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Combined Heat and Power

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ChrisOlson:
Is there any on the board that use CHP for winter time heating?

Some background - when we bought our new lake home it was off-grid, a seasonal home built by a doctor. It had a forklift battery with two Outback Radian inverters and a generator to charge the battery. We've since gotten rid of all that when we got grid power. Our house is 3,600 sq ft, split level, and we heat it in the winter time with wood-fired forced-air furnace in the mechanical room, and fireplace in the upstairs. Backup heat is provided by electric baseboard heaters in every room.

We wanted backup power so we bought a Caterpillar DE50 generator, rated 50KW on standby, 45 KW on prime. The generator was originally installed as a CHP unit for my 4,600 sq ft shop, while using the house as a load bank (with the electric baseboard heaters) to keep it at least 75% rated load. It heats the shop with no problem at -20F. I've read that CHP diesels can reach energy efficiency of 80-85%. I know what the electric power is from it, but for somebody that has experience with HVAC what is the set of calculations that would be normally used to calculate the BTU/hr for the heat I get from it? The generator burns 3.2 gal/hr at prime load (#2 diesel) so BTU/hr input is 440,000 BTU. At 188 amp load (split phase power) its electrical output is the equivalent of 153,500 BTU. Making the generator about 35% efficient on a fuel to electrical basis.

But how do I calculate the heat output for the heating part?

bigrockcandymountain:
A very approximate rule of thumb is 33% to mechanical energy, 33% to heat through coolant, 33% to exhaust heat.  Every setup would be different.  A long exhaust pipe inside the shop that acts as a radiant heater would help efficiency quite a bit.

I have a similar setup.  Mine is air cooled, deutz diesel and the exhaust is piped out through a thimble that is double wall, so the incoming combustion air gets heated by the exhaust a bit.  I definitely dont get all the exhaust heat, but i get all the engine heat.  I would guess i'm about 75% efficiency all said.

If you want a real redneck experiment, run the exhaust through a big tank of water and see how fast it heats up.  Then you'd know how much is going out as exhaust heat.  1 btu heats 1lb of water 1 degree F.

If i had to guess your heat output, i would say half of the btu that aren't making electricity. 

440 000 - 153 500 = 286 500btu

286 500 ÷ 2 = 143 250 btu

ChrisOlson:
The whole heating system is water (plus glycol). The engine cooling system is piped thru the exhaust, which has a marine riser on it. 180F thermostats in the engine, water temp going to the Modine heat exchanger in the shop is 240F (under 14 psi of pressure). The engine is turbocharged, exhaust temp under the turbo is 980F at full load, exhaust temp at the outlet is 120-125F, depending on when the thermostats in the engine's cooling system open or close. The coolant is moved by the engine's water pump, which moves about 7.5 gal/min with the thermostats open, 1.2-1.3 gal/min with the stats closed and cooling system operating on bypass (for warmup of the engine). The capacity of the cooling system is about 22 gallons with the piping, what's in the engine, and what's in the Modine heat exchanger.

I know there's some heat losses, whatever is radiated off the engine block and oil pan, and exhaust manifold under the turbocharger. But the turbocharger turbine housing and the marine riser is all wrapped with that silver high-temp insulation stuff that they use on marine engine exhaust systems.

It works good to heat the shop. 4,600 sq ft (well insulated), clear span with no internal walls in the shop, at -20F outside it easily heats the shop to 65 degrees inside. The air coming out of the heat exchanger is downright hot when it's running and warmed up, and there's a 26" diameter fan on the heat exchanger.

Seems to me there's a calculation for furnaces that uses square ft of heating area and temperature rise over time to calculate BTU/hr. But if I had to compare, I use a 225,000 BTU diesel-fired space heater (some call them "torpedo heater") otherwise to heat the shop when I'm not working in there. And it's easily equivalent to that.

I was interested in the "scientific" calculation that is used to figure BTU for space heating.

ChrisOlson:

--- Quote from: ChrisOlson on November 21, 2023, 10:56:53 PM ---It works good to heat the shop. 4,600 sq ft (well insulated), clear span with no internal walls in the shop, at -20F outside it easily heats the shop to 65 degrees inside. The air coming out of the heat exchanger is downright hot when it's running and warmed up, and there's a 26" diameter fan on the heat exchanger.

--- End quote ---

Or maybe that BTU/hr calculation deals with volume of air moved vs temp rise? I know it exists but I can't find it on the internet. I know the temperature of the air coming out of the Modine is about 140-145F, according to my wife's cooking thermometer. So it's heating ambient air in the shop to 140-145F as the air passes thru the Modine. But I don't know what the airflow volume of the fan is.

ChrisOlson:
I finally found it in one of my old engineering texts. Every internet search only returned calculators to figure furnace size for a given size of building and heating zone. The formula I was looking for is

BTU/hr = (∆ T) x CFM x (0.24 x Blower Air Density x 60)

The 0.24 x Blower Air Density x 60 is the 1.08 constant that's used. Based on my temperature rise of the air and a 26" axial fan with 1hp motor moving the entire air volume of the shop every 11 minutes it comes out to ~192,000 BTU/hr actual heat output. Giving an overall efficiency of ~78.5% for my CHP unit. So even though the whole thing is homemade, it's in the ballpark with industry standards for CHP plants.

The idea was to determine if one of these could heat the house. Our forced air wood furnace is 150,000 BTU. The fireplace is one of those EPA-approved ones with a heat exchanger and the catalyst above the firebox in the stone wall for the chimney. It is 100,000 BTU. Our house requires minimum 216,000 BTU in heating zone 7 at -40F. A portion of the power from the current one goes to resistance heating with the baseboard heaters (~50,000 BTU), so it's theoretical heat output for space heating would be around 240,000 BTU, plus the balance of ~30KW for electrical loads.

So again, in theory, I could plumb coolant lines into the house and put a heat exchanger in the forced air ductwork and it would probably be ok to heat both the house and the shop except in extremely cold weather when additional heat would be needed for the house. The generator currently runs about 900 hrs a year heating the shop from December to March. Increasing the run time to 2,000 hrs per year, I think, would be very competitive costwise with utility grid power rates combined with what it would cost to run traditional oil-fired furnaces in both buildings.

Grid power is not really all that economical. It is only about 38% efficient from the shaft turning the generator at the powerplant to the load in your house. There's losses all along the transmission lines and transformers, in the generator itself and in the loads. It doesn't matter if the powerplant uses solar panels or wind turbines, those didn't appear out of thin air - it took diesel fuel to mine the materials, process them, transport them and maintain them before they can even exist. The entire planet and all of society as we know it runs on diesel power, from the loaf of bread you buy at the supermarket to your plug-in EV, none of it exists or get into your hands without diesel power. As the consumer you have to pay for all that.

I'm convinced that a properly designed CHP plant running on diesel can be more efficient than traditional methods of home heating and grid power.

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