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I finally got the link to the MSR report to work. That one is 1.2 MW thermal, but only 125 KW electric as DC, and 100 KW electric as AC. It has transmission lines and power converter equipment, as well as an 80-degree partial shadow shield. 6.5 metric tons total as shipped. It appears to work only by radiating waste heat back to a 300 K sky. Not sure what I saw, but I think it is thermionic, when SAFE 400 is a heat engine.
GW
GW Johnson
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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What? The MSR report talks about a lot of things. It includes a number of nuclear reactor designs. SAFE-400 is discussed on pages 155-157 of that report. Using page numbers printed on the pages, not numbers the PDF reader uses. They can get out of sync when the report doesn't include the cover in page numbers, and gives roman numerals for pages with acknowledgement, abstract, table of contents, list of figures, list of tables, etc.
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Now to gain more energy out of the left over 300 kw one can change the loop of power to contain another heat exchange for another different power generation unit and you can also insulate the holding tanks for the exchange to control the heat loss as well as covering the conneting tubing throughout the reactor to power systems unit or units.
Be very careful. Typically, you cannot use waste heat from a thermal power plant. The reason is power is generated by the flow of heat from high temperature to low temperature. The rate of heat flow is directly proportional to the temperature difference, so if you increase the temperature of the lower temperature area then you restrict heat flow. That reduces energy production in the primary power converter.
For example, if a coal burning power plant is designed to dump heat into a lake in winter with temperature hovering around freezing (0°C), but instead you use waste heat to warm a greenhouse with interior temperature around 20°C (68°F), then the differential of the steam for the power generator has just been dropped by 20°C. A thermal power plant always produces super-heated steam, much hotter than 100°C. Typical operating temperature is 200°C. The reason is power conversion of a coal burning power plant works by boiling water to steam, which produces pressure, which drives a turbine, which drives a generator. As pressurized steam drives a turbine, it expands. The turbine is designed to allow steam to expand through the blades of the turbine. In order for the turbine to work, there must be pressure difference between the inlet and outlet of the turbine. The greater the pressure difference, the greater the force driving the turbine. As steam expands, it cools. Thermodynamics. Once steam cools to 100°C, it will go through a phase change to liquid water. When it becomes liquid, volume dramatically shrinks. That causes dramatic and rapid loss of pressure. This sets the limit for a turbine to get energy out of steam. That leaves hot water that can be used for other purposes, such as heating a greenhouse. In Winnipeg in the 1950s, a coal burning power plant downtown piped that hot water to radiators in office buildings near the power plant. It was shut down so long ago that I don't remember it. But thermal couples do not have the steam/water limit, so will not have waste heat. The schematic on page 156 shows a compressor and recuperator, which only exist to increase gas temperature in the radiator. That allows for more rapid heat loss in the vacuum of space. Some mechanical energy in the drive shaft from the turbine will be lost to the compressor. The only reason for doing this is to increase gas temperature in the radiator sufficiently that it can dump heat as radiant heat loss (IR) in the vacuum of space. It's not necessary for a coal plant on Earth that has a lake to dump its heat. My point is trying to use waste heat could compromise efficiency of the primary power converter.
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No heat engine can exceed the efficiency of a Carnot cycle engine, but a Sterling or Brayton cycle can theoretically achieve the same efficiency. Nobody builds a Carnot engine as this cycle would deliver so little power it probably would struggle to operate its own machinery, nevertheless the Carnot cycle demonstrates the total dependence of heat engine efficiency on the temperature at which heat is supplied to the system and the temperature at which it is dissipated. Rankine cycle engines (Roberts' coal fired plant) are a bit less efficient than the Carnot cycle, but are relatively easy to design, build and operate in very large sizes.
To improve the efficiency of any heat engine cycle, after you have selected the best working fluid, eliminated losses and minimised adverse mechanical and aerodynamic effects such as friction and turbulence, you have to raise the temperature of the heat source or lower the temperature of the heat sink or a bit of both. You cannot exceed the Carnot efficiency.
Frequently the factor limiting heat supply temperature is material availability and cost. In Space heat dissipation is more difficult than on earth as you can only radiate it. Radiation from a black body dissipates heat according to the 4th power of the absolute temperature, so a very small reduction in the temperature at which the radiator operates requires a huge increase in radiator size (and therefore its mass) so the limiting factors here are payload for your rocket and space available in its shroud.
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I saw the SAFE 400 stuff among several things in the MSR report document. It's not the one the report picked, although the report said SAFE 400 was a close second, and far better than the rest of the pack. The one they picked had about 10% energy conversion efficiency, I think it was thermionic instead of heat engine conversion to electricity. At the end of the document they gave its total mass-to-be-shipped as 6.5 tons, in a couple of places.
I'm going to guess that the SAFE 400 would be in that same few-tons class by the time you included all the same power transmission line stuff. Maybe the same, maybe a little less. I dunno.
But a working system ain't going to be a paltry ton. It'll be a few of them. Too much stuff is getting left out of the estimates we have been bandying about.
As for heat engines, you can estimate the Carnot efficiency fairly readily if you know the temperature at which heat is released Th, and the temperature at which heat is rejected Tc. Carnot efficiency is 1 - Tc/Th, using absolute temperatures. Most heat engines in the real world do much worse than that.
For a power plant running a lean flame at about 2000 F = 2460 R, and rejecting heat to a lake at 200 F = 660 R (actually you should be looking at the steam in the line, not the cooling water temperature, so this is optimistic), Carnot efficiency is at most 73%. The best power plant actual efficiency (steam or gas turbine) I ever heard of was 47%.
GW
Last edited by GW Johnson (2017-03-31 13:50:23)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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When doing this sort of speculating about efficiencies and utilization of "waste heat," it helps to define the system to which these calculations are applied. Does the system end with the steam pipes and condensate collected, or continue on outside the immediate reactor as some have suggested here. Taking a view that's too restrictive means simply letting 300 KW escape. Yeah, utilization for greenhouse heating might degrade the efficiency of the power generation a small amount, but the tradeoff is lots of heat for gardens. I think once the steam or other circulating medium has condensed, we end the Nuclear Reactor calculations at that boundary and call it a "system." This sort of cutoff is how most thermodynamic problems in Physical Chemistry are handled, otherwise the calculations aren't really possible.
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Ah! Another analogy. In the example of a coal-burning power plant. The boiling-hot water may appear to be waste heat, but if that water is cooled to room temperature before returning to the boiler for the new cycle, then that water has to be heated to boiling before it can be heated further to super-heated steam. If the water was returned barely below boiling temperature, then boiling it back to steam would not require as much heat. So every joule or calorie of heat consumed as "waste heat" must be added back. So using "waste heat" saps the primary energy conversion.
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The heat of evaporation needs to be rejected to turn the working fluid back to liquid before it can be pumped up from condenser pressure to boiler inlet pressure. Actually it must be subcooled a few degrees so that the pump doesn't cavitate and fail very quickly. The lower the condenser temperature, the more efficient your Rankine cycle machine will be, so direct use of Lake Winnipeg water in winter will be about as good a coolant as you will ever get. The low temperature in the condenser means very low vapour pressure of steam at the exhaust of the LP turbine and very low vapour density. The LP turbine would now get unreasonably large so they probably limit the condensing temperature to a much higher value than 4-5 deg C.
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What's being discussed here is the magnitude of second-order effects to the basic thermodynamic calculation. Yes, in principle, the utilization of the "wasted" heat will very slightly reduce the efficiency of the Carnot cycle, but the benefits in doing so will be significant. Using this dumped heat to warm a greenhouse during the Martian night will be offset by the gain in foodstuffs produced as a result. I also suspect that the reactor will be somewhat over-engineered to produce the desired Wattage regardless of what's done with the radiated heat.
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But oversizing the reactor will cause oversizing of everything else. Heat exchangers, pumps, turbines, alternators, condensers and radiators so the effect on mass will be very considerable. Same goes if they use a Brayton or Sterling cycle machine. The reactor should be exactly sized to match the capacity of the rest of the equipment. If the reactor controls go wrong the consequence must be limited by the capability of the rest of the system.
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I see no reason why an early stage colony on Mars shouldn't use solar reflector technology to generate steam and so power a generator turbine. The early colony should be able to manufacture polished metal reflectors with relative ease. Numbers will still be small at this stage: less than 100. The energy requirements will be fairly low. Even allowing for growing all their foodm with artificial light, probably around 800 Kws average. Weather conditions on Mars are relatively benign. Structures need not be too sturdy.
A combination of a large initial PV array, methane storage and solar reflect-powered steam generators should meet all the colony's energy needs.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Major sandstorms are a common phenomena on Mars, rendering reliance on a strictly solar power supply very unwise. The two systems should be complimentary, capable of providing abundant power to the initial research facility and associated agricultural endeavors.
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And that's why you have methane storage. The PV power system and methane manufacture machinery can be sent ahead of humans, so that even if they arrive in the middle of a sandstorm, they will experience no energy shortage.
Also, remember that Rover operation has shown that even during sandstorms insolation levels don't drop below around 20% of average. Most sandstorms are of short duration. The worst one I know of was 9 months.
Once the colony has been operating for a few years, they can always ensure they have a huge methane store in place to guard against prolonged sand storms.
I would not be against taking a few small RTGs as emergency back up. The more back up, the better. I would also land a tonne of batteries for the first colonists, giving them over 3 years of guaranteed power. You don't take risks with the energy requirement.
Major sandstorms are a common phenomena on Mars, rendering reliance on a strictly solar power supply very unwise. The two systems should be complimentary, capable of providing abundant power to the initial research facility and associated agricultural endeavors.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Please recall that methane requires 3x it's mass of oxygen for combustion. Nuclear is the ONLY safe backup to solar--at least in the early stages of settlement. The SAFE-400 system appears attractive at this point in time. It will be entirely inadequate once any type of manufacturing is started. RTGs simply don't have adequate power for what's needed, but could have several strategically located for emergencies. The ultimate reactor should be Thorium based, and not Uranium.
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With luck there will be fusion reactors by then (but we have been saying this for an awfully long time). Mars has deuterium in abundance.
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Yes, but Lithium is necessary for production of Tritium, which is the other component of the basic fusion reaction. That in itself requires a fission reactor for it's production. Yes, it would be nice to have a fusion reactor, but don't hold your breath. The He-3 on the Moon is also a useful commodity--once we get a fusion reactor working.
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I think all things are possible in time for mars and even at the large down mass levels but are we going to keep waiting decades for unobtainium or just the right power source or the ability to land mega sized loads as compared to what we know that we can do today?
We know that a larger diameter vehicles heatshield will allow for a greater down mass, we know how to do repulsive landings. We also know that any vehicle for mars landing is nearly if not totally a new design with the capabilities designed in. We also know that landing the chunks for what we need in smaller more managable sizes makes more sence than one mega sized lander for getting there sooner.
The same holds true for power as there is a break point for nuclear versus solar and we also have a break point for the wattage of nuclear for down mass. So designing a solar lander and a nuclear lander with in the common lander mass capabilities is what we should design for.
So lets set a target value of 10 mt for payload and stick with it, that said how large is the volume that can hold that, as it is dependant on density of the item. So we are still needing to nail down the volume to which we can use.
This will help in nailing down the lander mass dry versus wet plus payload for the on orbit mass before trying to land.
So what are the items that we need and how do we group them to stay with in the mass volume limit?
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SpaceNut-
I am in complete agreement with that sort of approach. I'll enumerate below what's really necessary--beyond the transportation to and from the planet.
(1) Shelter and protection from a hostile environment. Includes both adequate space suits and habitat.
(2) Breathable air supply, scrubbed for elimination of harmful gasses.
(3) Water. Required for drinking, food preparation, and sanitation.
(4) Food to the extent of provision of adequate calories and all other micro-nutrients (minerals, vitamins, etc.). Should be palatable.
(5) Power. For heating, running oxygenation apparatus, water reclamation, light. Production of O2 and electrolysis of H2O. Methane production.
(6) Transportation. Should be multipurpose, with front loader for moving heavy loads and digging, moving of regolith, construction.
The very first SpaceX Red Dragon vessels will give us an indication of what can be taken along. Forget the unobtanium items. Stick with solar arrays and a small but efficient nuclear power plant. Moxie unit for production of O2 from the CO2 atmosphere. Tracked and fully enclosed Bobcat with both front loader and auger drill bit. Can't go wrong with stockpiling food.
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There's no shortage of oxygen on Mars.
There is simply no need for any alternative to PV plus storage on Mars.
Let's remember we are (I presume) talking about the first few missions to Mars. The population of Mars is not going to be more than 100.
It's a tiny, tiny population total which solar power can easily cope with.
Once the colony is established, the colonists can make their own energy with solar reflectors (to name one technology) although there are other options as well.
Please recall that methane requires 3x it's mass of oxygen for combustion. Nuclear is the ONLY safe backup to solar--at least in the early stages of settlement. The SAFE-400 system appears attractive at this point in time. It will be entirely inadequate once any type of manufacturing is started. RTGs simply don't have adequate power for what's needed, but could have several strategically located for emergencies. The ultimate reactor should be Thorium based, and not Uranium.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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I don't accept your presumptions Spacenut...
A nuclear power source that goes wrong means death for the colony. One malfunctioning PV panel doesn't mean the death of your colony.
Pre-landing of PV power generation systems negates all the arguments against PV. The lead-in to any (human) Mars mission is 10 years minimum. Plenty of time to pre-land the energy generation system.
PV is wonderfully flexible - you can split it, move it and reconfigure it with ease. Invaluable for expedition beyond the base area. It poses no health threat to the early colonists.
I think all things are possible in time for mars and even at the large down mass levels but are we going to keep waiting decades for unobtainium or just the right power source or the ability to land mega sized loads as compared to what we know that we can do today?
We know that a larger diameter vehicles heatshield will allow for a greater down mass, we know how to do repulsive landings. We also know that any vehicle for mars landing is nearly if not totally a new design with the capabilities designed in. We also know that landing the chunks for what we need in smaller more managable sizes makes more sence than one mega sized lander for getting there sooner.
The same holds true for power as there is a break point for nuclear versus solar and we also have a break point for the wattage of nuclear for down mass. So designing a solar lander and a nuclear lander with in the common lander mass capabilities is what we should design for.
So lets set a target value of 10 mt for payload and stick with it, that said how large is the volume that can hold that, as it is dependant on density of the item. So we are still needing to nail down the volume to which we can use.
This will help in nailing down the lander mass dry versus wet plus payload for the on orbit mass before trying to land.
So what are the items that we need and how do we group them to stay with in the mass volume limit?
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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We have second order limits to a mars mission for the legs of the journey from earth and for return which can reuse parts of the design or be completely seperate from the start but to keep cost lower we will need to design in the least amount of expendability within the design.
This ship going to mars only needs a single lander for the crew with minimal supplies if a camp site has been preload with all that we need for the stay.
We can have the anti gravity and all that we need in the ship going and returning from mars but to keep it with a cost limit we will need to leverage what we already have for low cost pieces. To leverage from the ISS for the ship only makes sense for a huge amount of already made and designed items.
This ship will need to be pack as before with the list items from above to which we will need to quatatively set down a manifest mass budget and volumes for these. Even with an effiecent recycling of air and water there is still a starting point plus a buffer to be able to maintain for the duration of use out and back.
The methods for mass volume may make of break which type of ship design we can use for the artificial gravity layout.
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Louis-
I like using PV, but not as the mission's primary power source. They are great, but since Mars has an approximate 24+ hour diurnal cycle, another component of mass is required: batteries. Batteries are heavy, so there goes a lot of our mass transportation budget! My ideal nuclear plant would be Thorium based, as the fission by-products have shorter half lives and could simply be buried--or loaded onto a return spaceship and flown into the Sun. I'm a "belt and braces" kind of guy; never putting all the eggs in one basket, so to speak. Need BOTH solar and nuclear. The 300 We RTGs simply don't make enough contribution to the overall energy scheme to even be included in calculating the total energy required by the Mars Station1.
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Loius the nuclear versus solar also has to do with lander mass limitation... not just one or the other as you do want a back up and no solar panels once it gets smashed is useless, also if you lose a battery the voltage and current will not be capable of what we want and all the reconfiguring in to world does not make up for either loss.....
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This interesting paper suggests a baseline figure of 14.2 Kwe required to provide life support for six humans on a Mars transit:
http://www.marsjournal.org/contents/200 … 6_0005.pdf
If we take that as the minimum requirement, perhaps we can look to something like 25 Kwe being required for the Mars base, given experimentation and other work required.
This really isn't a large requirement. Less than an electric vehicle automobile.
I'd be happy to go up to 25 Kwe for a first mission. What's that in terms of PV? Maybe 1350 square metres of PV panelling? About 37 metres by 37 metres. I'm assuming 0.5 KweH per sol per square metre and 25% loss on battery storage.
We would need something like 300 KweHs of storage if power usage was steady throughout the sol. However, I think most work (e.g. powering of heat storage bricks, water heating, oxygen production, and water filtering) could be done during sol-light. Let's assume 5 KweHs average during night-time - so making the storage requirement about 60 KweHs, similar to one of the lighter Tesla batteries - think that would weigh in around 450kgs.
Ultrathin PV is already being produced. This one refers to cells weighing 5.2 grams per sq. metre.
http://spectrum.ieee.org/energywise/ene … plications-
Let's assume we can get down to say 20 grams per sq metre with a reasonably efficient system. 1350 sq. metres would be only 27kgs.
So if we throw in associated equipment say 500 Kg for cabling, converters and all other PV -related requirements, we would be talking about much less than 2 tonnes.
The chemical battery storage could be supplemented by methane/oxygen storage to provide an alternative power storage source (which could be built up over several sol years through pre-landing). I haven't looked into the mass requirements of that yet but probably no more than a few hundred kgs - Mars is the perfect place to store methane as clathrates of course. I think we would probably be looking to build up a good 3-6 months' methane-equivalent power storage (i.e. the ability to power the base without PV for that period).
Let's call the whole set up 3 tonnes. In terms of the overall mass requirement (which I put at something between 20-40 tonnes on the surface, depending on what we are trying to achieve) I think this is very reasonable and not at all burdensome.
An alternative storage approach is to use iron as a combustible fuel. It has lots of advantages. All we would need to do is put a processing unit (probably a rover robot) on the planet to convert iron ore into nano-scale iron particles. We could probably store that on the surface in light weight plastic bags, given the benign weather conditions on Mars. See link below referring to iron as a fuel.
http://www.mng.org.uk/gh/renewable_ener … rticle.htm
Not sure if I've got it right but I think a tonne of iron would provide enough power for four days (at the full maximum). So 45 tonnes would provide enough power storage to cover six months. We'd need 4,500 10 kg bags to facilitate the storage. If 2 roving robot processor unit could create 20 kgs of nano iron every day they accomplish the task in about 4 years.
There is of course additional tonnage involved of course in pre-landing methane or iron robots. Difficult to estimate that. A tonne? Call the whole energy package 4 tonnes then.
Some other points:
1. A nuclear reactor based energy system for a human mission is an untried approach. So we don't know how successful or otherwise it might prove.
2. You will have to take a spare back-up reactor unless you are going to take the dangerously arrogant approach of assuming your nuclear reactor cannot fail. A back-up reactor doubles your mass requirement near enough.
3. If you want to have the flexibility to mount expeditions, or power vehicles, you will need either more reactors or you will need to take supplementary PV, so don't forget that in your mass calculations.
4. PV is inherently safer. Spread out your PV and it is extremely unlikely that one meteor strike will take out your energy supply. Not true for nuclear.
5. How automated can the nuclear reactor be? Is there actually a built example of a fully automated nuclear reactor on Earth because we don't want to make unnecessary work for our early colonists.
6. Nuclear plus replacement reactor and PV back up is going to weigh in at something comparable to 3 tonnes - quite possibly more.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Louis-
I really appreciate your thoughts. I'm not ruling out or belittling solar panels, but we need to have systems in double redundancy. I was around an operational nuclear reactor many times during my grad school days, since the University of Wyoming had a critical mass Thorium reactor on the University Campus. My first 2 years of grad school, I was a teaching assistant for Dr. Victor Ryan, who was the Nuclear Chemist and University Safety Officer. After he passed on some years later, the University had the reactor decommissioned and subsequently dismantled. There were really no safety issues that ever arose, but the cost of insurance became excessive during the 1980s. In several of my previous posts, I suggest that we send at least 2 small nukes in these Red Dragon experimental landers, but certainly NOT on the first one!
The numbers you've stated seem pretty minimal when all energy requirements are taken into account; I'd personally at least double them. I own and operate a small cattle ranch (or I DID before I fully retired just 4 years ago), and a minimum rate of power consumption for cold weather seemed to be in the 8KWe to 13KWe, depending on the severity of weather. This includes heating the home (all electric except for hot water and cooking), keeping water tanks thawed for cattle (that's 5-7 KWE right there), and lighting.
I'm with Dr. Zubrin on this issue; there can never be TOO MUCH electric power available.
Last edited by Oldfart1939 (2017-04-08 18:56:24)
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