The simulation process explained

Overview of the simulation process. (Source: IBPSA-USA BEM Workshop)

When your model is ready, what happens when you press the button to start the simulation? The simulation engine captures the interactive effects of the weather, the zones in the building, and the systems serving the zones. The engine will calculate loads in the spaces, loads at the system, and energy consumption by the systems (and potentially other metrics such as carbon emissions and energy cost) on an hourly basis (or time steps less than 1 hour for additional detail of time response to loads and system behavior). This page provides a detailed description of the interactive calculations that happen during a simulation.

Most modeling tools provide calculations for both the individual zones, and for the HVAC systems to determine how the systems will meet the zone loads.

Zone model calculations

Zone model calculations. (Source: IBPSA-USA BEM Workshop)

The zone model calculations account for many factors such as:

• Zone characteristics: the characteristics of each zone, including its size, shape, orientation, and thermal properties (such as the insulation level and air tightness).
• Occupancy and equipment: the amount of heat generated by the occupants and equipment in each zone. This includes the number of occupants, their activity level, and the heat generated by lighting, computers, and other equipment. In addition to heat gains, the zone model also calculates energy consumption associated with lighting and plug loads located in the zone.
• Solar radiation: the amount of solar radiation that enters each zone through windows or other openings. This depends on the orientation and shading of the windows, as well as the time of day and year.
• Envelope heat gains and losses: heat gain or loss, such as conduction through walls and floors, and heat transfer between zones.
• Infiltration: heat gain or loss due to infiltration of outside air through the building envelope.

Zone model calculation approaches

Zone model calculation approaches. (Source: IBPSA-USA BEM Workshop)

The zone model calculations may use either the weighting factor method or the heat balance method.

Weighting factor method

The weighting factor approach involves assigning a weighting factor to each mechanism of heat transfer (conduction, convection, and radiation) based on its relative importance in the heat transfer process. These weightings can be based on factors such as the temperature difference, surface area, and material properties of the system. Once the weightings have been assigned, they can be used to calculate an overall heat transfer coefficient for the whole system. The weighting factor method is fast and accurate for conventional systems but cannot accurately model radiant systems that rely on surface temperature information.

Simulation engines that use the weighting factor method include the DOE-2.1E and DOE-2.2 simulation engines.

Heat balance method

The heat balance method relies on a more explicit surface-by-surface heat balance calculation at each time step where a radiative, conductive, and convective heat balance is done on each room surface (both inside and outside) using the first laws of thermodynamics. The heat balance method is accurate, and provides more explicit information about surface temperature, and time variant data than the weighting factor method. As a result it has greater capabilities that allow for thermal comfort analysis, accurate radiant system analysis, and detailed reporting of the components that make up zone loads.

Simulation engines that use the heat balance method include EnergyPlus and IES Apache.

HVAC model calculations

HVAC model calculations. (Source: IBPSA-USA BEM Workshop)

The HVAC model calculates the HVAC systems' response to the zone loads at each time step in the simulation. The HVAC system will attempt to meet the load by supplying heating and/or cooling energy to the zones. The performance characteristics of the systems and system components are used to calculate predicted energy consumption, usually with the ability to break down the results by the components of the system such as fans, pumps, chillers, boilers, etc.

The HVAC model also calculates the amount of airflow, and water flow in the systems, and temperatures at various locations in the systems. This information is useful for system design, and also performing QA/QC to ensure that the model is behaving as expected.

HVAC model calculation approaches

Approaches to capturing interactions between the zone and system model calculations. (Source: IBPSA-USA BEM Workshop)

The HVAC model may run separately (after completion) from the zone model, or it may be run in parallel with the zone model where both are calculated simultaneously.

Sequential calculations

In this approach, the annual calculations for the zone model are performed first, then the HVAC model calculations are performed separately. The benefit of this approach is that it is very fast but it does not provide feedback on how the zone temperatures respond to undersized HVAC systems.

Simultaneous calculations

In this approach, the zone loads and system response to the loads are calculated for each time step. This provides a more detailed representation of the HVAC system performance. For example, if a system is undersized, or turned off, then the zone temperature "drift" can be estimated. It is also able to calculate temperature conditions in unconditioned areas of the building.