Laminar Flow in Ground Collector Array

[bungledlow]

This might be too technical for this forum but giving it a go anyway on the off chance there is someone here who understand fluid dynamics more than me!

In designing a ground source heat pump system, specifically sizing the diameter of collector array pipe, the technical guidance is straightforward but quite grey in some areas.

 

My understanding of the design process:

 

Header pipes - Laminar flow is good as you don't want turbulence increasing heat transfer to the pipe wall

Array pipes - Laminar flow is bad, lack or turbulence means low heat transfer to the pipe wall

 

Whether your array is in laminar flow or not (and the pressure drop over it) is a function of the pressure drop per metre (itself a function of roughness and diameter) and flow rate:

1. Pressure drop per metre of pipe is a constant for the pipe diameter you use
2. Flow rate is a variable set by the heat pump chosen and is referred to as "minimum ground side flow rate".
3. Put both of these numbers on each axis of the chart below and you get your pressure drop.

(p23 of MCS Guidance Document - Hydraulics Design Guide for MIS 3005)

 

More often than not its marginal or very close to being within the laminar flow zone in the bottom left.

 

So by reducing the pipe diameter you can cause more turbulent flow but you also have to contend with the overall pressure drop and whether the pump can deal with that.

The difficulty I have found is not the overall pressure drop but actually getting comfortably out of the laminar flow zone due to the low flow rates quoted by the pump manufacturers.

 

 

Question:

What does "minimum ground side flow rate" of the heat pump actually mean?

In practice will the pump be running harder than this "minimum" because modern inverter pumps can vary their speed - is this just a theoretical worse case scenario for design purposes, meaning we are safe even if our numbers are marginal here?

 

Decision:

We have been advised that we should have bigger 40mm pipes, and there was some surprise from this advisor that some of the big names had specified smaller diameter (25-32mm) pipes. However when running the numbers, a smaller diameter would make more sense so that turbulent flow would be initiated at lower flow rates.

 

The advantage to using larger pipes is that https://www.gogeothermal.co.uk/ offer a crack resistant pipe in 40mm that does not require sand bedding, reducing time and cost of install and improving performance due to direct contact with the surrounding clay soil. 

 

Appreciate any advice/interpretation anybody can offer.

 

[SteamyTea]

https://en.m.wikipedia.org/wiki/Navier–Stokes_equations

 

Good luck.

 

[saveasteading]

5 minutes ago, bungledlow said:

any advice/interpretation anybody can offer.

How is the cost compared with ASHP? 

[bungledlow]

Just now, saveasteading said:

How is the cost compared with ASHP? 

Significantly more expensive

 

Going this route is more for technical elegance and the intellectual challenge versus the admittedly lower financial and installation requirements of ASHP.

The decision process was based on the fact being a new build it seems like a wasted opportunity not to make the most of a clean slate design given we have the land available with perfect soil conditions and a digger to do the groundworks.

Add in the slightly higher efficiency vs ASHP and the opportunity (however insignificant) to recharge the ground when the pump is in cooling mode, acting somewhat like a thermal store, and it feels like a long term investment opportunity, albeit not as financially sound as going with air source!

[Onoff]

28 minutes ago, bungledlow said:

This might be too technical for this forum

 

Cheek! There's some clever c**** on here.....and some plain c****.

[bungledlow]

5 minutes ago, Onoff said:

 

Cheek! There's some clever c**** on here.....and some plain c****.

 

I was referring to the subject matter of the forum rather than the members.

 

I would never imply people here are simple minded... its self evident ?

[Onoff]

32 minutes ago, bungledlow said:

self evident 

 

- ?

[pdf27]

1 hour ago, bungledlow said:

What does "minimum ground side flow rate" of the heat pump actually mean?

That's the minimum flow rate of coolant (glycol) passing through the heat pump. This will be set to ensure that in the worst case (lowest ground loop temperature + maximum heat demand) the coolant does not freeze up.

 

1 hour ago, bungledlow said:

In practice will the pump be running harder than this "minimum" because modern inverter pumps can vary their speed - is this just a theoretical worse case scenario for design purposes, meaning we are safe even if our numbers are marginal here?

Why would they run an inverter-driven pump on the ground loop side? To benefit from it you'd have to run a calibration routine for the ground loop and program it into the heat pump, and the savings would be trivial (tens of watts).

[SteamyTea]

I would think, without calculating, that variations in the ground makeup, depth of pipework, differences in radii of the loops, water retentions in the soil and any shading on the loop area, would make a larger difference than turbulence within the ground loop. 

[jack]

1 hour ago, bungledlow said:

simple minded... its self evident ?

 

*it's self-evident ? 

 

2 hours ago, bungledlow said:

Laminar flow is bad, lack or turbulence means low heat transfer to the pipe wall

 

What is "low heat transfer" in numerical terms, compared to turbulent flow? 2%? 20%? I mean, if it's just a few percent, can't you just make the larger diameter pipework a little longer to take that into account?

 

The other thing is that surely the pump manufacturer and/or installer should be the ones doing the calculations on this? I appreciate it's a little complex, but if you're in the industry, coarsely modelling a system presumably wouldn't take more than a spreadsheet with a small number of inputs?

 

One other thing: is the larger diameter pipework array typically the same length as the smaller diameter stuff? I ask because I understand that one of the major cost disadvantages with GSHPs is that the glycol needs replacing every few years, and if you have more volume in the pipes, it'll cost more. 

[pdf27]

Oh yeah, and the proposed way of calculating pump flow is wrong too. What you should instead be doing is plotting the pump pressure versus flow curve (typically called a P-Q curve) on the same axis as the graph shown above. Your operating point is where the two curves intersect, and the flow rate at that point needs to be greater than the minimum ground-slide loop flow rate. Many pumps have more than one operating curve (essentially there is one curve for each speed setting) and this needs to be considered as well.

 

Edited October 5, 2021 by pdf27
Forgot the graph!

[Onoff]

2 hours ago, bungledlow said:

people here are simple minded

 

Yep. That reminds me, I must try smaller diameter Redbull cans one day...

 

 

[saveasteading]

8 hours ago, bungledlow said:

recharge the ground when the pump is in cooling mode,

I would be interested to hear the latest views on GSHP from a slinky setup.

 

I have seen slinky heating in action at Mitsubishi HQ (Watford not Japan) and it clearly worked.

 

Yes or no to the following?

 

My understanding is that this is heat from the sun hitting the ground, and air touching it, so the thermal store from summer (less what is used for water) is the heat source for the winter.

Relatively no heat comes from the ground itself unless water flows through it to recharge it.

Warm rain soaking through will help and cold rain will hinder.

No use if shaded.

If you can recharge it in summer from cooling the house, then there is something wrong with the house, to get so hot. 

A lot of power is needed to circulate the liquid.

 

The elegance is in not having the ashp whirring outside.

Picks up heat from a big area of ground, if the heat is there.

Better in very cold spells of weather.

Good in granular ground.

[pdf27]

7 hours ago, saveasteading said:

My understanding is that this is heat from the sun hitting the ground, and air touching it, so the thermal store from summer (less what is used for water) is the heat source for the winter.

Relatively no heat comes from the ground itself unless water flows through it to recharge it.

Warm rain soaking through will help and cold rain will hinder.

No use if shaded.

If you can recharge it in summer from cooling the house, then there is something wrong with the house, to get so hot. 

A lot of power is needed to circulate the liquid.

1. Depends how deep you go. At the surface the ground temperature matches the air temperature and you've basically got an ASHP. As you start to go deeper, the thermal mass of the ground means that the temperature is the average of the last few days - and the deeper you go the longer this average gets until with a borehole it's pretty much constant at the average temperature over a year (~10°C).
2. Correct, heat is coming downwards from the sun. Water flowing through is effectively just widening the area of the collector.
3. Unless you're in monsoon conditions this isn't likely to have an enormous effect - essentially it makes the coil act as if it's buried less deeply.
4. Minimal impact - it really tracks air temperatures over a period of time, sun helps but really isn't a huge deal.
5. Correct. It's better thought of as a low-temperature heat sink which you can reject heat to very efficiently (possibly without even a heat pump) in summer for cheap cooling.
6. Not really - it's broadly comparable with fan power for an ASHP because the heat capacity of liquid is so much higher. COP values include pumping losses as I understand it. Care does need to be taken with the ground loop design though - if everything is in series the pumping losses can stack up, which is why manifolds are often used to run loops in parallel.

 

7 hours ago, saveasteading said:

The elegance is in not having the ashp whirring outside.

Picks up heat from a big area of ground, if the heat is there.

Better in very cold spells of weather.

Good in granular ground.

1. Pretty much, although in exchange you get a GSHP whirring inside - no fan, but you still have the compressor. This is probably the main selling point for me, and why I'm willing to pay a small premium for a GSHP over an ASHP.
2. Not sure this is a pro - I'd say large area of ground required, so neutral if you have the ground and major negative if you don't.
3. This is the averaging effect - cold snaps usually don't last more than a week or so, meaning you're taking advantage of the mild weather a few weeks (or months, depending on depth) ago when heating during a cold snap. You lose out when making hot water in summer though (the ASHP gets lovely warm air for this), so overall the effect isn't as big as it's cracked up to me.
4. Again, it really comes down to ground area - particular soil types need more or less ground array area for the same heat load. Unless you're drilling a borehole, the effect of this is modest in the grand scheme of things.

[jack]

1 hour ago, pdf27 said:

the thermal mass of the ground

 

Thermal mass? 

 

 

(I'm actually perfectly fine with the term "thermal mass", despite some of the hate it gets on BuildHub. I think the real problem is not the term itself, but the fact that self-builders, designers and architects don't really understand the mechanisms at work, and so the concept is misinterpreted.)

[saveasteading]

I admit to having a bias against GSHP as

1. There were once so many charlatans, possibly the companies that previously touted little wind turbines.

2. I helped tenants of 'affordable' housing where the GSHP did not work at all, and eventually got some of them changed to ASHP. It was clear that nobody understood how it worked.

Should never have been used 60m deep in clay, too close together, and shoddy work. In half the houses it was ok. 

I was impressed by the smallness of the pump in the broom cupboard, whereas the ASHP was a major nuisance in a small garden.

3. same point.  '.nobody knew how it worked'. Various parties said it would work, drilled the holes, put in the kit, fixed the pump, chose the wrong radiators, denied any problems. Are any of these parties still in business?

[Ferdinand]

I think I am declaring the benefits to be de minimis.

 

Now, I have a confused but highly intelligent bee that needs convincing it can fly...