IDA Indoor Climate and Energy

On the effects of decoupling air flow and heat balance in building simulation models

Per SahlinEQUA Simulation ABStockholm, Swedenwww.equa.se

IDA Simulation Environment

IDA Indoor Climate and Energy

IDA Room

IDA Road Tunnel Ventilation

Problem and Outline

• Why air-flow coupled thermal simulation by symbolic equations and general-purpose, variable-timestep DAE solvers?

• A decent GUI will be used

• Coupled models may be mathematically sticky:non-linearities + stiffness = short steps

• Will “ping-pong” decoupling help?simpler structure – stability problems > 0 ?

• Brief demo

• Test cases and results

• Alternative measures

• Summary and conclusions

Why DAE based building simulation?

Traditional approach:

– Flexibility

– Reuse

+ Speed?

Why DAE based building simulation?

GUI

Equation Based Framework

SymbolicProcessor

LanguageParser

NumericalSolver 1

NumericalSolver 2

Envelope & Zone Heat Balance

Models

Climate & Radiation

Models

Shading

Models

HVAC

Models

Industry independent Industry specific

Symbolic equation based approach:

+ Flexibility

+ Reuse

+ Secure model investment

– Speed?

The experiment

Full coupling:

m1-2Enthal(T1, H1), m1-2 > 0

q1-2 =

m1-2Enthal(T2, H2), m1-2 < 0

“Ping-pong” decoupling:

m1-2,oldEnthal(T1, H1), m1-2,old > 0

q1-2 =

m1-2,oldEnthal(T2, H2), m1-2,old < 0

Demo

Link to demo

Test cases and results

case decoupled execution time

family apartment two-zone constant air volume (CAV) office with open door -

13%temperature controlled variable air volume (VAV) ditto +

74%single-zone office with natural ventilation + 59%single family, naturally ventilated house 

Bedroom 1Living room

Entrance

Bedroom 1Kitchen

Bathroom

0,90m

Alternative measures

Apartment case:

fanout=5 simpl.

zone

kmax=2,

n_jac=4, tol=0.2

no internal LVOs

execution time

√     

- 46%

√ √   

- 59%

√ √ √ 

- 67%

√ √ √ √ - 79%

Summary and conclusions

•“Ping-pong” decoupling provided minor improvements for some cases.

•It also gave rise to fatal instabilities, making the approach unattractive

•A combination of other means proved effective in dealing with the problematic Apartment case

•Packaging and automating the application of such means seems to be a viable alternative

Questions and discussion

On the effects of decoupling air flow and heat balance in building simulation models

Per SahlinEQUA Simulation ABStockholm, Swedenwww.equa.se