Planet-Z

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Despite the variety and advanced power storage systems' technologies, it is hard to choose a suitable system to use on Venus because of its hellish environment; However, we managed to make a complete power storage system consisted of 3 integrated subsystems that can withstand these hard conditions by comparing different systems and searching for possible developments that can overcome these systems’ disadvantages. 

1. Liquid Metal Battery -LMB-: 

After searching for storage systems that operate within nearly ~ 460 ⁰C and more than 92 bars, we decided to work with liquid metal batteries (LMB). 

LMB is a chemical battery that depends on a redox reaction to conduct electricity, but in liquid form instead of solid states to increase the cell's life cycle.  

Taking Lithium-ion batteries, for example, there are two ways of distortion that can happen to the material after a redox reaction. The first distortion is the permanent damage to the solid electrodes, and the other is due to lithium dendrite that deposits through the electrolyte until it makes a short circuit causing explosions, however, if the material is in the liquid form, there is nearly no distortion as shown in Fig.1. 

A liquid metal battery consists of two liquid metal electrodes separated by a molten salt electrolyte that acts as a separator solution and because of different densities they are present in three layers. Determining the suitable material used is done experimentally between negative and positive electrode material to determine the highest possible potential difference. After some comparisons, we decided to choose Calcium (Ca) as a donor atom along with Antimony (Sb) as an acceptor atom. Calcium theoretical specific capacity is 1.34 Amp-h/g which makes it one of the suitable elements for storage systems. 

Although Ca has great experimental results, it has drawbacks; It has a high melting point compared to other cathodic materials which are equal to 842 °C. It is highly soluble in its salts which will lead to unacceptably high self-discharge. 

Thus, we are aiming to use Magnesium as a noble metal. Magnesium will lower the melting point by stimulating the ionization of calcium atoms and reducing the reactivity of the pure calcium resulting in decreasing solubility in its salts. However, using Ca-Mg alloy as a cathodic will decrease the cell voltage, but it still has an acceptable voltage range. The Ca-Mg || Sb cell can operate at high temperatures and deliver an average voltage of 0.99 V [6,7]. 

As a molten salt electrolyte, we found that Metal-Organic Frameworks (MOFs) will act better than regular electrolyte salts. It can decrease the slight movements of liquid particles and increase the conductivity paths resulting in a decrease in the response time [3].  

ZIF 68 is one of the recommended MOFs to be used as it can donate electrons and withstand up to ~ 800 °C. 

ZIF 68 consists of Carbon Nitrogen and here Nitrogen is acting as an electron donor which can stimulate Ca+2 movement.  

However, if the MOF is subjected to very high temperatures, it undergoes a carbonization process that will make it act more as a graphene but with the same atom structure.[8] 

Although MOFs will reduce movements of the liquids efficiently, we will fill the whole battery case with fluid to absorb any sort of sudden movements as the baby is in his mother's womb. This balancing system will reduce the movements of the sudden changes. 

1. Power generation system: 

Power Generation and Recharging of the system: Average rover power consumption (according to NASA’s mars rover) is 𝟏𝟎𝟎 𝑾𝒉. To live up to the target of the mission of 60 days on Venus, the system needs to continuously get charged. There are a couple of methods to generate power for the rover; either by using a solar panel or by making use of the high density and low speed of the wind on the surface to generate enough power to maintain the mission.  

Initially we used a single Favonius turbine as shown in fig.2 to generate power, but we eventually decided to use 4 of them as one was just not enough.  

Here are the parameters of a single Savonius Turbine we designed: 

𝑫𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝑫=𝟎.𝟓𝟓 𝒎 

𝑯𝒆𝒊𝒈𝒉𝒕 𝑯=𝟏.𝟏 𝒎  

𝑨𝒓𝒆𝒂 𝑨=𝟎.𝟔𝟎𝟓 𝒎  

𝑨𝒔𝒑𝒆𝒄𝒕 𝑹𝒂𝒕𝒊𝒐=𝟐  

𝑫𝒓𝒂𝒈 𝑪𝒐𝒆𝒇𝒇 (𝑪𝒘)=𝟏.𝟑𝟑 

𝑴𝒂𝒔𝒔 (𝑴)≅𝟑 𝒌𝒈 

𝑴𝒂𝒙 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒂𝒊𝒓 𝒊𝒏 𝒕𝒉𝒆 𝒈𝒂𝒑=𝟕.𝟕𝟐 𝒌𝒈 

𝑬𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 (𝜼)=𝟑𝟗% 

𝑴𝒊𝒏𝒊𝒎𝒖𝒎 𝒘𝒊𝒏𝒅 𝒔𝒑𝒆𝒆𝒅 𝒇𝒐𝒓 𝒕𝒉𝒆 𝒓𝒐𝒕𝒐𝒓 𝒕𝒐 𝒎𝒐𝒗𝒆=𝟎.𝟐𝟕𝟔 𝒎/𝒔 

The power generated from the four Savonius turbines can be calculated from the following equation:  

𝑷𝒐𝒘𝒆𝒓=𝟏𝟐 𝑨 𝝆 𝜼 𝑽𝟑∗𝟒 

𝑨𝒊𝒓 𝒅𝒆𝒏𝒔𝒊𝒕𝒚 𝒐𝒏 𝒗𝒆𝒏𝒖𝒔 (𝝆)=𝟔𝟓 𝑲𝒈/𝒎𝟑 

As it is shown, the power generated is related to velocity to the power of three; therefore, a minimal change in wind speed would result in an enormous difference in the power generated, we can consider that wind speed is main factor in this process. 

Given the worst-case wind speed of 1 m/s, a single turbine would generate about 𝟕.𝟔 𝑾, but with an average wind speed of 1.5 m/s, a single wind turbine would generate about 𝟐𝟓.𝟖 𝑾and a peak of 2.5 m/s would generate 𝟏𝟐𝟎 𝑾. 

Total power generated from the 4 turbines: 

    Wind Speed (m/s       Power generated – 4 turbines (W) 

1 (Wors-case)                                      31    

1.5 (Average)                                      104 

2 (Best case recorded)                      480 

Efficiency improvement:   

To improve the turbine efficiency, Nano Carbon tubes are used to reduce the weight of the turbine 

1. Battery Management System (BMS): 

Current: Monitoring battery pack current and cell is the road to electrical protection. The electrical safe operating area (SOA) of any battery cell is bound by current and voltage. A well designed BMS will protect the pack by preventing operation outside the cell ratings. In many cases, further derating may be applied to reside within the SOA safe zone in the interest of promoting further battery lifespan. A shunt resistor is used to measure the current of the battery pack in our system  

Voltage: The SOA boundaries will be determined by the intrinsic chemistry of the Liquid metal cell and the temperature of the cells at any given time. Moreover, since any battery pack experiences a significant amount of current cycling, discharging due to load demands and charging from a variety of energy sources, the SOA voltage limits are usually further constrained to optimize battery lifespan. The BMS must know what these limits are and will command decisions based upon the proximity to these thresholds. For example, when approaching the high voltage limit, a BMS may request a gradual reduction of charging current or may request the charging current be terminated altogether if the limit is reached.  

Temperature: A BMS can control the temperature of the battery pack through heating and cooling. Controlling the temperature of the battery pack is essential to ensure maximum efficiency of the cell and to cut off the charging or discharging of the battery if the temperature goes too high. A sensor is used to measure the temperature inside the bank and signal it to the BMS system.  

Capacity Management: Maximizing a battery pack capacity is one of the most vital battery performance features that a BMS provides. If this maintenance is not performed, a battery pack may eventually render itself useless. The root of the issue is that a battery pack “stack” (series array of cells) is not perfectly equal and intrinsically has slightly different leakage or self-discharge rates. The balancing process endpoints, before and after, are shown in the figure on the right.  

In summary, a BMS balances a battery stack by allowing a cell or module in a stack to see a different charging current than the pack current in one of the following ways:  

Removal of charge from the most charged cells, which gives headroom for additional charging current to prevent overcharging, and allows the less charged cells to receive more charging current  

Redirection of some or all the charging current around the most charged cells, thereby allowing the less charged cells to receive charging current for a longer length of time Diagram, schematic 

The schematic of the BMS and power controller (proof of concept):  

For the primary battery (Liquid Metal Battery) and the secondary one, each cell voltage, total voltage and current of the bank is monitored by BMS. A Temperature sensor is used to signal the temperature of the pack to the BMS. The BMS performs the algorithms required and outputs the SOC (State of charge), SOE (State of energy), SOH (State of health) of both batteries to the Power controller which in return, which is best to supply the demand of the load and which to charge. 

LMB Cell Performance @ 650 °C and 200 mA cm-2 

Theoretical discharge capacity: 1.08 Ah 

Achieved discharge capacity: 0.881 Ah 

Gravimetric discharge capacity density: 93.3 Ah/kg 

Self-discharge rate: ~4 mA cm 

Mass: 71.38 g 

Volume: 4.07E-5 m 

Battery Pack Specifications 

Volt: 24 V 

Total Capacity: 15 Ah 

Number of cells: 350 

Volume: 0.02835 m 

Mass: 25 kg 

Low self-discharge rate 

High temperature operation 

Safe launching and landing 

Shock resistance 

Average case (1.5 m/s): 104 W 

Best case recorded (2m m/s): 480 W 

We used proteus, SolidWorks, MatLab and Python programming language to calculate and simulate our data.  

1. 2019.Spaceappschallenge.org. (n.d.). Retrieved October 1, 2022, from https://2019.spaceappschallenge.org/challenges/planets-near-and-far/memory-maker/details  
2. Hall, L. (2016, April 1). Automaton rover for extreme environments (AREE). NASA. Retrieved October 1, 2022, from https://www.nasa.gov/feature/automaton-rover-for-extreme-environments-aree/  
3. Mars. (2020, September 11). Mars Rover specifications. NASA. Retrieved October 1, 2022, from https://mars.nasa.gov/msl/spacecraft/rover/summary 
4. Gipson, L. (2021, April 7). NASA seeks to create a better battery with sabers. NASA. Retrieved October 1, 2022, from https://www.nasa.gov/feature/nasa-seeks-to-create-a-better-battery-with-sabers 
5. Schlieder, S. (2020, February 18). Exploring hell: Avoiding obstacles on a Clockwork Rover. NASA. Retrieved October 1, 2022, from https://www.nasa.gov/exploring-hell-venus-rover-challenge   

By exploring Venus using NASAs’ resources, we learned valuable information about Venus’s surface, date & years, and weather. After exploring the target planet, we started reviewing the previous rover’s mission (such as mars 2020) to study the rover’s operations, power consumption, and main tasks. 

Then we assumed the required power for the challenge mission and started our research, based on another journey to other planets, we could design our new battery solution.

At the beginning of the competition, we hoped to achieve an effective solution to one of the problems of space.

Exploring Venus was our choice and finding an energy system that would work for 60 days in an atmosphere as intense as Venus was the real challenge.

After days of research and reading scientific papers, we came up with the perfect solution to the problem. At first week we began the journey by dividing the challenge into stages.

First stage was to define the problem and know our limitations; Extreme temperature in Venus prevents us from using RTGs or depend on regular batteries such as Li-ion battery. This paves the way for our next step.

The second stage was to start looking for suitable energy storage systems in the resources of NASA and previous space missions and started comparing the different storage systems to select the most suitable one that would supply the rover with its power requirements.

As soon as the Liquid Metal Battery was selected, we began to calculate all its ratings and protection system that were to be used in order to maintain the target of the mission.

In the latter stages, we realized that the system will not be able to run on its own for the duration of the missions, so we had to look for a method to generate power. We chose wind turbines as we found they were sufficient for the requirements of the mission and the energy storage system.

1. 2019.Spaceappschallenge.org. (n.d.). Retrieved October 1, 2022, from https://2019.spaceappschallenge.org/challenges/planets-near-and-far/memory-maker/details

2. Hall, L. (2016, April 1). Automaton rover for extreme environments (AREE). NASA. Retrieved October 1, 2022, from https://www.nasa.gov/feature/automaton-rover-for-extreme-environments-aree/

3. Mars. (2020, September 11). Mars Rover specifications. NASA. Retrieved October 1, 2022, from https://mars.nasa.gov/msl/spacecraft/rover/summary

4. Gipson, L. (2021, April 7). NASA seeks to create a better battery with sabers. NASA. Retrieved October 1, 2022, from https://www.nasa.gov/feature/nasa-seeks-to-create-a-better-battery-with-sabers

5. Pimonova, Y. A., Lastovina, T. A., Budnyk, A. P., Kudryavtsev, E. A., & Yapryntsev, M. N. (2019). Cobalt-based ZIF-68 and ZIF-69 as the precursors of non-platinum electrocatalysts for oxygen reduction. In Mendeleev Communications (Vol. 29, Issue 5, pp. 544–546). Elsevier BV. https://doi.org/10.1016/j.mencom.2019.09.022

6. Schlieder, S. (2020, February 18). Exploring hell: Avoiding obstacles on a Clockwork Rover. NASA. Retrieved October 1, 2022, from https://www.nasa.gov/exploring-hell-venus-rover-challenge

7. Ouchi, T., Kim, H., Spatocco, B. L., & Sadoway, D. R. (2016). Calcium-based multi-element chemistry for grid-scale electrochemical energy storage. Nature communications, 7, 10999. https://doi.org/10.1038/ncomms10999

8. Calcium-antimony alloys as electrodes for liquid metal batteries. (n.d.). Retrieved October 1, 2022, from https://www.researchgate.net/profile/Xiaohui-Ning/publication/270602166_Calcium-Antimony_Alloys_as_Electrodes_for_Liquid_Metal_Batteries/links/56a6e21808ae0fd8b3fc7579/Calcium-Antimony-Alloys-as-Electrodes-for-Liquid-Metal-Batteries.pdf

#Space, #Venus, #Batteries, #Rover