Geothermal Heat Pump Design 3

[lfc09]

I work for a MEP consulting firm and our client wants us to explore using geothermal as an option. My question is do MEP firms sub out the well design (open or closed) to a certified geo-exchanged designer? Is it delegated design for contractor?

 

[GMcD]

It depends. Many MEP engineers will take on the basic design and performance based specifications, but sub out the detailed soil conductivity tests and well drilling. The MEP engineer, in my opinion, should take responsiblity for a complete coordinated design that is based on a proper energy balance, heating and cooling load balance, and system operating parameters, and then work with a recognized geo-exchange design specialist/sub-consultant before tendering the job, so that the specific design is known, and the detailed performance is clear, so that Joe Smith Drilling Inc can bid on the job. There are far too many geo systems that have been "designed" (and I use that word loosely) based on rules of thumb which have resulted in permafrost, system capacity degradation, ground overheats, etc. etc.

The geo-exchange sub or contractor will have no clue what your building energy balance is, what your yearly heating/cooling load profiles are, or what the ground conditions are unless the MEP engineer gathers that information first and at least provides a clear scope design for the extent and performance requirements of the geo system side.

 

[Wareagl487]

GMcD knows the way it works - Nice Post!!

Andy W.

 

[ChrisConley]

When we were first asked to design a geothermal system we rolled up our sleeves and decided to learn how to design geothermal systems. We purchased design manuals, contacted 'experts' and harrassed manufactuers for quite a while before even starting the project.

One of the largest issue is the design of the field. We hired a geothermal installer to drill test holes and conduct the conductivity tests. We purchased software to help with the field design, and then completed 'spot check' hand calculations to verify the software (tedious, but I believe absolutely necessary).

We were able to tender the project out with enough information that the installing contractor had little to do besides actually install the field. This was particularily relevant, because most of the installers in our area have extremely limited background with systems larger than residential (leading to the problems GMcD mentions).

 

[AbbyNormal]

have you guys seen loops that would not that or would not take the cooling load?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[GMcD]

AbbyNormal: Cooling Load - Not in my area, which is heating dominated, so I've seen, and know of a few permafrost situations. The only one I know of "up north" here is a horizontal field that was undersized to start with where the system was allegedly sized for a balanced heating/cooling load with supplemental heat (condensing boilers) for winter peak operation. What happened was that the heat pump plant couldn't make the design cooling load as the ground temps and condenser water had gotten too warm so the heat pump ran outta jam, and the boilers were coming on earlier in the heating season than anticipated. That was just a poor application with not enough checking and allowances for soil conductivity and "nickel and diming" "value engineering".

 

[AbbyNormal]

value engineering is another term for "why doesn't it work"

Going back more than 20 years now, climate master ( american not the former climate master out of edmonton swallowed up by EngA) were the first to make the ground source push in Manitoba?North Western Ontario area.



Now, speaking in terms of residential applications, Climate Master was pushing a low 'feet per ton' and relying heavily on making ice underground to get the latent heat.Signle pipe at 4 ft, 300 per ton (1.25" HDPE) two pipes in a 6 foot trench (400 per ton)

As a contractor who went back to school, halfway through an engineering degree, I exceeded what clmate master said by 100 feet per ton and had the coldest loop I ever did.

I took a Kelvin Line source appraoch, while still a student. Some ASHRAE guide that Lakehead's Library borrowed from U of T's Library for me. With 2 pipes down 6 feet, it calculated out around 640 feet per nomial ton. I ennded up installing 600 per on about 5 other projects that year, and typically used 200 feet per ton more than my competitors.

Buried temperature probes in the ground on one project.

What were my conclusions, well in a severe cold climate with moist soil and good snow cover, you can put all the pipe in the world undergorund and you seem to hit an assymptotoe of the freezing point of water. Loop temps dropped down and hover around freezing for a significant portion of the year, then you hit March and you break the assymptote.

Also verified 'snow is a great insulator' as far as frost depths are concerned.

I think with wet soil you can get away with less pipe than a hotter climate needs for cooling.

Never had to consider taking a higher heat of rejection in cooling, was never a residentail problem.

Back then was waterfurnace guys in the CEEA, and all kinds of methods were being used to install smaller heat pumps. They had a balance point method that tried to incorporate solar/internal gains and construction quality to reduce the size of heat pump needed.

My view was smaller heat pump, less expensive unit, less pipe in the ground, lower costs and sells more heat pumps.

In fact the arguement of oversized cooling drives up RH and creates a meat locker was used. I targetted 80% of heat loss and even had one customer sign off and make me install one at 100%, not caring about the risk of driving up RH in summer.

So typically every geo I did was grossly oversized for cooling. No problem with high RH.

2 main reasons

1) Most of the homes were R2000 or close to, very air tight not much infiltration to worry about

2) Do not get the big ambient dewpoints a muggy day the dewpoint breaks 60, big deal



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

Interesting post Abby, my impression with the residential contractors in my neck of the woods was that they were unintentionally undersizing the loops, I didn't know that freezing was actually a goal. Wouldn't frozen ground around a pipe become pretty insulating?

As far as a field that wouldn't take the cooling, I'm in the same boat as GMcD. We've actually added 'trickle charging' systems to project that use a solar device of some kind to add additional heat to the ground in the summer to try and balance the loads.

 

[paultg]

Pretty much what GMcD said has been my experience. I work for am MEP firm in MA; my first geothermal project is in construction as we speak. We worked together on the project and coordinated the "inside the building" design with a recognized geo-exchange design specialist/sub-consultant out of PA. That included but was not limited to annual heating and cooling load simulation information, peak load calculation for heating and cooling, equipment selections, etc, which became difficult with the special equipment on the project.

It was an extremely interesting process, and the system is a "hybrid" closed-loop design with 400 wells, semi-custom energy recovery and VAV heat pump RTUs (the largest is 50,000 CFM), a cooling tower, and about 1000 vertical WSHPs that serve 900,000 sqft mixed use residential campus.

The "on" switch will be sometime a year from now, should be interesting!

Paul G.

 

[AbbyNormal]

Chris

You froze the water below grade to get the latent heat

Residential closed loop systems were always rated at an EWT of freezing

Open loop units 50F EWT

I watched some run, earlier in the heating season, Beginning of cycle the EWT be up around 40 and slowly dropped down after something like a 45 minute run time the temps would be down around 36 F

System cyles off, comes back on in say 15 minutes time, and the loop temp is right back up around 40 again.

Here are some numbers from 1990/91 comparing EWT after 1 minute and 20 minutes, "600 feet per ton" November 4 start up, predominately sand, not into the water table that much

Date EWT after 1 minute after 20 minutes

Nov 4 43.0 41.6

Dec 7 37.9 36.5

Dec 15 35.6 34.2

April 14 32.2 31.6

Here is another one with 600 feet per ton, varied from wet clay, sand and rocks

Oct 23 44.2 43.2
Dec 15 32.2 32
Feb 12 N/A 31.6
Apr 14 32.0 31.6

Nov 4 43.0 41.6


Here is some info on the Climate Master with the shorter loop, it was an earlier start up, clay, major water table

Oct 15 43.8 42.1
Nov 15 41.3 39.6
Jan 5 N/A 29.8

This winter was a little colder than normal, there was snow cover although less than typical

I skipped a bit of school that year starting up systems




Take the "V" out of HVAC and you are left with a HAC(k) job.