Technical Guidance: Building Energy Management Systems (BEMS)
The Accelerated Capital Allowances (ACA) list of eligible products includes Building Energy Management Systems (BEMS).
CONTENTS LIST
BEMS are computer-based systems that automatically monitor and control a range of building services, including:
Other facilities can also be integrated in the system, including:
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| - Energy monitoring and targeting
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A BEM system consists of one or more self-contained computer outstations or controllers that monitor and control the energy usage in a building.
The outstations are designed with customised software to control energy-using plant and equipment, report on the plant’s performance, and optimise building energy efficiency.
This guide:
- outlines the benefits, components and operation of a BEM system
- gives an overview of Energy Monitoring & Targeting Systems
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A Building Energy Management System (BEMS) can do a range of extremely useful things. It can:
- Switch plant on and off automatically
It can do this according to time, type of day, or environmental conditions. For example, it can control lighting to avoid unnecessary use of energy outside normal working hours or when ambient daylight levels are adequate. - Optimise plant operation and services
A typical BEMS application is the optimum start/stop routine for space heating. In this, the system can automatically adjust the start and stop times to compensate for external temperature changes and the thermal inertia of the building. - Monitor plant status and environmental conditions
A building manager can be alerted to ‘alarm’ conditions in time to take remedial action. The BEMS can therefore improve standards of operation and maintenance. - Provide energy analysis and management information
The BEMS provides you with easily available data on energy flows, consumption, trends and overall building performance. (See also What is an energy monitoring and targeting system? )
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The basic architecture of a BEMS is shown in Figure 1.
Figure 1: BEMS architecture
The components of a BEMS are:
- Outstations – direct digital controllers
- BEMS network and communications hardware
- Computer with supervisory and control software
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The outstation or controller is at the heart of the modern BEMS. It is designed to operate and control building services such as heating, ventilation, air conditioning, lighting, electrical demand and refrigeration equipment.
Outstations incorporate the following components or modules: Inputs, Outputs, Microprocessor, Memory, RAM, Eprom with Configurable Strategy, Modules, Time-clock, Power Supply, Local RS232 port for Supervisor.
Inputs: provided by sensors, relays and meters. The outstation can be configured to accept a range of analogue, digital and pulsed inputs. Pulsed inputs from energy meters are usually volt-free contacts.
Outputs: The outstation provides a range of analogue, digital and relay outputs.
Microprocessor or central processing unit (CPU): The microprocessor performs the control operations for the outstation.
EPROM – Strategy configuration: The outstation has a pre-programmed EPROM. Standard control functions or modules (firmware), provided by the BEMS manufacturer, are stored on the EPROM. These modules may be linked (softwired) and configured to provide the necessary outstation strategies.
The strategy configuration is usually programmed and set up on the central computer using software provided by the manufacturer and then downloaded to the outstation.
Strategies available include:
- Direct Digital Control: including proportional (P), proportional integral (PI) and proportional integral derivative (PID)
- Time-based functions/control
- Event-based control
- Optimum start/stop
- Data logging
- Metering
- Electrical demand management
- Lighting
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In accordance with the European Committee of Standardisations, communications within an intelligent building may be divided into three areas:
- Level 1: Field level, covering sensor and actuators, lighting systems
- Level 2: Automation level, covering the outstation/controllers
- Level 3: Management (i.e. supervisory) level
Communication protocols include: Ethernet, BACnet, ARCNET, ModBus, LonWorks, KNX and Internet.
The trend of development is that the BEMS will become an integral part of an information management system.
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A central computer with specialised software, usually called supervisory software, allows the user to operate the BEMS. The software provides a number of distinct components, including:
Engineering/strategy configuration tool: This allows the user to set up strategies and program the outstations.
Graphics software: Programmable graphic displays of plant operations with live point values that allow the BEMS to be operated and monitored.
Figure 2: Graphic display of plant operations (courtesy of Dublin Institute of Technology)
Scheduling: Scheduling software is provided to allow the user to time-schedule plant.
Data logging: The supervisory software provides a data-logging and archiving component.
Reports: A routine and an alarm-reporting component is provided.
User configuration and security: Remote access and level of access may be configured.
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Energy monitoring & targeting is the collection, interpretation and reporting of energy use. Its role within energy management is to measure and maintain performance and to locate opportunities for reducing energy consumption and cost.
Traditionally, energy monitoring & targeting has two principal functions:
- to control energy usage continuously
- to improve the efficiency of energy usage
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The benefits of energy monitoring & targeting include:
- Achieving energy consumption and cost savings, typically 7%-12%
- Reducing the environmental impact of energy usage
- Providing energy information for assessing energy projects and new plant acquisitions
- Improving preventative maintenance
- Avoiding waste and improving product quality through increased control
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Figure 3, below, outlines the main components of an automatic energy monitoring & targeting (M&T) system.
Figure 3: Outline of an M&T system (image courtesy of B. Swords)
The main components of an automatic energy monitoring and targeting (M&T) system are:
Fuel meters and sensors: These collect data on energy/fuel loading and consumption.
Most modern energy meters can provide an output signal e.g. a pulsed output, usually as volt-free contact closures, which represents an increment of energy consumption.
The pulse output from the energy meter is inputted to the data acquisition unit. Each site will have at least two main meters to account for electricity and gas/oil usage.
Data acquisition unit: The data acquisition unit is used for receiving and archiving energy data from energy meters and sensors, such as temperature and pressure.
Communication network: The network provides communication between the data acquisition unit and the central computer.
Central computer: The central computer has software to upload data from the data logger and provide energy analysis.
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Data may be automatically captured and key-inputted into the energy information software system. The types of data collected may be categorised as energy data and energy drivers.
Energy data
Energy consumption data is automatically captured from energy meters. Energy consumption data, energy load data and deduced costs are archived by the energy information software.
Energy data is monitored and collected for meters and for areas of accountability within an energy site. An area of energy accountability is generally termed an energy account centre (EAC).
The energy of an EAC should be measurable and manageable. An EAC, for example, could be a specific technology (e.g. lighting, compressed air), a department, a building, or an industrial production process.
Energy meters are usually categorised as follows:
- Main meter : A main meter is a supply meter for fuel supply (including electricity) to site.
- Sub-meter : A sub-meter is a meter installed after or downline from the main meter. Sub-meters are installed to measure energy within an EAC.
- Virtual meter: A virtual meter is a function of the measured energy from a number of other meters. A simple scenario might be the difference between the main meter and a number of sub-meters.
For large energy users in Ireland, energy consumption and load data is usually archived in 15-minute periods.
Energy drivers
An energy driver is a factor that influences the energy consumption and energy load levels overall for an energy account centre (EAC). Weather and occupancy are drivers that influence a standard office block. Production throughput, weather and occupancy are drivers that may influence an industrial site.
A common driver used in energy analysis is degree-days. Degree-days are used to compare building energy usage with the outside air temperature. Heating degree-days are the number of degrees by which the mean outside air temperature (over 24 hours) on each day is less than a given base temperature of 15.5°C.
Degree-days may be calculated if the outside air temperature is monitored; alternatively, the Irish Meteorological Service provides monthly degree-days totals for specific regions in Ireland.
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The data analysis undertaken may be divided into two categories, routine and investigative.
Routine analysis
This type of analysis is provided periodically. It includes analysis of: energy consumption, energy costs, energy performance and specific energy requirements for defined periods, e.g. daily, weekly, year to date.
Performance analysis techniques commonly used in routine analysis are outlined in Table 1 below.
| Routine analysis | Description |
| Specific energy ratio (SER) | Ratio of energy versus driver e.g. kWh/Ton, kWh/m², Cost €/ton |
| Normalised performance indicator (NPI) | Comparison of energy performance e.g. kWh/m², versus standardised values for building type |
| Descriptive charting | Graphics to display energy over time. Trend, combined, area charts |
| Historical comparison | Profiles and comparisons of energy over time e.g. this year vs. last year |
Table 1: Routine energy analysis
Benchmarks are widely used in industrial practice to improve performances through competition and comparison with others. Establishing Specific Energy Ratios and Normalised Performance Indicators (NPI) allow you to compare your energy performance against good-practice benchmarks and show how your organisation is performing.
Investigative analysis
A list of common investigative techniques is provided in Table 2. The investigative techniques used are similar to those used in quality-control statistics.
The techniques use and combine linear regression, cumulative sum (cusum) charts, and control charts.
| Investigative techniques | Description |
| Linear regression | Regression establishes the relationship between energy and driver(s) |
| Cumulative summation (CUSUM) | CUSUM is the cumulative sum of variances from target over time |
| Control charts | Charts displaying variances from target. The chart has control limits lines to indicate target breaches |
Table 2: Investigative energy analysis
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[1] CIBSE, Building Control Systems CIBSE Guide H, Butterworth Heinemann, Oxford, 2000.
[2] ETSU, GPG 31: Computer Aided Monitoring and Targeting for Industry, Energy Efficiency Office, Harwell, 1991.
[3] ETSU, GPG 125: Monitoring and Targeting in Small and Medium Sized Companies, Energy Efficiency Office, Harwell, 1998.
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