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Inverter drives cut HVAC system costs

21st February 2013


 

There are three trends that building managers and designers of heatingventilation and air conditioning (HVAC) systems need to be aware of: tightening legislation relating to carbon dioxide emissions; rising energy costs; and the desire to centralise control in order to reduce overheads.
Although the concept of an inverter with networking capability has been with us for several yearsthe three aforementioned trends make the technology more worthwhile than ever before.

 

Energy laws

 

In Europe the Energy Performance of Buildings Directive (EPBD) is forcing national governments to implement laws such as the Energy Performance of Buildings (Certificates and Inspections) (England and Wales) Regulations 2007.

Under this legislationEnergy Performance Certificates (EPCs) are being phased in for dwellings and other buildings.
In particularby 4th January 2009 all existing air-conditioning systems over 250kW must have been inspected
and all air-conditioning systems over 12kW must have been inspected by
4th January 2011. Similar legislation is either being implemented or actively considered in other nations around the worldwith the aim of reducing carbon dioxide emissions.

According to the UK Governmentnearly 50 per cent of the UK’s energy consumption and carbon emissions are accounted for by the nations 25million buildingsand air conditioning can account for one-third of an organisation's annual electricity costs.

While these figures for a country in a temperate zone cannot be extrapolated to others elsewhere on the globethey provide a useful indication of the scale of the issue.

Little needs to be said about rising energy costs. At the time of writingBloomberg shows the spot price of Brent crude at US$121which compares with approximately US$107 a year ago and US$71 in 2006.

In May 2008 Arjun Murtia Goldman Sachs analystforecast oil will reach between US$150 and US$200 within six months to two yearsso the upward trend looks likely to continue.

The third trend mentioned above relates to the centralisation of building management in an effort to reduce overheads. If HVAC andindeedother building functions can be monitored and controlled centrallyit reduces the need for personnelenables buildings to be managed in a more energy-efficient mannerand simplifies maintenance.

An important enabling technology for centralised control is networkingwith communications protocols such as BACnet (building automation and control network) and LonWorks (local operating networks) making it relatively straightforward and cost-effective to connectmonitor and control HVAClightingfiresecurity and other building automation systems.

 

Network choices

 

BACnet is a non-proprietaryopen protocol that requires no proprietary chip sets or protocols. It is also an American national standarda European standarda national standard in more than 30 countriesand an international standard ISO16484-5.

Initially LonTalk networks were implemented on hardware containing dedicated Neuron Chip microprocessors manufactured by Echelonbut the LonWorks protocol is now available for general-purpose processors and is in the process of being developed into an international standard ISO14908. The LonWorks protocol is also one of several data link/physical layers of the BACnet standard. Today BACnet and LonWorks are arguably the two dominant building automation networking protocols worldwidethough there are others as well.

Numerous inverter drives are available that can communicate via BACnet or LonWorkseither directly or by means of optional communications modules. In 2004 ABB claimed its ACH550 was the first variable-frequency drive to offer BACnet communications without the need for any additional hardware. Since then the company has continued to develop its drives for HVAC applications and has helped numerous organisations to reduce their HVAC energy costs (see panel).

Compared with traditional mechanical methods of controlling fans and pumpswhere throttles are used to regulate the flow while the motor operates continuously at high speedinverter drives control the motor speed andthereforethe flow. Because the motor typically operates below its maximum speed for the majority of the timesignificant energy savings can accrue.

 

cross the globe there are so many HVAC fans and pumps in operation that companies such as Mitsubishi have found it worthwhile to optimise inverter drives specifically for this type of application. For examplethe company's FR-F740 series inverters are claimed to enhance the energy balance of pump and fan applications – particularly in the low-speed range and in low-load operationwhere energy savings of up to 60percent can be achieved (Fig.1). Additional savings are made possible by the OEC (optimum excitation control) technology that ensures the optimum magnetic flux is always applied to the motorthereby reducing losses. The result is said to be maximum utilisation of the motor’s capacity at maximum efficiency.

The FR-F740 complies with standards such that it can be sold in EuropeRussiathe USA and elsewhere. Models are available with outputs ranging from 0.75kW to 630kW. An integrated control panel with the one-touch Digital Dial enables rapid entry of all necessary drive parameters. Alternativelyan alphanumeric control panel with multiple language support and copy functions is available.

Mitsubishi has equipped the FR-F740 series with connectivity for all major fieldbus systems such as LonWorksProfibus/DPDevicenet and CC-Link. Other communications facilitieslike the integrated RS-485/Modbus RTU interface and the optional USB portmake it possible to create multidrop configurations (with up to 32 stations) and to connect the inverters to PCs and process visualisation systems.

 

Compact and cost-effective

 

Earlier in 2008Control Techniques Driveswhich is a division of Emersonlaunched the Affinity range of HVAC drives that is focused on the needs of the building automation industry world-wide (Fig.2). This ruggedcompactenergy-efficient and cost-effective family of drives is offered with power ratings up to 132kW to suit supply voltages from 200-690 V. Affinity drives are manufactured in both IP20 and IP54 formats with BACnetMetasys N2 and Modbus communications all integrated as standard; LonWorks can be specified as a plug-in option.

Siemens is another company that has seen the potential for inverter drives dedicated to the HVAC market. Its SED2 variable-speed drive utilises insulated gate bipolar transistor technology (IGBT) and covers a power range from 0.37-90kW in IP20/21 housings and 1.1-90kW in IP54 housings. During the developmentparticular attention was paid to reducing harmonics without the need for additional expensive filters and chokes. The integrated human-machine interface (HMI) with pre-programmed HVAC functions is said to make commissioning quickand an integral PID (proportional-integral-derivative) controller enables flows to be controlled accurately. A LonWorks module is available as an option.

 

or applications where higher power ratings are requiredDanfoss offers its VLT HVAC drive with ratings up to 450kW. These drives are designed specifically for fanpump and compressor applications within the air conditioning market (Fig.3).

Power ratings start from 1.1kW and both IP20/21 and IP55 compact enclosure styles are offered; IP66 protection is also available as an option.

Danfoss describes programming the VLT HVAC drive as ‘child’s play’thanks to an intuitive and award-winning HMI that is similar in some respects to a mobile telephone. The graphical display also offers fewer codesmore clear textfewer rows of numbers and more graphs. Users can choose to work in any of 27 different languages.

An AEO (automatic energy optimiser) feature maximises energy savings by optimising the motor’s magnetic field under all load conditions. This is also said to simplify the drive’s set-up and reduce motor noise. Another interesting function is the drive’s Sleep modein which the drive detects situations with low or zero flow. Instead of operating continuouslyit boosts system pressure and then stops to save energy; when pressure falls below a lower set-pointthe drive starts again automatically.

VLT HVAC drives are compatible with Modbus RTUMetasys and Siemens FLN serial communications as standardand BACnet or LonWorks communications can be specified as built-in options.

Integral chokes and filters ensure that mains harmonics are minimised while output cable length to the motor is maximisedthereby eliminating the need for output line chokes.

Furthermoremotors do not have to be de-rated to allow for the harmonic content of the output waveform and the output circuit canif necessarybe broken under load without auxiliary control switching or concern for switching surges.

 

New HVAC projects

 

While the foregoing introduces some of the latest inverter drives available for networked HVAC applicationsthere are others either already on the market or in development.

New HVAC projects will almost inevitably include drives rather than physical throttlesand there are still ample opportunities to upgrade existing HVAC plant to use drives. Furthermoresome of the older drives installed when the technology was relatively new are now reaching the end of their operating livesso there will be advantages in upgrading these to the new genarationof networked drives. All of which will help building owners to comply with new legislationreduce running costs and benefit from the centralised controls.

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