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ORC Generators - Synchronous or Asynchronous?

The Organic Rankine Cycle (ORC) expander converts the recovered thermal energy from a low grade waste heat source into kinetic energy, which is then converted into electrical energy by a generator. There are many different combinations of expander technologies and working fluids used in ORC technology to achieve this, this article discusses the two different methods of generation used to convert the kinetic energy into electrical energy. The main Alternating Current generating methods used by ORC manufacturers are referred to as Synchronous Generation and Asynchronous Generation.

The operation and the power profile of the generated electricity are different and understanding these differences can aid in the understanding the type of ORC technology selected. The majority of ORC generators work on the principle of a rotating shaft in the center of the generator, called the Rotor. The static outer body of the generator is called the Stator. Both types of generators are based on the principle of Faraday`s law of electromagnetic induction and work with rotating magnetic fields, but they operate differently.

Synchronous Generator

In a synchronous generator the frequency of the output voltage is always directly related to the rotation of the Rotor, the speed of rotation and the output voltage are therefore synchronised. The basic three phase Synchronous generator consists of the Armature on the Stator with three windings and a displacement of 120O from each other.  These three windings generate the three phase alternating electrical output connected to the load. The Rotor can be equipped either with permanent magnet(s) (Synchronous Permanent Magnet Generator), or winding(s) connected to a DC current, called the Excitation Current to provide a constant magnetic field around the Rotor (Synchronous Generator using electromagnets).

The relationship between the speed of the rotor and the output frequency will be determined by the number of poles in the motor. The number of poles is an even number, with a minimum of 2. The poles represent the North and South poles of a magnet. In a 2 pole generator there is one North and one South pole. In a 4 pole generator there are 2 North and 2 South poles, in a 6 pole generator there are 3 North and 3 South effective magnetic poles.

The frequency of the output voltage is calculated by the following method:

Where:

 

is the frequency of the voltage (1 /s)

RPM is the revolutions per minute of the Rotor

P is the number of poles

 

The division by 120 is due to the RPM presence in the equation (1/60s) and that a North and a South magnetic pole pass by a winding will create a full cycle in the voltage output. See Figure 1 for a Synchronous Generator internal structure and voltage output, using electromagnet as the Rotor to create the rotating magnetic field.

Figure 1: Synchronous Generator Internal Structure and Voltage Output

 

If the generator is using Permanent Magnets, there is no need for electrical connection with the Rotor. However the Permanent Magnet construction causes difficulties at the time of generator assembly, and there is very little that can be done to regulate the output voltage and current due to the constant strength of the magnetic field. If the motor is using Electromagnets to generate the magnetic field, a constant DC current has to be provided to the winding on the Rotor, and this connection uses electrical commutators and slip-rings. These parts wear out after certain running time and need replacement. The DC current can be regulated, and therefore the strength of the magnetic field around the Rotor can be changed to affect the output voltage.

Due to the inductive nature of the generator windings the generator causes the current to lag behind the voltage, absorbing reactive power from the grid and producing a lagging Power Factor, this happens when the load it is powering is inductive (electric motor). Power Factor (cos θ) is calculated from the angle (θ) of phase difference between the Current and the Voltage in an AC power system.

When the Power Factor of an alternating voltage is 1, the current and the voltage are in the same phase, and all the generated power can be used to do useful Work by the load. When the Power Factor is less than one, there is a phase difference between the current and the voltage, and not all generated power can do useful work in the load, wasting generated capacity.

The use of capacitors can improve the output Power Factor of a generator. Capacitors cause the current to lead the voltage in an AC power circuit, providing a leading Power Factor “supplying” the generator with reactive power. This quality can compensate the inductive nature of the generator, bringing the power factor in the AC circuit closer to 1.

The synchronous nature of this generator means that the rotation of the prime mover providing the kinetic energy (and therefore the output voltage) needs to be strictly synchronised with the local electrical grid before making a connection to avoid disturbing the local power distribution system, and to avoid damage to the generator. This can necessitate a complex control method of the rotation of the prime mover and the grid connection isolator.

The other option to overcome the complication with synchronisation is to convert the AC output to DC power, and then convert the DC power into a 50 Hz 400 V AC electrical output that matches the grid signal. This is achieved using solid state power electronic semi-conductor components like MOSFETs and Thyristors. Filtering the output voltage may also be necessary. By de-coupling the grid frequency from the generated frequency, the Prime Mover can spin at any speed as long as it generates electrical output. This solution not only provides perfect match to the grid, it can also create an output with a Power Factor close to 1. The drawback of using power electronics is cost, and conversion losses due to the AC-DC-AC conversion.

Asynchronous Generators

Asynchronous generators or Induction Generators do not have permanent magnets or DC powered electromagnets, they normally use a copper “squirrel cage Rotor”. Without additional modification these generators need to be connected to an operating AC power system to generate electricity, without a connection the generator will not have a power output even if it is rotated.

The current from the power system is used to excite the windings of the armature, generating a rotating magnetic field. This rotating field interacts with the Rotor, generating a current in the squirrel cage, which then produces its own magnetic field and interacts with the rotating field of the Armature. For generation to occur the Rotor must be forced to spin at a slightly higher speed than the synchronous speed. If the Rotor was spinning at the same frequency to the rotating magnetic field, there would be no current induced in the Rotor, no magnetic field generated and no electricity generation. When the generator is operational, the grid frequency “lags” behind the rotation of the rotor, and there is a “slip” between the Rotor and the rotating magnetic field around the Armature of the generator.

Figure 2: Asynchronous (Induction) Generator internal structure

 

If more force is exerted on the Rotor, the torque increases and more current is generated in the Armature, if less force is exerted, less current is generated. This results in the rotation of the Rotor having a form of self-regulation, it can vary inside a certain range and does not need to be strictly controlled. This simplifies the connection of an Asynchronous generator to the local grid, and as long as the prime mover was designed to operate at a certain RPM range and torque, there is no need for synchronising power electronics or mechanical control of the shaft. Slip-rings and commutators are not needed either as there is no need to feed DC power to the Rotor, reducing manufacturing and maintenance cost.

The main drawback of Induction Generators are poor Power Factor due to the machine drawing reactive power from the grid. This can be improved with the use of capacitor banks, but this will increase cost.

Summary

In summary both generator types provide advantages and disadvantages. The ORC manufacturers make their choice of generator technology to compliment the technology that is used to provide kinetic energy from the heat source, and then equip the generator with the appropriate solution to make sure that the generator adheres to general electrical regulations. Some sites may need to consider additional technology to provide a seamless integration of the Waste Heat Recovery system. With careful financial consideration and correct engineering decisions both solutions can provide good quality power to the local loads.

Using a Synchronous Generator combined with automated Power Electronics can simplify the connection and provide very good Power Factor, and allows the Prime Mover to provide a varying speed of rotation.

An Asynchronous Generator combined with a capacitor bank can also provide a good results, without complex switching arrangements or Power Electronics, but the Prime Mover needs to provide the rotating movement at a fairly tight speed and torque band.

 

 

Sources:

Integration of Alternative Sources of Energy, Felix a. Farrett, M. Godoy Simones, IEEE Press, John Wiley & Sons 2006

Power Electronics for Renewable and Distributed Energy Systems, Sudibta Chakraborty, Marcelo G.Simones, William E. Kramer, Springer London 2013

http://ethw.org/Power_electronics

http://www.ee.lamar.edu/gleb/power/Lecture%2007%20-%20Synchronous%20machines.pdf

Figure 1, Syncronous Generator picture: http://www.alternative-energy-tutorials.com/wind-energy/synchronous-generator.html

Figure 2, Induction Generator Picture: http://www.alternative-energy-tutorials.com/wind-energy/induction-generator.html

Generator image in front: http://www.highspeedgenerator.com

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