[Virtual-sim] [Bldg-sim] One model

Tue Mar 24 00:24:13 PDT 2009

Regarding the modeling of water-cooled chillers, chilled water loops, and HVAC systems in IES <Virtual Environment>:

In response to user demand, IES is expanding the capability for HVAC systems modeling within IES <Virtual Environment> to more thoroughly represent both conventional and unconventional HVAC systems. 

While not yet publically released or announced, in-depth capability for modeling water-cooled chillers and chilled water loops within the IES <Virtual Environment> ApacheHVAC application has been developed and implemented. This capability is presently undergoing testing and will be available in the next release later this spring. 

The new water-cooled chiller model is described by three performance curves: 

1)      varies cooling capacity with entering condenser water temperature

2)      varies electric input ratio (EIR) with chilled water temperature

3)      varies electric input ratio (EIR) with part-load fraction. 

These scalable performance curves are pre-defined for common chiller types: screw; centrifugal (hermetic, with or without VSD, and open); and reciprocating (hermetic and open). Where water flow is variable, performance curves are provided for pumps. These include constant-speed/variable-flow "riding the pump curve" and electronically commutated variable speed. For expert users, parameter values are editable in the equations describing the performance curves for chillers and pumps. 

In addition to selecting appropriate performance curves, the user is able to specify interconnected performance inputs for capacity, COP, condenser and chilled water temperatures, flow rates, and loop delta-T at both rated and design conditions. An input is provided for the load fraction below which the chiller will cycle on and off. The user also has control of operating parameters such as condenser-water setpoint and variation of this setpoint via a formula profile (which may reference outdoor or indoor conditions and other sensed variables), chilled supply water temperature (SWT), and ramped SWT reset between high and low outdoor temperature setpoints. 

The cooling tower and condenser-water loop use essentially the same cooling tower model that is provided for the waterside economizer component in the current version of ApacheHVAC. The cooling tower model presently includes a two-speed fan with flow fraction setting for each speed, approach, range, and electric input ratio. This tower and condenser-water loop model accounts for changes in outdoor conditions, chiller heat rejection, and user-controlled tower operating parameters as described above. 

Primary and secondary chilled water loops are modeled for the first release of this new facility in a PRM-compliant "common pipe" configuration. For this configuration, the primary loop uses a constant-speed pump that operates whenever the chiller is running and the secondary loop with flow controlled to maintain the specified CHW supply temperature and loop delta-T. Either a constant-speed/variable-flow pump riding the pump curve or a variable-speed pump with variable speed drive (VSD) can be specified for the secondary (chilled-water distribution) loop. Cooling coils, chilled ceilings, etc. are then assigned to the secondary loop associated with a particular chiller.

The currently available release of the VE uses a simpler model that does provide means of describing the performance of a water-cooled chiller, cooling tower, and pumps in relation to both part-load fraction and outdoor wet-bulb or dry-bulb temperatures. This is presently done via a matrix of user input data points for the chiller COP, condenser-water pump power, cooling tower fan power, and chilled-water pump power. With appropriate inputs, the chiller COP in this simpler exiting model can be thus be varied according to both load fraction and outdoor conditions, while the power for the associated pumps and fans can separately be varied according to load fraction. However, because this simpler model does not use the type of curves that many EnergyPlus and DOE-2 users are accustomed to and does not explicitly model the thermodynamics of the CW, primary CHW, and secondary CHW loops or their controls, the new facilities described above have been developed to do just this.

It is worth noting that the USGBC generally supports the selection of the simulation tool that will most accurately represent what the user is attempting to model, while also suitably addressing the 90.1 PRM requirements for the baseline model. This requires a degree of professional judgment and USGBC approval is on a case-by-case basis. For example, one might be attempting to model a novel air handler configuration, such as a DOAS with indirect evaporative cooling, DX dehumidification, and a desiccant wheel regenerated by waste heat from the DX condenser coil, in a building that uses passive chilled beams fed by a water-cooled chiller. In such a case, it may be that the benefits of added detail in modeling the air handler configuration, desiccant wheel, and the combined radiant and convective effects of the passive chilled beams would outweigh certain limitations with respect to other aspects of the system, such as chiller sequencing. Similar professional judgment may need to be exercised to appropriately account for heat transfer mechanisms and system dynamics when modeling a UFAD system, displacement ventilation, double-skin façade, earth tube pre-conditioning, natural ventilation, mixed-mode operation, or highly customized controls. Ultimately the energy modeler has to make an informed selection of the most appropriate tool for the project at hand. 

Finally, with regard to the "One Model" topic that started this Bldg-Sim thread, the IES <Virtual Environment> is intended to address the need for an interdisciplinary range of technical analyses within a single, integrated model. In part, this is provided through interoperability with other tools, such as SketchUp, Revit, and other BIMs, thus minimizing and potentially eliminating the need for preparing a separate 3D model. The <VE> also includes a range of applications, several of which were described by users earlier in this thread, for integrated analysis of solar shading, daylight modeling, daylighting controls, thermal mass and dynamic load calculations, natural ventilation, thermal comfort, airflow, IAQ, conventional HVAC systems, building-integrated HVAC systems, etc. The <VE> provides a range of tools for analysis appropriate to each stage of the design process, facilitating both quick, early reduction of building loads followed by detailed investigation of technical opportunities. 

Hopefully this helps to clarify the capabilities of the VE for those concerned specifically with modeling HVAC systems and more generally with the "one-model" approach.

Timothy Moore 
Senior Consultant
Special Projects 

Office: 415 983-0603
timothy.moore at iesve.com <mailto:timothy.moore at iesve.com> 
www.iesve.com <http://www.iesve.com/> 

**Design, Simulate + Innovate with the <Virtual Environment>**

Email Disclaimer <http://www.iesve.com/disclaimer.html> 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Marcus Sheffer
Sent: Thursday, March 19, 2009 7:01 AM
To: 'm.lebourhis'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] [SPAM] Re: One model

USGBC does allow calculations outside the model for end uses which do not impact other end uses such as exterior lighting or domestic hot water.  If the cooling central plant can be completely isolated and you can demonstrate that the loads placed on it are exactly the same in both models then it may be allowed.  In order to do this with a high degree of accuracy you may have to build virtually the same model in both versions of the software.  You would also need to very thoroughly explain your modeling procedures and calculations.  By the time you did all that my guess is that you would be better off just submitting the whole model in eQUEST instead of justifying the combination.

So as a LEED reviewer I would be highly skeptical of the scenario you suggest and would likely scrutinize it to a significantly higher degree compared to all calculations in one model.  As I understand it IES-VE does not yet model water cooled systems very well so I might suggest that it is simply not an appropriate modeling tool to use for this particular project.

Marcus Sheffer

7group

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of m.lebourhis
Sent: Thursday, March 19, 2009 7:14 AM
To: bldg-sim at lists.onebuilding.org
Subject: [SPAM] Re: [Bldg-sim] One model

Hi all,

I am also currently working with IES-VE for LEED energy modeling. I am finding it very efficient to compute the loads on the project I am working on, which is quite huge and relatively complex in terms of geometry/envelope.

However, it gets harder to model the water chilling plant as required by Appendix G, and I am actually doing this part of the modelling with my own excel calculation sheets and with DOE-2 curves. 

I was wondering; how will this approach be received by the USGBC? Could they possibly require that the whole modeling is carried out with a single package? Has anyone had problems validating EA1 with part of the energy modelling being external to the simulation tool?

I have also been thinking of somehow using EnergyPlus or eQuest (preferably the latter) just for the water chilling plant modeling. Has anyone ever used one of these two tools to compute the systems energy from a cooling demand calculated in an other simulation package?

Regards

Mathieu Le Bourhis

________________________________

De : bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] De la part de Field, Kristin
Envoyé : jeudi 12 mars 2009 21:40
À : bldg-sim at lists.onebuilding.org
Objet : Re: [Bldg-sim] One model

I tried out IES-VE a few months or a year ago with some people because the potential for this type of integration seemed very exciting.  However, we found the HVAC side to be pretty basic.  For a lot of building types, there are significant savings to be had by changing HVAC control schemes and/or configurations and using HVAC types that were not well-represented in IES-VE the last time I saw it.  It seemed like a great interface to use if you were trying to design a building to use minimal energy due to envelope, orientation, natural ventilation, etc.  In other words, it seems strong on the loads calculation side.  On the system use side, though, it seemed a lot more limited than DOE2 or EnergyPlus.  For that reason, I have also stuck with the multiple-tools method.  I agree with all who wrote that it would be wonderful to have one integrated tool in the future!  Rob does bring up an interesting point about the resolution of spaces needed for daylighting vs. thermal vs. architectural modeling.  Architects especially have to be so detailed in their models that I've often just ended up using the floor plans they provided, deleting a bunch of elements out of those, and scaling wall heights and windows.  Other than that, their 3D models only serve to help me visualize the building myself.  It would be nice if we didn't end up tripling the effort like we all do now, but so far I don't see a great way around it (unless you don't need complex system modeling - then you could use IES-VE as discussed below).

Kristin Field

________________________________

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of CARNEY Ronan
Sent: Wednesday, March 11, 2009 10:34 PM
To: Rob Guglielmetti; Ross Harding
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] One model

Actually, I'm utilising IES to do HVAC design, Energy Analysis, PMV calc, lighting lux design, lighting power intensity design and natural daylight analysis all in line with the Australian Standards and Green Star (another form of LEED) requirements. 

Although the software costs money, I've found it to be worthwhile battling against it's flaws and it gives us the results which we require. 

Please note, this is a one model approach. It's easy to start a project at CD (concept design) stage within Google Sketchup and to utilise the freeware version of IES to do alternate design analysis. However, it's way better if the full suite is purchases and the model is crossed over into IES. Within IES multiple analysis and detailed designs can be modelled quickly and with fantastic final project results. I know, I've done it.

Also with the daylight analysis, if you've got a detailed building with over 100000 surfaces, it's going to take forever to do the analysis, that's my only issue with the software. However, the software is developing multithreading facility within it's software, therefore decreasing the time for analysis dramatically.

Should anyone want to discuss my approaches further, and are in Brisbane, I welcome the opportunity to meet and greet with you.

Regards,
Ronan Carney
Electrical Engineer
Project Services
*Level 5B 80 George St Brisbane
(Ph: (07) 3224 5983
2Fax: (07) 3224 6151
,Email: Ronan.Carney at projectservices.qld.gov.au <mailto:Ronan.Carney at projectservices.qld.gov.au>  

________________________________

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Rob Guglielmetti
Sent: Thursday, 12 March 2009 1:12 PM
To: Ross Harding
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] One model

On Mar 11, 2009, at 6:09 PM, Ross Harding wrote:

The ultimate goal for us would be to use one model for daylight and energy models, but I haven't found that quite that simple as most software requires varying drawing techniques.

Amen brother!  Another issue is that while the energy model has to include the total building -- all spaces and systems -- to be a complete picture of the energy use, a Radiance-based daylight model does not necessarily require all the spaces to be modeled simultaneously.  Indeed, the accuracy of the ambient calculation is directly affected by the maximum size of the scene, and so conducting a simultaneous daylight simulation of an entire building at an acceptably rigorous ambient resolution can be -- is generally -- time-prohibitive.

As mentioned in a recent thread on this list, Thomas Bleicher's "su2rad" plugin for SketchUp can export a SU model to Radiance format and there is also an E+ plugin to allow one to use the same SU model for E+ analysis; in theory, these three components (SketchUp, su2rad and the E+ plugin) allow for a "single model" approach to energy and daylight modeling. However, because the su2rad exporter wants to take the entire model and create a single Radiance scene description, that creates the aforementioned problem of the ambient calculation getting out of hand fairly quickly.  I suppose through intelligent layering and model structuring, one could create a model that could be exported to Radiance in "space components", but I'm not sure if this is compatible with the E+ plugin's layering/model organization requirements.

- Rob Guglielmetti

********************************************************

Kristin M. Field

Engineer II - Mechanical

Electricity, Resources & Building Systems Integration

National Renewable Energy Laboratory

1617 Cole Blvd.

Mail Stop #1725

Golden, CO  80401

Phone: 303.384.7376

Fax: 303.384.7540

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