Drives help efficient building design

By increasing airtightness and using mechanical ventilation with heat recovery, the energy consumption for space heating can be reduced by as much as 30%. The savings tend to be greater in commercial buildings than in domestic ones, as commercial buildings are more influenced by outside air movements due to their sheer size. Better sealed buildings mean higher energy efficiency, but unless effective ventilation is provided, it also means trapping stale air and humidity inside buildings, potentially leading to the so-called sick building syndrome.

Part L means more use of mechanical ventilation. Variable speed drives is the most effective way to achieve this.

A more robust HVAC system
A traditional mechanical control system for ventilation plant typically uses a mechanical arm on a jockey motor raising and lowering a damper to control air flow. This results in a system that is more costly, less efficient, more delicate and prone to disturbances and so requiring more maintenance, as well as being less flexible. The variable speed drive (VSD), by contrast, controls the flow of energy from the mains to the motor. Both voltage and frequency reference are fed into a modulator which simulates an AC sine wave and feeds this to the motor’s stator windings. This technique is called Pulse Width Modulation (PWM). The VSD controls the motor in the form of a PWM pulse train dictating both the voltage and frequency. A drive is robust, energy saving, easy to install and low on maintenance. Drives also have the flexibility to communicate with each other, with other devices on the network and with an overriding control system. A mechanical system can do none of this; apart from receive an analogue input.

Part L requires that ventilation fans should be capable of achieving a specific fan power at 25% of its design flow rate which is no greater than that achieved at 100%. It also says that all fans above 1,100 W should have a means of efficient variable flow control. With a variable speed drive, the motor power is proportional to the cube of the speed. This means that 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.

Many commercial buildings also require cooling due to high internal gains from people or machines. In such cases, heating may not be needed and better insulation will not improve the energy efficiency. However Part L still requires energy efficiency improvements compared to previous standards, so in this case, load reduction becomes the target. The energy savings offered by variable speed drives compared to traditional systems will be a significant help to achieve this.

These are concepts that designers need to take to heart. Many consultants rely on using systems they know, preferring to use tried and tested solutions from the past. Frequently, the result is a HVAC system that is not ideally matched to the application. In many cases, this approach fails to meet the more stringent energy efficiency requirements of Part L.

Better control at lower cost
There is often no benefit in using traditional systems without drives, not even cost. A ventilation system with drives has every possibility to be just as competitive on price. If the consultant wants a bare-minimum system, then a system with a drive and a motor at the end of the cable is hard to beat in terms of simplicity. But it is the programming and communications options offered by variable speed drives that primarily will help designers achieve more efficient systems. In buildings fitted with a Building Management System (BMS), variable speed drives can help adjust conditions to a high degree of accuracy.

Drives are relatively straightforward to connect to a BMS, either by hardwiring or via a fieldbus system. A fieldbus has the advantage of requiring only two wires rather than individual hard-wired connections for each drive. Fieldbus can also support an unlimited number of control points – systems incorporating hundreds of drives are not uncommon. This gives BMS designers more options, allowing them to use more of the drive’s features. If a drive is intended for use on a fieldbus system, it should have interfaces to all the major protocols, such as BACnet, LonWorks, Modbus and Trend IQ.

In smaller installations, intelligent drives can even act as a mini-BMS system. For example, ABB’s HVAC drive features a real-time clock with calendar function, enabling users to schedule activities for different times of the day or week, with the drive’s Inputs/Outputs (I/Os) being used to control other devices. An internal kilowatt hour meter enables easy monitoring of energy consumption.

Drives give much greater scope for controls provision, an area where we might see major improvements in the coming years. For example, demand controlled ventilation could become the norm. Substantial energy savings could be achieved in buildings with variable occupancy. Alternatively, they may be programmed to reduce consumption at peak times, which can be a requirement of electricity supply contracts.

There may also be increasing scope for combining ventilation with heating systems, even in domestic applications. Variable speed drives have in recent years reduced to a size and a price that would be acceptable in the domestic market.

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.

With or without Part L, the variable speed drive is one of the most effective ways to save energy in buildings, often offering payback in months on energy savings alone. Additionally, they also help other technologies become more effective as part of an overall package.