Question on a L2LHP with a Lake/Well GLHX

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Hello everyone,

I am trying to model a HVAC system that is composed of a central
geothermal system that uses a heater/chiller with back up boilers to
provide hot and chilled water to AHU for a VAV air-side system.
Following the advice I found on the archives from the Loop to Loop
Heatpump in eQuest discussion I modeled a system with a L2LHP that uses
a lake/well loop (see screen shot). I made sure to zero out the static
head for the GLHX and changed the max hot water temp in the hot water
loop to 130F to be compatible with a heatpump. I compared this system to
a basic chiller/boiler VAV system to see if it showed any signs of
improvement and the results have been very puzzling. The geothermal
system is using slightly less energy to cool but isn't providing any
heating to the system. Instead the boilers are providing all of the
heating. When I remove the boilers to make the L2LHP provide all of the
heating it is performing terribly using an enormous amount of
electricity to heat. Also, I assumed that there would be a pumping power
increase but the L2LHP is using about 3X the electricity as the
boiler/chiller system and this seems way too large.

Has anyone had any luck modeling a system like this or has any ideas why
my L2LHP model is using more energy than a standard chiller/boiler
system?

Taylor Roberts, EIT

TaylorRoberts at Eaton.com's picture
Joined: 2011-09-30
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Taylor,

I have not done this system and only have time for a quick reply, but when
eQUEST seems to just not be "seeing" a piece of equipment, I start checking
my schedules and pertinent keywords to make sure the equipment knows it's
supposed to be on.

Good luck!

Carol

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The added pumping power is likely from the default 200 ft of static
pressure on the lake/well ground loop. Try changing this.

I've only played with 1 of those system types but I found it was very
sensitive to the 4 temperatures on the "Miscellaneous" tab of the
chiller (and the defaults were not helpful -- but the values in the help
file were.

Wish I could be of more help -- good luck.

Brian

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Thanks for your responses, Brian.

Where do you find these help files that you mentioned? Are you talking
about the tutorial and reference PDFs in eQUEST, or is there something more
comprehensive like full project examples with the pd2 and inp files?

Patrick Keeney's picture
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Taylor,
Whenever anyone says VAV, I hear VAV with reheat so if you have reheat the following might help.

I believe the L2L heat pump assumes one refrigerant circuit, such that if it sees load on both loops it will have to meet both loop temperature set points regardless of the magnitude of either load. So a lift of around 88 (= 130-42). Do you want that and thus your boiler is truly back up? You can end up with the tail wagging the dog where you punish the efficiency of the heat pump in cooling to meet a little reheat load. Or would you rather have your boiler operating as a reheat boiler or in series with your condenser water heat exchanger?

I have worked more with the banks of water to water heat pumps that provide simultaneous heating and cooling by having some heat pumps in each mode, thus none working against the huge lift. I end up modeling those as separate chilled water and heating water plants. No its not perfect but with the lake/well loop you were already going to be assuming a temperature profile and disconnecting your equipment from any model representation of lake/well capacity.

Those are very different approaches in design and in modeling, so I'd consider which you want. Good luck.

Paul Riemer, PE, LEED AP

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Taylor,

Make sure you specify the size for all of the water side equipment
(Loop-to-Loop Heat Pumps, Boilers, and Pumps). If I let eQUEST size this
equipment with a central geothermal system the default sizes are way off
and the model does not give the results that you would expect.

You may want to consider using load management and equipment controls so
the system operates in eQUEST as you want.

When you get close I would recommend creating a ground temperature
schedule for the Ground Loop Heat Exchanger if the loop is a ground
source loop. The Lake/Well loop gets the ground temperature from the
weather file that is used (you have to select a lake/well loop for a
central system). Has anyone created a ground loop temperature schedule?
If you have, how did you determine the ground temperatures? My plan is
to create the model as a geothermal heat pump system, get the ground
temperatures from the hourly reports and create a schedule from the
results. This is very time consuming but it is the most accurate way
that I can think of to get the proper ground temperatures.

LAKE/WELL The loop return temperature is set equal to
the temperature given by LOOP-TEMP-SCH, if specified. If LOOP-TEMP-SCH
is not specified the monthly ground temperature from the weather file is
used. This option allows you to simulate water-wells, lakes, and rivers,
where the temperature of these sources is driven by factors other than
the loop thermal demands. Since the supply temperature is fixed by the
schedule, all keywords pertaining to the ground field definition are
ignored.

Note that a GROUND-LOOP-HX of TYPE =
LAKE/WELL can be attached to CIRCULATION-LOOPs of TYPE = WLHP, CW
(condenser water), or LAKE/WELL. This allows chillers or other
water-cooled components to be attached to heat sinks like a lake, well,
or river rather than a cooling tower or outside air.

Good luck,

Otto

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Joined: 2011-09-30
Reputation: 200

Do you mean the temperature of the fluid returning from the ground-loop heat exchanger?

I have relied on that at times, but it is not perfect. If someone knows a better way ....

______________
Demba NDIAYE

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I have also used eQuest as a "well field modeling tool," but have grown
to avoid this practice where I can - I'll get to why in a minute...

Convoluted and time consuming as it is, eQuest actually can make for a
pretty powerful tool for determining GLHX behavior in the near and long
term, but you must have solid inputs/assumptions for the important
variables (many) and a model that well-represents the heating/cooling
loads that are intended to be designed around. I've heard if from guys
who've done GLHX design/commissioning all day that eQuest is highly
regarded in some such circles from a "science of geothermal design"
perspective, as it allows for rigorous control of so many variables and
really does things on an hourly basis, but operating it for design is at
best iterative and can be very time-intensive if you aren't VERY
proficient and do it often.

It's fair to note there are software packages out there (free and
for-cost) which are much easier to use to determine an annual
temperature profile, even if they don't all have the same rigor as what
can be modeled on an hourly basis using eQuest. If your goal is to
simply come up with an annual temperature profile of reasonable
accuracy, and not necessarily to design/commission well field
installations, you can achieve your goal much faster by either (A) using
a separate loop design software, or (B) engaging whoever is/was
responsible for the well field design to produce an annual temperature
profile. Be prepared to bring your own monthly heating/cooling loads
incident on the loop to the table.

Best of luck!

~Nick

NICK CATON, P.E.

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Thanks Nick,

One of the software tools that will give an annual temperature profile
for the well field is GLHEPRO.

Otto Schwieterman's picture
Joined: 2011-09-30
Reputation: 200

I had a similar situation where I was trying to use multiple L2LHPs to
simulate a multistack system that used a geo loop for heat rejection per
the method that had been posted on the forum. The problem that I found
was that with the loop to loop chiller/heaters it is really hard to
determine how it is calculating the heating efficiency at part load
since there are only part load curves and EIR on the cooling side for
that type of equipment. Also, the heating performance seemed to get
worse as the ground source temperature increased (opposite of what you
would expect). I ended up using the regular heat pump system that has
both heating and cooling curves and EIR inputs. Two other things to
check are using the equipment controls/load management to force the heat
pump(s) to meet the heating load before the boiler and to normalize the
part load performance curves to what conditions the equipment was
selected for (most likely not ARI for a geo loop). Hope that helps.

Brendan Hall, M.Sc.

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