Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-06-29
2001-06-19
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06248461
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to an electric power generating system including a cell such as a fuel cell for outputting direct current power, and a conversion device having at least either an inverter for converting the direct-current power into alternating-current power or a converter for changing a voltage level of the direct-current power. More particularly, the invention relates to the art of improving energy efficiency of such electric power generating system.
2. Description of the Related Art
As such electric power generating system noted above, there is known a fuel cell electric generating system including a reformer for obtaining hydrogen from raw fuel material such as natural gas, a fuel cell for generating direct-current power through a reversed electrolysis of water between the hydrogen obtained by the reformer and oxygen obtained from air, with exhaust of high-temperature heat, and an exhaust heat recovery equipment for recovering thermal energy of the heat generated from the fuel cell.
Such fuel cell electric power generating system is an environmentally friendly electric power generating system due to its zero SOx emission, extremely low NOx emission, low CO
2
emission, low generation of noises and vibrations. The system provides the further advantage of effective energy utilization of about 40% electric power generating efficiency, and about 85% total energy efficiency combining the electric power generating efficiency and the heat recovery efficiency. For these reasons, interest in the fuel cell electric power generating system as a new promising energy source has been ever increasing in recent years.
The inverter mentioned above generally comprises an electric circuit using diodes, transistors or the like which are formed of silicon semiconductor. Hence, the upper limit of the operating temperature of the inverter is determined by the upper limit of the operating temperature of the silicon semiconductor. The ambient temperature for rated operation of silicon semiconductor is generally 25° C. And, the upper limit of the ambient temperature of the silicon semiconductor for general consumer use is 55° C.
Incidentally, the fuel cell generates high-temperature exhaust heat in association with the chemical reaction for the electric power generation. Then, for effective utilization of this heat, the exhaust-heat recovery equipment is provided, as described above, for recovering the heat energy in the form of hot water or vapor. On the other hand, the operating temperature of the fuel cell is 60° C. to 120° C. if it is the solid high molecular type, 170° C. to 220° C. if it is the phosphoric acid type, 650° C. if it is the molten carbon salt type, and even 1000° C. if it is the solid electrolyte type, so that the fuel cell generates such high-temperature heat.
However, as the exhaust heat from the fuel cell normally has the temperature higher than 60° C., the inverter cannot operate normally at such high temperature, hence leading to malfunction of the entire system. For enabling the inverter to operate at least at an ambient temperature below 60° C., the prior art has proposed to shield the inverter from the high temperature or cool it and also to provide means for reducing the high-temperature thermal energy within the exhaust-heat recovery equipment. Such cooling is believed to be responsible for about 5% reduction in the total energy efficiency.
The present invention has been made in order to solve the above-described shortcoming of the convention. A primary object of the invention is to provide improvement in the energy efficiency of such electric power generating system.
SUMMARY OF THE INVENTION
For accomplishing the above-noted object, according to one aspect of the present invention, an electric power generating system comprises a cell for outputting direct-current power, and a conversion device having at least either an inverter for converting the direct-current power into alternating-current power or a converter for changing a voltage level of the direct-current power, wherein the cell generates high-temperature thermal energy of 60° C. or higher in association with the generation of the direct-current power, and the conversion device comprises an element capable of operating at an ambient temperature of 55° C. or higher.
With the above feature, the conversion device can operate at a high ambient temperature of 55° C. or higher. Hence, the high-temperature thermal energy of 60° C. or higher may be utilized with higher efficiency, without being cooled unnecessarily. Consequently, the total energy efficiency combining the electric power generating efficiency and the heat recovery efficiency may be improved. Further, as the insulation and/or cooling of the conversion device may be simplified or even eliminated, it becomes possible to dispose this conversion device in the vicinity of the cell or the exhaust-heat recovery equipment. As a result, greater integration and compactness of the entire generating system may be achieved also.
According to a further aspect of the present invention, an electric power generating system comprises a cell for outputting direct-current power, and a conversion device having at least either an inverter for converting the direct-current power into alternating rent power or a converter for changing a voltage level of the direct-current power, wherein the conversion device comprises a semiconductor having a wider band gap than Si.
With above construction, the wider the band gap the semiconductor element has, the higher the temperature the element can operate, as will be detailed later. Therefore, the upper limit of the operating temperature of the semiconductor element which determines the operating temperature of the conversion device may be extended in comparison with the conventional conversion device using Si semiconductor. Consequently, in this case too, the high-temperature thermal energy of 60° C. or higher may be utilized with higher efficiency, without being cooled unnecessarily. And, the total energy efficiency combining the electric power generating efficiency and the heat recovery efficiency may be improved accordingly.
In addition to the above-described feature, preferably, the conversion device comprises a GaAs semiconductor element, an InP semiconductor element, an SiC semiconductor element, a GaN semiconductor element, or any combinations thereof. The GaAs semiconductor element, InP semiconductor element, SiC semiconductor element, and GaN semiconductor element are all capable of operating at an ambient temperature of 55° C. or higher. Then, the conversion device using such element is capable of operating at a high temperature of 60° C. or higher. So that, the total energy efficiency of the entire system may be improved
Explanation in qualitative terms will be given next on the operability of the GaAs semiconductor element, the InP semiconductor element, the SiC semiconductor element, or the GaN semiconductor element at a high temperature of 55° C. or higher.
A semiconductor has an energy-band structure consisting of a conduction band and a valence band which are separated from each other. And, the semiconductor exhibits its semiconducting property according to a value Eg of its band gap between the conduction band and the valence band, thus functioning as a diode or a transistor. However, the band gap Eg of most of semiconductors becomes narrower with rise in the temperature. And, this temperature variation can be approximated by the following mathematical expression (I), where t is a temperature and a is a coefficient of the temperature variation.
dEg/dt=−a×
10
−4
(
eV/K
) (I)
Accordingly, with rise in the temperature, the band gap Eg becomes smaller, i.e. narrower. Also, with application of significant thermal energy thereto, thermal excitation of free electrons from the valence band to the conduction band will readily occur. As the result, the semiconductor loses its semiconducting property, thus being disabled to function no
Abe Masayuki
Matsuo Hiroshi
Kalafut Stephen
Kansai Research Institute
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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