Building Automation Systems
A building automation system (BAS), also called an energy management system (EMS), controls energy-consuming equipment in a building to make it operate more efficiently while maintaining a comfortable environment (Figure 1). This system may include other features such as maintenance planning, fire- and physical-safety functions, and security services. Building automation systems, which are present in more than half of all buildings in the U.S. larger than 100,000 square feet, save an average of about 10 percent of overall building energy consumption. For older or poorly maintained buildings, the savings can be even greater. In addition to saving energy, these systems may also reduce the costs of overall building maintenance.
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Figure 1: How energy management system components fit together A building automation system (BAS) consists of sensors, controllers, actuators, and software. An operator interfaces with the system via a central workstation. Source: E Source; adapted from Portland Energy Conservation Inc. |
Unfortunately, many building automation systems save less energy than they are capable of saving. In one detailed study of 11 buildings in New England with BASs, five of the buildings were found to be underachievers, producing less than 55 percent of expected savings. One site produced no savings at all.
To improve the likelihood that the BAS you purchase or recommend will achieve the expected benefits, you should take advantage of advanced control strategies that use the computer-processing power of a BAS and adopt a comprehensive approach to quality control known as commissioning. This process is now required for some buildings, such as some public institutions and buildings certified by LEED (Leadership in Energy and Environmental Design). This process includes reviews and inspections throughout the design and construction process as well as rigorous performance tests that move the system through its sequences of operation before the building is occupied. Recommissioning–where building operators use trending and energy consumption data to periodically verify, document, and improve a building's operation–can be conducted throughout the life of the building.
What Are the Options?
There are many decisions that designers face when specifying a BAS, including:
- What control strategies to implement
- What type of system to specify
- Whether to use open or proprietary communications protocols
- Whether or not to add a web interface
Energy-Savings Control Strategies
Here is a list of the most common strategies that building automation systems employ in order to use energy more efficiently:
Scheduling. Scheduling is the practice of turning equipment on or off depending on time of day, day of the week, day type, or other variables such as outside air conditions. Improving equipment schedules is one of the most common and effective measures for saving energy in commercial buildings.
Lockouts. Lockouts ensure that equipment does not come on unless it's necessary. They protect against nuances in the programming of the control system that may inadvertently cause equipment to turn on. For example, a chiller and its associated pumps can be locked out according to calendar date, when the outside air falls below a certain temperature, or when building cooling requirements are below a minimum.
Resets. When equipment operates at greater capacity than necessary to meet building loads, it wastes energy. A BAS can allow equipment to operate at the minimum capacity required by resetting operating parameters. The traditional design practice is to use a proportional reset schedule based on outdoor temperature (Figure 2). Although that method works reasonably well, a more effective method is to base resets directly on building loads (Figure 3). Examples of building control parameters that can be reset include supply-air and discharge-air temperature for fan systems that use terminal reheat, hot-deck and cold-deck temperatures for multizone HVAC systems, and heating-water supply temperature.
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Figure 2: Proportional reset schedule As the outside air temperature decreases, the chilled water temperature is reset to a higher value. |
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Figure 3: Direct load information reset In this reset schedule, the cooling load is based on the number of chilled water valves that are greater than 90 percent open. |
Demand limiting. Because electrical demand charges can make up 40 percent or more of a utility bill, many building automation systems have demand-limiting or load-shedding functions. For example, when the demand on a building meter or piece of equipment, such as a chiller, approaches a predetermined setpoint, the BAS does not allow the equipment to load up any further. Another way to minimize peak demands is to program time delays between the start-up of major pieces of electrical load equipment, preventing a situation in which several pieces of equipment start up at the same time.
Diagnostics. Building operators who use a BAS to monitor information such as temperatures, flows, pressures, and actuator positions may use that data to determine whether equipment is operating incorrectly or inefficiently and to troubleshoot problems. A thorough job of building diagnostics typically requires the building operator to monitor more points than the minimal number needed to simply control a building, but a modern BAS gives users a good head start on a recommissioning or continuous-commissioning program. A modern BAS helped personnel from Texas A&M's Energy Systems Laboratory to cut energy bills at a state office building by 27 percent. The BAS helped to implement nighttime shutdowns and chart actual building temperatures to identify opportunities for temperature setback during unoccupied hours.
Control System Types
Building control systems largely fall into one of two categories:
Direct digital controls (DDC). These systems use electronic signals via computer to process data for direct system control. Although most BASs use electronically powered actuators, today a small proportion of BASs in older buildings use DDCs with outdated pneumatically powered actuators, which use compressed air to control valves and dampers.
Stand-alone controls. Buildings without BASs typically rely on the controls built into individual pieces of large equipment, such as packaged rooftop units and chillers, to maintain comfort conditions and manage energy consumption. Some of these stand-alone controllers are quite sophisticated and are capable of implementing many of the control strategies described above. However, they fall short of being able to appropriately manage the entire building unless they are "tied-in" to the BAS using a common communications protocol.
In general, full DDC systems (Figure 4) offer many more benefits over stand-alone systems that are not tied in to the BAS.
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Figure 4: DDC field panel This panel controls a large air-handling unit. Source: Portland Energy Conservation, Inc. |
Better building feedback. DDC systems typically provide much more information feedback from the building than stand-alone systems. For example, a DDC system could reset variable air volume (VAV) static pressure by scanning all the VAV damper positions and gradually changing the static pressure until only one damper was completely open. At that level of static pressure the fan draws the least amount of power required to distribute sufficient air to all the boxes. Stand-alone controllers usually cannot reset static pressure in this manner, because they typically have no way of sensing VAV damper position.
Centralized control. DDC systems allow for building control and information flows to be centralized at a single location (whereas stand-alone systems don't have the feedback mechanisms necessary for this capability). As a result, operators can readily view and control all building systems from a single computer terminal instead of having to keep track of a variety of different control locations throughout the building.
Communications Protocol
Two major communications choices are available for DDC systems: proprietary and open (or standard) communications.
Proprietary communications. These are used for intercommunication among different components of a particular manufacturer's equipment. The systems may allow backward or forward compatibility with equipment generations of the same manufacturer, but they don't allow ready intercommunication with other brands of equipment. Such systems are rapidly disappearing from the marketplace because they cannot be easily applied to very many types of equipment. Because a proprietary system does not communicate with other systems, the user's options for expansion of the BAS are limited. Choices are also reduced for the purchase of new equipment, which limits the user's bargaining power. However, proprietary systems do offer the advantage of a single source of responsibility when there are problems.
Open communications. Open communications use published protocols, which are open to all manufacturers. There are two major choices for open standards in the area of building automation. ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) published an open communications standard, known as BACnet, in 1995. The other major option is the Lonworks system from Echelon. Most manufacturers of building controls have allied themselves with one or both of these standards.
There are several advantages to using an open communications protocol for a BAS. First, there is the assurance that equipment from multiple manufacturers will be able to interact. Using BACnet products that are BACnet Testing Laboratory (BTL)–listed ensures that they have been tested to confirm compliance with BACnet standards. Using equipment with open protocols also creates a competitive bidding environment for system additions and renovations, which helps to limit costs. This situation also helps keep manufacturers that have on-site equipment from becoming too "comfortable," so it ensures a good level of service and response to problems.
Another advantage is the containment of expenses associated with interfacing the BAS to mechanical equipment. For example, it is normally difficult to extend the features of a BAS with proprietary communications to monitor temperatures, pressures, and flows of a new chiller. If all additions to a system are specified as open/standard protocol, interfacing becomes easier and less expensive.
In addition, by using an open protocol, the headend equipment from your chosen manufacturer can interface with all equipment in a facility via standard communications, without the need for separate gateways or a multitude of wires. The headend of a building automation system is the central controller–typically a personal computer that is set up to monitor all of the distributed processors in the system. This use of standard protocols reduces the need for multiple headends and specialized interface equipment. The result is lower system costs and training expenses, fewer maintenance agreements and spare parts, and a single mode of system access.
Web Browser Interface
The introduction of web browser interfaces may be the most exciting news in automation since DDC. A web browser is a piece of software that allows a user to access and view resources available through the Internet. Users can dramatically enhance their ability to manage a facility by networking the BASs for multiple buildings so that they can be controlled from one location through the Internet (see Figure 5). Networking may also allow the BAS to communicate with other computer applications like online weather forecasting services. The concept of enterprisewide management for facilities throughout the world is exciting, whether it concerns the management of HVAC control for building comfort, fire and physical safety, security, or buying power. Procurement of electricity in a deregulated world, for example, can become a real-time, dynamic activity facilitated by the BAS. The development of a language called XML may also help to boost the use of the Internet for building control (see box).
Figure 5: How a web browser interface works Controllers embedded in lighting, HVAC, and security equipment communicate with each other via a local area network. Each building is then connected to the Internet through a gateway that is protected by a security firewall. Because these networked building systems offer remote control capabilities, facility managers can monitor and control their buildings from any location with a web connection. They can also manage multiple sites simultaneously or aggregate them for load control. |
There are also some simple and pragmatic benefits of web browsers for building automation. All the proprietary hardware and software that was necessary for a conventional system interface is no longer needed. The same off-the-shelf technology used to surf the Internet becomes the controller interface, which opens up the system to anyone with a computer and a web browser. However, it is necessary to build firewalls and ensure that security is maintained. Through the web, building owners or operators can combine the power of automated controls with data from the world at large to create an effective building management tool.
XML: An Emerging Standard
Many of the technology companies involved in data exchange over the Internet have developed a language called XML (Extensible Markup Language). XML is emerging as the standard language for data exchange in many business sectors and is starting to gain attention in the field of building automation.
XML is similar to HTML (Hypertext Markup Language), the language used to create the web pages that you can view using your web browser. The XML technology uses tags, much like HTML data tags, to record the relationships among the data elements in a file. The data in an XML file can associate a device, such as a controller, with numerous objects such as points, messages, and alarms. A computer reading the file will be able to "understand" the physical capabilities of the objects and configure the system accordingly. By contrast, the same data written with HTML would associate a list with the controller, but it would not enable the computer to interpret the relationship between the controller and the items in the list.
By supporting XML for building automation, manufacturers give their customers the flexibility to configure the system on their own, use a configuration package from another manufacturer, or use a third-party software package that supports XML as a file format. Examples of the last include Microsoft Excel and Microsoft Access. Because Microsoft is freely distributing its XML software engine, it's much easier for manufacturers, software developers, or users to create custom applications that read and write XML data, possibly even reading proprietary configuration data files and exporting them in standard XML format. In the future, the use of XML may allow building automation systems to seamlessly communicate with other nonphysical systems such as accounting and scheduling packages.
How to Make the Best Choice
Clearly define goals for the BAS. Begin by talking to a variety of consultants and controls vendors. This process will provide the education you need to properly define your goals and needs and then match them with a good performance specification. Use the Internet to research the issues.
Today the choice of standards and communications dominates discussions about system selection and should be considered in light of system goals. Owners want an easy-to-use, single-seat interface (such as a personal computer) for the access and sharing of data among DDC systems in one or more buildings. Owners also often want to mix and match various components from different manufacturers in the same system–a feature called plug and play. The issue of how to integrate these complex systems raises questions about what is known as interoperability.
Interoperability means that controllers can work together in an integrated fashion. It requires that they operate as a unified system: a result that is not ensured simply by specifying standard communications. In other words–focus on specifying performance, not protocol.
By carefully considering the goals and objectives for the BAS, it will be possible to determine to what degree open communications are important and whether gateways or other networking technology will be required in order to combine existing equipment with the new system. Take time to understand and ensure that all the networking equipment and control equipment is compatible, will interact in the manner desired, and will provide the data necessary to manage the facility.
Clearly define lines of maintenance responsibility up front. Using open communication protocols introduces additional considerations to the specifying and contracting process. For instance, when a system has several manufacturers' headends and a host of third-party controllers, who is called when one part of the network stops functioning? Care should be taken in the maintenance contracting process to clearly delineate the areas of responsibility for maintenance activities.
Whenever system additions are installed in the field, the existing headend will require extra programming to support the additions. This could be done by the manufacturer, by a certified contractor who is proficient with the existing headend, or by in-house personnel. The same options apply to additions that affect only a distributed processor. There will be some programming requirements in the distributed processor whenever additions are made, so staff or maintenance contractors need to be familiar with all the on-site manufacturers of distributed processors.
Include commissioning considerations in the selection process. Commissioning is a systematic process improving the likelihood that all building systems will perform interactively as they were intended. It includes specifications in the design phase, inspections during the installation process, tests conducted after installation is complete, and operator training. Building owners and designers can do three things to ensure a successful BAS commissioning process:
- Select an able commissioning provider. Hire an objective engineer with commissioning experience to review design documents, help write specifications, design tests, observe the testing phase, and assist with operator training.
- Incorporate commissioning and recomissioning requirements into the specifications. These specifications should be detailed and should include roles and responsibilities of the project team, installation and initial checkout procedures, functional test requirements, training procedures, and documentation requirements.
- Ensure that the BAS is fully tested. There are many standard functional tests that describe requirements and procedures for testing building automation systems. In many cases it is necessary for a commissioning provider to customize these tests to more exactly match the technical requirements of the specific project.
In particular, open communication systems should be thoroughly commissioned, and the performance of all system components should be compared with the published and submitted manufacturer’s performance data. With BACnet standard systems, Protocol Implementation Conformance Statements (PICS) should be submitted before construction to ensure compatibility at all appropriate levels.
| Building Automation Resources |
The following are excellent sources for further information about building automation:
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What's on the Horizon?
BAS technology is advancing to offer the following new functionality in the future.
Increased multisystem operability. The BAS industry is currently doing extensive research into developing and testing web browser interfaces. BAS designers have recently begun using them for increased multisystem operability and to improve building operators' access to the system. ASHRAE has added web services to BACnet in order to standardize how BACnet systems run web services to exchange data with other computing applications over a network. One initial use of web services is to enable sophisticated functionality such as creating "virtual thermostats" that give users control over the temperature in their own area. Testing is also under way on using web services to integrate BASs with utility systems, which would implement control strategies based on real-time pricing.
Lower installation costs with wireless devices. Wireless networks offer future potential cost reductions that are dramatic for the primary controller architecture, because the cost of wiring and conduit is a major budget item. Although they are still relatively expensive, wireless devices are becoming more affordable and reliable with the development of new manufacturing techniques. Also, a new open communication standard, the ZigBee standard, is making possible reliable, low-power, wirelessly networked products that provide much more flexibility than proprietary wireless systems. However, because buildings typically require a large number of sensor nodes (which drives up the cost), designing wireless systems is currently a balance of functionality and cost-effectiveness, with designers cutting costs by removing functions and limiting compatibility with external systems. As costs continue to come down, wireless networks will likely become much more prevalent in BASs.
