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Re: [EnergyPlus_Support] HVAC modelling - Key definitions
1. REPORT VARIABLES are defined in the Input Output Reference document.
Search for the report variable name to locate the definition.
2. Yes, the COMPACT HVAC objects which you list are correct for this type of
system (without "reservoirs", thermal storage).
3. To change the boiler part load performance, run a simulation with the idf
file containing the COMPACT HVAC objects. Then take the expidf output file and
rename it to <newfilename>.idf. Find the BOILER:SIMPLE object and edit the
part-load coefficients.
4. Similar to the boiler, find the CHILLER object in the expanded idf file and
change the part-load performance. If the available inputs are not sufficient
to model the two-stage performance, you may find it necessary to model this as
2 different chillers, one that represents stage-one operation, and another that
represents stage two?
5. If I understand correctly, the boiler controls will turn on the boiler when
the hot water supply tempertaure falls below 40C and turn off the boiler when
the supply temperature reaches 45C? This type of control is not supported. In
the real system, the impact of the reservoirs will be to lengthen the cycle
time of the boiler (or chiller). The only suggestion I have for modeling this
would be to change the part-load curve for the boiler (or chiller) to show
improved efficiency at low loads compared to a system without the reservoirs.
Mike
On 2 Jan 2008 at 8:29, Ana Oliveira wrote:
> Hi.
>
> I've made a first approach to E+ HVAC models, but I find it difficult to
> correctly interpret all the relevant variables. I don't have much experience
> working with HVAC systems, so I apologize if some of the questions are very
> basic... If that is the case, please tell me so I can refer to more
> general/theoretical docs. I'll try to separate the questions in smaller
> batches!...
>
> I need to model a 4-pipe system for a large service building, served by:
> - a chiller (condenser on the roof) - Operating temperatures: 7º/14ºC;
> - and a set of 3 condensation boilers (fuel: natural gas) - Operating
> temperatures: 45º/40ºC; - with constant flow pumps on the return branches - both
> the chilled and the hot water systems include a reservoir - (plus an auxiliary
> set of vaccum tube solar collectors, that I dropped for now; they will only
> interact through a heat exchanger); - each room as an independent temperature
> control, achieved by a unit that simply recirculates the air (constant flow) in
> the room passing it through the internal heating and cooling coils.
>
> From what I've seen, I think I can properly model the essential of this system
> with the Compact HVAC: - Zone: Fan Coil model, and the respective Thermostat
> (dualband), Hot and Chilled Water Loops (constant flow), Chiller (electric, air
> cooled, constant flow) and Boiler (natural gas, constant flow)...
> ... RIGHT ?... remarks?...
> But:
> - How do I properly model the efficiency of a condensation boiler?
> - How do I model a chiller with a two stage operating compressor (50%-100% of
> maximum power)? - How do I model the thermal reservoirs, at aprox. 7ºC and 45ºC
> (because it will affect the on/off states of the chiller/boiler and thus the
> energetic behaviour of the building, right?....)?
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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