Keeping air conditioning costs in check

2014-03-19 - Air conditioning costs can be kept low by combining BMS and variable-speed drives into an energy management system.
When it comes to energy use league tables, buildings are near the top. They are in fact the largest end-users of energy, responsible for 40 percent, well ahead of everyone’s favourite villain, vehicles. The potential for energy savings in buildings is high and set to increase still further, with global energy consumption for buildings expected to grow by 45 percent between 2002 and 2025.

Much of this energy use of course goes into maintaining an equitable temperature within buildings, making them comfortable places to be in. Moving air around and cooling or warming it makes air conditioning and its associated motors among the largest consumers of energy.

Managing this energy use in air conditioning systems requires seamless control, which can be achieved by building management systems (BMS) combining both hardware and software. The hardware of a BMS typically consists of one or more control and processing units and some peripheral devices which control the operation of heating or cooling systems.

The control unit runs the system based on the information supplied by some of the peripherals or on pre-set instructions with commands sent from the central unit to the peripherals through communication cables. The software consists of the program and the instructions that allow the control unit to manage the operations of the peripheral devices and, through them, the appliances.

Drives and BMS – a perfect match
One component that can help the BMS in its efforts to keep energy use in check is the variable-speed drive, or VSD. These provide infinite control over the speed of motors driving pumps and fans. Compared to cruder systems, such as direct-on-line and star delta starters, dampers and valves, which choke off the flow of air or liquid produced by a fan or pump, can be controlled more precisely through PID loops utilising the VSD’s control capabilities which can drive the fan or pump at the exact speed needed.

Using VSDs ensures that the BMS can achieve maximum control over the building’s environment, matching the temperature and humidity to the demands of the prevailing weather and number of occupants. In this way, the building operator can make the most of the capabilities of the BMS’ software and sensors.

This alone can save huge amounts of energy, slashing energy costs by as much as 50 percent in many cases. Yet modern drives offer so much more, coming complete with their own communication and control features that make the BMS more capable and efficient, as well as being easier to build and install.

Many of today’s drives include built-in serial communications protocols such as BACnet, Modbus RTU and others. Several models will also offer a fieldbus adapter, allowing connection of LonWorks, Profibus-DP, Ethernet and several others.

This means the drive can connect to any BMS. For instance, BACnet, the open protocol serial communications standard, gives complete access to drive parameter information when connected to a BACnet control network. One particular model of modern drive has 13 I/O points, offering extensive control options, either hardwired or via the serial communication network.

Real-time clocks make it easy to schedule events, such as switching on air conditioning fan motors as the building fills up and turning them off at slack times. The timer allows the drive to act as a stand-alone control unit with no need for an input from the BMS. The clock can also be used to indicate the date of a fault, or to flag up that some inspection or maintenance is required.

Control at your fingertips
Modern drives often have numerous HVAC specific macros such as supply fan, return fan and cooling tower, which can dramatically reduce set-up time. The panel can help users start up the drive, maintain it and diagnose faults. An on-board fault history assistant can show the actual time when the fault occurred, speeding up drive repair. The assistant can detect, dirty filters and keep track of bearing and drive belt replacement intervals, as well as lubrication intervals for the driven equipment.

Many building operators are making dramatic savings by using VSDs to run their HVAC systems. Operators of the Omnibus building in Reigate, Surrey, are saving £20,000 a year on energy costs for the building’s HVAC system following the installation of ABB standard drives for HVAC.

An energy appraisal by Econowise Drives and Controls revealed that the motors driving the system pumps were all running at full speed, with flow control achieved by mechanical throttling. It was calculated that putting VSDs on the application could save around £20,000 per annum in energy costs, with a payback of under nine months.

Twelve ABB standard drives for HVAC, six on the chiller pumps rated at 15 kW, four on the LPHW secondary heating pumps rated at 15 kW and two on the LPHW primary pumps rated at 7.5 kW. All the valves were opened and the drives set to maximum speed. This was then reduced until the water flow rate met the exact requirements of the building.

The building also had two existing VSDs operating an air handling unit but these were not running in a speed control mode. Econowise reprogrammed the PID controller on the exhaust fan drive to accept signals from a CO2 detector and drive it at an appropriate speed to remove the CO2.

With better control, help with maintenance and the ability to rein in rising energy bills, today’s VSDs, combined with BMS’, offer a complete energy management system to building operators.

Omnibus building in Reigate is saving £20,000 a year on energy costs for the building’s HVAC system following the installation of ABB standard drives for HVAC.

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