An even bigger role for variable-speed drives

ABB Drives, variable speed drives
Part L of the latest Building Regulations requires that ventilation fans should be capable of achieving a specific fan power at 25% of design flow rate that is no greater than that achieved at 100%.

Steve Ruddell explains just why variable-speed drives have a key role to play in meeting the requirements of the Building Regulations, citing as an example an 11-month payback on a replacement project.

The new Building Regulations are designed to cut energy use in buildings, meaning that variable-speed drives and their energy-saving abilities will play an ever bigger role in the modern heating, ventilation and air-conditioning (HVAC) system.

Part L requires much stricter air-permeability standards. If buildings are more airtight, there is, of course, a need for improved ventilation to maintain a healthy atmosphere — meaning a greater role for ventilation fans.

Part L requires that ventilation fans should be capable of achieving a specific fan power at 25% of design flow rate which is no greater than that achieved at 100%. It also requires that all fans above 1100 W should have a means of efficient variable flow control.

How this flow control is achieved is left up to the HVAC system designer, but this is a specification that suits variable-speed drives (VSDs), which can drive motors at a wide range of speeds, reducing their power consumption. Because motor power is proportional to the cube of the speed, the power drawn drops away much faster than the speed when speed is reduced; for instance, at 80% speed, the motor only uses 50% of the power.

Change air to suit demand

This ability means that air changes can be exactly tailored to suit the needs of the building and its occupants, without over- or under- supply. Certainly, reducing the number of air changes per hour can drastically cut the energy bill. For example an office space of 50 by 20 m and 2.5 m high, we have a volume of 2,500 m3. With 20 air changes an hour, 50 000 m3/h needs to be moved. Adding up heating, cooling and fan power, this would give an electricity bill of around £150 000 per annum.

Simply reducing the number of air changes to six an hour would cut the electric bill to only £9000 per annum.

Saving £29,000

One building that is saving substantial amounts on its HVAC energy bill is Jackson House, an office block in Manchester. For several years Jackson House had air-flow problems throughout the tenanted areas. One issue was that the main ventilation plant’s fans were running constantly at full speed. The plant consists of four fans, ranging from 45 to 120 kW — two for supply and two for extract. The block also has a variable-temperature heating system comprising four pumps (two duty pumps and two standby pumps), also running at full speed.

It was decided to install VSDs that could be integrated with the new control system to improve the management of the building’s ventilation. The duty cycle of the fans and heating pumps was based on 2600 h a year, with electricity costing 7 p/kWh.

ABB Drives, variable speed drives
This office block in Manchester is saving over £29 000 a year on its HVAC energy bill following the installation of ABB variable-speed drives and achieving a payback of just 11 months.

For the air-handling fans, installing ABB standard drives for HVAC on each of the four fans resulted in a 49% energy saving. This equates to annual cost savings of between £4000 and £10 700 per fan, depending on rating, giving a total saving for the air-handling unit of £27 914 a year and a payback of about 11 months.

The pressure control of the fans was improved, so accurate air balance can be achieved on the floors — reducing noise, draughts and improving temperature control.

Similarly, the heating system pumps saved £1572 a year following installation of ABB standard drives for HVAC. Another benefit was the improvement in the pressure control of the pumps, allowing accurate water balance to be achieved on the floors. In a way similar to the air-handling fans, this will reduce noise and improve temperature control.

Additional energy savings are achieved as heating water will only be pumped around the building when it is required to match the building demand.

Better control

VSDs make HVAC applications easier to realise because they give much greater scope for control, making them ideal for demand-controlled ventilation. Drives can also communicate with one another, with other devices on the network and with an overriding control system. A mechanical system can do vary little, apart from receive an analogue input.

The control capabilities of VSDs can also give them the ability to use BACnet, which can give the control system access to on-board inputs and outputs, in addition to internal parameters such as a kWh signal.

An add-on benefit is the sub-metering of power through serial communications, making it easy to read from a drive just how much energy has been drawn. This means drives can help bill energy costs to a building’s occupants.

Wider use of VSDs means that the numbers of drives in individual buildings will inevitably rise. The new Building Regulations tighten up the rules on CO2 emissions and energy use and can only mean that VSDs have a bigger future in both new buildings and refurbishments.

Steve Ruddell is division manager — discrete automation and motion, ABB Ltd

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