I'm modeling a ground loop heat exchanger to provide condenser water to a large water-to-water heat pump which will be capable of providing simultaneous heating and chilled water to AHU coils in the building. The Water-to-water Heat Pump being designed around is similar to that manufactured by Multistack, Heat-Harvester, Climacool, etc.
I've successfully created a 2-pipe water loop with a water-to-water heat pump that gets its' condenser water from a ground loop heat exchanger (lake/well). When assigned to Fan Coils in the building this system appears to work just fine. However, no simultaneous heating/cooling capability.
So, I've now added a chilled water loop with a second water-to-water heat pump that gets its' condenser water from a seperate ground loop heat exchanger. I then assigned the cooling coils to the chilled water loop and the heating coils to the 2-pipe water loop. Trouble is I get the error that reads, "the chilled water loop has no load". If I manage to get this error to go away then my zones have 5,000 + hours of unmet heating loads.
I've been searching the archives and I see that this has come up several times but with no real solution other than "it should be able to be done". I'm posting the question again in hopes that someone who has successfully modeled this system can point the rest of us in the right direction. This type of system is coming up more and more (I have 4 projects considering them now) and as we strive toward greater building efficiency we are going to HAVE to be able to successfully model these system types.
Tim Dion, LEED(r) AP
Why are you using a separate lake/well loop and heat pump for the chilled
water loop? You should be able to have the same piece of machinery attached
to both a chilled water loop and a hot water loop. This way heat is
transferred from one loop to the other when there are simultaneous loads.
The ground loop is only used when there is net heating or cooling needed.
My beef with this system is that you can only use a loop type of lake/well
using a constant condenser water temperature. This is usually not the case
with ground loops, unless you are circulating water from an underground
reservoir!
--
Karen
If the chiller you are using is the "well-water-to-water HP" type
(CHILLER:HEAT-PUMP), I have successfully been able to run a simulation with
this chiller connected to a CHW loop. However, it was only connected to a
loop with a process chilled water load, and not connected to coils.
The loop-to-loop HP (CHILLER:LOOP-TO-LOOP-HP), which I believe Karen is
referring to, is intended to be attached to both a HW and CHW circulation
loops. As I understand the DOE-2 help, there is an important difference
between these chillers in that the LOOP-TO-LOOP-HP chiller allows for
exchange of heat between the HW and CHW circulation loops, where as the
HEAT-PUMP chiller only moves heat to-and-from the CW loop (LAKE/WELL type).
You can connect multiple HEAT-PUMP or LOOP-TO-LOOP chillers to a single
LAKE/WELL loop and heat exchanger, so you could potentially have one chiller
supply your HW and one chiller supply CHW by adjusting the controls of the
2-PIPE loop. Having them connected to the same CW loop, the total CW load
should be equal to the sum of the energy pulled out by the chiller in
heating, the energy put in by the chiller in cooling, and the loop pump heat
(plus loop thermal losses if simulated). However, this is somewhat
irrelevant because no matter what the CW loop return temperature is, the
LAKE/WELL loop and heat exchanger will always provide the scheduled supply
temperature (or by default, the hourly ground temperature in the weather
file).
To get around this, we have developed an approach that takes the building
loads from the LAKE/WELL loop and applies them to a DOE-2 well-field model,
and in turn, takes the resulting well-field temperatures for use in the
building hourly simulation. It requires post-processing and some iteration,
but I think this type of approach could be as good as we can do with the
existing tool.
Any other approaches out there?
David Reddy
Interesting David. We have been discussing how to deal with the same issue with the Lake/Well loop and heat exchanger always providing the default ground temperture as well.
When I review the line diagrams for the Loop-to-Loop HP it appears that it is moving heat from one loop to the other. However, what we are doing is using a large HP to supply the hot water / cold water to the AHUs inside the building. From the line diagrams for the Loop-to-Loop HP system it doesn't appear that this is what that system provides. Am I missing something?
Thank you all for your input.
Tim