WO2006135260A1

WO2006135260A1 - Cogeneration system with bypass exhaust passage

Info

Publication number: WO2006135260A1
Application number: PCT/NZ2006/000152
Authority: WO
Inventors: Donald Murray Clucas
Original assignee: Whisper Tech Limited
Priority date: 2005-06-13
Filing date: 2006-06-13
Publication date: 2006-12-21
Also published as: NZ540726A

Abstract

A Stirling engine based cogeneration system for generating heat and electricity comprises a combustion chamber (2) and burner (3) for supplying heat to the engine (Ia) and two exhaust passages (7, 8) from the engine to heat exchanger(s) (9a, 9b). A first exhaust passage (7) is arranged so that combustion gases first flow over the hot end heat exchanger(s)(6a, 6b) of the external combustion engine (Ia) to supply heat to the engine (Ia). A second exhaust passage (8) has an exhaust bypass valve (14) associated which closes the second exhaust passage (8) under lower thermal loads. Under high thermal loads the exhaust bypass valve (14) opens and increases throughput of the burner (3) such that the heat output of the cogeneration system is increased.

Description

"COGENERATION SYSTEM WITH BYPASS EXHAUST PASSAGE"

TECHNICAL FIELD The invention relates generally to cogeneration systems for combined generation of heat and electrical power, and particularly to cogeneration systems for meeting a variable thermal demand.

BACKGROUND

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

In many applications the maximum thermal demand exceeds the amount of heat that can be extracted from the exhaust when the engine is operating at capacity, requiring a supplementary heat input. Typical systems allow for a variable amount of supplementary heat output by burning additional fuel (i.e. auxiliary burning) in an auxiliary combustion chamber located in or near the exhaust gas heat exchanger. Hot exhaust gas from the engine includes some oxygen and is directed to the auxiliary chamber where the necessary amount of additional fuel and air is added and burnt via an auxiliary burner, to generate additional heat. The combined hot exhaust gas from the engine combustion and auxiliary burner is then directed to the exhaust gas heat exchanger or boiler. Such a system is shown, for example, in international patent application PCT/NLOl/00399.

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. They also add complexity, and may decrease reliability, due to the additional gas valve, burner and electronic controls. Cogeneration systems using a Stirling engine are known in which the hot end of the

Stirling engine is placed in the combustion chamber of the burner and the exhaust gas is directed to a heat exchanger for thermal energy recovery. A difficulty of this design is controlling the system when operated to meet a maximum thermal demand, to prevent overheating of the hot end of the Stirling engine. It is an object of some embodiments of the invention to address the foregoing problems or at least to provide the public with a useful choice. DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided a cogeneration system including: a combustion chamber having a burner for supplying heat to an external combustion engine; a generator driven by the engine for producing electricity; a first exhaust passage for exhaust gases, from the combustion chamber to an exhaust gas heat exchanger for supplying heat to an external thermal load, arranged so that combustion gases flow over hot end heat exchanger(s) of the external combustion engine to supply heat to the engine and then to the first exhaust passage; a second exhaust passage from the combustion chamber to an exhaust gas heat exchanger, via which combustion gases bypass hot end heat exchanger(s) of the external combustion engine; and an exhaust bypass valve associated with the second exhaust passage, the exhaust bypass valve being selectively operable in cooperation with the burner such that the heat output of the cogeneration system is increased when the exhaust bypass valve is open.

Preferably the first exhaust passage and second exhaust passage conduct exhaust gases to heat exchangers which are formed as one unit (hereafter often both referred to as "exhaust gas heat exchanger(s)") for convenience.

The heat transfer fluid used to cool the exhaust heat exchanger(s) may flow through the engine cooling system first or the exhaust heat exchanger(s) first or parallel flow with the engine cooling system. The flow through the exhaust heat exchanger(s) may be parallel flow, contra flow or mixed flow with respect to the exhaust gas flow.The exhaust gas heat exchanger(s) are preferably liquid cooled and have an inlet and outlet for circulating for example hot water to an external thermal load such as domestic radiators and or domestic hot water heating.

The exhaust gas heat exchanger(s) may be fixed to the cogeneration unit, or may be integrated with the cogeneration unit, in one embodiment with a part of the cogeneration unit which extends over the hot end of the engine to define the combustion chamber over the hot end of the engine, to thereby surround the combustion chamber of the engine. The burner and exhaust bypass valve are preferably operated to supplement the thermal capacity of the engine (i.e. the amount of heat which is supplied when the engine is operating at capacity). When operating at or up to a base heat load, the valve is closed and the exhaust gas heat exchanger is supplied solely through the first exhaust passage. To meet a thermal demand above the base load, the burner and valve may be modulated. The resistance to gas flow through the first and second passages are matched to ensure the engine receives sufficient heat to maintain the engine's nominal mechanical output power when operating above the base load.

Preferably the system further includes an exhaust fan for drawing or propelling exhaust gas through preferably both the first and second exhaust passages.

The exhaust bypass valve may be provided at any position along the second exhaust passage, but is preferably positioned at an outlet of the exhaust gas heat exchanger so that the fan operates at the lowest possible exhaust gas temperature.

The inlet to the second exhaust passage is preferably located opposite the burner in a central part of the combustion chamber. At least the part of the second passage closest to the burner flame is preferably liquid cooled to ensure surrounding material is not overheated by the hot combustion gases. Embodiments of the invention provide a cogeneration system which is efficient and versatile in operational use. In some embodiments it may be constructed at relatively low cost by employing only a single burner and associated combustion control equipment, while still being able to economically meet a variable thermal demand.

Any publication cited in this specification is hereby incorporated by reference, however this does not constitute an admission that the document forms part of the common general knowledge in the art, in New Zealand or in any other country.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interrupting independent claims including that term, the features prefaced by that term in each claim will need to be present but other features can also be present.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 is a schematic of a cogeneration system of the invention, and Figure 2 is a cross section through a preferred embodiment of a cogeneration system of the invention. DETAILED DESCRIBED OF PREFERRED EMBODIMENT

Referring to Figs 1 and 2 of the drawings, a cogeneration system includes a Stirling generator 1 for generating electricity and providing heat for an external thermal load. The Stirling generator 1 includes an external combustion engine comprising a Stirling engine Ia, having a combustion chamber 2 with a burner 3 supplied with air and fuel through inlets 4 and 5 respectively. The Stirling engine Ia is coupled to an electric generator Ib for producing electricity. The generator output may be alternating or direct current and may be supplied to the electricity network or an electric energy storage device such as a battery, for example.

In the preferred embodiment the heater head or hot end heat exchanger of the Stirling engine is positioned at the lower end of the combustion chamber 2 (see Figure 2). First and second passages comprising an engine exhaust passage 7 and a supplementary bypass exhaust passage 8 connect the combustion chamber 2 to exhaust gas heat exchanger(s) 9a and 9b. A cooling or heat transfer medium circulates through the exhaust gas heat exchanges 9a and 9b to take up exhaust gas heat. The cooling medium passes through the heat exchanger(s) via inlet 11 and outlet 12 (or inlet 12 and outlet 11). The cooling medium very preferably also passes through the engine for cooling the engine and taking up engine heat, and may pass through the engine before or after passing through the heat exchanger(s) 9, or in parallel split flow. Thus in the preferred embodiment shown particularly in Figure 2, the cooling medium passes into the engine Ia via inlet 10 to take up engine heat, and from the engine to the jacket 9c around the heat exchangers 9a and 9b via inlet 11, and then from the heat exchanger(s) 9, to the external circuit via outlet 12. This arrangement is shown by way of example. The cooling medium which passes through the exhaust heat exchangers and preferably the engine may be a liquid such as water, treated water, or oil or a gas such as air.

The exhaust passages 7 and 8 both pass from the combustion chamber 2 to the exhaust gas heat exchanger(s) 9, and then rejoin before an exhaust fan 10, which directs the exhaust to a flue 13. Alternatively separate fan units may be provided for each of the exhaust passages 7 and 8. The fan(s) may be positioned after or before the exhaust gas heat exchangers 9a and 9b.

In the preferred form, before entering primary exhaust passage 7 to exhaust gas heat exchanger 9a the hot engine exhaust passes through an inlet air pre-heater 17 where some exhaust gas heat is exchanged to preheat inlet air to the combustion chamber 2.

A valve 14 gates exhaust gas flow through the supplementary exhaust passage 8. When valve 14 is closed all exhaust gas flows through the air pre-heater 17 and then via exhaust passage 7 to the heat exchanger 9a. When valve 14 is open some exhaust gas flows from the combustion chamber 2 via supplementary exhaust passage 8 to the heat exchanger 9b, bypassing the Stirling engine hot end heat exchangers 6, air preheater 17, and exhaust passage 7. In the preferred embodiment illustrated, the Stirling engine is a multi-cylinder engine, having four cylinders each with a hot end heat exchanger positioned in a square or other symmetrical arrangement about a central axis 15. The inlet 16 to the exhaust bypass passage 8 is positioned centrally between the cylinders and hot end heat exchangers of the engine (two indicated at 6a and 6b), and opposite the burner 3 in a central part of the combustion chamber 2 aligned with the axis 15 of the engine. Where it passes through the engine block the bypass passage 8 is surrounded by a water jacket (not shown) in the preferred form, to reduce the temperature of the tube 8a forming the bypass passage between the engine block and exhaust heat exchanger.

The valve 14 in the outlet from the exhaust gas heat exchanger is positioned at an outlet of the exhaust gas heat exchanger 9b so as to operate at the lowest possible exhaust gas temperature.

In operation, with valve 14 closed, air and fuel enters the combustion chamber 2 where they are combusted, and the hot exhaust gases pass through the exhaust passage 7 to the heat exchanger 9a to meet the thermal base load. To provide additional heat to meet a demand above the base level, the valve 14 is opened to open the supplementary or bypass exhaust gas passage 8. This decreases the overall exhaust gas flow resistance, which increases the flow rate of air and fuel into the combustion chamber increasing the combustion rate and heat generated in the combustion chamber. The resulting exhaust gases exit the combustion chamber over both the primary and supplementary exhaust passages 7 and 8, to the heat exchanger(s) 9. The exhaust gas flow through the supplementary passage 8 bypasses the heater heads (e.g 6a and 6b) of the engine and the inlet air preheater 17. Greater heat is recovered in the exhaust gas heat exchanger(s) 9 to meet the thermal load, without requiring an auxiliary burner and associated control system. The flow path through the exhaust passage 7 and supplementary passage 8 are matched such that the necessary volumetric flow rate over the heater heads of the engine (sufficient for operation of the engine at its nominal capacity) is maintained when the valve 14 is opened.

The valve 14 and the burner 3 may be modulated by a control system to supplement the heat supplied to the exhaust gas heat exchanger(s) 9. The valve 14 may be controlled directly and may be an electrically operated valve for example, controlled over line 18 (see Figure 1). Alternatively the valve 14 may comprise a valve element which is free to move and is open when the fan 10 is operating (but is otherwise closed) and thus the degree of opening of the valve 10 may be controlled by controlling the speed of the fan 10 (or of a fan positioned on the other side of the heat exchanger 9b for example). Thus a single burner can be used to meet both a base heat load (with the valve 14 closed) up to the thermal capacity of the engine (i.e. the amount of heat which is supplied when the engine is operating at capacity), and also to meet a maximum thermal load exceeding this base load.

Preferably the valve 14 and burner modulation is controlled by a microprocessor having an input indicative of a level of thermal load i.e. from the home or space being heated. The microprocessor calculates the optimum combination of modulation and valve opening and/or fan speed to meet the heat demand.

In the preferred embodiment described the system comprising the supplementary exhaust gas passage 8 and valve 14 replaces an auxiliary burner, for providing increased thermal output to meet a thermal demand above a base load. In an alternative embodiment a supplementary exhaust bypass and associated valve and optionally a fan 10 or equivalent may be provided in addition to an auxiliary burner, to provide a maximum thermal output when the auxiliary burner operates and the valve 14 or equivalent is open to also open the supplementary exhaust gas passage. A control system may control the main burner, the valve 14 and/or fan 10, and the auxiliary burner.

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 including: a combustion chamber having a burner for supplying heat to an external combustion engine; a generator driven by the engine for producing electricity; a first exhaust passage for exhaust gases, from the combustion chamber to an exhaust gas heat exchanger for supplying heat to an external thermal load, arranged so that combustion gases flow over hot end heat exchanger(s) of the external combustion engine to supply heat to the engine and then to the first exhaust passage; a second exhaust passage from the combustion chamber to an exhaust gas heat exchanger, via which combustion gases bypass hot end heat exchanger(s) of the external combustion engine; and an exhaust bypass valve associated with the second exhaust passage, the exhaust bypass valve being selectively operable in cooperation with the burner such that the heat output of the cogeneration system is increased when the exhaust bypass valve is open.

2. A cogeneration system according to claim 1 wherein said exhaust gas heat exchangers are formed as one unit.

3. A cogeneration system according to claim 1 or claim 2 including a fan for drawing or propelling exhaust gases through the first and/or second exhaust passage(s).

4. A cogeneration system according to claim 1 or claim 2 including a fan for drawing or propelling exhaust gases through both the first and second exhaust passages.

5. A cogeneration system according to any one of claims 1 to 4 wherein the fan is on an outlet side of the exhaust gas heat exchanger.

6. A cogeneration system according to any one of claims 1 to 5 wherein the exhaust bypass valve is on an outlet side of at least one exhaust gas heat exchanger.

7. A cogeneration system according to any one of claims 1 to 6 including an inlet air pre- heater for exchanging heat from combustion gases to pre-heat inlet air to the combustion chamber, and wherein the second exhaust passage bypasses the inlet air pre-heater.

8. A cogeneration system according to anyone of claims 1 to 7 wherein the external combustion engine is a multi-cylinder engine and an inlet to the second exhaust passage from the combustion chamber is centrally positioned between the engine cylinders.

9. A cogeneration system according to any one of claims 1 to 8 wherein the exhaust gas heat exchangers surround the combustion chamber of the external combustion engine.

10. A cogeneration system according to any one of claims 1 to 9 wherein the external combustion engine is a Stirling engine.

11. A cogeneration system according to any one of claims 1 to 10 including a control system arranged to control opening of the second exhaust passage and comprising an input for receiving an input signal indicative of a level of an external thermal load.

12. A cogeneration system according to claim 11 wherein the control system is arranged to control opening of the second exhaust passage by directly controlling the exhaust bypass valve.

13. A cogeneration system according to claim 11 wherein the control system is arranged to control opening of the second exhaust passage by directly controlling the fan.

14. A cogeneration system according to any one of claims 11 to 13 wherein the control system is also arranged to control the flow of fuel to the burner.

15. A cogeneration system according to any one of claims 10 to 14 wherein the control system is arranged to control opening of the second exhaust passage to achieve an optimum combination of burner modulation and valve opening to meet the heat demand from the thermal load.

16. A cogeneration system including: a combustion chamber having a burner for supplying heat to an external combustion engine; a generator driven by the engine for producing electricity; a first exhaust passage for exhaust gases, from the combustion chamber to an exhaust gas heat exchanger for supplying heat to an external thermal load, arranged so that combustion gases flow over hot end heat exchanger(s) of the external combustion engine; a second exhaust passage from the combustion chamber to an exhaust gas heat exchanger, via which combustion gases bypass hot end heat exchanger(s) of the external combustion engine; an exhaust bypass valve associated with the second exhaust passage and operable to open the second exhaust passage; and a fan for drawing exhaust gases through at least the second exhaust passage.

17. A cogeneration system according to claim 16 including a control system arranged to control the exhaust bypass valve and comprising an input for receiving an input signal indicative of a level of an external thermal load.

18. A cogeneration system according to claim 17 wherein the control system is also arranged to control the flow of fuel to the burner.