Appendix G condenser water temperature reset in EnergyPlus

This question is following a thread on the EnergyPlus Support group.

Quoting Appendix G:

G3.1.3.11 Heat Rejection (Systems 7 and 8). The heat rejection device shall be an axial fan cooling tower with two-speed fans. Condenser water design supply temperature shall be 85°F or 10°F approaching design wet-bulb temperature, whichever is lower, with a design temperature rise of 10°F.

The tower shall be controlled to maintain a 70°F leaving water temperature where weather permits, floating up to leaving water temperature at design conditions.

For reference CTI rates cooling towers at 95°F DB/78°F WB with 95°/85°F condenser water temperature. That's what's used for the performance of the cooling tower in E+ too.

First question, is my design wetbulb temperature 78°F - meaning the design approach is 7°F - or is it my design day wetbulb temperature (eg: Ann Clg 1% Condns WB=>MDB) for my specific location, in my case 75.8°F meaning an approach of 9.2°F?

Second, as far as the reset strategy, I think SetpointManager:FollowOutdoorAirTemperature placed on the supply outlet of the condenser loop is appropriate.

Knowing that:

85°F = 29.44°C
70°F = 21.11°C

I would do something like this:

SetpointManager:FollowOutdoorAirTemperature,
    MyCondenserControl,      !- Name
    Temperature,             !- Control Variable
    OutdoorAirWetBulb,       !- Reference Temperature Type
    X,                       !- Offset Temperature Difference {deltaC}
    29.44,                      !- Maximum Setpoint Temperature {C}
    21.11,                      !- Minimum Setpoint Temperature {C}
    Condenser Supply Outlet Node;  !- Setpoint Node or NodeList Name

I'm wondering if this is the right approach and what value of Offset Temperature Difference I should use.

Should I use the calculated approach (7°F or 9.2°F) to keep a constant approach? Should I keep it at 0 and leave E+ to try and do as best as possible?

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Julien Marrec

asked 2015-11-05 10:41:51 -0500, updated 2016-07-20 03:47:31 -0500

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To be conservative, I would use designday wetbulb temperature to prevent the cooling tower from being over-sized in the baseline case.

I think your reset strategy looks good if you used a calculated approach - which means X would be replaced with 9.2°F. If you kept the approach constant (0F), there would likely be a net penalty because the increased cooling tower fan energy would negate any saving seen from reducing the lift across the chiller's compressor.

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Lincoln

answered 2015-11-05 11:23:39 -0500, updated 2015-11-05 11:27:34 -0500

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I've looked at all the E+ examples files, and whenever SetpointManager:FollowOutdorAirTemperature is used to do condenser water reset, the Offset Temperature Difference (deltaC) is set to 0.

I think this is potentially a misleading choice, because then the CT is basically controlled to deliver an unachievable approach of 0°F, and therefore you get high fan energy consumption that isn't compensated by a reduced lift on the chiller, as @Lincoln pointed out.

Here's total energy versus offset temp for the example file 2ZoneDataCenterHVAC_wEconomizer.idf, using chicago weather file. energy in Joules, offset temp in C.

foat offset