Transportation vehicles in use today are often powered by internal combustion engines (ICE) using either gasoline or diesel fuel. Due to the need to increase energy efficiency, reduce pollution and lessen petroleum dependence, vehicles using more advanced propulsion technologies are being developed. Of particular interest are alternatives to traditional ICEs such as electric motors, fuel cells and combinations thereof.
For example, hybrid electric vehicles use both ICEs and electric motors in the propulsion system. The ICE in a hybrid electric vehicle, as in any ICE-powered car, produces power through continuous, controlled explosions that push down pistons connected to a rotating crankshaft. That rotating force (torque) is ultimately transmitted to a vehicle's wheels.
The electric motor in a hybrid electric vehicle is energized by a battery. The battery is continuously recharged by a generator that—like the alternator of a conventional car—is driven by the ICE. In conventional ICE vehicles, energy from deceleration is wasted as it dissipates. In some hybrid vehicles, regenerative braking systems capture that energy, store it, and convert it to electricity to help propel the vehicle—hence increasing energy efficiency. Some hybrid vehicles use ultracapacitors to extend the life of the on-board battery system via the capture of power from regenerative braking and its release for initial acceleration.
Fuel cells are a clean and efficient power source. The fuel cells proposed for use in transportation vehicles produce electricity through electrochemical reaction between hydrogen and oxygen. In fuel cell powered vehicles, hydrogen may be stored as a pressurized gas in onboard fuel tanks. The electricity produced by the fuel cell charges a storage battery (similar to those employed in some hybrid electric vehicles) that energizes the vehicle's electric motor. Thus, a fuel cell vehicle may also be considered as a type of hybrid vehicle.
Despite the differences between traditional ICE powered vehicles and those powered by the various alternative power systems, i.e., electric, fuel cell and combinations thereof, most transportation vehicles require the management and control of excess thermal energy. That is, most transportation vehicles regardless of power source require a cooling system designed to shift or transfer undesirable accumulations of thermal energy or heat.
In most cooling systems, regardless of power source, a coolant is used to the heat from the ICEs, electric motors, fuel cell stacks, and/or other heat generating components in a vehicle. The coolant is then forced to flow through a heat exchanger (e.g., a radiator) to be cooled down by air. The cooled coolant flows back to the coolant tank where it can be pumped back to the heat sources of the vehicle to again remove excess or undesirable heat.
In order to maximize the amount of surface area available for transferring heat between the coolant and the environment, the heat exchanger may be of a tube-and-fin type containing a number of tubes that thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the coolant to the environment.
However, such traditional heat exchanger configurations require significant metal mass. High weight vehicle components are now undesirable in view of environmental standards that require improved fuel economy (mile/gal) and reduced emissions. According, it is now desirable for heat exchangers to be constructed entirely or predominately of lower weight metals, especially aluminum or aluminum alloys.
Several methods have been employed in the manufacture of heat exchangers for use in transportation vehicles.
In the past, mechanical expansion techniques have been used for mass-production of radiators. Mechanical expansion techniques rely solely on the mechanical joining of the heat exchanger components to ensure their integrity. Advantages of these methods include good mechanical strength and avoidance of joining operations that require the use of a furnace operating at high temperature. The disadvantages of these methods include inferior thermal performance and a relatively high final weight for the finished heat exchanger.
To overcome the disadvantages of the mechanical expansion-type heat exchangers, heat exchangers have been increasingly formed by a brazing operation, wherein the individual components are permanently joined together with a brazing alloy. Generally, brazed heat exchangers are lower in weight and are able to radiate heat better than heat exchangers formed by mechanical expansion.
Brazing operations used in heat exchanger manufacturing have traditionally occurring in vacuum furnaces. More recently, a brazing technique known as “controlled atmosphere brazing (CAB)” has become accepted by the automotive industry for making brazing aluminum heat exchangers. Illustrative examples of CAB brazed aluminum heat exchangers as that term is used herein include radiators, condensers, evaporators, heater cores, air charged coolers and inter-coolers.
CAB brazing is preferred over vacuum furnace brazing due to improved production yields, lower furnace maintenance requirements, greater braze process robustness and lower capital cost of the equipment employed.
However, in a CAB process, a fluxing or flux agent is applied to the pre-assembled component surfaces to be jointed. The fluxing agent is used to dissociate or dissolve and displace the aluminum oxide layer that naturally forms on aluminum alloy surfaces. The fluxing agent is also used to prevent reformation of the aluminum oxide layer during brazing and to enhance the flow of the brazing alloy. Illustrative fluxing agents include alkaline metal or alkaline earth metal fluorides or chlorides.
One widely used flux for brazing aluminum is sold under the trademark Nocolok®, which appears to be a mixture of K3AlF6, K2AlF5 and KAlF4. (see e.g., U.S. Pat. No. 3,951,328 and U.S. Pat. No. 3,971,501). Fluoride-based fluxes are generally preferred over chloride based fluxes for brazing aluminum or aluminum alloys because they are inert or non-corrosive as aluminum and its alloys but are substantially water insoluble after brazing. In fact, due to its non-corrosive nature and tolerance to brazing assembly fit-up and flexible control, Nocolok® flux brazing is a particularly advantageous method of joining of aluminum heat exchangers. It is now commonly used by the automotive industry in the manufacture of aluminum and aluminum alloy heat exchangers.
Unfortunately, it has been found that residual brazing flux remaining on an aluminum or aluminum alloy surface will leach out fluoride ions. It is believed that the leached fluoride ions can lead to localized corrosion on the metal substrate when it is immersed in the coolant of a cooling system. This undesirable localized corrosion resulting from residual flux has been found to occur in a number of currently available commercial coolants, including those containing organic acid based (or extended life) or hybrid-based (i.e., containing a smaller amount of silicate than traditional silicate based coolants and also one or more organic acids) corrosion inhibitors.
Moreover, the coolants employed in fuel cell powered vehicles have special requirements. For example, coolants are required to cool both the fuel cell stack and the “balance of the plant” systems, such as air compressors, electric motors, DC to AC converter, and other systems. However, the coolant used to cool the fuel cell stack must have a very low electrical conductivity, (e.g., less than 5 μS/cm) in order to minimize electrical shock hazard, corrosion and efficiency reduction.
A fuel cell assembly typically includes an anode (a negatively charged electrode where the oxidation reaction of a fuel, e.g., hydrogen, takes place), a cathode (a positively charged electrode where the reduction reaction of an oxidant, e.g., oxygen, takes place), and an electrolyte in between the two electrodes. To produce sufficient power for use as a vehicle engine, a fuel cell based engine needs to have many cells connected in series together to form a fuel cell stack. Each single cell will typically operate at a voltage of 0.6-1.0V DC. The proposed fuel cell stack for use in vehicles often has more than 100 cells connected in series. Hence, the DC electrical voltage across the fuel cell stack can be very high. The typical reported cell voltage in an automotive fuel cell stacks generally ranges from 125-450V DC.
In addition to generating significant electric power, a fuel cell assembly also generates substantial heat due to the exothermic nature of the electrochemical reactions involved and the flow of electrical current.
Thus, a fuel cell stack also contains coolant channels for the circulation of coolant to remove heat from the stack. In circulating a coolant through the coolant channels, the temperature of the fuel cell stack may be controlled at the desirable range for optimal operating conditions. The cooling system surrounding the fuel cell stack, however, is exposed to the same electrical voltage as the fuel cell stack itself. To prevent or minimize electrical shock hazard, the coolant must have very low conductivity. For example, the upper limits for coolant conductivity may be set to less than 5 μS/cm (e.g., U.S. Pat. No. 5,776,624). Low electrical conductivity for fuel cell coolant is also desirable for the reduction of shunt current in the coolant system and the minimization of system efficiency reduction.
Higher concentrations of fluoride ions in a fuel cell coolant from residual brazing flux produce an increase in the electrical conductivity of the fuel cell coolant. Thus, the interaction of residual brazing flux and coolant is particularly undesirable in a CAB brazed fuel cell heat exchanger, not withstanding the localized corrosion issues present in any CAB brazed heat exchanger.
Accordingly, the widespread use of CAB processes in heat exchanger manufacturing has led to concerns about: (1) localized corrosion of brazed aluminum surfaces due to the interaction of brazing fluxes and currently available coolants and (2) the effects of residual brazing fluxes upon the electrical conductivity of coolants currently used in aluminum based fuel cell heat exchangers, i.e., undesirable increases in electrical conductivity. These issues are of particular concern with regard to fluoride based brazing fluxes.