Solar Cell, from Past to Present

The recorded development of solar cell technology begins with the 1839 research of French experimental physicist Antoine-Edmond Becquerel. At the age of nineteen he discovered the photovoltaic effect while experimenting with an electrolytic cell containing two metal electrodes. He found that certain metals and solutions would produce small amounts of electric current when exposed to light. In 1883 Charles Fritts formed photovoltaic junctions by coating selenium with an extremely thin layer of gold. Russell Ohl invented the first silicon solar cell in 1941.

The era of modern solar cell technology began in 1954, when G. L. Pearson, D. Shapin and C. Fuller demonstrated a silicon solar cell capable of 6% energy conversion efficiency with direct sunlight. The first gallium arsenide (GaAs) solar cell was reported in 1956, with a photoconversion efficiency of 6.5%. In 1976 Carlson and Wronksi reported solar cells comprised of amorphous silicon.

Modern multi-junction solar cells can be viewed as a series of p-n junction photodiodes, each of different bandgap, that commonly include such III-V or II-VI materials as gallium arsenide (GaAs), gallium indium phosphide (GaInP), copper indium diselenide (CIS), copper indium- allium diselenide (CIGS), and cadmium telluride (CdTe).

In 1987, Jerry Olson reported a two-junction tandem photovoltaic device consisting of an upper GaInP layer and lower GaAs layer, with a photoconversion efficiency up to 29.5% under concentrated solar light. The addition of a third junction further increases the conversion efficiency, to 34% for a GaInP/GaAs/Ge solar cell, to 40% for a GaInP/GaAs/GaInAs cell. It is believed that the photoconversion efficiency of multi- (or many) junction solar cells can be increased up to 55%.

For example, highly mis-matched alloys such as Zn(1-y)Mn(y)O(x)Te(1.x) have shown utility in the high performance high dollar solar cell markets, such as in space satellites, where dollars are no issue but high photoconversion efficiency is. Unfortunately issues of cost limit application of solar cell technology in the ‘real’ world. The solar cell market continues to be dominated by silicon, the fabrication of which is energy-intensive requiring a manufacturing energy input equal to several years of energy output of the solar device.

Furthermore modest device efficiencies correspond to large land area requirements to meet the intrinsic energy demands of modern society. Generally speaking, as of today the cost of energy from solar cells is ˜ five times that produced by the burning of fossil fuels. However since fossil fuels are freely provided by nature, and still so cheap as to be commonly treated as free (and not treated as an irreplaceable precious commodity), the factor of five looks pretty modest.

Hydrogen Internal Combustion Engine

The volumetric energy density of H2 is less than that of gasoline. Therefore, to provide the same driving range, the hydrogen fuel tank needs to be three times the size of a gasoline tank. Today, a typical passenger car has a range of 575 miles and is provided with an 18-gallon tank, whereas an 18-wheeled semitruck has a 750 miles driving range and requires two 90-gallon tanks.

Actually, the volume of the hydrogen tanks can be somewhat smaller than three times because the efficiencies of hydrogen IC and fuel cell engines are better than the efficiency of gasoline engines (gasoline, 25%; hydrogen IC, 38%; and hydrogen fuel cell, 45–60%).

BMW, DaimlerChrysler, GM, Honda, and Toyota are in the process of placing both IC and fuel cell units into the hands of ordinary drivers to gain experience and to collect data. Their prototype units cost about $1 million each. The manufacturers aim for a “pilot commercialization phase” by 2010–2012 at a unit cost of $250,000. They expect full production by 2013 at a unit cost of $50,000, and this cost will drop as the volume of production increases.

The list of vehicles that can run on H2 is constantly growing. Quantum Fuel Technologies Worldwide converted Toyota Priuses to hydrogen fuel. BMW is marketing its 7 Series, 12-cylinder, 260-horsepower car with an IC engine that can burn liquid hydrogen or run on gasoline, whereas the BMW 750 hL is designed to burn liquid hydrogen. The IC engine of the Ford E-450 shuttle bus burns 5,000 psig hydrogen gas.

In connection with using H2 as a fuel for transportation, there is a lot of activity, but no firm direction or conclusion yet. In Iceland, one can rent a hydrogen-fueled car from Hertz. In Japan, as part of its national hydrogen program, a 200,000 m3 tanker ship has been designed for transporting H2. Also in Japan, an H2-fueled commuter train is in operation, using H2 at 35 mPa (5,000 psig or 350 bar) to fuel a 125 kW ”Forza” proton exchange membrane (PEM) fuel cell by Nuvera (http://www.rtri.or.jp).

Hydrogen buses operate in Montreal and Bavaria, an H2-powered passenger ship sails in Italy, and the 2008 Olympics in Beijing featured hydrogen vehicles. Russia has flown a jet, fueled partly by hydrogen. In the United States, the Defense Advanced Research Project Agency (DARPA), NASA, and the Air Force are jointly developing an Earth-orbit airplane fueled by

H2. Two teams (in Turin and Madrid) are converting two light planes so that they can use hybrid fuel cell–battery electric engines.

Hydrogen Production from Biomass

Hydrogen is the most environmentally friendly fuel that can be efficiently used for power generation. When oxidized in a fuel cell, it produces steam as the only emission. At present, however, hydrogen is produced almost entirely from fossil fuels such as natural gas, naphtha, and inexpensive coal. In such processes, the same amount of CO2 as that formed from combustion of those fuels is released during the hydrogen production stage.

Renewable biomass is an attractive alternative to fossil feedstocks because of the potential for essentially zero net CO2 impact. Unfortunately, hydrogen content in biomass is only 6-6.5% compared to almost 25% in natural gas. For this reason, on a cost basis, producing hydrogen by the biomass gasification/water-gas shift process cannot compete with the well-developed technology for steam reforming of natural gas. However, an integrated process, in which part of the biomass is used to produce more valuable materials or chemicals and only residual fractions are used to generate hydrogen, can be an economically viable option.

Biomass-based processes for the production of hydrogen can be either thermochemical or biological and can produce this clean carrier directly or through an intermediate, storable product. Also, the use of coproducts has to be addressed to improve the process economics and in view of the sustainability of using this natural resource.

Thermochemical biomass processing has two attractive features: (1) it is omnivoric, that is, a very broad range of biomass feedstock can be completely converted and (2) it can be integrated with fossil-based infrastructure for large-scale production of synthesis gas that is existing and foreseen to be biobased in the medium- to long-term future.