Micro Combined Heat and Power
Generator in Silencing Box
  Yonks ago when I used to go Amateur Radio VHF contesting we usually used a big 3kW Diesel generator but I also had a small 500W petrol for the small contests and emergencies. It had been kicking around for years unloved and unused since then. So one day I was thinking about my electric bill and that Gas is about a quarter the price of electricity /kWh... So if I had a generator that ran on gas I could use it to charge my PV batteries and use the "waste" heat to heat the house
gas Conversion
  It's never a good idea to start ripping the house heating system apart in the middle of winter but the workshop is always cold in winter and I use a superser gas heater to keep warm. As as I had the 500W generator kicking about I decided to have a small trial. I converted it to gas as shown. For a constant load it just requires a gas pipe up the air intake and a means of controlling the flow rate. It's that simple...
Gas Conversion
  .From a safety point of view however it would be nice to turn the gas off if the engine shuts down this helps keeps the roof on and the wall verticle... And of course it would help if the exhaust was dumped outside as I'm not suicidle yet. I had a temporary conversion once before when I produced methane from Manure and wanted to see if it would run my generator. But this time I made a tidier job of it. incidentally I did produce the methane (25lt) and it did run the generator but no for long at 6lt/min...
Flow Meter
  Flow control was manual for the trial and achieved by a needle valve and ball indicator. Engine starts up with 0.5 lt/min and runs full load cold 2.0 lt/min and warm 1.5 lt/min. The gas used for the trial was Propane there is as yet no gas in my workshop... But time has a habit of changing things...
Box with Cooling
  The generator is NOISEY so I decided it needed to be silenced this was partially achieved by sticking it in a box lined with foam and forced air cooling it through sound baffles. It was a lot quieter but not quiet... Most of the "waste" heat was dumped into the workshop some was lost through the exhaust being dumped outside.
  The electricity produced is used to charge the batteries of my PV System there is a Regulated 28 Volt Current limited (10 Amp) SMPSU that runs on the mains produced by the generator. It usually produces about 9.4Amps of Charge current.
After Crunching The Numbers
Data acquired from the net... several sources give similar figures
GAS Liquid @ Density kg/m^3 Energy kJ/m^3 (Gas)
-40 C
Natural Gas


Propane Consumption of generator
@ start up Set Rate to 0.5 - 0.7 lt/min
start and rapidly increase to 2.25 lt/min (till engine sounds stable) and monitor and adjust
after 5 min's running (engine warm) decrease flow rate till engine just starts slowing and increase
again till stable do this several times till you are happy you have the point of stability and min gas flow.
This occurs on my generator at about 1.8 lt/min

Why does the engine still regulate it speed with a constant Fuel flow... Answer

Energy Use
Propane contains 101,000 kJ/m^3 or 101 kJ/lt (Gas)
Therefore Propane consumed at 1.8lt/min is and energy release of 101 kJ x 1.8 / 60 = 3.030 kJ/s or 3.03 kWatts
Energy produced to charge batteries 9.8A x 25V /0.9 (SMPSU efficiency 90%) = 272W (what about the fans)
Electrical production efficiency = O/P / I/P = 272/3030 = 8.9%
Heat released = 3030 - 272 = 2758 kW about the same as a 3 kW fan heater They are usually 2.8kW

a 13 kg cylinder cost me £20 is
Therefore Propane cost/kg = £1.54/kg
Propane density is (liquid) 583 kg/m^
Therefore 1 lt Propane (liquid) is 0.583kg
Therefore 1 lt Propane (liquid) costs 0.583 x £1.54 = £0.90

When Propane expands to a gas from a liquid the volume increase about 250 times
Therefore 1 lt Propane (Gas) costs £0.90 / 250 = £0.0036

Total Energy Costing
Rate of gas consumption is 1.8lt/min (Gas)
Cost = 1.8 x 0.0036 x 60 = £0.39/hr
or 0.39/3.03 = £0.13/kWh

Electricity Only
0.39/0.272 = £1.43/kWh

Heat Only
0.39/2.758 = £0.1414/kWh

Conclusions for using Propane for CHP
Fuck me Gently... Turn it off. Turn on the electric fan heater it's cheaper and quieter and
there is sod all maintenance costs. The fan heater only cost £10 The generator cost £200.


Projected Results For Natural Gas...

I have shown similar Calc's for comparison all you really need is the cost/kWh and generator

Energy Use
Energy content of Natural gas is 39,300 kJ/m^3
then for same energy consumption and therefore o/p the generator would be expected to use
about 101,000/39300 x 1.8 lt/min = 4.625 lt/min

From my gas bill Gas / kWh is £0.03616 for the first 1063 kWh
They state the conversion from ft^3 is as follows (I'm not good at belief but I am lazy )
kWh = ft^3 x 2.83 x 1.022640 x 39.3 / 3.6 = ft^3 x 31.593611 = (kWh)
What The ****
Theirs looks like I read it wrong or they stuffed it up and forgot the E-2... Let take a look
(Glad that's sorted we'll use My Version shown below with an explanation of terms used)
kWh = ft^3 x 2.831685E-02 x 1.022640 x 39,300,000 / (3600 x 1000) = 0.316124216 kWh
...........^....... ^........................ ^................ ^................... ^......... ^...........^

1) ft^3 read from your (my) meter (not my gas bill) assume this to be 1
2) Conversion From ft^3 to m^3
3) The gas is at a small pressure so is compressed this is the expansion factor
4) The energy in 1 m^3 gas measure in J/m^3
5) converts Joules to Watt/hours in one ft^3 of gas
6) Converts Watt/Hours to kWh
7) Number of kWh in 1 ft^3

Therefore 1 ft^3 Gas costs/ft^3 £0.03616 x 0.316124216 = £0.01143/ft^3

1 ft^3 = 2.831685E-02 m^3
therefore cost of 1m^3 = £0.01143 / 2.831685E-02 = £0.403683707
Therefore Cost/lt = £0.4414332809 / 1000 = £0.000403683707
1 ft^3 = 28.31685 lt
therefore cost/lt = £0.01143/28.31685 = £0.0004036837 or // £0.0004

Projected Total energy costs
Rate of consumption = 4.625 lt/min
Predicted Cost of Gas in 1 hour = £0.000403 x 4.625 x 60 = £0.1118
Remember for that hour we produced energy at a rate of 3.03kW 0.272kW was electric and 2.758kW as heat
or £0.1118/3.03 = £0.03690/kWh
i.e. Close to the original cost /kWh of £0.03616 (should have been exactly the same but there was some rounding)

Projected Cost electricity only
Remember for that hour we produced energy at a rate of 3.03kW 0.272kW was electric and 2.758kW as heat
£0.1118/0.272 = £0.4110/kWh

Projected Cost Heat Only
Remember for that hour we produced energy at a rate of 3.03kW 0.272kW was electric and 2.758kW as heat
£0.1118/2.758 = £0.0405/kWh

Conclusions for using Natural Gas
As expected from the cursory look at cost/kWh and generator efficiency the electricity on its own is more expensive than Buying it from a standard supplier. But if we are using the heat the electricity cost/kWh is 3.6p rather than 14p (NPower 2006/04). Moving the heat to where you want it there will be some losses and allowing for this and adding the costs to the electricity costs say 4p a unit for electricity ain't bad... BUT IMPORTANTLY it should be noted with the efficiency of the generator only 9% it means MUCH more heat is produced than electricity. This suggests the best way to use a system like this is NOT to want the electric and use the heat but to want the heat and use the electric... i.e. Turn the generator on and off when you require heat and use the electric to supplemental charge my PV batteries...

Now where's my pipe cutters...

Thoughts for the future... Maybe