On other planets in the solar system there is no coal, oil or natural gas, so we will have to rely on a lot of the same technologies as those being promoted on earth for a greener future.
Solar Power and Batteries
This is the most obvious combination, and what probably all first missions and settlements will focus on. It is simple and easy to use. No movable parts and reliable up to a certain point. However there is no easy way of expanding power generation and storage capacity other than getting more stuff shipped from earth until you can your own sophisticated industrial base setup.
Solar Power and Fuel Cells
We can improve on the previous combination somewhat by using fuel cells. That allows us to easily expand the capacity of our energy storage by simply creating more tanks. It is easier to build more tanks than batteries from scratch with a limited set of materials, tools and expertise available.
What kind of fuel cells and tanks should be used though?
Hydrogen Fuel and Cells
The most efficient fuel cells use hydrogen. However there is a big problem with hydrogen, and that is storage:
- The atoms are small and can thus defuse through a lot of materials. While it might not be a problem in the short term it is a problem for long term storage.
- It has low energy density per volume. Reducing volume requires high pressure cryogenic storage, since hydrogen boils at −252.882 °C. One needs very good insulation and cooling, which makes it impractical to create systems for storage and transport.
Hydrogen Fuel Cells with Other Fuels
To get around the problem with hydrogen storage, one could use fuels which easily convert to hydrogen such as:
- Carbohydrates (polymeric C₆H₁₀O₅).
- Metal hydrides. Basically react hydrogen with a metal for creating a storable compound. Heat it up to release the hydrogen.
- Ammonia (NH₃), which is actually a great if, if on Venus since NH₃ may already double as a lifting gas.
However even if one can come up with a good way of storing hydrogen we still have the problem that fuel cells are not very durable. They wear out much quicker than batteries.
Other Types Of Fuel Cells
Natural gases like Methane is a lot easier to store and handle. And there are in fact fuel cells which can use them directly such as:
- Redox runs on natural gas or propane. Is a 1 m³ large cube producing 25kW. For comparison, an American supposedly consumes about 3–6kW on average.
- Bloom Energy Server is a fuel cell which can run on a variety of fuels.
Usually this is done with a solid-oxide fuel cell (SOFC) which are able to operate at higher temperatures like 700 °C or 1300 °F, which allows them to use other fuels than hydrogen. This gives 60% efficiency, similar to a gasturbine.
Solar Power and Steam Engines
Instead of using solar power create electricity directly, one can go retro and use lenses or mirrors to concentrate the sun to heat up water till it boils, enabling it to drive a steam engine. Solar steam power is in fact widely used. There are even systems for providing energy storage using some advance steam engine designs.
A steam engine could in turn power a generator to get electric power.
While a steam engine may require more maintenance and may break down more often than solar cells, they are easier to maintain and repair, which means there is no practical limit to how long you can keep them.
Steam engines are based on lower tech than solar cells which ought to be easier to recreate and maintain on another planet.
Solar Power and Sterling Engines
Solar power could also be used to power sterling engines, which is safer and more efficient than using steam engines. But as with any heat engine there are pros and cons. Here is an analysis of different kinds. E.g. comparing Sterling engine to steam turbine.
Sterling engines work great when you got a good temperature differential. So if you got ice on one end and steam on the other, that would increase your power output. That might be utilized as a method of melting ice on Mars to obtain water. Getting water on Mars requires melting ice dug up from the ground. This ice could be used to power an Sterling engine, which would help generate power while gradually melting the ice.
Algae and Bacteria Biofuels
Solar powered combustion engines don’t tell us much about how we store energy. We could store it in batteries, or use electricity for electrolysis to generate hydrogen. Hydrogen could either be used directly as fuel or to create other types of fuel such as Methane through the sabatier reaction.
However a better approach than what has been discussed thus far may be to use biofuels generated from algae or bacteria.
There are several reasons for this:
- You probably already want to grow algae for food, nutrients and oxygen production.
- Biological systems don’t break down as easily as machines, which gives very long durability.
- Simplicity. Growing algae does not require advance equipment or materials.
- It easily scales.
Why algae may not be as efficient at utilizing the sun as solar cells, they have the advantage that you can easily expand your production, since algae unlike solar cells duplicate themselves. The facilities to grow them does not require much more than a water pump and plastic tubes.
Algae can be used to produce various biofuels:
An alternative to algae is to use cyanobacteria producing sugar. These produce higher yields of biomass than algae up to 700% more and sugar is easier to filter and extract than oils from algae.
Another benefit of sugar is that it is a well known feedstock. Fermentation of sugar to produce ethanol has been known by humans for a long time. Ethanol is a great fuel because it is easy to store and has high energy density (unlike hydrogen). It burns clean avoiding sot. It can even be used as rocket fuel. That is what Germans used for their V2 rockets during WWII, since ethanol can be diluted with water which helps cool rocket parts.
Fermentation of sugar is also a pretty low tech technology, which is an advantage on other planets when you don’t have easy access to sophisticated suppliers.
These biofuels could be used either in an external combustion engine such as:
- Steam engine
- Sterling engine
- Steam turbine
As I’ve discussed earlier. However we also have the option of using an internal combustion engine. That is great if we need e.g. to create motorized vehicles. While I love electric vehicles here on Earth, they do require more sophisticated technology. Batteries will eventually wear out. An internal combustion engine Mars rover should be able to last longer with regular maintenance.
Just keep in mind that since there is no oxygen in the Mars atmosphere, you’ll actually have to bring your own oxygen tanks to run the internal combustion engine.
Metal Powder Fuel and External Combustion Engine
The short version is that when you turn anything into powder it is much easier to burn it, because it opens up the opportunity to increase the surface area exposed to an oxidizer such as oxygen. That is why liquid fuel has to be turned into a mist inside a combustion chamber both whether it is a rocket engine or internal combustion engine. In gunpowder it is solved by mixing the fuel (sulfur and coal) with an oxidizer (potassium nitrate).
This approach is actually already used in pulverized coal fire boilers.
Coal powder gets blown into the combustion chamber by hot air. Since it suspended in the air, all the particles are exposed to air and combust quickly.
Now the question is what is the big deal about using metal powder over coal? Or using metal powder over biofuels for that matter.
When you burn coal, it produces carbon dioxide which cannot easily be recaptured, and even if you could, there is no simple way of turning carbon dioxide back into coal.
However when you burn metal powder, say iron, you get iron-oxide or rust. Essentially you get iron ore back, which we know very well how to convert back to iron.
Keep in mind we are not using this approach to generate energy but to store it. Turning iron-oxide back to iron powder is going to take at least as much energy has we get out of it.
However if we use a solar smelter, we can essentially store the energy from the sun in the form of metal powder.
Metal as Fuel in Fuel Cells
Instead of burning the metal as powder in an external combustion engine it can be consumed in a fuel cell just like hydrogen. Or to be correct, when the fuel is no longer a gas we just call it a battery.
One promising approach to this is aluminum air batteries. Phinergy already offers this as a product. The difference between this an a regular lithium iron battery, is aluminum is being used like a fuel. When all the aluminum has been consumed and turned into aluminum-oxide, you just pull it out and install a new aluminum plate. The aluminum-oxide is recycles to produce new “fuel”.
Metal fuel does not make much sense on Mars and Venus since we already have access to carbon dioxide which is easier to utilize and we don’t have to worry about CO2 emissions. However on places like the moon where there is no carbon, metal fuels could be a very practical way of storing energy.
Carbon Dioxide Breathing Engines
Silane is a gas made from silicon and hydrogen. Actually it is a whole family of compounds made from silicon and hydrogen mimicking alkanes such as methane and ethylene (simple chains of carbon with hydrogen attached). Some of these will be liquid rather than a gas at room temperature.
But all of these silicon-hydrogen based compounds have an unusual property, which is that they burn in carbon dioxide.
SiH₄ + 2CO₂ ⟶ SiO₂ + 2C + 2H₂O
That makes it a very useful fuel for planets with a CO₂ atmosphere such as Mars and Venus, since you don’t have to carry with you oxygen, which would take up 73% of the total weight of fuel and oxidizer, for a typical hydrocarbon based fuel.
For Venus this is especially useful since the atmosphere is thick with carbon dioxide. In addition due to the high density of carbon dioxide, Silane (SiH₄) is a lifting gas. That is practical for an airship since storing Silane can be done without pulling the airship down.
The major downside of this fuel is that it produces solids such as silicon dioxide, which would clog up the cylinders in any internal combustion engine.
That means an external combustion engine such as a steam engine, steam turbine or sterling engine has to be used.
Silanes with longer silicon chains have been proposed as renewable fuels on earth by Dr Peter Plichta. One of the cited advantages is that it is able to burn with both oxygen and nitrogen, thus utilizing the air much better.
Compressed Air Energy Storage
Initially I did not write about compressed air systems, because they did not seem to have a very promising future, given their low efficiency and large storage volume required.
However at Low-Tech magazine they have several articles discussing compressed air storage systems. Some of the systems discussed achieve high energy efficiency by utilizing head and cold generated from compression and decompression.
The idea is to have compressed air storage in each home rather than at a central storage facility. When air is compressed with say an electric compressor heat is generated. This heat is used to heat up e.g. hot water tanks used for showering, heating the home etc. One could imagine separate heating arrangements for dishwashers, washing machines and cooking.
When compressed air is released it causes cooling. The cooling can be used to refrigerate food or airconditioning. This allows us to get high energy efficiency with small tanks with high compression.
Another way to get higher efficiency from compressed air systems is avoid taking the detour by generating electricity, but rather using mechanical conversion directly. That is what was done at large scale in the early 1900s. For instance on can a windmill to drive a compressor directly. The output from compressed air tanks can be used to drive a wide variety of pneumatic tools.
I have written several stories touching upon the usage of compressed air to drive things earlier:
- Building a non-electric economy on other planets. Here I discuss pneumatic tools as well as computers (fluidics) running on compressed air instead of electricity.
- A 3D printer operating without electricity. I discuss usage of pneumatic stepper motors and fluidics control logic.
- Fluidics and ink based displays. Using ink and compressed air you can essentially create an alternative to electronic displays based on electricity.
- Autonomous non-electric rover. Here I touch upon using compressed air for remote communication with an autonomous vehicle using accoustic transmitters and receivers.
For other planets compressed air systems have a number of advantages over batteries.
- Very high durability. Batteries do not last very long. Without easy access to repair and replacement on another planet, you want systems with high durability.
- Requires low tech to build. An society on another planet will struggle to have a diverse and sophisticated industrial base. It is thus preferable to rely on simply systems which can be built and maintained on site.
- Low energy usage in production relative to storage. Batteries require a lot more energy to build relative to the energy stored in their lifetime. Compressed air will pay for itself over time in other words.