Energy of Hydrogen

Scientists suggest using photolysis as a method to turn sunlight into useful fuel. If this could be realized, water photolysis would provide a pathway for efficient solar energy. Penn State researchers have a proof-of-concept device that can split water and produce recoverable hydrogen. “This is a proof-of-concept system that is very inefficient. But ultimately, catalytic systems with 10 to 15 percent solar conversion efficiency might be achievable,” says Thomas E. Mallouk, the DuPont Professor of Materials Chemistry and Physics.

Although solar cells can now produce electricity from visible light at efficiencies of greater than 10 percent, solar hydrogen cells have been limited by the poor spectral response of the semiconductors used. In principle, molecular light absorbers can use more of the visible spectrum in a process that is mimetic of natural photosynthesis. Photosynthesis uses chlorophyll and other dye molecules to absorb visible light

Mallouk and W. Justin Youngblood, postdoctoral fellow in chemistry, together with collaborators at Arizona State University developed a catalyst system that, combined with a dye, can mimic the electron transfer and water oxidation processes that occur in plants during photosynthesis.

The key to their process is a tiny complex of molecules with a center catalyst molecules surrounded by dye molecules which absorbs sunlight. When visible light strikes the dye, the energy excites electrons in the dye, which, with the help of the catalyst, can split the water molecule, creating free oxygen. These clusters are about 2 nanometers in diameter can cycle through the water oxidation reaction about 50 times per second. That is comparable to the turnover rate of Photosystem II in green plant photosynthesis. Photosystem II is the protein complex in plants that oxidizes water and starts the photosynthetic process. The water splitting requires 1.23 volts, and the current experimental configuration cannot quite achieve that level so the researchers add about 0.3 volts from an outside source. Their current system achieves an efficiency of about 0.3 percent.

Hydrogen Solar Ltd, the UK-based hydrogen production research and development company has developed a break-through Tandem Cell technology for producing high purity hydrogen by photolysis.

The Tandem Cell™ consists of two photo-catalytic cells in series: the front cell absorbs the high energy ultraviolet and blue light in sunlight, using nano-crystalline metal oxide thin films to generate electron-hole pairs. This cell does not generate enough voltage to split the water, so the electrons are connected to the back cell. The longer wavelength green and red light passes through the front cell and is absorbed in the back dye-solar cell producing electrical potential under nearly all light conditions. The two cells are connected electrically and together provide the potential required to split the water molecules in the electrolyte. This is not the arrangement used in real modules, but illustrates the principle, if this arrangement were used, a transparent membrane would have to be placed in the water cell to separate the hydrogen and oxygen.  No external electricity is required.

The key to the Tandem Cell™ is the performance of the metal oxides in reacting to the photons of the incident light. The metal oxides are expected to be the limiting feature of Tandem Cell™ efficiency. One of the benefits of Hydrogen Solar’s Tandem Cells is that they are inexpensive. The Cell is fabricated from widely-available and cheap materials, and as a result, the hydrogen production is competitive. On the small scale, producing hydrogen at one third the cost than from PV solar panel-electrolysis systems.  On the large scale it is about twice the cost of steam reforming with natural gas.

Tandem Cell array is capable of charging a domestic refueling station for hydrogen vehicles.  An 7m x 7m Tandem Cell unit, with 10% efficiency, covering double garage will produce enough hydrogen to fuel a production hydrogen vehicle for 11,000 miles over a year in Los Angeles light conditions.  Arrays placed on domestic rooftops or incorporated into industrial buildings will eliminate the transportation costs for the hydrogen.

The above narrative represents the companies views on their process. The researchers have a variety of approaches to improve the technologies oh photolysis. They can amend photolysis processes if investigate improving the efficiency of the dye, improving the catalyst and adjusting the general geometry of the system.

Resources:

EurekAlert

Hydrogen Solar Ltd, Guildford, UK

Fossil fuels are finite and increase in cost as supply dwindles. So the huge interest to the development of alternative energy souces occurs throughout the world. Consequently, battery-electric and fuel cells vehicles are on the road today. In 2007, there were 1.8 million alternative fuel vehicles sold in the United States, indicating an increasing popularity of alternative fuels. There is growing perceived economic and political need for the development of alternative fuel sources.

I think it’s appropriate to compare Battery-electric and fuel cells vehicles in this conversation. Firstly, let’s consider advantages of alternative fuel vehicles over vehicles with internal combustion engines.

· It is obvious that Battery-electric and fuel cells vehicles emit no pollutants. Battery-electric cars and fuel cell equivalents are each propelled by electric motors; however fuel cell vehicles create their own electricity.

· All internal combustion engines have efficiencies limited by the Carnot Cycle. Batteries and Fuel cell vehicles, not limited by the Carnot Cycle so it is very possible to achieve high energy efficiencies.

· Also they operate with electric motors which have very few moving parts, vehicle vibrations and noise are vastly reduced and routine maintenance (oil changes, spark plug replacement) are eliminated.

Above-listed advantages are typical for both classes of alternative fuel vehicles. In order to feel the difference let’s consider existing cars. Tesla Roadster will represent Battery-electric cars and Suzuki SX4 FCV will represent fuel cell cars.

Tesla Roadster is a lightweight (~1140 kg) two-seat sports car with 300-hp, 280 lb-ft of torque, a 244-mile range on a single 3.5-hour charge and 3.9-sec 0 to 60  mph time. Tesla’s cars run on a huge lithium-ion battery pack that can be recharged by plugging an adapter cord into a wall socket. Storage battery located behind seats is quite unusual: It’s a block consisting of 6831 lithium batteries which are identical with batteries used in cell phones and laptops. Weight of this block is 454 kg. There is a three-phased electric motor between rear wheels. Its size is impressive — diameter is 25 centimeters and length is 35 centimeters. Its efficiency is about 80-95 % depending on loading.

During braking kinetic energy isn’t spent in vain but recaptured by regenerative braking system to recharge the batteries.

The company estimates costs for trip approximately in one cent per mile. However, the company has calculated that the owner of a battery-electric car will have to replace (alas, for considerable money) the lithium-ionic accumulator, served its time, through 160-200 thousand kilometres. Nowadays the only problem of the high cost of such effective and high-capacious lithium-ion accumulators should be solved.

Overview of Tesla Roadster

*100% electrical
* 3.9-sec 0 to 60 mph, and single-speed gearbox couples the low drag and fuel efficiency of a manual transmission with the driving ease of an automatic.
* 300+ hp
* 244 mile driving range
* cost of journey is cent per mile.
*coefficient of efficiency is about 90%.

The SX4-FCV five-door hatchback uses a GM-made high performance fuel cell, a Suzuki-developed 70 MPa (10,000 psi) compressed hydrogen tank and a light, compact capacitor. This recovers energy during braking application and uses it to reduce load consumption during acceleration.

For the Suzuki SX4 FCV, General Motors is supplying the 80kw fuel cell. The Suzuki SX4 FCV also comes with a 68 kw/91 hp electric motor, a 10,000 psi compressed hydrogen tank that will help the vehicle achieve a range of 150 miles and a top speed of around 95 mph.

It should be noted that automotive fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles)

Overview of SX4-FCV
Fuel-cell output: 80kW
Motor output: 68kW
Fuel: High-pressure hydrogen (stored in 70MPa tank)
High-voltage battery: Capacitor
Maximum speed: 95 mph
Driving range: 150 miles

So, Tesla Roadster and Suzuki SX4 FCV are excellent state-of-the-art vehicles. I hope some day we can see these cars everywhere. It’s up to you which type of alternative fuel vehicles to use. I take notice of the benefits of fuel cell vehicles are not in it’s efficiency (NiCad has a 70-90% charge/discharge efficiency) but in it’s lighter weight than batteries and the users ability to fill a tank rather than waiting to charge.

The flow of a reacting gas mixture in small channels is used in microchemical reactors to intensify the heat and mass transfer at chemical conversions. The advantages of small channels are the large specific surface of the reactor and small external diffusion resistance. This makes possible chemical conversions of gas mixtures at small residence times in the reaction zone and reactions under conditions of considerable thermodynamic nonequilibrium. Microchannel reactors are used, for instance, to convert hydrocarbons and alcohols into a synthesis gas and hydrogen and for a number of other energy-intense chemical  processes. The advantages of such reactors are most evident in the case of strongly exothermal or endothermal reactions; for instance, at partial oxidation of methane into a synthesis gas or steam reforming of methane. The considerable interest in these reactions is due to the fact that they are used in fuel processors to obtain a synthesis gas (hydrogen and carbon oxide) for fuel cells. In such processors, a catalyst is usually used to increase the reaction rate at low temperatures. The main reactions of methane steam reforming are the following:

CH4 + 2H2O = CO2+4H2     H298 K ° = 165 (kJ/mol);

CH4 + H2O = CO + 3H2        H298 K ° = 206 (kJ/mol).

These reactions are strongly endothermal, and an external heat supply is needed to sustain them. One thinks that this requirement prevents the development of methane vapor conversion reactors due to a decrease in their effectiveness. However, the use of the heat of the chemical reactions in the neighboring reactor channels proceeding in parallel can make steam reforming an attractive alternative of other methods to produce hydrogen.

The methane steam reforming is a complex multistage process including not only the exchange of reactants and reaction products between the gas phase and the reaction zone. It also consists of a series of reactions proceeding on the catalyst. Xu J. G. and Froment G. F. proposed a three-step mechanism of methane steam reforming, and above mentioned reactions are complemented by chemical conversions of their products:

CO + H2O = CO2 + H2      H298 K ° = –41.2 (kJ/mol).

A peculiarity of methane steam reforming is the need to supply heat to the reaction zone. At small heat flux density, the methane conversion degree is small and increases with increasing heat flux density. It has been found that the steam reforming reaction is more intensive at the reactor inlet and a heat supply is most effective at the reactor inlet. The heat supply at the reactor outlet mostly heats the gas mixture, thereby sustaining endothermal reactions. Thus, not only the external heat flux density but also the way it is supplied along the channel length affects the degree of methane conversion. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for producing a synthesis gas.

rather more details http://www.springerlink.com/content/j634065qq85w1064/