery and Energy Reliability.
These storage advocates were assembled for the Energy Storage North America exposition, which doubled in size compared with last year, according to conference chairperson Janice Lin. Also co-founder of the California Energy Storage Association, Lin said that “energy storage is a game changer for the electric power system, and this year’s ESNA event truly represents that.”
Storage Belatedly Following PV Growth Curve
A number of investors in energy storage touted the wisdom of their bets. “Energy storage is one of the holy grails of renewable energy; the main issue is that costs still have to come down,” commented Andrew Chung, a partner at the venture capital firm Khosla Ventures, based in Menlo Park. “Energy storage wasn’t invented for grid-scale applications, but with it getting acceptance in solar, more states and nations will start to move toward implementing policy,” he said. Khosla lists six energy storage start ups within its portfolio now.
Similarly, Peter Rive, the co-founder of SolarCity said, “In three to five years, storage will be a standard component of residential solar power, not optional, but deployed with every solar system.” He continued, “It won’t look that different to the customer, but the fundamental value proposition will be cleaner and cheaper.” Rive also said that electric vehicle (EV) charging capability was likely to become a feature of the energy storage rollout.
Standout Projects Raise Storage Profile
Among standout energy storage projects highlighted at the show was the City of Santa Clara’s Levi Stadium, which features over 1,000 solar panels and six electric vehicle charging stations. The stadium also has installed Green Charge Networks’ GreenStationsystem to help curb its power demand and lower electric bills, which spike during certain hours on game days, compared with minimal power and energy consumption throughout the remainder of the year.
Developers like AES also are building a global network of pioneering energy storage projects associated with renewable energy. “We now have 200 MW of energy storage online, mostly in the United States and Chile, including four utility scale plants built, and 2,000 MW on order,” said John Zahurancik, the president of AES Energy Storage, based in Arlington, Va. “There is a demand for 30 GW of new peaker plants by 2024, and by that year, battery production capacity will exceed the peaker need by a factor of four,” he added, suggesting energy storage would be well positioned to compete.
Solar panel manufacturers now moving into energy storage also will drive adoption more rapidly. Kyocera, for example, recently has forged new strategic relationships with both Stem, the energy storage developers, and with Healthy Planet Partners, a clean energy solutions fund that finances, implements and maintains distributed energy and energy. Many energy storage companies have begun offering financing through their own balance sheets or through such partnerships with financial companies.
Ontario a North American Storage Leader
Apart from the now well-know U.S. state programs in California and Hawaii, the current energy storage initiative for the Canadian Province of Ontario is “to purchase 35 MW of varying storage at a rough cost of $42 million, “which will show up in couple of years,” says Kim Warren, the vice president of operations for the Independent Electricity System Operator (IESO) of the province, based in Toronto.
“We received 431 applications for 12 projects, with differing technologies, and we are doing a three to five year study of the combined storage system to see how storage will value, like California is doing; we may even conduct it in consultation with California,” Warren said. Describing the limitations of pumped hydro storage, and provincial plans to shut down 13,000 MW of nuclear power within five years, He suggested that wind and solar fit the ISO’s needs better. “We already have 9,000 MW of wind and solar contracted,” he said.
The Ontario IESO is studying the use of renewable energy storage not only for the primary grid, but for microgrids and remote off-grid locations, which abound in Canada. “We have purposely put renewables and storage in both congested and uncongested areas,” Warren said. “It is inevitable that as we move toward microgrids, especially with solar parity, we will need more intelligent conversation between the grid operators and the microgrids. The regulators will have to decide who pays for what,” he said.
The U.S. Environmental Protection Agency implemented a cap and trade program for sulfur dioxide (the primary contributor to acid rain) in 1995. This program was a great success, and essentially eliminated the acid rain program. California passed AB32 in 2006 to accomplish the same goals for CO2 emissions. This law sets a cap on emissions from almost all sources, and gives polluting companies a certain number of allowances. If companies reduce their CO2 emissions (with renewable power generation, more efficient processes or smokestack scrubbers), they can trade these emissions to companies that still pollute.
Many California companies succeeded in reducing their emissions. In fact, utilities installed so much wind and solar (courtesy of their RPS requirements) that they now have excess allowances to trade. But the gas refining industry didn’t act, and starting on January 1, 2015 they will have to purchase extra allowances. How much? When a gallon of gas is burned it emits about 20 pounds of CO2, which is 0.009 tons. At the current market price of CO2 allowances of $12/ton, that extra CO2 amounts to about 11 cents. So the downside of cap and trade is that the price of gas in California is likely to go up by about a dime. The upside is that we get cleaner air and an even stronger green economy.
But not everyone wants this outcome. In particular, the oil and gas refining industry tried to suspend AB32 in 2010 when they sponsored Proposition 23 (which was defeated by 62 percent of voters). This year they are sponsoring AB69, which will delay the application of cap and trade to transportation fuels. It’s shaping up as a battle between deep-pocketed dirty fuel polluters — and just about everybody else in California. I’m hopeful that California’s cap and trade program continues to succeed, and maybe someday soon will be adopted by the other 49 states. Please join me on this week’s Energy Show on Renewable Energy World as we talk about the real economic impact of cap and trade.
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As energy costs consume more and more of our hard-earned dollars, we as consumers really start to pay attention. But we don’t have to resign ourselves to $5/gallon gas prices, $200/month electric bills and $500 heating bills. There are literally hundreds of products, tricks and techniques that we can use to dramatically reduce these costs — very affordably.
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Barry Cinnamon is a long-time advocate of renewable energy and is a widely recognized solar power expert. In 2001 he founded Akeena Solar — which grew to become the largest national residential solar installer by the middle of the last decade with over 10,000 rooftop customers coast to coast. He partnered with Westinghouse to create Westinghouse Solar in 2010, and sold the company in 2012.
His pioneering work on reducing costs of rooftop solar power systems include Andalay, the first solar panel with integrated racking, grounding and wiring; the first UL listed AC solar panel; and the first fully “plug and play” AC solar panel. His current efforts are focused on reducing the soft costs for solar power systems, which cause system prices in the U.S. to be double those of Germany.
Although Barry may be known for his outspoken work in the solar industry, he has hands-on experience with a wide range of energy saving technologies. He’s been doing residential energy audits since the punch card days, developed one of the first ground-source heat pumps in the early ‘80s, and always abides by the Laws of Thermodynamics.
Lead image: Green microphone via Shutterstock
A year ago, Germany’s beleaguered solar industry rejoiced when the government-owned development bank and federal environment ministry announced loans and repayment support for combined rooftop PV and battery storage options of up to 30 kW. The sector thought it spotted an opportunity to redeem itself on the German and international markets with a technology that German PV and energy management firms had long been perfecting.
The logic of battery technology is pretty straightforward: By storing power generated when the sun is shining bright and demand is low, the owners of household battery systems can use more of the electricity they generate when they need it — twice as much, say experts. Larger storage systems, if connected to solar plants, could help integrate even more renewables into the system, dramatically reducing disparities in supply and demand — the bugbear of PV and wind power.
A schematic of a typical home energy storage system setup. Credit: Saft.
Yet, 14 months down the road, German PV and energy management companies have sold only around 10,000 decentralized battery systems (not all in Germany), with about 50 percent of the new German owners taking advantage of the subsidies. The Leipzig-based PV company Deutsche Energieversorgung says that only 15 to 20 percent of its battery customers use the supports. Roughly a third of RWE Home Power Storage’s clients have availed themselves of the subventions.
Critics say the bureaucracy required to obtain the credit is cumbersome, and that new laws in Germany that will tax self-consumption have put a damper on PV sales. The fact that these small-scale systems are designed only for self-consumption (in other words they’re not connected to smart grids or the larger power system) limits their usefulness for proving extra grid flexibility to balance renewables.
Indeed, the numbers so far are modest, even though both producers and suppliers of distributed storage hardware say business has been picking up as of late spring.
A study of 13 European (mostly German) firms conducted by the magazine Sonne Wind Wärme, found that Bavaria-based Sonnenbatterie led the field having sold 2,500 transportable intelligent lithium storage systems in 2013-14, mostly in Germany. It was followed by Deutsche Energieversorgung that sold just 100 fewer, and Nedap of the Netherlands with 2000. Saft Batteries out of Nuremburg moved roughly 1000 systems, half of those in Germany and the other half in Austria, Switzerland, Italy, and Luxembourg. A number of other firms, including E3/DC, Bosch Power Tec, and Frankensolar sold less than 1000 models.
About 85 percent of the units sold were the more traditional and substantially cheaper lead-acid batteries; while 15 percent were the newer lithium-ion-based models, which are more expensive but also tout more capacity. Not only can lithium-ion batteries store more power while requiring less space, they can be charged and discharged more frequently than lead-acid batteries. The prices of the systems range from 6,000 to 30,000 euros, depending on size, model, producer, and type. Most of the investors were households, followed by small businesses.
Since the program was launched last May, 4,000 solar battery buyers were awarded a total of 66 million euros of low-interest loans and grants of over 10 million euros. The state development bank KfW grants a low-interest loan for the purchase of a combined photovoltaic and solar-power storage system or for retrofitting an existing solar-power installation with a storage system. In addition, the state may cover up to 30 percent of the loan. Yet in 2013, more than half of the available monies went untouched.
“The support programs were slow getting started,” says Tom Rudloph of SMA Systems, one of the early Germany-based pioneers in battery storage. “But they’ve ironed out some of the wrinkles, so it’s working better now.”
The jury is still out on whether the batteries — with or without the supports — are worth it for small PV users.
Germany’s main solar advocate, the German Solar Energy Association (BSW), claims that small battery units won’t make or save homeowners a bundle, but — in economic terms alone — are probably a break-even proposition, depending on consumption behavior, type of PV system, location and other factors. “There wouldn’t be government incentives if small-scale battery storage was completely economical at the moment,” admits David Wedepohl of the BSW. “But at least half the systems in Germany sell without any incentive.»
The SMA Sunny Boy Smart Energy is a combination of a modern PV inverter and a battery with an effective capacity of 2 kWh. Credit: SMA.
“The point is to create a market for storage, create economies of scale, and generate experience with this new technology,” says Wedepol. “In the future, if we’re serious about running our economies on 90 percent or 100 percent renewable energy, we’re going to need it.” He notes there are factors besides economic viability that motivate people to invest in storage, such as energy independence. Most of the battery companies say independence is their customers’ primary motivation.
Lithium batteries have revolutionized consumer electronics and made EVs a mainstream reality.
But will they play a similar role in energy storage? That was one of the most prominent questions at Energy Storage North America, the second annual conference taking place this week in San Jose.
“Lithium ion was not fundamentally designed for grid scale storage,” said Andrew Chung, a partner at Khosla Ventures, during a panel at the conference. “Even with the Gigafactory, the cost won’t come down enough.”
Chung’s skepticism essentially revolved around the three complaints most often associated with lithium batteries: they cost too much, they can become a safety hazard and they have a limited lifespan. Utilities and commercial building owners want something that will last twenty years flawlessly.
One of the key problems with lithium ion cells is the liquid-solid interface that exists between the electrolyte and the cathode. Ambri, one of his portfolio companies, is trying to get around this problem with an all liquid batteries. Other companies are trying to commercialize solid electrolytes.
Lithium ion will play a major role over the next two years because it is the only rechargeable battery appropriate for grid-scale and commercial-scale deployment, but after that new chemistries will have to be developed.
Naturally, lithium ion manufacturers differed. “Lithium ion can be a 20 year asset,” said Bud Collins, CEO of NEC Energy Solutions. A lithium ion battery for cell phones might be designed for 500 charge cycles, but you can design long-lasting cells and supplement their performance and operation with advanced analytics so that they last 20,000 cycles, he said.
Other audience members also noted that “lithium ion” covers a lot of different chemistries. Lithium cobalt batteries are the ones subject to thermal runaway reactions. Lithium phosphate batteries don’t have that problem.
Then again, after the panel flow battery execs noted that lithium ion batteries are still cells. They are small. Something like a flow battery or a sodium sulfur battery is better designed for bulk storage or grid applications.
How will it work out? Chung has a great point: lithium ion weren’t designed for utility applications and they are expensive. However, the world has waited for years for an alternative — zinc, etc. — and nothing has come close to matching lithium ion. Ten years ago, a Sony product manager warned me that lithium ion was hitting its limit: if companies kept pushing it, we’d see failures and explosions in 2006. He was right on target, but we still don’t have the great alternatives.
«No one battery is perfect,» said John Jung, CEO of Greensmith, which produces software for managing energy storage systems, among others. Expect a variety. Even a Tesla exec told me they will examine all sorts of different technologies.
Other interesting tidbits from the conference:
PGE had some good news, and bad news about the future of storage. It has been using a 2-megawatt storage system to provide frequency regulation and then tracks the revenue and costs it incurs in selling power as well as performing frequency regulation. The system runs about six hours a day, from 11 a.m. to 5 p.m.
It basically breaks even. In September, the system generated a whopping $318.
So the good news? It is an indication that storage for smoothing renewables can be installed that effectively pays for itself. Solar critics have sometimes tried to add the cost of natural gas generators to the cost of solar systems to “prove” that solar is uneconomical. These results show you can have grid smoothing that doesn’t add to the cost of renewables.
- Four hours. That is the number everyone is shooting for. If your storage system can provide four hours of service, you can compete for a wide range of contracts.
- Policy is important but storage in many markets will stand on its own. 1.3 billion people are still not connected to the grid. China is choking in emissions. Storage systems that can somewhat economically bring the benefits of electrification to the still developing regions of Africa, Asia and Latin America will do well.
- Versatility. Virtually every vendor talked about how their storage system could be used for a variety of applications and functions. It’s firming solar power one moment, and providing demand response services the next. In theory, this sounds easy, but one of the big differences between the winners and the also-rans will be an ability to actually provide this sort of versatility.
“The completion of an integrated internal energy market will increase solidarity among member states, ensure safety of energy supplies and support integration of local renewable energy sources with a view to achieving energy self- sufficiency,” Italy, which currently holds the EU’s rotating presidency, said on its website.
The EU energy strategy includes developing interconnections, modernizing infrastructure and diversifying supply sources. Jean-Claude Juncker, the president-designate of the next European Commission, has vowed to move toward an energy union with forward-looking climate policy as a pricing dispute led to the cutoff of Russian natural gas supplies to Ukraine, the transit country for around 15 percent of the EU demand for the fuel.
The EU is trying to broker a compromise between the two nations and proposed a temporary deal to restore flows before winter. The next round of three-way talks will be set this week, the commission said in a statement on Oct. 3.
While Russian Energy Minister Alexander Novak on Sept. 26 called the EU “a big step” toward an agreement, Ukrainian Energy Minister Yuri Prodan said last week his country is “ready to reach an agreement, but not at the volumes and in the timeframes set by Russia.”
Concerns among EU governments over a possible disruption increase as Russia and Ukraine have been trading accusations of threats to EU-bound gas since July. European nations have already agreed to stress test of Europe’s energy system to help overcome a potential cutoff in the 2014-15 winter. Their leaders plan to decide on further measures to enhance the bloc’s energy security at the October summit.
“In the short term, the EU has the following overriding priority: to ensure that the best possible preparation and planning improves resilience to sudden disruptions in energy supplies, in particular during the coming winter,” the EU presidency said in a report sent to member states for discussion at today’s gathering.
The completion of the EU energy market by the end of 2014 and ending energy isolation of member states by 2015 remain “essential tools” for energy security, according to Italy. Investment challenges that member states face include the replacement of obsolete power plants and infrastructures to improve energy efficiency and lower energy costs.
The commission, the bloc’s regulatory arm, proposed in January the bloc adopt a binding goal to cut greenhouse gases by 40 percent by 2030, accelerating the pace of emissions reduction from 20 percent in 2020 compared with 1990 levels. It also recommended an EU-wide target to boost the share of renewables in energy consumption to 27 percent.
Energy efficiency is the third pillar of the strategy for 2030, to be decided by EU leaders later this month. The commission proposed nations increase energy savings by 30 percent by 2030 compared with 20 percent targeted for the end of the current decade.
Copyright 2014 Bloomberg
Lead image: EU and Ukraine flag via Shutterstock
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My 10 Clean Energy Stocks for 2014 model portfolio weathered the storm relatively well because of its emphasis on defensive and income stocks.
Since the last update, the model portfolio was down 4.8 percent, compared to 5.5 percent for small cap stocks (as measured by the Russell 2000 index ETF, IWM) and a 9.8 percent decline for clean energy stocks, as measured by PBW, the Powershares Wilderhill Clean Energy Index. The relative strength of the model portfolio was in spite of significant weakness in foreign currencies. The Canadian Dollar, Euro, and South African Rand fell 3.3 percent, 3.4 percent, and 5.8 percent for the month, dragging the model portfolio down 2.8 percent more in US Dollar terms than in local currency terms.
Since portfolio inception on December 26th, 2013, the model portfolio is up 1.5 percent in dollar terms, and 3.8 percent in local currency terms. The small cap stock index was down 3.9 percent, while the clean energy stock index eked out a tiny gain of 0.3 percent.
Stock market trends down in September. Image source Yahoo! Finance.
Individual Stock Notes
The chart and discussion detail the performance of individual stocks in the 10 Clean Energy Stocks for 2014 model portfolio, along with relevant news items since the last update.
(Current prices as of August 5th, 2014. The «High Target» and «Low Target» represent my December predictions of the ranges within which these stocks would end the year, barring extraordinary events.)
1. Hannon Armstrong Sustainable Infrastructure (NYSE:HASI).
12/26/2013 Price: $13.85. Low Target: $13. High Target: $16. Annualized Dividend: $0.88.
Current Price: $13.77. YTD Total US$ Return: 4.2%
Sustainable Infrastructure REIT Hannon Armstrong paid its regular $0.22 dividend for the third quarter. The REIT has a goal of paying dividends equal to 100 percent of distributable income, which were $0.20 in the first quarter, and $0.22 in the second quarter. I expect third and fourth quarter distributable income to be sequentially higher as the company deploys capital from its $75 million April secondary offering. This implies that we can expect a small increase in December’s fourth quarter dividend, which I expect to be approximately $0.24.
2. PFB Corporation (TSX:PFB, OTC:PFBOF).
12/26/2013 Price: C$4.85. Low Target: C$4. High Target: C$6.
Annualized Dividend: C$0.24.
Current Price: C$4.32. YTD Total C$ Return: -7.2%. YTD Total US$ Return: -11.7%
Green building company PFB re-authorized its normal course issuer bid to purchase up to 50,000 shares of its own stock over the next year. Over the past year, the company repurchased 19,500 of its shares at an average price of C$4.92. Given the current price of C$4.32, I would expect it to step up these purchases.
3. Capstone Infrastructure Corp (TSX:CSE. OTC:MCQPF).
12/26/2013 Price: C$4.44. Low Target: C$3. High Target: C$5.
Annualized Dividend: C$0.30.
Current Price: C$4.21. YTD Total C$ Return: 27.0%. YTD Total US$ Return: 20.9%
Independent power producer Capstone Infrastructure announced strong second quarter operating results based on higher wind production and increased income from its British water utility, Bristol Water. The results were generally in line with analysts’ forecasts, but Scotiabank increased its price target for the company to C$4.50 from C$4.00 while keeping its «Market Perform» rating.
Last week, Capstone closed C$76 million financing for its 25-MW Goulais wind farm, which is under construction in Northern Ontario.
In the October 3, 2014 issue of the journal Nature Communications, the researchers report that they’ve succeeded in combining a battery and a solar cell into one hybrid device.
Key to the innovation is a mesh solar panel, which allows air to enter the battery, and a special process for transferring electrons between the solar panel and the battery electrode. Inside the device, light and oxygen enable different parts of the chemical reactions that charge the battery.
The university will license the solar battery to industry, where Yiying Wu, professor of chemistry and biochemistry at Ohio State, says it will help tame the costs of renewable energy.
“The state of the art is to use a solar panel to capture the light, and then use a cheap battery to store the energy,” Wu said. “We’ve integrated both functions into one device. Any time you can do that, you reduce cost.”
He and his students believe that their device brings down costs by 25 percent.
Researchers at The Ohio State University have invented a solar battery — a combination solar cell and battery — which recharges itself using air and light. The design required a solar panel which captured light, but admitted air to the battery. Here, scanning electron microscope images show the solution: nanometer-sized rods of titanium dioxide (larger image) which cover the surface of a piece of titanium gauze (inset). The holes in the gauze are approximately 200 micrometers across, allowing air to enter the battery while the rods gather light. Image courtesy of Yiying Wu, The Ohio State University.
The invention also solves a longstanding problem in solar energy efficiency, by eliminating the loss of electricity that normally occurs when electrons have to travel between a solar cell and an external battery. Typically, only 80 percent of electrons emerging from a solar cell make it into a battery.
With this new design, light is converted to electrons inside the battery, so nearly 100 percent of the electrons are saved.
The design takes some cues from a battery previously developed by Wu and doctoral student Xiaodi Ren. They invented a high-efficiency air-powered battery that discharges by chemically reacting potassium with oxygen. The design won the $100,000 clean energy prize from the U.S. Department of Energy in 2014, and the researchers formed a technology spinoff called KAir Energy Systems, LLC to develop it.
“Basically, it’s a breathing battery,” Wu said. “It breathes in air when it discharges, and breathes out when it charges.”
For this new study, the researchers wanted to combine a solar panel with a battery similar to the KAir. The challenge was that solar cells are normally made of solid semiconductor panels, which would block air from entering the battery.
Doctoral student Mingzhe Yu designed a permeable mesh solar panel from titanium gauze, a flexible fabric upon which he grew vertical rods of titanium dioxide like blades of grass. Air passes freely through the gauze while the rods capture sunlight.
Normally, connecting a solar cell to a battery would require the use of four electrodes, the researchers explained. Their hybrid design uses only three.
The mesh solar panel forms the first electrode. Beneath, the researchers placed a thin sheet of porous carbon (the second electrode) and a lithium plate (the third electrode). Between the electrodes, they sandwiched layers of electrolyte to carry electrons back and forth.
Here’s how the solar battery works: during charging, light hits the mesh solar panel and creates electrons. Inside the battery, electrons are involved in the chemical decomposition of lithium peroxide into lithium ions and oxygen. The oxygen is released into the air, and the lithium ions are stored in the battery as lithium metal after capturing the electrons.
When the battery discharges, it chemically consumes oxygen from the air to re-form the lithium peroxide.
An iodide additive in the electrolyte acts as a “shuttle” that carries electrons, and transports them between the battery electrode and the mesh solar panel. The use of the additive represents a distinct approach on improving the battery performance and efficiency, the team said.
The mesh belongs to a class of devices called dye-sensitized solar cells, because the researchers used a red dye to tune the wavelength of light it captures.
In tests, they charged and discharged the battery repeatedly, while doctoral student Lu Ma used X-ray photoelectron spectroscopy to analyze how well the electrode materials survived — an indication of battery life.
First they used a ruthenium compound as the red dye, but since the dye was consumed in the light capture, the battery ran out of dye after eight hours of charging and discharging — too short a lifetime. So they turned to a dark red semiconductor that wouldn’t be consumed: hematite, or iron oxide — more commonly called rust.
Coating the mesh with rust enabled the battery to charge from sunlight while retaining its red color. Based on early tests, Wu and his team think that the solar battery’s lifetime will be comparable to rechargeable batteries already on the market.
The U.S. Department of Energy funds this project, which will continue as the researchers explore ways to enhance the solar battery’s performance with new materials.