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#101 2019-11-25 19:21:36

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,360

Re: Methane backup not nuclear vs solar

louis wrote:

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.


Calliban wrote:

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.

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#102 2019-11-25 19:22:05

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,360

Re: Methane backup not nuclear vs solar

Calliban wrote:
louis wrote:

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?

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#103 2019-11-25 19:22:46

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,360

Re: Methane backup not nuclear vs solar

louis wrote:

Well I would envisage meth-ox production being tailored to a PV power approach. In those circumstances for things like agricultural and propellant production, probably the two biggest calls on power, you would "make hay while the sun shines" as the old proverb goes which means you don't attempt to maintain production levels when solar radiation dips to very low levels. In such circumstances you would cut back on production of food and propellant. That means two things: (a) you need to have a large food store to see you through major dust storms (not likely to be much of a problem in the early stages of colony development) and (b) you need a larger propellant production facility than you would with nuclear.

You'd probably only be looking to use maybe 10% of your average energy use through meth-ox production. Estimates of how much energy is required to sustain life and basic services vary but 10KwEs per person seems a popular figure (probably an overestimate in my view).  For a 10 person colony producing enough propellant to return to Earth in one Starship that would mean 100KwE or 10% of the total energy supply (1MwE).

Even the severest dust storms do not stop solar radiation entirely, so PV will still keep working, albeit at a reduced level. From what I have read, you are maybe getting an average 40% of normal or thereabouts over the life of a dust storm.

The reality is there are unlikely to be any periods when your PV system could not cover the emergency baseload figure. But of course, you need to plan for calamity, so you would need to arrive with a large meth-ox supply and begin building up your reserve. Meth-ox production is a bit like a savings account - you only have to have a slight surplus and the account will build and build until you have a very large surplus available.  A reasonable compromise would be to look to build an energy reserve of 245,000 KweHs per ten people  comprising meth-ox, chemical batteries and maybe pumped storage - enough to maintain 100 Kwes over 100 sols.   

I think on Mars we have an opportunity to explore the possibility of storing methane as clathrates. Could we create those and bury them on Mars in a shadowed area?

https://en.wikipedia.org/wiki/Methane_c … ercial_use




Calliban wrote:
louis wrote:

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?

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#104 2019-11-26 05:52:23

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 222

Re: Methane backup not nuclear vs solar

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.

Last edited by Calliban (2019-11-26 05:57:05)


Interested in space science, engineering and technology.

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#105 2019-11-26 13:36:37

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,329

Re: Methane backup not nuclear vs solar

Yes, variable energy usage is the sensible way forward with a PV system.

On the day-night cycle, a lot can be done during the day:  washing machines can wash clothes, life support processes like oxygen separation can be carried out during the day, water purifiers can purify water, water can be heated for use in kitchens and bathrooms in the evening (having been kept well insulated). Batteries for lap tops, reading lights and so on, can be charged during the day and made available from evening onwards. During the night chemical batteries, recharged during the day, can supply power to the settlers.

Yes, I do envisage meth-ox as primarily about emergency supply in dust storms. But here's the beauty of the solution: you will always have a huge additional supply of meth-ox because that will be produced as propellant, but in any emergency you can divert it to  power supply.

Calliban wrote:

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.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#106 2019-11-26 13:47:15

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,329

Re: Methane backup not nuclear vs solar

When I looked into that re propellant production for one Starship return, I think it was a relatively trivial amount required to scale up for higher power output - less than 5 tons as I recall. One thing - solar energy on Mars does not fluctuate wildly from one minute to the next as there is virtually no cloud cover, no fogs.

Calliban wrote:

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.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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