Patent Application
20090142636
The present invention relates to a heating system for high pressure storage tanks for hydrogen and CNG gas fuel, or other gas, by compensating for thermal and mechanical stresses caused by a low temperature resulting from (1) gas decompression in the tank during driving as the gas is depleted from the tank and (2) environmental exposure of the tanks in low temperature climate conditions. The present invention heats the gas stored within the tank and ameliorates mechanical stresses to the tank and the component parts of the tank caused by the thermal conditions of the tank environment and thermal changes in gas temperature associated with the depletion of high pressure gas from the tanks.
Vehicles powered by compressed natural gas (CNGV) and hydrogen gas (FCV) typically include on board high pressure gas fuel tanks that may include gas absorbing materials within the tank interior. During driving, the gas inside the tanks becomes cold, caused by the tank pressure decreasing when gas is consumed by the vehicle power plant resulting in decompression of the tank. Gas absorbing materials used in the tank interior will absorb the intrinsic heat in the gas during the gas discharge from the tank during vehicle operation. In cold climates, the internal gas temperature in the tank can drop to −60° C. or below, a temperature that may be below the permissible operating temperature of 0-rings, or other rubber seals, or gas flow controls in the tank. An excessively low temperature in the tank may upset design tolerance limits for the seals and flow controls and cause the stored gas to leak as a result of temperature caused stresses in the tank system assembly. As an example, when the ambient temperature is −20° C., the reduction of internal tank temperature by an additional −40° C. due to gas decompression effects will result in an internal temperature in the gas tank of −60° C. Expansion and contraction of the tank and the component parts of the gas flow system associated with the tank may produce adverse mechanical stress effects. In the specification, reference to hydrogen fuel cell vehicles correlates with the use of the invention with CNGV's (compressed natural gas powered vehicles) and hydrogen powered fuel cell vehicles (FCV's) or internal combustion engine vehicles powered by either compressed natural gas (CNG) or hydrogen. Although hydrogen is typically referred to in the specification and examples, the term “hydrogen” is in most instances intended to be interchangeable with CNG and other fuel gases. Fuel gases are referred to as a “gas” or “high pressure gas.”
It is an object of the present invention to provide a warming system for a carbon fiber composite tank utilizing the intrinsic electrical conductive characteristics of the tank to warm the inside of tank and the gas therein during driving conditions. It is a further object to reduce the risk of a fuel gas leak in cold climate driving conditions caused by excessively low tank and/or gas temperatures. As a result, tank durability is increased because overall system temperature differences are minimized.
The invention is described more fully in the following description of the preferred embodiment considered in view of the drawings in which:
In brief, the invention provides a warming system for high pressure gas storage tanks utilized on high pressure gas fueled vehicles including vehicles powered by compressed natural gas, CNG, and fuel cell and internal combustion engines powered by hydrogen gas. In many examples, such vehicles include gas fuel tanks that may include gas absorbing materials in the interior of the tank. During driving, the gas cools because a decrease in the tank pressure occurs. When a vehicle tank includes gas absorbing materials, the gas absorbing materials absorb heat during the gas discharge from the tank further contributing to the cooling effect. Environmentally, a typical ambient temperature is approximately 20° C. In cold climates, the internal gas temperature in a vehicle tank can drop to −60° C. or below, a temperature that may be below the permissible operating temperature range of O-ring and/or other rubber or polymer seals used in the tank and the port inlet and outlet metal part assemblies that control the inflow and outflow of gas to and from the storage tank. Below the acceptable temperature range, variances allowable for seals, valves, control devices, and the like, may be exceeded by thermally caused mechanical variations in the tank and associated assemblies. Leakage of the stored gas may result.
The invention provides a solution that can efficiently warm the storage tank utilizing intrinsic electrical conductivity characteristics of the fiber and binder materials utilized in the fabrication of the tank. In examples, the carbon fiber layer in the tank itself can be used as a heater to warm the gas tank itself. Because carbon fiber layer has an electrical conductivity, the fiber can be heated up when an electrical potential is applied to an appropriate resistive expanse of a tank section. In another example, the metal end ports of the tank can be used as electrical terminals applying the current to the conductive tank fiber. Intrinsic extensions fabricated from the tank material and interconnected with the conductive tank fiber can be used as electrical terminals. In other examples, electrical terminals can be installed in the tank wall, connecting with conductive fibers, allowing a connection to the electric warming current. In instances wherein the carbon fiber of the tank is excessively conductive, conductive filler is mixed into the resin (epoxy) forming the tank is effective to reduce the resistance. Usually the resin will have a very high electrical resistance. Similarly, variations in the proportional mixture of conductive fiber and the carbon fiber are effective to reduce the resistance. In instances where the tank includes a metal liner, the metal liner has much lower resistance than the carbon layer and will produce an electrical short. An electrical isolation layer between the tank carbon fiber and the metal liner will prevent an electrical short from occurring.
Using the invention, only a small change in tank design is required and higher heating efficiency will result compared with an external heater system because the tank itself is heated up directly. Fiber composite high pressure storage tanks are formed from extending fiber strands, filler, and other materials embedded in a resin foundation. The invention provides a heating system for a fiber composite high pressure gas storage tank whereby the temperature of the gas within the tank and the gas flow components associated with the tank gas flow control system at one or more boss assembly at the tank ends are maintained above the lower temperature design tolerance limit for the tank and boss assembly. In the invention, the tank wall is formed from a polymeric binder having embedded therein longitudinally extending electrically conductive and resistive fiber material strands and an electric power source is interconnected with the conductive and resistive fibers forming the tank wall.
The electrically conductive and resistive fiber material strands in the tank intrinsically heat the tank and boss assembly system upon the application of an electric current. In examples, 1) electrodes are embedded in the composite fiber composition of the tank wall to define an electrically resistive path for current flow within the conductive fiber materials of the tank wall; 2) conductive metal bosses at either end of the tank are connected to an electrical power source providing a flow of warming current to the conductive shell; 3) an electric power source is interconnected to electrically conductive fiber extension elements intrinsically formed in the tank shell and lead from the conductive composite tank shell at each of the opposite sides of the tank ends to an interconnection with an electrical power source. When a metal boss is at either or both ends of the tank and a tank wall has an interior metal liner; an insulating layer sandwiched between the conductive fiber composite tank shell structure and the metal liner is provided wherein the insulating layer isolates the conductive wall of the tank from the electrically conductive fiber wall and the one or more metal boss from the flow of electric current.
A temperature control system utilizing temperature sensors may be utilized in embodiments of the invention to provide temperature stabilization. Measurement data is input into the control system for one or more of valve temperature, tank wall temperature, gas temperature and ambient temperature. The control temperature maintained by the system, as determined by the flow of electric current into the fiber components of the tank is such that the gas temperature and the temperature of the metal components associated with the tank does not drop below the lower tolerance temperature limit of the tank and the components associated with the tank gas flow control system at the one or more boss at the tank ends.
Electrically conductive materials forming the tank wall include a filler comprised of a metal powder composition; the powder composition may include one or more of aluminum, copper, nickel, silver, stainless steel, and titanium (Al, Cu, Ni, Ag, SUS, and Ti) powders, a carbon black powder, ceramic powder, and plastic powder coated with an electrically conductive metal. Preferred electrically conductive materials used in forming the tank wall include a filler that is usually a powder composition characterized by an average particle size diameter ranging from about approximately 0.1×10̂−6 m to about approximately 500×10̂−6 m. Electrically conductive carbon nano tubes may be embedded in the tank wall composition. Other electrically conductive fiber materials used in forming the tank wall include metal wires and carbon, glass, and plastic fibers coated with metal.
The resistive/conductive properties of the tank wall to which the electric current is applied range from 0.1 ohm to 100 ohm. The electric power source is interconnected with electrodes associated with the tank wall to form an electrically active circuit for current flow and the warming power input into the system. Warming power, P-warming [W], is determined by the tank and gas flow assembly tolerance temperature during the vehicle operation, wherein Warming Power Current (I) is determined by the formula: I-tank=(P-warming/R-tank)̂0.5; and Warming Power Voltage (E) is determined by the formula: EI-tank=I-tank×R-tank. The electric power source interconnected with the tank wall may be isolated from other electric lines in the vehicle and in such an instance, a DC/DC converter or DC/AC inverter comprises the tank warming power source.
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Having described the invention in detail, those skilled in the art will appreciate that, given the present description, modifications may be made to the invention without departing from the spirit of the inventive concept herein described. Therefore, it is not intended that the scope of the invention be limited to the specific and preferred embodiments illustrated and described. Rather, it is intended that the scope of the invention be determined by the appended claims.
