Energy Management System (EMS)

 

The Energy Management System (EMS) is one of the high-level control methods available in EnergyPlus. An EMS script is able to access a wide variety of “sensor” data and use this data to direct various types of control actions. The original concept was to emulate, inside EnergyPlus, the types of controls possible with the digital energy management systems used in real buildings. However there are many other application for the EMS including:

 

• Obtain more precise control over simulated behaviour for HVAC systems, glazing, lighting etc (e.g. override default EnergyPlus control linkage between economisers and heat recovery).
• Linking one system with another (e.g. link earth tube to HVAC inlet).
• Provide special outputs not provided natively by EnergyPlus (e.g. generate CIBSE TM52 or TM59 outputs).
• Researching new building systems.

 

The EMS allows the functionality of many of the existing high-level control objects to be overriden. For example the EMS can:

 

• Turn on and off pumps and fans and adjust their performance characteristics.
• Turn on and off lights and electric equipment,
• Change the set points on zone thermostats,
• Dispatch on-site generation.
• Change the set points on system nodes.
• Turn on and off supply-side heating, cooling, and heat rejection equipment such as boilers, chillers, cooling towers.
• Turn on and off on-site generation equipment.

 

The EMS uses the concept of sensors to obtain information from elsewhere in the model for use in control calculations. Actuators are the mechanism for influencing changes on the model and code is written to tell the Erl processor how to apply the control laws.

 

 

Sensor objects are used to declare Erl variables which are linked to EnergyPlus output variables or meters. Actuators map Erl variables to control actuators elsewhere in EnergyPlus. The EMS then initiates control actions by changing the value of this variable inside an Erl program.

 

The way it works is probably best understood by studying working EMS scripts such as those provided by DesignBuilder alongside this documentation and that in the EnergyPlus EMS application guide.

Note: EMS is an advanced feature of EnergyPlus and is not for beginners. To use it you will need to be comfortable writing your own custom code and have a thorough understanding of how you want your models to behave. If you are intimidated by the idea of writing programs to adjust the fine details of how an EnergyPlus model runs then EMS is probably not for you. However, if you relish the idea of being able to write code to override default behaviour, you will find that EMS can help you model many features to your exact specification, significantly extending EnergyPlus' normal range of capabilities.

 

DesignBuilder includes some important extensions to the standard EnergyPlus EMS and Erl system allowing DB EMS programs to reused and shared. DesignBuilder extensions provide:

 

• Portable Erl scripts
• Zone, surface and opening loops allow code to be written that doesn't reference specific object names in Erl scripts.
• All available actuators, sensors, internal variables and program entry points can be selected from lists.
• Connection between the script and the model, e.g. attribute settings from the model can be inserted in the script or used to control whether parts of the script are executed or not.

The DesignBuilder EMS functionality can be accessed from the Scripts toolbar icon and also from the Tools > Scripts menu option which opens the Script Manager dialog.

 

 

Previously created EMS programs are listed on the data panel to the right. These can be selected, copied, deleted and edited in the usual way.

 

Two steps are required to enable an EMS program:

 

1. The Enable scripts checkbox on the Script Manager dialog must be checked.
2. The Enable program checkbox on the Edit Script dialog must be checked.

Note: Selecting an EMS program on the Script Manager dialog is not enough to ensure its script is included in the simulation - the Enable program checkbox is the important setting.

DB Erl scripts are defined on the Edit Script dialog which provides utilities for quickly adding template script for new sensors, actuators, internal variables and programs.

 

As well using EMS for advanced control and customisation you can also use the system to include simple additional IDF script with the model. This is an easy way to add more outputs or to add HVAC or other EnergyPlus IDF objects not yet supported by DesignBuilder. It offers a more flexible approach than the Include IDF option on the Model and Simulation options dialogs because the scripting system saves the IDF data within the dsb file making it completely self-contained and portable. Also you can store and manage a list of various IDF datasets with EMS which makes it easier to include/exclude various models and/or outputs in a controlled way. A further advantage when adding IDF for outputs is that the Add Output dialog lists all outputs available for the current model which provides a convenient way to select any extra outputs that are required.

 

It is possible to write quite complex EMS programs and there is plenty of scope for making mistakes, which can take time to track down and fix. Please be aware that the DesignBuilder Support Desk is not generally able to offer a free service for debugging or advising on the possible cause of errors in EMS programs. See the Support Desk Policy.

 

The EnergyPlus EMS application guide provides a section on debugging EMS programs which is a good starting point for tracking down problems.