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The Accelerating Pace of Change Towards Variable Speed Drives

Geoff Pile is convinced that the time has never been so ripe for realising the substantial cost saving benefits of variable-speed drives

The future for the built environment seems especially bright just now. A concerted move by architects in recent years to focus on sustainable growth, has resulted in the emergence of some spectacularly successful ' lean, mean and green' developments in the commerce and leisure sectors.

Encouragingly for the HVAC and Facilities Management industries, the focus on whole life building costing rather than cost of construction means that, at long last, architects are opening the dialogue for the specification of building systems at the planning stage. Key to this process is a drive to deliver ongoing savings through reducing energy consumption and keeping maintenance costs down.

Equally challenging for HVAC consultants is the development of solutions which address issues in existing properties, a process complicated by overcrowding in plant rooms or the need to free up costly floor space and the integration of HVAC equipment with BMS systems.

In terms of whole-life costing, energy savings are particularly pertinent when it comes to HVAC. Studies have shown that somewhere between a third and half of the energy consumed in buildings can be accounted for by the daily use by centrifugal pumps, fans and compressors fitted to ventilation, space heating and cooling systems.

All too often however, capital cost continues to be the key driver in the specification and purchase of HVAC equipment.

One such technology that has suffered in the process is the variable-speed drive. Not a new technology, variable speed running offers potentially staggering energy savings which can realise an early payback and significantly reduce system running costs as well as prolonging plant life. Yet the use of this technology is far from widespread.

In part, lack of take up is due to historical reasons, with initial purchase costs for this technology being high and space requirements an issue. Today, however, variable speed drives need neither be costly nor impose space restrictions. Technology advances have driven down purchase prices down and produced drives with a much smaller footprint. Driven by 'smart' technology, my company’s solutions are representative of what the technology has to offer, with wall-mounting options and remote running potentially freeing up floor space and eliminating the need for dedicated plant rooms. Custom designed for HVAC systems, the new intelligent variable-speed drive links to building management systems to deliver lifetime operating efficiencies in older properties as well as new buildings whilst protecting HVAC plant in use.

It is in the long term, however, where variable speed technology really scores in saving energy, reducing the energy costs by as much as 50% on an ongoing basis to overshadow the capital costs of purchase by many times during a building's lifetime.

The 'law of the cube'

In most HVAC applications, electric motors run continuously at full speed around the clock. Traditional control methods such as vanes and throttles modify output to achieve the desired level of service. In practice however, the demand for services such as heating, cooling and hot water heating vary throughout the day, even in buildings running 24 hours a day, seven days a week. The result can be an excessive waste in energy.

Variable speed drives by comparison, reduce energy consumption by regulating motor speed to reduce flows in line with occupancy requirements. Power is significantly reduced by reducing the pressure and flow to devices such as fans and pumps. Any reduction in load results in a threefold decrease in power input. Reducing motor speed by just 20%, for example, could halve energy consumption, day on day, year on year. The difference between the most efficient and least efficient methods of operation, based on continuous running with a 75kW motor installed, is £14,000 saving in electricity costs per year while systems remain in operation, which could be up to 20 years.

Reducing Wear and Tear

At the same time, reduced wear and tear on HVAC systems' components offers maintenance advantages. For example, the DC` link choke fitted to the Alldales new 'G' series extends the life of capacitors by 3-4 times. The life of temperature dependant fans is extended by them being switched on only when required as opposed to running continuously. Additionally, the soft switching achieved by variable-speed drives is more motor friendly, with up to four times less occurrence of voltage peaks.

Today's HVAC-specific variable speed technology is particularly suited to retrofitting, being easily fitted to existing motors without the need for larger cable to improve the power factor of systems to near unity. Flexibility in commissioning allows the HVAC contractor to balance systems more easily to achieve the required airflow. Microprocessor-controlled, modern variable speed drives offer the potential for communications links into existing building management systems to allow information such as energy consumption to be exchanged for greater efficiencies in daily operation.

Worth a rethink

There has never been a better time to fit variable speed drives. The technology is one of several qualifying under the DETR's Enhanced Capital Allowances Scheme, which allows purchasers to offset the cost of purchase in the first year.

Added to the potential financial and energy savings fitting variable speed drives can deliver, this alone makes the technology a viable proposition which deserves a second look from HVAC specifiers.

As published in BSEE Vol 26 No 1 September 2002

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"It is in the long term,
however, where variable speed
technology really scores in saving
energy, reducing the energy costs by as much as 50% on an ongoing basis to overshadow the capital costs of
purchase by many times during a building's lifetime."