The Hydrogen Solar Ltd Tandem Cell™ is a self-contained unit which directly splits water molecules into high-purity hydrogen and oxygen using solar energy. It significantly reduces carbon emissions by eliminating the fossil fuels normally used in electrolysis or steam reforming to produce hydrogen.
Now for an explanation of their system. A flow cell, through which the water electrolyte passes, is in the front of the stack, the light passes through this cell to the Tandem Cell™ which consists of two photo-catalytic cells in series: the front cell, coated with nano-crystalline metal oxide thin films, absorbs the ultraviolet and blue light from sunlight and oxygen is formed on the surface. This cell does not generate enough voltage to split the water, so the electrons are connected to the back cell. The longer wave-length green and red light pass through the front cell and are absorbed in the back dye-solar cell, increasing the potential of the electrons, which then flow to the hydrogen cathode where the hydrogen is formed. 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 process is renewable, produces no carbon dioxide or other emissions.
The cells are made from low cost materials. The most expensive material used is the glass that the cells are made from. The thin film semiconductors are iron or tungsten based rather than more expensive silicon materials. The process is now competitive, on the small scale, producing hydrogen at one third the cost than from PV solar panel-electrolysis systems. On the large scale it was, mid 2004, about twice the cost of steam reforming with natural gas.
In October 2003 Hydrogen Solar and the Carbon Trust funded a 16 month, project to scale up the Tandem Cell technology from previous university research to a viable product. The project will undertake research and product development using nanotechnology processes to further raise the performance of the Tandem Cell.
In December 2003, a 200,000 grant was received from the UK Department of Trade and Industry's (DTI) New and Renewable Energy Programme to conduct twenty four months of extensive research into its Tandem Cell™ technology. The goal of this project was to achieve cells with 10% efficiency, with a “stretch target” of 15%, achievable with cost-effective manufacturing processes.
In August 2004, Hydrogen Solar announced it had doubled the efficiency of the units, converting more than 8% of sunlight energy directly into Hydrogen. Central to the increased performance is the use of nano-crystalline coatings within the Tandem Cell™. Using novel metal-oxides, the coatings produce high photocurrent densities, and create a highly efficient means of converting light and water into hydrogen fuel from one single unit.
In November 2004 Hydrogen Solar announced that it was awarded US$400,000 by the University of Nevada Las Vegas Research Foundation for the first year of a multi-year project to demonstrate solar hydrogen production using Tandem Cells™ in conjunction with vehicle refueling. The project involves joint research and development on a hydrogen refueling station in Las Vegas, Nevada, USA.
In October 2005 Hydrogen Solar received a grant from the BOC Foundation, for the development and demonstration of scalable hydrogen production using the Tandem Cell™ array at Beacon Energy Ltd. In the first phase of the project, Hydrogen Solar will design, build and install a 100 m2 array of Tandem Cells to generate high-purity hydrogen. The BOC Group will provide engineering safety expertise, compression units and engineering support during the installation. The second phase will be to demonstrate the Tandem Cell array over a six month period at the Beacon Energy Ltd site at West Beacon Farm, Leicestershire. The system will provide hydrogen for fuel cell applications, to provide electricity for recharging electric vehicles, and for heat and power for various buildings.
Early applications of the Tandem Cell™ will be for on site farm production, for refineries, for vehicle refueling and in supplemental energy for industrial and domestic buildings. During 2005 they expect to demonstrate a Tandem Cell array capable of charging a domestic refueling station for hydrogen vehicles. An 7m x 7m array of cells, with 10% efficiency, covering a standard southern California double garage will produce enough hydrogen to fuel a production hydrogen vehicle for 11,000 miles of driving per year. Arrays placed on domestic rooftops or incorporated into industrial buildings will eliminate the transportation costs for the hydrogen.
The US Department of Energy has awarded very large contracts to GE Global Research, University of CA-Santa Barbara, MVSystems, Inc. and Midwest Optoelectronics to work on similar projects, which I will investigate in the future for comparison.
The above narrative represents the companies views on their process. The company has made a lot of progress in the last two years, but I think that this system still has a ways to go before commercialization succeeds. Comparing their cost to PV-electrolysis may not be the fairest comparison, what about wind-electrolysis or concentrating solar power-electrolysis? The DOE contractors seem worried about corrosion of the semiconductors. Do these guys know something they don't? The increasing cost of fossil fuels is certainly makes their process more competitive.
Hydrogen Solar Ltd, Guildford, UK
"Direct Photoelectrochemical Production of Hydrogen", NREL Solar-Hydrogen Workshop, 11/9/04