[Equest-users] WSHP System with PS-H Report Circulation Loop Overload

Thu Jan 28 07:55:40 PST 2016

Nick, I worked on the model and checked some hourly results and came up with some interesting conclusions.

Note1: WSHP boiler sizing is related to the amount of heat required per square foot to heat the building, but only as this heating load affects the water loop’s ability to keep its water temperature between 70 and 85 F, the normal operating temperatures for water source heat pump loops.  If you perform a quick energy balance on a building using heat pumps with a COP of 3, the estimated water loop boiler or fluid cooler size for the water loop would be 0.66*maximum building load. When eQuest sized the boiler for this energy model, it selected a boiler 0.58*maximum building load. Why? The reason is that eQuest sizes primary equipment based on secondary (building loads) and the statistical probability that a certain amount of statistical data outliers is acceptable during the calculation process.  The following output reports show what I mean.

The PS-H report below shows the energy model’s maximum building load, net building load and a value it calls over load. The boiler size is represented by the value shown under the Heating Capacity (MBTU/HR) heading.

[cid:image004.jpg at 01D159A1.461B5C60]

The report shows that on January 7th at 8:00 a.m., an energy overload appeared in the water loop temperature calculation. Checking the hourly loop temperature report, shown below, confirmed that for this hour on this day, the water loop supply and return temperatures (columns AH and AI) go negative and the water loop average temperature (column AJ) is calculated at 74.4 F. The excess heating energy that cannot be handled by the water loop is passed on to the next hour’s calculation.  The next hour’s water loop temperature calculation shows the result passing this excess energy on by showing negative supply and return temperatures and an average loop temperature of 55.69 F. This is how eQuest accounts for the dynamic capacity of the water loop to absorb and release heat as a function of time. As long as the average water loop temperature calculation, which is the value eQuest uses to determine if the water loops ability to absorb or release is compromised, stays above 55 F, it continues calculation without issuing alarm messages.

The passing of excess water loop heat from one hour to the next occurs at 8:00 a.m for at least one day in the months of November through April and the number of hours that the building temperature remains below its required space temperature limits of 70 F and 75 F, or 55F night setback, are less than 10 hours out of 8760. Consequently, eQuest finds these results acceptable.

eQuest Water Loop Temperatures for PS-H report
[cid:image005.jpg at 01D159A1.461B5C60]

Adjustments can be made to the eQuest parameters if we wish to eliminate eQuest’s estimation of the dynamic abilities of the water loop to absorb and release heat over time. The most obvious adjustment is to increase the boiler size until there is no need for eQuest to pass along an excess heat requirement to the next hour’s calculation. Increasing the boiler size from 575 MBH to 1000 MBH eliminates the appearance of any overload heat in the PS-H report, but it also raises the energy model EUI from 18.6 kbtu/sqft-yr to 19.2 kbtu/sqft-yr due to inefficiency of the larger boiler cycling at low heat levels. The EUI increase can be compensated for by adding two boilers to the water loop sized with our usual 2/3, 2/3 arrangement which, in this case, would be two 660 MBH boilers.

The other adjustment that can be made to reduce dependence on the water loop’s ability to absorb and release heat over time is to minimize building heating requirement spikes that may occur in the water loop heating load.  The water loop overload calculations cease to exist if we adjust the heat pump start/stop schedules so that half of the building’s  heat pumps start at 7:00 a.m. and the other half start at 8:00 a.m. The original energy model has all heat pumps starting at 8:00 a.m.  For the staggered start/stop energy model, the boiler size remains at 575 MBH, and the EUI is 18.4 kbtu/sqft-yr, about the same as the original eQuest energy model.

The take away from this discussion is this: over sizing boilers for a building with a water source heat pump water loop is safe, but may costs energy savings. Another strategy that would not cost energy savings and achieve the same purpose might be to enforce heat pump start/stop schedules that minimize sudden demand heating requirements.

Note 2: Increasing heating efficiencies (COPs) for water source heat pumps does increase the water loop’s dependency on the boiler to provide make-up heat. See the table below for a comparison of the current energy model with a mix of heat pump heating COPs of 3.0 and 4.0 and the same energy model with heat pump heating COPs set at 4.2, the ASHRAE 90.1-2010 minimum requirement. The table below shows and increase in boiler size and energy model EUI. So the driver for optimizing water loop heat pump systems energy savings in our climate zone appears to be selecting multiple boilers to ensure minimum boiler cycling at low heat periods.

WSHP                    Heat                      Net                        Over                      EUI         Heating                 Boiler
Heat                      Load                      Load                      Load                                      over hours          Size
COP                       kBtu/hr kBtu/hr kBtu/hr                Hrs                         kBtu/hr
~3.5                       991.9                     576.2                     316.9                     18.6                        9              575
4.2                          1074                       816.5                     186.0                     19.1                        2              828

Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |

From: Nicholas Caton [mailto:ncaton at catonenergy.com]
Sent: Wednesday, January 27, 2016 10:47 AM
To: Kathryn Kerns <kathryn.kerns at bceengineers.com>; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] WSHP System with PS-H Report Circulation Loop Overload

Hi Kathryn,

I can’t answer without digging in myself, but I *think* if you follow along with the textual description of PLANT simulation in the DOE2 engineers manual (I’d attach, but don’t have my library handy atm – it’s out there in google!), leveraging your intimacy with the project-at-hand as a working example, you might come up with a better understanding of exactly what that “overload” figure is in isolation.  There’s a bulleted list somewhere in there that explains the order in which various parts of your model are considered before the remaining loop loads (for any loop type) are labeled “overload.”

A separate shot in the dark from the hip:  It may be possible the reason “net peak + overload peak =/= total peak” could have to do with one or more heatpumps on the loop operating in the reverse direction during the peak hour… maybe… or not… =)

~Nick

---------------------------------------------------------------------------------------------------------------------------------------------
Nick Caton, P.E.
  Senior Energy Engineer
  Energy and Sustainability Services
  North America Operations
  Schneider Electric

D  913.564.6361
M  785.410.3317
E  nicholas.caton at schneider-electric.com<mailto:nicholas.caton at schneider-electric.com>
F  913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org>] On Behalf Of Kathryn Kerns
Sent: Friday, January 22, 2016 2:10 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] WSHP System with PS-H Report Circulation Loop Overload

Everyone, I made an energy model of a building being served by water source heat pumps attached to a typical water loop with a fluid cooler and a boiler. I let eQuest size all the equipment including the boiler and water source heat pumps. The PS-H Water Loop report showed the following:

[cid:image003.png at 01D159A0.77826A10]

Notice that in January, the peak heating load is -991.9 kbtu/hr, the peak net load is -576.1 kbtu/hr, and the peak over load is -355.6 kbtu/hr. eQuest sized the water loop boiler to be -576.1 kbtu/hr. I believe these numbers are correct and the boiler size is correct but I cannot find an explanation in a document telling me how net load and over load is calculated and what they mean. I can explain how eQuest develops the heat load, but not the other two loads. I know a typical hot water coil system has heat load peaks equal to net load peaks and the over loads are usually zero. I am pretty sure the over load value is related to the heat pump COP, but I would like to be able to point to a written piece of documentation explaining this. Anybody know where to look?

Thanks.

P.S. the energy model ran successfully with less than 10 heating hours out of throttling range.

Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |

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