The Homemade Heat Pump Manifesto

[superlen]

Jeff,

For full disclosure, I am an engineer. I'm electrical though, not refrigeration in any means. I do agree with you that some engineers I have met are very inside the box. The ones at McElroy were not like that. I hope I didn't imply that they were. They were very excited about my system and interested that I did it myself. However, they liked to give me hell about my loop field pipe choice & even offered to loan me some fusion equipment to use if I needed it. Sadly, again this was 6months *after* I had pipe in the ground.

To keep more on topic, and upate:

I mowed the pasture a few days ago so I could start to locate the loops. I do dread trenching the new lines out there as a limestone shelf runs throughout the area as well as power lines from my solar grid, phone line, city water line, well water line/power, propane tank line, septic line, pool drain line, & another power/ethernet to a remote dish.

Dodging the lines are easy (but annoying). However, the limestone shelf makes digging ****sloooow*** when you hit it. It also tends to make it difficult to get a good clean 2' wide trench as it breaks out the side. I have used a trencher before putting in the city water line and it keeps a nice trench but comes to a snails pace in the limestone. I trenched almost a 1/4 mile to my house (2' deep) in about 8 hrs. It was nice good topsoil, minimum of rocks, through two hayfields, & one creek. When I got within 75yd I hit the limestone and it took another 6 hrs to make that distance. This was with a large rental trencher (rider, not walk behind).

Hmmm. That spring water is starting to look better and better vs the loops.


Len

[AC_Hacker]

Originally Posted by jeff5may

With engineers in general, they tend to pull ideas back inside the box of what is standard and comfortable to them. When certain ideas or parameters...

Originally Posted by stevehull

I often point to the Apollo project and engineers who not only thought out of the box, but thought...

Originally Posted by superlen

...For full disclosure, I am an engineer. I'm electrical though, not refrigeration in any means. I do agree...

This thread is very valuable real estate.

As far as I know, there is no other place on the Internet where the nitty gritty of fabricating a homemade heat pump system can be found.

Any discussion that mindlessly wanders from this topic only serves to dilute and reduce the uniqueness and usefulness of this thread.

If you want to vent your opinions about anything, create another thread.

Your opinions are absolutely not welcome here.


Sincerely,

-AC_Hacker

[superlen]

-- LOOP pumping costs analysis --

To resurrect my existing loop field I have to install some new supply/return lines. I was considering pros/cons of different piping methods and in determining these I did some calculations for pumping cost & would like to post for others to follow/learn and obviously to double check me.
DISCLAIMER: When I ran these numbers, the resulting cost seems too small. So I either have an error in my math or logic, or I'm not accurately modeling the system properly. Any input is appreciated.

To recap, the system has 5 - 3/4" pex loops in parallel. Each loop is 300ft total, 150 out, 150 back. The loop field is 200' from the house. This is a closed loop system, so the pump only needs to overcome pipe friction, not actually lift any water.

The method I use to calculate pump cost is as follows:

1. Calculate the friction loss (ft of head) for the piping system @ a given gpm.
2. Use GPM & head to arrive at ft-lbs needed to overcome the pipe friction.
3. Convert ft-lbs to kwH.
4. Multiply kwH by electric rate.
5. Adjust for pump efficiency.


--Step 1: Total system friction loss

Using a 15gpm (target flow) add up the total pressure across the supply + the loops + the return. Note: there are 5 loops in parallel, but because they are in parallel, the same pressure is across all of them. Therefore, we only need add the pressure for one of the loops in our total system pressure.

Using either a table of friction loss for your pipe (easier if you just have one flow rate you care about), or flow equations (handier if in excel and want to check multiple flow rates) calculate the head loss of each piece of the system.

In my case they are:

200' @15gpm Supply 1.25" pvc 6.55ft
300' @3gpm Loop 0.75" pex 16.41ft
200' @15gpm Return 1.25" pvc 6.55ft

Therefore my total head loss is 29.52ft.


--Step 2: Find ft-lbs to move the water through the loop.

@15gpm, we move 15*8.34lbs = 125.1 lbs of water every minute through the loop.

Remember our loop has 29.52ft of friction so we are overcoming 29.52ft * 125.1lbs = 3693 ft-lbs of energy every minute of operation.

Lets multiply by 60 to give 221598 ft-lbs of energy needed for every hr of operation.

--Step 3: Convert ft-lbs to kwH
For this we need the conversion ratio and it is: 1kwH = 2655223.737 ft-lbs
Doing the math yields that for every hour of pump operation we use: 0.083457617 kwh every hour of use.
Multiply by 24 to get daily usage: 2.002982807 kwH every day.

--Step 4: Show me the money!
Multiply your daily usage by your electic rate, lets use 10cents/kwh.
This give us 20cents/day to run the loop field pump continuously.

--Step 5. Pump efficiency
Pumps obviously don't run at 100% so we must adjust for the pump losses. A typical number I found online was 70% so lets use that. Our new daily rate would be 28.6 cents/day or $8.58 for the entire month.

So the resulting $9 ish seems like a very small cost to run this loop field. Obviously, it would only run when HP is operating, not 24-7 so the total pumping cost would be less.

Note: to simplify the pipe friction, I did not take into account any bends or other obstructions in the supply/return lines and any elbows would add to the total head loss & of course up the pumping cost. Depending on how many, it might significantly add to friction loss.

Edit - I just found this page: http://www.engineeringtoolbox.com/wa...ts-d_1527.html with a handy calculator that mimics the same pumping cost I came up with. It takes as a variable, head loss, so if I made a mistake in calculating the friction loss in my pipe, it wouldn't be caught by this online calculator. This is a handy simple calculator however, if you have a pipe loss vs flow rate chart of the pipe you are using, this would quickly give you a ballpark operating cost.

Len

[stevehull]

Len,

15 gpm is a lot of water to move through the pipe. Did you check Reynold's number to see if you are approaching turbulence? You will not in the parallel loops, but you may well be in the supply & return pipes.

Any small radius bends (U-tubes) in the setup? Those add a LOT of resistance.

Remember, those tables/calculators generally assume laminar flow and turbulence takes a lot more energy to push a fluid.

That said, if you remain in the laminar range, then I believe your numbers are spot on.

Love Kirkoff's law . . . . . .


Steve

[superlen]

Steve,

I haven't checked the Reynolds number, but was assuming that it was not close to turbulent. I'll double check. FYI, the 15gpm was just a swag at maximum flow I might need to give me a max pumping cost to expect.

(Edit - using this calculator, I'm showing well into turbulent flow even for loops! That's how handy assumptions are. - in this case..wrong) http://www.gcisolutions.com/flow.html I would need to drop the loop gpm to less than .5gpm to get back into laminar. I'm thinking that calculating pumping costs for turbulent flow is more math to figure out.


No tight u-bends at all. It's out and back in 2' wide trenches.

The other thought I had was to scrap the 1.25" supply/return lines and instead just extend each 3/4" loop line the 400' (200' each way) needed to reach the house. This would give me an additional 2000' ft of pipe in the ground which would be awesome for the heat load, but cranks up the daily cost to $0.37/day (again at max 3gpm/loop which would NOT be that high with that much pipe in the dirt).

It's also 5 times the digging to go that route & as I mentioned above, my field/yard is starting to resemble a small city with the amount of underground widgits running in it. I would like to have all five loop lines popping up in my basement for experimenting, but I think I'll probably just run 1.25" supply/return as before.

Len

[AC_Hacker]

Originally Posted by stevehull

Len,

15 gpm is a lot of water to move through the pipe. Did you check Reynold's number to see if you are approaching turbulence? You will not in the parallel loops, but you may well be in the supply & return pipes.

Remember, those tables/calculators generally assume laminar flow and turbulence takes a lot more energy to push a fluid.

That said, if you remain in the laminar range, then I believe your numbers are spot on.

Love Kirkoff's law . . . . . .

SH,

Are you implying that Kirkoff's law pertains to turbulent flow?

Also, if the purpose of the loop field is to operate with minimum power, then laminar flow would be the way to go.

However if instead, the purpose of the loop field is to transfer heat from the ground to the thermal transfer medium, and ultimately the heat pump, then turbulent flow should be designed-in to the system.

From what you have written, you seem to be advising Len to design for laminar flow. Any engineer who works with thermal transfer knows the advantages of turbulent flow.

Here's a page that explains what's going on, in simple non-technical terms, and why turbulent flow should be used for thermal transfer.

-AC

[superlen]

Steve,

I'm still a little confused on the power calcs as it pertains to turbulent vs laminar. The equation I used (or tables supplied by mnf which match my numbers) give out head loss vs flow rate. So aren't those head loss numbers already taking into consideration laminar vs turbulent? If so, then the equations would be valid no mater what range I'm in, wouldn't they? After all, once I have head loss, then ft-lbs is ft-lbs is ft-lbs.

AC,

I agree that the turbulent flow will transfer more heat. I'm struggling with the thought that is stuck in my head that a lot of one's potential COP is wasted with pumping power. I've seen that sentiment posted before and I have always assumed that pumping cost was a major cost of the energy in a geo-thermal. Obviously not as large as compressor amps, but still quite high. Using my example above, it doesn't seem to be as much as I suspected, so I think that minimizing loop pumping power, while certainly a design goal, isn't a great worry. Particularly if you drop below turbulent and tank your thermal transfer.

From the online calculator I checked, it looks quite difficult to get laminar flow with the typical ground loop sizes and flow rates I see used. The flowrate would need to be so low, that hardly any heat would be moved.

Len

[stevehull]

AC - you make us laugh!

The take home point is that you cannot have turbulent flows in any of the distribution fields as Len has described. I am wondering about just his supply and return pipes. In fact, I doubt if there ever is anything but laminar in vertical loops as well.

But that is my perspective based on some years of education, field and laboratory work.

You may want to consider the inlet diameter ratio where pipe velocity decreases in a parallel pipe network. In general, at about ten diameters downstream, turbulent flow becomes laminar. This is an accepted rule of thumb (is still on the EIT PE test) and is certainly accepted in straight pipes. A "loopy" horizontal field is still going to look pretty much like a straight pipe from a hydraulic perspective as the ratio of the loop radius to pipe diameter is so large.

This wound mean that all our loop fields (vertical or horizontal are likely laminar and would be very poor at transferring BTUs (your opinion).

Oh my - another deletion from the thread!!

We can continue to have fun with this - and expand the number of replies to the manifesto - or we can have more salient discussions . . . .


Steve
aka SH

[AC_Hacker]

SH,

I would much rather have this discussion with someone more intelligent.

Things being what they are, this will have to do.

The above photograph is of the cover of the International Ground Source Heat Pump Manual, often referred to as "The Basic Manual." I doubt that you ever heard if it, and I am certain that you never read it, else you would be giving good advice, rather than the advice you do give.

The book is prepared and published by the Oklahoma State University Division of Engineering Technology, Stillwater, OK 74078. I think you have heard of that school.

The book is a compendium of information that has been distilled from theoretical studies and real world application that date back more than fifty years.

The book actually has the expert information that you claim to have.

For example, I would like you to look at the photograph below that is from page 71 of the manual.

Originally Posted by stevehull

AC - you make us laugh!

Who is the laughing "us" you are referring to?

-AC

[superlen]

Hold on Steve.. Don't drag me into this.

I think my horizontal loops ARE turbulent. According to the online calculator I used, my 3/4" 3gpm are highly turbulent. I'm all ears if there is evidence to say otherwise.

My loops are not slinky, btw. They are 150' out, one 12" radius 180 deg turn and 150' back in the same 2' wide trench.

Len