Re: [EnergyPlus_Support] electric power (watts) of PTAC

On Sat, Nov 21, 2009 at 2:36 PM, YuanLu Li <yli006@xxxxxxxxxxx> wrote:

Hi, Hu Jia

I have not run the model you have quoted.

In general, the PTAC has only one fan, therefore it is used all the time for Winter and Summer. The split model has two fans, one for the compressor and one for the electric heater and coil unit (the fan coil unit) in the zone.

The size of the fan is related to the air flow rate. which is calculated from the maximum cooling demand and the minimum set point temperature. Once it is sized, the fan size does not change for the Summer and Winter use. The VAV models only varies the damper settings. The extra air from the fan is by-passed back to the return air path.

Therefore, the fan power or energy usage listed in the simulation run is usually simply calculated form the PLR and efficiency of the fan that you have entered.

HVAC,Average,Fan Delta Temp [C]
HVAC,Sum,Fan Electric Consumption [J]
HVAC,Average,On/Off Fan Runtime Fraction []
HVAC,Average,DX Coil Total Cooling Rate [W]
HVAC,Average,DX Heating Coil Runtime Fraction []
HVAC,Average,Unitary Heat Pump Fan Part-Load Ratio []
HVAC,Average,Unitary Heat Pump Compressor Part-Load Ratio []

These are from a heatpump model. I think, similar ones should be available from your model as well.

Although the DX bi-quard coefficients are given in the object, it is set to unity, so that the efficiency does not change with load. There are many fields you can set in the model, therefore, it is difficult to tell without looking at the IDF.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: hujia06@xxxxxxxxx
Date: Fri, 20 Nov 2009 17:31:47 -0500

Subject: Re: [EnergyPlus_Support] electric power (watts) of PTAC

Hello, Dr. Li

In my case, the heating coil type is electric and cooling coil type is SingleSpeedDX.

The power in the xls. is the total power of HVAC (including electric consumption of HVAC system) and not just the fan electric power. The maxmum air flow is autosized by EP.

I got the fan power through this formula: Maximum of the fan power = Max (for a timestep,Variable Fan Electric Power / variable Packaged Terminal Air Conditioner Fan Part-Load Ratio), which is suggested by Richard. I think the power of the fan may be small because the office is small and EP regards the small fan is enough for the HVAC ?

For the electric consumption (W) of HVAC indicated in slx, is it deviated from normal range?

Jia

On Fri, Nov 20, 2009 at 4:47 PM, YuanLu Li <yli006@xxxxxxxxxxx> wrote:

Hi, Hu Jia

I do not know how you get the fan power of 9W.

In your xls, the HVAC power goes to zero, because it is a on/off system. The power value has no real meaning.  PLR is varying and not the equipment power.

When the power reading averaged over an hour period, it may be very low or zero. That was why I said your numbers in the xls were reasonable. The maximum of 992 watt is just over one HP, which is the average size of a window unit.

A typical window unit has a half horse power compressor and a half horse power fan, or two quarter HP fans in a split unit. I assume that the wattage in the xls is not the fan power alone, and is the total power of the HVAC equipment.

A small window unit compressor is the same as the one used in your refrigerator.
A quarter HP fan draws about 200 wats. This amount of heat may be added to your cooling load, if the fan motor is in the air flow path. The exact amount depends on how you define it in the IDF.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: hujia06@xxxxxxxxx
Date: Fri, 20 Nov 2009 14:41:42 -0500
Subject: Re: [EnergyPlus_Support] electric power (watts) of PTAC

Thank you. And I will check the idf file.

On Fri, Nov 20, 2009 at 12:50 PM, Richard Raustad <RRaustad@xxxxxxxxxxxx> wrote:

9 watts is too low.

Hu,Jia wrote:
>
>
>
> Thank you for your specific explanation.
>
> HVAC I used is the templated PTAC provided by EnergyPlus, that is, the
> heating is provided by electric power and cooling is supplied by
> cooling coil.
>
> I calculate the fan electric power using the first method you
> provided, and find the maximum of fan electric power (W) is about 9W
> at the design day.
>
>
> On Fri, Nov 20, 2009 at 10:49 AM, Richard Raustad

> <RRaustad@xxxxxxxxxxxx <mailto:RRaustad@xxxxxxxxxxxx>> wrote:
>
>
>
> You need to identify the maximum power draw of your fan and
> compare that
> to your design intent.
>
> Report these 2 report variables:
>
> Output:Variable,*,Fan Electric Power,timestep;
> Output:Variable,*,Packaged Terminal Air Conditioner Fan Part-Load
> Ratio,timestep;
>
> and, if you are using an On/Off Fan object:
> Output:Variable,*,On/Off Fan Runtime Fraction,timestep;
>
> Maximum fan power is then roughly equal to Fan Electric Power /
> Packaged
> Terminal Air Conditioner Fan Part-Load Ratio or is equal to Fan
> Electric
> Power / On/Off Fan Runtime Fraction.
>
> Another method is to either look for a time during the simulation
> when
> the PTAC is operating at full load or force the PTAC to operate at
> full
> load and then look at the reported fan power for that time period.
>
> You will then need to determine if this maximum fan power is
> representative of your system.
> Fan power of 900 W sounds high for a PTAC (unless it is very
> large), my
> AC system at 3 tons (10.5 kW) gives an estimated fan power of 0.018 W
> per W of rated capacity.
>
>
>
> Hu,Jia wrote:
> >
> > Thank you, Richard.
> >
> > It is realistic then to have the HVAC consumption of 200 W per hour
> > (let's say on average) or in the range of 200-900 W per hour
> > depending on a day, time, that is, external weather conditions.
> >
> --
> 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
> <http://www.fsec.ucf.edu/>
>
> UCF - From Promise to Prominence: Celebrating 40 Years
>
>
>

--
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

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