Energy savings in pump systems: Case study 1
After introducing the way how energy and cost savings are calculated [1] we will demonstrate the theory on a real example from industrial practice. This article presents a case study of a boiler feedwater pump inside a thermal power plant. Let’s explore energy savings in pump systems like this one.
Variable speed pumping
A quick refresher before going deeper into the case study. There are several methods how to control the flow in pump systems:
– Re-circulation (bypass)
– Throttling
– Cyclic control (On/Off)
– Fluid coupling
– Variable frequency drive (VFD)
This particular case study compares a fluid coupling with a VFD.
Case study on energy savings in pump systems
The application is a boiler feedwater pump (BFP) inside a large thermal power plant. The power plant has total rating of 4’000 MW and consists of 5 blocks, each rated 800 MW. In base case the pump motors are driven via a hydrodynamic geared coupling. It is a combination of fluid coupling and integral speed increasing gear. Comparison of fluid coupling and variable frequency drive has been published earlier [2]. We will analyze for this specific case what is the energy consumption and operating cost of a combined fluid coupling with integral gear and variable frequency drive.
Pump and system data
Each pump has nominal discharge flow rate of 1’405 m3/h. Tapping head is 1’152 m and discharge head 3’511 m. The system water temperature is approx. 168°C. Water density is 898.3 m3/h. Efficiency of the pump at design point is 85.3%. The nominal rotor speed is 4’975 rpm and the corresponding shaft input power is 14’355 kW.
The operating profile is shown in Figure 3. Since thermal power plants mainly cover the base load we can consider 8’000 operating hours per year as realistic number. Because of design margins in such power plants the auxiliary systems such as feedwater pumps will almost never operate at their rated point. In this case study the maximum flow reached in normal operation reaches 95% of nominal design flow. In fact, as shown in Figure 3, the most common flow rate is 85% of design flow.
Compared flow control: Fluid coupling
Base case is a a geared hydrodynamic coupling. The efficiency of this hydraulic gear is 94.0% at its nominal (design) point. The efficiency drops quite quickly with reduced speed as per design data:
– 94.0% efficiency at 100% speed
– 93.6% efficiency at 99.7% speed
– 89.0% efficiency at 95.7% speed
Reducing the speed by less than 5% from nominal point the efficiency of the geared fluid coupling (i.e. combined efficiency of fluid coupling and gear) already drops below 90%. For wider speed range the results are even worse.
These are ‘as built’ data.
Variable flow control: VFD
When using VFD to control the flow of this specific pump there are two fundamental options:
– Option 1: VFD with conventional 4-pole motor and separate speed increasing speed
– Option 2: VFD with direct drive (‘high-speed’) where motor shaft has the same speed as the pump
Both above variants are technically feasible and well referenced in the industry. LCI drives have many references in the power range 12 – 18 MW with motors up to 6’000 rpm. These come already from 1990s (e.g. 9.6 MW @ 6’000 rpm installed in 1984 in South Africa or 13 MW @ 5’700 rpm installed in 1988 in Germany). VSI drives are technologically younger, but have already solid references up to very high speeds.
Nevertheless, as a conservative approach, we selected a conventional geared system driven by a VFD. It can be assumed that a direct gearless drive outperforms the results presented in this case study.
The VFD nominal efficiency is 98.7% and transformer nominal efficiency 98.9%. As explained in previous case study on fans [3] the combined transformer plus VFD efficiency is fairly flat curve. The percentage transformer efficiency increases at partial load while the VFD efficiency slightly decreases.
Motor data
Although the required mechanical input power demand is approx. 15’280 kW the motors were rated 18’100 kW applying decent safety margins. The same motor rating was selected for the VFD operation even if it seems little overdimensioned.
Exact same motor data are used for fluid coupling case and VFD case. To be fair, the true motor efficiency had been reduced by 0.3% for the VFD case considering additional losses due to harmonics:
– Motor nominal efficiency with fluid coupling: 97.3%
– Motor nominal efficiency with VFD supply: 97.0%
In other words the motor in VFD duty has 10% additional losses.
Economical results
The purchase price of geared fluid coupling (hydraulic gear) is not known. Therefore, payback time is not evaluated. However, based on above operating profile and energy cost 0.06 EUR/kWh the annual savings are 702’000 EUR. Therefore, payback time of less than 2 years is expected to offset the investment cost of VFD solution compared to hydraulic solution.
There are additional indirect benefits not considered in the economical results. One of them is the soft start of the pump motors with the VFD. It eliminates the inrush (starting) current of the motors contributing to longer lifetime. Soft starting also minimizes transient torque. That results into extended lifetime of shaft components such as couplings. Both benefits may positively affect the maintenance intervals and reduce maintenance cost.
Technical results
The solution with geared fluid coupling consumes 90’422 MWh/year.
As benchmark, solution with a variable frequency drive consumes 78’716 MWh/year.
Therefore, resulting annual energy savings yield (90’422 – 78’716) = 11’706 MWh.
Estimated carbon dioxide reduction equals to 9’873 t/year.
Annual savings are 11’706 x 0.06 kEUR = 702 kEUR as already mentioned.
Energy savings in pump systems
The graphical summary of above results in shown in figures 6 – 9. System losses are transparently shown for each system component.
Solution with fluid coupling includes losses of:
– pump
– fluid coupling with integral gear
– electric motor
Solution with variable frequency drive includes losses of:
– pump
– gearbox
– electric motor
– variable frequency drive
– input transformer
Although VFD system has more components, it still reaches overall better efficiency and considerable energy and cost savings.
Remark:
Note that larger amount of components in case of VFD is relative. In lower power range the input transformer can be integrated into the VFD – see [4]. Also, high-speed direct drives are available eliminating the need for a gear. In such case the amount of system components is reduced from 5 to 3 –> same same the hydraulic gear.
Summary
Although fluid coupling belongs to the more efficient flow control methods (compared to bypass valves or throttling) the VFD is still superior in terms of system efficiency. In our case of 18.1 MW boiler feedwater pump is a thermal power plant the annual energy savings reach 11’706 MWh. This amount of additional energy can be delivered into the power grid and transmitted to the households and industrial customers instead of being wasted in form of heat losses.
This case is just a demonstrator of the large energy saving potential in pump applications.
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References
[1] How to calculate energy and cost savings, https://mb-drive-services.com/how-to-calc-energy-and-cost-savings/
[2] VFD versus fluid coupling, https://mb-drive-services.com/comparison-of-vfd-and-fluid-coupling/
[3] Case study for a fan system, https://mb-drive-services.com/case-study-for-a-fan-system/
[4] Integrated versus external transformer, https://mb-drive-services.com/combined-transformer/
[5] M. Sirovy et al., “Sophisticated Software for Design and Optimization of Variable Speed Drives for High-Power Pumps: Hydrodynamic coupling versus Frequency Converter”, IECON 2011 – 37th Annual Conference of the IEEE Industrial Electronics Society , 7-10 Nov. 2011
[6] Energy efficiency in variable speed drives, https://new.abb.com/drives/energy-efficiency
