Mars Or Bust, LLC
1.0Mission Summary
2.0Systems Engineering and Integration
3.0Mars Environment and In-Situ Resource Utilization
4.0Structures Subsystem
5.0Electrical Power Distribution and Allocation Subsystem
5.1Overview
The power subsystem will manage, distribute and store power throughout the Mars habitat. Both mobile and stationary sources of power will be present within the habitat to provide for all the functions of the habitat and mission.
5.1.1 Level 2 Requirements
The following requirements were derived from the DRM and level 1 requirements:
Table 5.1: Level 2 Requirements
5.1.2Power Profile
The power profile (table 5.2) was created by collecting peak power usage from each subsystem during various mission modes. These modes include both non-crewed and crewed times as well as a survival added with battery and non-battery, in case the nuclear reactor is not operational during survival mode. The crewed active day and night modes push the limits of the allocated 25 kW to the habitat. However, these numbers could be reduced with further revisions from each subsystem. Also, the survival modes have relatively high power usage to allow the capability of an EVA. If the habitat were to go into a survival battery mode, ECLSS would not have the water filtration system on and the habitat would run off the storage tanks. This is why the battery and non-battery power survival values differ.
Table 5.2: Power Profile
5.1.3Mass/Power/Volume Breakdown
The mass of electrical cabling inside for the habitat can be estimated for different gauges of wire used. Aluminum wire would most likely be used due to its low mass and high conductivity. Below is a table showing several different gauges of wire as well as their mass for 5000 meters.
Table 5.3: Electrical Cabling Mass and Thermal Breakdown
Gauge / Diameter (cm) / Area (cm2) / Mass (kg/m) / Mass for 5000m of cabling / Resistance (ohms/km @ 20C) / % Loss for 25 kW1 / 0.73 / 0.42 / 0.11 / 572.30 / 0.68 / 2.35
2 / 0.65 / 0.34 / 0.09 / 453.76 / 0.86 / 2.97
4 / 0.52 / 0.21 / 0.06 / 285.41 / 1.36 / 4.73
6 / 0.41 / 0.13 / 0.04 / 179.46 / 2.16 / 7.51
8 / 0.33 / 0.08 / 0.02 / 112.91 / 3.32 / 11.54
10 / 0.26 / 0.05 / 0.01 / 71.00 / 5.29 / 18.35
12 / 0.21 / 0.03 / 0.01 / 44.64 / 8.40 / 29.16
14 / 0.16 / 0.02 / 0.01 / 28.10 / 13.35 / 46.36
16 / 0.13 / 0.01 / 0.00 / 17.65 / 21.26 / 73.82
18 / 0.10 / 0.01 / 0.00 / 11.11 / 33.79 / 117.34
20 / 0.08 / 0.01 / 0.00 / 7.00 / 53.48 / 185.69
The masses were calculated using simple volume calculations with 2.699 g/cm3 as the density of aluminum. Table 5.2 also shows the resistance and resistance losses using a total of 25 kW. Using this table, we can only estimate the total mass for electrical cabling in the habitat. We chose to use 5000 meters of cabling for an estimate, but the table also has a mass per unit length for future calculations, as the total cabling becomes more apparent. Using the table, we estimated the mass to be around 150 kg and heat losses from resistance to be around 10%. Any higher losses would be too inefficient and should result in choosing a larger gauge. We also realize that an actual habitat would use a combination of smaller and larger gauges depending on the load.
The mass of the power subsystem was calculated to include all of the equipment that would be used in a regulated system. This includes the charge control, batteries, regulation, conditioners, circuit breakers, and wire needed to carry out the full mission of the subsystem. Because this is the initial design phase and all of the components are not known as of yet, the mass of the power system is an estimation dependant upon the power output. The masses of the equipment were estimated using the ratio of 11.4 kg/kW for all equipment less the wiring and batteries [Larson and Pranke, 2000]. Having a 25 kW system the mass comes to 285 kg. The batteries were sized to provide 10 kW of power to keep the habitat running until power could be restored or provided from the rover. The habitat will have 24 hours to run on batteries. Using Li-ion batteries with a specific power of 170 W*h/kg and the specified power needed and time, a mass of 1411 kg of batteries is needed. There is also mass that needs to be allocated for spares. This was decided to be set at 100 kg for spare breakers, or other equipment that may need replacing. A table showing all of the mass calculations and the total mass required for the system is shown below. The estimated volumes are shown as well. The volume for the batteries was estimated by using the energy density of 160 W*h/L [Larson and Pranke, 2000]. Battery volume plus the estimates for the rest of the system gives a total of 3.6 cubic meters.
Table 5.4: Power System Masses and Volumes
Power System Masses and VolumesBattery Mass / W*h/kg / W / Time (h) / kg / Volume (m3)
Li-Ion / 170 / 10000 / 24 / 1411.765 / 1.5
Regulated system mass / 11.4 / 25 / 285 / 2
Spares (breakers, etc.) / 100 / 0.10
Total mass w/o wires / 1796.765
Total Volume / 3.60
The total mass and volume breakdown is shown in table 5.4.
Table 5.5: Mass/Volume Breakdown
Power Subsystem TechnologiesWires/Cabling
Component / # / Weight (kg) / Add. Weight (kg) / Total Weight (kg) / Power (kW) / Total Power (kW) / Volume (m3) / Total Volume (m3) / Crew Time (hrs/day)
Wires/Cabling / 150.00 / 2.5* / 0.10
Totals / 150 / 2.5 / 0.1
*Amount of heat generated
Batteries
Component / # / Weight (kg) / Add. Weight (kg) / Total Weight (kg) / Power (kW) / Total Power (kW) / Volume (m3) / Total Volume (m3) / Crew Time (hrs/day)
Li-ion / 1411.77 / 10* / 3.00
Totals / 1411.77 / 10 / 3
*Amount of power produced, not needed
Regulated System
Component / # / Weight (kg) / Add. Weight (kg) / Total Weight (kg) / Power (kW) / Total Power (kW) / Volume (m3) / Total Volume (m3) / Crew Time (hrs/day)
Regulated System / 285.00 / 25.00 / 1.00
Spares (breakers, etc.) / 200.00
Totals / 485 / 25 / 1
Grand Totals / 2046.77 / 37.50 / 4.10
5.1.4Input Output
The input/output diagram (figure 5.1) for the power subsystem is fairly simple. Power is taken in from the nuclear reactor and distributed throughout the habitat. Approximately 2.5 kW of heat is output to the thermal subsystem and command is input from the C3 subsystem.
Figure 5.1: Input/Output Diagram
5.2Design and Assumptions
5.2.1Assumptions
There were several assumptions made in the design of the power subsystem. Even while in transit, the habitat requires a minimum amount of power. It was assumed that this power would be supplied to the habitat before it arrives on Mars. Also, the nuclear reactor would already be on the surface of Mars to provide the habitat with power along with the cabling used to transfer power from the reactor to the habitat. An assumed 25 kW is allocated to the habitat with a maximum of 160 kW to the whole system.
5.2.2Functional Diagram
The power system will provide power from the nuclear reactor. There will be two reactors but only one will be used at a time, the other will serve as a backup. The power will flow through a three bus, regulated system. The chart below shows the diagram of the power flow. The system will allow for power to flow from the reactors to the conditioner and regulator or to the batteries via a charge controller if the batteries are in need of charging. The power can also flow from the batteries to the conditioner and regulator if the reactors are offline. After regulating the power to a usable voltage the power is transferred on either of three buses to the distribution hub. A three-bus system is used for redundancy and three-level redundancy is needed because it is a life critical system. From here the power is transferred to separate breakers that are specified for each subsystem with an extra breaker for life or mission critical systems that cannot be turned off. Figure 5.3 shows, as an example, the critical components of the Command, Control, and Communication under the life/mission critical breaker. Most ECLSS functions are considered life or mission critical and therefore kept under the ECLSS breaker. Under each subsystem there can also be branches to each component or a breaker to a few components depending on the need of the component. Figure 5.4 and 5.5 show examples of the ECLSS and CCC subsystem in the way that they are setup. The system uses circuit breakers to allow for equipment to be powered down while connecting or disconnecting the equipment, reducing the risk of arching. The structure of the system is designed to minimize the interference between subsystems while connecting or disconnecting components within a certain system. The figures show a basic structure for the layout of the power grid, however since all components are not known as of yet, a complete system cannot be mapped out.
Figure 5.2: General Functional Diagram
Figure 5.3: Life/Mission Critical
Figure 5.4: ECLSS
Figure 5.5: CCC
5.2.3AC vs. DC/120V
The nuclear reactor will be outputting AC power, which is easier to transport at high voltages over long distances. Based on requirements of today’s technology, it was decided to use a voltage regulator and operate the habitat on 120V. Also, this is a safe operating voltage because there is little plasma interaction, which will reduce the risk of arching. Since the habitat will be operating at 120 Volts and the reactor is 2 kilometers away, AC current is ideal. However, the habitat will be using DC power so the input current from the reactor will need to be converted. Coincidentally converting from AC power to DC is easier and more efficient than DC to AC.
5.2.4Contingency Power Supply
The power system, being life critical, needs two backups. There are two nuclear reactors that can be used. One reactor at a time supplies the power to run the habitat and the second reactor is on standby as the first backup. The second backup is the 10 kW power supply in the pressurized rover. This supply can be connected to the habitat and used to keep power supplied to critical systems. The batteries in the habitat will remain charged and be ready to supply 10 kW of power for 24 continuous hours while the second reactor is powered up or if needed, for the rover to return from a mission and be connected to supply power. The batteries will insure the habitat is never powerless during the times when the power load is switched from one source to another. While the batteries will weigh the most, they will last the entire mission, as they will not be cycled very often. Battery life depends on the amount of cycles. 10 kW is the initial full charge and with an acceptable rate of degradation they will last well over 500 cycles. The 10 kW of backup power will be enough to power the habitat and keep the occupants alive but may not be enough to sustain all or any current experiments or EVAs. Therefore when running on the emergency 10 kW of power all unnecessary equipment needs to be powered down and only life and mission critical systems will be powered.
5.2.5Mission Operations Overview
There are some operations that will be required by the power subsystem during the mission. In case of emergency, the power cutoff and restart was estimated to take 2 hours, occurring as little as possible. It will take 3 crewmembers with arching concerns and potential damage to equipment. General power maintenance, such as replacing fuses or resetting circuit breakers, would only take 20 minutes to perform with 2 crewmembers. The maintenance will occur as needed, with only general electrical safety concerns. Manual switching of lights, computers, etc. will only need one crewmember and occur as needed. When the equipment is brought on-line or the power is cycled, the initial power-up and breaker switching will take 2 hours with 2 crewmembers. There is potential for arcing and damage to equipment. This operation will occur at the initial habitat set up and as often as the emergency power cutoff and restart occurs. The majority of the equipment will have automated switching, which will occur as needed. There are only concerns with error in the automation.
5.3Verification of Requirements
5.3.1Level 2 Requirements
The power group was as able to design the subsystem to meet all level 2 requirements. Table 5.6 shows the level 2 requirements and the methods used to meet them. Mainly, the 3-level redundancy was met with a back up reactor and solar panels. Batteries were also implemented for safety purposes. Also, the mass came out to be 2050 kg, which is well below the allocated 3250 kg recommended by the DRM.
Table 5.6: Requirements Verification
5.3.2Failure Modes Effects Analysis
There are potentially two failures that can occur in the power subsystem. The first one is a power outage, which can arise two different ways. One is power outage to the whole habitat and the second possibility is power outage to specific subsystems. Following the flow charge, one can observe that the power outage to specific subsystems can occur due to many different causes, damaged power line, damaged transformer and damaged connector. If one of the above causes occurs, then the crews must fix or replace the specific technology that had been damaged. However, during this time, the backup power must be used. Whether the backup power from the power subsystem or the battery located at each subsystem will be used is determined by the technology that had been damaged. For instance, if the power line is damaged, then the power subsystem cannot provide the specific subsystem with backup power that the problem is being addressed at and the battery located inside the specific subsystem will be used for all the life critical components in the subsystem.
Power outage to the whole habitat can occur when the reactor or the transformer is damaged. Fixing and replacing will have to be considered. However, the time in which the problem can be fixed is indefinite so the battery will be used as backup power during the night cycle and the high efficiency solar panels will be used during the day cycle as back up power for life-critical components and charging the battery for night cycle usage until the problem can be fixed. Power supply from the rovers is also an option here.
The second major problem that the power subsystem can encounter is the voltage at outlets inside subsystems does not meet the requirement of 120 V. This can occur when the transformer is damaged and doesn't distribute the right voltage to each subsystem's outlet. The problem can affect the habitat two different ways. Either the whole habitat’s outlet is affected or some specific outlet is affected. Knowing which outlets have been affected will help the crew locate the problem. If this occurs, backup power will be used to makeup the voltage so life-critical components of each subsystem can operate properly while the problem is being repaired.
Figure 5.6: Failure Tree
5.4Future Considerations:
After this first iteration, some things need to be further analyzed before moving on to the next step in the design. First, a more detailed power profile could be constructed with more mission modes. Also, the hardware used to transfer and distribute the power will need to be specified. Although the system mass came out to be below the allocated budget, another iteration could be preformed to lower the mass even more. The effects of electromagnetic interface and electrostatic discharge also need to be taken into consideration.
6.0Appendices
6.1Appendix A: Acronyms
Please add any acronyms that you use in your section here.
DRM / Design Reference ManualKISS / Keep It Simple and Stupid
MOB / Mars Or Bust
6.2Appendix B: References
Please add any references that you use in your section here.
6.3Appendix C: Acknowledgements
Please add any acknowledgements specific to your section here.
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