Power--Landon Nemoto

As of October 17th:           

            The power supply is an integral part of the Mars rover. There are two main concerns: how much power should be supplied, and what type of power supply will be most efficient (personified by a power-to-weight ratio).

The most practical way to conclude how much power is supplied is to obtain desired power figures from the other subsystems observed. Once a total value of power drawn by the other subsystems is determined, this will be the minimum value the power supply must create. The power source will be designed to satisfy these needs as best as possible, but it is more likely that the power source will only output a certain amount of power, constraining the individual subsystems.

To determine efficiency of the power supply, the team weighs each characteristic of the supply against how much power the supply can give to the rover. One criterion here is power-to-weight ratio, but the team is also aware of cost, surface area, and mass being other constraining factors.

In order to determine a starting point in the evaluation of power sources, one can observe the lunar roving vehicles of the Apollo 15-17 missions. These LRVs were powered using two 36-Volt silver zinc batteries. This power was then distributed to motors on each wheel for propulsion as well as the video system.[1]

            However, since the group wishes to produce power without the concern of charging or re-charging batteries, alternatives to the batteries mentioned above are considered for the Mars rover. Three general power supplies are being considered for the design so far: Krypton-85 and Alkali Metal Thermal to Electric Converters (AMTEC), solar power, and alkaline fuel cells.

Utilizing AMTEC, one would have to maintain a 700-1000oK operating temperature. The heat released due to the process of creating power might be harnessed for the purpose of heating other subsystems to maintain correct operating temperatures. The power output for AMTEC has been shown to satisfy a power output of 520W for a Carnegie-Mellon lunar rover. Any increase in power necessary to the Tigernauts’ design will require the addition of AMTEC cells with the output of 8 W (electric) per cell.[2]

            Solar power can yield an output of 110-126 W/m2 on Mars, but this means that it would require 12 m2 of photovoltaic cell area to produce 1 kW.[3] The tremendous amount of surface area required would ruin any hopes of building a compact (and efficient) rover. The only possible use for solar power might be for an emergency back-up power source.

            Lastly, alkaline fuel cells are considered because NASA already uses them for other processes, including rovers. These cells can operate at a temperature of 150 to 200oC, and the power output is satisfactory for almost any power demands placed on the rover.

            After talking to the group advisor Dr. Humboldt Mandell, several other points were taken into account. If AMTEC should be the choice and the heat harnessing idea comes to fruition, Dr. Mandell explained that Marlow Industries, Inc. manufactures an item called a Peltier device, whose purpose is to harness and distribute heat. Also, Dr. Mandell advised that nuclear power can be considered using Radioisotope Thermoelectric Generators (RTGs), which provide an “excellent power-to-weight ratio” (although not yet quantified) and in no way incur a radioactive hazard upon an astronaut using the rover. However, Dr. Mandell states the only downside is cost (again, not yet quantified).

After considering all power supply options, the group has determined that AMTEC cells are the best option for the desired rover (smallest weight and cost). However, should calculations or a situation change the rover size, the group will still consider RTGs as an option.

As of November 14th:

Power

I.  Design Requirements

The penultimate requirement for the power subsystem is that enough power is supplied to satisfy performance of Red Rover’s other subsystems. Communications and navigation, computation and imagery, and propulsion must also constrain power consumption as much as possible in order to obtain the proper balance of power supplied and power consumed.

There are two main concerns regarding power: how much power should be supplied, and what type of power supply will be most efficient (personified by a power-to-weight ratio). To determine efficiency of the power supply, the team weighs each characteristic of the supply against how much power the supply can give to the rover. One criterion here is power-to-weight ratio, but the team is also aware of cost, surface area, and mass being other constraining factors.

 

II. Final Design

The group began observing the lunar roving vehicles of the Apollo 15-17 missions. These LRVs were powered using two 36-Volt silver zinc batteries.[1] However, since the group wishes to produce power without the concern of charging or re-charging batteries, alternatives to the batteries mentioned above were necessary. Krypton-85 and Alkali Metal Thermal to Electric Converters (AMTEC), solar power, and radioisotope thermal generators (RTGs) were considered.

            Solar power requires 12 m2 of photovoltaic cell area to produce 1 kW.[2] The tremendous amount of surface area required would ruin any hopes of building a compact (and efficient) rover.

            After deliberating the use of a Radioisotope Thermal Generator compared to the Alkali Metal Thermal to Electric Converter cells, the group has concluded that the AMTEC technology is the optimal alternative for Red Rover’s design. In terms of power conversion ratio (power generated versus power consumed), the ratio of the RTG is 6%. The AMTEC cells each have a ratio of 15%, considerably better than the RTG. In addition, the process of obtaining power through AMTEC technology ejects waste heat as a side effect of producing power satisfactory for the design. This waste heat can be harnessed and utilized in the form of thermal energy to maintain other subsystems at a proper working temperature.

            A Carnegie Mellon lunar rover employs this method of power. Much of the following data regarding the AMTEC cells can be directly attributed to Carnegie Mellon’s previous research. This data will be tailored to suit the specific needs of the Tigernauts’ Mars rover.

            Three pressurized vessels of Krypton-85 gas “rest in an aerogel and MLI insulation block. AMTEC cells are directly mounted to conductive spacers on the curved surface of the pressure vessels. An aerogel insulation block is lowered onto the AMTEC cells to allow unidirectional heat flow. Finally the power system’s radiator is directly mounted to the cold side of the conversion cells to complete the assembly of the power system…The krypton vessels are not pressurized until shortly before launch for safety and performance needs.”[3] Consult Figure 1 for a schematic of the basic assembled design.

Figure 1. Schematic of One Basic Assembled Design.

(Source: Design of a Day/Night Lunar Rover. Dr. P. Berkelman, Carnegie Mellon University.)

            Three vessels made of Astroloy contain Krypton-85 gas (with a half-life of 10.7 years) at a temperature of 1000K and a pressure of 100 atmospheres. This Krypton-85 gas emits beta particles, which is converted to heat in each vessel. Each vessel is cylindrical with a hemispherical end cap on each end. The length of the cylinder is 50 cm, the diameter is 23 cm, and the thickness of the vessel is 1.3 cm, including an overall factor of safety of three. These dimensions are chosen to not only absorb a majority of the energy of the beta particles, but to negate effects of radiation as well. The vessels are designed to maintain the Krypton-85 gas at a temperature of 1000K for a lifespan of two years.

            The AMTEC cells partially convert electric energy from the heat that has been emitted from the vessels. “The AMTEC cell is a thermally regenerative concentration cell utilizing sodium as the working fluid and sodium beta-alumina solid electrolyte (BASE) as the ion selective membrane through which a nearly isothermal expansion of sodium can generate high current flow/low voltage power at high efficiency…The conversion of thermal to electric energy occurs by using heat to produce and maintain a sodium concentration gradient across a BASE membrane…The liquid sodium in the heat pipe evaporator, evaporates and flows as a vapor to the heat pipe condenser inside the BASE. The vapor condenses and deposits its latent heat, picked up in the evaporator, inside the BASE tube. Then the sodium liquid returns to the heat pipe evaporator through the heat pipe wick. In the power loop, sodium liquid fills the wicks on the condenser, in the artery, on the outside of the heat pipe condenser and the inside of the BASE tube. The heat delivered by the heat pipe loop keeps the entire BASE tube region hot and raises the vapor pressure of the sodium inside the BASE…The condenser is kept at a low temperature. The sodium vapor pressure (and concentration) in the region of the condenser and the outside of the BASE tube is therefore much lower than inside the BASE tube. This pressure, or concentration gradient produces an electrochemical potential difference across the BASE tube wall…When current is drawn through the electrodes and current collectors on both sides of the BASE, energy is extracted from the cell in the form of electrical power.”[4] The electrical power is then channeled to the subsystems via 65 AMTEC cells, while the thermal heat is released through the radiator. 

            The Tigernauts have budgeted 1700 Watts for Red Rover. 1400W will be used for propulsion (two-wheel drive), 100W for communications, and 200W for onboard computing. The power output for the Carnegie-Mellon assembled design is 520 Watts of electric power. However, since the AMTEC cells are only 15% efficient, 3466W of thermal power is necessary. It is the judgment of the Tigernauts that the design of Carnegie-Mellon be utilized as four independent assemblies for a total of 2080W. One assembly will power the onboard systems and communication while the other three assemblies power the two-wheel drive propulsion system. The maximum power output will still be 520W and the thermal power still 3466W on each assembly. Thus, for Red Rover, 260 AMTEC cells and twelve Astroloy vessels of Krypton-85 gas will be necessary. The total mass for the Krypton-85 gas necessary is approximately 4.0625 kg and the volume is approximately .2788m3.

            Each assembly discharges 2946W of thermal power into an apparatus built over the radiators. A Peltier device will harness the sum of 11784W and control working temperatures of each subsystem. Peltier devices “are small solid-state devices that function as heat pumps. A "typical" unit is a few millimeters thick by a few millimeters to a few centimeters square. It is a sandwich formed by two ceramic plates with an array of small Bismuth Telluride cubes ("couples") in between. When a DC current is applied heat is moved from one side of the device to the other - where it must be removed with a heatsink… If the current is reversed the device makes an excellent heater.”[5] The Peltier device in question would have the ability to maintain a control temperature (the temperature of the particular subsystem) by controlling the current in the device to heat and cool according to the present temperature.

 


 

[1] Wright, Mike and Jaques, Bob. A Brief History of the Lunar Roving Vehicle. 3 April 2002. NASA Marshall Space Flight Center. 7 Oct. 2003. < http://history.msfc.nasa.gov/LRV.pdf>.

 

[2] VanderWyst, Anton. Power Generation Analysis. 18 March 2002. University of Michigan Mars Rover Project. 21 Sept. 2003.

[3]Berkelman, Peter. Design of a Day/Night Lunar Rover. June 1995. The Robotics Institute, Carnegie Mellon University. 7 Oct. 2003. <http://www.ri.cmu.edu/pub_files/pub1/berkelman_peter_1995_1/berkelman_peter_1995_1.pdf>.

 

[4] Ivanenok, Joseph F. III, and Sievers, Robert K. “Radioisotope Powered AMTEC Systems.” IEEE AES Systems Magazine p. 29-35, November 1994.

[5]General Information on Peltier Devices. 2003. Thermoelectric Peltier Device Information Directory. 31 Oct. 2003. <http://www.peltier-info.com/info.html>.

 

 


 

[1] Wright, Mike and Jaques, Bob. A Brief History of the Lunar Roving Vehicle. 3 April 2002. NASA Marshall Space Flight Center. 7 Oct. 2003. < http://history.msfc.nasa.gov/LRV.pdf>.

[2] Berkelman, Peter. Design of a Day/Night Lunar Rover. June 1995. The Robotics Institute, Carnegie Mellon University. 7 Oct. 2003. <http://www.ri.cmu.edu/pub_files/pub1/berkelman_peter_1995_1/berkelman_peter_1995_1.pdf>.

[3] VanderWyst, Anton. Power Generation Analysis. 18 March 2002. University of Michigan Mars Rover Project. 21 Sept. 2003.