Materials For Energy

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).

Transparent Conductive Material Could Lead to Power-Generating Windows

(Brookhaven National Laboratory)

Scientists have fabricated transparent thin films capable of absorbing light and generating electric charge over a relatively large area. The material consists of a semiconducting polymer doped with carbon-rich fullerenes. Under carefully controlled conditions, the material self-assembles to form a reproducible pattern of micron-size hexagon-shaped cells over a relatively large area (up to several millimeters). This is the first such material blending semiconductors and fullerenes to be formed to absorb light and efficiently generate charge and charge separation. The material remains largely transparent because the polymer chains pack densely only at the edges of the hexagons, while remaining loosely packed and spread very thin across the centers.

The scientists fabricated the honeycomb thin films by creating a flow of micrometer-size water droplets across a thin layer of the polymer/fullerene blend solution. These water droplets self-assembled into large arrays within the polymer solution. As the solvent completely evaporated, the polymer formed a hexagonal honeycomb pattern over a large area

Credit: BNL and Chemistry of Materials

The scientists verified the uniformity of the honeycomb structure with various scanning probe and electron microscopy techniques, and tested the optical properties and charge generation at various parts of the honeycomb structure (edges, centers, and nodes where individual cells connect) using time-resolved confocal fluorescence microscopy. The scientists also found that the degree of polymer packing was determined by the rate of solvent evaporation, which in turn determines the rate of charge transport through the material.

In terms of practical applications, the material could be used to develop transparent solar panels or even windows that absorb solar energy to generate electricity.

Structural dynamics and charge transfer via complexation with fullerene in large area conjugated polymer honeycomb thin films
Chemistry of Materials, Article ASAP DOI: 10.1021/cm102160m
Publication Date (Web): November 1, 2010

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