[bldg-sim] EnergyPlus - DOE

[John F. Kennedy]

Brent,
 
Our firm is very excited about the capabilities of EnergyPlus and look forward to its wide use on design projects, but we really have to justify when to use it for the questions we are trying to answer. 
 
Our view is that EnergyPlus is a tool that should be used where any of the following exist:
 
1. Unique systems are in the design and can not be modeled in DOE-2 (radiant, UFAD, etc.)
2. Comfort analyses
3. Moisture analyses
4. Large building natural ventilation
 
So, if one uses EnergyPlus for the above scenarios their model should be more accurate then a DOE-2 model for the following reasons.
 
1. Radiant systems and UFAD are very sensitive to view factors and temperature differences and must be modeled at the room level rather than a zone level for accurate and meaningful results.
2. Comfort analyses are highly dependant on several things including view factors of surfaces and a simplified thermal zone analysis equates to meaningless surfaces that do not exist from the occupants view point. Consequently, the simulation results of a comfort analysis would be very questionable in a highly simplified model.
3. Any moisture analysis must include all interior surfaces, interior materials, and conditions that can absorb and give off great amounts of moisture and simplifying the model by eliminating surfaces produces a simulation whose results are very questionable.
4. Natural ventilation is very sensitive to the accurate representation of external walls as well as interior partitions. Again eliminating interior surfaces/openings and simplifying exterior surfaces will produce a simulation whose results are very questionable.
 
If it is a typical building with a typical system that can be modeled in DOE-2 and you are trying to answer annual energy use questions, DOE-2 is the ticket. If any of the above scenarios are desired, then a detailed EnergyPlus model is required. Not a simplified one.
 
I am curious to know how many zones, surfaces, and systems your 16.6 minute run had. Do you mind sharing?
 
John F. Kennedy
President & CTO

Green Building Studio, Inc.
www.greenbuildingstudio.com <http://www.greenbuildingstudio.com/> 
707.569.7373 v
707.569.7313 f
707.235.1335 c 

________________________________

From: bldg-sim at gard.com [mailto:bldg-sim at gard.com] On Behalf Of Griffith, Brent
Sent: Thursday, January 19, 2006 10:56 AM
To: bldg-sim at gard.com
Subject: [bldg-sim] EnergyPlus - DOE


Here is some more information on EnergyPlus simulation run times.  
 
I recently did a study with 5,374 EnergyPlus baseline models covering virtually every kind and size of commercial building out there (as reflected in 1999 CBECS public use data).  These are all full-blown, annual building models with 5-zone-per-floor thermal zoning, 15-minute timesteps, PSZ and VAV HVAC systems, etc, etc.  
 
Here is a summary of the run times I encountered:
 
Average:  4.53 minutes
Standard Deviation: 3.15 minutes
Minimum: 0.92 minutes
Maximum: 16.6 minutes
 
These were run on 3GHz, 64 bit AMD processors running Linux with a 64-bit-compile of Version 1.2.3.  
 
I have learned the hard way that it is entirely possible (and quite tempting) to create overly-complicated EnergyPlus models that will lead to excessive run times.  To speed things up, use Zone multipliers wherever practical and keep the number of windows and surfaces low.   I also set Minimum System Timestep to 5 minutes, and Maximum HVAC Iterations to 5. 
 
Brent Griffith, NREL
 


 -----Original Message-----
From: bldg-sim at gard.com [mailto:bldg-sim at gard.com]On Behalf Of Mahabir Bhandari
Sent: Wednesday, January 18, 2006 11:41 AM
To: bldg-sim at gard.com
Subject: [bldg-sim] EnergyPlus - DOE



	Brian,

	 

	I think one additional difference from uses' point of view, which is not reported in this report, is simulation run time. It seems EnergyPlus takes 5-10 times longer to complete a whole year simulations than DOE2. I was running a simple 5 story building with VAV system and it took me more than 40 minutes for annual hourly simulations (1 hr time step) in EnergyPlus while eQuest could run the same building in less than 5 minutes. I am interested in hearing from others on the run time issue.

	 

	Mahabir Bhandari

	 

	 

	
________________________________


	From: Brian Fountain [mailto:bfountain at greensim.com] 
	Sent: Wednesday, January 18, 2006 12:12 PM
	To: bldg-sim at gard.com
	Subject: [bldg-sim] EnergyPlus - DOE

	 

	That is a big question.

	 

	For a comparison of current energy simulation software, you could look at Dru Crawley's paper comparing 20 tools:

	http://www.energytoolsdirectory.gov/pdfs/contrasting_the_capabilities_of_building_energy_performance_simulation_programs_v1.0.pdf

	 

	Main differences between DOE and EnergyPlus:  

	1)      DOE calculates the space loads for each hour then calculates the system and plant response for each hour.  EnergyPlus simultaneously solves the loads the system/plant response for each timestep.

	2)      DOE has a fixed 1 hour timestep, EnergyPlus has a variable timestep (down to 1 minute?).

	3)      There are also key issues with the weighting factor assumptions, the wall convective coefficients.

	 

	I haven't used EnergyPlus much yet.  What did I miss guys?

	 

	Brian Fountain

	 

	-----Original Message-----
	From: bldg-sim at gard.com [mailto:bldg-sim at gard.com] On Behalf Of Renato Sousa
	Sent: January 18, 2006 10:56 AM
	To: bldg-sim at gard.com
	Subject: [bldg-sim] EnergyPlus - DOE

	 

	Hello everybody.

	 

	I would like to know if anybody has experience on using both EnergyPlus and DOE.

	Currently I'm using an interface for EnergyPlus. From the software description I couldn't find out the main differences between EnergyPlus and DOE. Does anyone know the main differences or where can I find them?

	 

	From the description it is said that EnergyPlus was based on DOE and BLAST so I suppose EnergyPlus has some extra features but I'm not sure if that is right.

	 

	Best regards,

	Renato Sousa

	Tel: +351 919 316 221 | E-mail: renato.sousa at fluidinova.pt

	 

	___________________________________________________

	FluidInova, Engenharia de Fluidos, SA

	TECMAIA - Parque de Ciência e Tecnologia da Maia

	Rua Engº Frederico Ulrich, nº 2650 | 4470-605 Moreira da Maia | Portugal                  

	Tel: +351 229 408 265 | Fax: +351 229 408 266

	E-mail: geral at fluidinova.pt <mailto:geral at fluidinova.pt>  | Web: www.fluidinova.pt/

	 

	 

	 
	 
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