[BLDG-SIM] Mysterious space heating load spikes - RESOLVED

Xiaobing Liu XLiu at climatemaster.com
Fri Feb 16 13:17:48 PST 2007


Randy:
 
As you experienced, different from other HVAC equipments, the ground loop heat exchanger (GLHE) can not be auto-sized by eQUEST. There are couple of reasons for this. 
 
First, the characteristics of GLHE is quite different from other HVAC equipment (i.e . chiller and boiler), which can be easily and properly sized using the peak loads. In fact, due to the huge thermal mass of the ground, the history of rejecting/absorbing heat to/from ground will significantly affect the performance of  a GLHE. Therefore, to properly size a GLHE, not only the peak loads but also the loading history have to be accounted for. As a result, eQUEST can not properly auto-size a GLHE before the annual simulation is performed. 
 
Second, there are lots of variations in the design of GLHE, such as borehole field configuration (geometry pattern), borehole depth, grouting material, borehole spacing, etc. These variations may significantly affect the required size of GLHE. Since GLHE is a relatively expensive device, it is really necessary for the designer to try various possible GLHE designs, perform simulations, evaluate the results, and finally achieve the optimal design. Although technically speaking, it is possible to make eQUEST be able to auto-size GLHE (which required multiple eQUEST runs controlled by certain iteration procedure), the auto-sized GLHE is by no means the optimal design.
 
There are also couple of notes regarding eQUEST GSHP simulation you may be interested in:
 
1. Warning messages will be given in the detailed simulation output file (XX.sim) if GSHP Water Loop temperature has exceeded a hi/low alarm. For example, following is the warning message shown in your output file. This warning message has to be taken care of since the heat pump unit will be shut down when the loop temperature exceeds the hi/low alarm. It is true in both the reality and the simulation. As a result, the room temperature could not be maintained at the thermostat set point. If this happen, you will see (in BEPS or SS-R report) high percentage of hours that the room temperature is outside of the throttling range. In this case, you need adjust the design of your GLHE incrementally until this message disappear. 
 
**WARNING**********************************************************************

WLHP loop: GSHP Water Loop has exceeded a hi/low

alarm limit and will shut down associated HVAC equipment

for the hour. Loop T: 110.26 Limit: 110.00

Verify that alarm limits are reasonable, verify primary

equipment sizing, especially ground-loop heat-exchangers,

and verify that the HP performance curves are suitable for

the intended operating range. See Report PS-D for total

number of hours alarm limits are exceeded.

First occurrence: 7/19/17

Actually, the better way to watch the performance of a GLHE is to let eQUEST output the hourly report of the GLHE outlet temperature, which is the entering fluid temperature to the heat pump if the thermal loss from the loop and the pump temperature rise can be ignored. As an example, to output the hourly loads on GLHE, outlet temperature of GLHE, and the undisturbed ground temperature, you can save following text as a short inp file and add it to your input file by using the "Import files" feature of eQUEST in the detailed mode. For advanced users, you can directly define the hourly report in the detailed mode of eQUEST.

$ ---------------------------------------------------------
$              Hourly Reporting
$ ---------------------------------------------------------

"GHX" = REPORT-BLOCK    
   VARIABLE-TYPE    = "GLHX (VertWell-CM)"
   VARIABLE-LIST    = ( 1, 4, 5 )
   ..
"Hourly Report" = HOURLY-REPORT   
   LIBRARY-ENTRY "Hourly Report"
   ..
"Hourly Report 3" = HOURLY-REPORT   
   REPORT-SCHEDULE  = "Hourly Report Schedule"
   REPORT-BLOCK     = ( "GHX" )
   ..

2. Mutli-year performance of GLHE has to be considered when you size GLHE, especially when large borehole field is used and the heat rejected to the ground can not be balanced by heat absorbed from the ground on an annual basis. In this case, although the GLHE loop temperature is within the limit at the first year, it may be outside of the limit just couple of years later. To observe the performance of a GLHE after multi-year operation, you can change the input "Years of previous operation" from the default 0 to other positive integers, and run the simulation again.

3. The "Number of identical borehole field" is only an approximate approach to describe and simulate large borehole fields. DOE-2 assumes that the identical well fields do not interact with each other. This assumption is only valid when the identical well fields are apart from each other with enough distance. For large vertical well field with many boreholes placed closely, it is recommended to use the exact borehole field configuration in the simulation. Data for simulating more borehole field configurations than the 41 existing configurations are available upon request.

4. New reports for GLHE will be added to eQUEST in near future. Hopefully, it will make the the iterative GLHE design procedure described above a little bit easier.

Hope it helps,

Xiaobing Liu

 

-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com]On Behalf Of Randy Wilkinson
Sent: Wednesday, February 14, 2007 6:59 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Mysterious space heating load spikes - RESOLVED


Thanks to all,

Brody and at least one other suggested using a larger GSHP ground loop.  That fixed it.  Thanks for your help.

Randy

Brody Wilson wrote: 

Randal,
    I just ran into this issue myself, it looks like the temperature alarms on your GSHP Water loop are alarming and shutting off your equipment, then the loop returns to non alarm temps, then the equipment cycles back on, etc. 
It looks like you're alarming on both the high and low sides, so I'm guessing that whatever heat source or heat sink you have are too small. If you have a ground source heat exchanger make it bigger and see if that fixes it. 

Let me know if that's it.

--

Brody Wilson



Phone: 314.644.2629

Fax:   314.644.0691

Mobil: 314.651.8319



Energy Solutions Inc.

P.O. Box 300354



St. Louis, MO 63130



brody at energysolutions-stl.com 



www.energysolutions-stl.com




On 2/9/07, Randy Wilkinson < randy at hvacware.net> wrote: 

Hi All,

Our eQuest model has very  <http://www.lseng.com/%7Erandyw/stegasaurus.pdf> strange looking spikes in the space heating load profile.  We can't figure out why they would be there.  I put a PDF of the Monthly Electric Peak Load Profile on my web  <http://www.lseng.com/%7Erandyw/> page.  The is also a PDF of the complete simulation output <http://www.lseng.com/%7Erandyw/simulationoutput.pdf>  if that helps.  We did a sanity check of inputs...it still looks insane.  This model was done all with DD wizard inputs.  Anybody got any ideas? TIA.

http://www.lseng.com/~randyw/ <http://www.lseng.com/%7Erandyw/> 
http://www.lseng.com/~randyw/stegasaurus.pdf <http://www.lseng.com/%7Erandyw/stegasaurus.pdf> 
http://www.lseng.com/~randyw/simulationoutput.pdf <http://www.lseng.com/%7Erandyw/simulationoutput.pdf> 

Randy

-- 
Randall C. Wilkinson 
L&S Engineering
Spokane, WA

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