Clean Technology : Solar technologies


Solar Cells for Reducing HMF to Valuable Platform Chemicals

UW–Madison researchers have developed a high yield method for reducing HMF to valuable furan alcohols including BHMF.

The new method uses electrochemical cells (ECs) or solar-powered photoelectrochemical cells (PECs) to drive the reduction reaction. The cells feature cost-effective catalytic electrodes made of silver film on copper. The reaction takes place at ambient temperature and pressure using water as the hydrogen source.

The process also can be used to produce linear ketones such as 5-MF (5-methylfurfural) using a zinc catalyst.

Solar Cells Turn HMF to Valuable Platform Molecules

UW–Madison researchers have developed a new method using solar cells to electrochemically oxidize HMF to highly prized furan compounds, specifically FDCA (2,5-furandicarboxylic acid) and DFF (2,5-diformylfuran). These important compounds are used to produce polymer materials, pharmaceuticals, antifungal agents, organic conductors and much more.

The reaction takes place at ambient temperature and pressure using a TEMPO mediator. Unlike previous methods, the process does not require a precious metal catalyst.

More Efficient Water-Splitting Cells

UW–Madison researchers have developed a method for synthesizing nanoporous BiVO4 electrodes with large surface areas. The material is made up of a porous network of BiVO4 particles smaller than 150 nm and coated with oxygen evolution catalyst. The small size of the particles addresses prior drawbacks by increasing a property called electron–hole separation yield. The material is made by applying a vanadium solution to a type of bismuth crystal. The mixture is heated and converted into a porous network of BiVO4 particles.

Visible-Range Sunlight Drives CO2 Reduction Process for Cheaper Syngas

UW–Madison researchers have developed a new method of reducing CO2 to CO via a reverse water gas shift reaction using visible solar light. The reaction produces a syngas mixture which can be further converted to liquid fuels.

In this process, CO2 (which can be obtained from many industrial processes) is contacted with a plasmonic catalyst in the presence of hydrogen. The catalyst is exposed to visible-range sunlight so that it undergoes an optical phenomenon called surface plasmon resonance, which causes metal electrons to oscillate in a certain way and accelerates the rate of CO2 reduction.

The process results in CO2 being reduced to water and CO that can be collected for downstream products.

Zinc Oxide Nanowires for Photovoltaics and More

UW–Madison researchers have developed a process for synthesizing chloride- or fluoride-doped ZnO nanowires. The process involves growing nanowires from seed crystals in an aqueous solution. They can be grown on a wide variety of substrates including non-electrically conductive substrates, flexible plastic substrates and fibrous substrates.

Prolonging Solar Cell Life

UW–Madison researchers have developed a simple technique to open the ion channel blockages that cause non-liquid DSSCs to lose efficiency over time.

In the technique, a power source is connected to a solar cell’s electrodes to apply pulses of voltage and provide forward electrical bias. This injects new electrons into the electrolyte and redistributes its ions, forcing the channel-blocking ions to move. The result is improved efficiency.

Solar Cells Track Sun

UW–Madison researchers have developed a passive solar tracking system utilizing materials that move in response to sunlight.

In the system, a solar cell panel is supported by flexible posts. The posts are made from a composite material, including a liquid crystal elastomer. This material has properties that cause it to contract and tilt when exposed to heat. To further exploit such properties, the material is embedded with carbon nanotubes that act as miniature heat sources, absorbing sunlight and giving off warmth.

Rapid, Controlled Growth of Doped Gallium Arsenide for Solar Cells

UW–Madison researchers have developed a new method for growing layers of carbon-doped GaAs using a haloalkane dopant and HVPE.

First, a substrate is exposed to a gas mixture composed of gallium, arsenic and a haloalkane dopant. Conditions in the reactor, including gas flow rate, growth temperature and timing, are controlled to grow layers of carbon-doped GaAs on the substrate via HVPE, and are adjustable to maximize dopant concentrations.

The process may be repeated to deposit additional layers. Also, a variety of haloalkanes based on bromine, chlorine and iodine can act as dopant. Using the new method, the concentration of carbon within the layer can be monitored and altered as needed.

Photovoltaic Capacitor for Direct Solar Energy Conversion and Storage

UW–Madison researchers have developed a two-electrode bio-inspired photovoltaic capacitor that can directly convert and store solar energy in a single structure. The device includes a transparent electrode and a second electrode disposed opposite from the transparent electrode. The structure features an electrolyte slurry containing semiconducting particles along with particles of low ionic diffusivity. This medium exhibits a combination of photovoltaic and ferroelectric properties. The slurry is sandwiched between the transparent electrode and a membrane of low ionic diffusivity adjacent to the negative electrode.

To harvest energy, incident photons excite the electrons within the semiconducting layer and holes in the electrode to generate electron-hole pairs via the photovoltaic effect of solar energy being absorbed. The electrons attract ions to the cathode electrode, creating a concentration gradient across the device. The device is charged using this process until a saturated electric potential difference is reached. The diffusion force of the ions and electric field are counter-balanced and maintain a stable electrical double layer across the two electrodes.

Improved Heat Transfer Fluid Helps Sun Drive Steam Power Plants

UW-Madison researchers have developed a system and method for transferring heat using a variable composition heat transfer fluid that remains liquid over a wide temperature range, up to 500 ºC or above. Typically, low molecular weight fluids stay fluid at low temperatures but evaporate quickly at high temperatures, while high molecular weight fluids stay liquid at high temperatures but solidify at lower temperatures. This new system and method utilizes a heat exchanging fluid comprised of a mixture of a high boiling point, high molecular weight fluid (H), and a low freezing point, low molecular weight fluid (L).

As the combined fluid mixture heats up during the heat exchange process, L evaporates and the vapor is collected in a tank and condensed back into the liquid phase, leaving the heated fluid comprised mostly of H. As the heated H is cooled after heat exchange, the liquid L is added back into the mixture to prevent H from solidifying as it cools. The high boiling point component of the mixture is useful in increasing the boiling point temperature of the heat transfer fluid and lowering the vapor pressure of the heat transfer fluid at high temperatures. The low freezing point component of the mixture is useful in lowering the freezing point temperature of the heat transfer fluid, ensuring that it does not solidify during the temperature cycle.

The system of the invention includes a vessel for containing the heat transfer fluid, a heat source, an outlet for removing some of L as temperature increases during the cycle and an inlet for re-adding L as temperature decreases during the cycle.

Using UV Light to Functionalize Metal Oxides for Use in Biosensors, Solar Cells and Other Devices

UW-Madison researchers have developed methods for functionalizing metal oxides, including TiO2, with organic molecules.  The methods use UV light to covalently bond linker precursors to the surface of a metal oxide.  Then other molecules, such as dye molecules or biomolecules like DNA, can be coupled to the linker precursors to further functionalize the metal oxides. 

The functionalized oxides may be used alone or as coatings on various substrates.  They can be incorporated into devices such as biosensors or dye-sensitized solar cells. 

These methods provide functionalized metal oxides with higher densities of organic molecules and greater thermal and chemical stability than functionalized metal oxides prepared using conventional methods.  In addition, these methods do not require high temperatures or an ultra high vacuum, and are simpler and more reproducible than conventional methods.

Efficient and Economical Graphite-Based Photovoltaic Cells

UW-Madison researcher have now used lithographically patterned, thick stacks of carbon nanoribbons as the building blocks of an efficient and economical photovoltaic cell. The basic design of the cell includes several spatially separated graphite nanoblocks, each composed of many layers of graphite. The graphite is nanopatterned to mimic vertically stacked nanoribbons, which bridge electrically conductive contacts.

The electronic properties of the patterned nanoribbons depend on the direction and width of the nanoribbons, much like the electronic properties of single-walled carbon nanotubes. In fact, a nanoribbon may be thought of as an unrolled nanotube. By incorporating nanoribbons with different widths into the photovoltaic cell, the cell can be designed to absorb efficiently across the solar spectrum. However, unlike nanotubes, when carbon nanoribbons of different sizes are put in contact with a metal lead, approximately the same potential is generated at all the contacts, regardless of nanoribbon size. As a result, nanoribbons and nanoribbon stacks don’t need to be sorted during the production process, making nanoribbon-based solar cells easier to design and manufacture than nanotube-based cells.

Carbon Nanotube Schottky Barrier Photovoltaic Device

UW-Madison researchers have developed a cost- and energy-efficient photovoltaic cell that uses carbon nanotubes as the photoconducting material. Unlike semiconductor materials, carbon nanotubes absorb different spectra of light depending on the diameter and chirality of each tube. A variety of nanotube sizes and chiralities can be used within a photovoltaic array to significantly increase the efficiency over current technologies.

The invention also includes a novel method of manufacturing the nanotube array. Normally, nanotubes are grown with a catalyst and preserved in a fluid, which the end user must go through several steps to remove. The nanotubes described here can be grown and then directly attached to the array surface.

Because large numbers of nanotubes are needed to generate current efficiently, they are attached in a dense, but random arrangement. After the nanotubes are deposited on the surface, the metallic contacts from which the current is gathered are applied in a uniform grid over the nanotubes.

Efficient Boost Rectifier Employing Half-Rated Semiconductor Devices

UW-Madison researchers have developed a half controlled rectifier that can deliver performance similar to that of a fully-rated, three-phase, pulse-width-modulated (PWM) rectifier, while allowing the ratings of the switches and diodes to be reduced to half the rated power. Because the semiconductors devices need only be rated for half of the rectifier’s peak current, they can be significantly less expensive than those used in conventional rectifiers. This rectifier also eliminates the typical problems seen in other half-controlled rectifiers, such as low-order even harmonics on both the AC and DC sides.