Hi,

Thanks everyone for the various informative posts. I was considering the installation of earth tubes as part of my extension I am planning in Perth. The links provided here were useful in the research.

I thought I would share some of the information and the resulting conclusion I reached in deciding not to proceed.

One of the most important aspects in making a reasonable assessment of the likely effectiveness of the concept is reliable soil temperature data at a suitable depth. Although not perfect we appear to be lucky here in Perth with Murdoch University making 10 minute data available to download for 125, 250, 500 and 1000mm depths (http://wwwmet.murdoch.edu.au/).

Based on a quick analysis of the data for 2010 shows the annual average temperature of the soil at 1 metre deep was 21.1 degrees which compared to 18.1 degrees for the annual average ambient temperature. So the average soil temperature at one metre was 3 degrees above average ambient. Even greater than Catopsilia 2 degrees observed in Manilla.

The one metre depth temperatures ranged from 14.4 to 27.9 degrees (fairly similar to Catopsilia’s data, which isn’t too surprising given similar ranges in ambient temperature). For the shallower 125mm depth there were 6 days where the soil temperature exceeded 40 degrees.

I then extrapolated this data for 2 meters depths which is about the maximum depth that earth tubes would be installed.

My first method was to double the difference between the 500 and 1000 mm depth to guestimate the 2m temperatures. The graphed results are here:

http://www.scribd.com/doc/78724444/Soil-temperature-recorded-at-Murdoch-University-2x-factor-for-2000mm.

After a more detailed review some of the depth temperature tends with depth at various times of the year the 2x factor looked too optimistic and better data fit was achieved using a 1.3 factor:

http://www.scribd.com/doc/78724442/Soil-temperature-recorded-at-Murdoch-University-1-3x-factor-for-2000mm

The resulting estimates suggest that the January/February soil temperature at 2 metres will be 24-26 degrees. So allowing for some loss in heat transfer efficiency I doubt air from an earth tube system would be below 26 degrees in summer, which would barely be below the maximum house temperature. At such a high temperature I couldn't see the benefits for my circumstances and therefore have decided not to progress any further.

Are there any fundamental flaws in this logic?

A couple of cautions with respect to the above are:

1) Murdoch is situated on the Swan coastal plain, and although I don't know the exact location of their weather station it is almost certainly in an area of partially cleared bush with dark coloured sand and grassy weeds.

2) Only a single year (2010) of data was analysed.

3) I suspect sub-soil temperature has a lot to do with the soils ability to absorb solar radiation (and our dark sandy soils are good at absorbing heat).

4) My 2 metre depth estimation method is ‘quick and dirty’ (but I believe sufficient for my purposes)

The main assumptions/methodology used to generate the graphs:

1) The 125mm, 250mm and 500mm temperatures are a 7 day rolling average (this removes daily and weekly temperature change - does introduce some time lag). Perth’s weather typical repeats on a 7 day cycle.

2) The 1000mm depth reading is measured data.

3) The 2000mm is a rough approximation by subtraction of either twice or 1.3 times the difference between the measured 1000mm data and the 500mm rolling average. The result was then averaged over a further 14 day period (rolling average) to account for attenuation of the temperature changes. This second computational time lag was removed by moving the data a week earlier (although that doesn't really have a material effect given the purpose of the data analysis).

Posted Thursday 19 Jan 2012 @ 9:19:29 am from IP

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