The present invention relates to a Stirling engine assembly.
Stirling engine assemblies are known in the art, for example,
WO 99/40309 and WO 02/070887 having a recuperator at the Stirling engine head to
heat the air supplied to the burner with combustion gases from the burner. The exterior
surface of the recuperator is exposed to the air within the interior of an appliance
and hence any heat dissipated from the surface of the recuperator is dissipated
into the appliance.
The principal purpose of the burner/recuperator assembly
is to maintain the temperature of the engine head with the minimum amount of energy.
This minimisation of energy is achieved with the use of the recuperator which recovers
heat from the burner exhaust gas and uses it to preheat air/gas mixture supplied
to the burner. The degree of recuperation and thermal efficiency can be maintained
with the use of insulation around the outside of the appliance. However, two factors
must be taken into consideration.
Firstly, the temperature of the incoming air/gas mixture
must remain below a critical upper limit. If this limit is exceeded, auto-ignition
of the mixture can occur resulting in further over heating and potential damage
to the appliance.
Secondly, the appliance may house thermally sensitive electrical
components which have a nominal maximum temperature. Therefore, the amount of heat
dissipated from the burner/recuperator assembly must be maintained at a level to
prevent any damage to the electrical components.
According to the present invention the Stirling engine
assembly comprises a Stirling engine having a head; a burner surrounding the head
and comprising a burner element on which a flame is sustained, the burner being
fed with a combustible gas stream; and a recuperator to preheat the gas stream with
combustion products from the burner; characterised by a coolant circuit positioned
to absorb heat, which is radiated from the back of the burner element away from
the head, into a coolant stream separate from the gas stream.
The present invention uses a burner which surrounds the
Stirling engine head to provide particularly effective heat transfer to the head.
However, a significant amount of heat radiates away from the burner element which,
in turn, can radiate to the recuperator walls and subsequently into the appliance.
By removing some of this heat into a separate coolant stream, the temperature of
both the incoming gas and the interior of the appliance can be controlled.
The coolant stream for the burner element may be a dedicated
stream. However, preferably, the coolant stream is a stream which has cooled the
cool end of the Stirling engine. Such a stream is conveniently available and therefore
has cost and space benefits.
Preferably, the coolant stream is arranged to subsequently
receive heat from the exhaust gas from the burner. In domestic combined heat and
power applications, the coolant stream which is heated in this way can be used to
supply a domestic heating requirement such as the central heating or water heating.
With such an arrangement, a supplementary burner is preferably also provided to
supply further heat to the coolant stream to ensure that the domestic heat demand
can be met at all times.
A flexible seal may be provided between the burner and
the Stirling engine head in order to prevent the escape of gases from the burner
into the appliance. In this case, the seal may also be cooled by the coolant stream
which is used to cool the burner element. The cooled flexible seal arrangement is
the subject of a separate co-pending application GB 0211121.9. The burner element
and seal may be positioned such that a common duct for the coolant stream can cool
both the burner element and seal on a single pass around the head.
An example of a burner assembly in accordance with the
present invention will now be described with reference to the accompanying drawings,
- Fig. 1 is a schematic representation of a combined heat and power system incorporating
a Stirling engine assembly of the present invention;
- Fig. 2 is a cross-section through a Stirling engine assembly of the present
- Fig. 3 shows, in cross-section, part of a Stirling engine assembly according
to the present invention with a modified cooling arrangement.
The domestic combined heat and power system shown in Fig.
1 comprises a Stirling engine assembly 1 together with a supplementary burner 2
and a heat exchanger 3 in which water from a domestic central heating or hot water
system is heated by exhaust gas from the Stirling engine assembly and by the supplementary
The Stirling engine assembly 1 comprises a Stirling engine
4 supported on a resilient support 5. A fan 6 provides a supply of combustible gas
to a burner 7 surrounding the head 8 of the Stirling engine. The gas is supplied
along the gas supply duct 9 and combustion gases which have heated the head 8 subsequently
flow along exhaust gas duct 10 which is surrounded by the gas supply duct 9. Exhaust
gas is subsequently fed to the heat exchanger 3 where it combines with combustion
products from the supplementary burner 2 (which is also fed with combustible gases
by fan 6). The combined stream is then exhausted through a concentric flue 11.
The nature of the Stirling engine assembly is shown in
more detail in Fig. 2.
In addition to the engine head 8, the Stirling engine comprises
an engine cooler 12 and an alternator 13. The internal structure of a Stirling engine
is well known in the art and will not be described in greater detail here. An annular
absorber mass 14 surrounds the Stirling engine 4 and is resiliently mounted thereto
to counteract vibrations of the Stirling engine.
A recuperator 20 is positioned above and around the head
8 of the Stirling engine. The recuperator comprises an outer casing 21 in which
a block of insulation 22 is mounted. The gas supply duct 9 is defined between the
casing 21 and insulation 22, while the bottom surface of the block of insulation
22 is profiled to define the exhaust gas duct 10 between itself and the Stirling
engine head 8. This extends out through top of the casing 21 as shown at 10' in
Fig. 2, although this duct is out of the plane of the cross-section of Fig. 2.
The burner 7 has a flame distribution strip 23 which distributes
the gas more evenly to the annular burner 7. The majority of the heat from the burner
7 is transmitted by forced convection and radiation to the heater head 8, with the
absorption being aided by a system of annular fins 24. Some heat is radiated into
the recuperator or radially outwardly of the burner as described in greater detail
The Stirling engine 4 will vibrate to a limited degree
with respect to the burner 7 and recuperator 20. A flexible seal 25 is therefore
provided between the Stirling engine 4 and the burner housing. As shown in Fig.
2, this seal is positioned away from the burner 7 and separated from the burner
by a block of insulation 26 in order to limit the temperature that the seal 25 has
The cooling arrangement for the Stirling engine is as follows.
Cool water which, in a domestic combined heat and power system, has given up its
heat to satisfy the domestic heat requirement is initially passed around engine
cooler 30 to maintain the cold end 12 of the Stirling engine at the lowest possible
temperature. The water is then fed around a seal cooler path 31 surrounding the
insulation 26 to absorb heat at this point thereby limiting further the temperature
that the seal 25 has to withstand.
The water is then fed to an annular burner cooler path
32 surrounding the burner 7 which absorbs the heat which is radiated outwardly from
the flame distribution strip 23. As shown in Fig. 1, the water is then fed to heat
exchanger 3 where it is further heated by the exhaust gas from the Stirling engine
and by the supplementary burner 2 as described above.
Although the various water paths have been described above
as being in series, it is possible for certain of the paths to be arranged in parallel.
In particular, the seal cooler path 31 and burner cooler path 32 may be in parallel,
with manually adjusted flow control valves located in the parallel flow paths, to
enable the flows to be balanced.
As the burner cooler path 32 extracts heat from the burner,
the effect of using this is that, in order to maintain the nominal head operating
temperature of approximately 550□C, more heat is required from the burner.
However, this is offset by an increase in thermal efficiency due to the recovery
of heat from the burner that would otherwise have been dissipated into the appliance.
An alternative configuration for cooling the recuperator
is shown in Fig. 3. This figure shows only a top left hand portion of the Stirling
engine assembly as the remainder of the engine is as shown in Fig. 2.
In the example of Fig. 3, the seal cooler path 31 and burner
cooler path 32 have been replaced by a single cooling channel 40. A thermal bridge
41, which is an annular disk of material of high thermal conductivity, provides
a heat path from the burner 7 to the cooling channel 40, while the seal 25 is positioned
adjacent to the cooling channel 40. This allows the integration of the cooling arrangement
for the burner 7 and seal 25 into a single assembly thereby reducing manufacturing
costs and materials.