Heat output of engine

Discussion in 'Diesel Engines' started by ErikdeJong, Sep 21, 2012.

  1. Tackwise
    Joined: Mar 2010
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    Tackwise Member

    I have not yet seen the math for warming up 1200 litres so allow me:

    Saltwater has a specific heating capacity (cp) of 3,93 kJ/kgK
    Density of 1025 kg/m^3

    Assuming worst case scenario start temperature at 0 degrees Celsius,
    the Delta T becomes 85 – 0 = 85 degrees Kelvin

    Amount of energy needed to warm 1200 litres becomes:
    Qneeded = cp*Mass*deltaT
    Qneeded = 3,93*(1,2*1025)*85 = 410881 kJ

    Your engine is 70 kW mechanical
    Using the rough figures as already presented 1/3 mech, 1/3 coolant and 1/3 exhaust
    You will have at best 70 kW of heat energy in your engine coolant system.

    If you did indeed have 70 kW of constant input power (which you don’t!!!!)
    The heating up time would be:
    Time= Qneeded/Qinput = 410881/70 = 5870 seconds =>1,63 hours

    Now then the fine print:
    1) The heating power available through the coolant system estimated at 1/3 of the total power is overly optimistic due to engine specs/fouling/etc. Use the more conservative figures of 20-25%
    2) The full 70 kW of heat energy is not available for the full duration of the heating of the tank. Once the seawater starts warming up the delta T between the seawater and the engine coolant becomes smaller and smaller, therefore the energy transfer rate becomes slower and slower. In other words, the time to heat the seawater will take a lot longer than the 1,63 hours.
    3) 1200 litres is a relative big tank for a yacht. I am therefore assuming it will be an integrated tank in the ships construction. The heat loss through the hull and construction will therefore be quite extensive.
    4) Artic seawater has other characteristics then the figures I used.
    5) Etc
     
  2. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    "I will circulate water around a 300 degrees celcius exhaust pipe,"

    The folks that sell water heaters for dirt house roof tops sell Aquastats that will sense water temperature and start & stop the water circ pump at your desired temperature.

    The whole concept can be made seamless , you just watch as it happens.

    Diesels are more efficient at higher loading.700C exhaust temp,whatever the eng. mfg allows.

    Propulsion usually does not load the engine to 80 or 90% , so perhaps making electric , and dumping it into elements in the water tank might get more heat per gallon of fuel, and cut down on run time?

    FF
     
  3. Frosty

    Frosty Previous Member

    Trying to use every little BTU available from fuel has eluded engineers for a hundred years. I suspect you will have to put up with it and accept the inevitable. I would;nt feel bad about it --no one else has managed to do it.
     
  4. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Taking all energy out of fuel as heat is quite common technology in power plants, where you typically get over 95% of energy as heat. Getting mechanical work or electricity is more difficult.
     
  5. ErikdeJong
    Joined: May 2012
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    ErikdeJong Junior Member

    Good to know!

    Unfortunately they are not, it is a steel boat and the diesel tank is part of the bottom structure of the vessel for optimum weight location. Heating the deep tank is therefore not an option. Heating the daytank might be possible, but that one is only 130 liters without space to place a larger one.
     
  6. ErikdeJong
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    ErikdeJong Junior Member

    @ Tackwise, your math seems to be very similar to mine.

    Answers to your points:
    1)I want to "collect" the waste heat of 4-5 hours of motoring at the time. That is why I want such a big tank. Being conservative in my math, I come to the conclusion that my engine will need 3,5 to 4 hours to heat up the 1200 liter tank. In my math I use extrapolated numbers of the warm up time of my freshwater heater that has been in service for a few years now.
    2)See above.
    3)The tank will be made of foam core epoxy for optimum insulation and will be tightly placed between three steel bulkheads that are already there. I have no concerns about a tank coming loose or going on the run. If the conditions will get really tough out there (and they will!), I will just empty the tank. It is a luxurious item and not a necessary addition to the vessel.
    4) Arctic seawater has a little less salt content in general and will therefore take longer to warm up.
     
  7. ErikdeJong
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    ErikdeJong Junior Member

    I think you will have a problem as soon as the whole system reaches maximum temperatures and the water will not be cooled any more because the object that you would like to heat cannot absorb more energy. In that case the water around the pipe will start boiling and I will have to turn off my engine. That is not the objective of the whole exercise.
     
  8. Frosty

    Frosty Previous Member

    Well you got me then!!! I dont think there is anyone or is anything on the planet that can help you.

    I once thought of making an oven from the exhaust system. Thats all I got ---sorry.

    Last ditch attempt,------ use storage bricks from heating radiators and use them as ballast as well.
     
  9. ErikdeJong
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    ErikdeJong Junior Member

    Frosty, It is not that I want to be impossible, but it is the main reason why I was focusing on utilizing the existing coolant circulation circuit for this purpose rather than the exhaust heat.

    The only thing that can help me is a liquid that remains pumpable in the temperature range of 0 to 300 or 350 degrees and that is capable of carrying heat. All other systems are, as far as I know, much more complicated and expensive to make. I would love to use the exhaust heat, but do not see how yet.

    As for the coolant system, I have the concept in my head and the math adds up to what I have in mind. It is time to make a tank and run some tests!
     
  10. Joakim
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    Joakim Senior Member

    If you really want to, it is not that difficult. All you need to do is to put an heat exchanger in the flue gas pipe before you put cooling water into it. Of course you need to be careful not to boil the water (and keep the flue gas in the pipe). Thus you need a thermostat that puts the cooling water to flue gas pipe before the heat exchanger, if your water tank is already warm enough.

    Using exhaust heat you can get close to 100 C, which is much more than you can get from cooling circuit. But sea water + oxygen + 100 C is very corrosive. That is why sea water cooled engines are set to run at much lower temperature (thermostat opens ~50 C).
     
  11. ErikdeJong
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    ErikdeJong Junior Member

    Joakim, my coolant is 90 degrees and that is pretty much the maximum I would go to for seawater tank temperature any way.

    In regards to the corrosion abilities of hot seawater: that is why I like to go with a glass reinforced foam tank with Kevlar or thin walled fiberglass tubing where the coolant runs through. There is nothing that can corrode there, it is very easy to build and it is insulated by its own structure.

    I do see what you mean with the extra heat exchanger and the thermostat, but it leaves room for error. It can over-heat, back pressure in the exhaust might increase room for failure in the thermostat resulting in steam production.

    Adding parts that can fail is not something I'm keen of for a vessel that goes to remote areas. I would like a system that has no change of overheating (boiling) if I do something wrong or forget something or if a sensitive but important part fails.

    The 1/3rd rule says that the heat energy of the exhaust pipe is pretty much the same as the amount of energy that is taken away by the coolant.

    They will have to be two separate systems anyway because of the difference in operating temperatures and since the coolant is a lot easier to achieve, and at lower costs, that is what I prefer to use.
     
  12. jonr
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    jonr Senior Member

    > In that case the water around the pipe will start boiling and I will have to turn off my engine. That is not the objective of the whole exercise.

    No, in that case, your storage tank is near its maximum usable temperature (say ~90C) and you divert the exhaust flow to a different path.

    You can extract heat from both the coolant system and the exhaust at 20-90C. But I agree with keeping things simple.
     
  13. ErikdeJong
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    ErikdeJong Junior Member

    In that case it should be an open system and not a closed circulation. If it would be a closed circuit, there is nowhere to go except far adding another heat exchanger to get rid of the heat if I can't store it any more. If I would do that, I would already have 5 exchangers rather than 3.

    I have drawn my conclusion, I will implement the system in the existing cooling circuit and if it works well and salts or zeolite becomes more effordable, I will upgrade to exhaust heat storage as well.

    Thanks guys! I really appreciate your input!
     
  14. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Erik.

    can you answer a few questions for us.

    1. What is the target day/night temp in the boat?

    2. What is the measured temp of the coolant flow out of the engine. Just measure the pipe temp with a thermocouple with a bit of insulation on the pipe when running at your 6-7 l/h.

    3. What is the plan to distribute the stored heat? ducted/unducted/piped?


    There is a fair amount of things to worry and wonder at when doing this.
    I assume you are going to split the keel cooler loop and not run in series.

    Lets say you have about 60MJ/hour coming out the motor at a pipe temp of around 90 C. the flow rate outside the motor can be calculated from this and the return temp. Since you are concerned primarily with fuel economy, the issue is to provide optimal engine cooling and save the waste heat. Returning dead cold water to the tstat isn't usually the best thing to do, but I don't have any numbers for you regarding that.

    As a minimum, when the storage is hot, you need to return water about 15 C cooler. This is a blatant guess. I scaled up the flow rate of my little engine and assumed you were cruising at about 1/4 rated power.

    In order to keep the heat flow reasonable, you probably need the storage media to be 15 degrees cooler than that for a tube in tank setup. Now you are down to about 60 C. Conventional air handlers will output air about 10 degrees warmer than room. If you are going to pipe the water to the airhandler, there will be some more temp drops there, maybe 5 C. So if you want 18 C in the boat, the low end of effective storage is about 30 C.

    pumping 60MJ/hr = 1 MJ/min means 33 liters with a 30 C change = 8000lt in four hours. The rate of temp change is borderline but acceptably slow. It's about 0.125 C/min, which is an important consideration for many types of storage systems. It would pass to a higher value if you ran harder though.

    Two things make this interesting- you have a better quality source than what many systems are build to work with and the heat sent back to the motor will pretty much be returned next time around in the form of a higher flow rate assuming you do not run the keel cooler in series. You can improve the system by using PCMs because of your long discharge times. They generally accept heat at a higher rate than they give it up. Paraffin would store about 280kJ/kg over the same temp range. This is nearly ten times better and gets you down to your 1000 liter figure. So, $1000 worth of paraffin or there abouts. And most of that heat would come off at a very workable temperature. The trouble here would be the rate of temp rise in the liquid phase after most of the wax has melted. You would need to calculate the heat transfer rates above melting point and try to limit the temp rise rate. Use a high melt point paraffin to minimise this.
     
    Last edited: Sep 26, 2012

  15. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The unasked question is what about a fuel burning heater?

    Sure if the engine MUST be operated , attempting to get 100% of the fuel energy is a grand concept.
    '
    But if only HEAT , for interior comfort is required , what about a simple built fuel burning heater?

    With few moving parts to repair (just the pump) my guess would be a vented Primus , old style roaring burner , no hand wheel ,in a housing to capture/ spread the heat might give longer endurance if 4000-10,000 btu would be enough. Diesel fuel works .

    A long vent pipe would take every btu out .

    FF
     
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