Monday, December 26, 2011

Scientists create first solar cell with over 100 percent quantum efficiency

Researchers from the National Renewable Energy Laboratory (NREL) have reported the first solar cell that produces a photocurrent that has an external quantum efficiency greater than 100 percent when photoexcited with photons from the high energy region of the solar spectrum.

The external quantum efficiency for photocurrent, usually expressed as a percentage, is the number of electrons flowing per second in the external circuit of a solar cell divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell. None of the solar cells to date exhibit external photocurrent quantum efficiencies above 100 percent at any wavelength in the solar spectrum.

The external quantum efficiency reached a peak value of 114 percent. The newly reported work marks a promising step toward developing Next Generation Solar Cells for both solar electricity and solar fuels that will be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.

Multiple Exciton Generation is key to making it possible

A paper on the breakthrough appears in the Dec. 16 issue of Science Magazine. Titled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” it is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard. The research was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences. Semonin and Nozik are also affiliated with the University of Colorado at Boulder.

The mechanism for producing a quantum efficiency above 100 percent with solar photons is based on a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon.

NREL scientist Arthur J. Nozik first predicted in a 2001 publication that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as:

• Rpidly increasing bandgap with decreasing quantum dot size,

• Formation of correlated electron-hole pairs (called excitons) at room temperature,

• Enhanced coupling of electronic particles (electrons and positive holes) through Coulombic forces,

• And enhancement of the MEG process.

Quantum dots confine the charges and harvest excess energy

Quantum dots, by confining charge carriers within their tiny volumes, can harvest excess energy that otherwise would be lost as heat – and therefore greatly increase the efficiency of converting photons into usable free energy.

The researchers achieved the 114 percent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.

In a 2006 publication, NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35 percent relative to today’s conventional solar cells. Furthermore, the fabrication of Quantum Dot Solar Cells is also amenable to inexpensive, high-throughput roll-to-roll manufacturing.

Such potentially highly efficient cells, coupled with their low cost per unit area, are called Third (or Next) Generation Solar Cells. Present day commercial photovoltaic solar cells are based on bulk semiconductors, such as silicon, cadmium telluride, or copper indium gallium (di)selenide; or on multi-junction tandem cells drawn from the third and fifth (and also in some cases fourth) columns of the Periodic Table of Elements. All of these cells are referred to as First- or Second-Generation Solar Cells.

MEG, also referred to as Carrier Multiplication (CM), was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory. Since then, many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dots. However, nearly all of these positive MEG results, with a few exceptions, were based on ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions.

The new results published in Science by the NREL research team is the first report of MEG manifested as an external photocurrent quantum yield greater than 100 percent measured in operating quantum dot solar cells at low light intensity; these cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight. While these solar cells are un-optimized and thus exhibit relatively low power conversion efficiency (which is a product of the photocurrent and photovoltage), the demonstration of MEG in the photocurrent of a solar cell has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

Another important aspect of the new results is that they agree with the previous time-resolved spectroscopic measurements of MEG and hence validate these earlier MEG results. Excellent agreement follows when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell. In this case, the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incident photons are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

More information: Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell, Science 16 December 2011: Vol. 334 no. 6062 pp. 1530-1533. DOI: 10.1126/science.1209845

Thursday, December 22, 2011

Paint-On Solar Cells Developed

A team of researchers at the University of Notre Dame has made a major advance toward this vision by creating an inexpensive "solar paint" that uses semiconducting nanoparticles to produce energy.

"We want to do something transformative, to move beyond current silicon-based solar technology," says Prashant Kamat, John A. Zahm Professor of Science in Chemistry and Biochemistry and an investigator in Notre Dame's Center for Nano Science and Technology (NDnano), who leads the research.

"By incorporating power-producing nanoparticles, called quantum dots, into a spreadable compound, we've made a one-coat solar paint that can be applied to any conductive surface without special equipment."

The team's search for the new material, described in the journal ACS Nano, centered on nano-sized particles of titanium dioxide, which were coated with either cadmium sulfide or cadmium selenide. The particles were then suspended in a water-alcohol mixture to create a paste.

When the paste was brushed onto a transparent conducting material and exposed to light, it created electricity.

"The best light-to-energy conversion efficiency we've reached so far is 1 percent, which is well behind the usual 10 to 15 percent efficiency of commercial silicon solar cells," explains Kamat.

"But this paint can be made cheaply and in large quantities. If we can improve the efficiency somewhat, we may be able to make a real difference in meeting energy needs in the future."

"That's why we've christened the new paint, Sun-Believable," he adds.

Kamat and his team also plan to study ways to improve the stability of the new material.

NDnano is one of the leading nanotechnology centers in the world. Its mission is to study and manipulate the properties of materials and devices, as well as their interfaces with living systems, at the nano-scale.

This research was funded by the Department of Energy's Office of Basic Energy Sciences.

Ref: Science Daily

Friday, December 16, 2011

The way towards Cop 18 in Qatar

Countries at the United Nations climate change negotiations have publicly acknowledged their current pledges to reduce carbon emissions will not result in limiting global warming to less than two degrees Celsius.

To bridge this shortfall, delegates at the 17th Conference of Parties (COP 17) climate talks proposed on to address this so-called "emissions gap" at COP 18 in Qatar next year.

Documents negotiated in Durban, South Africa acknowledged the science-based emissions reduction target of 25 to 40 percent by 2020. Those reductions and that timeline are what is needed to stay below two degrees Celsius. The draft text says this would be the target to be agreed on at COP 18.

"We need agreement on that science-based target next year at the latest," said Karl Hood, Minister of Foreign Affairs of the Caribbean island of Grenada and representing the Alliance of Small Island States.

"And we want those targets to legally come into force before 2017."

Hood told IPS waiting to close the gap until after 2020 is "unacceptable" and a "disaster for small island states" who are already suffering the impacts of climate change.

The world has months to curb emissions from burning fossil fuels before two degrees Celsius of warming will be impossible to stay below. Delay a few years and the extraordinary emission cuts needed could bankrupt the world's economy and reverse development gains in most countries, climate experts warned at the largely deadlocked United Nations climate change conference here.

"We're here to warn policy makers that we are dangerously close to not being able to meet the less than two degrees Celsius target," said Bill Hare, Director of Climate Analystics, a non-profit climate science advisory group based in Germany.

The current pledges made by countries to cut emissions after the Copenhagen climate talks in 2009 will result in global warming of 3.5 degrees Celsius, said Hare a climate scientist. Two years later, those pledges remain essentially unchanged and that means the world's options to stay below two degrees Celsius are narrowing Hare said in press conference during the COP 17 negotiations that conclude Friday.

"To put it bluntly, the longer we wait, the less option we will have, the more it will cost ...and the bigger threat to the world’s most vulnerable," he said.

Global emissions of fossil fuels have increased 49 percent since 1990 and reached a record of about 48 gigatonnes (billion tonnes) of CO2 in 2010 and likely 50 gigatonnes (Gt) of CO2 this year, he said. Thanks to the moderating affect of the oceans, the world has warmed only 0.8 degrees Celsius on average, however, many parts of the world are much warmer than that.

The science shows that global emissions need to fall to 44 Gt by 2020 and continue to decline by two percent per year, a rate that our fossil fuel-dependent world will find "extremely challenging" but still doable, he said.

If countries live up to their pledges made in Copenhagen global emissions are likely to rise nine to 11 Gt above the 44 Gt target creating an "emissions gap" that is quite considerable, said Niklas Höhne, Director Energy and Climate Policy of Ecofys, an energy consulting organization.

"Our results are in agreement with the United Nations Environment Programme (UNEP) Bridging the Emissions Gap Report released at the opening of the Durban climate talks," he told IPS.

The new UNEP report calculates a similar emission gap and outlines the way reductions can be made between now and 2020 to bridge that gap. Shockingly many of the items under intense debate at here at the COP 17 — biofuels, agriculture, carbon credits for forest protection, carbon capture and storage — are not considered important pathways to reduce emissions by scientists.

"With biofuels you have to be very sure they won't result in a net increase in emissions," said Höhne.

A number of new studies of palm oil biodiesel and maize ethanol show their net emissions are higher than fossil fuels when their entire lifecycle is calculated.

Biofuels are unlikely to be a significant method for reducing emissions, agreed Höhne. Agriculture is in the same category. Farming practices could be altered to reduce emissions but based on analysis using various reduction scenarios they would only be a small part of the "bridge.”

The emissions gap can only be bridged with a combination of improving energy efficiency in all sectors, significant increase in renewable energy including biomass power and shifting from coal to natural gas. The cost of making this shift is relatively low at 38 dollars a ton of CO2 avoided.

Ref:The Madison Times

Sunday, December 11, 2011

1411 MW Wind capacity added in India during the first half of the Indian Financial Year

Wind power has shown remarkable growth over the past decade, with the global cumulative installed capacity reaching 215 GW by June 2011. By 2015, installations are predicted to grow at an annual average rate of 15.5%, with the annual installed capacity growing to 81.4 GW from the current 39.5 GW (2010), including offshore development. India has maintained its position as one of the leading wind power nations, remaining at fifth position worldwide in terms of cumulative installations in 2011. The Indian wind industry has successfully weathered the economic slowdown encountered by many other nations and is moving towards achieving maturity. Presently, the country has a cumulative installed capacity of 15,567 MW. The capacity addition for FY 2011-12 is expected to be around 3,000 MW, out of which 1,411 MW has already been achieved. According to the estimates, the annual capacity increase for the Indian wind market is expected to reach 5000 MW by 2015.

Tremendous achivement is there in India in terms of capacity addition in Wind Energy. Based on some reliable sources, around 1411 MW wind power capacity has been added in India during the period from April-2011 to Sept-2011 of the FY 2011-12 which is the highest capacity addition during the first half of any Financial year in India.

Indian State wise capacity addition during the period April-2011 to September-2011 is as under.

Sl.No. -    Indian State      - Capacity (MW)   

 1.       Andhra Pradesh        -    11.50

2.       Gujarat                      -  251.00

3.        Karnataka                 -   93.30

4.        Madhya Pradesh        -   54.20

5.        Maharashtra               -  150.80

6.        Rajasthan                   -  226.35

7.         Tamilnadu                 -  624.30

             Total                        - 1411.45

About 14 WTG manufacturers were involved in this capacity addition in various capacities, major being Suzlon Energy, Enercon, Regn Powertech, Gamesa, Vesta Wind, Inox Wind, Letner Shriram, Gobal, RRB Energy etc. Mjor addition is done by Suzlon Energy with 510.35 MW, Enercon added 315.20 MW, Regen Powertech added 199.50 MW, Gamesa with 165.75 MW and Vestas Wind with 86.10 MW.

This clearly shows the intensity of WTG capacity addition going on India which will continue in the later par of the financial year as well.

Despite the financial crisis affecting the western world, Indian Renewable Energy Sector shows strong growth which may be the reason for major forign players investing in India along with the domestic WTG manufacturers.

Wednesday, December 07, 2011

JNNSM Phase I Batch 2 -Solar Power reaching the Grid Parity in India

This was a fantastic news for all of us involved in Grid based Large Solar photovoltaic Projects. Jawaharlal Nehru National Solar Mission (JNNSM) is bringing Indian solar sector to the new heights. The turning point is that for the first time Solar Photovoltaic Power in India is closing to the grid parity with the lowest bid being Rs.7.49 ($ 0.146) per kilowatt hour (/kWh). I could sense the enthusiasm when I visited the SOLARCON conference at Hyderabad, where the speculations were there that this time the bids will be still coming down from the previous one. Lot of US and European based companies had set up booths at SOLARCON like Solar Semiconductor, Azure Power, BERGEN Power, SunEdison etc.

Aggressive bidding has taken place at the JNNSM Phase 1 Batch II. A very aggressive bidding round was seen at the Scope Complex Auditorium for the Batch II of Phase 1 of the Jawaharlal Nehru National Solar Mission (JNNSN). Solar Direct Emerges was the lowest bidder at Rs.7.49/kWh, while Green Infra was the highest bidder at Rs.9.39/kWh.

Other developers such as Welspun - it quoted three projects at Rs.7.97, Rs.8.05 and Rs.8.14/kWh; SunEdison at Rs.9.28/kWh; Mahindra Bids at Rs.9.34/kWh; Sai Sudhir at Rs.8.22/kWh; VS Lignite at Rs.8.54/kWh; and Sunborne Energy at Rs.8.99/kWh.

Meanwhile, Punj Lloyd offered no disount this time. Inderpreet Wadhwa led Azure Power and quoted a very aggressive price of Rs.7.91/kWh (50 megawatts); Sujana Energy at 9.09/kWh; and Kiran Energy quoted Rs.9.34/kWh for a 50 MW project. Green Infra emrged as the highest bidder, having quoted Rs.9.39/kWh.

This has set a trend on the one side making solar energy closer than ever to grid parity while on the other it has presented a big challenge for these projects to achieve financial closure and prove viability.

This bidding for the Solar PV project allotment under the second round of JNNSM phase I and the results from the bidding has shown signs of solar becoming genuinely cost competitive with grid power. While sub 10 bids were obviously expected, the bid price falling below 8 was indeed amazing for many developers and experts.

The Rs 10.59/kWh bid during the first round of bidding during the previous year, resulted in serious discussion over whether the developers are trying to pull wool over the eyes of the general public and more importantly the lender community.
Similar to the previous year results, the new numbers that were out yesterday too has resulted in a lot of debate. With severe penalty clauses in the second round this year and considerable learning during the last year, the discount that could be offered was never expected to cross 50% but all the assumptions and predictions are here to be bulldozed in the solar PV industry. Though the numbers might seem unreasonable at first sight, if one gives a proper thought over it, we would understand that these developers had every reason to submit such lower quote.

As earlier said, the French company Solaire Direct created shock in the industry with an unimaginable Rs7.49/kWh which was just close to grid parity.

Solardirect is the second largest solar power company in France. The company has quoted the so called ‘rock bottom’ price possibly as a part of market entry strategy as they have big plans of entering and growing in the Indian market. The latest report on photovoltaic support schemes released by the French government has caused haywire in the industry since the government threatens to severely curtail incentives including a potential annual cap of 500 MW. The French company has a pressing need to expand their market presence and India being one of the most promising market, the company had shown serious interest in the country with an anticipated target of 25 MW in the year 2011-2012. Also, the company has now saved loads of money which they would otherwise have spent on marketing. They have managed to hit all the headlines and gained extremely significant visibility in the country and all without any phenomenal marketing investments. All the revenues saved could come handy when they implement and operate the historic Rs 7.49/kWh project. The calculative risk and the low bid price is all a part of the market entry strategy.

Welspun Solar, with three different bid prices emerged second only to Solairedirect. Three different price levels and a 50MW clean sweep – and there is definitely some serious thought process behind their high discount bids. For Welspun, solar is of strategic importance and they are well on their way to become the bid daddy of Solar PV power generation sector in India. Project allotments in the first round of JNNSM phase I and projects from Gujarat state policy would have given them very good learning’s and confidence to go all out to secure 50MW and do not forget, they are on the fray to get projects allotted in Karnataka policy too. With past experience (something which is a rarity in the niche solar PV sector in India), the company definitely would have working relationships with EPC companies, module makers, inverter and other BoS manufacturers, lenders etc. The past experiences and associations comes in handy not only for Welspun, but a similar situation exist for a few other renowned solar PV developers like Azure Power, Mahindra Solar, Kiran Energy, Sai Sudhir Energy, Sun Edison India, Green Infra and Sun Borne Energy. All these companies have made the early mover advantage to their fullest benefit and managed to get projects allotted by quoting competitive yet viable prices.

There can be a number of arguments to justify the low bid prices, but we have to wait and see that how these companies are achieving financial closures with these lower tariffs.

Export-Import Bank of America, OPIC will be one of the sought after destinations for many of the victorious developers other than Azure Power. It is however surprising to know that two other well known developers – Punj Lloyd and Acme Solar who were EXIM bank beneficiaries in their previous projects, did not quote competitive prices to emerge victorious.

Deadline for achieving financial closure in the second round of JNNSM phase I has been raised to 210 days (7 months) from the earlier 180 days (6 months). The timeline for the commissioning of the project is also extended by a month – to 13 months from the date of signing PPA from 12 months earlier in batch I. With one more month of additional time coming in as a cushion for the victorious developers, one would have to wait and watch the actions that are to unfold in the days to come.

Now the challenge is to accomplish the projects as per the schedules after signing the PPAs. Of course, most of these developers can benefit from their previous experience in India in setting up Solar Photovoltaic Projects. I am sure that Solar will overtake Wind Energy sector in India if this enthusiasm and spirit prevails which can replace several tons of carbon dioxide otherwise would have produced with fossil fuel based power plants.

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