The crucial role of inverter drives in the efficient hospital
 As
our energy reserves dwindle, as the spectre of climate change looms
ever larger and as energy cost continue to rise, it becomes ever
more imperative to take action on energy savings. This requirement
is so critical that it has even been enshrined in law by the new
building regulations. That part concerning energy efficiency, Part
L, is now ‘live’ and mandates a substantial improvement of between
23% and 28% in energy performance of non-domestic buildings. This
applies equally to hospital buildings as it does to commercial
buildings.
While this sounds so draconian as to be unachievable, in many cases
it is actually more than achievable because the air conditioning
systems of modern buildings offer the greatest potential for energy
savings. The various fans, pumps, cooling towers and humidification
systems of today’s hospitals comprise some 40% of that building’s
energy costs. Particularly where older HVAC systems are concerned
there is invariably enormous scope for energy savings.
Inevitably, environmental conditions vary day to night, day to day
and season to season over an extremely wide range and air
conditioning systems are of necessity designed to cope with that.
Unlike many commercial buildings that shut down during the night,
hospitals operate 24/7, day in and day out and stable conditions
must be maintained throughout. Hospital indoor air quality is
critical. It cannot be too dry, too cold or too hot. Pressure must
be maintained higher in some areas, and lower in others yet doors
must open easily. Aspects of sterility and isolation further
complicate the control of HVAC systems in hospitals beyond that of
most commercial buildings.
Unfortunately, when the majority of our building’s air-conditioning
systems were designed and installed, energy efficiency was not a
priority and systems were designed to meet the lowest installed
cost. Despite the need for close building environmental control over
wide ranging outdoor conditions, the control mechanisms employed
were crude and simplistic. In variable air volume (VAV) systems,
damper and vane control of air handling fans, throttling of chiller
pumps and on-off control of many functions was the norm in matching
the capacity of the system to the actual requirements of the
building.
The systems being installed in modern buildings these days could not
be more different. This revolution in efficient air conditioning
systems is due to the development of the most improbable and complex
device to touch all of our lives – the microprocessor. These
min-computers in a single electronic chip have ushered in an age of
superior Building Management Systems (BMS) linked via fieldbus
networks such as BacNet, to a complex network of sensors and
variable speed drives controlling the motors that drive the numerous
fans and pumps that constitute a modern, efficient climate control
system. The result is a much more stable, comfortable working
environment, right through the year and even on the hottest days.
While the modern BMS has made an enormous contribution to overall
control of the system and fieldbus networks have simplified
connectivity throughout the building, the greatest contribution to
energy efficiency has come from variable speed control of the fan
and pump motors themselves.
This is where the largest consumption of energy takes place and
where the need for efficiency is greatest. Considering the widely
varying conditions under which a UK building climate control system
is required to work, it is inevitable that all of the fans and pumps
must by definition be oversized a great deal of the time. In point
of fact, even under the most extreme operating conditions, the vast
majority of systems are designed with what can only be described as
a generous ‘service factor’ and seldom work beyond 75% capacity. At
every stage of the design process, margins were built in to ensure
no-one erred on the side of incapacity.
Inefficient throttles and dampers were then employed pretty much all
of the time to control a system more than fit for purpose. The
problem with this is that all of the fans and pumps employed in
buildings are centrifugal devices that operate to well-established
and disadvantageous affinity laws governing their power consumption
and noise production. The most critical of these laws is that the
power absorption of a centrifugal fan or pump is a direct function
of its speed cubed. The noise of a fan is directly proportional to
its speed to the power five! This means that running fans and pumps
at full speed when full output is not required, i.e. most of the
time, is both energy inefficient and excessively noisy. Speed
control of these fans and pumps would therefore offer greater
efficiency, superior control and reduced noise levels.
The most practical form of fan and pumps control is to vary the
speed of the drive motor and while historically there have been a
number of alternatives available, these were complex, maintenance
hungry and expensive. In recent years however, variable frequency
inverter drives (VSDs) have become highly cost competitive and
significantly more efficient of themselves. Their big advantage is
that they offer close accurate speed control of standard ac
induction motors, themselves the most efficient and reliable prime
mover available. This is by far the most numerous type of motor used
on existing HVAC systems and it is from this that an enormous energy
saving opportunity stems. The cube law characteristic of fans and
pumps, while causing high energy absorption at full motor speed,
provides an enormous reduction in energy taken as speed is reduced.
Theoretically based upon the cube-law rule a speed reduction of 25%
would result in a 42% reduction in energy absorbed. A 50% reduction,
not at all uncommon, would reduce energy by 87.5%. In practice
however, damper and vane systems do offer energy reductions as they
are closed down, (see fig1) but nowhere nearly as significantly as
does variable speed control. At 50% flow, damper control of an air
handling fan might offer 30% reduction and inlet vane control 50%
reduction but this is still long way short of the 87.5% offered by
motor speed control.
Rapid advances in VSDs over the last 10 years have revolutionised
the technology for HVAC applications to the point where it is rare
for a major new hospital projects not to have full VSD control of
the HVAC system linked to the BMS. This ensures close control over
environmental conditions and, particularly important in hospitals,
control over flow between wards to prevent the potential spread of
infection.
Many viral conditions such as SARS, Bird Flu or a number of potent
viruses demand isolation of the patient in an environment from which
the virus cannot easily travel into the general environment. This
can be done by maintaining a negative pressure in the isolation
wards and this best done by accurately controlling supply and
extract fan speeds, maintaining a differential between supply and
return air flows. Velocity sensors installed in the supply and
extract ducts can be connected direct to Danfoss VLT HVAC Drives so
that all air-flow differential calculations are carried out directly
by the drive, and the motor speed finely regulated.
Similarly, for sterile environments where a clean filtered air
supply is required, variable speed control of the fans ensures that
air flow is maintained constant even over a dirty HEPA filter so
long as the fan is sized for ‘worst case’ conditions and run at
reduced speed while the filter is in clean conditions. Simply
installing a static duct pressure sensor just past the HEPA filter,
connecting that to the VSD and allowing this simple closed loop
control to maintain duct pressure and ensure air flow is delivered
at all times. Equally the drive can be programmed to flag up a dirty
filter condition, for maintenance purposes.
It’s application benefits such as these that are responsible for the
proliferation of VSDs. There has also been a huge reduction in cost
per kW of installed power and inverter drives themselves are vastly
more compact and efficient that previously. It is now common
practice in many installations today to install the VSDs near the
motor/pump/fan it operates instead of mounting it in a central
control panel. The main advantages of this ‘decentralisation of
control’ are that it reduces the size of the main switchboard or
control panel and results in simpler installation and easier
commissioning.
However, to ensure long-term reliability of the VSD and protection
of personnel in the plant room, for such an installation it is
important to ensure that the ‘enclosure protection’ of the drive is
not compromised. The availability of IP66 enclosures, facilitated by
the reduction in internal heat production ensures both the integrity
of the panel and the safety of the personnel.
Along with this is the array of software facilities that make their
integration into an overall building environment absolute child’s
play. Inverters are now effectively an intelligent part of the
intelligent hospital. Early inverters simply varied their output
frequency and voltage in ratio, varying the speed of the motor but
with no control over its torque or efficiency. No matter how
efficient at full speed and full load, an induction machine at less
than full load suffers a severe drop in efficiency and also exhibits
a very poor power factor. Modern inverter drives not only control
the speed of the motor but by virtue of powerful digital control
algorithms, Automatic Energy Optimisation (AEO) actually tunes the
inverter to the driven motor, forming a matched pair so that at all
loads and speeds, overall efficiency is maintained at the optimum.
In addition, the intervention of the inverter between the motor and
the mains supply means that the pair exhibits a power factor close
to unity, a significant benefit in its own right.
At the heart of the Danfoss VLT® inverter is a powerful mini
computer controlling not only the supply to the motor but also
offering important fan and pump control functions previously
supplied as separate discrete modules. Flow Compensation responding
to pressure sensors mounted close to the fan or pump, maintains
pressure constant at the discharge end of the system, offering
further energy savings. Inbuilt multiple PID loops and Smart Logic
Controllers offer advanced control of the system external to the
BMS, expanding its capacity.
Fire override mode ensures inverter operation to destruction if
necessary and the Stairwell Pressurisation feature ensures that
stairwells remain smoke-free in the event of a fire. Sleep Mode
detects low or no flow, boosts the system pressure then goes to
‘sleep’ pending a drop in pressure when it automatically ramps up to
operating conditions.
The energy saving capacity of VSDs is best illustrated where older
fan and pump systems have been refurbished and variable speed drives
incorporated, providing a before and after comparison. There are
many of these and in some cases the savings are quite spectacular
with rapid payback of the initial investment:-
• Belfast International Airport has fitted Danfoss VLT® inverters to
28 of its AHUs and expects to save over 1,000,000 kWhrs worth
£60,000 per annum and will recover the total installation costs in
around 8 months.
• Dairy Crest fitted a VLT® inverter to an 11kW boiler FD fan saving
47,000 kWhrs per annum and recovered the capital cost in under 9
months.
• Diageo fitted a Danfoss VLT® inverter to a 132kW duty / standby
pump pair and realised 37,000 kWhrs savings per year with investment
payback in 18 months at an energy cost of a margin 2p per unit.
Of course these savings represent an enormous saving in carbon
emissions, these 3 alone representing well over 500 tonnes of carbon
emissions. Danfoss estimate that the annual energy efficiency
contribution of one year’s production of their VLT® drives alone
represents the power output of a major nuclear power station.
Savings in carbon today can of course be traded on the carbon market
to further reduce the payback period of the initial investment.
In the UK, further cost reduction can be achieved by setting the
inverter costs against corporation tax as they attract an Enhanced
Capital Allowance (ECA), put in place by the government to stimulate
the uptake of energy efficient technology. The exact value of this
allowance varies with a company’s circumstances but as a rule of
thumb it is worth a reduction in capital cost of approximately the
ruling bank rate of interest. Equally other countries offer
incentives to stimulate the adoption of such energy efficient
technology.
The case for intelligent inverter speed control of fans and pumps in
buildings is therefore well made and more than adequately
demonstrated. The downside? Well, quite simply there isn’t one. Even
the initial capital cost is so quickly offset by the returns that
there is no more effective use of a company’s capital resources. The
upside of their adoption is a previously undreamed of efficiency,
putting substantial amounts of cash back where it belongs – on the
bottom line, enhanced levels of year round comfort and a vital
reduction in carbon emissions when the fight against global warming
is the duty of all of us.
Email:
sara.bernard@danfoss.com
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