• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Solar water heating idea

R

redly

Golden Member
The house I am buying has a pretty pathetic asphalt driveway. I'm planning on replacing the driveway next spring. My dad is a contractor, and he has the equipment to to cement finish work. I am thinking of hiring some grunts to do the cement work on a driveway, and then me and my dad will finish it over a case of beer.

One thing I've always been interested in is alternative energy. A thought came to me that I should install a couple of loops of PEX into the driveway to take advantage of some of the sun's heat. I've come up with two possible uses, and thus two loops:

1. warm weather use
this consists of a loop that I can transfer my potable water into. Presumably it would be in front of the hot water heater. The cold water front the city service would be routed out to the PEX tubing int he driveway, capturing the heat stored in the concrete mass and transfering it to the water, thus lessening or eliminating the energy usage of the hot water heater. Possible add ons would be a recirculation pump and a device to remove air from the lines.

the system would only be utilized in the warm season and would be purged in the cool months via compressed air and a system of valves.

2. cold weather loop
this system uses a solar panel mounted on the garage to capture heat from the sun. A pump would be used to curculate antifreeze liquid from the solar panel to the driveway, thus heating the driveway to melt snow/ice. The system could also have a diversion valve that would route the fluid to a heat exchanger in the house for either hot water or warm air.

have any of you played with systems/ideas like this?
I'm in Iowa, so I'm pretty confident that the system would be fairly practical at least in the summer as a hot water source. PEX tubing is fairly cheap (~$30/100ft in McMaster catalog for 1/2" tubing)

Any thoughts?
 
Good thinking, but I'm concerned about a few things:

The tubing would have to be pretty shallow in the cement to take full advantage of the heat, as cement takes a while to warm up. This could lead to a cracked driveway (though I haven't had much experience with cement work, I will admit).

What would be the water capacity of each loop? Too little and you won't get much benefit from the system, too much and you're wasting water when you purge in the cool months. Remember not to water the lawn the day before.

Make sure you have at least one overflow valve, as pressures can vary in any water system.
City water is often full of minerals and other things to help you clog up your pipes. How good is this tubing when you deal with corrosion and deposits?

What sort of antifreeze would you use for the cold weather loop? I'm thinking toxicity and environmental damage here.


But anyway, good use of the old noggin'. I've been thinking about alternative energy sources here and there but I haven't taken the time to study up on generators or PV cells.
 
#1:

If you have a flowing fluid, it will remove energy (heat) from the driveway medium very fast relative to the rate at which the medium absorbs the energy. There are a couple ways around this, but off the top of my head, I'm not sure how practical any of them are.

PEX tubing might not be the best choice if I understand your design. Polyethylene (and most plastics) have low thermal conductivity, which means there will be large temperature gradients within the pipe walls. Further, almost all plastics are made with plasticizers which can leach into transported liquids. Most of them are pretty dangerous, so I'm not sure if it's a good idea for drinking water. If you're fully encasing it in concrete, you could use copper instead (not sure about thermal cycling with copper - all my materials books are at school) or some other metal. Metals transmit heat very well relative to plastics, so you would be able to heat water at a faster rate. There are some very simple equations you can use to figure out the relative rates of heat transfer - let me know if you're interested in looking into it further.

One other possibility involves many small, evenly distributed tubes. This would mean that each locality within the driveway would only be supplying heat to a small amount of liquid. However, it also means complex piping and large pressure drops, which means you'd pay a lot more for pumping.

#2:

The key here is the rate that energy would be transferred from the fluid to the ice/snow relative to the rate at which the solar panels could heat the fluid. If the rate at which energy is leaving the fluid is greater than the rate at which it is absorbed, you'll be paying a lot to pump the fluid around but it won't really be doing anything. Since you're talking about phase change (ice/snow --> water), the heat will be removed from the fluid very quickly. I'd say you'd have to have some pretty fierce solar panels to account for that, though I'm not terribly familiar with solar panels, so maybe it could be done. I'd definitely put this system on a switch so you could let heat accumulate in the fluid at the panels, then activate the pump as necessary.

Of course, this probably isn't helpful as it sounds like you know what you're talking about, but I've been studying alone in a room with no windows all day, so I had to talk to somebody. 😀
 
PEX tubing is what is used in radiant heating systems. there are some variants of it that a rated for potable water. Similar to what newer style plumbing fixtures use...flexible plastic tubing. I would LOVE to use copper, but I'm afraid of what that might do to the budget of this little hobby project

good point on the toxicity of the antifreeze. I know there are some that are less toxic than others...but still

Cracking of the cement was one of my concerns also. This is where I was hoping someone would voice some knowledge or experience. There are plenty of websites that try to sell heated driveway products, but they don't go into much detail about the installation

to clarify on the solar panels...it's not the PV kind, but rather more of a heat exchanger. Most likely a 4x8 enclosure with finned copper piping running through it. A clear plastic face and a black background. This part will require a bit of experimentation.

I've also been thinking about finding a smaller hot water heater tank (insulation removed) and placing it and a series of heat collector piping in the attic during the summer months to heat the water/cool the attic.

 
Well, I cant say that I think It will be a very effective money saver, plus you will have to turn the pipe off and drain it for winter every year (oh what fun. Another possible problem is the fact the concreate is an off gray, And It is usually not very hot to the touch. You should have to put a coat of asphalt or some other black substance to take more advantage of the heat from the sun. That is my thinking
 
In the summer you?d want to split the feed into preheated and normal temperature water so as not to have preheated water from the cold valve on your sinks. In the winter the melting of the snow would have to keep pace with rate of snowfall. If not you would only get a layer of water, a layer of partially melted snow, a top layer of snow providing good insulation, in other words a big mess. I'd rather shovel 2 feet of dry snow than 6" of slush. If you provide a means to store a large mass of warmed water, you might keep up with a moderate snowstorm. IMHO, feasible but not practical. Stored water system are excellent for heating homes, sometimes eliminating the need for any other heating source in moderate climes.
 
and you might not want to use PEX tubing, becuase it might expand and contract th the weather, and then your concrete would crack, and you wouldnt be a happy camper.
 
PEX tubing is used in concrete slabs to heat houses in cold climates. You just run hot water through it to warm up the floor. I'm not sure how much heat the water would absorb through the driveway though, usually people put these things on the roof.
 
Originally posted by: bonkers325
unless your concrete slabs are painted black and are in direct sunlight for a good part of the day, it really isnt feasible
Click to expand...

True. However, the space your drive uses would be usefull for a ground heat exchange system (same basic idea of what your going for, just you bury the pipes deeper where you can exchange with the ground which stays pretty much the same temperature all year).

See here for more information.

Bill
 
Re #2:

Does Antifreeze work like that? Does it actually bring up the temperature of the solution it's in? Or does it just prevent water crystals from forming?
 
May I suggest asphalt instead of concrete for this scheme?

I think it's less likely to crack, and of course, the black material will absorb heat a LOT better.

I used to love shovelling asphalt driveways as a kid, because by the time I finished, all the little scraps i'd missed had melted away.
 
Originally posted by: Kibbo
Re #2:

Does Antifreeze work like that? Does it actually bring up the temperature of the solution it's in? Or does it just prevent water crystals from forming?
Click to expand...
Antifreeze would be used because it has a lower freezing point than water. If water was used, it would freeze in the pipes, since it's exchanging heat at or near its freezing point.
 
I've actually thought through a very similar design that I was tempted to use last winter after becomming extremely aggrivated with the cost of heating my home.

The design I envisions was a combination of the designs you specified for winter and summer use. Basically a large black tube laid in a snaking pattern down the slope of my garage roof. The design called for only a small booster pump because the pressure of city water would bring my supply to the entry point of the system near my chimney. Gravity would help take care of the rest with some help from a small booster. Something like this

Pump -----------------------|
|----------------------|
|-----------------------inlet into house

Once inside I would run the heated water through a series of heat exchangers in the basement and 1st floor. The heat would then rise through the house.

My biggest obstical has been freezing during the winter. During the day Im not concerned but at night, when the system is turned off, I feel there could be potential for freezing and breakage.

Another obstical was determining where the water would go after being ran through the heat exchangers. I though of bleeding some off for things such as dishwashers, sink, drinking water, etc. However, the majority of the water would most likely have to be reintroduced into the system and this may negate my desire to use city water pressure to bring the water to the starting point of the system.
 
FWIW, it depends on the type of antifreeze as to whether you'd want to run it straight, or in a mixture...
Propylene glycol - straight
Ethylene glycol - 70% solution with water provides best protection.
Text
 
You could use alcohol as an antifreeze agent (I use it in my windwasher fluid, and it hasn't freezed yet). You should experiment in your freezer to find the best concentration.
Not that alcohol would be incredibly environment-friendly...

I saw on some buildings metal pipes painted black, inside glass-walled pannels facing the sun. The sunlight is concentrated on the tubes by some cylindrical reflective surfaces. They would heat water quite well during warm seasons.
 
Umm if you have solar heating for the winter to defraust you drive way why not use the solar heating to pre heat your water?
 
I think PEX is exactly what you want to use for this idea ... it's very tough & flexible, it's even supposed to be able to sustain freezing without splitting. So crack in the concrete, etc. will be less likely to effect it. Reemember that if your driveway is any appreciable size, you'll have to put expansion joints in it, and depending on the subsurface, frost action, etc, those joints can move appreciably. Would definitely kill copper, but PEX might stand a chance. And it is used for potable water supplies as well.

I'd consider blacktop over concrete though ... both for the additional heat absorbtion, and the fact that its softer and likely deforms in a more continuous manner compared to concrete.

I'm not sure the winter idea is going to do you much good in Iowa though. Think of it this way, if your system was 100% efficient (and you won't get close to that), you'd be effectively increasing the amount of solar heat applied to the driveway by the surface area of your collectors. So if you have a 480 ft^2 driveway (16'x30') and 2 4'x8' collectors, you'd only be increasing the amount of heat dumped into the driveway by 13%. If your system is only say, 30% efficient overall, your only adding about 4%

Now, where it may help is if your driveway is on the north side and doesn't get much/any sun ... now your making a significant improvement wrt ambient. Of course, if its on the N. side, that kinks your summer plans a bit.
 
Originally posted by: DrPizza
FWIW, it depends on the type of antifreeze as to whether you'd want to run it straight, or in a mixture...
Propylene glycol - straight
Ethylene glycol - 70% solution with water provides best protection.
Text
Click to expand...

Also recall that most antifreeze reduces the specific heat of water ... ie. a given volume of water will take less heat to raise it a given dT ... ie. it carries less heat.
 
Yes, antifreeze will reduce the heat a certain volume of solution carry. I know as I had the car's engine cooler in the summer while using tapwater as coolant, and hotter in September when I changed the coolant with distilled water plus antifreeze.

Regarding the previous post, a driveway covered by snow will reflect most of the incoming solar energy. I would say that a black collector receive 5 times as much solar energy (heat) as one covered with snow. So I would say the added heat is more like 20%.
Also keep in mind that a black driveway will probably melt the snow starting from the "entry point" of the warm water, and it looks like once the melting process starts, the already cleaned driveway (exposed to sun) will help melt the snow on the rest of the driveway, accelerating the process.

What I don't know is, what will be the temperature limit of the melting system - you might end up with a skating ring (well, driveway) if the outside temperature is too low, as the system will melt some snow that will simply freeze over night.
Anyway, I am sure it can work for a good part of the winter, and also for a good part of the summer.
 
Originally posted by: Calin
Yes, antifreeze will reduce the heat a certain volume of solution carry. I know as I had the car's engine cooler in the summer while using tapwater as coolant, and hotter in September when I changed the coolant with distilled water plus antifreeze.

Regarding the previous post, a driveway covered by snow will reflect most of the incoming solar energy. I would say that a black collector receive 5 times as much solar energy (heat) as one covered with snow. So I would say the added heat is more like 20%.
Also keep in mind that a black driveway will probably melt the snow starting from the "entry point" of the warm water, and it looks like once the melting process starts, the already cleaned driveway (exposed to sun) will help melt the snow on the rest of the driveway, accelerating the process.

What I don't know is, what will be the temperature limit of the melting system - you might end up with a skating ring (well, driveway) if the outside temperature is too low, as the system will melt some snow that will simply freeze over night.
Anyway, I am sure it can work for a good part of the winter, and also for a good part of the summer.
Click to expand...

Good point on the snow covered driveway reflecting heat ... but I still doubt that overall you'll get much benefit from the system in Iowa. Might be better off just laying a dark tarp on the driveway.
 
There are lots of books & websites on the subject of solar heating ... you should run the numbers.
Where I'd start for a quick hack...

1. Find the expected heat output/ft^2 of solar panel for your location on a good day in the middle of winter.
2. Find the volume of concrete you want to de-ice - assume a 4" slab
3. Find the heat capacity of concrete
4. Figure out the heat required to raise the temp of the concrete slab by 10 degrees - from 22F to 32F
5. Figure out the heat required to melt (phase change) 1/4" of ice over that surface.

From this, you should be able to figure out how much solar panel you would need to accomplish the task in a given amount of time. This will be a very optomistic answer because, your not just heating the concrete slab & ice, but also the ground beneath the slab and the air above it. But its a starting point. If the answer from this is unreasonable, there's not much point in going further.
 
I'll take a bite on this one. I can only speak from heating with PEX.

PEX (Crosslinked polyethelene) is the best material for what you are planning to do. It not only will carry potable water, it is also impervious to most chemicals so you can run antifreeze through it to prevent freezing in colder climates.

I live in western Pa. and installed a PEX tubing hydronic heating system in my garage floor when I built it.
It's a 4 zone system that's heated with an oil fired hotwater boiler and circulator pump. I control the temp of the concrete with a bulb thermostat glued to the floor. It prevents the boiler from starting everytime I open a garage door. The antifreeze in the tubing lets me shut down the system totally in the winter if I'm not working in the garage.

I also have a separate PEX system that heats the side walk that runs from the garage to the house (about 70 ft). On that one I use a propane hot water tank and circulator pump. It's controlled by a bulb thermostat also and a shut off switch once the sidewalk melts and dries.

Both systems use common RV Antifeeze that campers use in their RVs when they store them for the winter. Both systems also have hot water bladder tank expansion tanks.

There is a maximum tubing length to follow from the source to the destination and back to the source. That would be the only limitation in heating a driveway. If the driveway is too long you would lose any heating advantage by the time it got to the farthest point.

I got most of my tubing and fittings through McMaster-Carr. You'll also need ties and angle supports for where the tubing exits the slab.
I have 4 Zones (250 FT of PEX in each zone) in my 40'x30' garage and 160' in the 70' sidewalk run.
 
Originally posted by: Home Built
I'll take a bite on this one. I can only speak from heating with PEX.

PEX (Crosslinked polyethelene) is the best material for what you are planning to do. It not only will carry potable water, it is also impervious to most chemicals so you can run antifreeze through it to prevent freezing in colder climates.

I live in western Pa. and installed a PEX tubing hydronic heating system in my garage floor when I built it.
It's a 4 zone system that's heated with an oil fired hotwater boiler and circulator pump. I control the temp of the concrete with a bulb thermostat glued to the floor. It prevents the boiler from starting everytime I open a garage door. The antifreeze in the tubing lets me shut down the system totally in the winter if I'm not working in the garage.

I also have a separate PEX system that heats the side walk that runs from the garage to the house (about 70 ft). On that one I use a propane hot water tank and circulator pump. It's controlled by a bulb thermostat also and a shut off switch once the sidewalk melts and dries.

Both systems use common RV Antifeeze that campers use in their RVs when they store them for the winter. Both systems also have hot water bladder tank expansion tanks.

There is a maximum tubing length to follow from the source to the destination and back to the source. That would be the only limitation in heating a driveway. If the driveway is too long you would lose any heating advantage by the time it got to the farthest point.

I got most of my tubing and fittings through McMaster-Carr. You'll also need ties and angle supports for where the tubing exits the slab.
I have 4 Zones (250 FT of PEX in each zone) in my 40'x30' garage and 160' in the 70' sidewalk run.
Click to expand...

How do you handle it where the tubing crosses an expansion joint? I was thinking about 1' piece of pipe insulation would probably allow enough movement.

 
In my garage I used metal keylock for all my slab joints to prevent the slabs from settling at different levels. I did this because I was doing each of the 4 zones in 4 separate pours. The keylock is a heavy galvanized metal tongue and groove designed joint material. It doesn't allow for expansion. All transitions of the PEX from slab to slab at these joint areas were sandwiched between 2 peices of regular 3/8" thick expansion joint material and are run below the concrete slab where one slab met the next. I wanted to make sure anything I used to protect the PEX wouldn't rot, get hard or be consumed by pests. Along the outside perimeter of the garage where the concrete met the walls I used the regular expansion material.

On my sidewalk I used extra heavy reinforcing (welded rebar) and heavy highway mesh in the forms. I didn't use any expansion joints. I also used fibreglass reinforced concrete. It's one solid smooth 6" thick by 3' wide pour the whole 70' length. The fibreglass strands in the concrete makes it very tough stuff.

 
You must log in or register to reply here.

TRENDING THREADS

Back
Top