[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [EnergyPlus_Support] Re: Residential heater sizing and cycling question



I get the feeling that you think you will see the heater turn on and off 
as it does in a residence. You will not see the heater turn off except 
when there is no load. What you will see is a reduction in the fraction 
Heater Rate/Heater Capacity or part-load ratio (PLR). This is the value 
sent to the Part Load Fraction as a function of Part Load Ratio curve. 
The output of the Part Load Fraction curve is then used to calculate 
runtime fraction as RTF = PLR/PLF. The report variable RTF is the 
cycling rate.


Troy Harvey wrote:

> Mike thanks for the reply.
>
> I'm not real familiar with the HVAC options, I normally use purchased air.
> But I've look through the docs, and I'm looking for clarification.
>
> What I'm hoping to be able to simulate is the effect of over-sizing
> residential forced air heaters, and the effect of variable BTU/air volume
> vs. Const. BTU/air volume, and their impact on heater efficiency (due to
> heater cycling effect on combustion efficiency, burner/flue reheat, etc).
> That is I'm hoping that, if I oversize, I should see more cycling, and 
> less
> efficiency. Is this feasible in E+?
>
> As Don and Rich have explained, the HVAC system timestep does vary, 
> and you
> can
> report results at the timestep level, but this is not relevant to what you
> are
> trying to model. Each HVAC timestep in EnergyPlus is a quasi-steady-state
> simulation. For the gas heating coil cycling loss is modeled by looking at
> the
> quasi-steady part load ratio and making an adjustment to the efficiency
> through
> the "Part load fraction correlation (function of part load ratio)" 
> curve in
> COIL:Gas:Heating.
>
> For example, if the load on the furnace from 9:00am to 9:30 am is 50% of
> capacity, it doesn't matter if EnergyPlus simulates two 15-minute HVAC
> timesteps, or 30 1-minute HVAC timesteps. Each timestep will evaluate the
> efficiency at 50% part-load. There may be minor differences in results 
> when
> the timestep shifts down, but you should not need to be concerned with 
> the
> length of timestep. But, you do need to make sure the Part load fraction
> correlation curve accurately represents the type of equipment you are
> studying.
>
> Mike
>
> On 4 Apr 2007 at 10:30, taharvey2001 wrote:
>
> > I'd like to be able to model a standard constant volume residential
> > forced air heater in EnergyPlus and see the impact of Sizing on
> > efficiency and heater cycling.
> >
> > I haven't been able to figure out how to model the heater
> > appropriately in E+ or and make sure the heater has sufficiently small
> > timestep resolution so the heater can cycle, and the difference in
> > efficiency can be seen.
> >
> > As I understand, the HVAC system timestep is decoupled from the 15
> > minute building timestep, allowing it to free-run with an adaptive
> > timestep (down to seconds if need be) - is this true, and is there
> > anything that need to be done to enable this behavior? Is there anyway
> > to see the high-res HVAC transients in the output?
> >
> > What is the best way to get a const. volume, const BTU heater to model
> > residential forced air systems?
> >
> > thanks,
> >
> > troy
>
>
> [Non-text portions of this message have been removed]
>
>  


-- 
Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
1679 Clearlake Road
Cocoa, FL  32922-5703
Phone:   (321) 638-1454
Fax:     (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years



[Non-text portions of this message have been removed]



The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/

<*> Your email settings:
    Individual Email | Traditional

<*> To change settings online go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/join
    (Yahoo! ID required)

<*> To change settings via email:
    mailto:EnergyPlus_Support-digest@xxxxxxxxxxxxxxx 
    mailto:EnergyPlus_Support-fullfeatured@xxxxxxxxxxxxxxx

<*> To unsubscribe from this group, send an email to:
    EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/