You are not logged in.
The atmospheric steam engine.
https://eprints.soton.ac.uk/348565/1/Mu … 202013.pdf
An interesting idea for converting low quality heat into mechanical power. The low pressures and temperatures lead to relatively low efficiency - around 10% for a 69C boiler temperature. However, the advantage of low pressures and low temperatures are simplicity. The system is very easy to build using common materials. There is no danger of explosion, as pressures are <1bar. The system does not need a pump, as the vapour pressure of water is 20KPa at 60C. A condensed water column just 2m tall would be sufficient to force return coolant into the boiler.
On Mars, we would probably use something other than water in low temperature heat engines. CO2 is the most likely choice, though it would mean operating at higher pressures.
1. Mars atmospheric pressure is the issue for how to make use of the co2 that it has 610 Pa (0.088 psi) or not even 0.1 bar as the chart of the triple point shows.
I think this would have to function as a closed system. The solar collector and boiler would be at the bottom of a tall hill on the south facing side and would generate vapour at a pressure of 30bar. The CO2 vapour would rise up an upcommer pipe, several hundred metres high and would enter the turbine at the top. The condenser would be another set of radiator panels on the north side that would condense the CO2 into liquid that would then reenter the boiler at the bottom. The weight of liquid CO2 in the condenser side would be such that a pump would not be needed. In fact, the only strictly necessary moving part would be the turbine, as all other components could work on natural circulation, with vapour pressure in the upcommer, balancing the static pressure in the downcommer.
Both the upcommer and downcommer could be shafts drilled into the solid rock, as would the boiler. The heater panels could be coiled hose beneath a thin layer of regolith. The condenser would be much the same at the top of the hill. All components aside from the turbine would be very low tech with low embodied energy.
This is another concept that would work on Mars but not on Earth. On Mars, the very thin atmosphere allows very large temperature variations between areas in sunlight and shadow. There could be very significant temperature variations between the south side and north side of a hill. The system would continue to function at night, thanks to the heat stored in the rock surrounding the boiler.
The most energy efficient option is to avoid high power processes when solar input is low. That negates the need for storage. That is the way our ancestors used the sun and wind. They varied demand inline with supply, not the other way around.
https://www.lowtechmagazine.com/2017/09 … ather.html
Thermal storage is quite easy, especially if heat is the end use. Battery, compressed air and pumped storage, can then be used to meet demands that cannot be postponed at night or close to sunset and sunrise. These storage options are affordable for meeting reduced short-term power demands, over a period of hours - say during the Martian night.
Methane-oxygen energy storage is too inefficient to be useful for routine power supply. We would use this to provide emergency power during dust storms. Basically, nothing beyond hotel requirements.
There is a price to pay of course. In addition to the costs of power supply and storage; demand management implies that high energy equipment can only be used when power is abundant. That is clearly problematic if that equipment is expensive and you are attempting to amortise capital cost over a greatly reduced production. Solar power on Mars (at the equator) will vary according to a sine function.
Solar powered dirigibles are an interesting idea, but depend upon the efficacy of extremely light-weight thin-film solar cells.
Going back to the example of a spherical hydrogen filled balloon 100m in diameter. The total lift provided would be 6.5tonnes. That is 0.207kg/m2 of envelope area. The solar cells, envelope, propulsion system and cargo, must weigh no more than 207 grams per square metre of envelope. Thin solar cells must operate at low voltage, which would lead to significant losses even across the circumference of the balloon. Maybe we can do clever things, like printing the cells on the inside of the envelope and using the envelope itself as a cover and UV shield for the cells?
A similar idea would be to cover the balloon in solar cells and tether it to a ground vehicle via an electrical cable. The balloon then carries the power supply for the vehicle; and the vehicle carries the payload. The top speed and payload capacity of the ground vehicle would be a function of sunlight intensity. Excess power would go into life support functions. At full sun on Mars, a 100m diameter balloon covered in 10% efficient solar cells would provide about 400kW of power. This would vary sinusoidally throughout the day. At night, power would be zero and driving would stop.
A historical example of gravity storage is the raised weight hydraulic accumulator (see below).
https://en.wikipedia.org/wiki/Hydraulic_accumulator
Useful in situations where relatively small amounts of energy need to be delivered at high power - e.g. raising a crane boom with load or raising a bridge.
Let's say I have a small power source delivering a few kW and I want to use it to power a device that consumes hundreds of kW of mechanical power for just a few seconds, every few minutes. A raised weight hydraulic accumulator is useful in these applications.
A cheaper way of doing the same thing would be a polyethylene bladder located at the bottom of a deep pit. Above the bladder would be a deep layer of Martian fine dust, which would put downward pressure on the bladder, essentially exerting hydrostatic pressure in the same way as water - but somewhat denser. We would inflate the bladder with fluid, either CO2 or water. The dust would be pushed up the shaft and would fall back down the shaft as pressurised water or CO2 was extracted.
Regarding the use of train cars to extract gravity energy - this idea was used from the 18th century onwards and could still be relevant today. Say you have a mine located within a hill. You want to remove mine tailings and transport them to a railway or canal boat several hundred feet below. The weight of the full trucks will carry them down the hill and will also pull empty trucks back up the hill in a loop. In principle, one could extract additional energy from the falling material and use it to power the mining operation. One could couple a generator to a cable that is fixed beneath the cars and pulls over a pulley system. The energy extracted could be electrical or even pneumatic - powering air tools for the mining process.
Interesting ideas on an established theme.
I think manufacturing methane and oxygen when you have an energy surplus is probably a quicker and better route to energy storage, since we will already be producing methane and oxygen for fuel-propellant. Methane and oxygen can be used directly for heating, for electricity generation or for transport power.
That said, the use of basic materials makes your proposal an attractive one.
Methane-oxygen synthesis for power production has extremely low cycle efficiency – about 5%. I examined this in a previous thread.
Assuming that the intention is to provide small amounts of power only very occasionally – say 10% of baseload power, for a dust storm lasting a couple of months occurring once every two years; then it might be an affordable expense, especially if it can be tailed onto the existing propellant production process. But it is wasteful in the extreme. Like everything else, these sorts of decisions will be made on the basis of cost benefit analysis.
Long-term energy storage is always problematic, because it requires large amounts of energy storage that is utilised very poorly. This has a terrible effect on the marginal cost of each kWh. This is why renewable energy is proving so poorly effective at replacing fossil fuels for power generation here on Earth. We can build pumped storage plants that balance supply over a period of hours, but it will never be affordable to store even 1 week of spare power in this way. I suspect that the 'solution' to this problem will have more to do with going without; or maybe just finding an energy source that doesn't fluctuate with the weather. Can anyone think of one?
Some thoughts.
1. A blimp is an interesting idea. A spherical hydrogen filled balloon 100m in diameter would lift about 6.5 tonnes on Mars. Subtracted from that would be the mass of the envelope, fuel and propulsion.
2. For short distances on well used tracks, roads could be constructed from corralled stone and gravel. Ice could be used as a bounding agent between stones, although gravity and friction would likely be sufficient.
3. As Mars is a dry and heavily oxidised environment, power could be supplied to some vehicles through a ground level power supply, i.e. through a third rail, embedded within the road. Two rails would be needed; one +ve and one –ve. Vehicles would draw power using pickup shoes much as subway trains do here on Earth.
4. A railway is a logical extension of the above idea.
5. For equipment that is used close to a specific location, power supply could be via cables, attached to a local grid. If power is delivered using DC, then loads could be directly coupled to solar panels. Motors will run more slowly or more quickly, depending upon the balance between load and supply, but battery storage is not necessarily needed.
6. Hydraulic capsule pipelines can be used to transport bulk materials. The capsules should be carefully ballasted for neutral buoyancy. The pipelines could be steel, plastic or maybe polymer lined trenches.
7. Pneumatic transport is also possible – basically blowing wheeled carriages through a tube.
8. Over rough terrain, ropeways can be used to transport bulk materials in hanging buckets.
The huge diurnal temperature fluctuations on Mars make this idea more workable there than it would be on Earth. Assuming a daytime average temperature of zero Celsius and a night-time panel temperature of -50C, say, Carnot efficiency for a vapour cycle would be 18.3%. A practical heat engine can get between half and two-thirds Carnot efficiency. So a flat plate solar thermal power plant would get ~10% efficiency on Mars.
The Martian atmosphere is so thin that convective heat losses from panels will be negligible. No cover glass would be needed. Panels could be slabs made from concrete or clay with plastic pipes running through them. A practical heat transfer fluid would be brine, containing concentrated chlorate salts. The heat exchanger could also be made from thermoplastics and would consist of essentially a coiled hosepipe within a large tank of water or brine. During the day, lightly salted ice-water would be melted in one tank by heat flowing from the panels at about zero C. This tank will provide a hot source. At night, a second tank containing concentrated brine would have heat removed through the panels and would freeze at -50C. Insulation for both tanks would be provided by regolith. The tanks could be nothing more complex than excavated holes lined with polyethylene sheeting.
A vapour generation cycle would run between the two tanks continuously, providing base load power. Compressed CO2 would make a workable secondary power-cycle fluid. Alternatively, there is SO2, ammonia, ethane, propane or various fluorocarbon compounds.
To cover outages resulting from dust storms, the tanks would need to be oversized to provide several weeks worth of storage. There will be capital cost implications to this. Overall, the system will be large but relatively low tech. It will need valves and pumps on the primary (brine) side and valves, pumps, turbines, generators and liquid-vapour separators on the secondary CO2 vapour cycle side. Lots of carbon steel on the secondary side; plastics on the primary side.
Power density will be limited by the amount of heat that the panels can dump into the Martian night. Assuming a -50C working temperature for panel radiating as a black body, gives a panel thermal power density of 140watts. The panel can only radiate at night, so time average thermal power density is 70w/m2. Assuming a 10% electrical conversion efficiency gives a panel electrical power density of 7W/m2. This is poor by most standards, but begins to look much better if panels can be made from clay and storage tanks are nothing more elaborate than polythene lined holes in the ground. A 1MWe power plant would cover 143,000m2 or 36 acres.
To store 4 weeks worth of power, would require some 6720MWh (24million MJ) of thermal heat storage in each tank. 1 litre of water has latent heat of freezing of about 400KJ. So each tank would need a volume of 60,000 cubic metres - a cube 39m aside. That is a lot. Especially considering that sourcing water from buried glaciers on Mars, will take about that same amount of energy as concrete manufacture on Earth. Heat could be stored in solid rock by drilling bore holes.
Gravity storage is already used extensively in the form of pumped storage. This involves pumping water from a low reservoir to a high reservoir and then releasing it back to the low reservoir through a turbine. Practical efficiency is about 75% in large scale facilities. Not bad. Relatively low energy density. A cubic metre of water raised through a head of 1km, will store about 10MJ. The energy stored is proportional to height and volume.
The advantage of using a liquid is that it can be pumped through pipes, which is logistically much easier than having to lift blocks by crane and then recover energy using some sort cable winch. That involves a lot of moving parts and is labour intensive.
At typical Martian temperatures, CO2 is far beneath its critical temperature.
https://en.m.wikipedia.org/wiki/File:Ca … iagram.svg
This implies that very little compressor work is needed to condense it into liquid.
Methane-oxygen would appear to be problematic as a bulk energy storage solution, on Earth or Mars.
https://en.m.wikipedia.org/wiki/Sabatier_reaction
"A variation of the basic Sabatier methanation reaction may be used via a mixed catalyst bed and a reverse water gas shift in a single reactor to produce methane from the raw materials available on Mars, utilising carbon dioxide in the Martian atmosphere. A 2011 prototype test operation that harvested CO2 from a simulated Martian atmosphere and reacted it with H2, produced methane rocket propellant at a rate of 1 kg/day, operating autonomously for 5 consecutive days, maintaining a nearly 100% conversion rate. An optimised system of this design massing 50 kg "is projected to produce 1 kg/day of O2:CH4 propellant ... with a methane purity of 98+% while consuming 700 Watts of electrical power. Overall unit conversion rate expected from the optimised system is one tonne of propellant per 17 MWh energy input.[25]"
By my reckoning, even an optimised system will have an efficiency of 18%. If the methane is then burned in an IC engine or gas turbine with an efficiency of 30%; overall efficiency drops to around 5.4%. Not something that would be affordable as anything other than a niche application, maybe used in emergencies.
Pumped storage has better efficiency on the face of it. However, water will not be cheap on Mars. It must be melted and pumped from deep buried glaciers at an energy cost of 1MJ per kg. It must also be kept liquid, which implies either heating it or storing it as brine, which would suffer corrosion problems. On Earth, in places where pumped storage is used, the water is available for free and it remains liquid at typical temperatures. A Martian system might use liquid CO2. But this must be kept under a pressure of at least several bars to remain liquid. That implies that the storage lake is built at a pressure vessel.
We have already discussed hydrogen. On Mars, we must keep both hydrogen and oxygen in separate pressurised containers. This would require either steel or polymer vessels, or underground reservoirs that are excavated and covered over with a mass of covering material. Either way, it is a lot of effort and a lot of embodied energy.
All things considered, the storage of energy would appear to be expensive, however it be done. On Earth, we tend not to do things this way. Most bulk energy supply is controllable according to demand. Wind and solar power are simply extensions of the fossil fuel energy system. Fossil fuels are used to refine and smelt the metals used to make these systems. They are used to transport and assemble the machines. And when power is produced by a wind or solar power plant, it offsets the output of another controllable (fossil) power plant.
Hydrogen has poor cycle efficiency as an energy storage medium. First, about a third of electricity is lost as heat in the electrolysis stack.
The energy cost of storing hydrogen depends upon its mode of storage. It is relatively cheap to store in a lightly pressurised gasometer arrangement. But capacity is limited. Compressing hydrogen to 300bar would consume another 10% energy content and the hydrogen would gradually leak through seals. Liquefaction allows hydrogen to be stored long-term as a liquid at 20K. But the energy cost is equivalent to about one third of the energy content of the liquid hydrogen. Ultimately, electrical power can be recovered in a fuel cell or, more likely, a combined cycle gas turbine plant. Both have realistic efficiency of about 50%. Add all efficiencies together and you get 22-33%.
In terms of bulk energy storage, it might be cheaper to store thermal energy in rock by means of electric heating elements. This can be recovered as steam passing through pipes in the hot rock, which generates electricity in a turbine. Storage efficiency would be 30-50%. Whilst this is better than hydrogen, it is still not fantastic. But the system is simpler overall and a single cubic metre of basalt heated to 1000C would store nearly 3GJ of energy.
In terms of whole system cost, thermal energy storage is hard to beat. Hydrogen has the advantage of being a useful chemical reagent. If you want to reduce metal ores or manufacture plastics, hydrogen is an intermediate product that is likely to be useful. Storage isn't very important, because the hydrogen is likely to be used rapidly after it is produced.
Carbon dioxide will boil at about 0 degrees Celsius at 30bar. And it will condense at -40C at a pressure of 10bar.
http://www.chemistry-blog.com/2009/02/0 … xtraction/
Solar heating panels can provide a hot source at about zero Celsius. The tower would be useful for drying the saturated vapour. Liquid CO2 droplets will drop back into the boiler by gravity, and the vapour will be much dryer at the top of the tower, whereupon the vapour will enter the turbine.
For Calliban re #10
Thank you for your (to me very) interesting addition to the topic.
I checked with Mr. Google, and found: https://en.wikipedia.org/wiki/Ambient_pressure
My intention was to try to follow your idea ... If I understand it correctly, at the center of Phobos, the gravitational force of the body is strong enough to deliver half an atmosphere of pressure.
Would it be your recommendation to set the habitat pressure to .5 bar, or would you increase it to a full bar, counting upon the gravitation of Phobos to reduce the burden on the evacuated region?
One ** definite ** advantage of your proposal is that the radiation protection in such an evacuated habitat would be about as good as it gets.
(th)
I think 0.5bar is the maximum pressure that could be achieved, as Phobos is a rubble pile with no cohesion. But within that limit, the gravitation of Phobos would balance internal pressure.
Phobos could be a very significant world in its own right.
Spectroscopy suggests that Phobos is a silicate rich body. It could be a useful place to build a refuelling station.
Phobos is also big enough that hydrostatic pressure at it centre is about 0.5bar. A sizable colony could be built at the centre of Phobos, provided one was willing to pay for equipment needed to drill some 10km into the body. Close to the centre, inhabited volumes would not need pressure vessels, as air pressure would be balanced by hydrostatic pressure. This means that a very large pressurised volume (perhaps a kilometre or more in diameter) could be excavated relatively cheaply.
The excavated materials could be used to produce fuel, air, reaction mass or even manufactured products.
The average UK citizen is 20.8% poorer now than they were in 2003.
https://surplusenergyeconomics.wordpres … unreality/
For the US, the decline has been less extreme but still significant. The average French citizen lost 40% of their prosperity between 2003 and 2019.
The growing public anger around falling real income and rising inequality, is a large part of the driver behind 'populist' politics in developed countries; think the Trump election; Brexit and the yellow vest protests in France. This things would have been unthinkable in the 1990s.
The ruling elites, who continue to model the economy as a purely financial entity, do not understand what is happening. In Britain, they will deal with popular unrest by turning on the people with more and more oppression. Expect to see more spying; more censorship and more imprisonment of anyone that dares to utter the wrong sort of opinion too publicly.
Ultimately, falling tax revenues are going to impede the Elites ability to oppress the people. At that point, it is entirely possible that the British political elites will face violent revolution.
Lithium ion batteries have energy density of 1MJ/kg. Jet fuel has an energy density of 43MJ/kg. DC electric motors have a peak efficiency of about 80%. High bypass turbofans have efficiency of about 33%. So 1kg of jet fuel yields almost 18x as much work energy as 1kg of batteries. And a jet fuel powered plane gets lighter as it consumes fuel.
Certainly a poor energy source for a flying machine in which range is proportional to energy per unit weight. Hydrogen or liquefied natural gas would be far more efficient fuels, at least at the point of use.
If the climate change is true and we are fighting against more CO2 in the atmosphere, if CO2 contributes to the global warming... how is it possible that almost all the atmosphere of Mars is composed by CO2 and it does not have extreme temperatures??? Mars should be a very hot planet!! However this is not certain. Is the climate change a lie and more CO2 does not contribute to more hot temperatures?? or maybe the atmosphere of Mars does not contain too much CO2 such as NASA tells us????
The atmosphere is thin, contains very little water vapour and the planet is 50% further from the sun than Earth. Without oceans, the surface of the planet has very little thermal inertia, causing temperatures to drop to very low levels at night. Whilst carbon dioxide is a greenhouse gas, it is not the only heat trapping mechanism at work in Earth's atmosphere. And being 50% further from the sun, it would take huge amounts of greenhouse gases, shutting off the entire infrared spectrum, to make Mars as warm as Earth.
I don't understand Louis' desperate need to avoid the deployment of nuclear energy, on a planet where it is clearly needed. Everything that humans do on Mars will be more energy intensive than it is on Earth. Even breathing, and especially eating. Water on Mars, has the same embodied energy as concrete on Earth. And we need lots of it for just about every human function and industrial process. All these energy needs in a place with half the sunlight intensity of Earth. To live in the place at any reasonable standard of living, will require bucket loads of energy, delivered very cheaply.
Using hydrogen-oxygen propellant, increasing dV from 3.4 to 7.93km/s, almost doubles the mass requirement. It therefore doubles the payload cost. Until humanity perfects a propulsion system that combines high exhaust velocity with high thrust; it will make sense to minimise dV at the expense of flight time.
Excellent idea from Void. Instead of building pressure shells for greenhouses, use hydrostatic pressure instead. One could line a crater or depression with a plastic lining; cover it with a glass greenhouse and fill it with water. Dissolved oxygen concentration is a function of pressure. The hydrostatic pressure at the bottom of the pond, assuming it is about 10m deep, would be sufficient to maintain enough dissolved oxygen for marine life.
The concept could be taken further by stratifying the pool using a layer of polyethylene to prevent mixing of water between two layers. The bottom layer would be warm and oxygenated; the top layer would be relatively cold and free from oxygen and salty to keep it liquid.
One stumbling block is the need for abundant water. Water is an energy intensive product on Mars. Melting buried glaciers would require about 1MJ per kg of water. So a lake 100m x 100m x 10m deep would require 100TJ of heat (27,800MWh) of heat. That is a lot. 1MW of thermal power constantly for 3.2 years. One of the reasons why Mars colonisation will depend on the liberal use of nuclear energy.
For Louis re #13 and topic in general ...
An option available using Newtonian physics is momentum transfer of mass travelling from Earth (or anywhere) to Phobos.
The engineering is beyond human capability at present, but were a solution to be found, the momentum given to a vehicle for a fast transfer to Mars would be given to Phobos, which needs help staying in orbit, instead of to heating the atmosphere of Mars, which is the only technique humans can manage at present.
(th)
How about a very long piece of elastic?
:-)
Asteroid Bennu is a B-type asteroid containing hydrated minerals and its orbital characteristics make it relatively accessible from the Earth. It is therefore a good candidate for asteroid mining and SPS manufacture. Its aphelion takes it out to 1.36AU, where sunlight levels are only 54% those at Earth orbit. So not an ideal destination.
https://en.wikipedia.org/wiki/101955_Bennu
https://en.wikipedia.org/wiki/B-type_asteroid
The presence of hydrated minerals and organic material is of great importance for space manufacturing, because hydrogen and carbon are the two most important reducing agents for iron.
The asteroid is nearly 500m in diameter, so it would not be practicable to enclose the body within a bag shipped from Earth. Instead, a rotating habitat and factory could be placed at one of the asteroids poles and mounted on thrust bearings. The gravity of the asteroid is only six-millionths of Earth's; so a habitat massing 10,000 tonnes would weigh only 60kg. Material could be gathered using robotic handlers and fed into an airlock at the base of the rotating factory.
A solar array could be installed on bearings at the opposite pole and would track the sun to provide continuous electric power. A solar panel 200x200m in area would weigh 80 tonnes and would provide some 16MWe of power at Earth distance from the sun, but only 8.5MWe at aphelion. So some high energy tasks must be carried out close to the asteroid's perihelion.
As manufacturing produces waste silicate materials, fibreglass fibres could be woven into rope and used to produce a net which would eventually be of sufficient size to surround the entire asteroid. At this point, the asteroid would be enclosed by the net and could then be spun up. At this point, tunnels could be dug into the asteroid, which could be pressurised and would have gravity due to the spin of the asteroid. Operations could then be expanded, with additional facilities constructed in excavated tunnels.
Excellent post Louis. I agree with your assessment. Most of the mass shipped to Mars should be capital equipment that allows other items to be produced using ISRU. We can divide pressurised areas into habitation space, industrial space and agricultural space. The first two can be subterranean, using corralled surface materials to ballast internal pressure and provide insulation. Agriculture is going to be quite difficult, as there is no escaping the need for either large amounts of electrical power with artificial lighting in underground growing systems or heating for surface polytunnels. Either way a lot a area is needed and energy requirements are high.
I think $20/kg is ambitious. It seems to require very rapid turnaround rates of rocket components. The problem is that these are subject to thermal and mechanical fatigue.
A specific problem with colonising Mars is its distance. It would take 2 years for Musk's Starship to make a round trip. How cheap would an aeroplane flight be if it lasted for 2 years?
I wish he hadn't called it the 'Starship'. That name has a very specific meaning and in this context it is a misnomer.
Calliban:
Does anyone yet know how to turn rock rubble into real engineering materials? If there is, I haven't heard of it. Not yet, anyway. Mass conservation is not the same thing as materials properties, even though both start with an "M".
Just because the atoms are there does not mean you can actually do anything useful with them. Especially in a vacuum/zero-gee environment. And anything you do besides simple masonry construction will require massive amounts of energy. It certainly does here on Earth.
Typical rocks and rocky materials have compressive strength, but little tensile strength, which means they have very little bending strength. That has driven masonry construction to compression-only structures for millennia now.
For just about any conceivable useful things in space, you need pressure shells. Those require materials with at least high tensile strength, and usually significant bending strength.
GW
Unanswered questions I would suggest. Aluminium and magnesium are produced via electrolysis of molten oxide rocks. The process requires carbon electrodes and generates CO2, which would presumably need to be recycled back into carbon. The scale of the operation is intimidating. Whether it could be done on a more compact scale is questionable.
https://aluminiumleader.com/production/ … _produced/
Typically, about 20kWh of electrical energy is required to produce 1kg of aluminium. To produce 100,000 tonnes of aluminium would require some 2billion kWh. Other a nominal five year period (2.3 tonnes per hour), that is equivalent to a constant power of 46MW. The solar array needed to produce this much power would weigh 460 tonnes. That is about 11% of the assumed mass budget.
