solar pavement

Karl_Northwind

Hey y'all,

I have an interesting application to pick brains about.  I have a prospective customer who wants to re-surface a piece of pavement with tubes in it.  it'd most likely be black pavement (concrete or asphalt if they can lay over tubes) about 1000 SF and be used to heat a pool. 

I want to as well do a glazed collector to handle shower water (100+ showers per day) and heat the pool as well.  glazed in this case makes sense because of the DHW consumption, and the pool heating shoulder seasons (april/may and Sept) and their low ambient temps.



what I'm trying to do is this:  figure out how many of the 1500ish Btu/sf that hit that pavement on a summer day (pulled from NREL/Retscreen) will actually make it 3" down to the tubes, and how many BTUs I can expect to make it to the tubes and pool water. 



The plan is to use Uponor aquapex in the slab and plastic manifolds to handle the pool water, and then blow out the system at the end of the year.  Kind of a pain, but the rest of the system gets drained and blown out at the end of the season as well.



has anyone got access to a whitepaper on the heat movement aspects of this?

My alternative, seat of the pants method is to use the delta T heat driving numbers from concrete slabs and guess at surface temps, and figure out a btu harvest. 



thoughts?  I will be doing figuring as well, but I'd like to know I'm not shooting in the dark.   I will do BTU monitoring on both systems if we go ahead.

Mark Eatherton

Mario Andretti has this on his house...

If memory serves me correctly, Heatboy had something to do with it, or was aware of someone who had something to do with it.



I've proposed it for a number of jobs, but none have come to fruition.



If you REALLY wan to maximize the effort, put glycol into the system, like a real snow melt system, then put a water source heat pump on it to extract the low grade heat. You could even do a decent job of preheating their DHW at that point as well (120 degrees F).



The hardest part is going to be the overlaying of hot asphalt over the tubing. you MUST maintain a constant flow of COLD water through the tubing, during, and after the pour in order to avoid tube collapse. Otherwise, the hot asphalt will melt the tube, and then you are screwed...



Best of luck



ME

Bob Gagnon plumbing and heating

Driveway Solar Collector

I thought Mad Dog was going to do one of these a few years ago. Does anyone know how it turned out? I have a customer interested in this also.



Thanks, Bob Gagnon

Bob Gagnon plumbing and heating

a

Karl_Northwind

efficiency

I've been trying to run the numbers, but it eventually made my head hurt and in order to run 200 BTUh thru the pavement it's ridiculous where the numbers end up.  the big unknown is the re-radiation numbers and heat loss from the pavement to account for th BTUs that don't end up in the fluid.



we have 1500 BTU/d that end up on a flat surface per july day here in central wi.  how many of them are harnessable with a super low efficiency collector like pavement is a good question.  if I can get 25% of them, it will give me 1000sf x (1500x.25)= 375,000 btu/d x 30days=11,250,000 btus, and at a burner eff of 80%, offset 14,062,000 btu or 150+ gallons of propane for each of june, july, and august.  not too bad. 



anyone have a good idea of what the efficiency of an old ray-pak pool heater is?  can't be too bad, but might be able to convince customer to install something condensing. 



how do the above numbers look to people?  do you thing 25% is too high an efficiency at near ambient temps?



thansk

karl

NRT_Rob

we never got to instrument

but working with Gary wallace we did this in a client of his, driveway, and I would definitely recommend dark concrete rather than asphalt/sand.



We were pulling over 100 degree water out on overcast days with system sized for snowmelt. how much, how long, all that is not really known with any good detail.



but you can do this, yes, and it does appear that dark concrete makes a pretty good solar collector. BTU monitoring would really be sweet!!!

Karl_Northwind

monitoring

I would really like to do a flatplate collector system here, for a number of reasons, and if that happens, i'm going to throw in (at least temporarily) BTU monitoring, and possibly solar flux monitoring as well



I'm hoping that we will get some minor gain in the later afternoon as the thermal lag in the mass of the concrete may help.  we'll see.

Karl