"Too many iterations (SOLVZP) in Airflow Network simulation"
This is caused by EnergyPlus failing to solve an equation internally due to the presence of very high airflow heat transfer between zones. The usual cause is a large horizontal hole between stacked zones. It is often possible to solve the problem by increasing the size of the tolerances as described in the program help on advanced calculation options. For example try multiplying both the "Relative airflow convergence tolerance" and the "Absolute airflow convergence tolerance" model option values by 10 until the simulation is able to proceed. In most cases it is sufficient to multiply both by 100.
Updated 02.Nov.2016
]]>There are in principle two possible approaches when modelling tall spaces such as atria using DesignBuilder with calculated natural ventilation:
1. Model the atrium as a single tall space or,
2. Model the atrium as a series of horizontal slices with air flow connections between them.
Option 1, where the atrium is modelled as a single space is the best option because:
a) There is no reliable way to get solar radiation pass through multiple horizontal slices in the general case.
b) EnergyPlus allows you to set up a dynamically varying temperature gradients to improve the accuracy of simulations in tall spaces. This can be accessed in DesignBuilder using the Temperature distribution data on the HVAC tab. Be warned though that, in current versions, the EnergyPlus temperature distribution model is not applied to airflow which occurs in the airflow network simulation so it is not possible for example to include the effect of relatively warmer air being extracted from a vent at the top of an atrium. In the simulation air would be extracted at the mean temperature. For more information on air temperature distribution in DesignBuilder/EnergyPlus see:
www.designbuilder.co.uk/programhelp/index.htm#air_distribution.htm
So at the moment we have to live with some inaccuracy in EnergyPlus models due to temperature gradients in tall spaces. Otherwise there is no problem. DesignBuilder also includes a CFD module which calculates spatially distributed temperatures within zones as a 'snapshot' but we are some way away from dynamic CFD simulations.
In summary, there are inaccuracies involved with large horizontal openings in airflow network simulations (a and b above), but these may not be 'show stoppers', especially if the openings are small. It is always worth checking that hourly temperature results in the various zones look believable. Considering all the other inaccuracies involved in building energy simulations you may find that the approximation involved with using algorithms intended for LVO to model LHO may not cause serious problems.
]]>See also Is DesignBuilder suitable for modelling natural ventilation?
]]>Scheduled:
You define the natural ventilation rate and its operation. The advantages of 'Scheduled' natural ventilation are:
Calculated:
You define opening and crack sizes, pressure coefficients and control strategies and EnergyPlus/AIRNET calculates the natural ventilation flow rates including consideration of wind and buoyancy pressure effects, natural ventilation setpoint temperatures and flow between zones. DesignBuilder automatically sets up default Wind Pressure Coefficients using standard AIVC data. The AIVC WPC are good for early design stage natural ventilation studies but should be replaced by WPC from CFD calculations or wind tunnel measurements for more detailed studies, high rise buildings or buildings using other features such as wind pressure induction vents which require special treatment. DB Version 2 will have integrated CFD for calculating WPC and also will take E+ output as boundary conditions for internal airflow analysis.
Advantages of 'Calculated' natural ventilation are:
One approach combines both methods to create a realistic model that simulates fast:
You can select 'Scheduled' or 'Calculated' natural ventilation from the Model Options dialog.
Check the Program Help for more information on natural ventilation modelling.
See also: Using the 'Calculated' Natural ventilation Model option
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