Patent Application
20110024150
The subject matter disclosed herein generally relates to thermal management and particularly to thermal management of current carrying conductors.
Power distribution in a high current environment requires current flow from a power supply to various components, for example, drive systems, motors, electrical loads, amplifiers, rectifiers, routers, servers, etc. Among the more common methods used to supply power are heavy gauge wire and cable, switchgears, circuit boards, and bus bars.
Typically, power distribution has involved one or more heavy copper bus bars that are provided with connectors or holes for connecting cables. Bus bars might be spaced apart from each other and isolated by insulating spacers. Large copper or aluminum bus bars and cables have been used to distribute power within industrial control systems. Such bus bars are large and can carry high power relatively easily. Traditionally, bus bars cooling techniques involved circulating air within a cabinet to cool the bus bars. In systems requiring isolation, bus bars are located remotely and coupled via cables to other components. However, cables are capable of handling lesser power compared to the bus bars. Efficient power distribution systems require higher operating current densities such that more power is distributed within the system. Additionally, increasing the power density through the bus bars has challenges such as airflow and ventilation, vibration, noise, and efficient use of space.
It would be desirable to provide a cooling system for conductors such as bus bars that increase the current carrying capacity of the bus bar, while reducing its size, thereby saving space and weight.
Briefly, a cooling mechanism for a current carrying conductor is proposed. The mechanism includes a first layer having plurality of micro fluidic channels. The first layer is thermally coupled to the current carrying conductor and configured to exchange thermal energy. A micro-pump is configured to circulate a heat exchange fluid through the micro fluidic channels to exchange thermal energy with the first layer and remove heat from the current carrying conductor. The heat exchange fluid and the current carrying conductor are electrically isolated.
In another embodiment, a heat exchanger to cool a current carrying conductor is presented. The heat exchanger includes a heat exchange layer thermally coupled to the current carrying conductor and having one of more micro-channels. A fluid path is defined within the micro-channels to transmit a heat exchange fluid. Plurality of air voids is disposed around the micro-channels in thermal communication with the fluid path. A self-regulating pump is configured to circulate the heat exchange fluid through the fluid path.
In another embodiment, a method of cooling a current carrying conductor is presented. The method include coupling a heat exchange layer around the current carrying conductor, the heat exchange layer thermally coupled to the current carrying conductor and electrically isolated from the current carrying conductor. The method further includes providing a fluid path via plurality of micro-fluidic channels defined in the heat exchange layer having open porous structure and circulating a heat exchange fluid having a phase changing material within the fluid path. The method further includes cooling heat from the current carrying conductor by thermal exchange through the heat exchange layer and cooling the heat exchange layer via circulating the heat exchange fluid in micro-fluidic channels coupled to air voids.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Typically, factors that limit a current carrying capacity of the bus bar are temperature rise that increases the resistance (due to heat) from the flow of electrical energy. Such increased temperatures and increase in resistance may lead to decreased current carrying capacity in the bus bars. Traditional approach to cool the bus bars include natural cooling, forced cooling techniques such as blowing air to achieve higher current density across the cross-section of the bus bar conductor. Other known techniques implement heat sink structures to dissipate heat by natural convection or forced convection. However, such approaches have marginal effect on the current carrying capacity. Further, such cooling methods require major modification when implemented in presently operational systems. Systems employing forced liquid cooling employ a conventional vapor compression refrigeration cycle or bulky cooling apparatus such as evaporator, condenser, or compressor. Certain embodiments of the invention disclose a retro-fit cooling mechanism 22 for bus bars, than needs minimal changes in the system design, and enable an increase in the current carrying capacity of the bus bar for a given cross-section and temperature rise.
In one embodiment the heat exchange fluid is configured for non-contact type circulation wherein the heat exchange fluid and the current carrying conductor are isolated. However, heat exchange fluid in certain embodiment, may be designed for contact type circulation wherein the dielectric breakdown voltage of the heat exchange fluid is configured to be higher than operating voltage to the conductor 20. A phase change material may be included within the heat exchange fluid. The phase change material include for example, inorganic or organic salts and the mixture forming a colloidal solution or slurry. Further the heat exchange fluid is configured to provide good surface contact with the first layer that may be hydrophobic or hydrophilic.
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In an exemplary operation, the current carrying conductor 20 carrying high amperage current result in generation of heat. As discussed above, if not cooled appropriately, the conductor 20 may develop excess heat that in turn reduces the current carrying capacity. In one embodiment, the first layer 24 absorbs the thermal energy from the conductor 20 that is facilitated by the interface layer 44. The micro fluidic channels 42 disposed within the first layer 24 are configured to carry a heat exchange fluid such as, for example, a colloidal mixture or an aqueous mixture.
In one embodiment the heat exchange fluid includes a phase change material. The current carrying conductor 20 may be subjected to peak current for short duration resulting in excessive heat generation. Phase change material is configured to change from a solid state to a liquid state or alternatively, from a liquid state to a vapor state during short intervals of excessive high temperature. Such change in phase results in effective cooling due to absorption of heat by the phase change material in the form of latent heat from the conductor 20. Further, the phase change material may return to original state during normal operation wherein thermal energy stored in the form of latent heat. The latent heat is stored within the phase change material once lower temperature is attained. In one embodiment, the heat exchange fluid and the current carrying conductor are electrically isolated. In another embodiment, the heat exchange fluid may be in contact with the conductor 24. In such scenario, the heat exchange fluid is designed to have a higher dielectric strength than the voltage handled by the conductor 24. A micro-pump (not shown) is configured to circulate the heat exchange fluid through the micro fluidic channels to exchange thermal energy with the first layer and cool the first layer. The micro fluidic channels in turn are cooled by the air voids around the channels and dissipate heat to the ambient. It may be noted that such self condenser mechanism for cooling, as described above, remove heat from the current carrying conductor effectively without requiring a separate condenser unit thus saving cost and space.
In operation, the current carrying conductor 20 carrying high ampere current produces magnetic flux. The power supply 56 is a self-regulated power supply having an output voltage 62 directly proportional to the current 64 flowing in the conductor 20. The micro-pump 36 powered by power supply 56, will automatically initiate the operation once the current 64 flowing through the conductor 20 increases beyond a threshold. Also, the speed increases with increase in the amount of current 64. The self-regulating pump circulates more heat exchange fluid (increased flow rate) through the fluid path as the current 64 increases. It may be noted that use of sensors to detect current flow is avoided by non-contact power supply and configured to self-regulating the speed (and in turn flow rate) of the micro-pump based on the current flow in the conductor 20.
Advantageously, such self regulating mechanism employed for cooling bus bars provide closed loop control on regulation of heat dissipation. Further, absence of condenser makes the cooling mechanism simple to maintain and reduce installation cost. Bus bars and current carrying conductors implementing such systems drive higher current density and help reduce the overall size of the system. Also, cooling mechanism as discussed above is easily retrofit on existing bus bar and switchgear installations. Accordingly, modular based systems can be built based on the bus bar size. Further, interconnect joints have high temperature that limit the current carrying capacity. Such modular design enables addressing specific hot pockets effective by localized cooling. Systems implementing such localized cooling mechanism have reduced by the temperature of the bus bars up to about 50%.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
