Materials For Energy

EQ Energy Sector Analysis: Variable States

Corinna Wu
Feature Editor: Paulina Jaramillo

(Article in Energy Quarterly (EQ) in the March 2011 issue of MRS Bulletin)

On August 14, 2003, a downed high-voltage power line in Ohio triggered a massive blackout across eight states in the Northeast and throughout Southeastern Canada. A total of 50 million people lost power for up to two days. The episode cost an estimated $6 billion and is considered to be the worst electrical system failure in the history of the United States.

The cascade of failures—due to both human error and equipment malfunction—made clear the problems of the United States’ aging and balkanized electrical transmission grid. The blackout spurred the U.S. Congress to pass the Energy Policy Act of 2005, which enforces mandatory reliability standards. Investment in the transmission grid had fallen during the 1980s and 1990s, but after the blackout, funding picked up again.

Against this backdrop, power companies and regulating authorities now have another challenge to face: how to integrate variable and intermittent renewable resources such as solar and wind into the mix. Currently, electricity from non-hydro renewable resources accounts for 3.8% of the total electricity produced in the United States, according to the U.S. Energy Information Administration. In 2009, half of non-hydro renewable electricity came from wind and less than one percent from solar.

Although wind and solar are attractive because they do not produce greenhouse gases, they also pose difficulties because of their inherent variability. If wind and solar must supply a third of the energy generated, the job of meeting changing demand for electricity throughout the day becomes much more difficult. Power plant operators must meet the net load—the difference between user demand and this highly variable renewable output—by scheduling power generation from conventional sources.

An Energy Sector Analysis article published in the Energy Quarterly section in the March 2011 issue of  MRS Bulletin discusses various options to manage net load variability in the electric grid, with a focus on energy storage technologies. For materials scientists, a lot of potential research for grid-level energy storage lies in developing advanced battery technologies. Superconducting magnetic energy storage (SMES) systems are another option and have been used since the 1970s, mostly to release short bursts of energy to smooth out fluctuations in the power supply.

_________________________________
Gopal R. Rao
Editor, MRS Bulletin
Materials Research Society (MRS)

 

Battery turns entropy into electricity

(Chemistry World)

When fresh water rivers flow into the sea the concentration difference leads to a change in entropy.  Researchers have developed a battery that generates power from this entropy difference. They were able to extract energy with 74 per cent efficiency using manganese dioxide nanorods and silver electrodes.

While entropy-based power generation has been done before but is most reliably done today by separating fresh and seawater with membranes and as ions travel through the membranes they generate currents. The new method extracts energy from the difference in concentration between two solutions by storing it chemically in batteries.

The battery extracts energy through sodium and chlorine ions' movements into and out of the crystal lattice of the electrodes. The battery discharges in seawater as chlorine is taken up by the silver electrode and sodium is taken up by the manganese dioxide electrode. The ions are released when the battery charges in freshwater. Because of the higher ion concentration in seawater, the electrical energy discharged is greater than that needed for the battery to charge in freshwater.

Credit: ACS/Nano Letters

The researchers estimate that if the technology was used in all of the world's rivers, this renewable energy technology would hypothetically generate 2 TW, or approximately 13 per cent of current global consumption.

Source: Battery turns entropy into electricity

Batteries for Efficient Energy Extraction from a Water Salinity Difference
Nano Lett., Article ASAP, Publication Date (Web): March 17, 2011
DOI: 10.1021/nl200500s

 

_________________________________
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

 

Flexible batteries based on graphene paper

(Chemistry World)

To fabricate roll-up displays and wearable devices, the power source that supplies them must also become more flexible.  The major challenge of developing a truly bendable power source has been the shortage of materials that are both highly flexible and have superior electronic conductivity. Polymers are typically used, but they are susceptible to degradation at relatively low temperatures.

Researchers have now developed a graphene-based hybrid electrode producing a flexible lithium rechargeable battery. The cathode material, in this case V₂O₅, is grown on graphene paper using pulsed laser deposition and graphene paper coated in lithium is used as the anode. The resultant battery is lightweight and flexible enough to be twisted or rolled.

Credit: Energy Environ. Sci.

The performance of the new battery appears to be promising. The electrode was shown to exhibit significantly improved electrochemical performance in almost all aspects of electrochemical properties, such as higher energy density, power density and better cycle life, compared with non-flexible conventional electrodes, according to the authors.

(Source: Bendy batteries a step closer)

Reference:
Flexible energy storage devices based on graphene paper
Energy Environ. Sci., 2011, DOI: 10.1039/c0ee00640h

 _________________________________
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

Learning from Energy Initiatives Conducted in Different Regions of the World

The second issue of Energy Quarterly has been published in the December 2010 MRS Bulletin. This month the editorial by Steve M. Yalisove emphasizes the importance of Regional Initiatives, those “far-reaching experiments in sustainable energy” performed in some region of the world, with lessons to be learned by other regions of the world as they set forth to find their own solutions. Energy Quarterly now has presented two such experiments:

• September 2010: How Sunlight Became a Commodity in Germany and
• December 2010: Maintaining Brazil’s ethanol fuel momentum.

Sometimes, such as in the case of Germany scaling up solar energy, it is a question of political will. Germany might not be thought of as the sun center of the world, yet they are producing more than a third of the world’s photovoltaic power. Incentives for individuals to install solar cells on their homes and sell energy back to the utilities moved this program forward. Scaling up solar in Germany also opened a market for thin-film solar cells produced in the United States.

In other cases, local resources play a significant role, such as Brazil’s use of ethanol from their plentiful sugarcane crops. Now Brazil not only produces a large fraction of their automobile fuel domestically, they export ethanol to other countries. With an established ethanol industry, will catalysts and other materials developments make possible further experiments with cellulosic ethanol or other second-generation fuels?

These and other regional initiatives, such as nuclear power in France, wind in Denmark, and Desertec to transport renewable energy from deserts to population hubs, make up a useful set of experiments to examine economic, public policy, and engineering challenges in the energy arena. Energy Quarterly’s approach is to present the underlying science behind the engineering challenges and identify the materials issues that may offer useful solutions. What other lessons can be learned?

 

_________________________________
Elizabeth L. Fleischer
Editor, MRS Bulletin
Materials Research Society (MRS)

Solid-State Thermoelectric Generation: Capturing Waste Heat

By David Nemir, Jan Beck, Ed Rubio, and Manuel Alvarado—TXL Group, Inc.

Solid-state thermoelectric (TE) generation is a heat to electricity conversion technology that requires no moving parts. It is unique because it can be applied to both small- and large-scale waste heat harvesting, and it can produce electrical energy from relatively small temperature gradients at relatively low operating temperatures. In fact, in many settings, thermoelectric generation represents the only means to capture value from a small ΔT temperature gradient. Thermoelectric phenomena arise out of the intercoupled electrical and thermal currents in a material. A two-element thermoelectric generator is depicted in Figure 1. It is constructed by connecting an n-type thermo-element and a p-type thermo-element in electrical series, with both elements in thermal parallel between a heat source and a heat sink. The electrical series connection is made by attaching conductors as shown. Ideally, these series connection elements will be both good thermal conductors and good electrical conductors.

 The key components of a thermoelectric device are the n-type and p-type thermo-elements, which are the active portions of the device that do the actual conversion of heat energy to electrical energy. Thermoelectric devices are generally formed by connecting many pairs (couples) of n- and p-type thermoelectric elements in electrical series and in thermal parallel. The open circuit voltage that is generated in a given thermo-element is proportional to the temperature difference, ΔT, across that thermo-element. The constant of proportionality is the Seebeck coefficient or thermo-power, S, yielding an equation for generated voltage, ΔV, of

                                            (1)

Besides the thermo-power, S, two other materials parameters of interest when analyzing a thermoelectric material are the electrical conductivity, σ, and the thermal conductivity, λ, which are important when analyzing losses in a thermoelectric device. The three key material properties governing thermoelectric performance are often lumped into a single thermoelectric figure of merit Z:  

                                              (2)

As the parameters σ, S, and λ are temperature-dependent, Z will also be a function of temperature. Often the thermoelectric efficiency of a device.is characterized by the dimensionless figure of merit ZT, where T is the temperature in Kelvin. Currently, the standard material for high ZT at room temperature is undoped bismuth telluride (Bi₂Te₃), with a ZT at 300 K of approximately 0.6. A review of typical state-of-the-art ZT values is shown in Figure 2.

 

Figure 2. ZT values for SOA p-type (right) and n-type (left) thermoelectric materials. ^{Download Design and Discovey}

 

            Thermoelectric generators generate electrical power from the heat energy flux that passes through the thermo-elements. Heat energy is delivered by the heat source, passes through the thermo-element, and is then carried away by the heat sink. Critical components of any thermoelectric device are the heat delivery to one side and heat removal from the other. So, it is a natural idea to add a thermoelectric generation capability to a device (or application) that is already set up to optimize heat delivery to one side and heat removal from the other. Such devices may be called boilers, condensers, recuperators, and radiators, but all fall under the generic name “heat exchangers.”

            As a generic description, heat exchangers move heat energy flux from one fluid to another through an intermediate surface. That surface can be a plate, a tube, or a wall. Commercial application proposals have targeted the application of a thermoelectric generation structure to the wall of a heat exchanger, with a particular focus on applications in which the heat exchanger is already needed and for which electric generation can occur as a bonus to the job of heat exchange. Adding thermoelectric generation to a heat exchanger may not be practical as a retrofit. It will have an impact on heat transfer and may require larger surface areas to accomplish a given heat transfer job.

 

Reference

1. J.-P. Fleurial, Download Design and Discovey, Jet Propulsion Laboratory/California Institute of Technology, 1993.

 

 

 

 

Graphene—the ultimate ultracapacitor?

By Timo A. Lähde, Helsinki Institute of Physics / Department of Applied Physics

The isolation of graphene, a single atom thick layer of graphite, in 2004¹ has propelled the study of two-dimensional electron systems to the forefront of theoretical as well as technological progress. This development has recently culminated in the award of the 2010 Nobel Prize to Andre Geim and Konstantin Novoselov of the University of Manchester, UK, for “groundbreaking experiments regarding the two-dimensional material graphene.” While the unusual electronic properties of graphene are exciting in their own right,² graphene is also having a major impact in the industrial sector, with applications being developed from ultra-sensitive nanomechanical resonators to transparent electrodes. Recently, graphene has become a hot topic in “macroscopic” applications as well, notably in the design and manufacture of ultracapacitors, which have recently become highly valued as power storage systems in electrical light-rail vehicles, diesel-electric battery drive systems, and forklift trucks.

Unlike traditional capacitors, ultracapacitors make use of the “electrical double layer” effect, whereby charge separation is achieved on a length scale of a few nanometers at the interface between the capacitor plates and an electrolyte.³ The “plates” in commercially available ultracapacitors are formed of nanoporous, sponge-like substances, such as activated charcoal. These maximize the surface area exposed to the electrolyte, thereby achieving capacitances thousands of times larger than parallel-plate or electrolyte capacitors. To be specific, the energy densities of current ultracapacitors reach ~10 Wh/kg, whereas lithium-ion batteries feature ~160 Wh/kg. In comparison, a typical gasoline-driven automobile operates at ~ 2400 Wh/kg. While ultracapacitors feature very high charge and discharge rates, little degradation over hundreds of thousands of recharge cycles and low toxicity of component materials, the amount of energy stored per unit weight is considerably lower than that of an electrochemical battery. As a result, finding improved materials that would rectify this situation is a big priority.

Recently, the group led by Rodney Ruoff of the University of Texas at Austin has demonstrated prototypical ultracapacitors with energy densities up to ~100 Wh/kg⁴ using a novel material, derived from exfoliated graphite oxide and dubbed “chemically modified graphene” (CMG). This research is driven by the high conductivity and extraordinarily high specific surface area of graphene, which reaches ~2630 m²/g, about the area of a football field in 1/500th of a pound of carbon. Graphene thus enables the storage of an exceptional level of charge by allowing a superior number of positive and negative ions to form.

The possibilities opened by graphene go beyond improvements in the energy density of ultracapacitors, as the team of John Miller, president of JME, an electrochemical capacitor company based in Shaker Heights, Ohio, and Ron Outlaw, of the College of William & Mary, Williamsburg, VA, have recently shown.⁵ By using electrodes made from vertically oriented graphene nanosheets, the team of Miller was able to improve drastically on the RC time constant of extant ultracapacitors, opening up new possibilities for the miniaturization of AC filtering and rectifier circuits. Other ultracapacitors fail in this respect, as the porous electrodes act like resistors in filter circuits. The graphene nanosheets resemble ~600 nm tall “potato chips” standing on edge in rows. The novel ultracapacitors charge and recharge in ~200 microseconds, compared to ~1 second for nanopore-based designs.

These exciting developments indicate that graphene-based ultracapacitors are poised to take a further step in the “graphene revolution,” which has been ongoing since the groundbreaking work of Geim and Novoselov in 2004. However, graphene is also facing stiff competition from a host of other revolutionary materials, such as carbon nanotubes, aerogels, and conductive polymers. The adventure of carbon-based, environmentally friendly energy storage is likely just beginning.

1. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306, 666 (2004).
2. A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81, 109 (2009).
3. J.R. Miller, P. Simon, The Electrochemical Society Interface 17, 31 (2008).
4. M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8, 3498 (2008).
5. J.R. Miller, R.A. Outlaw, B.C. Holloway, Science 329, 1637 (2010).

Hybrid Nanoparticle-Ionic Liquid Electrolyte for Better Batteries

(Chemistry World)

Batteries containing lithium as the ion source can store the highest known density of energy, but the most commonly used electrolytes do not work well in these batteries as they are unstable in typical operating conditions and may break down disastrously. 

Other electrolytes, including polymers and ionic liquids (ILs), have been explored. While polymer electrolytes generally have low conductivity at room temperature, ILs are salts in a liquid state and thus have good conductivity and stability under the battery operating conditions. They are, however, unable to prevent the formation of lithium networks called dendrites, which degrade battery performance. 

A research team has now developed a new family of solventless electrolytes made of hard zirconium oxide nanostructures linked to softer IL species, using a simple one-pot procedure. The electrolytes, which are a gel-like fluid at room temperature, are temperature- and redox-stable which makes them suitable for use in batteries, they transport lithium ions well and they also prevent the growth of dendrites.

Credit: Angew. Chem. Int. Ed. 

Ionic Liquid Tethered Nanoparticle Hybrid Electrolytes
Angewandte Chemie International Edition, 2010. DOI: 10.1002/anie.201004551

 

_________________________________
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

Nanoscale ion diffusion behavior in Li-ion battery revealed

(Oak Ridge National Laboratory/Nature Nanotechnology)

Credit: Kalinin et al., Oak Ridge National Laboratory

A research team has developed a new type of scanning probe microscopy called electrochemical strain microscopy (ESM) to examine the movement of lithium ions through a battery's cathode material. The method can provide a detailed picture of ionic motion in nanometer volumes, which exceeds state-of-the-art electrochemical techniques by six to seven orders of magnitude. They achieved the results by applying voltage with an ESM probe to the surface of the battery's layered cathode. By measuring the corresponding electrochemical strain, or volume change, the team was able to visualize how lithium ions flowed through the material. Conventional electrochemical techniques, which analyze electric current instead of strain, do not work on a nanoscale level because the electrochemical currents are too small to measure. These are the first measurements of lithium ion flow at this spatial resolution, according to the authors. The team's ESM imaging can display features such as individual grains, grain clusters and defects within the cathode material. The high-resolution mapping showed, for example, that the lithium ion flow can concentrate along grain boundaries, which could lead to cracking and battery failure.

Nanoscale mapping of ion diffusion in a lithium-ion battery cathode
Nature Nanotechnology Published online: 29 August 2010. DOI:10.1038/nnano.2010.174

_________________________________
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

Flexible paper Li-ion batteries fabricated

(Chemical & Engineering News and ACS Nano)

Credit: Yi Cui, Stanford University

Ultrathin rechargeable lithium-ion batteries have been fabricated on a single sheet of paper, resulting in highly flexible and lightweight portable power sources. Researchers coated a solid support with a thin film of carbon nanotubes and deposited a film of a metal-containing lithium compound on top of the nanotubes. Then the team deposited the double-layer films on both sides of ordinary paper. The lithium layers function as battery electrodes and the nanotube films serve as current collectors. The paper is the electrode separator and also serves as a mechanical support. The new batteries, which are just 300 μm thick, are thinner and more flexible, and exhibit higher energy density and other electrical advantages, compared to other types of thin batteries, according to the researchers.

Thin, Flexible Secondary Li-Ion Paper Batteries
ACS Nano, Article ASAP September 13, 2010. DOI: 10.1021/nn1018158

_________________________________
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

Energy and Expansion: The Tata story

By V.S. Arunachalam and Gopal R. Rao

 

Tata is a household name in India. Starting well over a century ago, the Tatas began their foray into industry, and the brand represents more than a diversified conglomerate. The Tatas are well-respected, having endeared themselves in every walk of Indian life. Their businesses are all-pervasive, from building automobiles and generating electric power to making steel and building supercomputers. Their charitable trusts and endowments are as extensive as their businesses. They have founded and funded outstanding educational institutions and human welfare organizations, and they have supported research on alleviating human suffering. The incumbent of the Tata Group is Ratan Naval Tata, a Cornell University graduate in architecture. In the 20 years since he took over the mantle, he has set the Tata industries on a steep growth trajectory, increasing the revenue of the Tata industries 12-fold, making automobiles—the famed Nano—available at very affordable prices in India, and introducing efficient steelmaking. The business empire he heads, the Tata Group, has over 90 companies with footholds in 80 countries.

In the midst of his busy globe-trotting schedule, we managed to steal an hour of his time at the Tatas’ “Bombay House” headquarters in Mumbai (formerly known as Bombay), India, for him to tell us how he sees the global energy challenges and the opportunities they create. A condensed version of the interview now appears in the first Energy Quarterly section of the September 2010 issue of MRS Bulletin. The interview reveals the breadth of Ratan Tata's knowledge on energy and materials, in line with the vast business interests of the Tatas. 

V.S. Arunachalam
Chair, Energy Quarterly Organizing Committee

Gopal R. Rao
Web Science Editor, Materials Research Society