Theia

Universal Event | Exploring Venus Together

Theia

High-Level Project Summary

Theia is a modified version of a standard radioisotope thermoelectric generator (RTG) that overcomes the high pressures and temperatures of Venus’ surface through redesigned component structures and materials. This includes an internal complex of 15 stacked modules containing Pu-238 with aerogel-insulated SiGe thermoelectric converters used to generate power from the temperature differential. This complex is contained within a gold-plated structure made of chromium-molybdenum alloy steel with 8 fins to help remove excess heat. This redesign creates an energy source/storage system that could effectively function for over 10 years on Venus.

Link to Final Project

https://mixolydian-chartreuse-1d7.notion.site/Theia-An-Iterated-RTG-for-Long-Term-Venus-Missions-8833cb086c52420791074981d2539cce

Link to Project "Demo"

https://youtu.be/3MRojZNB0I8

Detailed Project Description

The Problem 

Current energy sources don’t match the demanding needs of a long-term mission to Venus’ surface. The high-pressure environment (around 92 bar, which is 92 times greater than the atmospheric pressure on the surface of Earth) would heavily affect how systems function on Venus and how much power an object would require to operate. There are also extremely high temperatures of the planet’s surface (∼465 °C), making the design of an effective energy system challenging.

In the past, researchers have reviewed primary batteries, rechargeable batteries, solar arrays, ion harvesting, wind energy, and chemical energy for energy source/storage systems on Venus. The limitations to primary batteries and chemical energy were that they functioned on the basis of a 1-time use facility. The whole power output of the system is dependent on the capacity of the containment system being used for the batteries. 

Rechargeable batteries and solar arrays are viable but have more logistical/technological barriers to implementation. The use of solar arrays would require aerial vehicles due to the lack of sunlight that reaches Venus’ surface, which in themselves require their own power source. Rechargeable batteries would be possible, but there are no viable systems to use as an energy source in Venus conditions to recharge them. 

Ion harvesting also has a logistical barrier as a 120 km long tether alongside electronics would be required to process the ions into electricity. Wind velocities are also too sparse on the surface and to harness the wind velocities up in the atmosphere, would need a large tether and an aerial vehicle with turbines to collect the power, which would be another logistical barrier.


The Solution 

Theia instead proposes a redesigned radioisotope thermoelectric generator (RTG). RTGs work by using the heat from the natural radioactive decay of radioisotopes. There is a hot and cold shoe that creates a temperature difference between the 2 sides. This creates a magnetic field in which the electrons can flow, known as thermocoupling. There is a fundamental principle known as the Seebeck effect in RTGs which states that when there are 2 different materials at different temperatures, then the temperature differential creates thermocouples to convert heat into electricity.  

The type of RTG we focused on was with GPHS-RTGs as they showed potential for high-temperature usage and were common in previous missions. Current RTGs would only be able power for a few minutes to days on Venus’ surface. To accommodate for the high-pressure and temperature environments of Venus, Theia modifies the conventional design to be more fitting for the Venus surface alongside recreating the structure of the RTG. Below are all the modifications that were made:  



  1. Heat Shoe/Fuel Pellets: Iridium alloy clads, graphite impact shells, and carbon fiber would be the materials used for the fuel pellets containing Pu-238, and would be part of the GPHS modules. The materials present within the fuel pellets would also make for excellent heat shoes for the thermocouple system. 
  2. Insulation: We used a triple insulation technique to prevent heat loss to maximize power output and ensure the safety of the fuel pellets in the Venus surface-modified GPHS-RTG. The first layer of insulation would consist of silica aerogels whose maximum melting point is 1200 °C. Because of the ambiguity regarding the variables of the calculations (see final submission mathematical equations), we created the temperature gradient of the system based on the cold shoe being the external temperature of Venus and the hot shoe being around 1050 °C. The aerogels have very low thermal conductivity, making them deal insulators. The additional vacuum insulator works by slowing the finite distribution of heat in RTGs by restricting airflow. The last layer is titanium which is non-corrosive and contains everything regarding the heat shoe.
  3. Thermoelectric Converters: Silicon germanium (SiGe) would be the primary thermoelectric converter alloy used in the RTG. This alloy works better at higher temperatures than most other alloys which tend to fall off at the 400 - 500 °C range. SiGe is also more stable and less prone to changes caused by vibrations and impacts. 
  4. Cold Shoe/Outer Coating: Tungsten, beryllium oxide (ceramic), graphite, and copper & silver alloys have notably high thermal conductivities, which is essential as the cold shoe must dissipate heat. Layering these materials would help create a solid cold shoe that would be able to neglect radiation/heat loss functions. Gold would be used as the outer coating as it is highly resistant to single acids and sulfuric acids, which are in high levels of concentration through the Venus atmosphere. 
  5. Fins/Gas Management Assembly (Not Included in CAD): Pyrolytic graphite was selected as the material for the fins as it has excellent thermal conductivity (1700 W/m-k) and a high melting point at 2200 °C. The fins act as radiators in which the heat loss from the cold side travels to the fins to be slowly released into the atmosphere. The gas management assembly would be made of stainless steel due to its high melting point of 1400 - 1500 °C alongside the high-gas pressure containment capabilities of the metal. 


CAD Prototypes


Full Prototype (Translucent)


Full Prototype (Section View)


Module (Section View):

The Outcome

Ultimately, Theia encompasses a redesigned radioisotope thermoelectric generator optimized to function on Venus for multiple years. The heat source, Pu-238, is able to sustain a rover that consumes ~150W daily for the entire 60-day duration of the mission (plus more), due to its half-life of 87.7 years. Furthermore, the layers of insulation are a culmination of modern devices, each proven to be individually superior to other methods under harsh conditions like that over Venus. In addition to this, SiGe was used for the thermoelectric converters, which is generally accepted as the best material for that purpose. The cold shoe is also a composition of highly thermally conductive materials which are able to quickly dissipate heat, allowing for a larger internal temperature differential, resulting in more power output. Finally, the fins and gas management systems are used to maintain the systems by dissipating the heat and managing the small number of He particles released during the isotopic decay of Plu-238.

Space Agency Data

Our project used sources from NASA, science webpages, and various papers to come up with our design. We used academic papers to learn how RTG technology has already been used and how it is planned to be used in future space missions to serve as a starting point for our research. Other articles and simulations were used to understand thermoelectric power generation and how it could be used in our RTG models. Articles were also used to study the properties of materials such as pyrolytic graphite and platinum, and how they could be used in the RTG.

Hackathon Journey

What We Learned

Throughout our NASA Space Apps journey, our team learned many valuable skills that we will be able to apply in our lives and future hackathons, as well as technical information surrounding the project. Some of the most important skills we learned were how to manage our time to be as efficient as possible, how to brainstorm and manage ideas to decide on what we wanted to focus on, and how to collaborate as a team and communicate schedules, thoughts, and new findings. Some of the more technical skills include how RTG technology works and how it has been used, the math behind our system's efficiency and power output, and how different compounds and materials could be used to maximize the efficiency of an RTG.


Why Exploring Venus?

We chose the Exploring Venus Challenge because we believe that interplanetary exploration and expansion will be a major part of space advancement in the coming decades. We also believe that Venus is still incredibly under-explored and has the potential to reveal important information about the history of our Solar System and the Earth. It was an amazing journey to figure out how our power generation system would work out and we are very happy to have had the opportunity to work on it.


Our Approach 

Our approach to developing our idea was by segmenting our time into preliminary research, understanding the problem, brainstorming, solidifying our idea, and prototyping. Initially, we had to familiarize ourselves with the status quo of Venus exploration and energy storage systems, and why standard energy systems will likely fail on the surface of Venus. After understanding the main challenges energy systems would face in Venus’ harsh surface environment, we began brainstorming various ideas, which were each explored and eliminated until we landed on RTGs. We sought for issues that a regular RTG would face on Venus, and how we could eliminate those issues through structure and material redesigns. Finally, we started writing our project, calculating numbers, and prototyping a CAD model. Our team also stayed in communication regularly throughout the challenge, going on calls to discuss ideas and build together.


Thank you to NASA Space APPs and all other organizations involved for giving us the chance to undergo this exciting and innovative journey!

References

Tools

  • Onshape
  • SimScale
  • Notion


Sources

Tags

#energy #CAD #radioisotope #venus #energysystem

Exploring Venus Together

Your challenge is to design an energy storage system that will power a surface lander or rover on the surface of Venus for at least 60 days, so that there is a viable energy storage capability for long-duration exploration missions.