Spacecraft require power for electronics necessary for performing the vital functions on the spacecraft. This power is typically supplied by solar arrays, batteries, and/or fuel cells. Of particular interest to the present inventions is a fuel cell, which can be described as a device that converts chemical energy from a fuel into electricity through chemical reactions of reactants consisting of a positively charged chemical element, such as hydrogen, and an oxidizing chemical element, such as oxygen. As long as the reactants are continually input into the fuel cell, a continuous source of electrical power is output from the fuel cell. Along with electrical power, water and heat are produced from the reaction of the hydrogen and oxygen in the fuel cell. To ensure that the fuel cell does not over heat, a fuel cell coolant subsystem may be used to remove the waste heat from the fuel cell. Typically, in lower temperature fuel cells, this coolant subsystem comprises a coolant loop system that circulates a coolant through the fuel cell to absorb the waste heat from the cell stack, and a radiator that transfers the heat from the coolant into the ambient environment. Typically, in higher temperature fuel cell systems, the fuel cell stack is cooled by additional reactant flow through the fuel cell stack, which can then be exhausted or cooled through a heat exchanger and then recirculated back to the fuel cell.
In typical spacecraft, the main propulsion system and power systems are separate and operationally isolated from each other. For example, the space shuttle utilizes a fuel cell-based power system in which liquid hydrogen and liquid oxygen are supplied to the fuel cell from different tanks as those that supply liquid hydrogen and liquid oxygen to the thrusters of the main propulsion system. Main propulsion cryogenic propellants, in particular liquid hydrogen, have a propensity to absorb heat while the vehicle is operating during orbital coast or loiter. Because of this heat absorption, the liquid hydrogen will boil off and must be vented overboard into the external environment in order to control the pressure within the tank. The vented hydrogen gas or vapor can be considered a necessary waste to ensure that the pressure within the main propulsion tank remains within operational limits. Once placed in orbit, spacecraft require occasional positional altitude and orientation adjustments, and thus, utilize a set of small thrusters that typically provide thrust by expelling gas via gas thrusters or reactant thrusters into the ambient environment. This requires an additional source of fuel or reactant.
Significantly, it is very important that the power system of spacecraft, be as light-weight as possible and used most efficiently due to the launching cost and/or payload considerations. However, the additional tanks, and reactant stored therein, that are used for the fuel cell-based power system and positioning thrusters, as well as the relatively large radiator used to remove waste heat from the fuel cell, disadvantageously add weight to the spacecraft.
As such, there is a need to decrease the weight of spacecraft that utilize fuel cell-based power systems.