High-Level Project Summary
The N.E.V.E Storage System project (Nitrogen Expansion Vessel Energy Storage System) aims to develop an energy storage system that will power a stationary surface lander during a long duration exploration mission on the planet Venus. The goal will be achieved by exploiting the thermal expansion property of a cryogenic fluid (such es liquid nitrogen) that, stored in a tank, will build up pressure due to the high temperature of the planet surface providing a constant stream of cold gas that will power the circuits trough a small turbine generator, cooling down the electronics at the same time.
Link to Final Project
Link to Project "Demo"
Detailed Project Description
For a long term in situ planetary exploration missions on Venus, an innovative energy storage system is needed. Due to the extreme temperature, the acidic atmosphere and the high pressure, most of the common energy storage systems used in other exploration missions until now are very difficult to use, forcing us to explore other ways to face the problem.
The most important decision to make is the choice of the energy storage itself and we decided to go for a Cryogenic Energy Storage (CES). A CES uses a cryogenic liquid, such es liquid Nitrogen, to store thermal energy.
WHY?
The reasons for this choice are multiple:
- This kind of energy storage is relatively dense, up to 214 Wh/kg for liquid Nitrogen.
- ·It does not require any kind of combustion or inflammable material, just inert gases.
- It will use the heat of the surrounding environment to work, with no need of artificial heat sources.
- The exhaust cold gas will also be used to cool down the electronics protecting them from the extreme heat of the planet
- It can space travel without major energy losses due to the extremely low temperature of the outer space.
- It uses Nitrogen, which is abundantly available in loco, opening the opportunity to recharge the system (more on that later).
How does it work?
This system would work as shown below (fig 1):
- The Nitrogen is stored in an insulated pressurized vessel tank (1.a), connected with a heat exchanger (1.b) directly exposed to the external environment.
- As soon as the temperature start rising, the nitrogen expands, extracting heat from the ambient air, becoming nitrogen gas, and building up pressure inside the tank.
- When the desired pressure is reached a pressure-controlled valve (1.c) would let the resulting pressurized gas out, that with the help of a pressure regulator will create a constant stream of nitrogen gas.
- ·The gas will operate a small turbine generator (1.d) powering the electronics.
- ·A small lithium battery (1.e) is used as an energy buffer allowing higher power demanding operations when needed.
- Both the buffer battery and the circuits (1.f) will be stored in an insulated enclosure (1.g) and constantly cooled down by the exhaust nitrogen gas from the turbine.
- In the end another valve (1.h) will let the gas out of the enclosure (1.i) into the environment.
- ·As a safety measure one last pressure-controlled valve (1.j) is installed on the tank itself. This valve will just open if a peak pressure is reached, shooting a burst of gas directly into the environment, releasing pressure.
fig. 1: schematic
In this setup the overall duration of the mission will depend exclusively on the size of the nitrogen tank. As previously said nitrogen can store up to 214Wh/kg and similar engines have an efficiency of around 60-65%. This means that we could approximately extract up to 130Wh/kg from the surrounding environment.
In the hypothesis of a stationary lander that needs from 5 to 10 W per hour to power his circuitry and sensors, to achieve the assigned goal of 60 days we would need a nitrogen tank of at least 55kg resulting in a volume of liquid nitrogen of approximately 70 liters (approximately the size of an average car GPL tank).
This won’t account the weight of the tank itself. Since here on Earth common liquid nitrogen tanks are not pressurized and definitely not rated for Venus’s temperatures, a custom tank is needed. From previous missions we know that stainless steel and titanium alloy can withstand the extreme environment of the surface. The weight of a seventy liters tank made from those metals has to be defined, but using a low-density insulating material as casing, such es reinforced aerogel of some sort, can help reducing the amount of metal needed by slowing down the pressure build up inside the thank.
g. 2: Lander prototype design
The system will start to operate as soon as the tank is filled with the liquid nitrogen on Earth, but once in the outer space, the temperature will drastically reduce, and the inner pressure will stop rising. This will assure that during the flight there won’t be any major energy losses until the descent in the Venus’s atmosphere.
Since this system is relatively compact and it doesn’t have exposed moving parts, it should, to the best of our knowledge, tolerate the forces and vibrations due to launch, reentry, descent, and landing. Anyway, further investigations and research are needed.
This design requires the tank to work in an upright position (fig. 2): the liquid nitrogen will always sit on the bottom due to gravity, while the gas will try to escape from the top. In an upside-down position the expansion of the gas could eventually force the still liquid nitrogen into the heat exchanger and then into the valve itself; this probably won’t be fatal but could bring to unexpected inefficiencies in the system and nitrogen losses.
EXTRA
The atmosphere of Venus is composed of around 96.5% carbon dioxide and 3.5% nitrogen. Even if it doesn’t sound much, due to the extreme density of the atmosphere, the actual nitrogen content is four time the amount of nitrogen here on Earth. If only we could extract the liquid nitrogen from the planet’s atmosphere by using wind or solar power this would allow the lander to be recharged in situ.
However, the liquefaction of the nitrogen is an inefficient operation, extremely impractical, if not impossible to do on the Venus surface; but thinking at the future, in the hypothesis of a human floating base in the habitable part of the Venus atmosphere, high in the sky, where the thick sulfuric acid clouds won’t block the sunlight, the extraction of the Nitrogen from the surrounding environment would be plausible, opening up to countless new possibilities in the use of this technology.
Not only nitrogen, but other inert cryogenic fluids can be used to store the energy needed in the same way. Carbon dioxide, both in its liquid or solid form, is a valid alternative; it’s important to highlight that using CO2 won’t be as efficient, but due to its higher concentration in the planet atmosphere, it would be easier to extract in situ.
In this work we decided to focus on a stationary lander due to its low power consumption, however nothing stop us to use this technology to power a rover. The pressurized gas can directly operate a rotary motor that allows the rover to move and operate, but compared to a stationary lander the system autonomy would be drastically reduced.
Space Agency Data
In one of the "Venus resources" provided by NASA something called our attention. "To cool down the needed electronics a cryogenic fluid could be used". From here we started to think about using the cryogenic fluid itself to power the needed circuits. Fron here we based our idea on the nitrogen storage system already in use on Earth in bigger industrial implant, adapting it to our needs.
All the resources about the environmental condition of the surface have been really useful.
Hackathon Journey
Inspiration for selecting the challenge:
We decided to choose this challenge because the space exploration missions excite us. But also because it is one of the most suitable for our skills and knowledge.
Approach to reach the goal:
We approached the project by going for steps, in a sort of Top-Down scheme.
We began by choosing the right energy storage system, being the choice that impacted the whole system’s prototyping.
Then we considered the possible approach to solving the problem, using the resources given to us by NASA and the ones we found by ourselves.
The hardest part was to design a space and weight-efficient solution, while keeping it simple. We prototyped the system by ourselves, basing it on previous existing nitrogen storage metods in use for industrial application on large implants.
Problems faced:
- Find a space-efficient and weight-efficient solution
- store enough energy to withstand 60 days on venus
Tags
#cold #gas #liquid #energy #storage #venus