This invention relates to a metal-oxide-hydrogen cell and more particularly, to a variable conductant heat pipe which is an integral part of such a cell to improve thermal control.
Metal-oxide-hydrogen batteries are well-known as typified by U.S. Pat. No. 4,115,630. Typical is the nickel-hydrogen cell such as shown in FIGS. 5 and 6 of U.S. Pat. No. 4,115,630. Cells of that type have been used successfully as prime energy storage systems on earth satellites. The cell shown in U.S. Pat. No. 4,115,630 has been used, for example, aboard the NTS-2 (Navigation Technology Satellite-2) spacecraft. In such spacecraft designs battery modules can be located at various places on the spacecraft. For example, a preferred location is on an outside panel that may view cold space. Heat which is generated by the battery will be rejected to space as a result of a completely passive direct radiation system. In such passive systems, radiator sizing can be optimized to maintain battery temperatures at acceptable levels during various cyclic operations, for example, during period of trickle charge. However, passive systems are not generally effective during prolonged time periods where the radiator is exposed to the sun or heat generation rates vary vis-a-vis normal spacecraft orbital operation. For example, during periods of eclipse heat generation can cause significant rises in battery temperature.
For example, in the case of the NTS-2 a passive system was designed to maintain the battery at temperature levels of approximately 0.degree. to 5.degree. C. However, during eclipse battery temperatures were actually in the order of 15.degree. C. to 25.degree. C., that is, an increase of approximately 10.degree. C.
Accordingly, within this technology a need exists to define a system having higher heat rejection rates to flatten the battery temperature profile and thereby lower the average operating temperature of the battery.
Accordingly, a requirement of thermal control in such spacecraft batteries exists which cannot be adequately satisfied by using completely passive direct radiation systems. Such completely passive systems do not have the ability to provide active thermal control during periods of high battery heating such as during periods of high battery heat dissipations such as eclipse periods.
In spacecraft power systems there is a standing design goal to increase usable energy density. Increases in energy density are accomplished by reducing overall weight of such spacecraft cells and/or increasing the depth of discharge of such systems. These design techniques in turn aggrevate problems of thermal control by increasing both the end of discharge and the average operating temperature. Passive systems are simply not satisfactory for such storage systems having very deep discharge cycles. If such were used, unacceptable temperature swings could result during operation of the system. For example, in the case of a low orbit spacecraft application, with a 90 minute orbital period stored energy would have to be removed within 30 minutes yet completely returned within 60 minutes. The limited amount of time for energy dissipation results in higher temperatures. Passive systems do not have the capability for variable heat control.
In addition to the passive system as used in a spacecraft environment, the prior art defines other techniques of passive thermal control in context of a non-spacecraft cell designs. One such technique is shown in Sanderson, U.S. Pat. No. 3,498,844, which shows the use of a heating vent 25 in FIG. 2 of that patent, located at the center of a primary fuel cell evaporator 20. In this passive heat transfer system, heat produced at the electrodes 8 and 10 causes the temperature of the working fluid to increase until it is high enough to open a pressure regulating valve 27. As a result, water vapor will be exhausted through vent 25 as a result of the pressure increase in the system.
Another passive system is shown in U.S. Pat. No. 3,865,630 which in FIG. 2 also shows a heat pipe 27 utilized as a heating device for electrolyte 23 in a molten salt battery. As shown in that system, the heat pipe is a unitary body which is integrated into the framework of the battery. The heat pipe cannot actively vary its conductants in response to a change in temperature of the system.
Metal-oxide-hydrogen batteries, in particular, nickel-hydrogen batteries, are assembled utilizing a multitude of cells having an electrode stack contained within a pressure vessel. As shown in FIG. 5 of U.S. Pat. No. 4,115,630 the pressure vessel 90 contains an electrode stack which is axially aligned on a centrally disposed rod 52. Two sets of electrodes comprising a first back-to-back stack of positive electrodes 74 are coupled to a positive bus bar 92 and a second stack of negative electrodes 78 are disposed relative to a negative bus bar 94. At one end of the cell, coupled to the negative bus bar, a negative support 114 is used as a negative terminal. At the other end, a fill tube 126 is disposed coaxially within a positive terminal 112 coupled to the positive bus bar 92. As indicated, such a cell relies on passive thermal control. The predominant transfer path for heat which is generated in the electrode stack within the cell is radially through the hydrogen gas and then through the pressure vessel to the battery structure and finally, radiated to space.