Electric power conversion systems – Current conversion – Cryogenic
Reexamination Certificate
2000-06-13
2003-04-08
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
Cryogenic
C174S043000, C174S047000, C307S011000, C307S013000
Reexamination Certificate
active
06545880
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to the field of electrical power and energy distribution and conversion in large structures such as high-rise buildings, office complexes, factories, ships, large airplanes, etc. providing a higher efficiency than the existing system thus saving expensive energy.
It is believed that “related background art” does not exist. To the best of the inventors knowledge nobody has described or built an energy/power distribution system based on the integration. Background art in itself is presented by the existing energy/power distribution system based on 480 VAC/120 VAC distribution copper/iron-steel transformers and corresponding copper or aluminum power lines and cables.
Thirty years ago, the equivalent of today's desktop computer filled an entire office room, and yet had only a fraction of its computing power. This tremendous reduction in size, weight, energy consumption, and especially cost has not yet occurred in the field of power electronics, energy distribution, power conversion and control. The reason for this is that the silicon technology that led the computer industry to revolutionize the world has not yet been seriously applied to the copper-and-iron realm of power engineering. However, this is now a possibility through the application of two new technologies: Cryogenic Power Conversion (CPC) using Low-Temperature Operated Semiconductor Devices (LOTOS) and High-Temperature Superconductors (HTS). The basic research for CPC has been done already and awaits implementation.
The concept of Cryogenic Power Conversion is based on the fact that the conduction losses of power MOSFET switches (1) and the switching losses of IGBTs, MTOs, Thyristors, GTOs, etc. (2) are reduced by cooling the devices to the temperature of liquid nitrogen (77K). The first statement is confirmed by the on-resistance measurements of
FIG. 1
for the MOSFET APT10026JN (1000 V, 33 A, 0.26 &OHgr;). The area between the 300 K and 77 K curves represent the Cryo-Gain which is even larger taking into account the higher junction temperatures (400-425 K) in actual operation.
FIG. 2
shows the typical temperature dependence of the same conduction-loss producing parameter. The improvement factor for the on-resistance reduction between 375 K and 77 K is F=35, certainly an impressive number.
The physics behind this effect is the drastic increase at low temperatures of the majority carrier electron mobility in the drain-drift region of a high-voltage n-channel power MOSFET. MOSFETs are the fastest switching power devices available (1). Even further improvements, i.e. further reductions in on-resistance, are possible using the new Cool-MOS power devices developed by Siemens (3-5) and recently also by International Rectifier Corporation. Cryo-MOSFETs are best for applications below 1000 V. In minority carrier devices such as IGBTs, IGCTs, IEGTs, MTOs, MCTs, etc. for the higher voltage range (1 kV-6 kV) (6) charge storage limits the switching speed. These charges are proportional to the minority carrier lifetime which in turn is drastically reduced by cryo-cooling thus reducing switching times, and herewith also the switching losses considerably.
FIG. 3
shows how the resistive (100 &OHgr; load) turn-off time of a 1700 V IGBT is reduced from 288 ns at 300 K to 39 ns at 77 K when operated at 1200 V.
FIG. 4
shows the concept of Cryogenic Power Conversion. The power dissipation of a high power circuit is reduced at the source by cryo-cooling thus permitting a drastic size, weight and cost reduction by eliminating big heatsinks, etc.
The U.S. Department of Energy (DOE) is already funding several efforts to develop high-temperature superconducting cables. An added benefit of the proposed system is that only relatively short cable lengths are required, and these do not have to be designed for high-voltage (>100 kV) applications. Thus, the HTS industry finds a market for immediate implementation, and the entire system can be realized in a period of a few years for any new high-rise building or other large structure to be constructed. One can assume that sooner or later, such lossless cables will be available for applications in Cryogenic Energy Distribution. HTS cables will solve the key problem of CPC and the Cryogenic Energy Distribution System (CEDS), the cryo-cooling for the distributed cryo-power electronics by providing a cryogenic fluid such as liquid nitrogen. Cooling with liquid gases such as LN2 is the only practical solution for HTS cables today. Also, even if it should turn out that HTS cables are too expensive (7), the CED system could be implemented using high-purity liquid-nitrogen-cooled cooper or aluminum cables, whose DC resistance drops by about the same factor (~×7-8) that applies to LN2 generation (7-10 W/W). This improvement factor is much higher in very pure, but more expensive copper and aluminum cables.
Another advantage of cryogenic operation is the drastic reduction of the thermal conductivity of silicon and the usual substrate materials such as beryllium-oxide, etc.
In summary, it is believed that the prior art does not interfere with this disclosure. None of the referenced patents teaches the intricate combination of high-temperature superconducting cables with high-efficiency cryo-cooled silicon power electronics in a large-scale system. In addition, prior art represented by conventional transformers is totally different from the new technology proposed in this patent application. The conventional electrical power distribution system is basically about hundred years old, is not very efficient and did not yet profit from the many new technologies such as high-temperature superconductors and the whole semiconductor technology with its tremendous potential for size, weight and cost reduction at increased reliability.
SUMMARY OF THE INVENTION
The object of this invention is to provide an electrical power/energy distribution system which is more efficient than the existing one by using two new technologies thus saving a considerable amount of energy if implemented on a large scale in big cities. It will also reduce global warming.
The existing electric distribution infrastructure has remained unchanged for almost a century. A key component of this system is the so-called “distribution transformer” converting 480 VAC, 60 Hz into 120/240 VAC, 60 Hz. This transformer has a relatively low efficiency of ~95% and has been called by experts the “weak link” in the US energy distribution system. Accordingly, this invention introduces two new technologies to change this situation providing the benefits of higher efficiency, reduced energy consumption, and in addition power load shedding, voltage regulation, improved power quality, power factor correction and control. Additional benefits are possible and will be discussed.
The new Cryogenic Energy and Power Distribution (CED) system according to this invention is shown in
FIG. 5
based on the application of the two new technologies of High-Temperature Superconductivity (HTS) and Cryogenic Power Electronics (CPE). The CED system combines the following concepts, features and configurations:
A central high-temperature superconducting DC cable (650 VDC or higher) cooled by liquid nitrogen (LN2, 77 K) is installed and extends from the top floor to the basement of the high-rise (or any other) building through its center (FIG.
5
).
HTS Cable Power Distribution: High-Temperature Superconducting (HTS) DC cables supply electrical power and the cooling medium—liquid nitrogen—to the main power loads. The use of direct current permits a doubling of the transmitted energy per wire cross-section and the elimination of AC losses in the HTS cables, thereby allowing for reduced cable weight. The DC voltage is 650 VDC (480 VAC rectified), 2×325 VDC or any other suitable voltage (1.2 kV, 2.4 kV).
Cryogenic Power Conversion (CPC): The conventional heavy and bulky copper/iron core transformers (for example 460 VAC/120 VAC, 75 kVA: 500 pound, 1-2 square meter footprint) are replaced by ultra-small, lightw
Cooper Leonard
Laxton Gary L.
Sherry Michael
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