With increasing pressure on building managers to improve energy efficiency, there has never been a better time to look at how buildings use energy and what can be done to decrease their use of it.

In commercial buildings between 40 and 80% of the energy used will be down to the HVAC system. Therefore, one of the biggest opportunities to cut energy use in buildings is through greater use of Variable Speed Drives (VSDs) in HVAC applications. Used as an integral part of the Building Management System, they can cut energy consumption drastically.

Commercial buildings can gain heat from people or machines. These buildings may not need heating, and better insulation will not improve the energy efficiency. However, Part L of the building regulations requires energy efficiency improvements, so load reduction becomes the target.

According to the new regulations, ventilation systems must achieve the same specific fan power at part load as that achieved at the design flow. As part of this, it is proposed that all fans with a rated power greater than 1,100 W should be equipped with an efficient form of flow control such as a variable speed drive (VSD), or variable pitch fan.

Many consultants rely on using tried-and tested-solutions from the past, producing a HVAC system that is not ideally matched to the application. In many cases, this approach will fail to meet the new and more stringent energy-efficiency requirements of Part L.

These approaches include traditional mechanical control systems for ventilation plant, which typically 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 applications using variable speed drives. As a result, it needs more maintenance, with the added disadvantage of being less flexible.

The VSD, by contrast, is robust, energy saving, easy to install and low on maintenance. The specification of drives by consultants has been increasing for some time, with more pressure to use them coming from customers. Consulting engineers themselves have been increasingly interested in specifying drives and the use of drives will certainly help them achieve the air flow per sq metre targets demanded by the regulations.

Facing the HVAC challenge

A consultant or building operator wishing to take advantage of the benefits of VSDs for HVAC applications need to take several issues into account. Users will need to understand the available tariffs. There may be penalties for maximum demand, power factor or time of day.

Over supply or extraction of air may be an issue and building operators will want to make maximum use of outside air.

Reducing the number of air changes per hour can drastically cut the energy bill. If we take an office space of 50m by 20m and 2.5 m high, we have a volume of 2,500 m3. With 20 air changes per hour, this means a volume of 50,000m3 of air needs to be moved every hour. 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 per hour would cut the electric bill to only £9,000 per annum.

VSDs are ideal for this as they can be used to implement demand controlled ventilation – if there is no-one occupying the ventilated space, far fewer air changes are needed. If there are changes in the use of the area throughout the day, then thought needs to be given to issues such as zoning – dividing the building into zones with different levels of air movements - control methods and time schedules for switching HVAC equipment on and off.

A staged approach

Tackling these issues can be done using a staged approach.

Stage 1 is to identify suitable applications. The largest savings can be made with centrifugal machines – fans, pumps and compressors. These applications may also be subject to enhanced capital allowances, making them even more attractive. Savings will be optimised if the application has an output variable such as flow, pressure, temperature, allowing the VSD to control the speed of the pump or fan depending on the value of the variable.

Stage 2 involves gaining an understanding of the load. What are the load characteristics? This can be determined with the help of commissioning data, manufacturer’s data sheets, system calculations/curves and observations or data logging. The length of time the load operates is also important.

Using drives will help fine tune motors to the load point – with a typical 7.5 kW motor, the designer will have allowed a factor, as will others, perhaps the contractor. If a 5kW motor is all that is needed for the application, a drive will allow the motor to be fine tuned so it uses only the power needed.

Also needed is an estimate or measurement of energy use. This can be done through automatic monitoring. As well as information on energy used, continuous monitoring allows problems to be identified. Alternatively, energy used data can be gained manually from Sub Metering, when available, or from portable monitoring equipment.

Stage 3 involves thinking about control mechanisms - what ‘variable’ will be used, what other options are there and what should it (or can it) be linked to?

The control capabilities of VSDs often gives 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.

VSDs make HVAC applications easier to realise because they give much greater scope for control. These capabilities make 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 none of this, apart from receive an analogue input.

An add on benefit is the sub metering of power through serial communications. It is easy to read from a drive how much energy has been drawn. This means drives can help bill energy costs to occupants of buildings, for example using BACnet as previously described.

Wider use of VSDs means that the numbers of drives in individual buildings will inevitably rise. Each air handling unit has both extract and supply, so will need two drives. In addition, every heating circuit needs a pump and there are chilled water pumps to consider. There are also duty and standby pumps.

Because the purchase cost of drives has been falling, consultants can also avoid the need to use an interlock to allow one drive to drive two pumps, giving a drive for each pump to provide full redundancy.

With new, strict regulations on energy use, designed to reduce emissions, variable speed drives have a big part to play in HVAC applications, both in new buildings and in refurbishments.

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