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RE: [EnergyPlus_Support] Re: Heat Recovery Problems
Just to follow up, I now have been able to confirm that with the changes below, the input file DOES demonstrate a correct energy balance on the water heater. I think the SYSTEM CONVERGENCE LIMITS had a lot to do with the trouble you might have been seeing.
Some tips on verifying the energy balance:
1. Use the new report variable in V 1.4 called "Water Heater Final Tank Temperature" to find the final tank temperature at the end of the timestep, otherwise you get an average over the timestep. This is what you'll use to measure the actual change in energy in the tank.
2. Report your variables at the "timestep" interval instead of "hourly". Tank temperatures can change rapidly within an hour. The hourly values are averages of the timestep values and therefore will not be as accurate for checking the energy balance...that is assuming that your timestep is less than 1 hour.
3. The minor bug that I mentioned below pertains to the reporting of the variable "Water Heater Net Heat Tranfer Energy". I think the "Vent" energy is getting added into this with the wrong sign. When in doubt, manually add up the components for "loss energy", "use energy", "source energy", and "vent energy".
Let me know if you have any further questions.
P
-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of Ellis, Peter
Sent: Thursday, October 26, 2006 4:43 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] Re: Heat Recovery Problems
Hi Andrea,
I just wanted to let you know I have been looking at your input file. So far I have found a couple of things that will need to be fixed in your file before it will run properly.
1. You have SYSTEM CONVERGENCE LIMITS set as follows:
System Convergence Limits,
60, !- Minimum System Time Step {minutes}
2000; !- Maximum HVAC Iterations
The default "Minimum System Time Step" is 1 minute. 60 minutes means that the system is simulated AT MOST once per hour. This is giving some really strange results. I don't think the system simulation is completing correctly with this setting. (At least it is not greater than your timestep of 1 hour.) I would suggest leaving it at the default of 1 minute unless you have a good reason for doing otherwise.
2. You have TIMESTEP IN HOUR set to 1. This is not too bad, but usually the plant simulation will run better at 4 or 6 timesteps/hour. The problem is that the plant loop volume becomes very large at 1 hour timesteps. To account for the thermal mass in the loop you end up getting a very large temperature jump between the outlet of the demand side and the inlet of the supply side (or is it vice versa? I forget.) Once you are convinced that everything is running correctly, then you might try experimenting with a 1 hour timestep.
3. Don't forget Sankar's observation that you want the water heater branch to be ACTIVE:
BRANCH,
Water Heater Use Branch, !- Branch Name
0, !- Maximum Branch Flow Rate {m3/s}
WATER HEATER:MIXED, !- Comp1 Type
Water Heater, !- Comp1 Name
Water Heater Use Inlet Node, !- Comp1 Inlet Node Name
Water Heater Use Outlet Node, !- Comp1 Outlet Node Name
ACTIVE; !- Comp1 Branch Control Type
4. Your second water heater "secondo boiler" does not have any heating capacity. So it is not doing any auxiliary heating and you are probably not getting the space heating that you hope for at all times.
There may still be an energy imbalance that you've found here, but I'm not sure yet. There is at least one minor bug in the reporting for the net heat transfer that I've found, but that should not cause any energy imbalance. I will continue my investigation and let you know as soon as I have a definite answer. I do, however, want to point out that the energy balance on the SolarCollectorFlatPlate.idf file was corrected with the new release (I just double-checked). So if there is an energy imbalance in your new file, then this is a new bug!
Peter
-----Original Message-----
From: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com [mailto: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On Behalf Of ingdallolio
Sent: Tuesday, October 24, 2006 12:32 AM
To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
Subject: [EnergyPlus_Support] Re: Heat Recovery Problems
Peter,
If I understand correctly, your water heater is still too generous !
The energy is still not balancing in waterheater E+ v 1.4 ?
As usual, thank you and the E+ team for your work.
Andrea.
--- In EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com, "Ellis, Peter"
<peter_ellis@...> wrote:
>
>
> The energy numbers are actually correct for any given time step
because they are based solely on the tank temperature at that time
step and the various energy flows into and out of the tank. In other
words there is a correct energy balance at the time step level.
Unfortunately, the problem is with the thermal history of the
tank--the tank temperature is not propagating correctly from time step
to time step. It's actually moving ahead in time about 4 times as
fast as it should. That means the tank temperatures are wrong, and by
extension the energy numbers are wrong too. Ultimately, this messes
up the energy balance at longer time scales, e.g., over a day or a
year. So to sum up, both the outputs and actual performance are
incorrect.
>
> Peter
>
>
> -----Original Message-----
> From: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
[mailto: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On Behalf Of ingdallolio
> Sent: Thursday, September 21, 2006 11:00 AM
> To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
> Subject: [EnergyPlus_Support] Re: Heat Recovery Problems
>
>
>
> Thank you and the team for the work Peter.
>
> One more question, is the problem just on energy output numbers or is
> it effecting the component actual performance? I mean, beside the
> energy output values, are the other output of the model still good ?
>
> Andrea.
>
> --- In EnergyPlus_Support@
<mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com, "Ellis,
Peter"
> <peter_ellis@> wrote:
> >
> >
> > Andrea,
> > I have confirmed that there is indeed a bug with the water heater
> object. The thermal history of the tank is being incorrectly updated
> several times per system time step (should be only once per sys time
> step). This is causing the problems in the energy balance that you
> were seeing. Originally the water heater did have the correct energy
> balance. This bug was most likely introduced one or two releases ago.
> Although we do extensive testing to try to make sure that we don't
> "break" anything from release to release, this bug got by us somehow.
> >
> > The bug has now been fixed. You should be able to successfully
> confirm the energy balance in the upcoming release in October. I have
> also added a new report variable for the water heater that gives you
> the final tank temperature at the end of the time step. This should
> allow you to show an exact energy balance between the tank starting
> conditions and ending conditions and the various fluid flows.
> >
> > Thanks for your persistence and patience in dealing with this bug.
> >
> > Peter Ellis
> > National Renewable Energy Laboratory
> >
> >
> > -----Original Message-----
> > From: EnergyPlus_Support@
<mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
> [mailto: EnergyPlus_Support@
<mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On
Behalf Of ingdallolio
> > Sent: Tuesday, September 05, 2006 3:22 PM
> > To: EnergyPlus_Support@
<mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
> > Subject: [EnergyPlus_Support] Re: Heat Recovery Problems
> >
> >
> >
> > Peter,
> > I understand source and use power do not match in a water heater but
> > energy source and use should balance in a year period ( with stored
> > heat and losses of course ), they do not match in any water heater I
> > tried like the example file collectorflatplate.idf.
> > Mike Wite confirmed me the unbalance before summer but so far no info
> > about the issue nor it's in the known issue list. The present version
> > waterheater object can be considered reliable ?
> > Grazie (Thanks).
> > Andrea.
> >
> > I've also problem in balancing energy
> >
> > --- In EnergyPlus_Support@
> <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com, "Ellis,
> Peter"
> > <peter_ellis@> wrote:
> > >
> > >
> > > The "source rate" and the "use rate" refer to the heat gain rate (or
> > loss rate) through the two sets of plant loop connections to the water
> > heater. In your case, it sounds like you have the "source side"
> > connected to the IC engine for heat recovery. I would guess that your
> > "use side" is connected to some hot water load, like domestic hot
> > water, or maybe space heating. The heat gain rate (or loss rate) for
> > either side is calculated as the Q = mass flow rate * Cp * (Ttank -
> > Tinlet).
> > >
> > > That said, it is not often the case that the source rate and the use
> > rate will match each other. That is usually why the water heater has
> > some volume to it for energy storage, and why there's a burner or
> > electric heating element to provide auxiliary heat when necessary. If
> > your water heater has an auxiliary heat source, you need to take that
> > into account too. The report variable for that is "Water Heater
> > Heating Rate". You may also have parasitics adding extra heat to your
> > water heater also.
> > >
> > > Finally, keep in mind that the heat rates will only add up if the
> > water heater is kept at a steady-state temperature. If, for example,
> > you do not have a burner on your water heater and the use rate is
> > greater than the source rate, the water in the tank will cool down
> > until there's no more useable energy left. Or conversely, if you are
> > adding more heat through the source side than you are using, the tank
> > temperature will heat up until it hits the Maximum Temperature Limit
> > at which point the heat is vented away and lost.
> > >
> > > As for the IC engine heat recovery, you should be able to verify the
> > actual heat recovery rate delivered by the plant loop by looking at
> > the mass flow rate through the IC engine, the inlet water temperature
> > and the outlet water temperature. This calculation should match the
> > water heater source rate. There may be some reason why the "total
> > heat recovery rate" of the IC engine is defined differently, I don't
> > know. Hopefully, they match.
> > >
> > > If this hasn't explained your problem yet, you can post the file.
> > >
> > >
> > > -----Original Message-----
> > > From: EnergyPlus_Support@
> <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
> > [mailto: EnergyPlus_Support@
> <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On
> Behalf Of energypluser
> > > Sent: Sunday, September 03, 2006 3:19 AM
> > > To: EnergyPlus_Support@
> <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
> > > Subject: [EnergyPlus_Support] Heat Recovery Problems
> > >
> > >
> > >
> > > Dear all
> > >
> > > I connected a water heater to an IC engine for the purpose of
> > recovering the exhausted heat.
> > > When I was checking the results, I found something that I can not
> > understand.
> > > First, I found that the source rate of the water heater is much
> > larger than the use rate. The
> > > difference is around 12kW in the winter design day and 7kW in the
> > summer design day.
> > > I checked that the heat loss rate is around 0.3kW in design days.
> > > Why are some heat missing?
> > > Second, I found that the total heat recovery rate of the IC engine
> > was around 28kW in the
> > > design days. It is bigger than the source rate of the water heater!
> > So the same problem
> > > appears again: there are some heat missing.
> > > How can I explain this kind of things?
> > > Thank you in advance!
> > >
> > > mac.cz
> > >
> > >
> > >
> > >
> > >
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
[Non-text portions of this message have been removed]
[Non-text portions of this message have been removed]
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