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Ian B
12-21-2010, 04:37 AM
I’d like to install a ground array like this:

http://www.baxi.co.uk/docs/Baxi_Geoflo_Ground_Array_(Slinky)_Installation_Ins tructions.pdf

and a circulation pump (note – *not* a heat pump) to provide water at around 10 – 11 deg. C to circulate through pipes beneath a driveway of 50m2. The driveway will be insulated beneath and along the sides, and the blockwork + sub base will be around 20cm thick, above the insulation. Assume no heat losses through the bottom or sides.

The generally accepted figure for thermal power derived from a ground array of this type is 1Kw per 10m of trench (which contains about 60m of piping in coils). I have enough land and an excavator, and the pipe isn’t that expensive, so multiples of 10m trenching aren't a problem.

The idea is to switch the circulation system on whenever the drive temperature drops below, say, 2 deg C and to keep it running to maintain this temperature to prevent ice forming, and also to melt snow as it falls.

“Heavy” snowfall by Dutch standards seems to be around 2cm per hour – one night of this is what recently wiped Schiphol out, so let’s work on that as a rate of snowfall to continuously melt. Density of snow is about 1/10 that of water, so about 100 liters of water (in the form of snowflakes) falls on the total drive area per hour.

So my questions; is it feasible to prevent ice formation and snow buildup with a system of this kind, and if so, to keep the given size of drive free, how many Kw would I need from the ground array? What circulation rate would be required?

Ian

Rich Carlstedt
12-21-2010, 10:38 AM
your link does not work

Ian B
12-21-2010, 10:50 AM
Rich,

I just tried it, it worked for me; it's a PDF which seem to take longer than other links - could that be it?

Willy
12-21-2010, 10:53 AM
The link works for me.

Seems like an awful lot of expenditure of time and resources for a very limited return on investment.
The concept sounds interesting at first but I believe the reason it has not been widely accepted is the limited return on investment. Basicaly there are more cost effective means of snow and ice removal.

airsmith282
12-21-2010, 11:26 AM
i for see a few problems its only good on light snow, if you get a big snow fall and big drifts your screwed you still going out to clean the driveway, , i also for see huge hydro bills, and its never going to pay you back, ok so iam canadain and i know people that went for this sort of deal and belive me they are not getting and benifit at all out of it ,kinda of like in floor heating in the house there is an increas of 500.00 a month more is gas and water expenses not to mention the broliers cost of replacemnt as they do toast eventaully and then if you get a leak ever your screwed, of course heat rises so now youll have to heat your basment to so the pipes dont freeze heat goes up not down, bingo more money out the door..

its your choice and your money,

oh waite some one is about to call me a know it all again i can feel it coming so as an advance to that, experience matters, knowing people that done this matters , studing and talking to the inventers of this techknoledgy matters, of course they usual BS comes to you so they can sell you .then you pay pay pat later,

not my fault god gave me a brain and i tend to use it,

kendall
12-21-2010, 11:45 AM
How deep is your water table there?
Up here it's pretty shallow, and temps of the ground water run 45-50 degrees, would be fairly easy to run the 'slinky' to the water table and pump the heat out of it

Jim Caudill
12-21-2010, 02:22 PM
Seeing as how he mentions the Netherlands, I suspect he's Dutch, and their water table is very high (my assumption). I have operated a self-serve car wash for over 12 years that has an underfloor heating system to keep the concrete bays from freezing. My system has 16 complete loops of tubing running throughout the 4 bays. The longest runs, go pretty much straight out and back, whereas the shorter runs go out and weave back and forth before returning. What you are trying to cope with is heat loss during the runs.

You are talking about an open loop system. If you start with water that is 10degrees C and then start circulating it in a slab that is below freezing, your water is going to cool and freeze after it travels a certain distance and gives up its delta T.

I run a closed loop system and have a fluid that is freeze protected to some very low temperature. I also use a 500,000btu boiler to heat the fluid. I regulate the boiler based on the return temperature of the fluid. Ideally, I would look for a uniform return temperature of around 4 degrees C. Since there are so many variables, I wind up setting the return temperature to around 14 degrees C on a -7 degree C type of day. My natural gas bill can be quite "eye opening" for a small car wash! After paying over $2,500 in one month, I now shut down the car wash when it is going to be below freezing for an extended period of time. It has been closed for the past 2 weeks, and I may not open it again until New Years ( we've only had a few hours of above freezing temperatures in the last few weeks).

The principle is sound, the devil is always in the details. Since you have a relatively small delta T, you will have to figure your worst case scenario. Design a system with a relatively large flow manifold, and keep your loops fairly short. You would definitely want to insulate your slab.

vincemulhollon
12-21-2010, 03:20 PM
so about 100 liters of water (in the form of snowflakes) falls on the total drive area per hour

So, basically you're trying to melt 100 liters/hr of ice..

1 L takes about 400 KJ to melt, you got 100 L, so thats about 40 MJ
1 KWH is about 4 MJ, so thats about 10 KHW to melt that 100L of ice/snow.
No huge surprise that it takes 10 kilowatts to deliver 10 kilowatt.hours in 1 hours time.
So I'd say 10KW continuous needed to melt the snow. you'll need more to keep it warm. windy days will be excitingly expensive.

Is my number reasonable? Well, within an order of magnitude, keeping in mind coefficents of performance of fridge compressors, blah blah, if you wanted to make 100 liters of icecubes per hour, its gonna be vaguely ten kilowatts, right? My ice cream maker takes 100 watts while it makes a liter of ice cream and it takes an hour or so, plus or minus some scaling and square cube law this and that, its believable estimate.

You do way the heck better to buy an instant water heater and some sort of frost-proof lawn sprinkler system and hose it off. Heated pressure washer?

The killer problem is you need to EVAPORATE 100L not just melt it. Otherwise you end up with an ice rink just off the lower ledge. That might be much more dangerous than just light fluffy snow on the driveway.

Duffy
12-22-2010, 12:06 AM
50 square meters of driveway isnt very much. The cost of tubing and pump, invested in a small ELECTRIC snowblower and an extension cord would likely see you better served and have a lower operating cost. Why an electric unit? Because they ALWAYS start!

Dragons_fire
12-22-2010, 12:32 AM
Our transit here put in some electric heated sidewalks in some places. it works great when the snow falls when its only about -5C. then the next day it gets a lot colder, and the melted snow freezes, and now its solid ice. then its such a thin layer of ice that its noe easy to chip off, when they could have saved all that effort and just shovelled it in the first place.

Bill736
12-22-2010, 12:46 AM
True, 50 square meters is a small area. More conventional alternatives such as a snow blower would seem a better solution. Blowing and drifting snow plus very low temperatures might complicate your snow melting energy calculations, plus the problem already pointed out that you have to have somewhere planned for the melted snow to drain before it refreezes.
If I apply the same energy requirements to your driveway that I apply to my 20 square foot fish pond to keep it deiced during a snow fall at around 25 degrees F, (450 watts), I come up with about 11 KW of power.

Willy
12-22-2010, 12:58 AM
The killer problem is you need to EVAPORATE 100L not just melt it. Otherwise you end up with an ice rink just off the lower ledge. That might be much more dangerous than just light fluffy snow on the driveway.


At a 100 liters per hour the process of what to do with the melt water becomes a very serious problem after a 6 to 12 hour storm.
Storms are often followed by high pressure systems where wind and dropping temperatures are a very real concern.
The water must initially be drained or the threat of ice dams becomes a certainty.
But maintaining adequate temperatures in the driveway under windy cold conditions my overtax the system's ability to cope. The pump's flow rate would have to be increased during these conditions. Which would either mean increased complexity of controls, manual baby sitting, or an icy driveway.

A simple roof would keep the driveway relatively free of snow and ice. It would also be easy to install and be virtually maintenance free.

The construction of a heated driveway can be successfully executed, but unless you've got lots of money to throw at a very temporary problem, there are more cost effective alternatives.

RB211
12-22-2010, 01:16 AM
This seems like more fun!
http://www.youtube.com/watch?v=tPg1ZMiC9pA&feature=player_embedded

zerodegreec
12-22-2010, 01:24 AM
http://www.engineeringtoolbox.com/snow-melting-d_1082.html
http://www.heatdrive.com/

Refrigeration Mechanic (in Europe and Australia we are titled as Engineer's) This is done allot around here for commercial buildings and wealthy home owners.

Most systems use a glycol loop off of a boiler. Proper ground work is essential for proper operation.

BTW I am in Calgary Alberta, Canada. we get chilly winters with a fair amount of snow (average -15c but we get -30 typically once a month and a week of -20c is not uncommon)

have fun :)

Ian B
12-23-2010, 04:56 AM
All,

Thanks for the replies so far.

To answer some of the points; I'm English, living in the Netherlands (Friesland actually). Yes, the water table is very high - the tops of the slinky will always be below the water table, and use of slinkys is becoming the commonest method of capturing heat when using heat pumps to run underfloor heating systems, and they seem to be pretty efficient. Generally, these systems take water in at 10 deg C and return it to the slinkies at 2 or 3 deg C.

Pretty much all of the slinky systems are closed loops, using a glycol/water mixture, so freezing of the return water isn't a problem. That's the plan here. The open loop systems are usually water wells, but I'd need all sorts of permissions to drill one and then to return the water to a canal (you think that you're allowed to let ground water flow into a canal here? Ha!)

I certainly don't need to evaporate the water! The drive will have drains along the edges. We get enough rain here, and I'm not planning on having a lake every time it rains - melt water will run off into the (sometimes frozen) canals, as it does at present. The thin film of water that may remain shouldn't freeze, if the system keeps the drive just above the freezing point.

I can see that snow drifts would overwhelm most systems. That shouldn't be a major problem, as there are enough bushes & trees around to prevent major drifts. If the drive remains above the freezing point, then snow melts as it lands and the melt water drains away. Then it's a steady state - heat is fed in from the slinkies, and is lost in raising the temperature of the snow and melting it.

Willy, I liked the idea of a roof, but sadly the local planning department would find this highly amusing - planning is strictly regulated, and that wouldn't fit in with their views of what I can and cannot build. They even get to pick what shade of brick I can build with...

Jim, I can see that using a gas boiler to produce the heat will give horrendous heating bills - hence the use of low grade heat from the slinkies. Again, closed loop with antifreeze.

Vince, thanks for those calcs - I was coming out somewhere in the same region. 10Kw = 100m slinkies, not bad. That's about 700m of tubing at €0.50/m, plus a central heating pump and some valves etc. Installation will cost me some diesel for my excavator and some hobby time. Running costs will be power for the circulation pump.

Good point about the wind chill - reducing the wind on the drive seems like a good idea (evergreen bushes etc.)

Snow blowers. Fine if it's virgin snow. Sadly, this is a drive that gets walked and driven on (by visitors, postmen, paper deliver boys etc) and these footprints then tend to freeze into patches of ice that have to be chipped off with the blade of a snow shovel. I don't think that blowing on it will move the ice.

Clear it before anyone walks on it? I'm away a lot, and I somehow can't see SWMBO getting out of bed at 6am to run a snow blower! Oh, and use of any kind of machinery on a Sunday is highly frowned upon around where I live. I've been growled at for using an angle grinder inside my workshop on a Sunday before now. Hence my preference - if feasible - to go for a passive system.

Ian

Black Forest
12-23-2010, 07:14 AM
I have a friend that has a heated ground outdoor riding arena here in Germany. He heats the closed loop system with oil. it is a series of pipes underneath the plastic grid that has sand on top of the grid. He is in an area with a lot of snowfall in the mountains. It is quite nice and it keeps the snow off his riding arena. Of course he owns a big pharmaceutical manufacturing company in Switzerland and the 15 to 20 thousand Euro's per month doesn't seem to bother him. It would me.