Energy saving is just one benefit of variable-speed drives
As well as bringing great energy savings, variable-speed drives also help with the overall improvement of the building system, explains Pete Winterbottom.
Drives are increasingly used in building systems, mainly as a result of demand from end users. In addition to improved energy efficiency, they offer reduced maintenance and more flexibility. To the contractor, drives mean faster installation and commissioning.
Drives can also communicate with one another, with other devices on the network and with an over-riding control system. A mechanical system can do none of this — apart from receive an analogue input.
A traditional control system for a ventilation plant uses a mechanical arm on a jockey motor raising and lowering a damper to control air flow. This system is more costly, less efficient, more delicate and prone to disturbances than using a variable-speed drive. As a result, it needs more maintenance and has the added disadvantage of being less flexible. The variable-speed drive, by contrast, is robust, energy saving, easy to install and low on maintenance.
The variable-speed drive adjusts the output of a fan by controlling the speed of the fan motor, resulting in significant energy savings and improved control possibilities. The speed can, for instance, be adjusted in response to feedback from temperature sensors.
One obstacle to using high-efficiency equipment has so far been that the specification is usually made up by someone who will not be using the building. The contractor often influences the consultant, and short-term gains take precedence over lifecycle cost and long-term energy efficiency.
However, in most cases, there is no benefit in using a system without drives — not even cost. But some consultants and contractors prefer to stick with what they know, and the result is a HVAC system that is not ideally matched to the application. A ventilation system with variable-speed drives has every possibility to be competitive on price. If the consultant wants a bare-minimum system, one with a drive and a motor at the end of the cable is hard to beat in terms of simplicity.
When using variable-speed drives, the building-management system can adjust conditions to a high degree of accuracy. Drives are relatively straightforward to connect to the BMS. In many applications, they require only three hard-wired I/O connections (a start/stop, an analogue 0 to 10 V input for speed control and a fault relay output). The other main connection option is to use a fieldbus system, which has the advantage of requiring only two wires rather than individual hard-wired connections for each drive.
The variable-speed drive can also be a valuable tool for energy management. It is easy to read from a drive how much energy has been drawn. This means drives can help with the billing, to occupants of buildings, of the energy costs.
But the area where the drive really excels is energy saving — possibly the reason why its many other advantages are frequently overlooked.
As a retrofit item, the variable-speed drive can be one of the most effective ways to save energy. Insulation can give return on investment over 30 years, whereas the drive achieves this in two years or less. The impact is immediate. By reducing fan speed by a little, energy consumption can be reduced by a lot, because fan power reduces as the cube of the flow rate.
Substantial reductions in fan energy can be achieved in buildings with variable occupancy when variable-speed drives are used — for example by using demand-controlled ventilation or variable-air-volume systems.
Reducing the number of air changes per hour can drastically cut the energy bill. Variable-speed drives are ideal as they can be used to implement demand-controlled ventilation (if nobody is in the room, far fewer air changes are needed). If there are changes in the use of the area throughout the day, it might be possible to divide the building into zones with different levels of air movements, with control methods and time schedules for switching HVAC equipment on and off.
Variable-speed drives make HVAC applications easier to realise because they give much greater scope for control. This makes them ideal for providing a more comfortable indoor climate with reduced input of energy — the type of indoor climate that users demand in the 21st century.
Pete Winterbottom is UK manager for HVAC drives with ABB. ABB Ltd
