TECHNICAL FIELD

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

BACKGROUND

In a cogeneration system a heat engine is used to generate electricity, and heat exchangers are used for recovering heat from the engine cooling fluid and from the engine exhaust gases.

In many applications the maximum thermal demand can exceed the amount of heat that can be obtained when the engine is operating at capacity, requiring a supplementary heat input. Typical systems allow for a variable amount of supplementary heat output (in a ¾oost' mode) by burning additional fuel in an auxiliary combustion chamber located in or near the exhaust gas heat exchanger. The hot exhaust gases from the engine and the combustion gas from the auxiliary burner are both directed to the heat exchanger.

International patent application publications WO 2006/135260 and WO 2009/017431 disclose alternative systems for providing supplementary heat output.

DISCLOSURE OF THE INVENTION

In broad terms the invention provides a cogeneration system including:

a combustion chamber having a burner for supplying heat to an external combustion engine,

an electric generator driven by the engine for producing electricity,

an exhaust gas outlet for combustion gases after flowing over or around and/or otherwise g? ^vm g up heat to an engine heat exchanger of the external combustion engine,

an exhaust gas heat exchanger arranged to receive exhaust gases from the engine over an exhaust gas passage to the exhaust gas heat exchanger to recover heat from the exhaust gases for supply to an external thermal load,

a combustion gas bypass outlet from the combustion chamber, leading to said exhaust gas passage to the exhaust gas heat exchanger, and a combustion gas bypass flow control system between the combustion chamber and an inlet side of the exhaust gas heat exchanger, enabling when open direct exit of some of the combustion gas from the combustion chamber to said exhaust gas passage to the exhaust gas heat exchanger without first flowing over or around the engine heat exchanger, and

a control system arranged to operate the combustion gas flow control system to control gas flow from the combustion gas bypass outlet to vary the heat output of the cogeneration system.

When operating at or up to a base heat load, the combustion gas flow control system keeps the combustion gas bypass outlet closed and allows a normal flow of engine exhaust gases from the combustion chamber to the exhaust heat exchanger. These are predominantly gases which before exiting the combustion chamber have given up heat to the heat exchanging head(s) of the engine. Heat to meet the thermal load may also be adsorbed from the engine cooling system.

To meet a thermal demand above base load, (in a boost mode) the control system opens the bypass outlet from the combustion chamber so that some combustion gases flow directly from the combustion chamber through the bypass outlet(s) and to the exhaust gas heat exchanger, bypassing the engine heat exchanging heads and thus substantially without giving up heat to the engine. Opening the bypass outlet also allows a higher flow rate through the combustion chamber. The exhaust gas heat exchanger then receives heat from both the engine exhaust gases and the bypass combustion gas, to increase the thermal output from the system. The engine continues to receive the required heat from those gases which still flow over the heat exchanging head(s) of the engine.

In another embodiment, when operating at or below a base heat load the combustion gas bypass outlet(s) may be partially open, with the base thermal output of the system comprising heat obtained by the exhaust gas heat exchanger from the engine exhaust gases as well as some bypass combustion gases, optionally as well as heat from the engine cooling system. To meet thermal demand above base load, the control system further opens the bypass oudet(s) from the

combustion chamber, to allow a higher bypass flow rate of gases from the combustion chamber to the exhaust gas heat exchanger in a boost mode.

The burner and bypass outlet(s) from the combustion chamber, and the fuel flow rate, may be modulated. The resistance to exhaust gas flow may be matched to ensure the engine receives sufficient heat to maintain the engine's nominal mechanical or electrical output power when operating above the base load. Preferably the control system is arranged to control the combustion gas flow to provide sufficient heat to meet the thermal demand whilst mamtaining the optimum engine temperature to also provide the optimum mechanical or electrical output.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interpreting 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 cross-section diagram of one embodiment of a cogeneration system of the invention, and

Figure 2 is a schematic cross-section diagram of another embodiment of a cogeneration system of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the embodiments of both Figures 1 and 2, a cogeneration system includes a Stirling engine 1 coupled to an electric generator 2 for generating electricity and which provides heat for an external thermal load. 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. Heat from the engine and/ or exhaust may be used for space heating or water heater for example.

The Stirling engine 1 has a combustion chamber 3 with a burner 4 both of which may be in any form, supplied with air inlet 5 and fuel is added to the airflow at mixer 7. Alternatively fuel may be injected immediately prior to or direcdy into the combustion chamber. The fuel may be gas or a liquid fuel such as oil, or the combustion chamber may be arranged to combust a biomaterial to generate heat such as wood pellets for example. The one or more heater head(s) 19 or hot end heat exchanger of the Stirling engine are exposed in the combustion chamber 3 to receive heat to drive the Stirling engine.

In the embodiment of Figure 1 the interior of the combustion chamber 3 below the burner

4 comprises wall(s) 3a and floor 3b, and within the combustion chamber is provided a lining 20, which may be formed of a ceramic material for example, or alternatively steel. For example the lining 20 may have an approximate bowl shape comprising wall(s) 20a and floor 20b. The one or more heater heads 19 of the engine are exposed from the floor 3b of the combustion chamber and through the floor 20b of lining 20 and in particular through apertures 20c in the floor 20b. In operation combustion gases pass down between the heater heads 19 and the apertures 20c in the lining through which the heater heads extend, passing around and over the heater heads 19 and giving up heat to the heater heads to power the Stirling engine 1 as they do. The heater heads 19 may comprise radially extending fins for maximum heat transfer. Then as indicated by arrows B the exhaust gasses pass through a space or spaces between the lining 20 and the combustion chamber, and specifically between the floor 3b of the combustion chamber and the floor 20b of the lining 20, and then between the wall(s) 3a of the combustion chamber and the wall(s) 20b of the combustion chamber, to an exhaust oudet to passage or pipe 8, which then carries the exhaust gases to a heat exchanger 9. In the embodiment shown the heat exchanger 9 is a separate unit external to a housing 30 around the combustion chamber and engine unit. A cooling or heat transfer medium circulates through the heat exchanger 9 to take up exhaust gas heat. The cooling medium passes through the heat exchanger via inlet 10 and oudet 11 (or inlet 11 and oudet 10). The heat exchanger may be parallel, counter flow or mixed flow. The cooling medium may optionally also pass through the engine for cooling the engine and also taking up engine heat, and/ or through a jacket around the combustion chamber for taking up heat from around the combustion chamber, and may pass through the engine and/or combustion chamber jacket before or after passing through the heat exchanger 9, or in parallel split flow. The cooling medium may be a liquid such as water or oil or a gas such as air.

Optionally a combustion air inlet fan 6 and/or exhaust fan 12 is/are provided to drive air/ fuel/ combustion gases through the system. If provided the exhaust fan may be positioned after or before the exhaust gas heat exchanger 9.

In the embodiment of Figure 1 valve 14 controls bypass combustion gas flow from the combustion chamber 3 to the exhaust gas heat exchanger 9. The valve 14 opens and closes and optionally may also when open vary the size of one or more bypass outlet(s) 15 from which combustion gases may also exit the combustion chamber 3 under higher thermal demand. The bypass oudet 15, controlled by valve 14, is through an upper part of the ceramic lining as shown. The valve is operated by an electric drive device 16 such as a solenoid or motor for example.

In operation under normal thermal demand, air and fuel enter the combustion chamber 3 where they are combusted and then hot combustion gases flow over and down around the heater head(s) 19 to drive the Stirling engine, and then between the combustion chamber lining 20 and the combustion chamber to the pipe 8 over which the hot exhaust gases then pass to the heat exchanger 9 as described previously, to meet the thermal base load.

To provide additional heat to meet a thermal demand above base level, in a boost mode the valve 14 operates to open the bypass oudet(s) 15 from the combustion chamber. This both decreases the overall combustion gas flow resistance, increasing the flow rate of air and fuel into the combustion chamber thus increasing the combustion rate and heat generated in the

combustion chamber, and also bypass combustion gases flow direcdy to the bypass oudet(s) 15 in the upper part of the combustion chamber and then also to the exhaust gas heat exchanger 9 over passage 8, substantially without first giving up heat to the engine, thereby increasing the heat carried to the heat exchanger 9. The speed of inlet fan 6 and/ or exhaust fan 12, if either is provided, and the rate of fuel entry into the system, may also be increased. The exhaust gases may also exit the combustion chamber at a higher temperature and/or flow rate, and greater heat is recovered in the heat exchanger 9 to meet the higher thermal load, without requiring an auxiliary burner and associated control system. Standard electrical power output from the engine and generator may also be substantially maintained. The exhaust flow control valve 14 and optionally also the burner 3 may be modulated by a control system to control the heat supplied to the heat exchanger 9. For example a control system may comprise a microprocessor 17 which controls the valve 14 and which may control inlet air fan 6 and/ or exhaust fan 12, and the amount of fuel entry to the system, for example the amount of fuel injected at mixer 7. The microprocessor 17 may receive a temperature input signal which may be from a temperature sensor 18 for example associated with a space or load to which the cogeneration system provides heat, or may receive any other signal indicative of thermal demand, for example outdoor air temperature. The microprocessor 17 aims to control operation of the cogeneration system for a fixed, and generally maximum, mechanical or electrical output with a variable heat output.

In the embodiment of Figure 2 the bypass outlet when open is an annular outlet between a lower part 20d and an upper part 20e of the lining of the combustion chamber. The lower part 20d of the lining is moveable reciprocally in the direction of arrow A in Figure 2, via a mechanical mechanism schematically indicated at 21 including an electric drive device 22. Under increased thermal demand the microcontroller 17 causes drive device 22 to operate to lower the ceramic lining part 20d, via see-saw lever mechanism 21 about pivot 21a, thus opening an annular bypass outlet 25 from the combustion chamber, defined between the upper edge of the lower lining part 20d and the lower edge of the upper lining part 20e, as shown in Figure 2. Under normal thermal demand the lower part 20d of the ceramic lining is elevated closing this opening, so that substantially all combustion gases pass over the heater head(s) 19 of the engine. The lower lining part 20d comprises the floor 20a including the apertures 20c through which the heater heads pass, and the lower part 20d of the lining wall(s). The upper lining part 20e comprises the upper part of the lining wall(s). This embodiment may be advantageous because the flow of heated exhaust gases is equalised around the combustion chamber and heat exchanging heads of the engine.

Alternatively the upper part 20b of the lining may move, via the drive motor 22 and mechanism 21/22 or an alternative mechanism, or both parts 20a and 20b may move towards or away from one another to open or enlarge or close the bypass outlet.

The enlargeable annular bypass oudet 25 which is shown in Figure 2 and described above as a continuous annular bypass oudet around the Hning 20 may alternatively comprise a series of discrete annular oudets spaced approximately equidistandy (or non-equidistandy) around the lining 20 which are normally closed and which are opened, or are normally open to a reduced extent and are opened to a greater extent, in boost mode, by separation of the two lining parts 20c and 20d (as indicated by arrow A). For example the lining parts 20c and 20d may be arranged to overlap at their adjacent annular peripheries except where apertures through the linings are provided, so that these apertures are exposed or are open to a greater extent as the lining parts 20d and 20e move away from each other.

In both embodiments a single burner can be used to meet both a base heat load 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 without requiring an auxiliary burner. However in alternative embodiments an auxiliary burner may also be provided, to provide a yet higher thermal output when the auxiliary burner operates. The control system then also controls operation of the auxiliary burner.

In another embodiment, when operating at a base heat load the combustion gas bypass oudet(s) may be partially open, with the base thermal output of the system comprising heat obtained by the exhaust gas heat exchanger from the engine exhaust gases as well as some bypass combustion gases, optionally as well as heat from the engine cooling system. To meet thermal demand above base load, the control system further opens the bypass oudet(s) from the combustion chamber, to allow a higher bypass flow rate of gases from the combustion chamber to the exhaust gas heat exchanger.

In the embodiment of Figure 1 combustion gases pass down over the heater heads 19 and through the apertures 20c in the combustion chamber lining 20 then flow through a space or spaces between the floor 20b of the lining and the floor 3b of the combustion chamber and ultimately to the exhaust passage or pipe 8, but in an alternative embodiment an annular exhaust manifold may be provided around the base of each heater head 19 to capture the exhaust gases and a conduit or passage from each such manifold may then pass to the exhaust pipe 8 or join to a common conduit through a part of the combustion chamber to the exhaust pipe 8.

In the cogeneration system of the invention both the exhaust gases which give up heat to the engine and drive the engine, and the bypass exhaust gases when the system is operating in boost mode to meet higher thermal demand, exit the combustion chamber or at least the combustion chamber-surrounding enclosure indicated at 30, via a common exhaust passage or pipe 8 to the exhaust heat exchanger 9, rather than via two (or more) separate exhaust conduits, one carrying exhaust gases which have passed over the heater heads and another carrying bypass or boost exhaust gases (in boost mode). Also separate heat exchangers are not required, one for engine exhaust gases and another for bypass boost gases. The boost valve 16 in the embodiment of Figure 1, or the equivalent, is a 'hot' valve on the exit side of the combustion chamber-inlet side of the heat exchanger 9, rather than a valve on the outlet side of the heat exchanger 9.

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.