Simplifying control strategies with load-dependent voltage
Innovators at NASA’s Johnson Space Center (JSC) have developed a method for efficiently controlling parasitic power in fuel cell systems. “Parasitic power” refers to power required for internal system maintenance rather than for the system’s primary purpose of net power output. Originally designed for spacecraft, this novel method employs a single self-regulating control signal that does away with overly complex control strategies and external power controllers, such as electronic power control units, sensors, and thermostatic controllers. In situations where efficiency and reliability are crucial, this innovative method simplifies and reduces operating costs for fuel cell power systems.
Fuel cell systems typically must control several parasitic power functions such as pumps for circulating coolant or reactant gases, electric heaters for maintaining operating temperatures, and centrifugal water separators. Operating these functions steadily at their full power is wasteful and reduces the overall efficiency of the power system. This unique method allows smooth adjustment of parasitic control voltages in situations where the power demand from parasitic devices depends on the power output of the fuel cell power system.How it Works
The method takes advantage of the operating characteristics of fuel cell stacks, particularly the fact that voltage drops off with increasing current density in greater extremes than with battery-based power systems. In one configuration, the innovation uses two or more fuel cell stacks (one or more primary and a parasitic-load stack) in parallel. Between the negative ends of the fuel cell stacks, a device that is to be controlled from a parasitic power standpoint is placed with its positive electrical power input feed from the negative terminal of the primary stack. The negative output terminal of the parasitic powered device is attached to the negative terminal of the parasitic power stack (see Figure 1).Figure 1. The voltage applied to the coolant pump (V2 – V1) would increase as V1 decreases with increasing current through the load.
As the electrical power output of the fuel cell system increases, the voltage of the primary stack decreases as a function of the voltage versus current density response of the particular fuel cells incorporated into the primary stack. As the primary stack voltage decreases, the voltage difference between the primary and parasitic stacks increases, resulting in current flow through the parasitic device. By carefully selecting cell stack areas, numbers of cells in the two stacks, and resistance of the parasitic power device, the system will operate in a self-regulating, load-following manner.Why it is Better
Conventional methods of controlling parasitic power involve active control features such as electronic power units, electrical or mechanical thermostatic control, current sensors, and variable frequency inverters. Such features increase overall system complexity and reduce reliability. Most existing methods work by turning a parasitic device on and off as requirements change, which is less efficient than the continual adjustment offered by JSC’s novel method.Patents
Johnson Space Center has patented this MSC-24169-1 technology. (U.S. Patent No. 7,862,946)
This technology is being made available through JSC’s Technology Transfer and Commercialization Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the Fuel Cell Power System with Self-Regulating Control of Parasitic Loads (MSC-24169-1) for commercial applications.