Ground Arrays

Welcome to the Ground Arrays module.

As we have seen in the Heat Pump Basics module, the ground arrays are the part of a heat pump system that harvest low-grade heat from the ground (or from water).

In this section we will examine the different types of ground array, and consider the pros and cons of each type so that you can select the right kind for your project.

We'll go over sizing arrays in order to get the best performance and to comply with MCS guidelines.

We will look at how the arrays are installed in the ground, and illustrate a few important points to remember.

What kinds of ground array are there?

Straight pipe

We've seen that a heat pump uses pipe buried in the ground, filled with heat transfer fluid, to absorb energy from the surrounding earth.

The amount of pipework that's needed will depend on the thermal properties of the ground that they sit in.

Straight pipe is just that! In this case, the plastic pipe is usually 32mm or 40mm in diameter. It is available on reels, in various lengths depending on your supplier.

The pipe is buried just over a metre deep, in long trenches 1m wide. If the soil is stony, the trenches are lined with sand to prevent stones from damaging the pipe and causing leaks. A 1 metre gap must be left between trenches (centre to centre). MIS3005 actually stipulates 0.75m, but that's for 25mm pipe, so stick to 1m.

If you pack too much pipe into a small space, you will extract too much heat from that area of ground. It's possible to freeze the ground, because the heat transference in soil just doesn't happen fast enough to make up for the amount of heat being taken out.

On a small scale this can damage grass laid over the top of the trenches. At its worst, perma-frosted ground will buckle and crack, and when there is no more heat to be taken from the ground, the heat pump will stop working. It can even damage the heat pump beyond repair.

This is why the seperation distances are defined by the MCS guidelines.

The heat in the soil isn't "geothermal". It's not that deep! The energy we are trying to use is warmth from the sun and from rainwater.

Anyway, straight pipe is laid in a meandering pattern up and down. This means it's easy to avoid obstacles like trees.  Remember though that the ground-side pump in the heat pump has a limited amount of head, so you will probably have to install straight pipe in loops of no more than 300m each. You will use a manifold to link the loops together.

INSERT STRAIHT PIPE PHOTOS

Slinkies

Slinkies are simply straight pipe coiled up!

Kensa sell slinkies that are made with 32mm pipe, cut into 3 different sizes. We make a 30m, 40, and 50m slinky. A slinky that is 30 metres long actually contains 200m of pipe, coiled up. A 40m slinky is 250m of pipe, and a 50m slinky is 300m.

 

Slinkies are installed into trenches similar to straight pipe - at the same depth (1.2m). But because the slinkies are physically wider, the trenches now have to be 1.2m wide, and at 5m spacings (centre to centre). This means that metre for metre, you need the same amount of space for slinkies as you do for straight pipe. MIS3005 stipulates 3 metre spacings - Kensa have had thousands of slinky installations though, and to guard against over extraction, we stick to 5m. We've never once had an issue with frozen ground because of this extra caution.

Why bother? Well, slinkies can make life easier.

Because you have a bigger space between trenches, you have space to dump the spoil from the trench; you can just leave it alongside. When you are digging for straight pipe and only leaving a metre gap, you won't have anywhere to put that spoil - you have to dump it elsewhere and then move it again to backfill. If you are doing your own groundworks, this isn't generally an issue, but if you are paying groundworkers to do it for you, then time is money!

Over the few years that ground source heat pumps have started to gain popularity in the UK, slinkies seem to have attracted an unfair amount of criticism. Historically this was because often they were installed without enough spacing. MIS3005 actually specifies 3 metres; it's Kensa that stipulate 5 metre centres. In the thousands of slinky jobs we've sold, we haven't had any issues with over-extraction.

Like any ground array, when installed correctly, they work.

Boreholes

A borehole is a very deep, drilled hole that contains a U shaped length of pipe, filled with heat transfer fluid. The pipe runs from the top of the hole down to the bottom, and back up again. This pipe is known as the "probe" and is the equivalent to straight pipework, just running vertically.

Boreholes are drilled by specialist drillers using drilling rigs. They are backfilled with grout which helps the thermal conductivity between pipe and soil.

Boreholes should not be placed too close to building foundations (4 metres) and should be seperated from each other by 8 metres. MIS3005 stipulates 6m; Kensa are always cautious.

Because the ground temperature gets warmer the further down you go, boreholes are considered to be the best way to extract heat, metre for metre. The hitch is that they aren't cheap in comparison to other methods - depending on your driller, you could be looking at £40 to £50 per metre drilled.

Roughly speaking you need 20 metres of borehole per kilowatt of heatpump output, so for an 8kW heat pump you'd need around 160 metres drilled. You can see how this would stack up, especially for larger installations!

You can link multiple boreholes togethe, but to ensure balanced flow rates they should be kept to the same depth - or flow balancing valves will be needed.

Boreholes are really useful where you are pushed for space to fit straight pipe or slinkies. You just need to ensure there is space for a  drilling rig to get in - normally this means HGV access for unloading of the rig, but some drillers have small tracked rigs that can get through gates.

 

 

Drillers working at a social housing site

We are occasionally asked "I have a borehole for drinking water, can't we just use that?".  For closed loop, this would involve simply dropping the probe down the borehole. It sounds very neat, but there are a couple of catches.

1) Contamination. This is pretty unlikely, but it's possible - if that probe pipe is damaged, then you may end up with glycol contaminating your water supply. Although the glycol Kensa supply is "detoxified", that doesn't mean you would want to drink it - and if the borehole is your only source of drinking water, what happens?

2) Potable water boreholes are generally not deep - 10m to 40m depending on where the water table is. Boreholes for a heat pump need to be much longer - even given the added thermal advantages of having some of the probe pipe in water, you lose out if some of it is in air, and you can't add grout in this situation.

Pond Mats

A pond mat is a 40 metre slinky (so that's 250m of pipe!), attached to a metal frame, that is submerged in water.

Why would you do this?

Because water is a much better conductor of heat than the ground is. If your project is close to a lake or pond, then making use of this source of energy could be well worthwhile! Remember that the better the incoming temperature, the better the heat pump's performance - well, this is exactly what we meant.

It can also be a lot less work that digging. You will need to dig a header trench, running pipework from the heat pump to the lake, but there's no digging for slinkies or straight pipe.

The pond mats are floated out into position, and as they are filled with heat transfer fluid, they sink, eventually resting on the lakebed. The rigid frames ensure that they don't move around, and breeze blocks secured to the frame help to sink the mats where you want them.

It's possibly to secure the pond mats to the side of a harbour wall (you need permission!), or even to a jetty. Take a look at this video to see a clever pond mat installation completed by a Kensa Partner.

 

 

 

Water source systems

The other alternative to using the ground is to use water.

We've looked at this briefly, by using pond mats. This is called a closed loop system, because the lake water never enters the heat pump - the heat pump uses a sealed loop of pipe, full of heat transfer fluid (glycol) to absorb heat.

It's also possible to use open loop systems.

 

This implies that the water from your water source (let's keep the lake as an example!) is pumped directly into the heat pump. So you skip out the heat transfer fluid and pipework altogether, and pump water directly into one side of the evaporator.

Sounds good, doesn't it? No need for pipework, less work, no need for glycol - just add a pump.

It isn't quite that simple! If you pull water directly from a freshwater source, there is always a risk that there might be debris in there.  So you need filters to start with, and you also need a pumping arrangement that is properly sized to cope with the kind of flow rate you will need across the heat exchanger (which will vary, depending on how powerful the heat pump is).

The greater risk is this:  If you don't get that required flow rate for whatever reason (blocked strainer, failed pump), you would over extract from whatever you do have. If this occurs, you run the risk of freezing your heat source - just the same as if you over extract from the ground.

The implication here is that rather than freeze up a lake, you would in fact freeze up the evaporator heat exchanger in the heat pump. This is going to crack it, and you have basically written off your heat pump.

So in order to prevent this from happening, we add a second heat exchanger, and a very small closed loop of pipework containing glycol.

The "open loop" heat exchanger is gasketed, rather than brazed together, so if this does freeze then all that will happen is that the gaskets expand. Yes you get leaks, but the unit can be rebuilt, and you haven't ruined the heat pump.

We are often asked if it's possible to use a potable (drinking) water borehole for this use, too. Generally the answer is no, but this depends on the borehole. The important bit is the refresh rate of that borehole - like a flow rate in a river. If it's too slow, you end up over extracting, and you'll freeze the heat exchanger at the very least.

Pros and Cons

What's right for my project? Pros and cons at a glance

Array Pros Cons
Straight Pipe Simple - especially for self-builders doing own ground work Moving spoil can be laborious. Need space!
Slinkies No need to double-move spoil. Straightforward to use. Costs more than straight pipe. Like straight pipe, installation can be disruptive
Boreholes Efficient. Great for installations where space is at a premium Comparitively expensive
Pond mats Very efficient. No digging. Must have a body of water! Installation will almost certainly need a boat or the arm of a digger.
Open loop Efficient, cost effective The water source must be able to meet flow rate demands. Sometimes permissions are required. Additional pumping equipment will be needed
     

 

Sizing

How much pipe is enough?

How do you know how much of any type of ground array to fit?

The amount required depends on several things:

  • The size of the heat pump you're fitting
  • The number of hours the heat pump will run for over the year
  • The type of rock or soil the arrays will be in, as conductivity varies.

When Kensa quote for a heat pump, we will always quote for ground arrays (unless you ask us not to!). At first we will estimate what is needed, using assumptions about run hours and ground conductivity.

Once we get more details, we will ascertain what is needed.

The MCS design guidelines, MIS3005, tell us how to calculate this. We use something known as "Table 3" which takes all the relevant information and tells us how much array is needed.

This does not work for pond mats or open loop systems - we have to size these on a case by case basis, considering heat pump size, run hours, water temperature and flow rate. For more details on  sizing for water source heat  pumps please see the Heat Pump Specialist training module, where you will find an in depth discussion.

Here is an example of a Table 3 calculation. The heat pump used in this example is a 15kW Hybrid with four 50m slinkies.

It's possible to over-size ground arrays. Up to a point, this can be beneficial - but over around 20% oversized and you're spending more money on pipework than you will ever recover through efficiency gains.

If all you want is a rough "rule of thumb" just to be able to offer a client a quick price, we suggest you estimate using:

  • 6 kW per pond mat
  • 5kW per slinky
  • 20m of borehole per 1kW of heatpump output
  • 20m³ of open loop flow rate daily will supply up to approximately a 12kW unit (over this amount you will need an abstraction licence anyway)

MCS ground loop sizing look-up tables

Communal Ground Arrays

What's a communal array?

"Communal" ground arrays, also referred to as shared or common arrays, just mean that more than one heat pump uses the same ground array.

The array can be slinkies, straight pipe, boreholes, pondmats or open loop.

Here's a schematic showing what it looks like:

Why would I use one?

Firstly because it is probably a practical option.

 

If you have a project where more than one heat pump is being used in seperate buildings (let's say, a farmhouse and a workshop) then why bother with distinct and seperate arrays for each one? You would only do this if there was a likelihood that the properties would be sold individually.

if you know they are going to stay owned by the same person, then you may as well use a shared array.

Secondly, because it could well net you payments under the Non-Domestic RHI.

The payments on the non-domestic scheme are made over 20 years, not 7 like the domestic version, so the customer is ultimately in for more money.

This happens because OFGEM consider that any system using shared "plant" is a district, or micro-district, heating system. The ground array is considered plant.

This works out especially well for social landlords (housing associations) who know that properties aren't going to change ownership, but it can also work for clients who want to heat their own house plus, say, a holiday let.

Installation hints and tips

Slinkies

Slinkies are delivered pre-coiled.

DO NOT cut the BLACK cable ties on receipt!

The coloured cable ties indicate the slinky length:

  • white
  • blue
  • yellow

Remember if the soil is stony, line the trench with sand.

Slinky trenches can curve, they don't have to be straight. If you are making a sharp bend, maintain the 5m centre-to-centre spacing, and snip cable ties if you have to.

Do NOT cut slinkies to length. If they are too long, coil the excess into the trench. This way, all the slinkies are the same length, so you have balanced flow through each one.

Slinkies have 25m of "tail". If that's not enough, undo a couple of coils.

You should not have any mechanical joints underground. All joints to subterranean manifolds should be electrofusion.

Straight pipe

Straight pipe can make a lot of sense if you are doing your own groundworks.

Use a 1m wide digger bucket for your trenches and put the pipe right up against the sides of the trenches - this way you can get two runs of pipe per trench.

Remember to maintain the 1m spacing between trenches.

Blind the trenches with sand if the soil is stony.

 

Boreholes

Most drillers will leave you with pipes sticking out of the ground at the borehole, unless you have arranged for them to lay header pipework for you.

Ensure that any header pipework is properly sized.

Pond mats

Attaching small buoys to the pond mats before you sink them can prove a useful indicator of their location.

You can normally tow a pond mat to its location using a small boat - they will float because the pipework is full of air, until of course you fill the system with glycol.

Failing that, the arm of a digger can be used to swing the mat out over the water.

Open loop

Generally you will need an open loop installation specialist for this job.

It's vital that the open loop heat exchanger, the pipework and the pumping kit is all correctly sized which will depend on temperatures, flow rates and heat pump output.

The open loop pump needs to be controlled so that it doesn't run all the time.

Placement of the intake and discharge is also important.

Over 20m³ of water extracted per day will require licencing, and regardless of the amount you must be sure that the owner of the body of water gives permission.

Manifolds, header pipework, and antifreeze

Types of manifold

Kensa supply manifolds for linking together the ground arrays. You can select either above-ground manifolds, which are mechanical fittings affixed to a board, or subterranean manifolds, which are buried.

Any buried joints need to be extra secure so all buried joints are electro-fusion. Subteranean manifolds are a neat solution but this does mean that you will likely have to hire electro-fusion kit. Always practice, and follow  the instructions.

Above ground manifolds are ideal for fastening to the wall of an outhouse, but remember that they will always have condensation forming because the transfer fluid inside the pipework makes it very cold.

Antifreeze

Referred to as glycol, antifreeze, heat transfer liquid and occasionally, "brine" (although it's not!), this is the fluid that circulates around the ground arrays.

Its purpose is to absorb heat, but it also needs to prevent freezing which is why plain water isn't used.

It must also contain biocides to kill off algae and bacteria that can grow in the liquid.

The concentration is also important. The liquid is a mixture of glycol and water, so if the concentration is too low or light, then the freezing point is also too low. This means there is a greater chance of the liquid freezing in winter.

If the concentration of glycol is too high, then the liquid is thicker than usual - glycol is actually quite syrupy stuff when neat. This can affect how well the circulating pumps in the system can perform, which eventually would affect the amount of heat absorbed from the ground.

Kensa suggest that when you fill the ground arrays, you allow the purge pump to run for a couple of hours to purge out any air in the arrays, and to allow the glycol and water to mix properly. Only once you are happy that the fluid is well mixed should you take samples and return them to us for testing - or you can test them yourself.

To test samples you will need an instrument called a refractometer. 

These can be bought online for under £20 which is quite suitable for this purpose - often garages use them for testing coolant (which is all that we're doing - you can always ask a friendly local garage to do this for you!).

Take a pipette full of your antifreeze mixture and smear it onto the lens of the refractometer. Hold the refractometer up to the light and look at the scale. You should see an obvious line on the scale that will tell you the percentage concentration of the sample.

Most refractometers have scales for propylene glycol and ethylene glycol. The Coolflow DTX that Kensa supply is ethylene glycol that has had its poisonous components neutralised - ethylene glycol is normally poisonous, so do not drink it!

(Propylene glycol is harmless and is used in food and toiletries regularly - however it's slightly thicker. Make sure you know what you are testing)

Header pipework

When you are planning your ground arrays, you may find that you need to link boreholes or slinkies together with a length of header pipe. This is the pipework that links the heatpump itself with the manifolded ground array.

It's important that this part of the pipework is properly sized, in order to ensure you get the  required flow rate to the arrays. Undersize it and the flow to the slinkies would be restricted, and you would be under-extracting from the slinkies - and probably end up over-extracting from the header pipework itself.

You can't really oversize header pipework, but it's important to remember that the larger the pipework you use, the more expensive it is to buy. So there's no point oversizing the pipework just for the sake of it.

Kensa are always happy to size header pipework for you - we need to know which heat pump you are using, how many slinkies you are using, and the length of the header.

Header pipework is sized in just the same way that heating system pipework is sized. You can use an online tool such as pressure-drop.com, but it helps to know a couple of parameters to get accurate results.

You will need to know the inside diameter of the pipe you are using:

Pipe outside diameter mm inside diameter mm
25 20.1
32 25.8
40 34
50 40.4
63 50.9
90 72.9
110 89.1
  • Weight density for Coolflow DTX is 1036 kg per cubic metre.
  • Pipe roughness should be 0.002.
  • Viscosity is 2.7 CP.

To achieve a freezing point of -10° C, the concentration of Coolflow DTX must be 22%. Other types of glycol do vary, and generally around 20% is sufficient for ethylene types, but do check the data sheet of whatever you are using.

If you are requesting Kensa to cover your installation under the MCS Umbrella scheme, you should only use Coolflow DTX, unless you can provide a datasheet that confirms that whatever glycol you are using has equivalent properties.

 

Quiz - let's review what you've picked up.

True or False?

  • Slinkies need more space than straight pipe
  • Closed loop boreholes are efficient but expensive
  • If you are using open loop, you can take as much water as you like from a lake/river, no one will care
Do you think these statements are true or false?

What factors affect the size of a ground array?

  • Size of heat pump
  • Conductivity of ground
  • Number of hours the heat pump will run for
  • Time of year installed
  • Amount of antifreeze added to the system
  • Type of controls used on the system
Tick everything you think applies.