The present invention relates to an engine comprising a positive displacement isothermal air compressor provided with a liquid spray to spray liquid into the air as it is compressed so that the compression is substantially isothermal; a combustion chamber in which the compressed air is expanded to generate power; means to feed the compressed air from the isothermal compressor to the combustion chamber; a separator to remove liquid from the compressed air upstream of the combustion chamber; a primary heat exchanger for transferring heat from exhaust gas from the combustion chamber to compressed air upstream of the combustion chamber; a precompressor to compress the air upstream of the isothermal compressor; and means to feed the air from the precompressor to the isothermal compressor.
Such an engine, which will subsequently be referred to as xe2x80x9cof the kind describedxe2x80x9d, is disclosed in WO 94/12785.
A different heat engine is disclosed in U.S. Pat. No. 5,839,270. In this engine, ambient air is fed directly to the sliding-blade compressor. Water is sprayed into the air during compression to ensure that the process is generally isothermal. The mixture of water and compressed air is then fed to a separator where the water and air are separated. Some of the air is then fed to a combustion chamber where fuel is injected and the fuel/air mixture is burned. Combustion gas is then fed to a separate expander where it is adiabatically expanded to perform useful work and also to drive the compressor. The exhaust gas from the expander is fed to a recuperator in which it preheats the remainder of the cool compressed air from the separator. The preheated compressed air mixes with the cool compressed air in the combustion chamber. In one example, the expander is cooled by water from the separator. The warmed water leaving the expander is then fed to a boiler which is used to generate steam to provide useful work in a steam expander. Thus, this document does not disclose a combustion chamber in which the compressed air is expanded, nor does it disclose a precompressor, or a means to feed air from the precompressor to the isothermal compressor. This is therefore not an engine of the kind described.
In order to recover heat from the combustion chamber in WO 94/12785 a fraction of the compressed air from the separator is heated by passing it through a cooling jacket surrounding the combustion chamber. This heated compressed air is then expanded in a cylinder in order to do useful work. However, the temperature of the coolant is necessarily limited to that which the materials and/or the lubrication oil of the combustor can withstand. The amount of heat that can be absorbed in the cooling jacket per unit mass of compressed air is therefore quite small. In order to recover this heat, a considerable amount of extra compression work is required from the isothermal compressor, thereby reducing the benefit of the heat recovery.
According to a first aspect of the present invention, an engine of the kind described is characterised by a secondary heat exchanger provided to transfer heat recovered from a part of the engine to the compressed air from the isothermal compressor upstream of the primary heat exchanger.
The present invention therefore offers a more efficient heat recovery process than that disclosed in WO 94/12785 because it makes more economical use of the air that is compressed. This saves compression work and increases the overall efficiency. The present invention recovers heat from a part of the engine and uses this to preheat the compressed air produced by the isothermal compressor. Unlike the invention described in WO 94/12785, no additional air needs to be compressed in order to recover this heat. Furthermore, unlike WO/12785, the present invention does not need to have a separate expansion chamber for the expansion of air used to recover the additional heat.
The heat recovered by the secondary heat exchanger may be recovered from any part of the engine including the exhaust gas. In this latter case, if the precompressor is arranged to be driven by the exhaust gas, for example by means of a turbine or other form of expander, the temperature of the exhaust gasses which have been used to drive the precompressor may still be high enough that heat can be recovered at the secondary heat exchanger from these gasses. However, it is primarily intended to recover heat from the combustion chamber, which passes into a combustion chamber cooling system, and/or from the air which is compressed by the precompressor.
For the most efficient engine cycle, heat should be recovered from both the combustion chamber and from the air from the precompressor. In this case, the secondary heat exchanger comprises a precompressor heat exchanger and a combustion chamber heat exchanger connected in parallel, and wherein the compressed air from the isothermal compressor is split into two streams, one stream being fed to the precompressor heat exchanger to receive heat from air from the precompressor, and the other stream being fed to the combustion chamber heat exchanger to receive heat from the combustion chamber; the engine further comprising means to control the split of the flow of air from the isothermal compressor into the two streams.
The advantage of providing two heat exchangers in parallel, each of which recovers heat from a separate component of the engine, lies in the fact that the range of temperature of the heat available from the separate components is broadly similar, but can vary according to engine load. A series arrangement presupposes that one heat source is consistently at a significantly higher temperature than the other. This is not the case with the heat from the precompressor and the heat from the engine cooling system, since both produce heat over overlapping temperature ranges. A parallel arrangement is more flexible in this situation.
Also, in spite of the reduced air flow in each of the parallel flow paths, it is still possible to provide sufficient capacity to absorb most of the available heat by adding water to the compressed air entering the secondary heat exchanger in a quantity which will be vaporised in the secondary heat exchanger. The high latent heat evaporation of water allows a considerable amount of heat to be absorbed by a small amount of water. The benefit of the evaporated water is that it provides additional working fluid to the engine, which is gained without any additional compression work. This results in a higher engine efficiency and a higher power output. It is generally found that the optimum cycle efficiency is achieved if the system is configured to maximise the total enthalpy of water vapour which is produced upstream of the primary heat exchanger. Carryover of excess liquid water into the primary heat exchanger is generally found to be detrimental to the cycle efficiency, although it may benefit the power output.
Adding the liquid to the compressed air creates a two-phase stream. It is difficult to control the composition of such a stream when the stream is distributed amongst a number of heat exchange elements within the secondary heat exchanger. Therefore, preferably, when the secondary heat exchanger has a number of heat exchange elements carrying the compressed air, the liquid is directly injected into each element. This allows direct control of the phase composition throughout the heat exchanger.
The liquid may be taken from any source. However, it is most conveniently taken from the separator.
When heat is recovered from both the air from the precompressor and the combustion chamber, liquid is preferably supplied to each of the compressed air streams in the precompressor heat exchanger and combustion chamber heat exchanger respectively. In this case, the engine further comprises means to control the flow of liquid to the precompressor heat exchanger and combustion chamber heat exchanger.
The streams from the precompressor and combustion chamber heat exchangers may be fed separately into the primary heat exchanger. However, they are preferably combined upstream of the primary heat exchanger.
Preferably, the engine comprises a coolant circuit containing liquid which extracts heat from the combustion chamber and transfers the heat in the secondary heat exchanger to the compressed air. When the secondary heat exchanger comprises a precompressor heat exchanger and a combustion chamber heat exchanger as described above, the coolant circuit will, of course, transfer its heat in the combustion chamber heat exchanger.
The use of the liquid coolant provides a more efficient cooling of the combustion chamber than the compressed air of the prior art, thereby allowing the power of the engine to be increased.
The liquid in the coolant circuit may be unpressurized. However, in order to prevent boiling, which would degrade heat transfer and cause local overheating, the liquid is preferably pressurized.
The coolant circuit preferably has a liquid pump for driving the liquid around the circuit. In order to enhance the degree of cooling provided by the liquid in the coolant circuit, it may be preferable to provide an auxiliary cooler downstream of the secondary heat exchanger.
In its simplest form, the coolant circuit comprises a single circuit having a single heat exchanger. The combustor will generally have a lubricated portion which must be maintained at a relatively low temperature (for example, less than 200xc2x0 C.) to prevent degradation of the lubricant. On the other hand, the combustor will generally have a non-lubricated portion such as the cylinder head which can be allowed to reach higher temperatures. If a single coolant circuit is used, then it should be arranged so that the coolant passes to the cooler lubricated surfaces before the hotter non-lubricated surfaces. However, in order to provide sufficient cooling to a highly rated combustor with a relatively small temperature difference between the metal surfaces and the coolant, it is necessary to have a high heat transfer coefficient and therefore a higher flow rate. Unfortunately, if the flow rate is high, the coolant temperature does not rise very much, with the result that the final coolant temperature is too low at the secondary heat exchanger. Therefore, for some applications, the engine may be arranged such that the secondary heat exchanger comprises a high temperature heat exchanger and a low temperature heat exchanger, and the coolant circuit comprises a high temperature circuit and a low temperature circuit, the high temperature circuit having a high temperature pump arranged to circulate liquid past a relatively high temperature part of the combustion chamber and through the high temperature heat exchanger, the low temperature circuit having a low temperature pump arranged to circulate liquid past a relatively low temperature part of the combustion chamber and through the low temperature heat exchanger, and means to feed the compressed air from the separator through the low temperature heat exchanger to receive heat and subsequently through the high temperature heat exchanger to receive further heat prior to entry into the primary heat exchanger. The combustor coolant flow rate, and hence the heat transfer coefficient, can be maintained at a high level in both circuits, with a relatively small temperature change in each circuit.
The maximum coolant temperature can in each case be maintained at the highest level acceptable to the lubricated and non-lubricated surfaces respectively.
In this case, the engine preferably further comprises means to add liquid into the compressed air in separate elements of the low temperature heat exchanger all of which liquid is evaporated in the low temperature heat exchanger, and in separate elements of the high temperature heat exchanger all of which liquid is evaporated in the high temperature heat exchanger. This again solves the problem of controlling the phase distribution of the air/liquid entering the low and high temperature heat exchangers.
As an alternative to the above mentioned arrangement with low and high temperature heat exchangers, the coolant circuit may comprise a main pump for pumping liquid around a main circuit including the secondary heat exchanger, a low temperature circuit which is fed with liquid from the main circuit, a low temperature pump for pumping liquid around the low temperature circuit past a relatively low temperature part of the combustion chamber, a high temperature circuit which is fed with a bleed flow of liquid from the low temperature circuit, a high temperature pump for pumping liquid around the high temperature circuit past a relatively high temperature part of the combustion chamber, and means to feed a bleed flow of liquid from the high temperature circuit to the secondary heat exchanger.
This arrangement allows the high and low temperature circuits to be maintained close to their respective maximum temperatures and allows high flow rates through the combustor, while still allowing the primary side of the secondary heat exchanger to have a modest flow rate. This has the advantage over the previous arrangement that it only requires a single heat exchanger. On the other hand, three pumps are required. The choice of system therefore depends to some extent on the relative cost of the components.
The precompressor may be arranged to be driven entirely by the exhaust gas as in WO 94/12785. However it has been found that the power output of the engine can be substantially increased if the precompressor is arranged to be driven only partially by exhaust gas. Also it is found that the engine is more flexible in the sense that it can change load much more quickly if the precompressor has an additional source of power, which is independent of the exhaust gas.
The power output of the engine is primarily determined by the mass flow of air and by the final pressure delivered by the compression system to the combustor cylinders. The higher the mass flow of air, the more fuel can be added at constant fuel to air ratio, subject to thermal loading limits of the engine. Also higher pressures increase the scope for expansion of the combustion gases and it is possible to increase the fuel to air ratio, subject to limits on emissions, to take advantage of this. Both the efficiency and power output improve with increasing fuel to air ratio, providing that the efficiency of the combustion process itself can be maintained.
The isothermal compressor is capable of providing the largest part of the overall pressure ratio of the compression system. Typically, an isothermal compressor as used in the present invention is most suitable for a compression ratio in the range of about 10:1 to about 25:1. If, for example, an overall pressure ratio of 100:1 is required, this could be achieved if the isothermal compressor delivers 25:1 and the precompressor delivers 4:1. Higher pressure ratios can be achieved by using two isothermal compressors in series. Both isothermal compressors would preferably be driven from the same crankshaft.
With inlet air pressure of 105 Pa, depending on the exhaust gas temperature, a conventional turbocharger can deliver a pressure of about 4xc3x97105 Pa to the inlet of the isothermal compressor, which can therefore deliver a pressure of about 107 Pa, without any form of supplementary compression. However if the pressure at the inlet to the isothermal compressor is doubled to 8xc3x97105 Pa during periods of high power demand, then it is possible for an isothermal compressor of the same physical size to deliver twice the mass flow of air at the same outlet pressure of 107 Pa. The efficiency of the engine would be reduced by a few percentage points because the power required to drive a supplementary compressor would be greater than the saving caused by the reduction in the pressure ratio of the isothermal compressor to 12.5:1. However the increase in power output achieved by this method amounts to nearly 50%. This is one example of the increased power output and flexibility, which can be achieved by providing supplementary compression.
Another advantage arises since the supplementary compression does not depend entirely upon the available energy in the exhaust gas. Engines, which rely only on a turbocharger to provide air compression upstream of the reciprocating unit, take a significant time to respond to a demand for additional power. This is because of the time lag, which exists between an increase in fuelling and the consequent increase in speed of the turbocharger.
A further major advantage of supplementary compression arises because it allows the possibility to increase the maximum pressure. This allows the engine to achieve high power output simultaneously with high efficiency.
This arrangement thus provides an engine in which the net engine power is significantly increased, while the high cycle efficiency of the prior art is either maintained or not affected too adversely. Also, this arrangement is capable of providing a substantial improvement in the control and speed of response of the engine.
In addition to being driven by the exhaust gas, the precompressor may be driven either by a power source external to the engine cycle, such as an electric motor, or it may be driven by power generated by the expansion of gases acting on a piston in the combustion chamber. In this latter case, when a main crankshaft is driven by gases acting on a piston in the combustion chamber, this crankshaft is used to drive the precompressor, preferably via a gearbox.
A further alternative is to provide a heat source such as a burner to increase the temperature of the exhaust gas which drives the precompressor. This provides a simple way of increasing the power to the precompressor and is particularly suited to increasing the power for a short period.
The precompressor may be an single compressor with a dual power source. However, preferably, the precompressor comprises two compressors connected in series, one of which is driven by the exhaust gas, and the other of which is not. In this case, it is preferable for the one compressor to have a higher compression ratio than the other compressor, and wherein heat is recovered downstream of the one compressor. In other words, the exhaust gas is used to provide a higher degree of compression than the other power source. Typically, the one compressor may have a compression ratio of at least twice that of the other.
If the precompressor consists of two compressors placed in series, one of which is driven by the exhaust gas and the other by a separate source of power, then it is possible to have one or the other in the upstream position. The choice may be made on the basis of availability of suitable components and on the pressure and flow characteristics of the component compressors over a range of part load conditions.
The other compressor, i.e. the compressor not driven by the exhaust gas, may be any suitable compressor. However, it is preferable for the other compressor to be a rotary compressor as this can handle large flow volumes at relatively low cost. In this case, the rotary compressor is preferably a single-stage centrifugal compressor. In order to provide the necessary degree of control, the other compressor may either by driven by a variable speed motor, or it may be a variable geometry compressor. A further option is for the other compressor to have variable inlet guide vanes as these can reduce the compression ratio of a fixed speed compressor by altering the inlet swirl angle.
In order to improve the efficiency of the precompressor, an intercooler is preferably provided to cool the air between the two compressors of the precompressor. A further improvement can be provided by using an air pre-cooler to cool the air between the precompressor and the isothermal compressor. This ensures that the air entering the isothermal compressor is at the lowest possible temperature. When the secondary heat exchanger recovers heat from the air from the precompressor, the air pre-cooler is positioned downstream of the secondary heat exchanger. Further cooling of the air entering the isothermal compressor can be provided by a spray of water which cools the air prior to its entry into the isothermal compressor.
The positive displacement isothermal air compressor may be a compressor such as a sliding vane or screw compressor. However, it is preferable for this to be a reciprocating compressor in which a piston reciprocates in a cylinder to compress the air.
Preferably, in order to provide an engine which is readily able to cope with large variations in power demand, there is provided a storage chamber, means to feed the compressed air from the isothermal compressor to the storage chamber, means to feed compressed air from the storage chamber to the combustion chamber, and a valve system for selectively feeding compressed air from the isothermal compressor either to the storage chamber or to the combustor, and for selectively feeding compressed air from the storage chamber to the combustion chamber.
Thus, at times of low power demand, air can be compressed and stored, for example in pressure vessels, high pressure piping or an underground cavern of some kind. This compressed air can then be used for power generation during periods of high demand. The engine is particularly suited to storage of compressed air, as the air leaving the isothermal compressor is at a pressure at which it can be used in the combustor, so it can be stored without requiring further compression and used directly without wasteful pressure reduction. As the engine can operate efficiently over a range of system pressures, there is no need to compress the air to a significantly higher pressure than the pressure at which it will be used and then throttle the air to the correct operating pressure before it is used. Also, as the air compressed by the isothermal compressor is cold, there is no need to remove heat prior to storage.
In order to improve the flexibility of the system with the storage capability further the isothermal compressor and combustor are preferably connected by a drive shaft, and a motor/generator is coupled to the shaft between a pair of clutches. Also, a generator may be provided which is driven by exhaust gas from the engine and an independent power supply may be provided for the precompressor.
If a means is provided to recover heat from a part of the engine and transfer the heat to a site external to the engine, the engine can be used as a combined heat and power (CHP) system. In this case, it is preferable for there to be means to control the relative amounts of power and heat for external use generated by the engine. The heat can be recovered from any suitable part of the invention. However, one current preference is to provide a bypass line transporting a proportion of the compressed air and/or the exhaust gas past the primary heat exchanger and a bypass valve for controlling the flow through the bypass line. This allows some of the compressed air to be fed directly to the combustor without passing through the primary heat exchanger with a consequent reduction in the temperature of the compressed air entering the combuster. However, the exhaust gas will have given up less of its heat and hence leaves the primary heat exchanger at a higher temperature. This higher temperature exhaust gas can be used for external heating. The valve allows the amount of compressed air bypassing the primary heat exchanger to be controlled by allowing the flexibility to generate different proportions of heat or power as required. Alternatively to the bypass line for the compressed air, a bypass line could be provided which bypasses a proportion of the exhaust gas around the primary heat exchanger. This has a similar effect to the compressed air bypass line. The bypass line and valve also have value independently of a CHP system. It could be used to have a similar effect to the burner, namely to raise the temperature of the exhaust gas which drives the precompressor, allowing rapid adjustment of the engine load.
To provide high grade heat for an industrial purpose, the primary heat exchanger may include a number of flow paths connected to an external heating circuit.
The invention also extends to a method of generating power, the method comprising compressing air in a positive displacement compressor; spraying liquid into the compressor as the air is compressed so that the compression is substantially isothermal; removing liquid from the compressed air; feeding the compressed air to a combustion chamber; injecting fuel; combusting the fuel and expanding the combustion gases in the combustion chamber to generate power; and feeding the exhaust gas from the combustion chamber to a primary heat exchanger to heat the compressed air upstream of the combustion chamber; characterised by recovering heat from a part of the engine and transferring this heat to the compressed air from the isothermal compressor upstream of the primary heat exchanger.
Preferably the method further comprises the step of compressing the air in a precompressor upstream of the positive displacement compressor, and wherein the step of recovering heat comprises recovering heat from the precompressor in a precompressor heat exchanger and recovering heat from the combustion chamber in a combustion chamber heat exchanger.
In this situation, the heat recovery can be improved by the steps of splitting the air from the positive displacement compressor into two streams and feeding one stream to the precompressor heat exchanger and the other stream to the combustion chamber heat exchanger, and controlling the split of the air into the two streams in such a way as to optimise the recovery of heat.
In order to optimise the performance of the two heat exchangers, the method preferably comprises controlling the fraction of air directed to each stream by means of a look-up table which specifies the required air split as a function of the various inlet conditions to the precompressor heat exchanger and combustion chamber heat exchanger.
A further improvement can be achieved by the step of supplying liquid to each of the two streams upstream of the respective heat exchangers and controlling the amount of liquid supplied to each stream.
As a further improvement, the method may comprise the steps of monitoring the temperature differential between the incoming and outgoing streams at the hot end of each of the precompressor and combustion chamber heat exchangers, and controlling the water flow to each of the heat exchangers so as maintain the temperature differential at a desired level.
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