Energy savings in fan systems (case study)
Fans (also called ventilators) belong to common drive applications. Most of them can significantly benefit from the use of variable frequency drive (VFD) technology. Many statistics claim that fan, pump and compressor systems consume up to 60% of worldwide generated electricity. And there clearly is a lot of energy that can be saved. However, not every application has the same potential for energy savings. Sophisticated calculators identify the services with largest potential and shortest payback – the low hanging fruit to start with! After introducing the tools for energy and cost savings [1] we present a real case study for of energy savings in variable speed fan systems.
System description
The case study is a forced draft (FD) fan in a medium size thermal power plant in Asia. FD fan provides the required quantity of hot air into the furnace for smooth and uniform combustion of fuel. The fan is of an axial type.
– Type of gas: air
– Gas density 1.276 kg/m3
– Nominal system pressure 3500 Pa
– Nominal system flow 345 m3/s
– Nominal fan efficiency 82%
Operating profile and current flow control
The energy and cost savings depend heavily on the operation profile. The profile defines how many hour per year is the application in operation and how is the load distributed. For existing plants customer might have historical measurements of power consumption of the fan. This is the most reliable input data. If the past power consumption is not known, a qualified estimation shall be made. The operators might be valuable source of information.
In our case the fan runs most of the time -> 8’000 hours per year were selected. The auxiliary systems in thermal power plants are often dimensioned with considerable margin. We can see it in the operation profile as well. The fan operates most of the time with 87-89% of nominal flow.
Existing flow control is inlet damper. It will be compared with a variable frequency control, i.e. VFD driven fan.
Motor data
The mechanical power for both cases is 1’700 kW. Existing motor is 6 kV asynchronous machine. The nominal efficiency is 95.5% and nominal power factor 0.9. VFD driven motor is assumed to have the same parameters (for simplicity – roughly correct).
Reasoning:
– Case new motor:
New motor would have slightly better efficiency neutralized by additional losses due to VFD.
– Case existing motor:
For existing motor a motor friendly VFD (eventually with output sine filter) is selected so additional losses are negigible.
VFD data
Combined efficiency of VFD and input transformer (integrated or external) is assumed 96.7% at nominal load. The curve showing efficiency versus load can be provided by manufacturer for most accurate data. The combined transformer-VFD efficiency is usually quite flat curve. At partial load the transformer efficiency improves while the VFD efficiency drops. The one offsets the other resulting in quite flat curve.
Economic data
Economic data consists of total investment cost (CapEx), operating cost (OpEx) per year, energy cost (€/kWh or $/kWh), interest rate and service life time.
We calculate the variant where the existing motor remains operational (assuming it is still in a good shape). Therefore, the existing solution has practically no investment cost. The VFD solution has a price tag of 215’000 € for the VFD plus input transformer and includes also power cables, installation and commissioning. The service lifetime is 20 years.
The energy cost is 0.07 € / kWh. Interest rate of 3% is considered.
Energy savings in fan systems: Results
1. Economic results
The annual savings are 99’000 €. Accordingly, the payback time is slightly more than 2 years. Net present value (NPV) is 1’467’000 €.
2. Technical results
Energy consumption of current solution with inlet damper is 10’174 MWh/year.
Energy consumption with VFD is 8’765 MWh/year. Energy savings in fan system are therefore 1’408 MWh/year or 13.8%, respectively. The VFD would save 704 t of carbon dioxide per year.
Energy savings in fan systems: Conclusion
This case in just one of many examples where VFD can save substantial amount of energy. In our case study the forced draft fan with VFD has approx. 13.8% lower energy consumption compared to the inlet damper. The payback time is approximately 2 years. It is, therefore, not only environmental friendly, but also economically attractive solution.
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References
[1] Energy efficiency – How to calculate energy and cost savings, https://mb-drive-services.com/how-to-calc-energy-and-cost-savings/
[2] M. Sirovy, Z. Peroutka, J. Michalik, M. Byrtus, “Medium Voltage Fan Save: Energy Efficient Fan Systems in Power Engineering, Part 1”, 38th Annual Conference on IEEE Industrial Electronics Society, IECON 2012, pp. 1368 – 1373
[3] ABB, Energy Efficient Design of Auxiliary Systems in Fossil-Fuel Power Plants. USA: ABB, Inc. in collaboration with Rocky Mountain Institute.
