The present invention relates to a combined heat and power system.

In particular, it relates to a system which uses a Stirling engine as the prime mover. Stirling engines are well known to be particularly effective in combined heat and power systems, particularly for small to medium scale devices. A Stirling engine such as the Microgen™ lkWe engine is a compact sealed unit which is able to generate electricity when its head is exposed to a heat source. The waste heat is usable to provide water or space heating. The Stirling engine is particularly versatile as it can be used with various heat sources. In a domestic

environment, it can conveniently be powered by natural gas where the infrastructure exists to provide this. However, it can also be powered by alternative sources such as biomass or solar making it suitable for use in remote locations where no gas infrastructure is available.

The present invention relates to a combined heat and power system using a Stirling engine which is powered by a solid fuel. The solid fuel is preferably biomass such as wood chips, municipal solid waste or crop material, but may also be a fossil fuel such as coal or peat.

Such solid fuel burners are well known in the art.

known arrangement comprises a hopper into which pellets the solid fuel are deposited and a feed screw system to transfer small amounts of solid material to a combustion chamber where they are burned.

A number of aspects of a solid fuel burner do not sit well with the requirements of a Stirling engine.

Particularly during the initial start-up of a solid fuel burner, the combustion process generates an amount of particulate contaminants in the form of tar and ash. The head of the Stirling engine is generally provided with an array of fins which serve to enhance the heat transfer to the head. If these are exposed to the tar and ash, they will foul the head and quickly reduce the operating

efficiency of the engine. Secondly, a requirement of a Stirling engine is that it can quickly be shut off due to a loss of electrical power or an electricity grid fault condition. On the other hand, a solid fuel biomass burner does not lend itself to such a rapid change and, indeed, in many circumstances, it will be desirable to continue to operate the burner in order to generate heat. If the Stirling engine was exposed to this heat after it has been shut down, it will overheat.

According to the present invention, there is provided: a combined heat and power system comprising a source of solid fuel;

a feed mechanism to supply the fuel to a combustion chamber ;

a burner within the combustion chamber to combust the fuel, the combustion chamber having first and second outlets for the hot exhaust gas; a first heat exchanger fed with hot exhaust gas from the first outlet, the first heat exchanger being connected to a water supply to provide a supply of hot water;

a Stirling engine with a head, the head being in a second chamber to which the second outlet leads, the

Stirling engine being arranged to generate electricity when exposed to the hot exhaust gas and having an electrical power outlet;

a second heat exchanger downstream of the second chamber, the second heat exchanger being arranged to receive hot exhaust gas once it has passed the Stirling engine head, the second heat exchanger being connected to a water supply to provide a supply of hot water;

a fan to draw air through the combustion chamber; and a damper to control the flow of gas through the second chamber and second heat exchanger.

Thus, the flow of exhaust gas is separated into two flow paths, the first supplying heat to the first exchanger, and the second supplying heat to the Stirling engine head in the second chamber and the second heat exchanger. The damper allows the flow through the Stirling engine chamber and second heat exchanger to be controlled. Thus, during start-up when the combustion is particularly dirty, the damper can substantially prevent any flow past the Stirling engine head. Once combustion is fully established and the contaminants largely eliminated, the damper can be opened to allow flow past the Stirling engine head. In normal operation, the damper is effectively able to vary the relative proportion of hot gases flowing to the first heat exchanger and the Stirling engine head/second heat exchanger. This provides the flexibility to run the system in a manner compatible with the heat and electrical output demand. Should the Stirling engine be required to shut down while the burner continues to operate either because its heat output is still required or because it cannot be shut down as quickly as the Stirling engine, the damper can close, thereby substantially preventing the flow of hot exhaust gases past the Stirling engine head.

As can be appreciated from the above, the damper, on its own, has a reasonable degree of control of the system. However, preferably, a second fan is provided for the gas stream through the second chamber and the second heat exchanger. This provides a further degree of control as this allows the flow through the second chamber and the second heat exchanger to be actively boosted. For example, in a mode in which the damper is fully open and the second fan is running, while the first fan is run at a relatively low speed, the electrical output is proportionately

increased, as is the proportion of the exhaust gas passing through the second exchanger in preference to the first. This mode of operation provides maximum electrical output with minimal thermal output. Alternatively, with the damper open, the second fan running and the first fan running at full speed, both the electrical and thermal output can be maximised . The first and second heat exchangers may be fed by separate water supply sources. However, preferably, they are fed by a common water supply. In this case, the water is preferably supplied to the second heat exchanger upstream of the first heat exchanger, as the second heat exchanger will generally have a lower thermal output. The first and second heat exchangers are preferably in a common block. Preferably, also, the water supply is supplied to cool the Stirling engine upstream of the first and second heat exchangers. This not only provides the required cooling for the Stirling engine, but also serves to pre-heat the water. The system would work with the components in any suitable orientation. However, preferably, the first heat exchanger is positioned directly above the combustion chamber and the second outlet into the second chamber is in the lower portion of the combustion chamber. As well as the closed damper inhibiting the flow of contaminants towards the Stirling engine head, this arrangement also helps to prevent these from entering the second chamber.

As a further refinement, there is preferably at least one baffle extending across a substantial portion of the upper region of the combustion chamber. This ensures that the majority of tar and ash produced in the early stages of combustion will impinge on the baffle (s) and will not reach the first heat exchanger.

An example of a system constructed in accordance with the present invention will now be described with reference to the accompanying drawing which is a schematic view of the system. The present invention requires a Stirling engine 1 and solid fuel burner 2, both of which are well known in the art and will not be described in detail here. The Stirling engine is preferably a linear free piston

Stirling engine. One such engine is the Microgen™ lkWe engine. This is described, for example, in WO 02/14671 and WO 04/101982. The Stirling engine has a displacer and a power piston which reciprocate out of phase with one another when heat is applied to the head 3 of the engine. An alternator is positioned adjacent to the power piston to provide an AC output 4 as the power piston reciprocates with respect to a magnet.

Similarly, the solid fuel burner is also well known. A suitable burner is incorporated in automatic pellet boilers such as those from Froling, Okofen and Baxi Brotje. In broad terms, the solid fuel burner comprises a hopper 5 and a removable lid 6. This allows the hopper 5 to be manually filled with a regular supply of solid fuel pellets P. A feed screw 7 is provided beneath the hopper. This is controlled in accordance with the system requirements in order to deliver the desired amount of pellets to the burner 8. The burner 8 burns the pellets in the combustion chamber 9 and the ash left over following combustion drops into an ash pan 10 from which it is subsequently recovered by an operator .

A second chamber 11 which contains the head 3 of the Stirling engine 1 is positioned immediately adjacent to the combustion chamber 9. A passage 12 connects the lower part of the combustion chamber 9 with the lower part of the second chamber 11. A fan 13 is provided adjacent to the combustion chamber 9 to feed air into the combustion chamber 9. A baffle 14 is provided immediately above the burner 8 to trap particulates in a combustion gas. Although shown as a simple plate, this baffle can take any configuration, for example, it may form a tortuous path for the combustion gases .

A first heat exchanger 15 is provided directly above the combustion chamber 9 and is provided with a wound tube through which water is circulated. Similarly, above the second chamber 11 is a second heat exchanger 16 which is also provided with a wound tube. A partition 17 separates the two heat exchangers.

The water circuit for the heat exchangers comprises an inlet pump 18 which pumps water along a line 19 around a central part of the Stirling engine 1 in order to cool it and pre-heat the water before it enters the tubes of the second heat exchanger 16. It then subsequently flows through the tubes of the first heat exchanger 15 before exiting via a hot water outlet 20 where it can be used for space or hot water heating. A series arrangement of first and second heat exchangers is preferred, although these may be arranged in parallel or even have separate water

circuits . Above the second heat exchanger 16 is a damper 21 and a second fan 22. The damper may be any suitable element which can selectively reduce the cross-sectional area of the duct 23 above the second heat exchanger 16, for example, a flap valve. However, it is preferably a butterfly valve which is rotatable about a horizontal axis as shown in Fig. 1. The second fan 22 is arranged to be operated to selectively increase the flow rate through the second heat exchanger 16 as desired. The exhaust gases are drawn out through the flue 24.

On initial start-up of the burner 2, the damper 21 is horizontal thereby providing maximum obstruction in the duct 23 and the fan 22 is off. When the first fan 13 is

operating, pellets P are fed to the burner 8 by the feed screw 7 and are ignited. The combustion gases flow up through the first heat exchanger 15 with the majority of the particulates in the gas being trapped by the baffle 14. The position of the opening 12 and the back pressure caused by the damper 21 ensure that little, if any, of the combustion gases enter the Stirling head combustion chamber 11 at this time .

Once full clean combustion is established and

electrical output is required, the damper 21 can be moved to a more open position allowing combustion of gases to be drawn into the Stirling engine head chamber 11 and up through the second heat exchanger 16. In order to encourage this flow through the second heat exchanger 16, the fan 22 may be operated at this time.

In the event that the Stirling engine stops operating either because the demand for electricity drops, or because an emergency shutdown is required, the burner 8 may continue to operate. In this case, the damper 21 is closed and the fan 22 switched off such that the flow reverts to being essentially via the first heat exchanger 15.

In an alternative arrangement, the fan 13 could be replaced by an exhaust gas fan either immediately downstream of the first heat exchanger 15 or in the flue 24 such that it draws gas through both heat exchangers. In this latter case, there may be no need to have the additional fan 22 in the duct 23.

Although described with reference to a single Stirling engine 1, a number of Stirling engines could be provided with their heads positioned in the Stirling second chamber 11 in order to boost the electrical output of the system.