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DX Geothermal systems

Started by Thehardway, January 13, 2014, 12:13:22 PM

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Thehardway

So I found myself involved in research on a large Commercial Geothermal proposal as an alternative to conventional gas boiler/electric chiller and rooftop units.  As I went through the numbers, the same issues kept popping up which seem to be the problem most folks face when considering geothermal heat pump systems.  Cost of the ground loops, and space to install them.

Geothermal heat pumps are probably the most efficient means of both heating and cooling a building in temperate regions.  So why aren't more people using them? Cost and space.  Up until recently, most geothermal systems were based on circulating water through ground loops or open loops in wells and open loops in wells are usually frowned on due to potential contamination of the aquifer.  Plastic pipe which is a relatively poor conductor of thermal energy (compared to copper) was used and the loops we either in expensive, vertically bored holes with difficulty in predicting output due to varied soil conductivity, or in horizontal loops which required reserving large amounts of real estate and a lot of directional boring or open trench digging at a large expense.

After realizing both these methods are project killers for any capital cost comparison, I began looking for alternatives and found the small world of DX Geothermal.    It seems that Geothermal HVAC contractors are falling prey to the same pitfalls as all the rest of the HVAC contractors.  Ask just about any local HVAC contractor you can find that does Geothermal and they will automatically presume you are talking about water source geo. It has become their bread and butter and they don;t like to change their profit margins or recepie.  Ask them about DX geo and they will likely look at you as if you had a third eye in the middle of your forehead or give you a song and dance about potential ground contamination as an excuse for their ignorance on the topic.

DX geothermal can be done much cheaper, with much less space, much less land disturbance and is more efficient, has fewer moving parts and requires less maintenance.  Environmental risks are no greater than with other ground source systems.  It just takes more attention to detail and better quality installers.

It works like this,  Instead of installing thousands of feet of plastic pipe and circulating water/glycol solutions through the ground with circulation pumps and then having an exchanger to a gas based refrigerant system (in essence two independent systems),then having to distribute the air from  central unit to point of use by means of ductwork,  the DX Geothermal system uses a compact bore field with copper pipe installed.  It circulates a refrigerant through the copper pipe rather than water/glycol using a compressor.  The copper pipe and gas is much more efficient at thermal transfer.  In addition, the gas can be distributed to remote units similar to a ductless mini-split system.  There is one compressor and no water to have to keep conditioned and no circulating pumps to power and replace. 

This is a super efficient system and can be installed with a small, directional boring machine.  The bore field for the average home (up to 7 tons of AC) would take up an area no more than about 12'X 12' and approx 125' deep.  With a de-superheater they can supply most of your residential hot water needs as well.

For folks interested in energy savings or building off grid, these offer some very interesting options and they move geothermal prices into the average working mans budget on up-front costs.  The ROI is very good.

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Al_Smith

The bore field for the average home (up to 7 tons of AC) would take up an area no more than about 12'X 12' and approx 125' deep. end quote

Is that a miss print? Reason I ask is that is a lot of cubic yards of dirt not to mention rather deep.In this area that would be in the bedrock .

Now the idea sounds great but the hole size makes me wonder .

BTW my geo is just a conventional ran off a well no buried lines .

samandothers

Given a 7 ton system if a conventional Geo unit with plastic pipe and water/glycol solutions what is the percentage reduction in piping if using the gas with copper?

Wellmud

I believe they make several angled bores inside the 12x12 area.
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Thehardway

Sorry for misleading, they do not excavate a 12' X12' hole 125' deep.  That would be a pretty expensive undertaking.  What I meant was, they can install the bore field in that amount of real estate.  Most people have at least this much space on their property.

Usually they set up a directional boring machine and drill one 2.5" -3"  hole straight down, Each hole represents 1 ton or more of cooling capacity (varies with soil heat conductivity, water table, etc)  then almost from the same setup position they make several additional slant bores. Bedrock is not an issue as it can be bored through and is acutally preferable to some other geological formations as it is dense and conducts heat fairly well.  Where you run into trouble is with dry soils that are loose or have a lot of annular space.  They conduct poorly and saturate quickly.

125' "U loops" of copper pipe are then placed in the bore holes and a conductive grout material is pump in to fill the void around the pipe.  All U-loops are then tied into a central header which uses a common supply return line to the compressor unit.  The typical reduction when comparing DX to GSW is 50% reduction in bore diameter, 60% reduction in bore depth.

There is also a system called Kelix which claims 5T of cooling capacity in a single 6" X 300" bore using an aqueous based solution.  These comparisons are all closed loop.

An open loop system with a high water table is obviously much more efficient than either as the ground water carries the heat/cooling away into the aquifer but they are not permitted in all areas due to "potential" issues of possible aquifer contamination.  There have been some pretty expensive lawsuits where neighbors claim their water supplies were negatively impacted by a nearby open loop geo-system.  This is much like the fracking issues.  Many of the claims are anecdotal and not supported by evidence but a few bad apples where it is real, ruin the whole barrel and give reason for regulation and concern. 

The way to think about DX is that it is the equivalent of having the outdoor coil unit of a mini-split, buried in the ground in a perpetual ambient temp of 50-60F with optimal transfer conditions and no electric fan needed.  What would that do to the efficiency rating? 

Is your system open loop Al?  Does it circulate pure well water or does it have a closed loop with Water/ethanol or Water/Glycol circulating in it.

Likewise, closed loops in ponds or lakes can be cheaper to install and maintain as they require no boring or grouting ans in cases of problems the loop is easily removed and replaced or prepared. Most folks don't have this as an option though.

The whole concept tempts me to try hacking a 1ton ductless minisplit and turning it into a 1 ton DX geothermal.  It would certainly be cheaper.  Geo units seem to carry inflated price tags and are treated as luxury units.  Mini splits are very competitive.  Or better yet, get an old refrigerator and re-pipe the coil to a buried copper coil.  Lets see how efficient we can make that fridge.

There are some issues with needing a refrigerant oil trap so it does not all go to the bottom of the coil and cause issues and a refrigerant that is not prone to fractionation is neccessary.

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Wellmud

Early open loop geo-thermal systems (often called pump and dump) used a 2 well system, one to pump water from and one to discharge the water back into. These seldom worked for very long as the second well receiving the discharge water would plug up with iron and just run over the top, now most open loop system discharge into a pond or just into a tile. What most people dont take into acount on open loop systems is that the well pump pretty much runs 24-7 and on average will have to be replaced about every 5 years.
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Sonofman

Wellmund, around here they just call that a water source heat pump. I looked at it a little 2 years ago when I replaced my heat pump, but the unit I was looking at needed 1.5 gpm per ton. I needed a 4.5 ton unit, and I do not know for sure if my well could handle it, but I did not want to put it through it. Seven gpm might not sound like much, but like you said, the well pump would be running when the heat pump was. Think of filling up seven milk jugs in a minute. That is a lot of water for a residential well.
Located due west of Due West.

Al_Smith

I think this system in discusion is very inovative and interesting.

Now let me describe two systems I am totall familar with which will take several posts because I haven't mastered the forum software and it always shuts me off .

My system is open loop from a well .However it does not run water 7 and 24 365 .It has a 20 second lag from the time a temperature setting is detected which opens an ASCO valve prior to the compresser starting .The water is throttled though a valve to around 5 gallon per minute water temp 53.9degrees F .

Al_Smith

More .A post run water ciculation is continued for three minutes after the compressor shuts down because in theory you could freeze it up with no water flow .The discharge temp is 43 degrees heating and 72 cooling .Above 75  on cooling it shuts down and below 40 on heating it shuts down .

I designed the water management portion of the refridgeration controls myself .Some systems use a modulating water valve that works of compressor head pressure .Real old system sometimes did use constant water flow 

Al_Smith

Now a giant system .I work in an auto engine assembley plant making aluminum engines .When working with aluminum temperature control is paramont to get a good product .

The system at work is a closed loop .30 inch liquid lines cooled from two 10 acre quarry ponds spring fed .Believe it or not the bottom water temp is below 42 degrees .

At the moment it strictly water to water heat exchanger .During summer there is a huge refridgeration system involved .Huge is over 2 million sqaure feet of building plus associated machine coolant

Al_Smith

Now the problem of using two wells .You have cones of convergence and cones of divergence .It is next to impossiible to suck water out of a well and stuff it all back in another one .Some will fit but not all of it .

Wellmud

Sonofman, you are right, it takes a pretty good well when the same well is used for the geo-thermal as well as for the domestic uses. If your geo unit uses 7 gpm, the "average" home at peak demand also uses about 7 gpm, that puts a 14 gpm demand on your well, thats usually not a problem in my area but that is a lot of water in some places.
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submarinesailor

Just out of curiosity.  Has anyone installed or used this product:  Slim Jim Geo Plates for lakes and ponds.

http://www.awebgeo.com/Slim_Jim_Geo_Lake_Plate_Home.html

Bruce

Al_Smith

I think from the general desciption they are just talking about a multi plate heat exchanger .If so I'm familiar with the style .Fact the large system at my place of work has 6 or 8 large ones on that huge system .They are not all in service at once .Perhaps 4 or 5 with several in stand by and allowing several to be dissassembled for routine maintainance .These are probabley the size of a small pickup truck .

Thehardway

It would probably be best for us to define some terms here as I have seen some used incorrectly in previous discussions of loops.

For those unfamiliar, or new to this discussion, an open loop system is one that does not recirculate the same finite amount of water/ solution.  In the case of a geothermal heat pump, it takes water out of a well, pond or lake,  uses it as a heat transfer medium and then dumps it back into the same or or different location at a different temperature which is then dissipated by the earth and atmosphere.

In a closed loop system, a solution is used that is typically not pure water due to the potential of freezing as all of the heat is transferred out of it.  It can be Methanol, Ethanol, Ethylene Glycol, Propylene Glycol, or in some cases Calcium Chloride.  These solutions are circulated within a finite loop to maintain a specific concentration and volume.  Heat exchange takes place with the earth in the earth loop and with a refrigerant gas heat exchanger at the actual heat pump unit.  Circulation pumps, pump the fluid in a continuous loop.

In some other discussion threads I have seen the term "open loop" used to describe non-pressurized hydronic systems.  This is inaccurate.

Al's system would be considered an open loop system.  It uses pure water without additive to transfer heat and then dumps the water. Water is not recycled without addition of new or fresh water.  The only potential for contamination would be if the exchanger at the heat pump unit began leaking refrigerant gas/oil into the water loop.  That contaminated water could be reintroduced into the ground water and pollute the aquifer.   

The primary limitation of this type of system is that the water temperature can only be reduced to just above the freezing point, or just below the boiling point. He has chosen 43F as the limit on the low end.  This prevents freeze up as well as prevents the well from freezing up and becoming saturated. (Saturation is the point at which the earth can no longer accept cold or heat)  Saturation has been known to occur where systems are inadequately sized, are run for extended periods of time without reversing heating/cooling modes or during extreme weather conditions.   

Using Antifreeze solutions allows lower temps and greater temp differential but risks freezing the water in the well or the moisture in the earth around the well.

The two well open loop or "pump and dump" system has issues in that the dump water is not absorbed into the ground as rapidly as it might be pumped in.  Its sort of like a percolation test.  You can deplete the water standing in a well quicker than it flows in from the ground and conversely you can flood a well with water quicker than it can absorb it back into the aquifer.  These are the cones Al speaks of.  There are a lot of factors that play into this, geology, hydrology, water table, etc.

Another factor is if the wells are too close together, saturation will effect efficiency.

Open loop well systems are still built, but there is considerable effort being made to ban them, especially in large commercial type systems.  I would not advise anyone to use this systems today due to potential regulatory restrictions and liabilty.

Closed loop systems which use a solution are also facing scrutiny as there is the potential for cross contamination with domestic water should the loop begin leaking into the well.  For this reason, Propylene Glycol is a popular antifreeze as it is perhaps the least toxic of the solutions and can be consumed without immediate adverse effect.  I would not want it in my drinking or bathing water.

Enter  closed loop DX Geothermal.  Refrigerant gas used in the system can be compatible with domestic water and non-toxic to the environment.  The cutting edge of DX geothermal is using "green" refrigerants such as R744 (CO2).  Worst case scenario, you will get sparkling water in your well.

Good discussion.  Hopefully people will learn that Geo systems don't have to be big and expensive or risky. 
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Al_Smith

Bore holes weather water filled or this direct gas transfer obviously take up less room .

Horizonal trench systems in my part of the country require 700 liniar feet of loop per ton of refridgeration .Figure it out ,6 tons times 700 feet .8 tenths of a mile of trench and pipe ,mercy me .

Al_Smith

There's basically nothing new regarding the use of refridgeration to provide heat and cool .Fact the early geo system simpley used a water A coil as a cooler and refridgeration on the heat side .

Some of the larger grocery stores captured the latent heat from their chillers and coolers and helped heat the stores .

As time has progressed in this field while the basics have not changed the technoligy has provided much more energy efficient systems than previous .

NHMike

We installed an Open Loop system in 2010.  I did not have the room for Closed Loop system.  There were no companies doing the DX system in my area, and that was just starting to make some buzz around here.

I have NO regrets on our system.  My cost for heating 6 months ( Nov - Apr ) has never been higher than $750 for Both the geo unit and the well pump.  We keep the house set at 72.

I agree, that Open loop does not work for some locations, but for me it has been Great.

I am surprised that it is taking so long for this technology to catch on.

Wellmud

I drill water wells for a living, all kinds of water wells from 5 inch residential wells to large diameter irrigation wells, as well as pumping systems (this supports my sawing hobby which is why I am here ;D). I don't know alot about the actual geo systems just the water source that is required to run the open loop system. Many open loop systems are still installed in my area, I just done the water system for one last week. We have heating contractors in the area that tell customers that the open loop system is more efficient. I have no idea. Personally, I don't like to see the water wasted as I work some areas where people barely have enough water for household needs.
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sperry

Same here Wellmud. This is my 44th year in the Drilling business. I have to say though that cutting lumber is much more fun.

Wellmud

sperry, sounds like we have a lot in common. I started going on drilling jobs with my dad when I was small, worked every summer with him, my son done the same with me and works with me now. I really enjoy my drilling work but if I were not in the drilling business I would certainly be in the wood business.
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hvac101

DX (Direct Exchange geothermal systems are a very considerable option to look at when looking into geothermal systems for heating/cooling and domestic hot water usage.  They are very efficient.  I am a fan and believer in DX technology.  I would recommend anyone interested to go check with your local public library and see if they have the book (many do) by Donal Blaise Lloyd called "The Smart Guide to Geothermal."  Very good book you can understand.  The author explains both water source and DX geothermal.  This book was published in 2011 so the information is pretty current.  Check out chapter five which goes into DX geothermal, it's advantages over water source and how it is different.  I found the book to be good reading.  Can be bought on-line also through various sources.  BTW:  The HDPE pipe used for water source gives 2.7 BTU'S per square inch and copper gives you 19 BTU'S per square inch of heat transfer.  DX has one less transfer process than water source and less complicated in how it works.  This all adds up to better performance and efficiency with less maintenance and repairs.  Isn't that what we all look for?

hvac101

Quote from: Al_Smith on January 16, 2014, 11:24:18 AM
Bore holes weather water filled or this direct gas transfer obviously take up less room .

Horizonal trench systems in my part of the country require 700 liniar feet of loop per ton of refridgeration .Figure it out ,6 tons times 700 feet .8 tenths of a mile of trench and pipe ,mercy me .

hvac101

Horizontal trench type fields with DX geothermal in your area only need 250 feet of 1/2 copper piping per ton of capacity and can be laid in a 125 foot trench four feet wide with the loops on each side of the trench.  One trench per ton needed.  There are various patterns allowed and two types of horizontal fields.  That being horizontal pit and horizontal trench.  Each takes different lengths and diameter of copper tubing loops with various patterns to choose from to meet your space and application.

Thehardway

HVAC101,

Thanks for your replies and let me say welcome to the FF Forum.  I have been doing some further study and you state the case very well.  IMHO it is probably one of the most efficient means of providing heating and cooling in any climate and offers the fastest ROI with less peices and parts to go bad and break.   The more I read, the more a fan of DXGHP I become.  It is a shame so many HVAC installers are not more informed on it and that it is not better promoted.  It makes me feel like some of them are purely profit motivated and willing to sell inferior equipment and systems to achieve a larger profit margin.  Becuase of this, most people still believe GHP systems require lots of land, deep wells, and a deep pocket.

Perhaps you can tell us where you hail from. Do you make sawdust like most of us here?



Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

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