Ask the supplier for the density, thermal conductivity and specific heat capacity figures. Then work out the expected inputs and outputs for all the expected temperature ranges.

Then apply this formula to work out the power delivery:

Thermal Inertia
Thermal inertia is defined as [units in square brackets]:

I = (kρc)½ [tiu = J m-2 K-1 s-½]
where:
k = thermal conductivity [W m-1 K-1]
ρ = density [kg m-3]
c = heat capacity [J kg-1 K-1]

Or just pick anything as it probably wont make much of a difference as I suspect that most mixes have similar enough properties.

Are you going to put some temperature probes in? You can get them on a string or fit individual ones spaced to where you want them.

25N concrete should be fine, we usually space the pipes at 150mm centers.

Posted By: TimSmalland increase thermal conductivity

Do you want to do that though?
The Heat Capacity and the Density of concrete does not vary much.
The Thermal Conductivity varies greatly though, by a factor of 17. This is the real problem as it governs how fast you can transfer the energy into and out of the slab.

<blockquote><cite>Posted By: SteamyTea</cite><blockquote><cite>Posted By: TimSmall</cite>and increase thermal conductivity</blockquote>Do you want to do that though?</blockquote>

My intuition is yes, BICBW. You might need to tweak other parameters tho' - e.g. thickness.


As to a rough indication of how much difference you'd get, it might be worth looking at table 5, and page 34 of:

http://www.osti.gov/bridge/servlets/purl/573177-l5FI05/webviewable/573177.pdf

The example from that paper is table 5 - which shows two identical mixes (a couple of the 'flintshot' ones), except one's a lot wetter than the other.

When cured and dried, the wet mix ended up at 1.5 W/m·K and and dry mix at 2.3 W/m·K

As this mix is designed to be pumpable and self-compacting in a borehole, it'll be a lot wetter than a slab mix, but it still shows that the variation is considerable.

BTW, on page 36 it says:

"The results obtained for cement-sand grouts can be compared with published data for
concrete. Thermal conductivity of concrete is dependent on the mineralogical composition of
aggregate used in the mix, as well 11,s that of the cement paste. Ranges of values for different
aggregates are provided by Neville (1996), Mindess and Young (1991) and AC1 207.1R (1982). For
example, quartzite aggregate results in thermal conductivities around 3.5-4.3 W/m.K, whereas basalt
aggregate concrete has values around 1.4-2.0 W/m.K. Degree of saturation of concrete also
determines the thermal conductivity. "


so the locally available aggregate is likely to have a big effect - which could be anything from granite based, to mixed marine aggregates, to limestone...

Posted By: Novice1The concrete company will not suggest and the structural engineer has never worked with a solar slab before.
The BCO and structural engineer insisted on 1m deep footings to support the walls. ...
What concrete should I order?

Approaching the problem from a different direction, does the BCO insist that the engineer signs off on the floor? If so, and I expect the answer is yes, then is your engineer happy to sign off on some concrete spec, despite not having worked in this situation before? If so, then accept what your engineer says. If not, I think you'll need to find another engineer who will take responsibility, perhaps via advising your engineer or perhaps by replacing him.

Posted By: Novice1Few questions
1. Temperature probes
a. Does anyone have the specs or source of temperature probes.
b. String or fit individually?? Which is easier to install?
c. Anyone in the East Anglia area who could oversee the installation and integration of this part of the system?

I would go with the 1 Wire sensors from Dallas, they can be either individual or on a string as each has a unique identifier. Singles will take more wiring, single string may have reliability issues if the wire gets broken.
Alternatively you could go for holes in the slab with iButtons dropped down them. That way you get individual loggers as well. Just pull the iButtons out when you want to read the temperature.

The spacing is the tricky one as without being able to modify their position if cast in place you will get what you get.
A bit of statistics could help you decide on the number for each position though. Would have to think a bit about that though. Some position will be fixed. Centre, corners, any areas of concern (different heights or slab thickness). The rest could be randomly assigned either by the Monte Carlo method (just random scattering) or by creating a grid and randomly assigning a probe to that grid.
Sampling frequency is also tricky, and may exclude the iButtons.
Would you want to sample at just fixed times, just when the slab is being charged or discharged, or a combination of both? Fixed times is easiest, but the resolution becomes important, but that can be established after a few weeks from the data (standard error and degrees of freedom calculations will help you there)
What you are trying to show is important as well (where I came a bit unstuck once when doing this). Is it just that the slab is hotter than the room or is it that the slab can heat up faster than the room, or cool down slower than it can be heated.

How long have you got till you have to decide about fitting them?

Thanks djh and Steamy...

1. Temperature probes
a. Is this what we mean.
http://www.ebay.co.uk/itm/DS18S20-Temperature-Sensor-1-Wire-Dallas-Maxim-/130621920626?pt=UK_Computing_Other_Computing_Networking&hash=item1e69ac5572

b. How does knowing the temperature of each part of the slab change what we do? Surely we will not have any control of different parts of the slab. Does it vary by that much? How do you manage your slabs during the seasons.

c. Anyone in the East Anglia area who could;-
i. oversee the installation and integration of this part of the system?
ii. Has a solar slab installed and functioning. The Viking 3Ds make it look so simple

2. Concrete spec - So 25N concrete ( 70% GGBS and 30% portland cement) + a super-plasticiser
a. @djh who is your eco-friendly structural engineer who has worked with these slabs and understands the system
b. What salient points would I need to consider wrt function, longevity and strength of the solar slab?

3. Thermal Conductivity –
a. What aggregate should I specify if I do have a choice?
b. @djh did you get a dry mix?


@steamy - I was planning to pour the slab in the next 2 weeks but the info and questions probably mean I'll delay and get it right. Just need a basic understanding of how experienced guys on here have installed and used the slab through the seasons



Thanks

Thanks. Beginning to understand. Less questions

Solar slabs - Anyone East of Steamy who could;-
i. oversee the installation and integration of this part of the system?
ii. Has a solar slab installed and functioning through the seasons.

2. Concrete spec –
a. @ Viking - Any arguments I can put forward for the use of 25N concrete?
b. What are the temperature ranges we are talking about and how does it compare to a south facing passive house slab?
c. Should I ask the concrete suppliers to add reinforcing fibres?
d. Has anyone with a Solar slab experience problems – eg. cracking etc


3. Thermal Conductivity –
a. What aggregate should I specify if I do have a choice?
b. @djh did you get a dry mix?

Thanks

@SteveZ
LOL - I know exactly what you mean... I keep wondering if the plumber, concrete company, BCO and structural engineer are going to have me sectioned as well!!

Posted By: Novice1Temperature probes - Is this what we mean.

Yes, or
http://www.ebay.co.uk/itm/Waterproof-Digital-Thermal-Probe-or-Sensor-DS18B20-DS18S20-Arduino-Sensor-924-/350708625289?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item51a7dcef89

FWIW, I haven't bothered with any sensors under the slab or EPS. I have put a piece of drainpipe vertically alongside the house to a depth of 1.3 m, so I can drop any kind of probe down that to get a general idea of conditions down below. But then I'm not building a solar slab, or even UFH.

Regarding your other questions: please note that I do NOT have a solar slab, I am building a concrete-raft-in-EPS-bucket. The engineer is expert in that system. I have no idea whether he knows anything about solar slabs. Also, we are going to pump the concrete so no, it won't be a dry mix.

Right, after letting it cool for a while I started to heat it up. Then looked at the data and realised that I was reading one sensor twice. So changed the code and let it cool again. I turned the pump off as well.
Cooled quite nicely.
Have now turned the pump and heater on and shall log it over night (Friday the 13th should not make any difference, I expect to have a shed in the morning).

So below is the cooling curve. I should have put a logger in the shed to see what the ambient temp was, but may do that when and if I run the test again.

Pretty graph ST
Wonder if you could simulate the 4 seasons :-) and mimic the slabs function inter-seasonally

About to order So 25N concrete + a super-plasticiser + mesh + 3 sensors + expansion joints