Konstantinos Kanellakis, Marine Business Development, Group Product Manager Motion & Drives, Schneider Electric presentation during the 2015 GREEN4SEA Forum.
The presentation is about the energy efficiency potential of using variable speed drives to run fans and pumps. Usually valves and dampers are used to regulate the flow of air or liquids, which demands more energy than controlling the flow with drives. Two of the most promising solutions are mentioned and a case study of an actual installation and the energy efficiency achieved. All the issues that may arise by the use of drives on ships, like harmonics, electromagnetic interference (EMC) and voltage peaks are analysed.
There are many ways to reduce a ship’s energy consumption, one of which is with the use of variable speed drives. Variable speed drives have a low investment cost and are easy to install, even when the ship is travelling. Another important factor is that it is also one of the solutions with the fastest return-on investment.
A variable speed drive is a device which gets power from the network (440V-60Hz) and transforms it so that it will run a motor at the speed we want with the torque our application needs. There are many ways to save energy by using variable speed drives, depending on the type of application. For example, in the case of a crane, we can regenerate power back to the network, while for variable torque applications, like fans and pumps, simply reducing the speed will lead to a significant reduction of energy consumption. Fans and pumps are the applications we should target on-board a vessel. We call them variable torque applications, because at low speeds we need low torque. If a fan has stopped, we can even rotate it by simply using our hand, while at nominal speed this is impossible. As speed increases, the torque increases exponentially. If we can run at a reduced speed, we need less torque, which means less current and less energy.
We use fans and pumps to create a flow, of air or liquid. The usual way of regulation of this flow is with dampers and valves, which is energy inefficiency. To reduce energy consumption, instead of using dampers and valves, we can use variable speed drives. We can achieve significant energy efficiency which of course will depend on many parameters of the application (whether existing damper is before or after the fan or the throttling of the valve).
The diagram below shows in general what we can expect to save. The x axis is the flow of air or liquid and y axis is the energy. At 100% flow obviously we consume 100% of nominal energy. The red line shows energy consumption, depending on the flow, when we use dampers and valves as flow regulation methods. At 80% of the flow, we have almost no energy efficiency. However, if we use a drive, then at 80% of the nominal flow, we can achieve 50% energy efficiency.
Most of the fans and pumps operational on ships are usually oversized, so there is a potential of operating at reduced speed. Two of the applications which have the fastest return-on investment, in some cases even less than a year, are:
- Sea water pumps
- Engine room fans
In cooling sea water pumps, sea water is used to cool down fresh water, with the use of a heat exchanger. Then fresh water cools on-board machinery. Through the engine room fans, air is supplied to the main engine and the engine room temperature is regulated. To estimate the potential of energy efficiency, we need to go back to the design phase of the vessel. Depending on the nominal load and speed, decisions are taken regarding:
- Main engine
- Auxiliary machinery
- Cooling needed for above applications
However this is based on the worst case scenario, which is 32oC sea temperature, full load, nominal speed and over sizing due to foul hull, bad weather etc. However, all these rarely happen at the same time. Additionally, a very important factor is slow steaming. Ships today operate at a slower speed than the nominal, in order to burn less fuel, a practice which leads to less heat production and thus the need for less cooling. A case study of the results, from using drives for engine room fans and sea water pumps is shown in the following table, whereby we see the energy efficiency achieved on the “SEAPRINCESS”, an Aframax sized tanker of Thenamaris Ships Management.
Annual savings for both applications were almost 110.000 US dollars. The return on investment was accomplished in less than 12 months. The analysis is from 2013, the year of the system’s installation, when oil prices were $630/tn for HFO and $900/tn for diesel, so the payback period may vary, as the price of oil has since been reduced. Additionally, the environmental impact is significant too, as the CO2 emissions are reduced by approximately 470 tons/year.
Up to the present date, we have experience from installations on more than 50 vessels and the results are more or less the same. However, the correct way to assess the efficiency potential of a specific vessel is by conducting an Energy Audit. The methodology is to first study the applications of a ship
and then go on-board for a walk-through audit, where an Energy Efficiency Expert will discuss with an on board engineer, who is familiar with the ship’s operations, for each one of the vessel’s applications. This is important, as we may even have sister-vessels, travelling on different routes (high or low ambient temperatures), which will have completely different energy profiles. If we make this assessment, we can first identify and fix “the basics”. This may be as simple as turning off the lights when they are not needed. It is what we call “passive energy efficiency”. As an example, we can also stop one of the engine room fans and immediately reduce energy consumption by 25%. In that case we may save $1000/month. If we go to the next step, of active energy efficiency, then we install automation systems, which make sure that not only do we just save energy, but we also maximize the energy efficiency achieved. This may be between $3000 or $4000/month. Through automation we also guarantee that the ship’s operation will not be put in jeopardy. If we stop two fans to reduce energy consumption, we cannot be sure that the air flow will be enough at the current speed of the vessel. However, with the right sensors and through PLC automation, we can be sure that the air flow will always be enough.
The next topic is about what issues may arise by the use of variable speed drives on ships.
One of the issues is the generation of harmonics. Normally, we should expect something like the blue line for voltage and current waveforms, as they are sinusoidal. Since speed drives have power electronics, which are non-linear loads, current waveform appears to be closer to the red line. Harmonics are currents at higher frequencies, most importantly 5th harmonic (5 x 60Hz = 300Hz) and 7th harmonic (7 x 60 = 420Hz). The sum of harmonic currents leads to a non-sinusoidal waveform.
Harmonic currents flow back to the network of the ship. The result is that the voltage will be distorted too and if we have other instruments or electronic devices, they will be supplied by a power of low quality and may malfunction or give false readings. Also, components may need to be oversized, for the additional harmonic currents. We may also have resonance with capacitors and destroy them. With the use of speed drives for engine room fans and sea water pumps, THDU is about 1%-1.5%, which is much less that what is required by the classification societies. The only harmonic mitigation solution used, are simple DC chokes, which are embedded in most drives. Even if we already have high harmonics or we want to use drives for many motors, we can choose among many harmonic mitigation solutions.
The next issue is the electromagnetic compatibility. The problem here is that the voltage output of the drive is not a sinus waveform. As shown in the diagram below, the voltage output of the drive is a series of pulses, what we call PWM (Pulse Width Modulation). This leads to a current which is close to sinus and can supply the motor with enough torque to run the application. These pulses have a dv/dt of hundreds of volts within a few micro-seconds.
This creates radio frequency interference which is emitted around the drive and close to the cable which connects the drive to the motor. This interference may lead sensors to show false readings or the fire alarm to go off while there is no fire. We may also face issues with network communications. All we have to do is use EMC/RFI filters and pay attention to the guidelines of wiring:
- Use shielded cables
- Avoid loops in ground connections and
- Do not put command cables close to motor cables etc
If we follow EMC wiring guidelines, we should have no issues. Also, most drives have embedded filters to reduce emissions. In cases of long motor cables, motor chokes may also be used to reduce the effect of EMC perturbation.
Another issue to consider is the leakage current. These filters have a leakage current to the ground, which of course will give an alarm through the insulation monitoring system. These leakage currents come through the filters of electromagnetic compatibility, so we have to disconnect them from the ground through a special switch which each drive has.
Last but not least, we may have voltage peaks. As we explained, the voltage output of the drive is a series of pulses, which lead to a sinus current. These pulses are not perfect. If we observe through an oscilloscope, we will see that during the rise phase we have a small overshoot. This overshoot will be much higher if the capacitance of the cable is high. As we have to use shielded cables, we are going to have a high capacitance and increased overshoot. A depiction of this phenomenon is shown in the following table:
For the drive used for the tests, at a cable length of 0,5m the peak voltage is 664V. The second graph shows us that at 4m it goes to 824V and at 42 m to 1300V. This leads the PWM pulses to be distorted, which will lead to a low quality of the power supply to the motors. The result is motor noise or even a burnt motor, as old motors especially, were not designed to withstand such high voltage peaks. If we have long motor cables, we need to check what the manufacturer suggests for the mitigation of the above described phenomena. Usually this is solved by the drives’ functions for overshoot mitigation or by using motor chokes.
As a conclusion, we can say that the use of speed drives is an effective way to reduce energy consumption on vessels:
- It is cheap compared to other solutions
- It is easy to install
- It has a very fast return on investment
- Issues associated with the use of drives on ships (EMC, Harmonics, Voltage peaks) can easily be solved
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Above article is an edited version of Konstantinos Kanellakis presentation during the 2015 GREEN4SEA Forum
You may view his presenation video by clicking here
Click here to view all the presentations on this GREEN4SEA Forum |
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About Konstantinos Kanellakis
Marine Business Development, Group Product Manager Motion & Drives, Schneider Electric
Konstantinos Kanellakis is an electrical and computer engineer (Aristotle University of Thessaloniki), with postgraduate studies in business administration (MBA, Athens University of Economics and Business). Has been working for Schneider Electric for more than 8 years, with extensive experience on variable frequency drives and automation.
In the starting, I was straightforward with you propecia before and after has changed my essence. It has become much more fun, and now I have to run. Just as it is fabulous to sit.
It is an nice informative and interesting blog to read. Really feeling updated with lots of information and all the analysis.
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The global Variable Frequency Drive market is expected to experience steady CAGR of over 7% from 2016 to 2024.
Energy efficiency can be achieved only through variable frequency drives and they are designed especially for them. This is a brilliant post as people who are looking for equipment regarding the smooth functioning of their machines and energy efficiency will surely find this helpful. Keep sharing!