Building Automation Systems: The Smart Buildings of Future
What is Building automation System?
Complete autonomous control of an entire facility is the goal that any modern automation system attempts to achieve. The distributed control system – the computer networking of electronic devices designed to monitor and control the mechanical, security, fire, lighting, HVAC and humidity control and ventilation systems in a building or across several campuses.
Building Automation System (also known as Building Management system(BMS)), is used primarily for Centralized Control & Monitoring of various services of the Building like Air-Conditioning, Lighting, Fire Alarm System, Power Systems and Security systems. Its core functionality is to keep building climate within a specified range, light rooms based on an occupancy schedule, monitor performance and device failures in all systems and provide malfunction alarms. Automation systems reduce building energy and maintenance costs compared to a non-controlled building. Typically they are financed through energy and insurance savings and other savings associated with pre-emptive maintenance and quick detection of issues.
A building controlled by a BAS is often referred to as an intelligent building or “smart building”. Commercial and industrial buildings have historically relied on robust proven protocols like BACnet.
Almost all multi-story green buildings are designed to accommodate a BAS for the energy, air and water conservation characteristics. Electrical device demand response is a typical function of a BAS, as is the more sophisticated ventilation and humidity monitoring required of “tight” insulated buildings. Most green buildings also use as many low-power DC devices as possible, typically integrated with power over Ethernet wiring, so by definition always accessible to a BAS through the Ethernet connectivity. Even a passivhaus design intended to consume no net energy whatsoever will typically require a BAS to manage heat capture, shading and venting, and scheduling device use.
A Simple Example of a Intergrated Building Automation System
Buses and protocols
Most building automation networks consist of a primary and secondary bus which connect high-level controllers with lower-level controllers, input/output devices and a user interface devices. ASHRAE’s open protocol BACnet or the open protocol LonTalk specify how most such devices interoperate. Modern systems use SNMP to track events, building on decades of history with SNMP-based protocols in the computer networking world.
Physical connectivity between devices was historically provided by dedicated optical fiber, ethernet, ARCNET, RS-232, RS-485 or a low-bandwidth special purpose wireless network. Modern systems rely on standards-based multi-protocol heterogeneous networking. These accommodate typically only IP-based networking but can make use of any existing wiring, and also integrate powerline networking over AC circuits, power over Ethernet low power DC circuits, high-bandwidth wireless networks such as LTE and IEEE 802.11n and IEEE 802.11ac and often integrate these using the building-specific wireless mesh open standards.
Current systems provide interoperability at the application level, allowing users to mix-and-match devices from different manufacturers, and to provide integration with other compatible building control systems. These typically rely on SNMP, long used for this same purpose to integrate diverse computer networking devices into one coherent network.
Types of inputs and outputs
Analog inputs are used to read a variable measurement. Examples are temperature, humidity and pressure sensors.
A digital input indicates if a device is turned on or not.
Analog outputs control the speed or position of a device, such as a variable frequency drive or a valve or damper actuator.
Digital outputs are used to open and close relays and switches. An example would be to turn on the parking lot lights when a photocell indicates it is dark outside.
BAS Controllers are purpose-built computers with input and output capabilities. These controllers come in a range of sizes and capabilities to control devices commonly found in buildings and to control sub-networks of controllers. Inputs allow a controller to read temperatures, humidity, pressure, current flow, air flow, and other essential factors. The outputs allow the controller to send command and control signals to slave devices, and to other parts of the system. Inputs and outputs can be either digital or analog. Digital outputs are also sometimes called discrete depending on manufacturer.
Controllers used for building automation can be grouped in 3 categories.
Programmable Logic Controllers (PLCs), System/Network controllers, and Terminal Unit controllers. However an additional device can also exist in order to integrate 3rd party systems (i.e. a stand-alone AC system) into a central Building automation system).
System/Network controllers may be applied to control one or more mechanical systems such as an Air Handler Unit (AHU), boiler, chiller, etc., or they may supervise a sub-network of controllers. In the diagram above, System/Network controllers are often used on the IP backbone.
Terminal Unit controllers usually are suited for control of lighting and/or simpler devices such as a package rooftop unit, heat pump, VAV box, or fan coil, etc. The installer typically selects 1 of the available pre-programmed personalities best suited to the device to be controlled, and does not have to create new control logic.
Most air handlers mix return and outside air so less temperature/humidity conditioning is needed. This can save money by using less chilled or heated water (not all AHUs use chilled/hot water circuits). Some external air is needed to keep the building’s air healthy. To optimize energy efficiency while maintaining healthy indoor air quality (IAQ), demand control (or controlled) ventilation (DCV) adjusts the amount of outside air based on measured levels of occupancy. Analog or digital temperature sensors may be placed in the space or room, the return and supply air ducts, and sometimes the external air. Actuators are placed on the hot and chilled water valves, the outside air and return air dampers. The supply fan (and return if applicable) is started and stopped based on either time of day, temperatures, building pressures or a combination.
Constant volume air-handling units
The less efficient type of air-handler is a “constant volume air handling unit,” or CAV. The fans in CAVs do not have variable-speed controls. Instead, CAVs open and close dampers and water-supply valves to maintain temperatures in the building’s spaces. They heat or cool the spaces by opening or closing chilled or hot water valves that feed their internal heat exchangers. Generally one CAV serves several spaces
Variable volume air-handling units
A more efficient unit is a “variable air volume (VAV) air-handling unit,” or VAV. VAVs supply pressurized air to VAV boxes, usually one box per room or area. A VAV air handler can change the pressure to the VAV boxes by changing the speed of a fan or blower with a variable frequency drive. The amount of air is determined by the needs of the spaces served by the VAV boxes.
Each VAV box supply air to a small space, like an office. Each box has a damper that is opened or closed based on how much heating or cooling is required in its space. The more boxes are open, the more air is required, and a greater amount of air is supplied by the VAV air-handling unit. Some VAV boxes also have hot water valves and an internal heat exchanger. The valves for hot and cold water are opened or closed based on the heat demand for the spaces it is supplying. These heated VAV boxes are sometimes used on the perimeter only and the interior zones are cooling only. A minimum and maximum CFM must be set on VAV boxes to assure adequate ventilation and proper air balance.
Chilled water system
Chilled water is often used to cool a building’s air and equipment. The chilled water system will have chiller(s) and pumps. Analog temperature sensors measure the chilled water supply and return lines. The chillers are sequenced on and off to chill the chilled water supply.
A chiller is a refrigeration unit designed to produce cool (chilled) water for space cooling purposes. The chilled water is then circulated to one or more cooling coils located in air handling units, fan-coils, or induction units. Chilled water distribution is not constrained by the 100 foot separation limit that applies to DX systems, thus chilled water-based cooling systems are typically used in larger buildings. Capacity control in a chilled water system is usually achieved through modulation of water flow through the coils; thus, multiple coils may be served from a single chiller without compromising control of any individual unit.
Chillers may operate on either the vapor compression principle or the absorption principle. Vapor compression chillers may utilize reciprocating, centrifugal, screw, or rotary compressor configurations.
Reciprocating chillers are commonly used for capacities below 200 tons.
centrifugal chillers are normally used to provide higher capacities; rotary and screw chillers are less commonly used, but are not rare. Heat rejection from a chiller may be by way of an air-cooled condenser or a cooling tower (both discussed below).
Vapor compression chillers may be bundled with an air-cooled condenser to provide a packaged chiller, which would be installed outside of the building envelope. Vapor compression chillers may also be designed to be installed separate from the condensing unit; normally such a chiller would be installed in an enclosed central plant space. Absorption chillers are designed to be installed separate from the condensing unit.
Hot water system
The hot water system supplies heat to the building’s air-handling unit or VAV box heating coils, along with the domestic hot water heating coils (Calorifier). The hot water system will have a boiler(s) and pumps. Analog temperature sensors are placed in the hot water supply and return lines. Some type of mixing valve is usually used to control the heating water loop temperature. The boiler(s) and pumps are sequenced on and off to maintain supply.
The installation and integration of variable frequency drives can lower the energy consumption of the building’s circulation pumps to about 15% of what they had been using before. If that sounds hard to believe, I’ll explain, and we can do the math. A variable frequency drive functions by modulating the frequency of the electricity provided to the motor that it powers. In the USA, the electrical grid uses a frequency of 60 Hertz or 60 cycles per second.
Variable frequency drives are able to decrease the output and energy consumption of motors by lowering the frequency of the electricity provided to the motor, however the relationship between motor output and energy consumption is not a linear one. If the variable frequency drive provides electricity to the motor at 30 Hertz, the output of the motor will be 50% because 30 Hertz divided by 60 Hertz is 0.5 or 50%. The energy consumption of a motor running at 50% or 30 Hertz will not be 50%, but will instead be something like 18% because the relationship between motor output and energy consumption are not linear.
The exact ratios of motor output or Hertz provided to the motor (which are effectively the same thing), and the actual energy consumption of the variable frequency drive / motor combination depend on the efficiency of the variable frequency drive. For example, because the variable frequency drive needs power itself to communicate with the building automation system, run it’s cooling fan, etc., if the motor always ran at 100% with the variable frequency drive installed the cost of operation or electricity consumption would actually go up with the new variable frequency drive installed.
The amount of energy that variable frequency drives consume is nominal and is hardly worth consideration when calculating savings, however it did need to be noted that VFD’s do consume energy themselves. Due to the fact that the variable frequency drives rarely ever run at 100% and spend most of their time in the 40% output range, and the fact that now the pumps completely shut down when not needed, the variable frequency drives have reduced the energy consumption of the pumps to around 15% of what they had been using before.