the single most important question for any alternate-fuel ships?

Discussion in 'Boat Design' started by ijason, Jan 28, 2009.

  1. marshmat
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    marshmat Senior Member

    Yes, I think that would make sense, if one has the space for a solar array that big. Solar desalination might be a good solution for a land-based system, ie. near a major port. I think you'd be hard pressed to get enough array area on a boat for it to work though.

    Wind is essentially a condensed form of solar energy- it's created by temperature gradients over enormous areas of land or water. Technically, one could think of a sailboat as being a solar-powered vessel that uses large areas of nearby land or water as its solar array. The actual solar flux in good sunny weather is around 1.0 kW/m^2, much less if cloudy, which makes it very difficult to collect significant amounts of energy from a small object like a boat. But given a few square kilometres of Aussie desert.....

    I saw a Mercedes-Benz recently that was set up to run as you describe. It worked remarkably well, in fact. But you're still stuck with the limits of the IC engine- theoretical maximum thermodynamic efficiency of 50 to 60 percent at best, minus friction, minus pumping losses, etc. and it becomes really, really hard to get more than about 30% of the chemical energy in the fuel extracted in the form of usable work. Thus, even with a fair bit of tankage, I seem to recall that the Benz had somewhat lower range than comparable fuel cell cars. Although, it was substantially cheaper, something like $200k instead of $700k.
     
  2. ijason
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    ijason Junior Member

    really? i could have sworn i had seen endless reports about how fuel-cells were going to be the answer for storing energy in cars... i guess i may have misunderstood or - more likely - they were misreporting. all of the shows i saw talking about them referred to the fuel-cell as providing electricity to an electric drive, without mention of on-board hydrogen stores. but i think they were all fuel cells using a petro-chemical from which the hydrogen was cracked and converted to energy???

    as to salts and other impurities fouling the electrolysis process, it would seem this would be easily solved by using a distillation process or reverse-osmosis filtration on the water you intend to use as fuel. i would prefer to have a set up for purifying sea water for drinking, anyways, so i don't see why the system couldn't do double-duty. as Robherc mentioned, on a boat you generally need a supply of fresh water, if you could avoid bringing 100s of pounds of it with you by having a system to make your own, win win.

    a question at the crux of our theory would be : is it more efficient to run a mechanical compressor than it is a mechanical drive engine? if the answer is yes, then it just seems a question of how much storage you need. it seems like a compressor with a really big fly-wheel, hooked up to a wind-turbine would be a fairly simple solution... and require less torque than it would to turn a drive prop.

    so if you have a solar/turbine set up that is cracking your hydrogen, and then that gets shunted to a holding tank for the compressor to have at it when there is enough of a breeze, and then you've got a storage tank to hold the fuel for your fuel-cell to power your electric drive motor... whew! how long do you have to sit around in the windy sun before you can power your boat anywhere useful? or, as it's been mentioned again and again, far superior to rely on all this solar/hydrolysis/fuel-cell to take the place of your generator and being a sailing boat?
     
  3. robherc
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    robherc Designer/Hobbyist

    Actually, I don't think you'd need a very big array if a 36"x12"x14" "solar still" science fair project can claim to distill bout 1 gallon of water/day...I was planning on using the Sun to directly heat the water for distillation, no need for expensive solar arrays...PV cells are FAR too inefficient, but direct heat transfer, that tends to work fairly well (in my experience, at least). I was thinking a bit along the lines of placing a couple mirrors on deck & focusing 3-4x the sunlight on your solar-heat still, maybe put the mirrors on top of your cabin, or somewhere else out-of-the-way, where the light's not doing any good anywise.

    Yes, IC piston engines are EXTREMELY inefficient, but for constant-load applications (like propelling a boat), a turbine engine can be >90% efficient, and rival fuel cells & batteries for efficiency of energy harvest.

    One more idea (and I'm working on a prototype for this one), since you're getting VERY close to it here anywise: If you heat your water to the boiling point (which is lower in the presence of salt), then you can distill purified water from it by harvesting the thermal energy in a steam-turbine. The steam-turbine can then be used to either generate electricity directly, or compress the hydrogen that's being split from the water in hydrolysis. Requires another (relatively very small) piece of equipment, but it allows you to gain a significantly more EFFICIENT harvest of energy from the Sun. I was actually, originally, working on the [solar boiler -> steam turbine -> electricity & distilled water] device concept as a possible way to improve on the electrical efficiency of PV cells, still haven't proved that concept yet, but I'm working on it.
     
  4. 2farnorth
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    2farnorth Junior Member

    Sun21

    Am I the only one that thought the SUN21 (that catamaran that crossed the atlantic on only solar power) was a big breakthrough. What is wrong with 5 knots with or without wind. Dont most sailboats end up motoring half the time. I thought the solar panels charged the batteries AND drove the electric motors during the day, then at night they dropped thier speed a bit and cruised on the batteries. I think a simple solar electric propulsion is the answer. K.I.S.S.
     
  5. marshmat
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    marshmat Senior Member

    Not sure what you're referencing here. I suspect you're talking about isentropic turbine efficiency, ie. the actual work per unit mass of combustion products, divided by the value that quantity would take if the expansion were isentropic. That's typically around 0.7 to 0.9 for well designed turbines. But most of that work is needed to drive the compressor, and there are other losses throughout the system. To make a fair comparison against a piston engine, though, one would more likely use the efficiency of the entire system, which looks more like:
    (rate of mechanical energy delivery at output shaft) / (rate of heat input, or rate of chemical energy input)
    which is in the range of 0.2 to 0.4 for typical mobile turbines, and about 0.5 for a very well optimized stationary generator; a good combined-cycle turbine setup like the 480 megawatt GE H-system can hit 0.6 (ie, 60%). Ultimately, with any heat engine- be it piston, turbine, whatever- you are always going to have the Carnot efficiency (1-Tc/Th) as an absolute upper limit on efficiency, and getting to more than half of the Carnot efficiency in something that is still compact and light enough to move around is very, very difficult.
    Fuel cells can, in theory, get around this limitation because they are not heat engines- the electricity is produced directly from the fuel by electrochemical, not thermal, processes.
    That was a pretty impressive voyage. I do think there is a lot of potential in that line of thought, especially if we can get the cost of the solar cells down and the durability up. It may not be the solution for everyone- SUN21 was extremely light and had relatively little accommodation space and cargo capacity. Still, I'd love to see more boats like that around....
     
  6. venomousbird
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    venomousbird Junior Member

    I think everyone is forgetting that running a fuel cell is an energy transfer based on a chemical reaction, so producing new fresh water isn't important. . . The water can be reused after the gasses are recombined!
     
  7. Tcubed
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    Tcubed Boat Designer

    That is correct if the energy is being produced on board.

    In the alternative situation that i think is a pretty realistic future scenario, where the energy is produced and packaged in the form of pressurized hydrogen, you would actually be exhausting pure water. In fact using fuel cells and filling up with H2 + O or just H (in which case the oxygen just comes from the air) you would barely need fresh water tanks as the engine is continually providing water for drinking, etc.
     
  8. robherc
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    robherc Designer/Hobbyist

    T^3:
    About how much water does a fuel-cell ACTUALLY exhaust while it's operating? I seriously doubt that it will be enough for anything other than a minimal amount of drinking water. But, as I am NOT "in-the-know" on fuel cell tech, I'm only guessing here.
     
  9. Tcubed
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    Tcubed Boat Designer

    Let me check this out right now and will post what i find.

    It's funny because you just say thee opposite of what someone said a few posts back (maybe Marshmat?) that to split water into H2 + O would require quite a bit of water as in difficult to solar distill fast enough given the limited deck space of a deck.

    Remember that a fuel cell is a mirror image process off electrolysis. (well almost , as there are losses both ways) That is why some fuel cells are reversible.
     
  10. robherc
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    robherc Designer/Hobbyist

    Yes, and I was the one arguing FOR doing on-board solar distillation of the water...but I guess it would all depend on how much energy you were demanding of the fuel-cell, and how efficient it is. How many gallons of water-worth of Hydrogen are you anticipating using per day?
     
  11. Tcubed
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    Tcubed Boat Designer

    Well i'm still looking for exactly what i want to see but i did find that gasoline contains a bit over a third as much energy per mass as water so i guess figure the rate of gasoline consumption if it were to be fitted with a gasoline engine , divide by three and that would be roughly the rate of water through put . So that is definitely more than enough to keep humans from getting thirsty onboard. If the energy is being produced onboard then the water would not be drunk , rather it would be a closed loop system where the fuel cell puts the water back into the electrolyser.

    I did find some other good info though.

    ***

    (FUEL CELLS)
    Efficiency


    [edit] Fuel cell efficiency

    The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)
    For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these numbers represents the difference between the reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.
    Fuel cells do not operate on a thermal cycle. As such, they are not constrained, as combustion engines are, in the same way by thermodynamic limits, such as Carnot cycle efficiency. At times this is misrepresented by saying that fuel cells are exempt from the laws of thermodynamics, because most people think of thermodynamics in terms of combustion processes (enthalpy of formation). The laws of thermodynamics also hold for chemical processes (Gibbs free energy) like fuel cells, but the maximum theoretical efficiency is higher (83% efficient at 298K [14]) than the Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio of 1.4). Comparing limits imposed by thermodynamics is not a good predictor of practically achievable efficiencies. Also, if propulsion is the goal, electrical output of the fuel cell has to still be converted into mechanical power with the corresponding inefficiency. In reference to the exemption claim, the correct claim is that the "limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems".[15] Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.

    [edit] In practice

    For a fuel cell operating on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and dehumidifying it. This reduces the efficiency significantly and brings it near to that of a compression ignition engine. Furthermore fuel cell efficiency decreases as load increases.
    The tank-to-wheel efficiency of a fuel cell vehicle is about 45% at low loads and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure.[16] The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a car with fuel stack claiming a 60% tank-to-wheel efficiency [17].
    It is also important to take losses due to fuel production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.[18] In addition to the production losses, over 70% of US' electricity, used for hydrogen production, comes from thermal power, which only has an efficiency of 33% to 48% resulting in a net increase in carbon dioxide production by using hydrogen in vehicles[citation needed].
    Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions.[19] While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.
    Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a micro combined heat and power (m-CHP) application. When the heat is captured, total efficiency can reach 80-90% at the unit, but does not consider production and distribution losses. CHP units are being developed today for the European home market.

    [edit] Fuel cell applications

    Further information: Fuel cell vehicle, Stationary fuel cell applications, and Portable fuel cell applications [​IMG] [​IMG]
    Type 212 submarine with fuel cell propulsion of the German Navy in dry dock


    Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability.[20] This equates to around one minute of down time in a two year period.
    Micro combined heat and power systems such as home fuel cells and cogeneration for office buildings and factories are in mass production phase. The stationary fuel cell application generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90%[21] (35-50% electric + remainder as thermal) Molten-carbonate fuel cells have also been installed in these applications, and solid-oxide fuel cell prototypes exist.
    [​IMG] [​IMG]
    The world's first certified Fuel Cell Boat (HYDRA), in Leipzig/Germany


    Since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).
    One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative[22] has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 gallon tank at 200 PSI, and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. The SIEI website gives extensive technical details.
    The world's first Fuel Cell Boat HYDRA used an AFC system with 6.5 kW net output.


    source; http://en.wikipedia.org/wiki/Fuel_cells#Fuel_cell_efficiency


    ***


    High pressure electrolysis

    From Wikipedia, the free encyclopedia

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    [​IMG] [​IMG]
    High-pressure PEM electrolyser.


    [​IMG] [​IMG]



    High pressure electrolysis (HPE) is the electrolysis of water by decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water[1]. The difference with an standard proton exchange membrane electrolyzer is the compressed hydrogen output around 120-200 Bar (1740-2900 psi)[2]. By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal differential pressure compression is around 3%[3].
    Contents

    [hide]
    //
    [edit] Approaches

    As the required compression power for water is less than that for hydrogen-gas the water is pumped up to a high-pressure[4], in the other approach differential pressure is used[5].

    [edit] Ultra high pressure electrolysis

    Ultra high pressure electrolysis is high pressure electrolysis operating at 5000-10000 psi. [6] At ultra-high pressures the water solubility and cross-permeation across the membrane of H2 and O2 is affecting hydrogen purity, modificated PEMs are used to reduce cross-permeation in combination with catalytic H2/O2 recombiners to maintain H2 levels in O2 and O2 levels in H2 at values compatible with hydrogen safety requirements.[7][8]

    [edit] Research

    The US DOE believes that high pressure electrolysis, supported by ongoing research and development, will contribute to the enabling and acceptance of technologies where hydrogen is the energy carrier between renewable energy resources and clean energy consumers.[9]
    High pressure electrolysis is being investigated by the DOE for efficient production of hydrogen from water. The target total in 2005 is $4.75 per gge H2 at an efficiency of 64%.[8] The total goal for the DOE in 2010 is $2.85 per gge H2 at an efficiency of 75%.[9] As of 2005 the DOE provided a total of $1,563,882 worth of funding for research.[8]


    source;http://en.wikipedia.org/wiki/High_pressure_electrolysis


     
  12. Tcubed
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    Tcubed Boat Designer

  13. Tcubed
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    Tcubed Boat Designer

    Another point i would like to remind everyone about is that once we're using free energy , the efficiency becomes somewhat irrelevant.

    What does matter is the cost per watt hour. Basically how much human effort ($) is required to fabricate, install, setup and maintain a given natural energy capture system, per energy.

    Example; suppose system B is half as efficient as system A , but costs a quarter as much in the long run. You just make B twice as big as A would have been and it still cost half as much as A.

    On a boat it's different, because of weight and drag considerations , of course.
     
  14. 2farnorth
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    2farnorth Junior Member

    Keep it simple

    Dont you guys think this is all a little too complicated for the average boater/builder such as myself? You guys sound very intelligent but I start reading the theories you present and my eyes and mind start to blur. It could just be my ADD kicking in, but I believe a pleasure craft with a crew of 2-4 souls has to be simple enough for the skipper to understand and redundant also. I could see these complicated systems on a naval vessel where you have a good size crew with specialists on all the systems just like the nuclear powered ships of our fleet. I am a master auto technician by trade (also a 10 year Navy veteran on surface ships) so I think I may be above average on the knowledge scale as far as mechanical and electrical systems go, but I would not want to go to sea with my wife and kids/friends in a vessel that I did not have complete understanding of how my propulsion and other systems work. Just my 2 cents. :)
     

  15. robherc
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    robherc Designer/Hobbyist

    2farnorth:

    You make a very valid point, but please also realize that all of the systems in a car, when discussed conceptually, would sound the same. There's a TREMENDOUS difference between discussions of efficiency/feasibility and user's manuals. You don't have to understand EXACTLY how your alternator in your car generates & regulates the electrical output to the battery/stereo/ignition system/etc. in your car to know that if its putting out >16 volts, or <10 volts, you have a serious problem and need to replace it. I'm quite sure that if any of these ideas get fully developed, the "user's manual" on them would be very similar.

    Cheers, and thanks for the "back down to Earth" reminder! ;)
     
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