• We are using Boreholes because the site area is far too small to consider horizontal ground source. 
  • We have Two boreholes because the machine required to drill 48m boreholes is a small fraction of the size of the ones required for 80-100m.
  • We did not realise in 2006 how wonderfully lucky both of these decisions would have been, because the arrangement we now have is perfect for solar charging.
  This 3D illustration demonstrates the relative scale of the house and the boreholes below. The two holes are 48m deep, and another important factor is their 'reach', i.e. how much of the soil around will be thermally responsive to our cooling and heating.
    Initially, one imagines deep overlapping Cylinders, but in all likelihood, the shape is more like two tapering interlocked Bottle shapes, with rounded bottoms.
    Near the ground surface, the ground temperature varies with the seasons and the days, so only the volume immediately round the pipes is relevant - so it looks like the neck of a wine bottle.
    As you go deeper, the radius of active soil around them increases, perhaps to 3.6m, so it looks like a widening Claret bottle. As it deepens further, it widens to something like 4.5 metres depending on conductivity, the delta-T that we are working with. The deeper parts are no longer affected by the seasonal changes at the surface or in the upper layers. At the bottom of the borehole, the bottle-shape is likely to be domed at the bottom. The two bottles overlap, sharing warmth between them - but it is very unwise to have them closer than 5m apart.

Video of the borehole being drilled, the pipes pushed down and the hole backfilled 

  This model is to scale (using ArchiCAD 3D), but it's a perspective projection, foreshortening the bottle-shapes. I am able to discover the volume by building these to real scale, intersecting the bottles (to delete the overlap), and then interrogating ArchiCAD to find out the volume.
The Planview shows how adjacent boreholes will 'nurse' heat
if you put solar heat down - but this arrangement
would be poor for a twin borehole set that was not being charged.  
   The model also assumes consistent conductivity. All we know is from what the drillers told us, who reported that for the entire depth, it was pretty consistent 'Marl', a mixture of dense clay and broken limestone - a mixture resulting from the retreat of the glaciers. It sounds right, there was a huge amount of clay slurry coming up (and much of it is still splattered on the wall) interspersed with sections of easier drilling where they found limestone. If the soil is stratified with different mixtures of clay, rock or sand, the density and conductivity would vary - the bottle is basically correct, but it would have thin and wider zones.
Jackson Drilling, men at work
by the end of the day, they have changed colour
completely coated with clay slurry!
  The CAD model tells me that the volume is 3600 cubic metres which amounts to about 6,800 tons of storage battery if the average density of deeply compressed Marl is assumed to be 1.9 tonnes/m3. The soil around and beyond is 'infinite', but the bottle shapes represent what our ground source heatpump can expect to be interacting with, annually.
  The heat pump is expected to draw about 9,000 kWh annually from the ground for heating the house and hot water. The sunboxes are able to put down 3,000 kWhr. On this basis I can draw two useful conclusions:
1. Heat or Defrost? We can never expect to raise the temperature of the earth, merely to reduce the rate of chilling - we are still drawing a net 6,000 kWh from what the Sun gives to the ground.
Drilling the holes is a lot easier than
deeply excavating the very small garden
2. Efficiency? As the rate of pulling is approx 3 times the rate of pushing it's fair to guess that ALL the heat we put down comes back to the house. That heat doesn't have time to escape.

So, even if we only raise the deep ground temperature by 2-3 degrees in summer above a base of 12.0º, that is a helluva lot of heat to be storing, in such a large mass, equivalent to 6,000 kWh. The highest summer deep ground temperature reached was 14.0º (in August 2010) and on 31 December 2010 it was still 10.3º. One year earlier, in 2009, without any solar charging, it was 13.0º in August and 5.0º at the year end.

Borehole questions
The house has a driveway, with enough space
to park a small drilling rig
Who?: The holes were drilled by Jackson Drilling of Shepton Mallet, based on a word of mouth recommendation.

Why?: We had to drill vertical boreholes because the available garden is no larger than the footprint of the house, and has a wall of high trees on the border, ruling out the chance of horizontal pipes or grids. Also, with a newly built house, it is simply too risky to dig deep trenches so close to freshly laid foundations, when the house itself has not completed its settlement process.
How?: We had been instructed to dig one hole of 85m deep but this would require a large drilling rig the size of a furniture van, which would not be able to get out of or turn round on our cul-de-sac. That neat tarmac in the photo was a sea of mud at the time. The alternative was two 50m deep holes which could be drilled with a portable landrover drawn rig, plus a trailer based compressor on the other landrover. They must be at least 5m apart.

Heat Source: Many people think that the borehole is drawing up deep Geothermal heat, but the reality is that most of the energy pulled up is Solar. At the limited depth of our borehole, only about 0.1 W/m2 is rising up from the greater depths, the rest is from Solar heat that falls on the earth, soaks in and finds its way down. At the rate of heat transfer, this must take decades to reach 40 or more metres deep. There are wide seasonal temperature variations in the upper layer down to 5metres, and lesser variations down to 15m, but below that, the temperature is almost static - until someone with a heat pump starts drawing it up.

How to Calculate?: The assumption is that you are drawing heat from each metre depth of the borehole, although the temperature varies as the depth increases. As a rule of thumb one expects an average of: 20W/m from poor/dry soil, 40W from clay, 50W/m from v good rock/v damp soil, 70W/m from solid/high-conductivity soil with solar access to the surrounding site (Veissman Technical Guide 9447529). So if you want a peak GSHP output of 6,000W, that is drawing 4,000W from the soil. You divide one by the other.... So if I have 90-96m of borehole, I am hoping to draw 42-44W/m from the hole. This is on the optimistic side (assumes good clay soil), and over 5-10 yrs, I would expect to see progressive degradation of performance if the soil could not keep up. Therefore there is a case for Solar Earth Charging.
Underground Thermal contours:
Right: Summer. Left Winter
During Equinox these are going up and down daily
resulting in 3 dimensional thermal Rings.

The Process: As the drill goes down, they add extra sections of pipe. On the first hole they had on a number of occasions to withdraw and clean the bit, as the clay was so extremely gooey. As the drill bit is grinding down, the compressor is pumping a mixture of compressed air and compressed water down the centre, to bring the pulverised rock and clay to the surface. It runs down the road into the gutter. Every time they hit chunks of rock they cheered, because the drill progresses more quickly and cleanly. The second hole was quicker (because they knew more about the soil below) and cleaner - less slurry squirted on the walls. They stopped when they got 48m down. There is a YouTube movie of the process, taking with a shaky mobile phone.
When?: November 2006. Because the small driller is lower powered, it needed 2.5 days to complete.
Completing: The 40mm plastic ground source pipes  were supplied precut for holes of that depth with a U-bend at the bottom, and the guys threaded the pipes down, leaving the ends sticking up. They then poured down granular particles of Bentonite, that look like pouring cat-litter down. This trickles down to fill the hole, and then they pour some water down to compact it, add a bit more Bentonite, until they reach the surface. The Bentonite is a 'non-setting putty' substance that fills the space and provides conductivity to the surrounding soil. It is also used in the foundations of tall buildings to provide seismic resilience.
Finish: The heat pump installer joins the pipes together in a small manhole near the house with a manifold and a simple flow and return are led into the utility room of the house.
The single 'energy bulb' model (left)
what we actually have (right)
Comment: For the purpose of solar charging, this arrangement of two shallow holes has been particularly beneficial as the space between the holes gets warmer, and less likely to lose the heat to the surroundings.
How long: The boreholes, being in seismic resistant Bentonite will last hundreds of years, and the physics of thermal conduction and capacity are eternal. Therefore our descendants could change the heatpump every 25-30 years, and completely rebuild the house every 100 years, and the same holes would suffice. Future houses will be of better insulation even if they have slightly larger footprint, so the ones we have drilled will be sufficient.
How much: The holes were £4,500 including VAT, and because the builder was delaying and obstructive some hours were lost, so we finished paying £5,000. However, we feel it is worth it for the infinitely long timescale, especially in the light of the solar charging project, because they enable us to heat the entire house for less than the amount of electricity provided by our PV power generation = Carbon Zero!

The alternatives
Why not use a horizontal loop? A surprising amount of land is required for this, and a loop in too small a space will cause ground swelling, frost damage to the flora above (and the organisms in the soil). I feel that if you have enough land to lay a long loop, you can afford to have a vertical borehole nearer the house, with shorter pipe runs, and the chance of storing solar heat. If you have a nice garden, would you really want it all excavated with a JCB? Consider the re-landscaping costs!
Single Borehole? The other alternative is a single deep hole of 85metres or more. The Thermal Modelling project that I have done has been using the assumption of a single borehole, with the Energy Bulb that is represented by the borehole metaphorically expanding and contracting as energy is extracted or restored.

This diagram on the NeoEnergy website includes a dynamic animation of the way that deep ground cools over successive years. Our big idea is to maintain warmth down below by direct solar injection.

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