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Author Topic: Overall CHP efficiency of Lister Veg Oil Genset  (Read 3412 times)
Ivan
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« on: November 12, 2006, 04:40:18 PM »

I spent about 30minutes running through the calculations, based on exhaust temperature of 50C and exhaust temperature of 100C (POST-HEAT EXTRACTION), to give an idea of the overall efficiency. This assumes that any radiated heat from the engine is 'useful' heat - ie heats the room it is located in.

The calculations (below) demonstrate that with exhaust temp of 50C, the efficiency could be as high as 99.2% and with exhaust temperature at 100C (importantly, this means that the water is present in vapour form), then the efficiency drops to 86%.

If the exit temperature of the exhaust can be reduced to below 100C, then the efficiency is quite a bit higher ('condensing mode'!). If the engine is run at low speed, this is more easy to achieve, as the water jacket has more time to absorb exhaust gas temperature. So even if the efficiency of electricity generation is reduced, it may be compensated for by higher overall 'CHP' efficiency.


CALCULATION
nb rough data only - the calculation is over-simplified for the benefit of laziness.


vegetable oil C66H100O6 density 0.91Kg/Litre Mr988.     1litres =33.72MJ/litre
Change of state enthalpy for water - steam = 40.7kJ/mol
C66H100O6 +113O2 = 66CO2 + 100H2O

1 litre of veg oil (0.92mol) when burnt will create 61mol of CO2. i.e.  2,684 kg. and 100mols of water ie 1.6kg

Assumining exhaust is under 100C, so H20 is produced in liquid form, then the exhaust volume is 61x22.4litres (assuming 25C) = 1366litres of exhaust. The actual exhaust temp is say 50C, so the the temperature-compensated volume is 273+50/(273+25) ie 1481litres.
The heat capacity of CO2 is 0.87kJ/KgK at 50C, so the 'lost' heat from exhaust is 2.684 x 0.87 x 30 (assuming 30C difference between ambient temp and exhaust temp) = 70kJ.
Thus CHP efficiency would be 99.8% (this assumes that condensate water is run through a drainage point where it is collected and allowed to cool where heat is absorbed. If the water in the exhaust is lost....as liquid... then the heat taken away would be 1.6kg x 30degrees x 4.2kJ = 201kJ. Thus CHP efficiency would be 99.2%If exhaust is 100C with water as vapour, then the exhaust would consist of  (CO2 Heat Capacity = 0.918kJ/KgK)  197kJ  (CO2 heat capacity) + water (liquid) heat capacity of 537kJ. If the water is present as vapour in the exhaust (ie lister CHP running in non-condensing mode!) then the additional heat lost would be 4070kJ. So overall CHP efficiency would be 86%.
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KenB
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« Reply #1 on: November 13, 2006, 05:33:17 PM »

Ivan,

Interesting calculations.

I have been running the engine in stealth mode again over the weekend.

At just 370 rpm  (about half speed) it was supplying 11 amps into my 48V battery pack at about 58V terminal voltage.  About 640W of power.

The vegetable oil consumption was just 0.342 litres per hour, giving an electrical efficiency of 20.6%.  This would be ideal to keep someone windgen battery topped up in calm weather, and probably more cost efective, particularly in winter when the heat can be used directly.

A collegue has developed a stainless tube and shell heat exchanger for the exhaust which we hope to be testing over the next couple of months.


Ken

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Ivan
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« Reply #2 on: November 13, 2006, 08:57:27 PM »

I MUST get mine up and running sooner rather than later!

Can I ask another few questions:

1. What is the typical exhaust temperature in 'stealth mode' with your load of 670w? (with/without heat exchanger?)

2. do you do anything to get rid of the water in the wvo before use, or do you simply filter the chips/solid fat out of it before it goes into the tank?


Did I mention my suggestions for heat exchangers - 1. heat exchanger out of a gas boiler - these are quite cheap as brand new spares for older machines 2.the water jacket for an oil boiler.   Both are designed for high efficiency removal of heat from hot gases, so might be a good starting point.


Ivan
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KenB
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« Reply #3 on: November 14, 2006, 09:38:32 AM »

Ivan,

I will measure the exhaust temperature next time I run in stealth mode.  Later today probably.

My waste veg oil filter barrel allows the water to descend to bottom of the drum, with the filtered oil being drawn off about 200mm up from the bottom of the barrel.

I let my waste oil settle for many weeks before I use it.  Let nature (time, temperature and gravity) help do the hard work for you first.

When I pour the WVO into the filter barrel, I stop pouring immediately when I see the white trace of fatty material appear in the flow. This way you exclude almost all the fatty material from the filter bag.

After you have poured off several 20 litre cans of oil, you can combine the dregs from several cans and let it settle agin.  This will allow you to get a 2nd pouring from the can - perhaps another 5% of oil, leaving most of the residual fats in the one can.

I am going to find a use for these fatty wastes - probably burning them in a turk or babbington type burner, or adding them to the woodchip feed stock for the gasifier.




Ken
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« Reply #4 on: November 14, 2006, 11:06:21 AM »

Ivan & list,

The Lister is now running and exhaust gas temperaturesmeasures approximately 200mm from the manifold.

After 40 minutes of run time the temperature is 190 C.

The Lister is driving my dc generator as a load, topping up the 48V battery pack at a nominal 60V, 10A.   This is ideal for anyone who wants to float charge a 48V inverterr pack, and also generate a few hundred watts to meet the mean load of a typical household.  There is no need to generate a full 3kW if the household doesn't need it. 

The engine rpm is 300, and is extremely quiet.   Fuel consumption is about 300g per hour once the engine has reached operating temperature after about 20 minutes.   A litre of veg oil will last for 3 hours at this rate.

As an aside:  190 C to me would appear to be the ideal temperature for drying wood chips, prior to burning or gasification. It will drive the water vapour off, but not begin to torrefy the wood.  The latent heat of vapourisation could be recaptured using a condensing boiler type heat exchanger.

There is at least one company selling a stainless steel basket insert that allows you to burn wood pellets or woodchips in a conventional woodstove.



Ken



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