Green Building Forum - Absolute energy

Absolute energy

tony — Sun, 19 Jun 2011 17:47:07 +0100

I have just read the latest GBM and could barely find any reference to actual heat loss or energy use figures

Lots of confusing percentage savings and comparative figures some U values but nothing hard and fast

I would like to see us all talking about kWh/m^2/y then we can compare and evaluate what is being claimed.

Absolute energy

SteamyTea — Sun, 19 Jun 2011 18:57:15 +0100

Even that only tells part of the story. Could also use a ratio of external area to floor area. But I think we have done that one somewhere else.

Absolute energy

fostertom — Mon, 20 Jun 2011 10:24:46 +0100

The influence of wall:floor ratio is incorporated into the kWh/m^2/y figure - as is local climate (less challenging in warm wet Cornwall that in Scottish highland or inland Germany). So the kWh/m^2/y figure is the primary criterion, but shd be commented to show how jolly difficult it was to reach that figure in this case because of these factors ...

Absolute energy

SteamyTea — Mon, 20 Jun 2011 11:27:38 +0100

I don't see how it is Tom, or are you saying it is in the Passiv or Sap methodology?

Absolute energy

fostertom — Mon, 20 Jun 2011 15:57:29 +0100

not clear what you don't you see?

Absolute energy

qeipl — Mon, 20 Jun 2011 20:03:53 +0100

I think kWh/person/day would be a good baseline to use for domestic energy comparisons.
We can see what we used yesterday, or the average for last month, or last year.

kWh/m2/year is too abstract for most people, too retrospective (by the time you know the figure it's too late to turn down the thermostat), and too difficult to compare from one house to another.

My house used 60kWh/m2 last year, which probably looks good compared to many, but I'm in the house on my own for most of the time - 16kWh/person/day looks a bit greedy and might encourage me to try a bit harder.

Absolute energy

seascape — Tue, 21 Jun 2011 00:48:32 +0100

qeipl: well it's worked - pulled out my electricity bill and did a sum - horrendous and I haven't even got to the kerosene or petrol yet.

Absolute energy

skyewright — Tue, 21 Jun 2011 11:12:53 +0100

Posted By: qeiplI think kWh/person/day would be a good baseline to use for domestic energy comparisons.
We can see what we used yesterday, or the average for last month, or last year.

A value I monitor is kWh/HDD15 for the heating side of things (relatively easy to do with THTC electric heating, and that all on a separate meter).
Averaged over a month we find the figure is quite remarkably stable over the main heating months at between 3.5 and 3.9.
On a day to day basis figures might vary from 3.0 to 4.5, with such extremes usually being closely tied to a sudden change in the weather (as we have a substantial element of storage heating, even with active management of the settings, it's easy to get involved in "catch up").
I've no idea whether 3.5-3.9kWh/HDD15 is good or bad, but at least its a figure that seems to have some logic and that we can work around and hopefully improve on.

During 2010 we significantly improved insulation. In the winter of 2010/2011 we had roughly the same kWh/HDD15 as for 2009/2010, but we were more comfortable (the area I use for the indoor temp averaged around 18C rather than 16C). The next stage is to maintain the comfort but reduce the kWh/HDD15...

One thing I've noticed is that though HDD15 give the most consistent figures, the critical "do we need heating" outdoor temp is ~12C average midnight to midnight, with the range from 10C to 12C being transitional.

If the midnight to midnight average is >= 12C we don't need heating. If the midnight to midnight average is <= 10C we need all the storage heaters on at least a little. Between those situations the heating requirement depend on what temps were like over the previous couple of days, and on the levels of sun light (a sunny afternoon can give us a decent solar gain). A single day dip below 12C but still above 10C can be bridged without reaching for any of the switches, but after a couple of days of colder and dull weather the retained heat in the fabric is used up and a bit of help is needed...

My guess is that as we improve further on insulation and air tightness the temps at which those stages occur ought to reduce?

I've tried plotting HDD12 against usage, but that hasn't proved anywhere near as "useful" as HDD15.

Absolute energy

DamonHD — Tue, 21 Jun 2011 11:19:08 +0100

I use HDD12 since it seemed a better fit to my data and my aim is to continue to meet these targets for our family of 4:

Electricity: 4.5kWh/d+0.2kWh/HDD12
Gas: 5kWh/d+3kWh/HDD12

http://www.earth.org.uk/saving-electricity.html#meter2011

Much of the electricity variation with HDD is to run the gas combi's circulation pump and fan I think, but some is extra lighting (etc) and heating of water for the washing machine and dishwasher to fixed temperatures (when not running cold).

Rgds

Damon

Absolute energy

djh — Tue, 21 Jun 2011 12:44:35 +0100

Surely kWh/HDD misses the point? Houses built in colder climates need better insulation. kWh/person makes it clearer in my mind. It's 'fair' for everybody to have access to the same quantity of energy.

Absolute energy

DamonHD — Tue, 21 Jun 2011 13:08:22 +0100

kWh/person/HDD is fine by me also (0.75kW/person/HDD12 for us over our baseline 1.25kWh/person/day).

Usually there won't just be one normalisation that makes sense, though I am fundamentally in favour of per-capita measures of some sort, since building huge palaces that are low consumption per m^2 but high per person is not good IMHO.

Note that you can't just ignore climate: however well I insulate the house I still need to raise incoming water by more delta-T in the winter to have a shower or wash (at least some) clothes/dishes at a given temperature.

Rgds

Damon

Absolute energy

skyewright — Tue, 21 Jun 2011 13:49:20 +0100

Posted By: djhSurely kWh/HDD misses the point? Houses built in colder climates need better insulation. kWh/person makes it clearer in my mind. It's 'fair' for everybody to have access to the same quantity of energy.

It's not missing the point, rather it's "horses for courses".

kWh/HDD (add a /person by all means if you wish) is a useful tool if you are trying to do a fair comparison of "how are we (in this house) doing compared to last year/month/day (in this house)".

Without adding HDD into the equation any improvements in insulation, air tightness, etc., could be masked by variation in the weather.

Absolute energy

djh — Tue, 21 Jun 2011 15:50:11 +0100

Posted By: skyewrightIt's not missing the point, rather it's "horses for courses".

Yep, I'll accept that.

Absolute energy

djh — Tue, 21 Jun 2011 15:52:04 +0100

Posted By: DamonHDNote that you can't just ignore climate: however well I insulate the house I still need to raise incoming water by more delta-T in the winter

Water's a rather poor example, I think. Regardless of how cold the climate gets, the temperature of the water supply in winter is likely to be pretty much the same!

Absolute energy

SteamyTea — Tue, 21 Jun 2011 16:15:06 +0100

If you get an 8C change in the water temperature and, like me you keep your hot water at 50C then it does make a difference, especially as the SHC of water is around 4 times that of air and it is only exposed to the atmosphere for a few minutes a day.
I am going to ponder all this a bit more as I have not got my 'temperature' head on today as been working on 'forces'.

Absolute energy

DamonHD — Tue, 21 Jun 2011 17:31:52 +0100

Our mains water varies between 6C and 21C while I've been measuring it, see towards the bottom:

http://www.earth.org.uk/note-on-data.html

Doing a 30C wash (our most common) that more than doubles the energy required to heat the water.

Even for the 50C Eco dishwasher setting that's nearly a third extra energy needed.

And considering that the washing machine and dishwasher are up to about 1/3rd of our daily consumption, that really is noticeable when reading meters and on our footprint, I promise.

Rgds

Damon

Absolute energy

qeipl — Tue, 21 Jun 2011 17:41:08 +0100

I've looked and can't find - what does HDD15 mean?

Absolute energy

DamonHD — Tue, 21 Jun 2011 17:59:34 +0100

HDD15 => Heating Degree Days with a base temperature of 15C

See the 'for beginners' part here: http://www.degreedays.net/

Rgds

Damon

Absolute energy

djh — Wed, 22 Jun 2011 14:41:25 +0100

I think we're talking at cross-purposes. Damon and skyewright are talking about measuring the performance of an existing system and making sure it is working well. I think this thread is actually about comparing the performance of different designs and designs in different places and climates.

Hence the confusion about which unit is "better".

Absolute energy

tony — Wed, 22 Jun 2011 16:18:48 +0100

It is quite nice to be talking about units -- a lot are talking about meaningless percentage savings over something

I would like us to talk in absolute terms about the energy used

I cannot see the point in involving degree days as these should have been taken into account at the design stage.

There is the argument about occupancy and I am open to persuasion but would prefer to talk about the building

Absolute energy

SteamyTea — Wed, 22 Jun 2011 22:45:32 +0100

Tony
You can state the annual energy use for any building you like and compare it to any other building you like, that will tell you nothing. It is like telling a builder you want a house built and you are expected to quote a price per square meter.
What could be done is to partition the country (or globe) into temperature regimes numbers (TRN) and also specify the surface area to floor area as a unitless number (N). You would have to agree a standard here (either Floor Area/Surface Area or Surface Area/Floor Area, I would favour the first one). The climate regime number should be comparative, so a number 300 would be twice as hot as a number 150 (no sniggering children). If the base was 0 K there would be no negative numbers. This may well be better if it was based on climate rather than temperature alone, but again a standard would have to be created and tested.
You could then end up with the equation: Climate Regime Number(kWh/y/(Floor Area m^2)/Total Surface Area m^2)))
think that could be re-written as TRN.kWh.y^-1.N
It would create a unitless number that could then be compared. It would tell you nothing about design of the house, its location, occupancy, internal temperature, insulation, airtightness, heating system, age or anything else 'physical' about it, that would be for further investigation and case studies. But you could compare on the same base.

Absolute energy

GBP-Keith — Thu, 23 Jun 2011 12:29:20 +0100

I can get on-board with the concept of trying to get authors to comply with regularised terminology but whether they are able to do so would remain out of my control.

I also need to confess to being a fan of the of 'doing one's best' approach.

Interestingly we have had a lot of praise for this most recent issue because it is less 'scientific' than we have been publishing over the past couple of years and certainly since it came out more authors than normal have come forward with story proposals.

Green Building magazine always flies by the seat of its pants and we are guided by 'inspired authors' rather than editor or advertiser led.

Absolute energy

djh — Thu, 23 Jun 2011 12:56:28 +0100

I say well done on the quality, Keith.

I'm only part way through reading it but I must say I am finding the articles more interesting than sometmes. It would still be nice to see more numbers though, and along with the technical numbers it would be nice to know how much some of these projects/materials cost!

Absolute energy

seascape — Thu, 23 Jun 2011 20:04:40 +0100

Enjoyed magazine but there is a typo in bit about me - problem is raft is UNinsulated, I'd feel rather more buoyant if it was insulated....

Absolute energy

SteamyTea — Thu, 23 Jun 2011 22:12:12 +0100

Keith
Get them all to read this and to stick to it. It is all very simple really.
http://www.kayelaby.npl.co.uk/units_and_fundamental_constants/1_1/1_1_1.html