Energy Assessment: What does efficiency mean?

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- CommentAuthortony
- CommentTimeOct 10th 2011
The efficiency of an appliance especially a heating appliance is frequently quoted and it is generally accepted that the higher the efficiency the better

The efficiency of the use of fuel is slightly different

as is the overall efficiency of the system as a whole

What does efficiency mean? should we qualify what aspect of efficiency we are talking about in our discussions
- CommentAuthorSteamyTea
- CommentTimeOct 10th 2011
Yes we should. Who is going to start the lessons off then?
- CommentAuthorqeipl
- CommentTimeOct 11th 2011 edited
Useful work done divided by the reduction in the potential (stored) energy applied to the task?

I think it's important that the potential energy is considered from source, e.g. the potential energy of mains gas is taken at its subterranean position, not at the gas meter in your house. So every bit of energy used to find, extract, distribute and burn the gas is counted as a 'loss' against the useful work done of heating water for a shower.
- CommentAuthorSteamyTea
- CommentTimeOct 11th 2011
I tend to agree there but then you have a problem about the 'useful work done at the shower'. If you could say halve your shower time, would this double your efficiency. Then you have issues around opportunity costs. Could you have used that energy (or a fraction of it) for something better/more useful.
- CommentAuthorJoiner
- CommentTimeOct 11th 2011
Which hints at the suggestion that there is no such thing as Absolute Efficiency, it's always going to be a case, then, that efficiency is relative to use?
- CommentAuthorSteamyTea
- CommentTimeOct 11th 2011 edited
Efficiency related to use is called exergy I think, a strange made up term but valid all the same.
Was it not Einstein that showed that mass and energy are the same thing (well till a couple of weeks back) and the product of mass and the speed of light squared is the energy, So if looking for the 'true' efficiency then this is the starting point.
Though Hawkins may have something to say about it with virtual particles evaporating from the very edge of a black hole. So maybe the theoretical energy in the information of the virtual particles that escape may be a better gauge.
It is claimed by some that the universe has a net energy content of zero, makes the sums very easy that does.
- CommentAuthorCWatters
- CommentTimeOct 11th 2011
Different industries have different "standard" definitions of efficiency. Did you know it's possible for a boiler to be more than 100% efficient :-)

http://www.blesi-evans.com/102%20percent%20efficiency.pdf
- CommentAuthorJoiner
- CommentTimeOct 11th 2011
And then just rusts away!

Have you not noticed that ALL athletes and sportsmen give more than 110%?
- CommentAuthorSteamyTea
- CommentTimeOct 11th 2011
110% of what, the mean speed/score rate/points of the average person? 110% of what they normally do.

50% of the time you can't see a wind turbine, does that mean it is only 50% efficient or does that mean it is 150% efficient (or even somewhere between 0 and 150%).

Then how do you work it out arithmetically:

a) Energy Out divided by Energy In

b) Energy In minus Energy Out all divided by Energy In

c) Energy Out divided by the sum of Energy In minus Energy Out

Examples:

Energy In = 10
Energy Out =3

a) 3/10=0.3
or
b) (10-3)/10=0.7
or
c) 3/(10-3)=0.43

I have seen all three used in one article.
- CommentAuthorqeipl
- CommentTimeOct 11th 2011 edited
Five posts into the game and my head hurts already.

Tony appears to be interested in a practical and universal definition of efficiency that can be applied across a range of energy types and conversion technologies, which suggests to me that Newton and Kelvin, rather than Einstein and his chums, should be our guides.

The first thing we have to do is define, in each case, the useful work done.

If we take my example of heating water for a shower we first have to agree a value for a useful shower, which I would define as water taken from the mains at 8C and delivered to the shower head at 38C at a flow rate of 8 litres/min for 3 minutes. So, if we agree on the parameters, the useful work done is:

4.19kJ/kg.C x 24kg x 30C = 3016.8kJ = 0.838kWh (please check this, someone).

Working back from there to agree the losses between the gas meter and the useful work done should be fairly simple.
The losses attributed to exploration, extraction, processing and distribution are impossible for me to define without lots of research but I bet someone, somewhere has had a good stab at it. When agreed, the value can be applied to every kg of gas delivered through the meter for whatever purpose.

Using this ratio - useful work done divided by the potential energy of the resource in its natural state - would give an efficiency comparison matrix of hydrocarbons, nuclear, renewables, and the various energy transfer technologies that are employed to do useful work.

Discuss.
- CommentAuthorSteamyTea
- CommentTimeOct 11th 2011 edited
Anyone going to mention the differing entropies of energy types/sources?
Really comes down to the context.
- CommentAuthorwookey
- CommentTimeOct 11th 2011
Following on from cwatters comment, it seems to be the case that euro-mainland boilers are rating in a way that allows > 100% efficiency (my Viessman claims 104%) , whilst UK (SEDBUK) ratings are done in a way which doesn't. (I've not seen better than 94%). It depends what you include and exclude from the initial and final states. This is quite a good example of the general problem of how to measure efficiency.
- CommentAuthorfostertom
- CommentTimeOct 11th 2011
Posted By: SteamyTeamaybe the theoretical energy in the information of the virtual particles that escape
Now yer talking!

Each of the fields that are apparent to our 4-dimensional senses (3 spacial + 1 time) has a characteristic speed of propagation. The speed of propagation of the electromagnetic field is known as the speed of light. There are other fields that we're aware of (tho apparently if you chuck in a few more dimensions they (may) turn out to be all one unified field). What are the characteristic speeds of propagation of each of these others - or is that a silly question.

What is the speed of propagation of the information field? Just as mass may be destroyed resulting in energy release, at a next level up may information be similarly interchangeable with energy?
- CommentAuthorCWatters
- CommentTimeOct 11th 2011
In general Efficiency = Energy Out/Energy In. Any other meaning or interpretation should be defined when you use it.

The problem with boiler efficiency ratings is the definition of "Energy In" (eg what is the energy content of the oil or gas?). If you were to ask Einstein he would have said E=MC^2 but no conventional boiler could ever get anywhere close to extracting all that!

Traditionally the energy content was defined as that released when oil is burnt without condensing the flue gasses. With the advent of condensing boilers we should have redefined how much energy was available in the oil to be extracted. That's why it appears we can extract more than 100%.
- CommentAuthortony
- CommentTimeOct 11th 2011
I knew this wasn't an easy question to answer
- CommentAuthorJoiner
- CommentTimeOct 12th 2011
Which, perverse person that you are, is why you posed it in the first place!

I think you lie awake at night thinking up these thread subjects. Or is there a book for forum admins titled "Thread topics to really wind the bastards up."
- CommentAuthorrenewablejohn
- CommentTimeOct 12th 2011
The one that annoys me is adiabatic efficiency which companies quote for efficiency of turbines. A bristle turbine I was looking at was supposed to be 65% efficient compared to my 12% Bellis & Morcom steam engines. Turns out we were comparing apples and pears.
- CommentAuthortony
- CommentTimeAug 23rd 2012
I am back on this again

A lot of the time when "energy efficiency" is used it really means reduction of energy use.

Is improving the energy efficiency of a house the same as reducing its energy use? Why dont we just say saving money on energy bills, I'm sure that is what the all mean isnt it?
- CommentAuthorjamesingram
- CommentTimeAug 23rd 2012
Yes, "energy efficiency" with regard to buildings use, to me means "saving money on energy bills"

could have a much wider meaning with regard to 'construction of ' though
- CommentAuthorfostertom
- CommentTimeAug 24th 2012
Energy efficiency means achieving the same (or more) with less.

'Efficiency' measures which just result in people using even more of whatever it is, are only 'efficient' when the criteria are artificially restricted, to disregard the inconvenient bits - a lot of that goes on.

There's a growing branch of social/environmental crossover research and consultancy, which looks at what actually happens, incl what people actually do, compared to the intended or theoretical result of efficiency measures, new green technologies etc.
- CommentAuthorbarney
- CommentTimeAug 24th 2012
Or you could argue that "energy efficiency" with regard to buildings use is "doing more - with less"

Regards

Barney
- CommentAuthorSeret
- CommentTimeAug 24th 2012
Posted By: tony
What does efficiency mean?

It's relevant to the context, you aren't going to get just one answer.
- CommentAuthorcrusoe
- CommentTimeAug 27th 2012
Yes, context is important, and to most people I think JI's point about saving money is quite intrinsic to the question. I was once taking off-the-cuff questions at a CAT conference and was asked a question which was semi-political. The question was 'What kind of person, politically, is most likely to spend money on energy-conservation measures?'

I didn't know the answer, or the political persuasion of the person asking the question, so after a moment, I attempted this: "I don't know, but I suspect that whether they are blue, yellow or red, or any shade in between, when I start talking big fuel savings, they will all go green."

I think that still holds true

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