[Bldg-sim] eQuest: Ground Source Heat Pump + DHW + Glazing

[Keith Swartz]

Gaurav,

 

Your question #2 can be explained in part using the pump affinity laws.
The pump power increases approximately with the cube of the flow. So if
the flow increases 2% (because the load increased 2%), then the pump
power will be about 1.02^3 = 1.06 (a 6% increase). Going the other way,
the cube root of 1.11 (11%) is 1.035, which corresponds to a flow
increase of 3.5%. The affinity "law" calculation generally understates
the actual power consumption. It assumes that the system curve (head vs.
flow) is a parabola that goes through the origin (0 feet of head at 0
gpm flow), but this is often not the case. There are two common cases
where the system curve will not go through the origin:

 

1)       The system is an open loop with static head to overcome, such
as a cooling tower water loop. In this case when the flow drops near
zero, the pump head approaches the static head of the system (elevation
of the cooling tower, etc.).

 

2)       The system is a closed loop, but the pump speed is controlled
to maintain a differential pressure somewhere in the system. In this
case when the flow drops near zero, the pump head approaches the
differential pressure setpoint, because the pump is still trying to
maintain that differential pressure even when flow is low.

 

Your hydronic system is most likely a closed loop. The pumping power
would increase even if the pump is constant speed due to the additional
piping losses requiring more pump head. For constant speed closed loop
systems, the error from the fan "law" calculation isn't as significant
as it is with the two cases above.

 

The glass change would not only increase your peak load (2%), but would
also increase the amount of time during the year that the system would
need to run to meet the load. This would also increase the annual energy
consumption, which is what I presume you mean by "pump energy."

Sincerely,

 

Keith Swartz, PE, LEED(r) AP

 

Energy Center of Wisconsin

455 Science Drive

Suite 200

Madison, WI 53711

 

Phone: 608-238-8276 ext. 123

Fax:     608-238-0523

www.ecw.org

The Energy Center of Wisconsin is an independent, non-profit
organization that seeks solutions to energy challenges.

________________________________

From: gaurav mehta [mailto:gmehta75 at gmail.com] 
Sent: Monday, January 07, 2008 2:30 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] eQuest: Ground Source Heat Pump + DHW + Glazing

 

Hi,

 

I have two questions:

 

1. I am modeling a single family residential building in the pacific
Northwest with a Ground Source Heat Pump which apart from providing hot
water for the radiant slabs also provides heating for domestic hot water
(DHW). How does one model DHW in this situation? I have calculated the
flow rate (gpm) required for the house and have an annual schedule. I
tried to assign a process load to the Water Loop HP and when i modeled
one case with the process load and the the other without the process
load the difference in the pump energy does not confirm to the hand
calculation. 

 

2. This question is independent of the first one (ie no process load
attached to the Water Loop HP). I modeled one case with double low e
glazing on all orientations and the other case had South facing glazing
changed to double clear and rest of orientations with the same double
low e glazing. Both the glazing types were modeled using the library
method. The difference in the results show a penalty of 2% in space
heating for the case with South facing glazing changed to double clear.
Understandable, the building configuration (amount of glazing + shading)
is such that insulating property of glazing provides more benefit than
solar penetration. However, I also note there is a penalty of 11% for
the pump energy. There is no daylighting and nothing different in the
two models accept the South facing with different glazing. There is no
cooling modeled for the house as well and hence no difference in that
end use as well. I can understand some increase in pump energy, but can
anyone provide some insight as to why such an impact on pump energy? 

 

 

I would greatly appreciate your response.

 

Thanks.

 

Gaurav

 

Sustainable Building Analyst,

Stantec

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