How are boilers tested when measuring their efficiency.

[fixitflav]

I would guess every boiler has a sweet point where it is at most efficient.

The trouble with all that is that the required heat output varies with the weather. The boiler could run On/Off, set up to run at the sweet spot, but that would be against current trend, which is modulating gas flow, and maybe variable-speed fan, so output can follow the demand. I wouldn't bet my shirt on which would be more efficient, but I'd guess the latter.

 

[bernardgreen]

set up to run at the sweet spot,

Which is probably how they are operated during the "independent" testing commissioned by the manufacturers.

 

[hard-work]

Sometime in the early 1970's a group of experimentors converted an engine recovered from a car to run on gas and this provided heat and electricity to a couple of small eco homes. They claimed it was the way forward. I suppose gas turbine power stations are similar but on a much larger scale.

An internal combustion piston engine mainly produces heat it is so inefficient. 80% of the fuel in a car's tank is wasted. If all the turning power and heat of an ICE is used it is efficient. If 5 homes use little electricity and mainly have demands for heat, then that is fine. In winter the efficiency would rise, but lower greatly in summer as all the heat used would be wasted. It is highly inefficient running an engine/genny to supply a few lights and TVs. I would think it would be fine these days with battery storage so much more efficient. The engine would need to store heat via a thermal store and electrical batteries with the engine coming in and out to suite.

For this sort of application the rotary engine (Wan-kel) is much more efficient, it is also one third of the size and weight and very smooth having no reciprocating parts. The efficiency rises greatly when running at a constant speed and load. Mazda are to introduce a hybrid using a rotary next year.

Continuous combustion, as in boilers and Stirling engines, is more efficient than explosions as in internal combustion engines. It is also better on emissions.

 

[hard-work]

Bernard, they get over 100% efficiency in boilers as they do not take into account the latent heat. When they do it gets to over 100%. About 110% is max.

 

[hard-work]

Which is probably how they are operated during the "independent" testing commissioned by the manufacturers.

SEDBUK's method is different. I think the method's outline is on their web site.

 

[fixitflav]

Good question Bernard. Don't SEDBUK give their testing methods? Many years ago I read their method, escapes me now.

I googled SEDBUK. Apart from something obvious like efficiency = heat out/heat in, I couldn't find anything about testing methods or calculations.
There's this about condensing, poorly worded IMO

"What is a Condensing boiler?
Condensing refers to the condensation which is created within the boiler as a byproduct of heating water.

In a traditional boiler, this is turned to water vapour so it can be expelled as a waste gas. However, a condensing boiler will turn this vapour to energy to be recycled back into the system. As less energy is lost in waste gases, the boiler doesn’t need to use as much fuel. This makes them both more energy efficient and cheaper to run."

Surprisingly, there's no comment about input being based on NCV or GCV, and apparent efficiency > 100%. But it lists efficiencies of loads of boilers, most in range 88 - 92%, so clearly based on GCV. Figures given for Intergas are comparable to others, nothing about 122 or 127%

 

[fixitflav]

An internal combustion piston engine mainly produces heat it is so inefficient. 80% of the fuel in a car's tank is wasted.

IC engines aren't that bad, and it's putting it a bit strong to say wasted IMO It's the 2nd law of thermodynamics at work. I've read comments from the green lobby implying that the efficiencies of IC engines and thermal power stations (~ 25 - 40%) is on a par with oil companies only extracting 10% of the oil from a well, then drilling another, as it's more profitable (dirty word!) than using secondary recovery.

Mazda are to introduce a hybrid using a rotary next year.

Be interesting to see how it goes. Mazda had rotary in the 90s, a colleague had a Ro80, but they stopped making them, seal problems I believe. Maybe running at constant speed it will be better.

 

[hard-work]

IC engines aren't that bad, and it's putting it a bit strong to say wasted IMO

Are you serious? When 80% of the energy in car's tank is wasted most would say that is very inefficient. They would. I am not kidding.

The new breed of rotary engines are greatly improved. Seals have never been a problem, that was solved in the 1960s. When rotary engines rev up and down continuously, as in normal road driving, they are very inefficient. At low rpm ranges, or under low-load conditions, gas pressure in the combustion chamber can cause the apex seals to lift off the surface. This results in combustion gas leaking into the next chamber. A sustained constant speed that does not lift off the apex improves efficiency. When running at a constant speed at their sweet spot, the efficiency rises greatly as does longevity.

 

[bernardgreen]

https://www.engineeringtoolbox.com/gross-net-heating-value-d_824.html

The heat value is the amount of heat produced by combustion of a unit quantity of a fuel


We differentiate between gross and net heating values:


Gross (or high, upper) Heating Value


The gross or high heating value is the amount of heat produced by the complete combustion of a unit quantity of fuel.


The gross heating value is obtained when

• all products of the combustion are cooled down to the temperature before the combustion (*)
• the water vapor formed during combustion is condensed (*)

In thermodynamics, the term standard heat of combustion corresponds to Gross heating value.
See Heat of combustion for examples of calculation of gross heating values from standard heat of formation of substances.


Net (or lower) Heating Value


The net or lower heating value is obtained by

• subtracting the latent heat of vaporization of the water vapor formed by the combustion

from the gross or higher heating value.

(*) Cooled to the temperature of the air entering the boiler. ( my addition )

 

[hard-work]

I would guess every boiler has a sweet point where it is at most efficient.

On condensing boilers that tends to be on a lower return water temperature promoting more gains from latent heat. This is where thermal stores come in. The boiler is hydraulically detached from the heat demands. Using blending valves the boiler can be pretty much running at maximum efficiency heating up the large volume of water. It also heats the volume of water one long efficient burn.

 

[fixitflav]

Are you serious? When 80% of the energy in car's tank is wasted most would say that is very inefficient.

Yes I am. For a start, petrol engines a few years back showed 25%, diesels 35%, latest ones are better. The best thermal power stations 42%. And you can't compare heat engine efficiency with boiler efficiency. That's making the same mistake as the greens. There's a theoretical limit to the fraction of heat that can be converted to mechanical power, depending on the temperatures of the heat source and sink for rejected heat. That's the 2nd law of thermodynamics I mentioned. The first practical steam engine, Newcomen's atmospheric engine, was < 1% efficient. Steam locomotives about 5%. We've come a long way since then!

 

[hard-work]

Yes I am

My word! He is serious. He thinks car engines are efficient, at 20%. Look at the efficiencies of boilers, even the most inefficient are 80% or so. Look at the efficiencies of an electric motor - in the over 90%.

 

[fixitflav]

My word! He is serious. He thinks car engines are efficient, at 20%. Look at the efficiencies of boilers, even the most inefficient are 80% or so. Look at the efficiencies of an electric motor - in the over 90%.

I was prepared to support you in the thread about Navien boilers, but now you're being silly. You clearly haven't read and/or understood my posts. I can't help it if you don't understand basic thermodynamics, I've tried.
Car engines are more than 20% efficient.
The efficiency of electric motors is irrelevant. Electricity doesn't occur naturally, it has to generated. If the generator is driven by a heat engine the efficiency of that has to be taken into account, between the heat in the fuel and the motor shaft power.

 

[hard-work]

I was prepared to support you in the thread about Navien boilers, but now you're being silly. You clearly haven't read and/or understood my posts.

I did it was nonsense. To claim 20% is efficient is complete nonsense.

 

[fixitflav]

I did it was nonsense. To claim 20% is efficient is complete nonsense.

As I've said a few times, car engines are quite a lot more efficient than 20%, and in any case, as I've tried to explain, you can't compare efficiencies of boilers and engines.
I prefer to avoid sarcasm, let alone direct insults, but I'm beginning to think Desperate Dan is right about you.