Efficiency how is it measured?

27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
In a discussion we were comparing heating measures and which type of fuel is the most efficient.

Clearly put an electric heater in centre of the room and all energy feed into the heater is turned into heat for the room but with a gas fired unit some will leave via the flue.

However one must also include the loses in getting the fuel into the home.

This is where I run into problems. Energy used to process gas or to generate electricity transport oil or wood even the amount of petrol used in the chainsaw.

I am not talking about cost. But how many joules and used to release one joule into the home.

Electric is a real problem. If a wind turbine has air passing it at 5 mph over an area of 20 square feet and leaves at 4 mph should we consider the turbine is 20% efficient? The same with water turning turbines. We all know it would be impossible to extract all the energy and undesirable.

It all started with an off the cuff statement that wood burning stoves were very efficient. And it was pointed out that to be efficient the combustion air would need to come from outside the building or it would cause drafts, the flue would have to exhaust nearly cold gas, and the wood would need to be reduced to ash with no charcoal in the ashes. Also it would need to be controlled so the room was maintained at the required temperature. To do this likely only way would be to use a central furnace with an auto wood chip feed and motorised draft control. Then one has to include to power used to control it and reduce the wood to wood chips able to be precisely metered.

In the main wood burning stoves are very inefficient with much of the heat going out through the flue and also causing drafts. But look at any large electric power station and we see cooling which puts a lot of energy into the atmosphere or water. Plus we as the user have no idea as to how the kW of power was generated.

Look at Dinorwig for example and we have a very well defined efficiency 275 MW for 7 hours will give 288 Mw for 5 hours and the station uses 12 MW when generating. So 71.6% efficient. If we assume half the losses when pumping and half when generating then we are looking at around 86% efficient but with any hydro power station some water has to be released even if not generating unlike the wood in a wood stove which could be stored and compressed and extracted millenniums latter as coal.

So I came to the conclusion it’s impossible to compare the efficiency across different types of fuel. The closest we can hope it to measure the cost of production and assume that is an indication of efficiency. Clearly we can’t include subsidies to this it has to be real cost.

I asked about the amount of wood which could be extracted from a woodland area and the only way one can economically grow wood for burning is coppices and that method of growing and harvesting wood does not allow trees to grow to an age where cavities form or allow the rich under canopy growth to take place.

As far as burning wood already used before goes we are able to recycle this wood as MDF and paper so burning is really wasteful.

I have watched as foresters place wood to rot so encouraging the rich ecosystem within the managed wood. Clearly anyone collecting this wood is damaging the ecosystem.

To me there is only one reason why most people use wood burning heaters. They are avoiding paying taxes on the fuel used and often steal the fuel and by not paying their way make it even more expensive for those of us that do to live.

So I am throwing the gauntlet down. Please try and put the case why we should not tax all people with wood burning stoves to redress the tax they are not paying on the fuel they are using.
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What if they are used as an ornament? How would you police the tax collection? What are you doing in this forum suddenly?
You need to get out more, especially on Saturday nights. :mrgreen:

And keep away from the Philosophy corner down the pub. :D
i think you are getting bogged down in detail
you could argue about windfarms extracting energy depriving some area in nature or human life causing a negative effect

you could also say electricity is less efficiant than gas if you include transmission losses

it all depends on where the efficiency is measured from :eek:
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Auto Express published a letter of mine in this week's edition in edited form & their response was similar to your point:


I think Ed's response is stupid, frankly, as the emissions they mention would apply to any vehicle. Still, they (and we for that matter) are perfectly entitled to our opinion!
Please try and put the case why we should not tax all people with wood burning stoves

Well not all people steal the fuel. I don't have a stove but a big open fire.
Much more inefficient.

You'd be better off going on a crusade to have all homes in the uk fitted with water meters.
Thats said chopping down the trees and concreting over is probably attributing to the water shortages.
There's just too many people with not enough resources to go around. The country and the world needs a population cull.
As you rightly point out not confined to wood burning stoves but near every item we see economy ratings for seem to have a flaw in the calculations.

One of the big problems is it seems items are tested in a large warehouse with not heating or cooling. But we in fact use the items in premises where heatings and cooling are a big factor.

Light bulbs (tungsten) heat the room with radiated heat which is not lost with air changes so may save more energy than discharge lamps plus no mercury.

Tumble driers (with heaters) blow heated air outside cause drafts and danger where combustion products can be drawn into the house those with heat exchangers don't impact on the house heating.

Induction hobs don't produce water like a gas hob does and put the heat directly into the pan and not the room so there is not the need for cooker hoods in the same way as with cooking on gas.

Two major problems:-
1) Laws have been passed which result in wasting energy from the cat v lean burn engine, to tungsten light bulbs during winter, to condensating boilers rather than being able to fit a sterling engine to extract the last bits of power.
2) Misinformation where people are told what they are doing saves the environment when in real terms it's no better than all the other methods.

Mars bars were advertised as "Helps you work, rest and play" and this had to be either proved or stopped. It's about time all the other advertising came under the same rules.

Prove it saves the ecology or don't use it as an advertisement.

I had a Vaxhall Agula (Manual 1.2)and Toyota Yaris (Auto 1.3) at same time and used them alternative fortnights to travel North Wales to Heathrow both were supposed to do same miles to gallon but in real terms Yaris was far better because of the VVT but that's not shown with government testing methods.
I had a poo earlier, used up energy therefore creating more poo :mrgreen:

I had a virtual reality poo earlier. My missus copied me then started singing that Smokey Robinson and the Miracles classic "I second that e-motion."
There's just too many people with not enough resources to go around. The country and the world needs a population cull.

A nice world war would do it but that would also use up a lot of resources to fight it. :mrgreen:
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