RE: [EnergyPlus_Support] Solar Collectors

[Edwin Cox <ebcox@xxxxxxxxx>]

Thanks. I didn't consider shaft power.

Looking at the HTML report, I see that the collector loop plant loop volume is 67.5 liters compared to maximum loop flow rate of 0.060 liters/sec. So the volume amounts to over 20 minutes of flow. Qualitatively, that would be the "big slug of hot water" that I was referring to, so that loop capacitance is probably the best explanation.

Thanks again!

Edwin

--- On Tue, 5/26/09, Griffith, Brent <brent.griffith@xxxxxxxx> wrote:

From: Griffith, Brent <brent.griffith@xxxxxxxx>
Subject: RE: [EnergyPlus_Support] Solar Collectors
To: "EnergyPlus_Support@xxxxxxxxxxxxxxx" <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Date: Tuesday, May 26, 2009, 5:38 PM

The pumpâ??s shaft power is still added to the loop. (If you want to turn that off, set the pressure to something close to zero.)  See the write up in the Engineering Reference under â??Pump Heat Addition to the Loop .â??

 

Brent Griffith

EnergyPlus Development Team.

 

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From: EnergyPlus_Support@ yahoogroups. com [mailto: EnergyPlus_Support@ yahoogroups. com ] On Behalf Of Edwin Cox
Sent: Tuesday, May 26, 2009 3:25 PM
To: EnergyPlus_Support@ yahoogroups. com
Subject: RE: [EnergyPlus_ Support] Solar Collectors

 

Thanks for your response, Brent.

The "fraction of motor inefficiencies to fluid stream" is set to 0 for the collector loop pump, so I don't think that is the source for the extra heat.

Regarding loop capacitance, for the tank rate to be so much higher at the beginning of the day's operation than the collector output, wouldn't there have to be a big slug of very hot water sitting in the tubing? It's not making much sense to me, but I'll play around some more and see what I can figure out.

Edwin

--- On Tue, 5/26/09, Griffith , Brent <brent.griffith@ nrel.gov> wrote:

From: Griffith, Brent <brent.griffith@ nrel.gov>
Subject: RE: [EnergyPlus_ Support] Solar Collectors
To: " EnergyPlus_ Support@yahoogro ups.com " < EnergyPlus_Support@ yahoogroups. com >
Date: Tuesday, May 26, 2009, 4:36 PM

I suspect that there are at least two things you may not understand.  First is pump heat.  The work of pumping adds additional heat to the loop.  Second is loop capacitance.  EnergyPlus includes a time delay effect between the supply side outlet and the demand side inlet.

 

---

From: EnergyPlus_Support@ yahoogroups. com [mailto: EnergyPlus_Support@ yahoogroups. com ] On Behalf Of ebcox
Sent: Tuesday, May 26, 2009 2:04 PM
To: EnergyPlus_Support@ yahoogroups. com
Subject: [EnergyPlus_ Support] Solar Collectors

 

I have a question about the SolarCollectorFlate PlateWater example.

I ran it unaltered with the Chicago O'Hare TMY2 .epw weather file. I read the .CSV output into Excel and I'm comparing column L (STORAGE TANK:Water Heater Source Rate) against column AA (Collector1: Solar Collector Heat Transfer Rate) multiplied by 6 (there are 6 collectors). The value for timestep July 01, 8:30 AM is 6620 for the former, 3770 for the latter. Going forward through the time steps, the values become fairly close but not nearly identical.

I would expect from conservation of energy that the heat energy going into the tank would equal the heat energy leaving the collectors. What do I not understand about Energy Plus?

Thanks!

Edwin Cox

 

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