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

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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

Gaurav Mehta2's picture
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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

Keith Swartz's picture
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Gaurav,

Addressing your first question with eQuest is a difficult task. First off,
if you don't have Climate Master's GSHP eQuest "expansion pack," get it
(http://www.climatemaster.com/index/equest). It adds many GSHP modeling
functions, including DHW, which would accurately capture your DHW energy
requirements. I think this still requires a dedicated HP to supply DHW.

The real difficulty arises when you plan on using a single HP unit to supply
both radiant heating and DHW since there are no commercially available HP's
(that I'm aware of) that will operate the desuperheater option without a
call for radiant heating. Obviously, in situations where no radiant heating
call exists, you run the risk of quickly exhausting your DHW supply (unless
you have auxiliary heating). A practical solution to this involves using an
Indirect Hot Water tank as your radiant buffer tank. Kudos if you can
figure out a way to model the latter!

Anthony Hardman

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