How to convert Insulation Loss (in Watts/meter) to kWh energy savings per year?

[laurenrose01]

Hi, I'm working for a company that want me to calculate something for them. I haven't done physics since A level so have forgotten a lot of it and am now very stuck!

They want to know the kWh per year they will be saving by reducing their heat loss (which they have given me in watts per meter) through using insulation on pipework.

The inputs they have given me include:
- insulation diameter and thermal conductivity
- pipe diameter and thermal conductivity
- boiler efficiency
- the operating temperature of pipe (same as pipe contents)
- ambient temperature of the room

From this, I need to work out the U-values of uninsulated pipe and insulated pipe, and then work out Q (in Watts/meter) which is the heat loss per meter of pipe.

Through doing this for uninsulated pipe vs insulated pipe, I can see how many watts/meter they are saving through using insulation.
- They want it in watts/meter, and not meter3 apparently.

I then have to use this information to find out how many kWh they are saving in a year.

For example, how would I calculate the kWh annual savings for reducing the heat loss in watts per meter from 130 to 115 W/m?

I can't seem to figure out the spatial element of watts/m to the time element of kilowatts per hour.

Any help would be greatly appreciated!

Thank you in advance!

 

[fresh_42]

I looks as if you needed the velocity of the flow as well.

 

[anorlunda]

For example, how would I calculate the kWh annual savings for reducing the heat loss in watts per meter from 130 to 115 W/m?

You already made it simple by expressing it in watts. The only thing left is time.

130-115=15 W/m

15W=0.015 kW
If it is maintained for 1 hour, the loss is 0.015 kWh.
If it is maintained for 1 year, the loss is 0.015*24*365=131.4 kWh/year.
If the cost of electric energy is $0.10/kWh, then the savins is $13.14/year.

But there is something else wrong. Where does the area come in? Your number 130 W/m. Do you really mean 130 W/m²?

 

[berkeman]

Hi, I'm working for a company that want me to calculate something for them. I haven't done physics since A level so have forgotten a lot of it! They want to know the kWh per year they will be saving by reducing their heat loss (watts per meter) through insulation. For example, how would I calculate the kWh annual savings for reducing the heat loss in watts per meter from 130 to 115 W/m? Does anyone know if this can be done or what inputs are needed for me to find this out?

I can't seem to figure out the spatial element of watts/m to the time element of kilowatts per hour.

Any help would be greatly appreciated!

But there is something else wrong. Where does the area come in? Your number 130 W/m. Do you really mean 130 W/m3?

Probably it would help the OP if he could use more standard units for R-value:

https://en.wikipedia.org/wiki/R-value_(insulation)
Also @laurenrose01 -- Heat is not just lost through walls. There are considerations for ventilation, HVAC, windows, etc. See this previous thread for some ideas of things to include in your calculations:

https://www.physicsforums.com/threa...ity-of-housing-materials.966846/#post-6137789
Finally, paging @russ_watters for a professional opinion...

 

[essenmein]

Hi, I'm working for a company that want me to calculate something for them. I haven't done physics since A level so have forgotten a lot of it! They want to know the kWh per year they will be saving by reducing their heat loss (watts per meter) through insulation. For example, how would I calculate the kWh annual savings for reducing the heat loss in watts per meter from 130 to 115 W/m? Does anyone know if this can be done or what inputs are needed for me to find this out?

I can't seem to figure out the spatial element of watts/m to the time element of kilowatts per hour.

Any help would be greatly appreciated!

The heat loss unit feels like is missing some letters or numbers. Usually there would be a per degree or kelvin there, the temperature differential is driving the heat loss, so larger temperature differential results in more loss.

The way you have written it could be bulk thermal conductivity (W/m/K) or a film coefficient (W/m2/K).

Then to answer your kw/hr question you'd need to know the average temperature difference you are trying to maintain and the area, or, since its linear, reducing the heat loss from 130 of the things to 115 of the things is a 13% reduction of those things.

 

[OmCheeto]

Probably it would help the OP if he could use more standard units for R-value:

I swear, that every year, I run across an "insulation" problem, and have to re-work out why nothing makes sense, again...

From your "wiki link on R-value"[with some Om doodling, to remind myself why this is so confusing]:

Now, for me, their "RSI per inch" units make total sense.

Given an "R-value", the thickness of the insulation, and the temperature differential, how many watts per square meter will be leaking?

In any event, I believe the inverse of the "RSI per metre" may be what is confusing the OP.

I can't seem to figure out the spatial element of watts/m

bolding mine

Wiki's:
1/"RSI per meter" is Watts/(m K)
makes no sense to me whatsoever.
Other than the fact that the units work out...

 

[essenmein]

Just looked at the wiki on R value since this should not be hard.

RSI per meter, is "R" value in "SI" units, and it is m2*K/W, R is the inverse of thermal transmittance, ie heat transfer coefficient or film coefficient, unit is W/m2/K.

Ie how many watts are needed per square meter to maintain a temperature difference of one K (C).

 

[laurenrose01]

You already made it simple by expressing it in watts. The only thing left is time.

130-115=15 W/m

15W=0.015 kW
If it is maintained for 1 hour, the loss is 0.015 kWh.
If it is maintained for 1 year, the loss is 0.015*24*365=131.4 kWh/year.
If the cost of electric energy is $0.10/kWh, then the savins is $13.14/year.

But there is something else wrong. Where does the area come in? Your number 130 W/m. Do you really mean 130 W/m³?

Hi, they want it in watts / meter apparently, and have given me an equation to find Q which is heat loss per meter length of pipe!

Q (W/m) = U*(temperature of pipe - ambient temperature)

where U is the overall heat transfer coefficient.

 

[russ_watters]

They want to know the kWh per year they will be saving by reducing their heat loss (watts per meter) through insulation. For example, how would I calculate the kWh annual savings for reducing the heat loss in watts per meter from 130 to 115 W/m? Does anyone know if this can be done or what inputs are needed for me to find this out?

I can't seem to figure out the spatial element of watts/m to the time element of kilowatts per hour.

Welcome to PF.

Like the others were getting at, I think the main issues here are units issues. I don't know what W/m is supposed to mean and I don't see how it could be useful. Also, there's no such units as kilowatts per hour: kWh is kilowatt-hours (kilowatts times hours, not divided by hours).

So, as said you want to look at the insulation's R-Value or better yet U-Value, which is the inverse: w/(m²-K). As you can see from this, the heat transfer rate is a simple function of the area of the wall, the temperature difference between inside and outside and the insulation value.

So the hard part here is the temperature difference. It obviously changes with the weather. For the purpose of an example, I'm going to say the UK is a miserable 5C and foggy all day, every day of the year.

...A better model would use "heating degree days", which is the number of degrees times number of days below equilibrium temperature (the outside temperature at which no heating or cooling is done). The number for London is 1420. You can google for this.

Let's say you have 100 square meters of wall. If your wall is R-2, then your annual heating energy use is 1/2*100*1420*24 = 1704 kWh.

To find savings, add insulation and re-calculate.

 

[russ_watters]

Hi, they want it in watts / meter apparently, and have given me an equation to find Q which is heat loss per meter length of pipe!

Q (W/m) = U*(temperature of pipe - ambient temperature)

where U is the overall heat transfer coefficient.

Ooooooooohhhhhh, you didn't say "pipe" in your OP! Someone assumed "house" and everyone else (including me) ran with it. That makes a lot more sense...but doesn't change the calculation much.

 

[essenmein]

Seems the original question is now at revision 2!

 

[laurenrose01]

Ooooooooohhhhhh, you didn't say "pipe" in your OP! Someone assumed "house" and everyone else (including me) ran with it. That makes a lot more sense...but doesn't change the calculation much.

sorry I've just edited my post to be more clear!

 

[laurenrose01]

Ooooooooohhhhhh, you didn't say "pipe" in your OP! Someone assumed "house" and everyone else (including me) ran with it. That makes a lot more sense...but doesn't change the calculation much.

They've shown me the online Paroc Calculus as a basis, which is an online calculator, and that also gives its output in watt/meter - I am not sure if you have seen it before, but when i go into work on monday i will let my boss know what you have said! thank you

 

[russ_watters]

They've shown me the online Paroc Calculus as a basis, which is an online calculator, and that also gives its output in watt/meter...

And you understand how to use that? I assume it uses a pipe size, surface temp, room temp and insulation value to get w/m. From there you just multiply by the length of piping you have and 8760hrs/yr.