13-07-2012, 04:50 PM
FUEL CELL SYSTEM COUPLED FOR A PORTABLE COMPUTING DEVICE
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INTRODUCTION
A Fuel Cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as its by-product.
Since conversion of the fuel to energy takes place via an electrochemical process, not combustion. It is a clean, quiet and highly efficient process- two to three times more efficient than fuel burning.
These are generally related to sys¬tems that use fuel cells to provide electrical power.The fuel cell system which is designed to both provide electrical power to and receive electrical power from a portable computing device.
Many country's depend on fossil fuels. These problems have led to the use of renewable energy sources. For example, the Electronic Product .Environmental Assessment Tool (EPEAT) is pres¬ently used to produce data that helps consumers evaluate the environmental friendliness of electronic products. Moreover, the EPEAT score for an electronic product can be increased by providing a renewable energy source for the product. As a consequence of this increased consumer awareness, electronics manufacturers have become interested in developing renewable energy sources for their products, and they have been exploring a number of promis¬ing renewable energy sources such as the hydrogen fuel which is used in hydrogen fuel cells.
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied.
There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety of sizes. Individual fuel cells produce very small amounts of electricity, about 0.7 volts, so cells are "stacked", or placed in series or parallel circuits, to increase the voltage and current output to meet an application’s power generation requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.
Hydrogen fuel cells have a number of advantages. Such fuel cells and associated fuels can potentially achieve high volumetric and gravimetric energy densities, which can potentially enable continued operation of portable electronic devices for days or even weeks without refueling. However, it is extremely challeng¬ing to design hydrogen fuel cell systems which are suffi¬ciently portable and cost-effective to be used with portable electronic devices.
The design of a fuel cell system which is capable of both providing power to and receiving power from a rechargeable battery in a portable computing device. This eliminates the need for a bulky and heavy battery within the fuel cell system, which can signifi¬cantly reduce the size, weight and cost of the fuel cell system. This fuel cell system includes a fuel cell stack which converts fuel into electrical power. It also includes a controller which controls operation of the fuel cell system.
The fuel cell system also includes a power link that transfers electrical power between the fuel cell system and the portable comput-ing device, and a communication link that provides commu¬nication between the portable computing device and the con¬troller for the fuel cell system.
The controller can regulate both the electrical power provided by the fuel cell system to the portable computing device and the electrical power pro¬vided by the rechargeable battery to the fuel cell system. While regulating the electri¬cal power provided by the rechargeable battery to the fuel cell system, the controller can monitor an operational parameter of the fuel cell stack during a boot-up process of the fuel cell system, and subsequently regulate a discharging current pro¬vided by the rechargeable battery to the fuel cell stack based on the value of the operational parameter.
While regulating the dis¬charging current based on the value of the operational param¬eter, the controller determines if the value of the operational parameter is less than a set-point value. If so , the controller maintains the discharging current provided by the recharge¬able battery to the fuel cell stack. Otherwise, the controller terminates the discharging current provided by the recharge¬able battery to the fuel cell stack. The operational parameter is a hydrogen pressure within the fuel cell stack.
The fuel cell system does not have an internal rechargeable battery. While regulating the electri¬cal power provided by the fuel cell system to the portable computing device, the controller can monitor an operational parameter of the fuel cell stack and regulate a charging cur-rent provided by the fuel cell stack to the portable computing device based on the value of the operational parameter.
While regulating the charg¬ing current based on the value of the operational parameter, the controller can communicate with a charging circuit in the portable computing device, wherein the charging circuit is configured to charge the rechargeable battery based on the charging current. while regulating the charg¬ing current based on the value of the operational parameter, the controller first determines whether the value of the opera¬tional parameter is greater than or smaller than a set-point value.
The value of the operational parameter is greater than the set-point value , the controller regulates the charging cur¬rent so that the value of the operational parameter decreases.
If the value of the operational parameter is greater than the set-point value, the controller can continue regulating the charging current until the value of the operational parameter decreases to the set-point value. On the other hand, if the value of the operational parameter is smaller than the set-point value, the controller can continue regulating the charging current until the value of the opera¬tional parameter increases to the set-point value.
If the value of the operational parameter is substantially equal to the set-point value, the controller can maintain the charging current so that the value of the operational parameter remains the same.
When regulating the charging current to decrease/increase the value of the operational parameter, the controller can transmit a second control signal to the charging circuit in the portable computing device. The first/second control signal causes the charging circuit to decrease the charging speed for the rechargeable battery, which subsequently causes the charging current to decrease and the value of the operational parameter to decrease/increase.
The controller transmits the first and the second control signals through the communication link. The controller routes the charging current through the power link. The charging circuit converts the charging current into a charging voltage suitable for charging the rechargeable battery.