Fuelcell Working – an overview

Source: http://www.esru.strath.ac.uk/

Fuel Cell System is a complex one including the interactions of mechanical, chemical, and electrochemical subsystems present, the stack being the heart of the system. The control of fuel cell systems under a variety of environmental conditions and over a wide operating range is a crucial factor in making them viable for extensive use in every-day technology. So, it is necessary to understand the heart of the system i.e. the fuel cell stack properly.

Fuel cells are chemical engines that generate electricity by converting the chemical potential of the fuel into electrical power. Since they are not based on temperature differences, they are not subjected to Carnot’s limit of efficiency. In addition, common pollutants such as sulfur dioxide and nitrous oxides are avoided since the process does not involve combustion. These advantages, together with the reduction of greenhouse gases and fuel consumption due to higher efficiencies and the possibility of alternative energy sources, have generated enormous interest in fuel cells for stationary as well as mobile applications.

A fuel cell is a class of galvanic cell based on oxidation-reduction reaction. The most basic system uses pure hydrogen as fuel, which is oxidized at the anode, producing electrons and protons:

Oxidation Reaction: H₂ –> 2H⁺ + 2e^-

The electrons are released to an external circuit, where they can be used to perform work, while the protons diffuse through an electrolyte to the cathode. At the cathode, oxygen reacts with the electrons from the external circuit and protons from the anode reaction, forming water:

Reduction Reaction: 0.5O₂+2H⁺+2e^- –> H2O

Therefore, the overall chemical reaction of the PEMFC is:

Overall Reaction: H₂+0.5 O₂-> H₂0+electricity+heat

Renewable Energy-the present state of affairs

As the name suggests, renewable energy means energy that can be renewed in the sense that the resource of energy is inexhaustible. It includes energy from such natural resources as wind, solar, hydel-power, tides, geothermal heat, bioenergy, wave energy, etc. In this article and those following this, we will discuss in details about these energy sources-both technical and non-technical.

Source: REN21-http://ren21.net/

Wind Energy:

Source: World Wind Energy Association-http://www.wwindea.org/home/index.php

One of the earliest resources of energy known to man, wind energy is coming to man’s rescue. In the year 2007, 19.696 MW of new wind energy capacity were added summing up to a global installed capacity of 93.849 MW by the end of December 2007. The added capacity equals a growth rate of 26.6 %, after 25.6 % in 2006. The currently installed wind power capacity generates 200 TWh per year; equaling 1.3 % of the global electricity consumption – in some countries and regions, wind energy already contributes 40 % and more. Total installed capacity has increased from 40 GW in 2003 to 93.85 GW in 2007. The following chart gives a country-wise installed capacity of a few countries.

Solar Energy:

Source: International Energy Agency: Solar Heating and Cooling Program-http://www.iea-shc.org/

Sun is an eternal source of energy. However, its potential as a source of energy is only being appreciated now. With an installed capacity of 70 GWth (as in 2001), solar thermal is one of the leading sources of renewable energy worldwide, and its potential is much higher. A number of solar thermal and solar photovoltaic power plants are coming up, which will generate hundreds of MWs of power. Solar energy can also be used in a variety of other applications such as cooking, heating, ventilation and air-conditioning, water heating, desalination and disinfection, lighting, etc. Presently the largest solar thermal power plant is in the Mojave Desert, California, with an installed capacity of 354 MW. The largest photovoltaic plant, however, is much smaller, having a capacity of 23 MW. Plants in excess of 500 MW capacities are under construction, thus promising solar energy a bright future.

Source: REN21-http://ren21.net/

Grid-connected solar photovoltaic (PV) continues to be the fastest-growing power generation technology in the world, with 50 percent annual increases in cumulative installed capacity in both 2006 and 2007, to an estimated 7.8 GW by the end of 2007. This capacity translates into an estimated 1.5 million homes with rooftop solar PV feeding into the grid worldwide.

Hydropower:

Hydropower includes all kinds of energy sources harnessing the power of running water. They are:

1) Hydroelectricity

2) Tidal power

3) Wave power

4) Micro-hydel power-both dammed and run-of-river

5) Ocean thermal Energy Conversion

Hydroelectricity is the most widely used form of renewable energy. Hydroelectricity supplies about 715,000 MW or about 19% of world electricity generation, accounting for over 63% of the total electricity from renewable sources in 2005.

The world’s first commercial wave farm is planned for Portugal, at the Aguçadora Wave Park near Povoa de Varzim. The wave farm will use three Pelamis P-750 machines with a capacity of 2.25 megawatts, enough to meet the average electricity demand of more than 1,500 Portuguese households.

Ocean thermal energy conversion (OTEC) is a method for generating electricity, which uses the temperature difference that exists between deep and shallow waters to run a heat engine. OTEC projects on the drawing board include a small plant for the U.S. Navy base on the British-administered island of Diego Garcia in the Indian Ocean. OCEES International, Inc. is working with the U.S. Navy on a design for a proposed 13 MW OTEC plant, which would replace the current power plant running diesel generators. The OTEC plant would also provide 1.25 MGD of potable water to the base. A private U.S. company also has proposed building at 10 MW OTEC plant on Guam.

During 2005, small hydro installations grew by 8% to raise the total world small hydro capacity to 66 GW. Over 50% of this was in China (with 38.5 GW), followed by Japan (3.5 GW) and the United States (3 GW). China plans to electrify a further 10,000 villages by 2010 under their China Village Electrification Program using renewable energy, including further investments in small hydro and photovoltaic.

Geothermal power:

Geothermal provides almost 10 GW of power capacity, growing at roughly 2–3 percent per year. Most of this is in Italy, Indonesia, Japan, Mexico, New Zealand, the Philippines, and the United States, with additional capacity in several other countries. Iceland gets a quarter of all its power from geothermal.

Bioenergy:

Source: REN21-http://www.ren21.net/

An estimated 45 GW of biomass power capacity existed in 2006. The United Kingdom has seen recent growth in “co-firing” (burning small shares of biomass in coal-fired power plants). The use of biomass for district heating and combined heat-and-power (CHP) has been expanding in Austria, Denmark, Finland, Sweden, and the Baltic countries, and now provides substantial shares (5–50 percent) of district heating fuel. Bioenergy also includes energy from ethanol and biodiesel. Production of fuel ethanol for vehicles reached 39 billion liters in 2006, an 18 percent increase from 2005. Biodiesel production jumped 50 percent in 2006, to over 6 billion liters globally.

Solar Cell Breakthrough and its Economic Feasibility

Solar Concentrator Innovation:

The goal of any solar concentrator is to concentrate the light that falls on a large area to a smaller one. Usually concentrators are large mirrors, lenses or other devices. The focused light increases the electrical power obtained from each solar cell “by a factor of over 40”.

MIT engineers report a new approach to harnessing the sun’s energy that windows can be used for this purpose. As a result, rather than covering a roof with expensive solar cells (the semiconductor devices that transform sunlight into electricity), the cells only need to be around the edges of a flat glass panel .Light is collected over a large area [like a window] and gathered, or concentrated, at the edges. Solar concentrators, in use today, track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain. Also solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators.

Mechanism of the Window Concentrator

The MIT solar concentrator involves a mixture of two or more dyes that is essentially painted onto a pane of glass or plastic. The dyes work together to absorb light across a range of wavelengths, which is then re-emitted at a different wavelength and transported across the pane to waiting solar cells at the edges. A mixture of dyes in specific ratios, applied only to the surface of the glass, that allows some level of control over light absorption and emission. The mixture was made so that light can travel a much longer distance and light transport losses can be reduced, resulting in a tenfold increase in the amount of power converted by the solar cells.

Economic Feasibility:

Because the system is simple to manufacture, the team believes that it could be implemented within three years–even added onto existing solar-panel systems to increase their efficiency by 50 percent for minimal additional cost. That, in turn, would substantially reduce the cost of solar electricity.

Continuous Improvement in Solar Cell Efficiency:

Most of today’s solar cells are between 12% and 18% efficient. Some of the ones used to power satellites are around 28% efficient. In 1954, 4% efficiency was state of the art. In 2006 Boeing-Spectrolab managed to create a solar cell with 40.7% sunlight-to-energy conversion efficiency. Recently, just after two years scientists at the US Department of Energy’s National Renewable Energy Laboratory have developed solar cells with efficiency 40.8%. These cells are basically multi-junction solar cells and composed of several layers, with each slice capturing only a portion of the solar spectrum; this method of optical concentration is what has allowed cells to surpass the 12% to 18% efficiency barrier faced by most traditional modules.

References:

(1) http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=196602149

(2) http://web.mit.edu/newsoffice/2008/solarcells-0710.html