Generating Power from Medium Temperature Heat Sources
This invention relates to the generation of mechanical power from medium temperature heat
5 sources.
Mechanical power is commonly recovered from external heat sources, such as combustion
products, in a Rankine Cycle system, using steam as the working fluid. However, in recent
years, as interest has grown in using heat sources at lower temperatures for power recovery,
10 there has been a growing trend to look for alternative working fluids and for heat sources at
temperatures of less than about 200°C. In most cases, it has been shown that organic fluids
such as light hydrocarbons or common refrigerants are appropriate. These fluids have
unique properties and much of the art of getting the best system for power recovery from a
given heat source is based on the choice of the most suitable fluid.
15
Those fluids most commonly used, or considered, are either common refrigerants, such as
R124 (Chlorotetrafluorethane), R134a (Tetrafluoroethane) or R245fa (1,1,1,3,3-
Pentafluoropropane), or light hydrocarbons such as isoButane, n-Butane, isoPentane and nPentane.
Some systems incorporate highly stable thermal fluids, such as the Dowtherms
20 and Therminols, but the very high critical temperatures of these fluids create a number of
problems in system design which lead to high cost solutions.
There are, however, numerous sources of heat, mainly in the form of combustion products,
already used for other processes, such as the exhaust gases of internal combustion (IC)
25 engines, where the temperatures are rather higher, typically having initial values in the range
200°- 700°C, where organic working fluids are associated with thermal stability problems and
their thermodynamic properties are less advantageous. Unfortunately, at these
temperatures, conventional steam cycles also have serious deficiencies.
30 Russian patent publication no. RU2050441 discloses a method of producing electrical power
by recovering energy from steam that is available as a waste product produced by an
industrial process. The dryness fraction of the steam is maintained in the range of 0.6 to 1,
hence the steam is relatively dry. The expansion of steam may be carried out in a twin
screw machine.
35
The present invention is concerned with optimising the power recovery from .external heat
sources in the temperature range of 200°C-700°C. The invention is basea on me
appreciation that the use of wet steam (even steam having a low dryness fraction) can
provide higher efficiency power recovery from medium temperature heat sources such as
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those in the 200°C-700°C temperature range than known power generation cycles such as a
Rankine cycle operating with water or organic fluids as the working fluid, when the working
fluid is condensed at the same, or even a slightly lower temperature.
5 According to one aspect, the present invention provides a method of generating power from
a source of heat at temperatures in the range of 200° to 700°C comprising the steps of
heating water in a boiler with heat from the source to generate wet steam having a dryness
fraction of0.1 to 0.9 (1 0% to 90%), expanding the wet steam to generate the power in a
positive displacement expander, condensing the expanded steam to water at a temperature
10 in the range of 70°C to 120°C and returning the condensed water to the boiler.
15
Such a system is most suitable for obtaining power outputs in the 20 - 500 kW range, from
hot gases such as IC engine exhausts or other hot gas streams in this intermediate
temperature range.
According to a further aspect, the present invention provides apparatus for generating
mechanical power comprising a source of heat, a steam boiler arranged to receive heat from
the source at temperatures in the range of 200° to 700°C, and thereby generate wet steam
having a dryness fraction of 0.1 to 0.9 (10% to 90%), a positive displacement expander to
20 expand the steam and thereby generate further mechanical power, a condenser sized to
condense the expanded steam to water at a temperature in the range of 70°C to 120°C and
a feed pump for returning the water to the boiler.
The invention will now be further described by way of example with reference to the
25 drawings in which:-
30
35
Figures 1A and 1 B show respectively the cycle (temperature plotted against entropy)
and the system components of a Conventional Steam Rankine Cycle;
Figure 2 shows a Saturated Steam Rankine Cycle;
Figure 3 shows boiler temperature plotted against heat transfer for Superheated
steam;
Figure 4 shows boiler temperature plotted against heat transfer for Saturated steam;
Figures SA and 58 correspond to Figures 1A and 1 B for a recuperative Organic
Rankine Cycle (ORC);
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Figures 6A and 68 correspond to Figures 1A and 18 for a wet steam Rankine cycle;
Figure 7 shows an arrangement for generating power from the heat of exhaust gases
5 of an internal combustion in accordance with Figures 6A and 68;
Figures 8A and· 88 show a combination of a Wet Steam Rankine Cycle and an
Organic Rankine Cycle;
10 Figure 9 shows an arrangement for generating power from exhaust gases using an
Organic Rankine Cycle;
Figure 1 0 shows an arrangement for generating power from the heat of a cooling
jacket of an internal combustion engine by means of a Vapour Organic Rankine Cycle
15 (ORC);
Figure 11 is a diagram similar to Figure 7 of a Superheated Organic Rankine Cycle
(ORC);
20 Figure 12 shows an arrangement for generating power from both exhaust gases and
25
cooling jacket of an IC engine using a Vapour Organic Rankine Cycle (ORC);
Figures 13A and 138 show alternative operating cycles for a combined steam and
ORC System for generating power from two heat sources at different temperatures;
Figure 13C shows an arrangement for generating power from exhaust gases using a
steam cycle and supplying rejected heat to an ORC system which also receives heat from
the cooling jacket of an IC engine; and
30 Figures 14A and 148 are side and end elevational views of expanders such as are
35
employed in the system of Figure 13C.
In the following description, the same reference numerals are used wherever possible to ·
refer to the same components.
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Rankine Cycle Systems
A basic Rankine cycle system, using steam, is shown in Figure 1. Points 1 to 6 on the
Temperature-entropy diagram correspond to points 1 to 6 in the system diagram. The basic
5 Rankine cycle comprises only four main elements, namely, a feed pump (10), a boiler (11) to
heat and vaporise the water, an expander (12) for generating mechanical power, and a
condenser (13) coupled to a generator (14) to reject the waste heat and return the water to
the feed pump inlet. Hot fluid enters the boiler at A and cooled fluid leaves the boiler at B.
Normally, the expander (12) is a turbine, when it is preferable to superheat it in a
10 superheater (15) before expansion begins in order to avoid condensation of vapour during
the expansion process. This is important because steam velocities within the turbine are
very high and any water droplets, so formed, impinge on the turbine blades and erode them
and also reduce the turbine efficiency.
15 By using special materials on the turbine blade leading edges it is possible to reduce the
blade erosion problem and thereby steam can enter the turbine in the dry saturated vapour
condition, as is done in some geothermal systems. Such a cycle is shown in Figure 2, and
this allows for increasing wetness in the latter stages of expansion at the sacrifice of some
efficiency. However, no turbine has yet been constructed that can safely accept wet fluid at
20 its inlet.
A problem then exists with admitting superheated or even dry saturated steam to the turbine
inlet, which becomes more pronounced as the initial temperature of the heat source is
reduced. This is the matching of the temperatures of the heat source and the working fluid
25 in the boiler if all the recoverable heat is to be used. This is best understood by reference to
Figure 3, which shows how the temperature of the working fluid and the heating source
change within a boiler, when hot gases are cooled from an initial temperature of 450°C to
150°C to heat pressurised water, evaporate it and then superheat it.
30 As can be seen, because water has the largest latent heat of any known fluid, the greatest
part of the heat received by the steam is required to evaporate it and this occurs at constant
temperature. However, the gas stream temperature continuously decreases as it transfers
heat to the steam. Accordingly, the evaporating temperature of the steam must be very
much lower than that of the initial gas stream temperature and in this case, despite the
35 relatively high initial temperature of the gas stream, the steam cannot evaporate at
temperatures much above 120°C. Moreover, if superheat is eliminated, as shown in Figure
4, the evaporation temperature can only be raised by a few degrees.
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This great degradation of temperature needed to evaporate the steam results in a poor
power plant cycle efficiency, because high cycle efficiencies are only achieved by increasing
the evaporation temperature.
5 Higher evaporation temperatures are attainable if the exit temperature of the hot gas stream
is increased. However raising the gas stream exit temperature reduces the amount of heat
recovered. In that case, despite the higher cycle efficiency, the net recoverable power output
will be reduced.
10 In contrast to this, organic fluids have a much lower ratio of evaporative heating to feed
heating and hence can easily attain much higher temperatures, therefore giving better cycle
efficiencies. An example of this is shown in Figure 5 where, using the same heat source, it
is possible to evaporate pentane at 180°C. This is generally considered to be a safe upper
limit for pentane in order to avoid thermal stability problems associated with chemical
15 decomposition of the fluid. The cycle of Figure 5 includes feed pump (10), boiler or feed
heater (16), evaporator (17), expander (18) and desuperheater-condenser (19).
It can be seen in this case that, unlike steam, starting from saturated vapour, the working
fluid becomes superheated as it expanas. There are therefore no blade erosion problems
20 associated with its use. In order to improve the cycle efficiency at the end of expansion, the
low pressure superheated vapour can be passed through a counterflow heat exchanger, or
recuperator (20), to recover the heat that would otherwise be rejected in the condenser and
use it to preheat the pressurised liquid leaving the feed pump before it enters the boiler (16).
Thus, using pentane, higher cycle efficiencies are attainable.
25
Thermal stability problems are not limited to the bulk temperature of the working fluid, where,
in the case of pentane, much higher temperatures are attainable, but with the temperature of
the boiler surface in contact with the pentane, which will be far higher, at the hot end. There
is also the risk of fire or explosion in the event of any rupture occurring in the heat exchanger
30 wall separating the working fluid from the heating source.
A further problem associated with steam is that it has very low vapour pressures at normal
condensing conditions required in vapour power plant rejecting heat either to a cooling water
stream or the atmosphere. Thus, at a condensing temperature of 40°C, the vapour pressure
35 of steam is only 0.074 bar. This means that the density of the expanded steam is very low
and huge and expensive turbines are required, while there are problems with maintaining a
vacuum in the condenser. In contrast to this, pentane at 40°C has a vapour pressure of 1.15
5
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bar. It is therefore far more dense and consequently, the expander required for it will be
much smaller and cheaper.
Screw Expanders
For units of relatively small power output, in the range of 20 kW to 1 MW, it is possible to
consider the use of positive displacement machines such as screw expanders, as an
alternative to turbines.
10 As shown for example in EP0898455, a screw expander comprises a pair of meshing helical
rotors, contained in a casing which surrounds them. As they rotate, the volume trapped
between the rotors and the casing changes. If fluid is admitted into this space at one end of
the rotors, its volume will either increase or decrease, depending only on the direction of
rotation, until it is finally expelled from the opposite side of the rotors, at the other end.
15
Power is transferred between the fluid and the rotor shafts by pressure on the rotors, which
changes with the fluid volume. Moreover the fluid velocities in such machines are
approximately one order of magnitude less than in turbines. Thus, unlike the mode of power
transmission in turbomachinery, only a relatively small portion of the power recovered is due
20 to dynamic effects associated with fluid motion. Fluid erosion effects are therefore eliminated
and the presence of liquid in the machine, together with the vapour or gas being compressed
or expanded, has little effect on its mode of operation or efficiency.
On this basis, steam can be used in a cycle in which it enters as very wet fluid, typically with
25 a dryness fraction of the order of only 0.5, as shown in Figures 6A and 68 which includes
feed pump (1 0), boiler (11) a screw expander (21) and a condenser (13). This value can
then be adjusted to give the best match between the heat source and the working fluid.
Under these operating conditions, it is easy to attain wet steam temperatures of 200 to
240°C. Temperatures much above this value are limited by thermal distortion of the casing
30 and the rotors.
A positive feature of steam is that at these higher temperatures, the pressure is not too high,
being only a little over 15 bar at 200°C and 30 bar at about 240°C.
35 This and the much higher specific energy of steam than that of organic fluids, implies that
the feed pump work required for pressurising the working fluid is much less in a steam cycle
than in an organic fluid cycle.
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In order to lubricate the bearings of the expander, a line (L) may tap off a small stream of
water from the outlet of the pump and supply this water to the bearings. The wet steam itself
will tend to lubricate the rotor surfaces and reduce clearance leakages.
5 The main problem remaining with utilising wet steam with screw expanders therefore lies
only with the large size of machine needed to expand to low condensing temperatures.
As will be illustrated by the following two examples, this can be done by raising the
condensing temperature of the wet steam, and preferably to approximately 1 00°C or more.
10 At this value, this vapour pressure of steam is just over 1 bar and though less than that of
the most commonly used refrigerants and hydrocarbon working fluids at the same
temperature, is of comparable value.
Some important benefits of raising the condensing temperature of the wet steam, and
15 preferably to approximately 1 00°C or more include:
20
i) the avoidance of problems associated with maintaining a vacuum in the
condenser;
ii) the need for a smaller screw expander to be employed with a reduced ratio of
expansion; and
iii) enabling the condenser to be effectively air cooled in any region of the world
compared to power generation systems operating with lower condensing temperatures
25 which require either excessively large and expensive air cooled condensers which absorb
too much parasitic power, or water cooling which is rarely practical and available in the
locations in which stationary internal combustion engines are commonly installed.
Where cooling water is available or where the ambient temperature is unusually low, the
30 efficiency of the process can be further improved by supplying the rejected heat from it to an
Organic Rankine cycle system, as discussed in more detail below.
It is known to use an internal combustion engine driven generator in a Combined Heat and
Power (CHP) system in order to maximise the usage of the available energy generated by
35 the internal combustion engine. In such systems, the exhaust gas heat from the IC engine is
recovered in a boiler to raise either hot water or steam to be used for heating purposes.
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A problem with all CHP systems is that the ratio between power generated and heat
recoverable is not always favourable and, in many cases and especially in summer, the heat
recovered is simply thrown away because there is no other practical use for it.
5 The apparatus for generating mechanical power of a preferred embodiment of the present
invention rejects heat from the condenser at a temperature of approximately 100-120°C. It is
possible to recover this rejected heat which remains at a temperature of around 85-90°C or
approximately 85-90% of the total available energy of the exhaust gases to heat water or
steam circulating through in an external hot water system. This provides a CHP system in
10 which 1 0-15% of the energy of the exhaust gases that is no longer available for heating
purposes has been used to produce additional power, thereby offering a more favourable
ratio between generated power and heat available for heating.
An arrangement for recovering power from waste heat in the steam of exhaust gases (22)
15 produced by the internal combustion engine (23) of a motor vehicle is shown in Figure 7.
The motor vehicle has radiator (24) and jacket cooling circuit (25). Boiler 11 may be a feed
heater -evaporator.
In motor vehicles, the energy released by combustion of the fuel is used in the form of
20 mechanical power developed by the engine, in heat rejected to the exhaust gases and in
heat rejected to the cooling jacket, in roughly equal proportions. Cost effective recovery of
any of the rejected heat to generate additional power would be highly desirable, especially,
in the case of large, long distance transport vehicles, where the annual fuel costs are very
large.
25
A major problem associated with conversion of low grade heat in motor vehicles is to find
space for the condenser (13), since the low rejection temperatures required to obtain good
cycle efficiencies, require it to be very large. However, if the exhaust gas heat only is used
and the condensation temperature is approximately the same as that of the engine jacket
30 coolant, then an air-cooled condenser need be no larger than the engine radiator (24).
35
Typically, the coolant enters at approximately 90°C and is returned to the engine jacket at
about 70°C. Thus, by condensing at approximately 80°C, it should be possible to fit a waste
heat recovery unit into the vehicle.
The following table compares what is possible from a pentane waste heat recovery unit, in
which the working fluid enters the expander as dry vapour at 180°C and the expanded
vapour is condensed at 77°C, with the recoverable power from a steam system, where wet
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steam enters the screw expander at 200°C, with a dryness fraction of 0.45, and is
condensed at 1 00°C. In both cases, it is assumed that the exhaust gases enter the waste
heat boiler at 450°C and leave it at 150°C and, in the process, 200 kW of heat is transferred
from the exhaust gas to the working fluid. All component efficiencies assumed are identical
5 in both cases.
Steam Pentane
Gross Power Output (kW) 25.46 25.69
Feed Pump Power (kW) 0.37 3.89
10 Coolant Fan Power (kW) 0.44 0.44
Net Power Output (kW) 24.65 21.36
Relative Feed Heater Surface 1.31 1.36
Relative Evaporator Surface 0.61 0.39
Relative Recuperator Surface 0 3.12
Relative Desuperheater Surface 0 1.27
15 Relative Condenser Surface 3.80 8.87
Total Relative Surface 5.72 15.01
Expander Volume Flow (m"/s) 0.128 0.056
As can be seen from the table, despite the higher condensing temperature of the steam, the
20 steam recovery unit generates 15% more net output and, if, as a good first approximation, it
is assumed that the overall heat transfer coefficients in the feed heater, evaporator,
recuperator, desuperheater and condenser are all equal, then the steam plant has a total
heat exchanger surface only one third of the size of the pentane plant. In fact, due to the
superior heat transfer properties of water/steam, this advantage may well be greater. The
25 steam screw expander size would need to be 2.2 times that of the pentane expander but
these machines are relatively cheap and the additional cost of this would be far less than the
savings made on the steam condenser, apart from the large savings.in space.
More significantly than any of the cost and efficiency advantages of the steam unit is that
30 steam is thermally stable and presents no fire hazard, whereas hot pentane, circulating in a
motor vehicle, presents a significant risk.
When there is no restriction on the size of the condenser, as in the case of heat recovery
from boiler exhaust gases in a stationary plant, much lower condensing temperatures are
35 then possible. Accordingly the heat rejected from the wet steam cycle condenser can be
supplied to a low temperature ORC system (26) in order to recover further power, without
incurring the problems of large machine sizes required to expand steam to low
temperatures. The proposed arrangement for this is shown in Figure 8A showing steam
9
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envelope (S) and organic fluid envelope (F), and corresponding to Figure 88 which includes
water feed pump (10), boiler (11), steam expander (18) and steam condenser-ORe feed
heater-evaporator (27), and low temperature ORC system (26) including ORC feed pump
(28), ORC expander (29) and desuperheater-condenser (30).
A typical case study was carried out for the recovery of power from a hot gas stream, initially
at 412.8 °C (775 °F), cooled down to 200.5 °C (393 °F). The total heat recoverable from this
source was 673 kW. Abundant cooling water was available at 10 °C (50 °F).
10 An established ORC manufacturer proposed to install an exhaust gas heat exchanger to
transfer this heat to a water glycol mixture, which would enter the ORC boiler at 130.5 °C
(267 °F) and leave it at 79.4 °C (175 °F) as shown in Figure 10. By this means, it was
estimated that 58 kW of power was recoverable. The cycle of Figure 10 includes internal
combustion engine (23), jacket cooling circuit (25) and ORC system (31) including feed
15 heater-evaporator (11 ), screw expander (21 ), condenser (13) and feed pump (28),
However, with steam condensing at a higher temperature than in known systems, and
preferably at approximately 1 00°C, it is possible to reject the heat from the wet steam cycle
and evaporate the vapour in the ORC system (31) shown in Figure 9 at an even higher
20 temperature. The cycle of Figure 9 includes exhaust gases (22) passing through exhaust
gas heat exchanger (32), coolant circuit (33) and ORC system (31) including feed heaterevaporator
(11), expander (29), desuperheater-condenser (30) and feed pump (28). By this
means, it was estimated, that after making due allowance for realistically attainable
efficiencies of both the wet steam and ORC components and allowing for pressure losses in
25 the pipes, it should be possible to obtain an additional 85 kW of power, bringing the total
power output to 142 kW from the combined wet steam ORC system i.e. nearly 2.5 times as
much. The overall thermal efficiency of the combined cycle would then be approximately
21%.
30 A further feature of this combined cycle is that its cost per unit output, would be
approximately 20% less than that of the ORC system, together with the exhaust gas heat
exchanger. This is because the additional expanders and feed pump are relatively
inexpensive, the ORC condenser of the combined system will be smaller because it has to
reject less heat than if the entire exhaust gas heat is supplied to the ORC system alone and
35 the intermediate heat exchanger that transfers the heat from the condensing steam to the
organic working fluid will be very compact due to the exceptionally high heat transfer
coefficients of both the condensing steam and the evaporating organic vapour.
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Stationary gas engines are widely used today to generate power, especially from landfill gas.
To maximize their efficiency power can be recovered from the heat rejected both by the
exhaust gases and the jacket coolant. A study of what is possible in such a case was made
for a typical gas engine. This was a GE Jenbacher J320GS-L.L. This engine has a rated
5 electrical power output of 1 065kW. The recoverable heat from the exhaust gases in cooling
from 450°C to 150°C is 543kW, while the heat that has to be rejected from the coolant to the
surroundings is 604kW to return it at 70°C, after leaving the jacket at 90°C. Using an
Organic Rankine Cycle (ORC) system for the conversion of the heat to power, there are two
simple arrangements possible. The first is to use separate units for recovery of heat from the
10 coolant and the exhaust gases as shown in Figs 10 and 11, respectively.
The cycle of Figure 11 includes internal combustion engine (23), jacket coolant circuit (25},
coolant heat exchanger (34), exhaust gases (22) and ORC system (31) including feed heater
(35), evaporator (36), superheater (37), expander (29), desuperheater-condenser (30),
15 recuperator (38) and feed pump (28). The recuperative superheat cycle is shown to
maximise the cycle efficiency.
The second possibility is to recover the heat from the exhaust gases by transferring it to the
jacket coolant and then transferring the entire recovered waste heat to a simple ORC
20 system, as shown in Fig 12. The cycle of Figure 12 includes internal combustion engine
(23), jacket coolant circuit (25), exhaust gases (22), exhaust gas heat exchanger (32) and
ORC system (31) including feed heater-evaporator (11), screw expander (21), condenser
(13) and feed pump (28).
25 A further possibility is to use a wet steam system (39) to recover the exhaust gas heat,
condensing at approximately 1 00°C and supplying the rejected heat to a lower temperature
ORC system (40), which also receives the jacket heat, as shown in Figure 13C. The wet
steam system includes boiler (11 ), steam expander (18), steam condenser-ORe evaporator
(27), feed pump (10) and line (L). The ORC system includes steam condenser-ORe
30 evaporator (27), ORC expander (29), desuperheater-condenser (30), feed pump (28) and
feed heater evaporator (41).
In this case, there are two similar organic cycles. In Figure 13A, the vapour admitted to the
expander is dry, hence the expanded vapour has to be desuperheated before it begins to
35 condense.
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In the cycle shown in Figure 138, the vapour admitted to the expander is slightly wet. This is
only possible with a screw expander (or for smaller powers scroll type expander) and
eliminates the need for desuperheat, thereby raising the ORC efficiency.
5 All these cases were analysed, assuming that the heat is finally rejected from the waste heat
power recovery system to the surrounding atmospheric air is at a temperature corresponding
to annual average ambient conditions in the UK.
In all four cases, the organic working fluid was taken to be R245fa. This was selected in
10 preference to n-Pentane because it is a better fluid for low condensing temperatures, where
it leads to cheaper and more compact expanders and condensers as well as a better
bottoming cycle efficiency.
15
The results of the study are contained in the following table.
Total Net Power Output (kW)
Single ORC Unit as in Fig 12 81
Two Separate Simple ORC Units as in Figures 9 and 10 96
Two Separate ORC Units with Superheat and
106
Recuperation as in Figs 9 and 11
Wet Steam Cycle System Coupled to Low Temperature
140
Simple ORC System as In Fig 13C
The superiority of the steam-organic combination is both obvious and overwhelming and its
use could lead to a 32% boost in the total power output of the system.
20 Screw Expander Arrangements
As already stated, screw expanders rotate with much lower tip speeds than turbines.
Accordingly, it is possible to design them to be directly coupled to a 50/60 Hz generator
without the need for an intermediate gearbox, as shown in Fig 13. However, since most of
25 the applications of concern for this invention, are for relatively small power outputs, they can
be coupled to a generator, by a simple belt drive to allow for more flexibility in selecting the
expander operating speed by appropriately sizing the belt pulleys.
In the case of their being used to boost the power and efficiency of an IC engine, then a
30 further possibility is to eliminate the need for a generator and couple the screw expander to
the main drive shaft of the IC engine.
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Screw expanders have a more limited range of operation than turbines, if they are to be
efficient and for best results, the pressure ratio of expansion should not much exceed 4:1. In
the case of this invention, where pressure ratios of the order of 15:1 are required for the
5 steam expansion, a two stage configuration, comprising two expanders in series, is therefore
required. Again, the two stages can be coupled either to the main IC engine, where
appropriate or to a generator.
In the case of a wet steam topping cycle, linked to an ORC bottoming cycle, in which both
10 units use screw expanders, all three units can be linked to a common drive, as shown in
Figures 14A and 148 where a high pressure twin screw steam expander 22 feeding a low
pressure steam expander 23 and an ORC expander 24 all have their power shafts
connected by belts 25, 26 and pulleys.

Claims
1. A method of generating power from a source of heat (A,22) at temperatures in the
range of 200° to 700°C comprising the steps of:
5 heating water in a boiler (11) with heat from the source to generate wet steam having
a dryness fraction of0.1 to 0.9 (10% to 90%),
expanding the wet steam to generate the power in a positive displacement steam
expander (21),
condensing the expanded steam to water at a temperature in the range of 70°C to
10 120°C, and
returning the condensed water to the boiler.
2. A method according to claim 1 wherein the pressure of the wet steam does not
exceed 30 bar.
15
3. A method according to claim 1 or 2 wherein the steam expander (21) is of the twinscrew
or the scroll type.
20 4. A method according to claim 3 wherein the expansion is effected in at least two
stages.
5. A method according to any preceding claim wherein the expanded steam is
condensed by heat exchange with a pressurised organic fluid operating in an organic
25 Rankine cycle (31).
30
6. Apparatus according to claim any of claims 1 to 4 wherein the expanded steam is
condensed by heat exchange with a fluid in a heating system thereby providing a Combined
Heat and Power system.
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7. A method according to any preceding claim wherein the source of heat is a stream of
exhaust gases (22) from an internal combustion engine (23).
8. A method according to claim 7 as appendant to claim 5 or 6 wherein heat from a
5 cooling jacket (25) of the engine is added to the heat from condensing the expanded steam.
9. Apparatus for generating mechanical power comprising:
a source of heat (A,22),
a steam boiler (11) arranged to receive heat from the source at temperatures in the
10 range of 200° to 700°C, and thereby generate wet steam having a dryness fraction of 0.1 to
0.9 (1 0% to 90%),
a positive displacement steam expander (21) to expand the steam and thereby
generate further mechanical power,
a condenser (13) sized to condense the expanded steam to water at a temperature
15 of 70°C to 120 °C, and
a feed pump (1 0) for returning the water to the boiler.
10. Apparatus according to claim 9 wherein the condenser (13) is an air-cooled heat
exchanger.
20
11. Apparatus according to claim 9 wherein the condenser ( 13) is formed by a boiler of
an organic Rankine cycle (31) power generator for generating additional power.
12. Apparatus according to claim 9 wherein the condenser (13) is formed by a heater for
25 heating a fluid for circulation through a heating system.
13. Apparatus according to claim 11 or 12 wherein a cooling jacket (25) of an internal
combustion engine (22) is connected to deliver further heat to the boiler of the organic
Rankine cycle power generator (31).
15
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14. Apparatus according to any of claims 9 to 13 wherein a supply of water (L) leads
from the delivery side of the pump to bearings of the steam expander or expanders
(18,21,29).
15. Apparatus according to any of claims 9 to 14 wherein exhaust gases (22) from an
internal combustion engine (23) form the source of heat.
16. Apparatus according to 15 wherein the internal combustion engine (23) providing the
10 source of heat is the internal combustion engine of a vehicle and the condenser (13) is sized
to condense the expanded steam at 70°C to 120°C.