Energy Recovery

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Dear eQuest users,

In my models, I realized that using energy recovery in the cooling season
always gives higher cooling energy consumption results than turning off the
energy recovery and using economizer only instead. I tried using OA
Heat/Cool with Bypass OA or Bypass Exhaust thinking that such configuration
would capture the best benefit of energy recovery and let the
outside/exhaust air bypass when economizer is more beneficial. Then, I read
in the eQuest manuals that this configuration still lets the energy
recovery run when it is not beneficial in warm months and just opens up the
economizer dampers to compensate for the unnecessary heating of the outside
air. So, the loss caused by the energy recovery eats up the benefit that is
provided with the economizer.

Thus, I thought, may be, I should use two separate files to maximize the
benefit from both the economizer and the energy recovery. The first file
would be for heating (with OA Heating) where the energy recovery would be
on whenever it can heat the outside air. Then the second file would be for
cooling where there would be no energy recovery but the economizer would
be available whenever the outside air is favorable for cooling. Finally, I
would get heating energy consumption from the heating file and the cooling
energy consumption from the cooling file.

The only thing that stops me from following this approach is that I am
wondering whether there was a logical reason why bypass configurations of
eQuest give higher cooling energy consumption than the economizer only
case. Shouldn't they be giving exactly the same cooling energy consumption?
Is it because part of the outside/exhaust air is being bypassed in the
bypass configurations instead of fully? or may it be because it is known
that setting the energy recovery and economizer for the best benefit is
pretty hard in reality and that's why it was assumed that it would not
function ideally? Or, is there a completely different reason that I do not
know?

Would you please help me with this question?
I'd appreciate it very much.

Thanks,
Best Regards,
S. Andolsun, PhD

SAndolsun's picture
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how are you controlling the erv/energy recovery? the default is for it
to run when the supply fan runs. this will cost you more in energy than
it saves. set the control method based on a delta T between the exhaust
air and the outside air you'll see the savings in cooling mode. in my
climate zone (2b/southern arizona) the best delta t i found is 27 d f.

once you set up the erv to run based on a delta t you can set up some
parametric runs (set up one & then copy & rename it in the .inp file to
save time) to try different delta t values without having to change each
hvac unit every time you want to try a different delta t.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
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Dear Patrick,

Thanks for your response. I actually had set my schedule to OA Exhaust DT
when get these results. The delta T I used was "0" so that the system would
benefit from every bit of temperature difference.

Thanks,
Best Regards,
S. Andolsun

SAndolsun's picture
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I see. Thanks for this information. When I model the cooling season
separately in another file with economizer and without energy recovery,
then I am avoiding having to use these inefficient motors, which saves
energy. Can we say that modeling cooling season with no energy recovery is
a good idea then? Can you think of any problem that might arise with this
approach? Would you please let me know?

Thank you very much,
Best Regards,

Simge Andolsun, PhD.

SAndolsun's picture
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Simge,

The higher cooling consumption comes from increased fan power due to the added pressure drop of the heat recovery unit. As your fans are working harder, they are emitting more heat into the supply air stream that the cooling coil needs to address to meet set point.

Make sure you have appropriate pressure drops: 0.5" for run around loop, 1.0" for wheel are good rules of thumb. Pressure drops are found under the "Heat Recovery 2" tab. I typically account for the pressure drop in my main AHU fan power and set the pressure drop and fan power to zero in tab 2. Either way works; just make sure you're not double counting fan power.

A 5 degree deltaT is pretty common for control. Also make sure the heat recovery is modulating so you're not overshooting your set points and having to reheat. Enthalpy control is not commonly done because of maintenance issues for RH sensors. Enthalpy controls saves more on paper, but reality will be dictated on how well the system is maintained.

As for applicability to your climate and design outdoor air fraction, ASHRAE 90.1 provides guidance on that, which is generally pretty good. I've had baselines operate poorly by requiring heat recovery due to utility rates (expensive electricity, cheap natural gas), but that's uncommon.

Hope that helps.

Fred

Fred Betz's picture
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