Some interesting comments about the benefits of stuffing as much pipe down the hole as possible...
What difference can an inch make... [
Quite a lot in my personal experience!]
Because we sell Geoclips, and I want to sleep well each night, we undertook a desk study of the effect of changing the Average Drill Penetration Rate, conductivity of the grout, borehole diameter, Loop pipe diameter and pipe position within the borehole [with or without GeoClips], for a specific DEsign Load. We got some interesting results - published in the November 2009 issue of Geodrilling International magazine.
For arguments sake, we assumed a total UK cost of £125 per working hour and an Average Drill Penetration rate of 10m (33') per hour. I appreciate these figures will vary from region to region, but you've got to start somewhere... I would be interested in feedback regarding your ADPRs. We also explored the effect of changing the Drill penetration rate by +/- 80% (2m - 18m (6'6" - 59' 3") per hour), to see how this affected things. Total costs included all costs of Rig ownership; including manpower, utilisation,
finance, insurance, depreciation, consumables (Drill bits, etc.) etc.,
as well as the cost of materials, such as the Loop pipe and grout, for
the drilled length(s).
The short version is that, if you:
- Use Single or Double Loop GHEx in the borehole,
- Take all of the true installation costs in to account,
- Can design the borehole taking account of the known Thermal conductivity of the formation to reduce the total borehole length to a safe minimum,
After taking the cost of buying the GeoClips in to account, GeoClips produce the lowest cost borehole every time; unless you are cutting through warm butter at more than 19m/hour (62' per hour), then you reach parity. In this situation the only benefits of using GeoClips are:
- An improvement in thermal performance, helping to reduce the future operating costs for the client - and the Environment,
- Knowledge that you have a high quality borehole producing the same (maximum) performance as any other Geoclipped borehole, in the same formation; unrestrained Loops are all unique - who knows where the pipes are, in relation to each other down the hole? The only way of finding out is to conduct a TRT on every borehole.
On the subject of Single / Double Loops being best:
Double loop installations definitely require the shortest boreholes, reducing Design Borehole Length required by 8%, if both single and double Loops are unrestrained, and 14 - 15% if the Double Loop uses GeoClips, whilst the single Loop is unrestrained.
However, Double Loops are not the lowest cost option; we found that a Geoclipped Double Loop is about 33p ($ 0.50) per foot more expensive than the equivalent unrestrained single Loop [UK prices] - due to the cost of the GeoClips and extra plastic pipe required - the unrestrained Double Loop is about 14p ($ 0.20) per foot more expensive, due to the extra cost of the plastic pipes alone.
The lowest cost installation, for the Design Load considered, was achieved using a Single GeoClipped Loop in (
surprisingly) a slightly narrower borehole - 130mm ( 5" ), as opposed to 150mm (6") - this reduced the Design Borehole Length required by approximately 4% and the cost by 53p ($ 0.80) per foot.
This data is shown on our (under development) web site: www.geo-clip.co.uk/home/grouting/thermally-conductive-grouts
The Study was made using Gaia GLD software, and the techniques and formuli contained in the IGSHPA Certified GeoExchange Designers book - "Grouting for Vertical Geothermal Heat Pump Systems - Engineering Design and Field Proceedures" Chapter 2.
Annex C also gives data relating to the Field verification test made by Smith (1997) and Remund(1997) which compared Configuration B (Unrestrained) Loops with Configuration C (GeoClipped). I believe this is where the GBT Inc. graphs come from.
Happy to send a copy of our full Desk study findings to anyone interested.