energy efficient variable frequency drives and submersible pumps

ENERGY EFFICIENT VARIABLE FREQUENCY DRIVES AND SUBMERSIBLE PUMPS
1
Scott Barnes
1. Unitywater, Caboolture, QLD
ABSTRACT
This paper explains the process and results of
trialling new sewage pump station technology with
the aim of determining the technologies energy
saving capabilities, when compared with existing
equipment. The paper includes the methodology
adopted while conducting the trials and a financial
analysis of the resultant energy savings.
INTRODUCTION
The electricity costs in Queensland are forecast to
increase above inflation over the next three years
resulting in a substantial increase in operating costs
for water utilities to operate both Sewage Pump
Stations (SPS) and Water Pump Stations (WPS).
This increase in electricity expenses and a need to
reduce the carbon footprint has driven Unitywater to
investigate energy efficient pumping technologies.
Unitywater commissioned a field trial at sewage
pump station BLI131, to determine the energy
reduction capabilities of four products. Three of the
products trialled are variants of Variable Frequency
Drives (VFD), which are used to control the motor
speed and torque of submersible pumps by varying
motor input frequency and voltage. In addition to
this, an energy efficient submersible pump (Flygt
Experior) that is designed to operate with a partner
pump controller (Flygt SmartRun) was also trialled.
Five trials were conducted to assess the energy
reduction properties of each technology and
included:
 A standard VFD connected to the in situ
submersible pump (trial 1);
 The Flygt SmartRun pump controller combined
with its partner energy efficient submersible
pump, the Flygt Experior (trial 2);
 The Flygt SmartRun pump controller connected
to the existing in situ submersible pump (trial
3);
 The energy efficient Flygt Experior pump
without the partner SmartRun pump controller
(connected to the in situ auto transformer) (trial
4); and
 A new technology VFD connected to the in situ
submersible pump (trial 5)
PRODUCT DESCRIPTIONS
Standard VFD
VFD units reduce energy consumption by
controlling the speed of the electric motor driving
the pump. Typically if a VFD or equivalent is not
installed the pump will start immediately at the
maximum 50Hz, operate at this speed and finally
instantaneously stop at 0Hz when the command is
given.
This type of operation cycle is very
inefficient as the energy consumption spikes when
the pumps start and stop. VFDs smooth out this
cycle of energy spikes by ramping up and down to
the required pump speed in a controlled fashion.
VFDs are also able to be controlled through the use
of a Programmable Logic Computer (PLC) that can
be programmed to operate the pumps in an energy
efficient manner that utilises reduced pump speed,
dependent on the hydraulic load. In this trial
Unitywater flow pacing software code was used to
control the VFD.
Flygt Experior Pump and Flygt SmartRun Pump
Controller
The Flygt Experior pump and Flygt SmartRun pump
controller are new products to the Australian
market.
Both products have been designed
specifically to be used together to improve pump
reliability and efficiency.
The Flygt Experior pump uses a newly designed
energy efficient electric motor that is engineered to
concentrate energy losses around the stator which
keeps the motor as cool as possible. This prolongs
the lifespan of the motor, increases efficiency and
protects the pump bearings.
The Flygt SmartRun pump controller is an
integrated control unit that is pre-programmed to
optimise pump operation by continually assessing
and responding to the hydraulic load on the pump.
This achieves energy savings by the pump
continually operating on the optimal point of the
pump curve.
New Technology VFD
The new technology VFD that was trialled is the
same as a standard VFD, however, unlike a
standard VFD this product also optimises the mains
power supply into the motor of the submersible
pump. The product detects the motor load
variations at a high sampling rate and regulates
both the current and voltage delivered to the motor,
producing energy savings by continuously
operating at the optimal point of the power curve.
Figure 1: SCADA Page Displaying Power
Consumption and Total Daily Flow
METHODOLOGY
The energy reduction capabilities of each
technology were compared against the energy used
by the existing in situ equipment at the chosen
sewage pump station.
Trial Site
SPS BLI131 located at Oro street Bli Bli, on the
Sunshine Coast was constructed in 1977 and
subsequently upgraded with new pumps and a new
switchboard in 1999, which are still in operation.
The specification for BLI131 for the upgrade in
1999 is listed in table1.
Table 1: Pump Station Specifications
Flow rate (l/s)
57
Head (m)
No. of pumps
28.5
2
wet well
submersible
63%
Pump Type
Pump Efficiency
Electrical Requirements
(kW)
kilowatt hours/ kilolitres
pumped
Flow Meter Installed
25.3
.1407
Trials
Once the measuring devices were verified as
accurate and functional, the in situ equipment was
trialled to give a baseline figure for comparison,
followed by trials designed to measure the energy
saving effectiveness of the different technology.
Table 2: Equipment Trials
Each trial was designed to examine the energy
reduction capabilities of one piece of equipment
except for trial two where the combined energy
reduction capabilities of both Flygt products were
examined. This trial was performed because the
manufacturer specifies that in order to achieve the
maximum efficiency and reliability gains, both the
Flygt Experior pump and Flygt SmartRun pump
controller are designed to be installed together.
yes
RESULTS
The two submersible pumps are connected to
autotransformers in the switchboard. All energy
savings in this trial are based on energy
consumption comparisons with the in situ
installation i.e. a 10% energy saving means a 10%
energy saving when compared to the normal in situ
installation.
Energy and Flow Measurement
An alteration was made to the SCADA system at
BLI131 to enable measurement of total daily energy
usage (kilowatt hours (kWh)) and total daily flows
(kilolitres (kL)) of pump number one. This allowed
the specific energy of the SPS to be calculated
(kilowatt hours per 1000 litres pumped) and,
therefore, direct energy consumption comparisons
between the products being trialled.
Figure 1 displays the amended SCADA page for
BLI131 and the totalised energy and flow amounts.
Figure 2 displays that, in terms of a single piece of
equipment, the Flygt Experior pump has the largest
energy saving impact. However, the Flygt Experior
pump combined with the Flygt SmartRun pump
controller produces an energy saving of almost
57% when compared with the in situ equipment.
A 57% energy saving at BLI131 correlates to an
electricity cost saving (at current electricity prices)
of $13,230 and a reduction of 72 tonnes of CO2 per
annum.
Figure 2: Percentage Energy Savings of Each Trial
greater return when compared to trials 1 and 5.
The three trials that resulted in the highest IRR
involved either one or both of the Flygt products
and also resulted in the same payback period of
three years.
Trial 2 that assessed both the Flygt controller and
pump, has the greatest Net Present Value (NPV)
over the five year study period. Trial 2 also has the
highest initial capital cost, however, the NPV
combined with the greatest reduction in CO2
emissions could be used to justify the added initial
expense.
CONCLUSION
Figure 3: Electricity Cost Savings and CO2
Emission Reduction per annum.
The trial of the Flygt system also highlighted
additional operational benefits that included:
 Ease of system installation, which required no
software coding or specialist engineering;
 Zero pump blockages throughout the trial, due
to the newly designed Flygt pump impeller and
pump cleaning function built into the SmartRun
pump controller.
Financial Analysis
The financial analysis of the five trials is based on
monthly electricity bills supplied by Unitywater’s
energy retailer for BLI131. All energy efficiency
figures used in the analysis are derived from the
five trials and based on comparisons with the in situ
installation.
The financial analysis was done over a five year
period and the assumptions include:
 No inflation has been applied;
 No tax has been included in the analysis;
 No escalation factor for electricity pricing has
been applied;
 The study period for the analysis is 5 years;
 A 7.11% discount factor has been applied;
 The energy savings have been applied from the
results of each trial.
Table 3: Financial Results of Each Trial
The Internal Rate of Return (IRR) of trials 1, 2 and
3 are very similar (18-19%) and provide a much
Trial 2 involving the Flygt Experior pump and Flygt
SmartRun pump controller resulted in the largest
energy saving of 57%, which in turn results in a first
year electricity cost saving of $13,230 and a 72
tonne reduction in the carbon footprint of BLI131.
Over a five year period the NPV of this option is
$19,124, with the only downside being the upfront
capital cost which is the largest of the five trials.
The initial upfront cost is offset by the strong
financial and environmental outcomes when both
the Flygt products are installed at the SPS.
The Flygt Experior pump and SmartRun pump
controllers range in sizes from 2.4 kilowatts to 70
kilowatts with this size range accounting for over
70% of Unitywater’s sewage pump stations. A five
site expanded trial of the Flygt technology is
currently underway at Unitywater, with the aim to
determine whether the same energy savings are
realised at SPS of differing system curves. This
expanded trial aligns with Unitywater’s commitment
to drive down operational expenditure and lower
Unitywater’s carbon footprint.
Trial
Number
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
New Equipment Trialled
Standard VFD
Flygt Experior Pump and Flygt
SmartRun Controller
Flygt SmartRun Controller only
Flygt Experior Pump only
New Technology VFD
In Situ
Autotransformer
used in Trial
No
No
In Situ Pump
Used in Trial
Yes
No
No
Yes
No
Yes
No
Yes
Table 2: Equipment Trials
Equipment Trialled
Trial 1 - Standard VFD Only
Trial 2 - Flygt Experior Pump and Flygt
SmartRun Controller
Trial 3 - Flygt SmartRun Controller only
Trial 4 - Flygt Experior Pump only
Trial 5 - New Technology VFD Only
Net Present
Value ($)
-2218
19124
Internal
Rate of
Return
-10%
18%
Discounted
Payback
Period
N/A
3 years
5834
15279
3232
19%
19%
9%
3 years
3 years
4 years
Table 3: Financial Results of Each Trial

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