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
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
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.
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
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,
The more ground contact the better, so they have to be separated & buried
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
& 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
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.
On Jun 30, 6:19 am, email@example.com wrote:
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.
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
Key to swapping heat is based on a difference in temperature, the larger the
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.
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.
On 1 Jul, 21:08, .p.jm@see_my_sig_for_address.com wrote:
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.
On 1 Jul, 23:22, .p.jm@see_my_sig_for_address.com wrote:
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
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.
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