It cd be important to develop ways to input ultra-lo-temp solar-derived heat to near-PH houses, that aren't UFH.

With UFH the possibility of taking advantage of coupling the interior to subsoil mass via uninsulated slab (perimeter downstand insulated instead) is eliminated. That leaves the walls e.g. inside face of external walls as candidate for very large heat emitters.

Even in a near-PH the emitters have to be v large in order to run at ultra-lo temp e.g. 1C above target room temp at the wall/emitter's room face. That ultra-lo temp is essential in order that the collectors can run at ultra-lo temp e.g. 25C in order to minimise simultaneous re-radiant loss from collectors to cold scenery/sky in Dec/Jan. That re-radiant loss (when collectors run hotter than that) is the reason for the conventional wisdom that it's unfeasible to collect solar thro Dec/Jan.

Up till now, full-PH has been able to rely upon incidental gains - solar thro windows, occupants' heat, cooking, electrics - to provide the tiny bit of heat still needed, with 'backup' heating system only for exceptional moments. That calculation is coming to an end, as new technology begins to reduce the two largest contributors to insignificant.

LED lighting is already here; enormous reductions in appliances consumption esp on standby are continuing (over-balanced at present by increasing nos of appliances); and lo-energy cooking is in the wings.

Present PHs will find their 'supplementary' heating systems (often on-peak electric) working harder, and future PHs will need another more full-time source of Dec/Jan heat - solar (other than thro windows) - which PH as a method is presently ill-equipped to calculate.

With such 'free' solar heat available, the fullest rigours of full-PH are no longer necessary, which will be a great relief to eco-builders, and to the cost of retrofitting the nation's building stock.

Tom

I'm very interested in this myself - my own house isn't insulated enough at present (I still have to run my heating at 50 degC) but I have recorded 50 degC surface temperature on a south facing olive green fence panel in mid-November and I believe with a large enough collector that it is viable down the line.

I am working on the design of a zero carbon house at present (not PH certified but <15kWh pa). I have proposed an oversized south facing vertical ST bank to extend the solar collection into the winter.

Although the heat will just be used for hot water (there is no wet heating) there will be lots of monitoring equipment available as it wil be used for R&D. We should be able to get some useful data next winter.

Tom,

how would this work in a normal house that is just well insulated.

The problem with PH is that it is a very small market

a) it works best on individual new builds,
b) is expensive to do
c) very few builders who will produce this unsupervised
d) people will ask 'why bother' when it becomes apparent that their 5 year old PH house no longer performs to the same standard as new.

It would be really interesting if someone came up with a method that made near PH levels on existing properties

Jonti

Posted By: MarkBennettHowever, in this case, is it worth thermally decoupling the internal walls from the external ones

Or build a passive house inside the existing one

Mark, those fears have to be considered against the fact that we're envisaging heat-emitter temp (external walls' internal surface) only 1C above target internal air temp. At that, temp gradients and convection currents wd be v feeble.

With such a small delta-t between emitter and air, the air temp is essentially lock-stepped to the emitter temp. The slightest drop in air temp causes a significant increase in that delta-t, so heat flow into the room increases sharply, quickly restoring the status quo - without thermostats etc. This principle is one of the plus-points of UFH; v much more so with even bigger (hence even lower-temp) emitters i.e. the walls, and in addition lower emitter temp due to much lower heat demand than typical for UFH (near-PH compared to 'normal' UFH wisdom).

Yes, the interior partitions could also, or alternatively, serve as the heat emitters. And upper floors could use UFH - if indeed the upper floors need heat at all.

With such large emitter surface, the ratio of radiant:convective heat transmission to bodies in the interior is about as high as it gets - which is best for subjective comfort.
Warm-air heating needs 21C air temp for comfort because the radiant:convective ratio is low - high air temp must combat relatively low radiant temp from surrounding heat-losing surfaces.
Hi-output 'radiators', tho still principally convective, improve slightly on warm-air heating.
Large simple single-panel radiators do better again in radiant:convective ratio, hence 20C is the comfortable air temp.
UFH is a big step on, with high radiant:convective ratio, hence 19C is the comfortable air temp.
Walls as heat emitter will be roughly double the area avail to UFH, hence have even better radiant:convective ratio, and we're expecting comfort temp to be 18C.

Each of these steps, as well as being more subjectively comfortable, also reduces fabric and ventilation losses pro rata, as comfortable air temp comes down. Also, humans are more tolerant of temp swings when the radant:convective ratio is high.

If using the external walls as heat emitters, solar heat has to be put into the external walls. Attached are pics of how we've done it - small-bore UFH-type pipes arrayed on the walls' external face, embedded in render for thermal contact with the wall (method currently subj to re-think) and EWI'd externally.

With room temp 18C, internal wall face 19C, external wall face 20C, mean temp of water thermofluid in the pipes 22C. So that's hardly an elevated temp as far as outward loss thro the EWI is concerned. Note that in calc'ing fabric losses from the room, loss thro walls drops out, as the heat flow is inward thro the walls, and the wall loss to outside is satisfied directly from the pipes out thro the EWI.

Posted By: fostertomThe slightest drop in air temp causes a significant increase in that delta-t

That depends on how and from where you measure it, work with absolute temperatures and you will find that the difference between 293K and 294K is 0.0034% while the difference between 20°C and 21°C is 4.76%.

Posted By: fostertomWith such large emitter surface, the ratio of radiant:convective heat transmission to bodies in the interior is about as high as it gets - which is best for subjective comfort.

No it isn't. The surface area of the absorber (a person in this case) becomes important, so unless they are the size of the room they are not getting all the radiated energy.

Do the sums rather than speculate and quote anecdotal evidence. It is the only way.

Posted By: fostertomPH as a method is presently ill-equipped to calculate

Oh, and why do you say this? Have you looked at how PHPP does calculate it? Which bit is ill-equipped for the scenario you describe?

I think it's all very interesting and would agree that speculating/experimenting is a good.

If you take DHW out of the equation for the moment and just think of it as a heating device for December to February, which you turn off for the other months.

Does the interior wall/floor ('emitter)' if concrete/block/brick become part of the main storage volume? Do you need an insulated storage tank at all or just a small one to fine tune the individual system to maintain the correct temperature? Would it matter if occasionally the water temperature got to 24 degrees?

For instance, my sisters ST set up (2 or 3 x12 tubes can't remember exactly) heats the top half (100l) of the tank to between 22-18 degrees throughout the winter. If she reduced the tank capacity would this increase the temperature?

As I'm writing this I have been thinking about when you would turn it on - from 11-4.00pm probably. So would the house hold the heat for 19 hours - ahh so maybe you do need storage and lots of panels....

Posted By: SteamyTeaDo the sums

You do the sums e.g. about the red herring of abs temps - you'll find I'm right - I have done that sum in the past, published here, because misunderstanding of S-B was rampant, confusing 'fourth power of difference in abs temps' with 'difference between fourth powers of abs temps'. And here it is again.

Posted By: fostertom

Posted By: GaryBI have proposed an oversized south facing vertical ST bank to extend the solar collection into the winter

Good - but at conventional DHW-producing temps, however oversized, it will produce little or nothing when cold outside, because simultaneous re-radiation loss to cold scenery and sky will dissipate whatever you collect.

To collect thro Dec/Jan, the operating temp of the panel's receiving surfaces must be reduced as far as possible - that means the thermofluid flow and return temps must be v low e.g. 21C return, 23C flow for example (which is useful for space heating all as described above). There's other optimisation of the collector design for ultra-lo-temp collection, that we're investigating.

Why are you hung up with re-radiation losses. If you use fibre optics then you dont have the re-radiation problems and your solar collector can be sited anywhere within the insulated fabric of the building not attached to the external fabric of the building as in traditional systems.

Posted By: fostertomAre fibre optics not bi-directional - as free to pass radiation outward as in?

Not when you have an absorber at the end of it such as a CSV unit found on the Zenith Z20 and built by the European Space Agency.

Could the GF ceiling be used as emitter?
Maybe integrated with first floor UFH
Isolated from external surface, so no losses to ambient.

Ref Ed Davies blog, is it nearly economic to use PV as (part of) the collector, this would allow resistance or heat-pumped top up of the ST temp, to reduce emitter area or to do DHW.

Right

Material,Delta T,q (Watts)
Wall (plaster),2,90
Human Skin,10,90

So to have a wall radiate the same amount of energy as a naked human there is a 8.1:1.5 ratio (5.4 times more wall area than human area) This wall will have to totally surround the person but not touch (conductance will take over then)

I have used a 2K temp difference for the walls and 10K for a human body.

Posted By: Viking Housea 67% reduction in running costs when a 40m2 solar roof was installed on a 150m2 house

Posted By: Viking Houseby 90% with 16m2 of Solar Thermal

Is that for 90m^2?

Posted By: Viking HouseThis 300m2 house we're building has a 60m2 Solar Array

Let's tabulate that:
Collector area, House Size
40,150
16,90
60,300

And then chart it to establish the slope.
Looks like you need 20% of the floor area plus 3.5m^2