DIY ground source heating

[markzoom]

Hi folks, I've been thinking about a simplified ground source heating setup for DIY and would value constructive and educated input.

You hopefully already know how ground-source heating works: Water is pumped through buried pipes and fed through a heatpump to concentrate the collected heat.

My simplified method would be to do away with the heatpump and water systems and suck (not blow!) air through slightly larger diameter buried pipes directly , with a constriction at the intake end. Since the air would be at a lower pressure inside the tube it would heat up as it regains ambient pressure indoors (similar to how heat pumps work). The air would obviously be fed over the pump/fan motor to salvage heat from that too.

(At daytime the air could be sucked through solar boxes which are warmer than the ground would be, but my main concern is night time heating.)

My rough calculations say that the low pressures achieved by a high powered vacuum cleaner are adequate (remembering that degrees Kelvin must be used for the maths). Obviously I don't plan to use a vacuum cleaner for anything other than tests but a purpose built vacuum pump.

So the big question is: Could this setup approach the energy efficiency of an air conditioning unit or existing ground source heating system? Mechanically it would be a much simpler system to build and maintain.

The second idea is for a refinement to above: Instead of sucking air through a constriction, how about recouping that energy by powering a smaller air motor from it and mechanically returning some energy to the vacuum pump? In practice, the unit would be designed to do both, running on a single shaft with the air motor being a slightly smaller capacity version of the vacuum pump doing the suction work.

[markzoom]

[geothermaljones]

Figure in the energy required to make the air move through those "slightly larger diameter buried pipes" You will need to push & pull the air at both ends. There's a reason they're called blowers, not suckers... The Static pressure will be a killer, & blowers don't handle Negative Suction Head Pressures as well as a pump.

High velocity systems use short lengths of small duct, & need much larger blowers to do it. A typical 3-4 Ton High Velocity system can require a 3/4 HP motor (Shhh, that's a secret) or more to deliver the air, in contrast to a standard low static system using a 1/4 to 1/3 HP motor. Variable speed is not an option either... They also have that huge short return connection so it can use near every bit of its power to push that air.

In geothermal systems the tubing can be anywhere from 300'-1000' in length, per ton. The more ground contact the better, so they have to be separated & buried deep. Vertical loops contact much more ground that horizontal "slinkies" so the tubing is much shorter, 300-400' vs. 800'-1000'. Upsizing the tube will get you more surface area ground contact, but the price of buying & burying bigger pipe will need consideration. Vertical loops would be out of the question.

Since Air has a HC (heat constant) is roughly .075 vs. the HC of 1 for water, this means a pound of air can transfer a lot less heat than a pound of water... & the air will occupy roughly 12-14 cu.ft volume per lb, vs. the pound of water only taking the space of this nice cold beverage next to my keyboard.

Since air can be compressed & decompressed, blah blah blah...

Take a look at the 3-6" tubes installed below walk in freezers & the like. They keep the ground from freezing & frost heaves from busting up the floor. & they only need to deliver about 2-4 btu/sf...

Of course I could be wrong...

Now let's talk about those outfits burying soft copper & pumping refrigerant through it...

goodluck geothermaljones st.paul,mn.

[markzoom]

No. The whole point of the setup is to have negative pressure air in the tube which gives off the gained heat when it recompresses to ambient on exit past the vaccum pump into the house. There can't be any "pushing", for heating it's got to be suction only. The tubes can be paralleled to make suction easier.

For heating the air has to be pulled through the tube to create the negative pressure needed for the heatpump effect.

Yes, point taken. The heat capacity of air is pretty bad and it takes more energy to move the heat around in air as it does in water.

Using refrigerant isn't an option at the place I want to do this, though Butane and Propane are easily available. I want to avoid fiddling with gases.

You seem to be savvy with geo, there's a slim chance that might be of use as the place is partially low activity volcanic (Azores). Any tips for DIY slow drilling of f%^%ing hard basalt? Hiring a drilling rig is out of the question so it would have to be be a purely DIY effort. The only tube available is galvanised steel water pipe, scaffold and plastic. An Ultrasonic drill bit crossed my mind.

[Bruce Richmond]

It can be made to work either way, with vaccum or pressure. All you need is a pressure difference. When you compress the air in the room it gets warmer. Let that heat disipate into the room so that the compressed air is back down to room temperture before it leaves the room. Allow the air to expand in the ground tubes, recovering some energy with a turbo fan if you like. The expansion will lower the temperture of the air enough so that it can absorb heat from the ground before returning to the compressor.

The only reason I can see for going with vaccum is that it allows the living space to be at ambient pressure. It does seem like a good idea after thinking about it though. It eliminates the need for any heat transfer from the hot side of the system to the room's air since the room's air is the working fluid of the pump. Even though the room is at ambient pressure it is the high pressure side of the system.

geothermaljones is correct in what he says about blowing vs sucking, but I think the "Negative Suction Head Pressures" will be low enough so that it wont make much difference. On the calculation of that pressure you used 323/290 to come up with .9 Bar. 13C is 286 Kelvin. Why so warm on the hot side? Normal room temperture is more like 23C or 296 Kelvin. That would give a pressure differential of .034 Bar. While you would need more to get a temperture difference for heat transfer through the wall of the tubes it shouldn't take anywhere near .9 Bar. Along the same lines, heat transfer is better when the air is under pressure. More molocules contact the surface in a given time. But the pressure difference is so small it shouldn't make much difference if you are + or - .05 Bar from ambient.

Biggest problem I can see is the cost of installing the buried pipes.

Using air as the working fluid should not be a big problem. A Sterling engine is like a steam engine without the liquid phase. It produces mechanical energy by allowing heat to flow from hot to cold. If you reverse the cycle, putting mechanical energy in, you can transfer heat from the cold side to the hot side.

Check your actual ground temperture. You may find it higher than 13C.

[geothermaljones]

I give up...

How can you draw a vacuum on pipes of this size length without a huge pump? Have you ever used a vacuum pump on a refrigeration system? These units are built like tanks, weigh a ton & have huge power to draw down very small (relatively) quantities of copper linesets, You plan to do it how?

Key to swapping heat is based on a difference in temperature, the larger the better. At what pressure in side the pipe will you be able to transfer 12,000 btuh of heat to any amount of 60dF air in order for it to deliver that ton of heat to the space at say 80dF?

Burying the pipe in Basalt will incur a massive expense. Even a conventional system will prove more cost effective in the long run. VFRZ with inverter drives will do even better.

I've always been intrigued by Sterling engines, but haven't spent the time on learning the process... Dean Kamen has one in his home doesn't he? More as an inspirational ornament, than an actual tool.

goodluck geothermaljones st.paul,mn.

mechanically

[Bruce Richmond]

Let me say up front that I am not a professional in this field, but I have studied thermodynamics and know how heat engines and pumps work.

In the refrigeration system you are working with much higher pressure differences than we are talking about here. Given P1/T1=P2/T2 with a room temp of 73F and a tube temp of 37F to draw heat from the 55F ground with the room at 14.7 psi the tube would be at 13.7 psi. If you want to use a blower just blow the air out of the tube into the room ;) It shouldn't take all that heavy duty a blower to deal with 1 psi of pressure. How much power will be needed will depend a lot on how much actual flow is taking place.

Delta T is part of the equation, but not all of it. The surface area of the pipe is also a big factor. We established a 1 psi pressure diferential to provide air at 18F deg colder than the ground. We should be able to estimate the surface area needed from there. Going higher on the pressure will require less surface area in the pipe but will require more power to maintain the pressure.

Why heat the air to 80F? We only want the room to be at 73F. The nice thing about the set up is that no temperture differential is needed at the hot end. The heat doesn't need to pass through a heat exchanger, it is just released directly into the room.

There is no sand/soil on the island? The OP should be able to figure out how much work/cost is involved to bury the pipe after calculating the surface area needed.

Sorry, I don't know who Dean Kamen is.

Bruce

[markzoom]

If you check an ordinary blow heater, the temp is much higher than the required one because the air gets mixed with the cool air in the room (cooled by the walls).

That's my point, all that's needed is a centrifugal sucker, no waterpumps, heat exchangers, heatpumps, control system etc. The only thing to wear out is the bearing on the motor and the on-off switch.

[.p.jm]

You must be f****ng smart. You just invented free heating.

When the rest of the world finds out, BOY HOWDY are they gonig to be impressed ! They never thought of it !

Carry on.

[markzoom]

Not quite, but there's a chance I've invented something more energy efficient (in many conditions) than a bar fire and much simpler and reliable than anything using refrigerant. Furthermore it's not patented, so anybody with diy skills could make it, courtesy of me.

!!!

[.p.jm]

Well, so much for any hope of you being a businessman ....

!!!

>

[markzoom]

Already my dosh with another invention, thanks, which is why I'm semi- retired at 45. You might not be able to grasp this but my interest now is to A) pay out as little as possible to energy companies and in the process hopefully: B) come up with green energy stuff for the masses.

The thing is there are/were plenty of ideas out there but greedy wankers wanted to make filthy lucre from it, so the ideas either never made it to market at all or they got snapped up by other firms and made too expensive to be an alternative.

If something is any good on the green energy scene it's far more important to get it out there than for one little twat to *maybe* make some lucre from it, so I don't mind at all if people copy such an idea or even improve on it.

[snip]

[.p.jm]

Would this include your 'previous dosh' ?

You don't mind ignoring the basic laws of thermodynamics either, it seems.

Here's rule # 1 for you : There aint' no free lunch.

( unless you're a public school kid, maybe )

[Joseph]

A public elementary school here has an eating program while school is out for the summer!

M-F Breakfast and lunch free to anyone under eighteen years of age.

[no spam]

Call my lawyer, I'm suing!!! Age discrimination. Age discrimination. Age discrimination.

[no spam]

Ah, to see a find mind working the art of debate. I wonder if google could find that quote about vulgar language being the sign of a small intellect. . .have to check on that. .