Something new under the sun: With strict weight and performance criteria imposed by NASA, one company specializing in applied R&D electrochemistry answered the challenge with an innovative design for test systems

EE-Evaluation Engineering, March, 2002 by Matthew Steinbroner

When you're flying 20 miles high, it's not the time to run out of gas. Such was the major challenge facing engineers developing an innovative fuel-cell-based power system during the design of Helios, the latest unmanned aerial vehicle (UAV) built by NASA and its partner Aero Vironment.

Helios is specified to remain airborne for up to six months at a fraction of the operational cost of traditional aircraft. One intended use is for scientific missions that are considered dull, dirty, or dangerous, such as atmospheric testing over volcanic plumes. With a wingspan longer than a 747 but weighing less than a ton, the UAV has flown via remote control to an altitude of 96,500 ft off Hawaii's coast, setting records for both solar-powered and propeller-driven aircraft. Only rocket-fuel aircraft have flown higher.

Regenerative Energy System

Earlier Helios designs used solar panels as their energy source for daytime flying, with excess solar energy charging batteries for overnight flight. The battery system proved heavy and was not a viable solution.

A lightweight alternative comprising solar panels, a fuel cell, and an electrolyzer was the answer. In flight, the solar panels' excess daytime power operates the electrolyzer to produce hydrogen and oxygen. At night, the fuel cell converts those gases back into water and electricity, keeping Helios aloft.

After successful initial design and testing, Aero Vironment selected Giner Electrochemical Systems to develop a regenerative power system for the Helios project. In business since 1973, we conduct applied R&D in electrochemistry and related areas.

We first designed a 50-cell proton exchange membrane (PEM)-based electrolyzer that converts liquid water into gaseous hydrogen and oxygen by electrochemically splitting water into hydrogen and oxygen gas (Figure la). Next, we designed a 60-cell PEM-based fuel-cell stack that combines gaseous hydrogen and oxygen into liquid water, producing electricity.

The electrolyzer stack produces nearly 4,000 1/h of gaseous hydrogen, operating at a current density of 1,000 A/[ft.sup.2], with individual cell voltages of 1.73 VDC. For earthbound testing, the driving force in this reaction is a power supply. The 5-kW fuel-cell stack operates at 500 A/[ft.sup.2], providing 850 mV per electrochemical cell (Figure 1b). A load bank draws power from the fuel-cell stack for testing.

The innovative concepts used in the fuel-cell and electrolyzer stacks result in stacks weighing approximately 20 lb each. These stacks operate at 400 psig, high pressure compared to typical fuel-cell operation at 30 psig.

Testing Environment

The test goals were two-fold: evaluate the performance and endurance of the regenerative power system to meet weight and performance goals and speed the process of data acquisition, measurement, control, and data reduction.

The test system was designed to reliably monitor and control high-pressure testing of explosive gases over a range of currents, voltages, pressures, gas flows, water flows, and temperatures. This system includes a variety of autonomous safety measures and provides long-term operation with minimal user intervention.

The test system had to be reliable and provide the precision data acquisition and control required to demonstrate the capabilities of the stacks. During testing, the total voltage was 51 VDC (0.85 VDC/cell) on the fuel-cell stack and 86.5 VDC (1.73 VDC/cell) on the electrolyzer stack. These measurements needed a precision of [ or -]0.5 mV.

The pressure and flow values were measured in the 0 to 5 VDC range, based on the individual sensor outputs. Other measurements included low-voltage (K-type) thermocouple and shunt readings, both in the millivolt range.

The test stands operated at pressures ranging from 0 to 400 psig, temperatures ranging from 60 to 180[degrees]F, gas flow rates up to 4,000 1/h, voltages up to 100 VDC, and currents up to 175 A (Figure 2).

Testing Challenges

The land-based test stands presented a variety of difficult problems. For example, the cells in the stacks, which act similar to batteries in series, create low point references on cells up to 98 VDC. In previous test systems, voltage divider arrays connected to a data acquisition system and a PC were the means to measure voltage, pressure, temperature, flow rate, and current, as shown in Figure 3a.

The use of divider arrays introduced errors, which were unacceptable for the precision measurements required of the test system. The inherent error in each resistor could be corrected in the data acquisition software but required standardization of each resistor array. This, in turn, caused each array to have a different scaling factor, increasing the complexity of the data acquisition programming manyfold. A more reliable solution was needed to continue testing of the electrochemical stacks and process variables.

The DT9800 Series Data Acquisition Modules from Data Translation, selected for the new test system, enabled direct readings of the stack properties. We could accurately detect which membrane electrode assemblies (MEAs) were operating within or outside of the specified operating parameters of DC voltage and AC resistance at specific current densities.