WO2012015318A1

WO2012015318A1 - Variable thermal output cogeneration system

Info

Publication number: WO2012015318A1
Application number: PCT/NZ2011/000145
Authority: WO
Inventors: Florin Silviu Gabriel Chokardell; Andrew Edward Kimber; Geoffrey Mentink; Donald Murray Clucas
Original assignee: Whisper Tech Ltd
Current assignee: Whisper Tech Ltd
Priority date: 2010-07-30
Filing date: 2011-07-29
Publication date: 2012-02-02
Anticipated expiration: 2013-01-30
Also published as: NZ587125A

Abstract

In a cogeneration system the control system is arranged to increase the rate of air and fuel supply to the engine and to cause a flow or flows of additional air onto the heater heads of the engine to mix with combustion gases around the heater heads, in response to a demand on the co-generation system for a boost level of thermal output, such that thermal output of the cogeneration system is increased proportionately greater than any increase in electrical power output.

Description

"VARIABLE THERMAL OUTPUT COGENE RATION SYSTEM" TECHNICAL FIELD

The invention relates generally to cogeneration systems for combined generation of heat and electrical power having a variable heat output, which may have one burner.

BACKGROUND

In a cogeneration system a heat engine is used to generate electricity, and engine coolant and exhaust gas heat exchangers are used for recovering heat from the engine coolant and exhaust of the engine.

In many applications the maximum thermal demand exceeds the amount of heat that can be extracted from the engine when operating at capacity, requiring a supplementary heat input.

Typical systems allow for a variable amount of supplementary heat output by burning additional fuel and air via an auxiliary burner, to generate additional heat. The hot exhaust gases from the primary and auxiliary burners are directed to the exhaust gas heat exchanger or boiler.

A disadvantage of cogeneration systems comprising an auxiliary burner is that the auxiliary burner and associated combustion control equipment for the auxiliary burner add to the capital and operating costs of the cogeneration system, as well as to the size (and weight) of the cogeneration unit.

It is an object of at least some embodiments of the invention to address the foregoing problems or at least to provide the public with a useful choice.

SUMMARY OF THE INVENTION

In broad terms in a first aspect the invention provides a cogeneration system comprising:

an external combustion engine;

a generator driven by the external combustion engine for producing electricity;

a heat exchanger for recovering heat from the engine exhaust gases;

an electronic control system arranged to increase the rate of air and fuel supply to the engine and to cause a flow or flows of additional air onto a heater head or heads of the engine from above, to mix with combustion gases around the heater head or heads of the engine, in response to a demand on the co-generation system for a boost level of heat output which is higher than a normal level of thermal output such that thermal power output of the cogeneration system is increased proportionately greater than any increase in electrical power output.

In broad terms the invention also provides a method of operating a cogeneration system comprising an external combustion engine, a generator driven by the external combustion engine for producing electricity, and a heat exchanger for recovering heat from d e engine exhaust gases, comprising under higher thermal demand on the co-generation system increasing the flow rate of air and fuel to a burner of the engine while also causing a flow or flows of air additional onto a heater head or heads of the engine to mix with combustion gases around the heater head or heads of the engine to compensate for the higher mass flow rate which would otherwise increase heat input through hot end of the engine and increase generator output.

Preferably the flow or flows of additional air onto the or each heater head from above is or are substantially centred on the heater head.

Preferably the flow or flows of additional air as onto each heater head from above is or are directed onto the heater head from an outlet or conduit above the heater head and having an axis substantially coincident with a central axis of the heater head.

Preferably the electronic control system is arranged to increase the rate of air and fuel supply to the engine and to cause said flow or flows of additional air in response to a demand on the co- generation system for a boost level of thermal power output, in relative proportions such that thermal output is increased substantially without any increase in electrical power output.

The air supply means and fuel supply means to the engine may also be arranged to supply air and fuel to the engine at a first rate and in a first airfuel ratio under said normal level of thermal output and at a second, higher rate and in a second, higher air:fuel ratio under said boost level of thermal demand. Alternatively the air supply means and fuel supply means may be arranged to supply air and fuel to the engine in a first ainfuel ratio under said normal level of thermal demand and to vary the rate of supply of air and fuel over a higher range, at a second higher air:fuel ratio, in response to a varying level of thermal demand over a range above normal thermal output. The control system may be also arranged to vary the ainfuel ratio over a range in response to said varying level of thermal demand above normal thermal output. The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example only and with reference to the following drawings in which:

Figure 1 is a schematic diagram of a first preferred embodiment of a cogeneration system of the invention,

Figure 2 is across-section diagram through the burner and combustion chamber and the hot end of the engine, of the cogeneration system of Figurel Figure 3 is an enlarged schematic diagram of the burner of the embodiment of Figures 1 and 2,

Figure 4 is a schematic diagram of a second preferred embodiment of a cogeneration system of the invention, and

Figure 5 is a schematic diagram of mass flow rates of air and fuel into the system DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, a cogeneration system comprises a Stirling generator for generating electricity and heat. The Stirling generator comprises an external combustion engine comprising a Stirling engine 1 coupled to an electric generator for producing electricity. The Stirling engine 1 has a combustion chamber 16 over the hot end of the engine, comprising a burner 2 which is supplied with air and fuel over conduit 3, which are premixed at a mixer 4. The flow of air is controlled by an air control means 5 and the flow rate of fuel , such as natural gas (LPG or CNG), is controlled by a fuel control means. The air control means 5 may be a fan with a variable speed electric motor or a flap moved by a solenoid or motor to vary the size of an air inlet aperture for example, and the fuel control means may be an electronically controlled valve or injector. Combustion occurs at burner 2 in an upper part of the combustion chamber 16, and hot combustion gases in the combustion chamber 16 flow down and over one or more heater heads 9 extending into the combustion chamber of one or more engine cylinders, of the Stirling engine, to heat the heater heads 9 and drive the engine and generator to generate electrical power. Exhaust gases pass from the combustion chamber 16 as indicated at 10 to an exhaust gas heat exchanger. A cooling or heat transfer medium such as water or other liquid or air or other gas circulates through the exhaust gas heat exchanger to take up exhaust gas heat which is used typically for water or space heating.

Additionally or alternatively a cooling or heat transfer medium may also circulate through the engine to extract heat at the cold end of the engine.

In normal operation, air driven by fan 5 and fuel are mixed in mixer 4 and are supplied to burner 2, and an air bypass valve 15 is closed. In boost mode under higher thermal demand, the control system increases the speed of fan 5 which increases the mass flow rate of air and of fuel drawn into the air flow, but at the same air:fuel ratio to the burner 2 as in normal mode, while the electronic control system 11 also causes air bypass valve 15 to open. This diverts some air from fan 5 to flow directly onto the heater heads 9 over conduits 14 which extend into the combustion chamber and comprise outlets at their ends so that this air flows onto the heater heads additional to the hot combustion gases. This additional air flowing onto the heater heads 9 via the conduits or outlets 14 mixes with the combustion gases around the heater heads, which compensates for the higher mass flow rate which would otherwise raise the temperature of the heater heads 9 and increase heat input through the hot end of the engine, and increase engine and generator output. Thus the increase in mass flow rate through the system to meet the higher thermal demand in boost mode is compensated for by the addition of cooling air to maintain the temperature of a heater head or heads of engine, so that the heat input to the engine under the higher mass flow rate remains substantially the same.

Figure 2 is a cross-section diagram through burner 2 and combustion chamber 16 and the hot end of the engine of a cogeneration system of the invention and Figure 3 shows an enlarged schematic diagram of part of the burner and the outlet end of a single air conduit 14. In normal operating mode, the air and fuel mix entering the burner 2 passes through an apertured distribution plate 6 and an apertured backing plate 7 and then combustion occurs at and below the surface of a burner mesh 8. Combustion occurs spread over the area of the burner mesh 8, causing heat to be evenly distributed across the combustion chamber, so that the heater heads 9 below the mesh are heated evenly and the risk of local overheating is reduced. In normal operation bypass valve 15 is closed, or is substantially closed to allow only a small amount of cooling air through manifold 13 and conduits 14 and the regions or parts of the burner mesh 8 under the exits of each of the cooling tubes 14 and immediately above or axially aligned with each of the heater heads 9 are at normal operating temperature (same temperature as the balance of the area of the burner mesh 8). In boost mode under high thermal demand, the cooling air passes through manifold 13 and exits from the oudet at the lower end of each conduit 14 as a flow substantially centred on the axis of a heater head 9. Each of the conduits 14 and outiets is positioned over a heater head 9, so that a downward air flow is centred on the heater head, and the air flow spreads over the heater head and dilutes the hot combustion gases immediately around the heater head, thus compensating for the higher mass flow rate of combustion gases which would otherwise increase the heat input to the hot end of the engine and increase generator output. The cooling air from outlets 14 may mix with the combustion gases as it moves through heat exchanger radial fins typically provided around the heater heads, through turbulence. Each of the outlets 14 terminates immediately above the burner mesh 8, and in the embodiment shown at d e backing plate 7. The mixed combustion gases and cooling air flow between each heater head and the ceramic combustion chamber liner, having apertures through which the heater heads 9 extend to be exposed to the combustion chamber, may maintain the temperature of the local parts of die ceramic liner immediately surrounding each heater head 9 at a similar temperature as in normal operation mode also.

The cogeneration system comprises an electronic control system 11 which can be implemented in hardware or software or both. The electronic control system controls the speed of the fan 5 and operation of the bypass valve 15. The electronic control system 11 receives an input signal or signals indicative of any one or more of thermal power output of the system, for example via a temperature sensor of the exhaust gases, air flow rate, determined from the speed of fan 5 for example, fuel flow rate, and combustion chamber temperature.

Figure 4 shows a further embodiment which is similar to that of Figures 1 to 3 except that in this embodiment in boost mode under higher thermal demand, the control system increases the speed of fan 5a which increases the mass flow rate of both air and fuel drawn into mixer 4.

Simultaneously the control system energises a secondary fan 5b to drive additional air to flow through non-return valve 19 and then enter cooling manifold 14.

Figure 5 shows mass flow rates of total air and fuel, air and fuel mix, and the cooling air, into the cogeneration system respectively in normal and boost modes. Line 20 represents the total mass flow rate of air and fuel through cogeneration system. Line 21 indicates the mass flow rate of air and fuel mix to the burner, from mixer 4 of Figure 1. Line 22 indicates the mass flow rate of cooling air to the heater heads, through bypass valve 15 in Figure 1. As can be seen from Figure 5, the mass flow rate of cooling air to the heater heads, through bypass valve 15, is zero or low in normal mode, to maintain the burner mesh 18 at normal operating temperatures. When d ere is an increase in thermal demand, the mass flow rate of air and fuel to the burner increases as indicated by the positive slope of line 21. The mass flow rate of cooling air to the heater heads through conduits 14 also increases to prevent an increase in generator output which would otherwise caused by the increase of air and fuel mix, as described previously.

In the embodiment of Figure 4, the mass flow rate of air flowing through non-return valve 19 is determined by single fan 5b, the speed of which is controlled by the electronic control system 11. Therefore the change in mass flow rate of cooling air will follow control curve 22 in Figure 5. However, in the embodiments of Figures 1 to 3 where a single fan 5 supplies air to both combustion and cooling conduits, the system is arranged so that pressure drops through bypass valve 15, manifold 13, conduits 14, and the sections of burner mesh 8 direcdy underneath the oudets 14, and through the gas mixer 4, distribution plate 6, backing plate 7, and burner mesh 8 except the sections direcdy underneath the oudets of pipes 14, are balanced so that the

corresponding flow changes in normal and boost mode also follow the control curves in Figure 5, and generaEy to divide die air flows between combustion and cooling in the required

predetermined ratio in boost mode.

Optionally the control system 11 may be arranged to also vary the air:fuel ratio to the burner between normal and boost modes. In normal operation (non-peak thermal demand condition) fuel and air are mixed and supplied to the engine in a predetermined ratio, producing combustion gases at a temperature which may be designated a normal operating temperature T_{9 NormiJ} ,which transfers heat to die engine, reducing their temperature to T_{U) Notmal}, and then pass to the exhaust heat exchanger. Under higher or peak thermal demand, the control system 11 increases both the air flow rate and the fuel flow rate to the burner, but in different proportions so that die air:fuel ratio is also increased , to increase the combustion gases mass flow through the engine whilst at the same time reducing the combustion temperature. The temperature of the combustion gases in boost mode may be designated to temperature T_{9 BcK>st} , which is lower than T_{9 Norma!} . The power output of the Stirling engine is primarily proportional to the mass flow of the combustion gases multiplied with the temperature differential between the temperature T₉of the combustion gases before contact with the engine's hot end and the temperature T_u) of the gases after contact with the engine's hot end. The thermal output of the cogeneration system is proportional to the mass flow of the combustion gases through the engine multiplied with the temperature differential between the temperature T_lnof the combustion gases entering the exhaust heat exchanger and the temperature of the cooling medium which passes dirough the exhaust heat exchanger (water or air). If the air:fuel ratio is increased in boost mode, this reduces the burn temperature in the combustion chamber below T_{g Ntmra)} to T_{9 Boost} which further reduces the heat input to the hot end of the engine. The control system 11 may simply switch the air control means 5 and fuel control means 6 between a lower air and fuel input level at a first air:fuel ratio for normal mode operation, and a higher air and fuel input level, at a higher ak:fuel ratio for boost mode operation, or may be arranged to vary the air and fuel supply and the ainfuel ratio over a range in response to a level of higher thermal output demand which also varies over a range.

In the embodiments described above die air control means is between an air inlet into the cogeneration system and the combustion chamber. Alternatively the air control means such as an electric fan may be positioned after an exhaust gas outlet from the combustion chamber and control the flow of air into the combustion chamber by controlling the flow of exhaust gases from the combustion chamber.

While the invention has been described in relation to a cogeneration system comprising a single burner, as an alternative to providing a second or boost burner to enable variation or increase in thermal output, it is possible that the invention may be applied to a cogeneration system

comprising multiple burners, to enable variation in the thermal output obtained from one burner, with switching in or out of a second burner enabling further variation in thermal and/or electrical output of the cogeneration system.

Optionally the control system may be arranged to maintain the air and fuel to the combustion chamber for combustion i.e. continue combustions and increase the flow rate of the additional air onto the heater head or heads of the engine, in the event of a failure of the external combustion engine or generator such as due to working fluid loss or mechanical failure, which may be detected through lack of electrical output from the cogeneration system, to maintain a heat output from the cogeneration system even without electrical output.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the accompanying claims.

Claims

CLAIMS:

1. A cogeneration system comprising:

an external combustion engine;

a combustion chamber having a burner for supplying heat to the external combustion engine;

a generator driven by the external combustion engine for producing electricity;

a heat exchanger for recovering heat from die engine exhaust gases to produce a thermal output;

air supply means capable of varying the flow rate of air to the engine;

fuel supply means capable of varying the flow rate fuel to the engine; and

an electronic control system arranged to increase the rate of air and fuel supply to the engine and to cause a flow or flows of additional air onto a heater head or heads of the engine to mix with combustion gases around the heater head or heads of the engine, in response to a demand on the co-generation system for a boost level of thermal output which is higher than a normal level of thermal output such that thermal output of the cogeneration system is increased proportionately greater than any increase in electrical power output.

2. A cogeneration system according to claim 1 wherein said flow or flows of additional air onto the or each heater head is or are directed to be substantially centred on the heater head or heads of the engine from above.

3. A cogeneration system according to either claim 1 or claim 2 wherein said flow or flows of additional air onto each heater head from above is or are directed onto the heater head from an oudet or conduit above the heater head and having an axis substantially coincident with a central axis of the heater head.

4. A cogeneration system according to any one of claims 1 to 3 comprising in the combustion chamber below the burner a burner mesh at or below which combustion occurs spread over the burner mesh to distribute heat across the combustion chamber and the heater head or heads.

5. A cogeneration system according to claim 4 wherein said flow or flows of additional air onto the or each heater head is or are directed onto the heater head or heads through one or more outlets which terminate at the burner mesh.

6. A cogeneration system according to any one of claims 1 to 5 comprising a ceramic combustion chamber liner having apertures through which the heater head or heads extend to be exposed in the combustion chamber and between which mixed combustion gases and additional air flow to maintain the temperature of local parts of the ceramic liner surrounding the or each heater head at a similar temperature when the cogeneration system is providing a boost level of thermal output to when the cogeneration system is providing a normal level of thermal output.

7. A cogeneration system according to any one of claims 1 to 6 wherein said air supply means capable of varying the flow rate of air to the engine includes an air bypass valve arranged to open in response to a demand on the co-generation system for a boost level of thermal output to divert some air directly onto the heater head or heads.

8. A cogeneration system according to claim 7 wherein said air supply means capable of varying the flow rate of air to the engine includes a primary fan for supplying air to the engine and said air bypass valve.

9. A cogeneration system according to any one of claims 1 to 6 wherein said air supply means capable of varying the flow rate of air to the engine includes a fan arranged to drive additional air directly onto the heater head or heads in response to a demand on d e co-generation system for a boost level of thermal output.

10. A cogeneration system according to claim 9 wherein said air supply means capable of varying the flow rate of air to the engine includes a primary fan for supplying air to the engine and said fan arranged to drive additional air directly onto the heater head or heads in response to a demand on the co-generation system for a boost level of thermal output.

11. A cogeneration system according to any one of claims 1 to 10 wherein the electronic control system is arranged to increase the rate of air and fuel supply to the engine in relative proportions and to cause a flow or flows of additional air onto a heater head or heads of the engine to mix with combustion gases around the heater head or heads of the engine, in response to a demand on the co-generation system for a boost level of thermal output, such that thermal output is increased substantially without any increase in electrical power output.

12. A cogeneration system or method according to any one of claims 1 to 11 wherein the electronic control system is arranged to increase the rate of air supply to the engine proportionally more than the rate of fuel supply, in response to a demand for a boost level of thermal output.

13. A cogeneration system according to claim 1 to 12 wherein the air supply means and fuel supply means are arranged to supply air and fuel to the engine in a first air:fuel ratio under said normal level of thermal output and in a second, higher air:fuel ratio under said boost level of thermal demand.

1 . A cogeneration system according to any one of claims 1 to 13 wherein the air supply means and fuel supply means are arranged to supply ait and fuel to the engine in a first air: fuel ratio under said normal level of thermal demand and to vary the rate of supply of air and fuel over a higher range, at a second higher air:fuel ratio, in response to a varying level of thermal demand over a range above normal thermal output.

15. A cogeneration system according to any one of claims 1 to 14 wherein the control system is also arranged to vat}' the air: fuel ratio over a range in response to said varying level of thermal demand above normal thermal output.

16. A cogeneration system according to any one of claims 1 to 17 wherein the electronic control system is arranged to receive control input signals indicative of any one or more of air flow rate, fuel flow rate, exhaust gas temperature, and combustion chamber temperature.

17. A control system according to any one of claims 1 to 16 wherein the control system is arranged to maintain air and fuel supply to the combustion chamber for combustion and increase a rate of flow of said additional air onto a heater head or heads of the engine, in the event of a failure of the external combustion engine or generator, to maintain a heat output from the cogeneration system.

18. A cogeneration system according to any one of claims 1 to 17 wherein the external combustion engine is a Stirling engine.

1 . A method of operating a cogeneration system comprising an external combustion engine, a generator driven by the external combustion engine for producing electricity, and a heat exchanger for recovering heat from the engine exhaust gases, comprising under higher thermal demand on the co-generation system increasing the flow rate of air and fuel to a burner of the engine while also causing a flow or flows of additional air onto a heater head or heads of the engine such that the thermal output of the system is increased proportionally greater than any increase in electrical power output.

20. A method according to claim 19 comprising directing said flow or flows of additional air onto the or each heater head from above and substantially centred on the heater head or heads.

21. A method according to either claim 19 or claim 20 comprising directing said flow or flows of additional air onto each heater head from above from an outlet or conduit above the heater head and having an axis substantially coincident with a central axis of the heater head.

22. A method of operating a cogeneration system comprising an external combustion engine, a generator driven by the external combustion engine for producing electricity, and a heat exchanger for recovering heat from the engine exhaust gases, comprising under higher thermal demand on the co-generation system increasing the flow rate of air and fuel to a burner of the engine while also causing a flow or flows of additional air onto a heater head or heads of the engine to compensate for the higher mass flow rate of combustion gases which would otherwise further raise the temperature of the heater heads and increase heat input to the engine and increase engine and generator output, such that the thermal output of the system is increased proportionally greater than any increase in electrical power output.

23. A cogeneration system comprising an external combustion engine, a generator driven by the external combustion engine for producing electrical power output, a heat exchanger for recovering heat from the exhaust gases of the engine to produce a thermal output, and an electronic control system arranged to increase the rate of air and fuel supply to the engine and to cause a flow or flows of additional air onto a heater head or heads of the engine to mix with combustion gases around the heater head or heads of the engine, in response to a demand on the co-generation system for a boost level of thermal output which is higher than a normal level of thermal output such that thermal output of the cogeneration system is increased proportionately greater than any increase in electrical power output.