[Equest-users] WSHP System with PS-H Report Circulation Loop Overload
Nicholas Caton
ncaton at catonenergy.com
Sun Jan 31 16:27:53 PST 2016
*Great* findings and writeup Kathryn – thanks for sharing!
~Nick
*NICK CATON, P.E.*
*Owner*
*Caton Energy Consulting*
306 N Ferrel
Olathe, KS 66061
office: 785.410.3317
www.catonenergy.com
*From:* Kathryn Kerns [mailto:kathryn.kerns at bceengineers.com]
*Sent:* Thursday, January 28, 2016 9:56 AM
*To:* Nicholas Caton
*Cc:* 'equest-users at lists.onebuilding.org'
*Subject:* RE: [Equest-users] WSHP System with PS-H Report Circulation Loop
Overload
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.
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
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
<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
F 913.564.6380
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States
[image: http://10.218.104.225:8090/Images/SE.comCapsule.png]
<http://www.schneider-electric.com/ww/en/>
*Please consider the environment before printing this e-mail
*From:* Equest-users [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
*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:
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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