Small and crude GSHP for a Michigan garage

[mrd]

2931 W is the spec under expected conditions, but submerging the evaporator in water could certainly change that number. If you know your test flow rate accurately, then you could plug in your actual dT and solve for Q as the variable..

[pick1e]

Originally Posted by mrd

2931 W is the spec under expected conditions, but submerging the evaporator in water could certainly change that number. If you know your test flow rate accurately, then you could plug in your actual dT and solve for Q as the variable..

Exactly right. Especially because I wanted do a quick and dirty test- I used a (relatively) large tub instead of making a small box simply because it was quick and easy, so heat transfer was quite inefficient. Plenty of water was flowing down the drain before giving up its heat.



This was taken this morning because I forgot to take one last night before firing it up. The water is extra dirty only because I used a dirty bucket from outside to scoop out the top bit of water so I could remove the drain hose without mess

It started to freeze up after about 15 minutes:



While I let it thaw I turned off the water supply and fashioned a cardboard shroud around the coil to keep the warm water closer to the coil. This gave me an actual dT of about 5.6°C and a dQ of 1955 W.

With this value I come up with a theoretical rise in temperature assuming no other heat loss from the garage (false) of 102m^3 at 5°C of 54°C per hour or 13.5°C per 15 minutes, about how long I ran the system at a time. Considering there is no insulation on the building and it's full of stuff, I'd say that's a fair calculation.



I still had no forced convection in the tub aside from the trickle of water which the evaporator was certainly not designed for.

But, it did work for a while which was cool for about 1 hour of effort putting it together and about an hour of testing. In 15 minutes it was able to raise the air temperature in the garage by 4ºF which was fair considering the garage is completely uninsulated at the moment, it was below freezing outside, dark, and the door was cracked to let the hoses in.

So I'm just about done! All I need to do now is drill some holes in the ground, build some ground coils, build some solar panels, reconfigure the AC unit with proper heat exchangers, develop a control system and insulate the garage!

[Daox]

Good luck with the digging. Let us know how it goes.

BTW, why are you digging now that its cold out?

[pick1e]

Originally Posted by Daox

BTW, why are you digging now that its cold out?

Because I have a bug up my @$$, as they say

I have a drive to do it because it IS so cold and I really need to be able to work in the garage. I am working on refinishing a dining room table in my living room- complete with tarps and window fans (it won't fit down the basement stairs). My girlfriend is really psyched about it

I started thinking about it seriously a few months ago believing there must be a better solution than electric resistive heat that sucks a bajillion watts. So now I'm bound and determined to make it work- hopefully before winter is over. Then I will bask in my warm garage.

[pick1e]

Originally Posted by Daox

BTW, why are you digging now that its cold out?

A more succinct answer: why not?

A nice crisp morning to start digging before heading to work. Here is my newly acquired post hole auger, the first in a long string of tools I'm sure:



Root at 9"! Easiest just to start over.



Started to get yellow clay at about 16"



And dry silt at about 2'.



With pea sized gravel in it, some larger gravel up to maybe 1".



Here are the soil layers (right to left from surface).



This was the biggest rock I ran into so far, about egg sized. The narrow auger seemed to push it into the soil, away from the wall of the hole, as well as steer itself around it (slight bend in the hole). After moving the auger end up and down to remove the soil below the stone seemed to be working its way loose so I just pulled it out before it fell to the bottom. Luckily it was only arm's length down. Hopefully I don't see anything bigger. It was recommended to me by a fence pro that if I hit something larger to abandon the hole and move on, it is just easier.



Time for an extension! This only took about a half hour so that was cool.



Pouring some water in the hole helped bring up the fine, dry silt. It turned to really heavy, thick muck when wet, so I think it is the same material as the upper layer, just dry.

While I was out and about I picked up a piece of pipe for an extension and used it later in the evening to get down to about 5'4". I'll pick up a longer piece tomorrow.

I measured the temps at the bottom of the holes non-scientifically at 35F at 9" and 46F at 36".

[Daox]

Haha, fair enough! Good to see progress being made. How many holes are you going to have to dig?

[pick1e]

Originally Posted by Daox

How many holes are you going to have to dig?

Not sure yet. "The best laid plans of mice and men oft go awry."

I plan on digging the first hole as far as I reasonably can, use that hole to conduct a heat test, then use that to figure out how many more similar holes I need.

[Ryland]

I would say that if you make "U" shaped loops dropping down then you should insulate between the leg going down and the leg coming up unless they are a good distance appart, otherwise you have your coldest water passing right next to what should be your warmest water, so a chunk of ridged foam separating them is a good idea.

[pick1e]

Originally Posted by Ryland

I would say that if you make "U" shaped loops dropping down then you should insulate between the leg going down and the leg coming up unless they are a good distance appart, otherwise you have your coldest water passing right next to what should be your warmest water, so a chunk of ridged foam separating them is a good idea.

Thanks for that suggestion. Any chance you have any references or data on this? It would be interesting.

I was reading something about borehole tube design saying that concentric tubes (small tube inside larger tube) were nearly equal in efficiency as a U tube since the warm water preheats the cold water. Also that a double U tube like W in one hole was more efficient than a single U but only marginally. Unfortunately I don't remember the numbers or where I was reading that.

I was planning on concentric tubes because the preheating makes sense to me, but so does insulating between the U tube legs to maximize the temperature difference like you mentioned. I would be willing to bet that their relative efficiencies depend largely on the desired difference in temperature between water entrance and exit.

It would be cool to test these designs against one another. Maybe I should install a different design in each borehole? I could design them with fittings to be able to test them separately for comparison, but then eventually link them together for the final system...

[Ryland]

My sources for insulating between legs is an engineer who's straw bale house I worked on, he had radiant heat floors with a bunch of different loops and valves so he could test different configurations, but he was the one that pressed on me how important it was to insulate between any supply and return line.
After all you are relying on the thermal conductivity of the soil, if it's not conductive then you want to do everything you can to keep the hot or cold that you put in the soil there, if the soil is a good thermal conductor then you have a greater chance of heat transferring from one leg to the other leg.

One other idea is instead of having a "U" shape is to have an insulated line run straight down then a loose spiral back up, that way you have more surface area.