Arusha Rover Power Subsystem Trade Study: Fuel Cells vs. Nuclear Power

Kamwana N. MwaraNSBE Space SIG

Houston, TX, USA

Margo BatieNSBE Space SIG

Madison, WI, USA

Onwar ShaheerNSBE Space SIG

Lakewood, OH, USA

Abstract - For a lunar rover being designed by the National Society of Black Engineers Space Interest Group, specifically for the Arusha lunar project, a trade study was initiated between fuel cells and nuclear power as a long-term power source for the Arusha lunar rover. Various fuel cell and nuclear power options were examined, to investigate how they align with the Arusha rover’s mission requirements. Power would need to be supplied to all of the systems on the vehicle, when it is on excursions away from the Arusha lunar base. This includes selecting a primary power subsystem, which would provide an average of 270 kW and possibly operate continuously for at least 30 days. An auxiliary power system would need to be selected, to provide “lifeboat” survival mode power of 10 kW to a non-mobile vehicle, for up to 20 days.

Acronym List:

PEMFC = Proton Exchange Membrane Fuel Cell

NSBE = National Society of Black Engineers

SOFC = Solid Oxide Fuel Cell

SAFE = Safe Affordable Fission Engine

SNAP = Systems for Nuclear Auxiliary Power

1 IntroductionArusha is a design project of the Space Special

Interest Group (SIG) of the National Society of Black Engineers (NSBE). Swahili for “he makes fly (into the skies),” Arusha represents a 48-person, multi-site lunar surface outpost. It is intended as a follow-on to an initial return to the Moon program. Its purpose is to begin to demonstrate capabilities for industry and government to expand scientific discovery and to utilize resources and opportunities found on the Moon.

The Arusha concept assumes a primary base at South Pole and additional facilities across scattered across the lunar surface. A long range, pressurized rover with lunar circumnavigation capability is used for transportation of six-person teams between these lunar facilities. The rover must sustain the crew for an up to 30-day mission excursion. A CAD model of the rover’s cabin pressure vessel is shown in Figure 1.

Power needs to be supplied to all of the subsystems on this rover, such as communications, life support,

habitation, and mobility when it goes on excursions away from the Arusha lunar base. A bottoms-up power analysis has not yet been conducted, but a preliminary estimate of 270 kW is being used as a design point for the power subsystem.

The Arusha Rover Power Subsystem is tasked to select a primary power subsystem, which would provide 270 kW and operate continuously for at least 30 days. An auxiliary power system would need to be selected, to provide “lifeboat” survival mode power of 10 kW to a non-mobile vehicle, for up to 20 days. This would allow for a rover with a failed primary power subsystem to remain alive while awaiting rescue.

Figure 1. Arusha Lunar Rover Cabin Pressure Vessel

2 Power Source Options & Research2.1 Trade Space

There are only a handful of potential power sources for space vehicles. Typically vehicles are powered by batteries, fuel cells, solar energy, or nuclear power. For obvious reasons hydrocarbons (internal combustion engines) are not practical options outside the Earth’s atmosphere. Batteries are also immediately eliminated as preposterous – the mass required for 30 days worth of batteries is an immediate nonstarter. Solar energy is also a non-starter, though this may be less obvious. The rover must conduct a 30-day mission, regardless of whether that mission carries it across lunar day or lunar night, in view of the sun, or hidden in craters and canyons. Line of sight to the sun cannot be sustained in such traverses, thus there is no way to use solar electricity as either a direct power source or as a means to charge an energy storage system.

This leaves only fuel cells and nuclear power as options, both of which are explored in this study.

3 Fuel CellsFuel cells are devices that use hydrogen as a fuel to

produce electrons, protons, heat and water. The electrons that are produced can be harnessed to provide electricity in a consumable form, through a simple circuit with a load. The technology is based on the reaction equation given below [2]:

2H2 + O2 ↔ 2H2O

Fuel cells have been used as power sources in previous space missions and are currently being investigated for use in future spacecraft. This section examines the primary types of fuel cells investigated for space missions at NASA (past and present), and also describes the characteristics of each type of fuel cell that could affect the alignment with the Arusha Rover mission system requirements. Initial rough system mass estimates are also given as well, as a starting point for future work.

3.1 Alkaline Fuel Cells

Alkaline fuel cells have historically been used in human spaceflight. They were first used in the NASA Apollo lunar program. [1] They were also used in NASA’s Space Shuttle program. [1] Figure 1 shows an alkaline fuel cell from the Apollo program, while Figure 2 shows a Space Shuttle alkaline fuel cell.

Figure 2. Apollo Alkaline Fuel Cell

Characteristics of the Alkaline fuel cell include extensive spaceflight experience, low system cost, and operating temperature of ~ 230oC. Downsides to the usage of Alkaline cells include their corrosive issues [2]. This tends to affect their ease of use, ruggedness, and safety.

This has led to a decline in their development and usage in the space program.

Figure 3. Space Shuttle Alkaline Fuel Cell

However, a very rough initial mass estimate of the fuel cell stacks and balance of plant system for the Arusha rover, based off of the mass estimates from the Space Shuttle 12 kW fuel cell power plant [1], comes to 3470 kg, for a rover system requiring 270 kW for 30 days. Reactant tank mass estimates include 18,000 kg for the H2 tank and reactants [1] [2]. For the required O2 reactant and tanks, there is a mass estimate of 52,960 kg [1] [2]. By the end of mission, all of the oxygen and hydrogen has been converted to water, which must be carried by the rover for reclamation back at the South Pole outpost. Assuming the tank is a cylindrical aluminum 2195 pressure vessel that maintains the water at approximately 8 psi, the tank mass is approximately 317 kg.

An alkaline fuel cell stack can also be used for the auxiliary power needs of 10 kW for 20 days. This system has a mass of 130 kg for a fuel cell power plant, 400 kg for the H2 tank and reactants, and 1177 kg for the O2 reactant and tanks. This yields a total alkaline fuel cell system mass of 76,454 kg.

3.2 PEM Fuel Cells

Proton Exchange Membrane fuel cells (or PEMFCs) have been previously used in NASA’s Gemini space program, and have been considered as a possible option for future missions.

Characteristics listed for this type of system include an operating temperature of ~80oC, quick start-up, and existing spaceflight experience for this particular type of fuel cell. Downsides to using a PEMFC include active cooling required for the system and water management that would be needed (adding system complexity). [3]

A very rough mass estimate for the Arusha rover power system can also be calculated based on a PEM fuel cell system. The fuel cell stacks and balance of plant system, based off of the mass estimates from 3 kW fuel cells investigated by NASA, total a mass of 13,050 kg [3] for a rover system requiring 270 kW for 30 days. Reactant tank mass estimates include 18,000 kg for the H2 tank and

reactants [2] [3]. O2 reactant and tank mass estimates equal 52,960 kg [2] [3]. The water reclamation tank mass is 317 kg, just as with the alkaline fuel cell because the reactant masses are identical.

The auxiliary power fuel cell to supply 10 kW for 20 days is 484 kg for a PEM fuel cell power plant. The reactants are the same as in the alkaline case - 400 kg for the H2 tank and reactants, and 1177 kg for the O2 reactant and tanks. Thus, the total PEM fuel cell system mass is 86,388 kg.

3.3 Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFC) have also been examined, as a possible future option for long duration missions. They have especially been considered, when coupled with other system architecture, such as LOX/CH4 propulsion systems and In-situ Resource Utilization architecture [3]

Characteristics listed for this type of system include an operating temperature of ~800oC, less sensitivity to reactant purity, high quality waste heat, and no active cooling required for this particular type of fuel cell. Downsides to using a SOFC include longer start-up time required for the system, and a lower Technology Readiness Level (no spaceflight experience). [3]

Estimating the mass of a Solid Oxide fuel cell solution for the rover, based on mass estimates from 3 kW fuel cells investigated by NASA [3], yields approximately 10,620 kg in fuel cell power plant mass for a rover system requiring 270 kW for 30 days. Reactant tank mass estimates are 18,000 kg for the H2 tank and reactants [2] [3] and 52,960 kg for the O2 reactant and tanks [2] [3]. As would be expected, the water reclamation tank mass is again 317 kg.

The solid oxide auxiliary power fuel cell to supply 10 kW for 20 days is 394 kg, with the same reactant sizing as in the previous cases, 400 kg for the H2 tank and reactants, and 1177 kg for the O2 reactant and tanks. The total solid oxide fuel cell system mass is 83,868 kg.

4 Space Nuclear Electric SystemsThis section will look into usage of space nuclear

electric systems as a power option for the Arusha Rover. Nuclear electric includes both radioisotope power systems and fission power systems. Power sources from previous space missions are investigated, as well as ones currently being examined for future space missions. Due to greater number of power sources evaluated, for this particular section, only the preferred power option will have more in-depth examination (including an initial mass estimate).

4.1 Radioisotope Power Systems

Radioisotope thermoelectric generators (RTGs) have been a main power source for the US space program since 1961 [4]. Missions where RTGs have powered US space vehicles include the Apollo, Pioneer, Viking, Voyager, Galileo, Ulysses, and New Horizons space missions, as well as various civil satellites. For example, the New Horizons spacecraft had a 250 watt, 30 volt General Purpose Heat Source RTG [4]

Figure 4 shows typical optimal operating ranges, for various power sources [5]

Figure 4. Power Source Operating Ranges

Other radioisotope power systems include radioactive heater units (RHUs). Both types of systems produce low levels of power, for example the RHU being developed by the Idaho National Laboratory’s (INL) Center for Space Nuclear Research (~100 watts). [4]

In all uses of RTGs, the power level is generally measured in watts, with maximum power far below the 270 kW needed by the Arusha rover. Consequently, RTGs are not a viable solution for the Arusha rover’s power requirements.

4.2 Fission Power Systems

The energy density of nuclear fission makes it a potentially attractive solution. As a point of comparison, the energy obtained from fissioning a coke can sized chunk of uranium is equal to fifty times the amount of energy contained in the Space Shuttle External Tank. [7] The United States has explored a wide variety of fission power systems, though virtually all of them have been restricted to the laboratory level.

The only fission reactor that the U.S. has flown in space is a Systems for Nuclear Auxiliary Power (SNAP) unit. [6] The U.S. SNAP-10A unit produced 650 watts using a thermoelectric converter. The US also made SP-100 (Space reactor Prototype), which was a fast reactor unit and thermoelectric system delivering up to 100 kWe as a multi-use power supply for various orbital and lunar/Martion missions. But this program was terminated in the early 1990s. [4]

Other flown fissions systems include the Topaz reactors with thermionic conversion systems, generating about 5 kWe of electricity for on-board uses. They were flown in 1987 & 1990 by Russia, but discontinued in 1993, due to budget restrictions. [4]

Figure 5. SAFE-400 Nuclear Fission Reactor

Heatpipe Power System (HPS) reactors are compact fast reactors producing up to 100 kWe for about ten years to power a spacecraft or planetary surface vehicle. They have been developed since 1994 at the Los Alamos National Laboratory as a robust and low technical risk system with an emphasis on high reliability and safety. They employ heatpipes to transfer energy from the reactor core to make electricity using Stirling or Brayton cycle converters. [4] A heatpipe is a heat transfer device that uses both thermal conductivity and phase change to transfer energy. Essentially, at the hot end a liquid vaporizes and at the cooler end it condenses. The liquid returns to the hot end, producing a repeating cycle. This conducts energy from the reactor’s fuel pins to heat exchangers, where the

energy is then transferred to power converters that convert the thermal energy to electricity. [4]

The SAFE-400 (Safe Affordable Fission Engine) reactor, shown in Figure 5, is a 400 kWt HPS that generates 100 kW of electricity using two Brayton cycle gas turbines. Core mass is approximately 512 kg, with each of two heat exchanger approximately 72 kg each [4]. SAFE-400 is based on existing technology [5] and appears to be the most feasible solution for the electrical power needs of the Arusha rover. Three of the reactors can be used to meet the Arusha rover’s power requirements of 270 kW.

For the auxiliary power requirements for the Arusha rover, there are very few nuclear fission options. A possible candidate is a smaller variety of the SAFE-400, the HOMER-15 (the Heatpipe-Operated Mars Exploration Reactor). It is a 15 kWt thermal unit designed to produce 3 kW electricity. It stands 2.4 metres tall including its heat exchanger and 3 kWe Stirling engine. Heatpipe length is 106 cm, and fuel height is 36 cm. In general, total mass of reactor system is approximately 214 kg. [4] Given that four of these would be needed to produce 10 kW of electricity at a mass of 856 kg, it is more reasonably to provide a fourth SAFE-400 as a redundant power source, or claim greater reliability of nuclear fission power over fuel cells as an argument against the need for an auxiliary power source. This paper will assume a fourth SAFE-400 reactor, though arguably any of the fuel cell auxiliary power systems could also be used should dissimilar redundancy be desired.

The SAFE-400 reactors can easily be mounted beneath the rover cabin. Figures 6 and 7 show two different configurations that can accommodate three SAFE-400 reactors, either in-line or abreast. Shielding and thermal control components would surround the reactors in either configuration. It should be noted that with the reactors mounted beneath the cabin, shielding would not only serve the role of protecting the crew from radiation, it would also protect the reactors from inadvertent contact with the ground.

Figure 6. SAFE-400 Mounted In-line

Figure 7. SAFE-400 Mounted Abreast

5 Conclusions & RecommendationsSumming up the various mass estimates for the four

primary power systems compared in this paper yields the mass summary in Table 1. These mass estimates include both the primary power system to produce approximately 270 kW for 30 days and an auxiliary power system to produce approximately 10 kW for 20 days. There are, however, additional masses required for all SAFE-400 system.

Table 1. Primary Power System Mass Estimates

Primary Power System (~270 kW)

Total System Mass (kg)

Alkaline Fuel Cell 76,454PEM Fuel Cell 86,388Solid Oxide Fuel Cell 83,868SAFE-400 Fission 2,624

The SAFE-400 estimates did not include shielding hardware or heat rejection, both of which will be required and may mass in the tens of thousands of kilograms. It is likely that the shielding and heat rejection system will consist of a water jacket surrounding the reactor assembly with a heat exchanger that transfers heat from the reactor to a secondary fluid, to the water, and then to an as of yet to be defined heat rejection system. Radiators will be problematic as the size required for radiation will be excessive. A possibility is a capacity for the rover to scoop up quantities of regolith, transfer heat to the regolith, and dump the heated regolith then repeat the process. However, this will have to be a sufficient quantity to endure traverses into rocky regions where loose regolith may not be available. This is forward work for a thermal analysis.

This shielding and thermal analysis will be the next step in the trade of fuel cells versus nuclear fission. Initially, nuclear fission appears to be a superior mass solution. However, it is unclear how much mass will need to be added to the fission solution. If the shielding and thermal control systems can remain below roughly 73,000 kg then it will emerge as the recommended solution for the Arusha long range rover power subsystem.

AcknowledgmentsThe authors would like to thank NASA Nuclear

Experts Jason Wolinsky and Mike Houts for their explanations and assistance in researching this paper. The authors also express thanks to all of the other Arusha project team members. Finally, the authors would like to thank each other for their volunteer time and participation.

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