|
The application of Variable Speed Drive (VSD) technology can reduce the annual energy consumption of HVAC plant by up to 30 percent, resulting in a significant reduction in CO2 emissions. Andrew Layland, Director of Field Sales (UK and Ireland)at York International,recently recognised by the US Environmental
Protection Agency for its pioneering work on VSDs applied to
chillers, outlines the environmental and cost benefits of these energy efficient devices.
It is a fact that the indirect effect of electricity consumption is the
largest contributor to greenhouse gas emissions from chillers.
Average annual energy savings of 20 percent, typically
achievable with the addition of a VSD, will significantly reduce
these emissions, resulting in substantial climate protection.
Since York pioneered the technology in 1979, the HVAC industry in
the USA has provided over 6,000 centrifugal chillers with VSDs,
representing an estimated reduction in total CO2 emissions of
almost 400,000 tons per year, as demonstrated in the table. With
our lower load factor here in Europe, compared to the USA, much
greater savings are possible.
York was the first manufacturer to introduce and actively promote
the application of VSDs to centrifugal chillers, demonstrating to the
industry and its customers that it is a robust and reliable technology.
Based on York’s efforts, VSD technology has evolved from large,
expensive floor mounted drives to today’s compact, unit mounted
and factory installed units. Current VSD products are more
economical and energy efficient than early generations, offering
chiller purchasers attractive, and rapid, returns on investment.
The technology is also now being extended for use on other
chiller types including air-cooled screw compressor models. In this
growing market, with higher energy levels than water-cooled
equipment, the opportunity to reduce power consumption and,
therefore, CO2 emissions is even greater.
VSDs are able to achieve such high energy efficiency because
they are specifically designed to match output with demand by
working at speeds that automatically vary according to the building
or process load. They are particularly beneficial where there are wide
variations in building occupancy and ambient temperatures, either
between seasons, or between day and night.
The VSD serves as both the motor starter and capacity controller.
It controls chiller capacity by varying compressor speed and, in the
case of centrifugal models, fine tuning the use of inlet vanes to
maintain the optimum compressor efficiency at all loads. It varies
the compressor speed by controlling the frequency of electrical
supply to the motor. Because the motor power is proportional to the
cube of the speed, the power drawn drops away much faster than
the speed when speed is reduced; for example, at 80 percent of
maximum speed, the motor only uses 50 percent of the full load
power.
On a screw compressor, by replacing
conventional slide valve capacity control with
a VSD, power consumption is minimised
throughout the entire load range; however, maximum energy
savings are gained at part load. Given that less than two percent of
chiller run hours are at design full load, it is clear that the
introduction of a VSD can play a major part in increasing the energy
efficiency of the chiller.
While VSDs help reduce energy consumption by making motors
draw less power, energy savings can also result from the design
flexibility the device allows specifiers and consultants. There is a
wide range of programming and communications options offered by
VSDs that allow designers to achieve more efficient systems. VSDs
have the ability to communicate with one another, with other devices
on the network and with an overriding building management system
to optimise conditions to a high degree of accuracy. Some models
with adaptive capacity control are additionally able to learn and
remember optimum speeds for various loads and watertemperature
combinations to maintain chiller operation at its peak
performance.
Further benefits are derived from a VSD’s other role as a starter.
Electro-mechanical starters usually use a full-speed start, with
inrush currents reaching as high as 600 percent of full-load amps.
This can cause heat build-up and flexing at critical points in the
motor windings that, over time, can damage the motor. For this
reason, a 30-minute cool down is mandatory before a constant
speed chiller can be restarted. By replacing the electro mechanical
starter with a VSD, the chiller’s motor starts more slowly, never
drawing more than 100 percent of its full load amps. Consequently
motor heat is reduced, as is the likelihood of electrical shorts and
burnouts. Chillers fitted with VSDs can be restarted in a little as three
minutes, making quick-turn, emergency restarts possible. In
applications requiring back up generators, the lower start-up amps
will also allow the specification of a smaller generator.
For further protection, VSDs can reduce electric-current harmonic
distortion, which can damage other equipment in the building. York
has designed its OptiSpeed drive to reduce electric-current
harmonic distortion to less than four percent compared to up to 80
percent caused by competitive drives.
While electrical safeguards protect the electrical components of
the chiller, the mechanical components benefit from the lower
operational speeds. Since approximately 98 percent of operating
hours are at less than full speed, moving parts
experience less component wear, resulting in greater
reliability and longer life.
All of these features lead to easier commissioning
and ongoing maintenance benefits. Users
buying into the original manufacturer’s
maintenance programme will also ensure their
equipment continues to operate at peak
efficiency and reap long term energy saving
benefits.
Finally, there are acoustic benefits associated with the use of
VSDs as slower speed means less noise, which could be as much
as 10 dBA.
The majority of new centrifugal chillers from all manufacturers are
equipped with VSD devices. These tend to be factory packaged with
the VSD unit mounted and complete with all the required sensors
and interconnecting wiring. All of the chiller and drive parameters
are displayed via an integrated control panel. VSDs will provide
savings in both single chiller and multi chiller applications. In many
multi chiller cases only one VSD will be required to provide control
across the chiller system. This is achieved by switching the machine
operation to keep the non-VSD equipped chillers at 100 percent load
and using the machine with the VSD to trim the balance of the load.
It is possible to retrofit VSDs to existing equipment in the field.
This is especially cost-effective if combined with a refrigerant
conversion or driveline retrofit. It should also be considered where
a starter has to be replaced, as it is far superior to conventional
types. However, retrofitting must be carried out by experienced
engineers, as, should the motor not be designed to accept variable
speed operation; the installation could result in lubrication and
overheating problems.
By accruing energy savings during 98 percent of a chiller’s
operating season, the payback for the extra cost of a VSD can be
fast; in many instances as little as one to three years. York has newly
developed advance computer analysis software that will calculate
the energy savings (or potential energy savings) due to the VSD and
display it for customers. The result is to reinforce the decision to use
a VSD or encourage the addition of a VSD to capture the additional energy savings.
Chiller Emission Impact – USA Example
Average chiller refrigeration capacity (kW) 1400
Average chiller COP (chillers from 1994 –2004) 5.65
Equivalent full load hours per year (EFLH) 2000
kWh per chiller per year 495,500
Energy reduction with VSD (%) 20%
CO2 equivalent saved per chiller per year (tons – based on 0.65 kg CO2/kWh) 63.3
Number of chillers with VSDs 6000
CO2 equivalent saved per year (tons) 379,464
