What an interesting thought. The transparent grains are probably quartz. If the rock surface lets some of the energy in through the top layer of crystals so that grains a few millimetres in can warm up, then the rock might warm up more quickly. Actually, I rather suspect that the effect you noticed has some other explanation, such as a difference in surface colour, smoothness, etc., but be that as it may, an application comes to mind. If a south-facing wall is constructed using glass bottles as the 'bricks', the bottle bottoms exposed to the sunshine, then the mortar should be able to absorb solar energy throughout the length of the bottle and therefore warm up much more quickly than a conventional wall. I've seen folk make walls in this manner, but more as a way to recycle bottles. Doubtless someone has done it as a solar collector as well. Anyone know of an example?

It is not an interesting concept at all if you think about it. In fact, every house has such a concept built-in already - and silicon based too (like the quartz in the Stonehenge rocks). It's called a window.
Amazingly, it lets sunlight (and hence heat) into your house. The downside is that it also lets it out as well - windows have far poorer thermal performance than a well insulated wall. There is some mitigation to be had with appropriate low-e coatings though - they can be designed to let shortwave IR in and reflect longwave IR back into the house (or do the opposite if you live in a hot climate).

Also, it's worth doing the calculation of how much energy the sun really supplies on the typical winter day in the UK - you'll find it's quite dismal. I have the figures for my house in Montreal in Canada - which is substantially further south than anywhere in the UK and much sunnier in winter too - the usable solar gains are pretty small - and the low-e windows we have reduce these even further.

In my humble opinion, the only reasonable way to get solar energy into a well insulated structure like a well designed house is not by clever means of using thermal mass or nonsense like that (I'm being deliberately provocative here) - it's by using a heat transfer fluid that allows large amounts of heat to be moved through a small opening in the building. This could be short-term solar collected on a daily basis through a water-based roof collector, or long-term solar collected through a ground source heat pump (or air source for that matter since the source of the heat is pretty much the same).

For a really interesting take on this concept, see the Drake Landing Solar Community in Alberta - admittedly at one of the sunniest sites in Canada, but an example of what can be achieved with very simple technology and some collective action. http://dlsc.ca

Paul in Montreal.

Posted By: Paul in MontrealThis could be short-term solar collected on a daily basis through a water-based roof collector

I may agree, but can we spell out why a solar collector would be more effective at i) collecting solar heat and ii) moving it to the interior, than the equivalent area of window? OK, being on the roof it gets a clearer view of the sun and it leans back square to the sun's rays. Beyond that, the amount of energy it absorbs must be down to the temp differential between a) the incoming radiant temp and b) the temp of the fluid in the tubes, plus c) the absorbtivity of the tubes' coating. Comparing windows to panels, a) will be the same, b) panels will be higher (so lower temp differential), c) panels will be better. So, short of heat-pumping, to increase temp differential by lowering fluid temp, it's hard to see why a panel is so much better at collecting solar heat, than a window.

Tom,

even vertical borehole GSHP systems are essentially "shallow" as the temperature of the ground at the bottom of the borehole is equal to the annual average air temperature. We've had this discussion before and I'm not going to repeat it - if you weren't convinced the 1st time, I doubt I will convince you again, despite presenting you figures which back up my case.
All that said, I have heard of cases in the southern US where, due to undersized loops or some other combination of factors, the ground gradually heats up over the years - this is where GSHPs are used mainly in air conditioning mode, the heat being rejected into the ground. Often times it is because the ground gets too dry - and there are systems available to ensure that it stays wet enough for good thermal contact. I don't know of any cases where the ground gets cooler progressively - but most places that have GSHPs also use them in airconditioning mode which, of course, rejects heat back into the ground. GSHPs are used in permafrost though without issue in the far north of Canada (and in the antarctic) without problems.

Deep borehole geothermal energy (such as the system in Southampton - drilled to a depth of 1800m where they found water at 70C- see http://www.dti.gov.uk/energy/sources/renewables/renewables-schools/case-studies/geothermal/page22986.html ) is a whole different beast and is not replenished by solar energy.

As for solar panels being better collectors than windows - there's several reasons for this. First of all, windows are not oriented to maximize insolation, being constrained (usually) to be vertical. Also, windows have far inferior insulating properties compared to walls and so are a source of increased heat loss. Solar panels on a roof can be oriented correctly to maximize gain, plus can be a larger area than windows would be and, finally, the transport of that captured heat can be made through a couple of openings of only 2" or so through the wall (for the pipes) compared to the square metres of windows that would be required otherwise to try and capture the insolation (which is a losing proposition in a cloudy climate like the UK anyway).

As for ASHPs being used rather than ground source? ASHPs work very well - but have two undesirable characteristics. The capacity of an ASHP is proportional to the air temperature - so the colder it is, the lower the capacity. This can be mitigated by using a larger ASHP - but this, of course, is more expensive. If used for air conditioning (as they sensibly are in pretty much everywhere except the UK) - a large enough capacity for heating in cold weather would be massively oversized for air conditioning and would lead to humidity problems due to short-cycling. The other undesirable characteristic of an ASHP, particularly in a damp climate like the UK, is the tendency for the evaporator coils (the external bit in heating mode) to ice up. This ice drastically reduces efficiency (and output) and has to be periodically removed - usually every hour or so by what is called a "defrost cycle". Essentially the heatpump goes into reverse and the evaporator coils heat up (by extracting heat from inside the house or running electric resistance heaters) and thus the ice is melted. This reduces the overall COP significantly. A GSHP doesn't suffer from either of these problems and so is preferable in a northern climate where the air temperature is such that defrost cycles are required and such that oversizing would be needed to provide sufficient heating capacity.

Paul in Montreal.

Tony,

yes, I did live in the UK and I remember my glass of water freezing at night due to all that good insulation! I'll bet the new house we built has a much lower energy requirement per square metre at our extreme conditions compared to many new houses in the UK at UK conditions - probably largely due to air tightness. Perhaps it would make an interesting thread for people to post their annual energy consumption - I certainly would find it interesting to correlate various variables related to the climate etc. to actual consumption (which is what I do - I read the meters at least once a week and record the heating/cooling degree days that we actually have).

As for the sun helping heat the building, I also remember being in Manchester and not seeing the sun for 3 straight weeks one year. Windows are great for letting in light, but, even in the temperate UK, there is just not a lot of heat to be had, particularly in winter. I do believe that solar collectors on the roof are a more reliable way of getting heat into a building without compromising the performance of the windows / walls. A good compromise might to be to fit thermal shutters (like they do in Germany for example) and so keep the heat inside that managed to come through the windows in the day time. There's many low-technology solutions which could help. I was just trying to make the point that a "transparent rock" is essentially the same as a window and has losses associated with it because it is transparent.

Regards,

Paul.

No, none at all. The borehole is 420 feet deep so the only direct way to tell is to monitor the loop temperature. I don't have the sensors for that. The indirect way to tell is to correlate electricity usage versus heating degree days. If the ground is getting colder, the heat output goes down and so electricity usage should go up. We've only had two winters with the system so it's still early days but the most recent winter the consumption per heating degree day was sightly less than the previous winter (and it was a colder winter overall). The hard part is to guess the baseload and factor that in correctly so as to know how much the heating actually uses.

That said, I do have a friend who has a GSHP (again with a vertical borehole) and he does have temperature sensors on the input and output side of the groundloop. His temperatures are the same from year to year.

So, I'm pretty confident we're not cooling the ground long-term.

Paul in Montreal.

Tom,

I don't have any experience with horizontal loop GSHP installations.

I'm sure groundwater has a significant impact on heat transport. We're half way up a hill in one location (with the 420 foot borehole) and there a good couple of hundred feet of hill above us. The other location (where we have a 500 foot borehole) is next to a lake that's at the bottom of a smallish hill but with a lot of ground water (but not enough to allow and open-loop system [which is probably a good thing due to the complicating factor of mineralization etc.]). I'm sure there are aquifers in the UK that are more than 500 feet down so water certainly helps transport solar energy into the ground. Perhaps that's the mechanism that makes the ground temperature at that depth exactly equal to the annual average air temperature at that location.

Paul.