Energy of Hydrogen

Archive for the ‘Hydrogen production’ Category

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

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/

Since hydrogen doesn’t exist on earth as a gas, we must separate it from other elements. We can separate hydrogen atoms from water, biomass, or natural gas molecules. Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via biological production (scientists have even discovered that some algae and bacteria give off hydrogen), by electrolysis, by chemical reduction or by thermolysis.Hydrogen can be produced at large central facilities or at small plants for local use. Every region of the country (and the world) has some resource that can be used to make hydrogen. Its flexibility is one of its main advantages.

• Steam reforming is currently the least expensive method of producing hydrogen. The production of hydrogen by reforming is a well-established process with large-scale production (330,000 kg hydrogen/day) and large facilities that produce hydrogen at costs that approach the DOE target ($2.00−$3.00/gge) (gge is gasoline gallon equivalent on an energy basis). Steam methane reforming accounts for 80% of the hydrogen produced. The remaining 20% is a by-product of chemical processes such as chlor-alkali production.
• Electrolysis is a process that splits hydrogen from water. It results in no emissions but it is currently a very expensive process. New technologies are being developed all the time. Water electrolysis represents only a niche segment of the merchant hydrogen market. There are two ambient temperature electrolysis processes for producing hydrogen: alkaline electrolysis, which uses concentrated potassium hydroxide (KOH) as the electrolyte, and PEM
  electrolysis, which uses the ionomer Nafion™ as the electrolyte. The membrane for PEM electrolysis is similar to that used in PEM fuel cell. Alkaline electrolysis stacks can be either monopolar or bipolar; PEM electrolysis stacks are bipolar.
• Coal gasification breaks down the coal into smaller molecular weight molecules, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide (CO) and hydrogen (H₂).

Today, approximately 9 million tons (~9 billion kg) of hydrogen are produced annually. More than 95% of the merchant hydrogen is captive for industrial applications―chemical, metals, electronics, and space. The cost of hydrogen produced safely and efficiently from on-site hydrogen generators must be lowered enough to be competitive with gasoline on a cost per mile driven basis, without adverse environmental impacts. Today the cost of high-volume hydrogen production and delivery is two to three times the DOE target of $2.00−$3.00/gge untaxed.

This figure depicts the reductions in hydrogen production costs that need to be achieved for distributed steam methane reforming and electrolysis to be competitive with gasoline.