Driving down energy costs

Vacon. VSDs, variable speed drives, quick fixes
Time to ditch energy-guzzling fixed-speed starters — Stephen Takhar.

Fitting variable speed drives to pumps and fans used in building services is one of the best and easiest ways of boosting energy efficiency, but it’s important to choose the right drives, says Stephen Takhar of Vacon.

It may be old news that using AC variable-speed drives (VSDs) can make a big difference to the energy efficiency of building-services systems, but there is still a shockingly large number of installations where this simple step hasn’t been taken. Possibly this is because not everyone has yet realised just how much energy can be saved, so let’s briefly restate the facts.

Many older installations — and some quite recent ones — have pumps and fans that run at fixed speeds. Flow regulation is achieved by throttling and the use of dampers, and both techniques waste energy rather than conserve it. Now bear in mind that the motors will have been sized for worst-case conditions, often with a little bit of extra capacity added ‘just to be on the safe side’, which means that it is rarely, if ever, necessary for them to be running flat out.

In other words, if a VSD is fitted, the motor can run well below its maximum speed most of the time, with absolutely no impact on the performance of the system in which it is used. And here’s the important bit — reducing the speed of the motor by just 10% cuts energy usage by around 25%. With larger speed reductions, the energy savings are even more impressive — a 20% speed reduction, for example, gives an energy saving of almost 50%.

Clearly fitting VSDs in place of fixed-speed starters is a very good way of achieving impressive energy savings; in most cases, the necessary work can be carried out quickly and easily.

Variable-speed drives designed for HVAC applications have built-in features fitted to the task — and they can also be interfaced to a BMS.

Modern VSDs in the size ranges typically used in building-services applications are compact self-contained units that can often be accommodated either within the control panel that houses the existing single speed starter or, if an enclosed VSD is used, on the wall or other convenient surface adjacent to the starter. There are even VSDs, like those in the Vacon 100X range, that can be mounted directly on the motor.

For easy installation and set up, as well as for the best possible energy-saving performance it is important to choose the right drive. General-purpose industrial types will work, but types specifically designed for HVAC applications are a much better choice.

As standard, VSDs for HVAC applications typically include functions like real-time clocks to shut the system down automatically outside normal occupancy hours, PID controllers that allow the motor speed to be controlled directly from flow or pressure sensors, and a sleep option that stops the motor completely when the demand falls to zero. All these features can, of course, be added to general-purpose drives, but only by using costly and inconvenient add-ons.

Another key point about the best HVAC drives is that they have excellent harmonic and EMC performance. This makes it much easier to meet the increasingly strict demands made by supply authorities for harmonic mitigation, which is a particularly important consideration where large numbers of VSDs are installed on the same site.

Depending on the application, it may also be worth taking into account the communication options offered by the VSD.

It’s true that the single-speed starter the drive is replacing is unlikely to have any network connections, but there are many cases where linking the new VSD to the site building management system (BMS) opens up additional possibilities for reducing and monitoring energy usage. Setting up a link to the BMS is, however, only conveniently possible if the drive has a suitable communications interface; an Ethernet port is most often what’s needed.

Landmark project — over 210 Vacon HVAC 100 drives have been installed in The Shard.

So you’re convinced about the benefits of fitting VSDs, and you know what to look for when you’re choosing them, but where’s the money going to come from to cover the cost of buying and installing them? The trite answer is that the VSDs will very quickly pay for themselves because of the energy they save.

That’s true, of course, but energy costs usually don’t come out of the same budget as capital costs, so it’s worth knowing that VSDs from some manufacturers — including many in the Vacon range — qualify for the Government’s Enhanced Capital Allowances (ECA) scheme. This means that all their cost can be offset against tax in the year they are installed, which translates into a very significant saving.

Hopefully, this article has made clear the benefits of fitting VSDs and has explained that, provided the right drives are chosen, this is a straightforward task that will start delivering substantial savings immediately. When you take into account that there’s also a valuable ‘sweetener’ available from the tax man, surely now must be the right time to ditch those energy-guzzling fixed-speed starters.

Stephen Takhar is managing director of Vacon.

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