GB2540191A

GB2540191A - Electric power generation

Info

Publication number: GB2540191A
Application number: GB1511992.8A
Authority: GB
Inventors: Geary Paul
Original assignee: LINORC Ltd
Current assignee: LINORC Ltd
Priority date: 2015-07-09
Filing date: 2015-07-09
Publication date: 2017-01-11
Also published as: GB201511992D0

Abstract

A power generation system has a prime mover, ideally an internal combustion engine 1, driving an alternator 2 to create a primary source of electric power and a power converter, ideally an organic Rankine cycle engine 8, having a secondary electric generator 108 which utilises energy contained in the exhaust and/or cooling system of the prime mover. An electrical output of the secondary generator is used without rectification or inversion as a power source to enhance a driving torque applied to a rotor of the alternator by the prime mover by; driving an electric motor 11 connected to apply a driving torque to the output shaft of the prime mover, and/or driving an electric motor which is an integral part of the alternator, and/or powering the exciter of the alternator. Preferably, liquid working fluid supplied to an evaporator of the power converter is preheated in a recuperator by heat exchange with the gaseous working fluid of the secondary generator. The arrangement improves the fuel efficiency of the system without the need for costly rectification inversion and synchronization circuitry.

Description

Electric Power Generation

Field of the Invention

This invention relates to electric power generation and provides a power generation system which may be a static generator or a mobile generator.

Static generators include standby generators for supplying power to a national grid and site generators for supplying electric power to a specific site such as an off-grid domestic or commercial property or group of properties. Such properties may be off-grid because of their location (not all countries have a national grid power supply network) or because of a temporary failure in the existing national grid power supply. Hotels and hospitals, for example, demand a reliable stand-by power supply to replace any failure in the normal national grid power supply, and such static generators are generally specified in the design and construction of such properties as a means of ensuring continuity of electricity supply.

Mobile generators range from small-scale generators for use in the automobile industry to larger scale generators for powering railway locomotives or ships. Of such mobile generators, those designed for the automobile industry generally run at a constantly changing engine input speed, whereas those designed for rail locomotives or for shipping tend to be run at a more constant optimum engine speed.

Background of the Invention

Typically all such electric power generators, whether static or mobile, comprise a prime mover driving an alternator or electric generator which creates the electrical power output. The prime mover and the electric generator are typically packaged together as a so-called generator set. The prime mover can be a driving engine such as an internal combustion engine (gas, petrol or diesel) or a turbine, for example. The prime mover generates heat as well as kinetic energy, and early designs of such generators tended to treat this heat as unwanted waste energy. In the case where the prime mover is an internal combustion engine, for example, it can be cooled using a heat exchange fluid such as air or a liquid coolant, with the extracted heat being discharged to the atmosphere via an engine radiator and cooling fan. Hot exhaust gases are also discharged to the atmosphere. A portion of the heat extracted by the engine cooling system may have been utilized in a heat exchanger, but in general the heat contained in the engine cooling system and in the engine exhaust gases has not been regarded as a useful energy source.

The development of combined heat and power (“chp”) systems marked a significant change in the design philosophy of both static and mobile electrical generators, with attempts being made to treat as a useful energy source the heat generated by the prime mover, e g., the internal combustion engine. Prominent among proposals to convert the heat generated by the internal combustion engine driving the alternator is the use of a Rankine cycle engine. A Rankine cycle engine used in this way passes the hot exhaust gases or the hot coolant from the internal combustion engine through an evaporator which is a heat exchanger which causes evaporation of a working fluid. That evaporation is accompanied by expansion of the working fluid, and the fluid expansion is sufficient to drive an electric generator which may comprise a turbine or a reciprocating piston, for example a free floating piston, generator. The expanded working fluid after powering the electric generator is then typically cooled and condensed in a condenser before being passed to a working fluid reservoir from which it is pumped back to the evaporator.

The use of a Rankine cycle engine to convert exhaust gas heat into electrical energy provides a combined heat and power system with two electrical outputs. The primary electrical output from the alternator of the generator set needs to be matched to the power requirements of the national grid or other user network for which it is intended. That generally involves the prime mover speed and alternator frequency being synchronized with the grid frequency that is demanded, before the synchronizing breaker will connect the grid directly to the alternator. The secondary electrical output from the Rankine cycle engine must be rectified and synchronised, typically using AC to DC plus DC to AC conversion. The cost of this additional rectification and synchronization is generally considered to be excessive for the energy saving involved using a Rankine cycle engine. Moreover the AC-DC-AC conversion involves power losses, and the use of a Rankine cycle engine to utilize that waste heat is therefore seldom commercially attractive.

This invention seeks to provide a combined heat and power system which vastly increases the commercial advantage of incorporating a Rankine cycle engine into the chp system.

The Invention

This invention provides an electric generation system as set forth in claim 1 herein.

The power converter which utilizes energy contained in the exhaust or cooling system of the prime mover (e g., an internal combustion engine) is preferably a Rankine cycle engine or other power converter which incorporates an expander as the secondary electric generator. That expander may be a turbine, a turbo generator or, preferably, a floating piston expander. The working fluid of the Rankine cycle engine may be organic, for example toluene, or it may be aqueous. The working fluid is preferably preheated in a recuperator by heat exchange with the gaseous working fluid which has been passed through the expander. The gaseous working fluid is preferably then condensed and stored in a liquid reservoir before re-use. To re-use that stored liquid working fluid it is pumped from the reservoir to the recuperator and from there to the expander. The pump may if desired be an electric pump powered by the electrical output of the expander.

One means of using the electrical output of the secondary electric generator without rectification or inversion to enhance a driving torque applied to the rotor of the alternator comprises an electric motor which is driven by the output of the secondary electric generator and which is connected to apply a driving torque to the output shaft of the prime mover. That electric motor may be connected to drive the output shaft at its output end where it is connected to the alternator, or it may be connected at any other suitable location, for example anywhere along the drive train of the prime mover (e.g., along the engine crankshaft), creating a drive connection through the prime mover output shaft to the alternator. The electric motor is preferably connected via a one-way clutch so that it does not impose a drag on the prime mover when the exhaust gas energy is insufficient to drive the secondary electric generator, for example on initial start-up. The electrical output of the secondary electric generator used as a power source for the electric motor is used without rectification (AC to DC) or inversion (DC to AC). In particular, it does not have to be matched in voltage, frequency and synchronism with the electrical output of the primary source of electric power output from the alternator. It may be used as either an AC or a DC power source for the electric motor and is effective to create a torque output from that electric motor which is applied to the output shaft of the prime mover. A positive torque assist to that output shaft will have the effect of attempting to increase prime mover speed, and if that prime mover speed is governed then the ultimate result will be a reduction in the fuel consumption of the prime mover. It is estimated that the resulting increase in fuel consumption may be as high as 10%.

An alternative or additional means of using the electrical output of the secondary electric generator without rectification or inversion to enhance a driving torque applied to the rotor of the alternator comprises using that electric output to power the exciter of the alternator.

In situations where the prime mover does not have sufficient fuel to power the alternator at 100% of load, this invention will allow the prime mover to produce more power so that the alternator can generate an increased load. Conversely, in situations where the prime mover would otherwise have had sufficient fuel to power the alternator at 100% of load, this invention will allow the prime mover to produce that 100% of load at reduced fuel consumption.

The prime mover and the alternator (or primary electric generator) can form a generator set. In one arrangement, the power converter can be integrated with the generator set. In another arrangement, the power converter can be a stand-alone component that can be retrofitted to existing generator sets.

Drawings

The invention is illustrated by the drawings, of which:

Figure 1 is a schematic power flow chart illustrating the flow of electrical and mechanical power through a power generation system according to the invention; and Figure 2 is a similar schematic power flow chart through essentially the same power generation system but showing the elements of the Rankine cycle engine component in more detail.

In Figure 1 an engine 1 (e.g., an internal combustion engine) is shown as driving an alternator 2 (or primary electric generator) which has main and exciter windings. The exciter winding is shown schematically as winding 3. The electrical output from the alternator 2 is connected to a synchronization breaker circuit 4 before being fed at 5 to a mains electric grid such as a national grid. The electrical feed 5 must match the voltage, frequency and synchronism of the grid. Although not depicted in detail in Figure 1, the synchronization breaker circuit 4 would comprise a typical breaker switch that will isolate the alternator 2 if it is not providing the correct A.C. voltage, frequency and phase.

The rotor of the alternator 2 is driven by an output shaft (not shown) of the engine 1.

Hot exhaust gases from the engine 1 are used to power generator power converter 8 which has an electrical output which is used as a power source without rectification or inversion of the current flow to enhance a driving torque applied by the internal combustion engine to a rotor of the alternator, as will be described in greater detail below. It is important to note that the electrical output of the power converter 8 is not converted from AC to DC or from DC to AC before being used to enhance the driving torque applied to the rotor of the alternator.

The hot exhaust gases from the engine 1 are directed to the power converter 8 via an exhaust pipe 10. The hot exhaust gases are cooled to a useful extent in the power converter 8 before passing as flue exhaust to atmosphere. That flue exhaust is cooler than the hot exhaust gases issuing initially from the engine 1, so there is an advantage of less waste heat to the atmosphere by having passed the hot exhaust gases through the power converter 8, as well as the advantage of a useful electrical output from the power converter 8.

Figure 1 shows three alternative electrical outputs of the power converter 8 which can be used without rectification or inversion of the current flow to enhance the torque applied to the rotor of the alternator. A first possible output is shown as a direct electrical feed 6 to the exciter 3 of the alternator 2. Any increase in the current flowing through the exciter 2 causes an increase in the power output of the alternator. A second possible output of the power converter 8 is shown as a direct electrical feed 7 to the rotor of the alternator 2, and such an electrical feed 7 presupposes that the rotor of the alternator 2 has been wound with an additional motor winding, so that applying the electrical feed 7 to that motor winding enhances the torque provided by the internal combustion engine to drive the alternator rotor. A third possible output of the power converter 8 is shown as a feed 9 to an electric motor 11 which applies a driving torque through a shaft connection 12 to the output shaft (not shown) of the engine 1. Advantageously a one-way drive connection (not shown) connects the electric motor 11 output to the engine output shaft, so that on start-up the electric motor does not actually brake the engine output shaft.

When the engine 1 has been running long enough to generate hot exhaust gases the power converter 8, utilizing the heat energy in the hot exhaust gases of the motor 1, provides an electric power output through feed(s) 6, 7 and/or 9. Electrically generated torque is added to the engine torque driving the alternator 2, in a direction which tends to increase the output speed of the engine 1. However an engine governor senses that attempted increase in engine speed and acts to reduce the fuel supply to the engine so that the engine speed is held constant. Fuel consumption is thereby reduced. The alternator 2 output is sent to the mains electric grid via the synchronization and breaker unit 4 which requires no modification from the addition of the power converter 8 and the electric feed(s) 6, 7 and/or 9, the result being an increase in the fuel efficiency at the engine 1 without the requirement of costly modifications to connect the power converter 8 to the main electric grid.

The power converter 8 of Figure 1 is preferably a Rankine cycle engine which utilises the otherwise waste heat from the engine 1 exhaust to generate electricity. It is a secondary electric generator in the system, with the alternator 2 being the primary electric generator.

Details of a preferred Rankine cycle engine are illustrated in Figure 2 which shows the power converter 8 of Figure 1 in greater detail. The reference numbers used in Figure 2 are otherwise the same as those used in Figure 1. The hot exhaust gases in the exhaust pipe 10 pass through an evaporator 107 where they come into heat exchange relationship with a preheated working fluid of the Rankine cycle engine. In the evaporator 107 the working fluid is vaporized and expands to drive a secondary electric generator 108. The secondary electric generator 108 could be a turbine generator or a free piston expander, or any other generator appropriate to the intended speed and speed variations of the engine 1, such as a turbo generator.

From the secondary electric generator 108 the expanded working fluid is passed through a recuperator 111 which is a heat exchanger used to preheat the liquid working fluid before it enters the evaporator 107. From the recuperator 111 the working fluid is passe at 113, still in its vapour phase but now slightly cooler, to a condenser 114 where it condenses to its liquid phase. The liquid working fluid is stored in a working fluid reservoir 117 before being pumped at 118 to the recuperator for preheating and a repetition of the Rankine cycle. Coolant fluid 115 extracts heat from the condenser 114 to a cooling bed 116, the heat being discharged to atmosphere from the cooling bed 116 and from the flue exhaust gases issuing from the evaporator 107 being considerably less than the heat in the initial exhaust gases from the engine 1 because a commercially useful part of that heat energy has been converted to electrical energy by the Rankine cycle engine illustrated.

None of the electrical connections 6, 7 and 9 from the secondary electric generator 108 to the alternator exciter 3, to the alternator main winding or to the electric motor 11 require rectification or modification to match the voltage, phase and frequency of the mains electric grid. The incorporation of the electric motor 11 to add drive torque to the output shaft of the engine 1 permits the engine fuel efficiency to be increased by a commercially significant amount without additional electronic control of the electric output of the secondary electric generator 108. This makes the invention particularly suitable for modification of existing electric power generating systems currently running without the benefit of Rankine cycle exploitation of the engine exhaust gases, as well as for new installations.

Claims

1. A power generation system comprising: a prime mover having an output shaft driving an alternator to create a primary source of electric power; and a power converter having a secondary electric generator which utilizes energy contained in the exhaust and/or cooling system of the prime mover; wherein: an electrical output of the secondary electric generator is used without rectification or inversion as a power source to enhance a driving torque applied to a rotor of the alternator by the prime mover, by driving an electric motor which is connected to apply a driving torque to the output shaft of the prime mover, and/or by driving an electric motor which is an integral part of the alternator, and/or by powering the exciter of the alternator.

2. A power generation system according to claim 1, wherein the secondary electric generator includes an expander which is a gas turbine.

3. A power generation system according to claim 1, wherein the secondary electric generator includes an expander which is a turbo generator.

4. A power generation system according to claim 1, wherein the secondary electric generator includes an expander which is a free piston expander.

5. A power generation system according to any preceding claim, wherein liquid working fluid supplied to an evaporator of the power converter is preheated in a recuperator by heat exchange with the gaseous working fluid issuing from the secondary electric generator.

6. A power generation system according to claim 5, wherein gaseous working fluid from the recuperator is condensed to its liquid phase in a condenser and is then stored in a liquid working fluid reservoir for re-use in the power converter.

7. A power generation system according to claim 6, wherein liquid working fluid is pumped from the liquid working fluid reservoir to the recuperator and from there to the expander.

8. A power generation system according to claim 7, wherein a pump which pumps the liquid working fluid from the reservoir to the recuperator is an electric pump which takes its power from the electrical output of the secondary electric generator.

9. A power generation system according to claim 1, wherein the power converter is a Rankine cycle engine.

10. A power generation system according to claim 9, wherein the working fluid of the Rankine cycle engine is an organic working fluid.

11. A power generation system according to claim 10, wherein the working fluid of the Rankine cycle engine is toluene.

12. A power generation system according to claim 9, wherein the working fluid of the Rankine cycle engine is an aqueous working fluid.

13. A power generation system according to any preceding claim, wherein the prime mover is an internal combustion engine.

14. A power generation system substantially as described herein and with reference to the drawings.