Advanced Calculation Options - Cooling Design

Cooling Design tab on Model Options dialog and General tab on the Cooling Design Calculation options dialog.

General Solution

Inside face surface temperature convergence criteria

The surface heat balance model at the inside face has a numerical solver that uses a convergence parameter for a maximum allowable differences in surface temperature. This field can optionally be used to modify this convergence criteria. The default value is 0.002 and was selected for stability. Lower values may further increase stability at the expense of longer runtimes, while higher values may decrease runtimes but lead to possible instabilities. The units are °C or °F.

Temperature and loads convergence

The temperature and loads convergence values represent the maximum allowable difference in zone temperature and loads between successive daily iterations before convergence is considered to have been reached during warmup.

Convergence of the simultaneous heat balance/HVAC solution is reached when both the loads and temperature criteria are satisfied. Both tolerances work the same way, one looks at temperatures and one looks at heating and cooling loads. After the second warm-up day, the program compares the maximum temperature experienced in a space with the maximum temperature from the previous day. If those two temperatures are within the tolerance, then it has passed the first warm-up check. It does a similar comparison with lowest temperatures experience within all the zones. If the current simulation day and the previous day values are within the tolerance, then it has passed the second warm-up check. A similar comparison is carried out with the loads tolerance and the maximum heating and cooling loads that are experienced within the spaces. Those are compared individually to the values for the previous day. If they are both in tolerance, then the simulation has passed the third and fourth warm-up check. The simulation stays in the warm-up period until ALL FOUR checks have been passed.

Convection

Inside and Outside convection algorithms

You can select from a range of EnergyPlus inside convection algorithms for calculating the convection between internal zone surfaces and the rest of the zone air in the simulation calculations. More details on this and external convection can be found under Constructions Model Data > Surface Convection header.

Shading

Maximum number of shadow overlaps

Enter the maximum number of figures per shadow overlap. The shadow overlaps is a measure of the amount of complexity in the shading calculation and this maximum value allows you to limit the amount of time spent in the solar initialisation calculations.

Note: entering a small value here can speed up simulations in complex buildings. If you plan to use Maximum shadow overlaps below the default value of 15000 you should check accuracy of solar gains relative to results using default value

Polygon clipping algorithm

This is an advanced feature. Prior to V7, the internal polygon clipping method was a special case of the Weiler-Atherton method. Now, two options are available:

1-Sutherland Hodgman (default)
2-Convex Weiler Atherton

Theoretically, Sutherland-Hodgman is a simpler algorithm but it works well in cases where receiving surfaces (of shadows) are non-convex. The Weiler-Atherton implementation is only accurate where both casting and receiving surfaces are convex. Warnings/severe errors are displayed when necessary. More details on polygon clipping are contained in the Engineering Reference.

Other

Surfaces within zone treated as adiabatic

EnergyPlus support recommend modelling surfaces wholly contained within a zone as adiabatic and this option allows you to follow this advice. We have found that in practice this option does not make much difference in results or in simulation speed so for most cases you can leave it in its default state.