Measuring ground thermal conductivity

[Nostrum]

How do you measure this?

Do you study the ground type and use the MIS3005 lookup tables, if so how? Or commission a report from a specialist, or use the BGS website for reports?

This is my biggest concern with ground source, getting this wrong!

thanks in advance

 

[Worcester]

Long or short ....

As it happens I did this as part of my degree way back when.... so fortunately have some experience to fall back on.

In practice 90% of the UK is a variation of a silty sandy clay, and so you will not go wrong by using a figure of 1.7W/m/°K

The way we do it is. as a first start we use the BGS web site both for the superficial deposits and the local borehole records to gather data, Geology of Britain viewer | British Geological Survey (BGS) we then talk to local farmers and landowners and inf necessary dig a trail pit to confirm our findings - on one job a rock outcrop discovered when we ran a 40m long trial trench across the field totally changed our loop layout. - As per above, I understand the data I'm reading and seeing.
i.e observation is usually sufficient.

If we still have any slight concerns, we then order a report from the BGS £45 with the data. - We ALWAYS do that for boreholes, else we don't know deep we will have to go.

For domestic sized heat pumps we would normally stop at that unless we had serious concerns.

For a commercial system - i.e anything over 45kW we would have tests carried out on actual soil samples taken from a number of places across the site. - The report to linked below was prepared by the BGS, so will give you an insight into the complicated nature of the tests and samples that need to be taken.

AS we have not )yet) done any very large commercial borehole schemes, we haven't yet done the trial borehole and thermal conductivity tests, if it was for a new major development where performance is critical, say a hospital with large heat demand, then trial boreholes with core samples for lab testing and onsite thermal conductivity tests would be a normal part of the site survey and feasibility process. Think £10k+ just on testing...


See this document
http://www.gshp.org.uk/pdf/CambridgeSeminar2011/JonBusby_BGS_Thermal_Conductivity.pdf

The advantage of the BGS data is that it has been collected over 100's of years, and the land surface isn't something that changes very often, so a 100 year old borehole record is still valid today.

Bearing in mind that when burying HV electric cables, the ground thermal conductivity is also important, The IEEE has also developed a tool for measuring up to 1.5m deep - right for cables and ideal info for ground loops - testing of this nature isn't too expensive. see here: Soil thermal properties - Wikipedia, the free encyclopedia though might be too costly for the average 15kW domestic system.

p.s. see this also https://shop.bgs.ac.uk/GeoReports/examples/modules/C011.pdf

 

[Nostrum]

Thanks for the reply, much appreciated. Encouraging to know the data available is up to the job. Would you ever over size the loops at all to allow any leeway in the design?

 

[Worcester]

Would you ever over size the loops at all to allow any leeway in the design?

Rarely, as there is already a certain amount of overdesign in the MCS process anyway - the kWh figure is always higher than in practice, which therefore is giving you a higher FLEQ figure, and hence a lower allowable heat collection, so you are getting longer loops anyway.

Ironically, I find the most difficult part of the design is to ensure turbulent flow and to still get an acceptable head loss across the loops, sometimes, it means juggling between the number and hence length of loops, SDR11 and SDR17 pipe, swapping ground loops and or header diameters, just to get the pump to pump the glycol! (How many plumbers actually do a head loss on their heating layouts )

 

[Nostrum]

Thanks again. What formula do you use for this, as there appears to be a lot of variation in advice?

The one that seems to use the most commonly available data is:

Re = density (kg/m3) * fluid velocity (m/s) * diameter (m) / viscosity (N*s/m2)

Re > 2500 = Turbulent flow.

 

[Nostrum]

Ah Ha,

I have found the supporting documents on the MCS website. I thought MIS3005 was all a bit vague. This should answer most of my questions!

 

[Worcester]

If you've not yet done it download all the reference material on heat pumps, and then with the documents open and a spreadsheet in your hands, spend the 3 hours plus running through the videos / webcasts - you'll learn a lot - I put one of my apprentices through it and he is now really quick at heat loss calcs

Watch out for the spreadsheet though - it has some errors in the formulas, can't remember what they are now as we've corrected and highly customised ours.

 

[Nostrum]

I made my own spreadsheet for BS EN 12831 heat loss. MCS inspector was impressed and is much more user friendly than that. I watched the webinars when they were first released though.

I've just made a spreadsheet today for ground loop sizing to make the whole thing easier. Unfortunately I can't take the data off the look up tables easily enough to automate the whole process.

I'm fairly happy with the process now, the reference documents filled in a lot of the blanks!

Out of interest, which ground source products do you choose to deal with?

I'm looking at the Stiebel Eltron and Heliotherm units currently.

 

[Worcester]

We Use Vaillant, Stiebel Eltron and Kensa.

Stiebel;s technical support is very good, though last year they lost their technical sales guy to vailant

Kensa really do keep things simple (mimimal stuff to configure on the heat pump) and their boxes are so much smaller than the others becasue they don't fill them with empty space Their technical support is excellent, challenge is delivery time - each HP is made to order so 6 - 8 weeks is normal. Bigger heat pumps are a LOT easier than the stiebel solution as they have integrated twin compressor models

Here's a Stiebel Cascade - before we insulated it and built the outer cover - it took one heck of a lot of plumbing and a lot of expensive flexible hoses.

 

[Nostrum]

Not to keen on the Kensa stuff, I struggle to see where the value for money is. plus I prefer the more technical stuff!

Not looked at the Vaillant stuff in any great detail, how do you rate it?

What's with all the flexi's then?

 

[Worcester]

Flexi's - there aren't any inside the HP, and it's a Stiebel requirement. The Vaillants have flexis inside the box.

Also the Stiebel price when quoted is JUST for the HP, then add in all the extras including the Controls and Brine pump(s), additional circulation pumps (DHW), plus installation/fabrication time .... it quickly adds up.

Vaillant have a bivalency capability built in, so for us it's a choice of which pump best fits the client's needs.

 

[Nostrum]

One more question when using the lookup tables and referring to FLEQ hours.

If for example you calculate FLEQ at 1810, would you use the 1800 tables or the 2400 tables? At what point do you decide which to use, or would you make an assumption based on both!

 

[Worcester]

It's about being sensible at 1810 I'd use the 1800 figures at 2200 i'd use the 2400 tables in between I'd interpolate . Remember that the whole of MCS Dicumentation had been designed for someone with no knowledge and in that case I'd say over 1800 use the 2400 tables as they won't be wrong