Patent Application
20110167825
The present invention relates to a refrigeration, heat and/or work production plant.
Thermodynamic machines used for refrigeration, heat production or energy production all refer to an ideal machine called the “Carnot machine”. An ideal Carnot machine requires a heat source and a heat sink at two different temperature levels—it is therefore a “dithermal” machine. It is called a “driving” Carnot machine when it operates by delivering work and a “receiving” Carnot machine (also called a Carnot heat pump) when it operates by consuming work. In driving mode, the heat Qhi is delivered to a working fluid GT from a hot source at the temperature Thi, the heat Qlo is yielded by the working fluid GT to a cold sink at the temperature Tlo and the net work W is delivered by the machine. Conversely, in heat pump mode, the heat Qlo is taken by the working fluid GT to the cold source Tlo, the heat Qhi is yielded by the working fluid to the hot sink at the temperature Thi and the net work W is consumed by the machine.
According to the second law of thermodynamics, the efficiency of a dithermal (driving or receiving) machine, that is to say a real machine whether operating in the Carnot cycle or not, is at most equal to that of the ideal Carnot machine and depends only on the temperatures of the source and of the sink. However, the practical implementation of the Carnot cycle, consisting of two isothermal steps (at temperatures Thi and Tlo) and two reversible adiabatic steps, encounters a number of difficulties that have not been completely solved hitherto. During the cycle, the working fluid may still remain in the gaseous state or it may undergo a liquid/vapor change of state during the isothermal transformations at Thi and Tlo. When a liquid/vapor change of state occurs, the heat transfers between the machine and the environment take place with a higher efficiency than when the working fluid remains in the gaseous state. In the first case and for the same exchanged thermal power levels at the heat source and at the heat sink, the exchange areas are smaller (and therefore less expensive). However, when there is a liquid/vapor change of state, the reversible adiabatic steps consist in compressing and expanding a liquid/vapor two-phase mixture. The techniques of the prior art do not allow two-phase mixtures to be compressed or expanded. According to the current prior art it is not known how to carry out these transformations correctly.
To remedy this problem, it has been envisaged to approximate the Carnot cycle by isentropically compressing a liquid and isentropically expanding a superheated vapor (for a driving cycle) and by compressing the superheated vapor and isenthalpically expanding the liquid (for a receiving cycle). However, such modifications induce irreversibilities in the cycle and very significantly reduce its efficiency, that is to say the efficiency of the motor or the coefficient of performance or the coefficient of amplification of the heat pump.
The object of the present invention is to provide a thermodynamic machine operating in a cycle close to the Carnot cycle, which is better than the machines of the prior art, that is to say a machine that operates with a liquid/vapor change of state of the working fluid in order to maintain the advantage of the low contact areas required, while still substantially limiting the irreversibilities in the cycle during the adiabatic steps.
One subject of the present invention is a refrigeration, heat and/or work production plant, comprising at least one modified Carnot machine. Another subject of the invention is a refrigeration, heat and/or work production process using a plant comprising at least one modified Carnot machine.
A refrigeration, heat or work production plant according to the present invention comprises at least one modified Carnot machine formed by:
In the present text:
The refrigeration, heat and/or work production process according to the invention consists in making a working fluid GT undergo a succession of modified Carnot cycles in a plant according to the invention comprising at least one modified Carnot machine. A modified Carnot machine comprises the following transformations:
The process is characterized in that:
In one embodiment, the work is received or delivered by the plant via a hydraulic fluid which flows through a hydraulic converter during just one of the adiabatic transformations. In this embodiment, the modified Carnot cycle and the modified Carnot machine are referred to as being “of the 1st type”.
In one embodiment, the work is received or delivered by the plant via a hydraulic fluid which flows through a hydraulic converter during both adiabatic transformations. In this embodiment, the modified Carnot cycle and the modified Carnot machine are referred to as “of the 2nd type”.
a and 12b illustrate schematically two embodiments of modified driving Carnot machines operating between the same extreme temperatures Thi and Tlo, these figures indicating the direction of heat exchange and work exchange between these machines and the environment.
a to 15h show schematically the heat and work exchange between a modified Carnot machine (or combinations of such machines) and the environment, and also the heat source and sink temperatures, for 8 examples involving various working fluids.
In a plant according to the present invention, a modified Carnot machine may have a driving machine configuration or a receiving machine configuration. In both cases, the machine may be of the 1st type (work exchange between the transfer liquid and the environment during one of the adiabatic transformations) or of the 2nd type (work exchange between the transfer liquid and the environment during both adiabatic transformations). A modified Carnot machine may also have a configuration that allows, depending on the choice of the user, operation in driving (1st or 2nd type) mode or in receiving (1st or 2nd type) mode.
The process for controlling a driving machine comprises at least one step during which heat is supplied to the plant, with a view to recovering work during at least one of the transformations of the modified Carnot cycle. The process for controlling a receiving machine comprises at least one step during which work is supplied to the plant, with a view to recovering heat at the hot sink Thi or removing heat at the cold source at Tlo during at least one of the isothermal transformations of the modified Carnot cycle.
The process according to the present invention consists in subjecting a working fluid GT to a succession of cycles between a heat source and a heat sink. In what follows, for the sake of simplification and because this does not affect the operating principle of the modified Carnot machine, no distinction is made between the temperature of the hot source or sink and that of the working fluid that exchanges with this source or sink, these temperatures being denoted by Th. Likewise, no distinction is made between the temperature of the cold source or sink and that of the working fluid that exchanges with this source or sink, these temperatures being denoted by Tlo. Thus, any heat exchange is considered to be perfect.
The working fluid GT and the transfer liquid LT are preferably chosen in such a way that GT is weakly soluble, preferably insoluble, in LT, GT does not react with LT, and in the liquid state GT is less dense than LT. When the solubility of GT in LT is too high or if GT in the liquid state is denser than LT, it is necessary to isolate them from each other by a means that does not prevent work exchange. Said means may consist for example in interposing a flexible membrane between GT and LT which creates an impermeable barrier between the two fluids but which offers only very slight resistance to the displacement of the transfer liquid and slight resistance to heat transfer. Another solution is formed by a float that has a density intermediate between that of the working fluid GT in the liquid state and that of the transfer liquid LT. A float may constitute a large physical barrier, but it is difficult to make it perfectly effective if it is desired for there to be no friction on the side walls of the chambers CT and CT′. On the other hand, the float may constitute a very effective thermal resistance. The two solutions (membrane and float) may be combined.
The transfer liquid LT is chosen from liquids that have a low saturation vapor pressure at the operating temperature of the plant so as to avoid, in the absence of a separating membrane as described above, limitations due to diffusion of the GT vapor through the LT vapor at the condenser or at the evaporator. Provided that there is the abovementioned compatibility with GT, nonexhaustive examples of LT may be water or a mineral oil or a synthetic oil, preferably one having a low viscosity.
The working fluid GT undergoes transformations in the temperature/pressure thermodynamic domain preferably compatible with liquid-vapor equilibrium, that is to say between the melting point and the critical temperature. However, during the modified Carnot cycle some of these transformations may occur completely or partly in the supercooled liquid or superheated vapor domain, or the supercritical domain. Preferably, a working fluid is chosen from pure substances and azeotropic mixtures so as to have a one-variable relationship between temperature and pressure at liquid-vapor equilibrium. However, a modified Carnot machine according to the invention may also operate with a nonazeotropic solution as working fluid.
The working fluid GT may for example be water, CO2, or NH3. The working fluid may also be chosen from alcohols containing 1 to 6 carbon atoms, alkanes containing 1 to 18 (more particularly 1 to 8) carbon atoms, chlorofluoroalkanes preferably containing 1 to 15 (more particularly 1 to 10) carbon atoms and partially or completely fluorinated or chlorinated alkanes preferably containing 1 to 15 (more particularly 1 to 10) carbon atoms. In particular, mention may be made of 1,1,1,2-tetrafluoroethane, propane, isobutane, n-butane, cyclobutane, and n-pentane.
A fluid that can be used as working fluid may act as driving fluid or as receiving fluid, depending on the plant in which it is used, on the available heat sources and on the desired objective.
In general, the working fluids and transfer liquids are firstly chosen according to the available heat source and heat sink temperatures and the desired maximum or minimum saturation vapor pressures in the machine, and then according to other criteria, such as especially toxicity, environmental impact, chemical stability and cost.
The fluid GT in the chambers CT or CT′ may be in the liquid/vapor two-phase mixture state after the adiabatic expansion step in the case of the driving cycle or after the adiabatic compression step in the case of the receiving cycle. In this case, the liquid phase of GT accumulates at the GT/LT interface. When the vapor content of GT is high (typically between 0.95 and 1) in the chambers CT or CT′ before said chambers are connected with the condenser, it is conceivable for the liquid phase of GT in these chambers to be completely eliminated. This elimination may be carried out while maintaining the temperature of the working fluid GT in the chambers CT or CT′ at the end of the steps of bringing the chambers CT or CT′ into communication with the condenser, at a value above that of the working fluid GT, in the liquid state in the condenser, so that there is no liquid GT in CT or CT′ at this moment.
In one embodiment, the plant comprises means for heat exchange between, on the one hand, the heat source and the heat sink, which are at different temperatures, and, on the other hand, the evaporator Evap, the condenser Cond and possibly the working fluid GT in the transfer chambers CT and CT′.
When the hydraulic converter of the modified Carnot machine is a hydraulic motor and the temperature of the source is above the temperature of the sink, the modified Carnot machine is a driving machine. A plant according to the present invention may comprise a single modified driving Carnot machine, or such a machine coupled to a complementary device, depending on the intended objective. The coupling may be achieved thermally or mechanically.
In a modified driving Carnot machine of the 1st type, the device PED consists of a device that pressurizes the working fluid GT in the saturated liquid or supercooled liquid state, for example an auxiliary hydraulic pump AHP1.
In a modified driving Carnot machine of the 2nd type, the pressurization or expansion device PED comprises, on the one hand, a compression/expansion chamber ABCD and the transfer means associated therewith and, on the other hand, an auxiliary hydraulic pump AHP2 that pressurizes the hydraulic transfer fluid LT.
In a process according to the invention implemented according to a modified driving Carnot cycle, the cycle comprises the following transformations:
an isothermal transformation during which heat is delivered by GT to the heat sink at the temperature Tlo below the temperature Thi; and
an adiabatic transformation with an increase in the pressure of the working fluid GT.
When the process of the invention is a succession of modified driving Carnot cycles, the heat source is at a temperature above the temperature of the heat sink. Each cycle is formed by a succession of steps during which there is a change in volume of the working fluid GT. This variation in volume causes a displacement of the liquid LT that drives a hydraulic motor or is caused by a displacement of the liquid LT which is driven by an auxiliary hydraulic pump. Thus, the plant consumes work during certain steps and this is recovered during other steps, whereas over the complete cycle there is a net production of work to the environment. The environment may be an ancillary device that transforms the work delivered by the plant to electricity, to heat or to refrigeration power. A process for operating a modified driving Carnot machine is described in greater detail on the basis of the machine shown schematically in
The evaporator Evap and the condenser Cond contain exclusively the fluid GT, in general in the liquid/vapor mixture state. However, depending on the working fluid GT and the temperature of the hot source Thi, said working fluid GT may be in the supercritical domain at said temperature Thi and, under these conditions, the evaporator Evap contains GT only in the gaseous state. It is the liquid LT that passes exclusively through the motor HM and the pump AHP2. The elements ABCD, CT and CT′ constitute the interfaces between the two (GT and LT) circuits and they contain the hydraulic transfer fluid LT in the bottom portion and/or the working fluid GT in the liquid, vapor or liquid/vapor mixture state in the upper portion.
ABCD is connected to Cond and to Evap by circuits containing GT that can be closed off by the solenoid valves SV3 and SV4 respectively. Evap is connected to CT and CT′ by circuits containing GT that can be closed off by the solenoid valves SV1 and SV1′ respectively. Cond is connected to CT and CT′ by circuits containing GT that can be closed off by the solenoid valves SV2 and SV2 respectively. In the embodiment shown in
In the embodiment shown in
ABCD is connected in its lower portion to the downstream end of HM by a circuit containing the transfer liquid LT and comprising, in two parallel branches, the auxiliary hydraulic pump AHP2 and the solenoid valve SVr. When LT flows from HM to ABCD, it is pressurized by AHP2 and SVr is closed. When LT flows from ABCD to MH, it flows under gravity, SVr is open and AHP2 is stopped. Since the transfer liquid LT is finally transferred to CT or CT′, it is necessary for ABCD to be above the chambers CT and CT′.
In
A modified Carnot cycle may be described in the Mollier diagram used by refrigeration engineers, in which the pressure P is plotted on a logarithmic scale as a function of the enthalpy per unit mass h of the working fluid.
Depending on the fluid GT used, the step of isentropically expanding the saturated vapor at the outlet of the evaporator may result in a two-phase mixture or in superheated vapor. In
The modified driving Carnot cycle is formed by four successive phases starting at times tα, tγ, tδ and tλ respectively. This is described below with reference to the a-b-c-dsv-e-a cycle of the Mollier diagram shown in
αβγ Phase (Between the Times tα, and tγ):
At the time immediately preceding tα, the level of LT is low (denoted by L) in ABCD and the cylinder CT, and is high (denoted by H) in the cylinder CT′. At the same instant, the saturation vapor pressure of GT has a low value Plo in ABCD and CT and a high value Phi in Evap and CT′. It is this instant of the cycle that the configuration of the plant shown schematically in
At time tα, the opening of the solenoid valves SVlo′, SV2, SVhi′ and SVlo and the engagement of the AHP2 cause the following effects:
The pressurization of GT from the low pressure Plo up to the high pressure Phi in ABCD must be carried out before it is introduced into the evaporator, which is still at the high pressure Phi. It is therefore only at the time tβ that the solenoid valve SV4 (which may be replaced by a nonreturn valve) between ABCD and Evap is opened. This requires there to be a stock of GT in the liquid state in the evaporator at the start of this phase, which stock is reconstituted at the end of this step.
From an energy standpoint, during this αβγ phase, heat Qhi has been consumed at the evaporator at Thi, heat Qlo, has been released at the condenser at Tlo (Tlo<Thi) and a net work Wαβγ has also been delivered to the outside.
γδ Phase (Between Times tγ and tδ):
At time tγ, that is to say when the level of LT has reached the predefined values (I in CT, J in CT′ and H in ABCD), the valves SV2, SVlo and SVhi′ are left open and the solenoid valves SV3 and SVr are opened. As a result:
From an energy standpoint, during this γδ phase, heat Qea is released at the condenser at Tlo, a little heat (taken from the hot source at Thi) is possibly consumed in CT′ to provide the d→dsv superheating and work Wβγ is also delivered to the outside.
The second portion of the cycle is symmetric: the evaporator, the condenser and ABCD are the sites of the same successive transformations, whereas the roles of the chambers CT and CT′ are reversed.
δελ Phase (Between Times tδ and tλ):
This phase is equivalent to the αβγ phase but with the transfer chambers CT and CT′ reversed.
λα Phase (Between Times tλ and tα):
This phase is equivalent to the γδ phase but with the transfer chambers CT and CT′ reversed.
After the λα phase, the modified driving Carnot machine of the 2nd type is in the α state of the cycle described above. The various thermodynamic transformations followed by the fluid GT (with the d→+dsv transformation considered as optional) and the levels of the transfer liquid LT are given in Table 1.
The states of the actuators (solenoid valves and clutch of the pump AHP2) are given in Table 2, in which x indicates that the corresponding solenoid valve is open or that the pump AHP2 is engaged.
Work production is continuous throughout the duration of the cycle, but not at constant power, either because the pressure difference across the terminals of the hydraulic motor varies, or because a portion, which can vary over time, of this work is recovered by the auxiliary hydraulic pump AHP2. This is not a problem if the work delivered to the outside is used directly for a receiving machine that does not have to be constant within the cycle, such as a water pump or a modified receiving Carnot machine. Of course, the average power over a cycle remains constant from one cycle to another, when a steady operating state is reached and if the temperatures Thi and Tlo remain constant.
Moreover, the evaporator is isolated from the rest of the circuit during the γδ and λα phases, whereas the heat supplied by the hot source at Thi is a priori continuous. Under these conditions, during these isolation phases there will be a temperature rise and therefore a pressure rise in the evaporator followed by a sudden drop at times tα and tδ when the valve SV1 or SV1′ reopens.
In a preferred method of implementing the process of the invention, the fact that the transfer liquid LT is incompressible and the fact that the variations in level which occur simultaneously in the three chambers ABCD, CT and CT′ are therefore not independent are taken into account. Moreover, these variations in the level of LT result from or involve concomitant variations in the volume of the fluid GT. This is represented by the following equation between the densities of GT at various stages of the cycle:
ρe−ρa=ρc (Equation 1)
ρi being the density of GT at the thermodynamic state of the point “i”, “i” being e, a, dsv and c respectively.
When the temperatures of the condenser and the evaporator are very close (or even coincident), the point “e” in the Mollier diagram is close to the point “a” (or coincident therewith) as shown schematically in the a″-b″-c″-dsv-e″-a″ cycle. As the temperature difference between the heat sink and the heat source increases, the point “e” moves away from the point “a” and approaches the point “dsv”. The a′-b′-c′-dαy-e′-a′ cycle represents an intermediate case and the a-b-c-dsv-a cycle represents the extreme case in which the points “e” and “dsv” are coincident. As the efficiency of the modified driving Carnot cycle increases with the temperature difference between the heat sink and the heat source, the a-b-c-dsv-a cycle is preferable provided that there is a heat source at the temperature Thi sufficient for a fixed sink temperature Tlo.
In this preferred case (in which ρe=ρdsv), equation 1 reduces to ρc=ρa as shown in
Thus, the temperature difference (Thi−Tlo) between the two isothermal transformations of the modified driving Carnot cycle cannot exceed a certain value ΔTmax which depends on one of the temperatures (Thi or Tlo and on the chosen working fluid GT. Now, the performance of the modified Carnot machine depends especially on this value ΔTmax. To obtain the maximum performance with a given fluid GT and a given temperature Thi or Tlo, it is necessary to choose the other operating conditions such that the ρa/ρc ratio is as close as possible to 1 (but always less than 1), or preferably such that 0.9≦ρa/ρc≦1 and more particularly 0.95≦ρa/ρc≦1.
The various thermodynamic transformations of this preferred method of implementation are given in Table 3 and the states of the actuators (solenoid valves and clutch of the pump AHP2) are given in Table 4 in which x means that the corresponding solenoid valve is open or that the pump AHP2 is engaged.
The steps of the modified driving Carnot cycle of the 2nd type in the preferred configuration are explained in detail below if they differ from those described above for the general configuration.
Starting from an initial state in which, on the one hand, the working fluid GT is maintained in the evaporator Evap at high temperature and in the condenser Cond at low temperature by heat exchange with the hot source at Thi and the cold sink at Tlo<Thi, respectively and, on the other hand, all the GT and transfer liquid LT communication circuits are closed, the working fluid GT is subjected to a succession of cycles comprising the following steps:
αβγ Phase (Between Times tα and tγ):
At time tα, the opening of the solenoid valves SV1′ and SVhi′ the engagement of AHP2 cause the following effects:
In the Mollier diagram (
a→b in the chamber ABCD;
b→c in the Evap-CT′ assembly.
The pressurization of GT from Plo to Phi in ABCD must be carried out before it is introduced into the evaporator, which is always at the high pressure Phi. It is therefore only at time tβ that the solenoid valve SV4 (which may be replaced by a nonreturn valve) between ABCD and Evap is opened.
From an energy standpoint, during this αβγ phase, heat Qhi has been consumed at the evaporator at T′hi and net work Wαβγ has also been delivered to the outside.
γδ Phase (Between Times tγ and tδ):
At time tγ, that is to say when the level of LT has reached the predefined values (J in CT′ and H in ABCD), the valves SV1′ and SV4 are closed, SVhi′ is left open and the solenoid valves SV2, SV3, SVlo and SVr are opened. As a result:
From an energy standpoint, during this γδ phase, heat Qda is released in the condenser at Tlo, a small amount of heat (taken from the hot source at Thi) is possibly consumed in CT′ in order to provide the superheating d→dsv and work Wγδ is also delivered to the outside.
As in the general case of the method of implementing the process of the invention in a modified driving Carnot machine of the 2nd type, the other half of the cycle is symmetric:
After several cycles, the plant operates in a steady state in which the hot source continuously delivers heat at the temperature Thi to the evaporator Evap, heat is delivered continuously by the condenser Cond to the cold sink at the temperature Tlo, and work is delivered continuously by the machine.
In this preferred case of the modified driving Carnot cycle of the 2nd type, there is, for a given working fluid and for whatever temperature of the condenser Tlo, a maximum value of the temperature Thi-max of the evaporator such that the densities ρc and ρa are equal. However, if there is a heat source at a temperature Thi well above Thi-max, it is possible a priori for the machine to have a higher efficiency, either by combining, in cascade, two modified driving Carnot machines in the plant of the invention, or by using, in the plant, a modified driving Carnot machine of the 1st type.
In a modified driving Carnot machine of the 1st type, the pressurization/expansion device placed between the condenser Cond and the evaporator Evap comprises an auxiliary hydraulic pump AHP1 and a solenoid valve SV3 in series.
In
The various steps of the cycle and the states of the actuators (solenoid valves and AHP1 pump) are explained in detail below and given in Tables 5 and 6.
The steps of the modified driving Carnot cycle of the 1st type are described below for the points that differ from what has been described above for the modified driving Carnot cycle of the 2nd type in its general configuration. The first cycle is carried out from an initial state in which the working fluid GT is maintained in the evaporator Evap at high temperature and in the condenser Cond at low temperature by heat exchange with the hot source at Thi and the cold sink at Tlo, respectively, and all the communication circuits for the working fluid GT and for the transfer liquid LT are closed off. At time t0, the auxiliary hydraulic pump AHP1 is actuated and the GT circuit between Cond and Evap is opened (by SV3) so that a portion of GT, in the saturated or supercooled liquid state, is taken in by AHP1 in the lower portion of the condenser Cond and discharged in the supercooled liquid state into Evap where it heats up, and then GT is subjected to a succession of modified Carnot cycles, each of which comprising the following steps:
αβ Phase (Between Times tα and tβ):
At the time immediately preceding tα, the level of LT is low (denoted by L) in the cylinder CT and high (denoted by H) in the cylinder CT′. At the same instant, the saturation vapor pressure of GT has a low value Plo in CT and a high value Phi in Evap and CT′. It is this instant of the cycle which is shown schematically in
At time tα, the opening of the solenoid valves SV1′, SV2, SV3, SVhi′ and SVlo and the operation of the AHP1 cause the following effects:
In the Mollier diagram (
a→b between the condenser and the evaporator;
b→b1→c in the Evap-CT′ assembly;
dsv→e in the CT-Cond assembly.
It is preferable for the auxiliary hydraulic pump AHP1 not to be operating and for the solenoid valve SV3 not to be open if there is no liquid GT upstream of this pump. A liquid level detector may be placed as safety element to stop the pump and close the solenoid valve if necessary. The evaporation of GT in Evap is continuously compensated for by supplies of liquid GT coming from the condenser so that the level of liquid GT in the evaporator is approximately constant.
From an energy standpoint, during this αβ phase, heat Qhi has been consumed in the evaporator at Thi, heat Qde has been released in the condenser at Tlo (Tlo<Thi) and net work Wαβ has also been delivered to the outside, said work Wαβ being the difference between the work delivered by the hydraulic motor HM and that consumed by the auxiliary hydraulic pump AHP1.
βγ Phase (Between Times tβ and tγ):
At time tβ, that is to say when the level of LT has reached the predefined values (I in CT and J in CT′), the solenoid valve SV1′ is closed, the valves SV2, SV3, SVlo and SVhi′ are left open and the pump AHP1 is operating (if liquid GT is present upstream). It follows that:
From an energy standpoint, during this βγ phase, heat Qea is released in the condenser at Tlo, a small amount of heat (taken from the hot source at Thi) is consumed in CT′ for the d→dsv, superheating, and net work Wβγ is also delivered to the outside.
The other half is symmetric: the evaporator and the condenser are the sites of the same successive transformations, whereas the roles of the chambers CT and CT′ are reversed.
γδ Phase (Between Times tγ and tδ) and δα Phase (Between Times tδ and tα):
These phases are equivalent to the αβ phase and the βγ phase respectively, but with the transfer chambers CT and CT′ reversed.
More particularly:
After several cycles, the plant operates in a steady state in which the hot source continuous delivers heat at high temperature Thi in the evaporator Evap, heat is continuously delivered by the condenser Cond into the cold sink at Tlo and work is continuously delivered by the machine.
In this configuration (of the 1st type), equation (1) linking the densities of GT in the various steps of the cycle is still valid, i.e.:
ρe−ρa=ρdsv−ρc (equation 1)
However, the density of GT leaving the condenser, i.e. in the saturated liquid state (point “a” in the Mollier diagram) is always much lower than that of GT leaving the evaporator, that is to say in the saturated or superheated vapor state (point “c” or “csv” in the Mollier diagram) irrespective of the temperature difference between Thi and Tlo. Thus, the following double inequality is still satisfied:
ρa<ρc<ρdsv (inequality 1)
The point “e” is always between the points “a” and “dsv” in the Mollier diagram and the temperatures Tlo and Thi may be fixed completely independently without this affecting the operation of the modified driving Carnot machine of the 1st type.
The modified driving Carnot, machine of the 1st type is simpler in its operation and comprises fewer constituent elements. However, as in the case of the Rankine cycle, the b→b1 transformation generates appreciable irreversibilities, this having an unfavorable effect on the efficiency of the cycle. However, since the increase in the difference (Thi−Tlo has, conversely, a positive effect on this efficiency, it is possible, depending on the thermodynamic conditions and the fluid GT that are chosen, for the efficiency of the modified driving Carnot machine of the 1st type to be finally higher than that of the modified driving Carnot machine of the 2nd type, including in its preferred configuration.
When the process of the invention is a succession of modified receiving Carnot cycles, the heat source is at a temperature Tlo below the temperature Thi of the heat sink. Each cycle is formed by a succession of steps during which there is a change in volume of the working fluid GT. This variation in volume causes or is caused by a displacement of the liquid LT. Thus during certain steps, the plant consumes work and restores work during other steps, but over the complete cycle there is a net consumption of work delivered by the environment via a hydraulic pump HP.
In a modified receiving Carnot machine of the 1st type, the adiabatic expansion step is isenthalpic rather than isentropic. This is because the work that can be recovered during the isentropic expansion is low in comparison with the work involved during the other steps of the cycle. The isenthalpic expansion requires only a simple irreversible adiabatic expansion device, the pressurization or expansion device may be a capillary tube or an expansion valve. In a modified receiving Carnot machine of the 2nd type, it is necessary for the pressurization and expansion device to be an adiabatic compression/expansion bottle ABCD and the associated transfer means. Thus, in this preferred configuration of the 1st type, the coefficient of performance or the coefficient of amplification of the modified receiving Carnot machine will be slightly reduced (while still being higher than the equivalent machines of the prior art) but with a significant simplification of the process and a lower cost.
When the process of the invention is a succession of modified receiving Carnot cycles, the heat source is at a temperature Tlo below the temperature Thi of the heat sink. Each cycle is formed by a succession of steps during which there is a change in volume of the working fluid GT. This variation in volume causes or is caused by a displacement of the liquid LT. Thus during certain steps the plant consumes work and restores work during other steps, but over the complete cycle there is a net consumption of work delivered by the environment via a hydraulic pump HP.
The evaporator Evap and the condenser Cond contain exclusively the fluid GT in general in the liquid/vapor mixture state. However, depending on the working fluid GT and the temperature Thi of the hot sink, said working fluid GT may be in the supercritical domain at Thi and under these conditions the condenser Cond contains GT only in the gaseous state.
Passing through the pump HP is exclusively liquid LT. The elements ABCD, CT and CT′ constitute the interfaces between the two (GT and LT) circuits. They contain the hydraulic transfer fluid LT in the lower portion and/or the working fluid GT in the liquid, vapor or liquid-vapor mixture state in the upper portion. ABCD is connected to Cond and to Evap by circuits containing GT that can be closed off by the solenoid valves SV3 and SV4 respectively. Evap is connected to CT and CT′ by circuits containing GT that can be closed off by the solenoid valves SV1 and SV1′ respectively. Cond is connected to CT and CT′ by circuits containing GT that can be closed off by the solenoid valves SV2 and SV2′ respectively.
In general, the liquid passing through a hydraulic pump always flows in the same direction. It is this most common option which is shown in
ABCD is connected in its lower portion by two parallel branches of the circuit containing the transfer liquid LT. The branch that can be closed off by the solenoid valve SVi is connected to the high-pressure LT circuit and the branch that can be closed off by the solenoid valve SV, is connected to the low-pressure circuit. When LT flows from ABCD into the transfer chamber CT or CT′, it flows under gravity and it is therefore necessary for ABCD to be above the chambers CT and CT′.
The shaft of the hydraulic pump HP must be connected to one or more drive devices (i.e. delivering work) either directly or via a conventional coupling, such as a universal joint, a belt or a clutch (whether magnetic or mechanical). For example in
The modified receiving Carnot cycle followed by the driving fluid GT is described in the Mollier diagram shown in
Depending on the fluid GT used, the step of isentropically compressing the saturated vapor at the outlet of the evaporator may result in a two-phase mixture or in superheated vapor. In
The device for introducing the working fluid GT into the evaporator is designed so that GT is introduced in the liquid state into the evaporator, but after the saturated liquid (point 3 in the Mollier diagram of
The modified receiving Carnot cycle is formed by four successive phases starting at times tα, tγ, tδ and tλ respectively. Only the 1-2sv-3-4-5-1 cycle is described below since the variant with the “1sv” point does not modify the principle.
Starting from an initial state in which all the communication circuits for the working fluid GT and for the transfer liquid LT are closed off, at t0, the hydraulic pump HP is actuated and then GT is subjected to a succession of modified Carnot cycles, each of which comprising the following steps:
At the instant immediately preceding tα, the level of LT is high (denoted by H) in ABCD and the cylinder CT, and is low (denoted by L) in the cylinder CT′. At the same instant, the saturation vapor pressure of GT has a high value Phi in ABCD, Cond and CT and has a low value Plo in Evap and CT′. It is this instant of the cycle which is shown schematically in the configuration of
At time tα, the solenoid valves SVr, SVlo and SVhi′ are opened. The isentropic expansion of GT to the liquid/vapor mixture state (but with an almost zero vapor content by weight) in ABCD discharges LT through HP. At the same time, the very small amount of saturated vapor and the transfer liquid LT that are contained in CT follow the same pressure variation, which, owing to the small amount of vapor, is not accompanied by a significant variation in the level of LT in CT. The transfer liquid LT downstream of HP isentropically compresses the GT vapor contained in CT′. The pressures upstream and downstream of the pump HP are balanced at time tβ. Between tα and tβ there is theoretically no net consumption of work delivered by the pump HP. The time interval tβ-tα is short, since during this step there is no heat transfer.
At time tβ, the solenoid valves SV1 and SV4 are opened. The consequences are the following:
3→4 transformation in ABCD;
4→5 transformation in the Evap-CT assembly; and
1→2sv transformation in CT′. The compression is isentropic and it is assumed that, for the fluid GT used, this ends up in the superheated vapor domain.
From an energy standpoint, during this αβγ phase, heat Q45 has been pumped into the evaporator at Tlo and work Wαβγ has also been consumed by the pump HP. This work has been delivered by the outside with a power increasing from tβ since the pressure upstream of the pump remains virtually constant (=Plo) after this instant, whereas the downstream pressure increases up to Phi.
At time tγ, i.e. when the level of LT has reached the predefined values (L in ABCD, J in CT and I in CT′), SV1, SVlo and SVhi′ are left open and the solenoid valves SV2′, SV3 and SVi are opened simultaneously. As a result, the GT vapor continues to be produced in the evaporator and to be expanded in CT (5→1 transformation), thereby again discharging the transfer liquid taken in by the pump into the cylinder CT′, which is this time connected to the condenser. The GT vapor contained in CT′ is desuperheated (partly in CT′) and completely condenses in the condenser (2sv→3 transformation) in which the vapor does not accumulate since it is discharged under gravity into ABCD. In parallel, a portion of the transfer liquid LT output by the pump is discharged into ABCD, in order to reestablish the high LT level therein.
From an energy standpoint, during this αβ phase, heat Q51 is pumped into the evaporator at Tlo, heat Q23 is released in the condenser at Thi (with Thi>Tlo), which requires work Wγδ delivered by the outside. This work is at almost constant power since the pressures upstream and downstream of the pump are also practically constant (with nonlimiting heat exchangers at the condenser and the evaporator).
At time tδ, one half of the cycle is complete. The other half is symmetric: the evaporator, the condenser and the chamber ABCD are the sites for the same successive transformations, but the roles of the chambers CT and CT′ are reversed.
δελ Phase (Between Times tδ and tλ) and λα Phase (Between Times tλ and tα):
These phases are equivalent to the αβγ phase and to the γδ phase respectively, but with the transfer chambers CT and CT′ reversed.
More particularly:
After several cycles, the plant operates in a steady state.
For refrigeration, in the initial state GT is maintained in the condenser Cond at high temperature by heat exchange with the hot sink at Thi and in the evaporator Evap at a temperature equal to or below Thi by heat exchange with a medium external to the machine, said medium having initially a temperature Thi. In the steady state, net work is consumed by the hydraulic pump HP, the condenser Cond continuously removes heat to the hot sink at high temperature Thi and heat is continuously consumed by the evaporator Evap, with extraction of heat from the external medium in contact with said evaporator Evap, the temperature Tlo of said external medium being strictly below Thi.
For heat production, in the initial state GT is maintained in the evaporator Evap at low temperature by heat exchange with the cold source at Tlo and GT is maintained in the condenser Cond at a temperature Thi≧Tlo by heat exchange with a medium external to the machine, said medium having initially a temperature ≧Thi. In the steady state, net work is consumed by the hydraulic pump HP, the cold source at Tlo continuously supplies heat to the evaporator Evap, the condenser Cond continuously delivers heat to the hot sink, the plant producing heat to the external medium in contact with said condenser Cond, the external medium having a temperature Thi>Tlo.
After the λα phase, the modified receiving Carnot machine of the 2nd type is in the α state of the cycle. The various thermodynamic transformations undergone by the fluid GT and the levels of the transfer liquid LT are given in Table 7. The states of the solenoid valves are given in Table 8, in which “x” means that the corresponding valve is open.
Work consumption is continuous over the duration of the cycle (excluding between the times tα and tβ on the one hand and tδ and tε on the other), but not always at constant power insofar as the pressure difference at the terminals of the hydraulic pump may vary. Of course, the average power over a cycle remains constant from one cycle to another, when a steady operating state is reached and if the temperatures Thi and Tlo remain constant. Moreover, the condenser is isolated from the rest of the circuit during the αβγ and δελ phases, whereas the removal of heat in the hot sink at Thi is a priori continuous. Under these conditions, during these isolation phases there will be a temperature drop and therefore a pressure drop in the condenser and then a sudden rise at times tγ and tλ upon the valve SV2 or the valve SV2′ reopening.
Since the transfer liquid LT is incompressible, the variations in level that occur simultaneously in the three chambers ABCD, CT and CT′ are not independent. Moreover, these variations in the level of LT result from or involve concomitant variations in the volume of the fluid GT. This is expressed by the following equation between the densities of GT at various stages of the cycle represented in
ρ5−ρ3=ρ1−ρ2sv (equation 2)
ρi being the density of GT in the thermodynamic state of the point “i”, “i” being the points 5, 3, 1 and respectively. Examples of curves at constant density are shown as dot-dash lines in
Unlike the modified driving Carnot cycle of the 2nd type, here there is no limit to the temperature difference between the cold source at Tlo and the hot sink at Thi. Since the density at the point “3” is always the lowest of the cycle, the following double inequality again applies, irrespective of Thi and Tlo:
ρ4<ρ5<ρ1 (inequality 2)
In a modified receiving Carnot machine of the 1st type, the pressurization/expansion device is inserted in series between the condenser Cond and the evaporator Evap; it comprises a simple expansion device, such as for example an expansion valve EV or a capillary tube, and possibly in series a solenoid valve SV3. Such a device is shown in
The various steps of the cycle and the states of the solenoid valves are explained in detail below and given in Tables 9 and 10. The solenoid valve SV3 is not essential since, when the machine is in operation, it is always open. Its only benefit is to be able to isolate the condenser from the evaporator on stopping the machine.
The steps of the modified receiving Carnot cycle Of the 1st type are explained in detail below when they differ from those described above in the case of the modified receiving Carnot cycle of the 2nd type.
Starting from an initial state in which all the communication circuits for the working fluid GT and for the transfer liquid LT are closed off, at time t0 the hydraulic pump HP is actuated and the GT circuit between Cond and Evap is opened (by SV3) and GT is subjected to a succession of modified Carnot cycles, each of which comprising the following steps:
αβ Phase (Between Times tα and tβ):
At the instant immediately preceding tα, the level of LT is high (denoted by H) in the cylinder CT and low (denoted by L) in the cylinder CT′. At the same instant, the saturation vapor pressure of GT has a high value Phi in Cond and CT and a low value Plo in Evap and CT′. It is this instant of the cycle which is shown schematically in
At time tα, the opening of the solenoid valves SV1, SV3. SVlo and SVhi′, has the following consequences:
In the Mollier diagram (shown in
the 3→4 transformation between Cond and Evap;
the 4→5 transformation in the Evap-CT assembly; and
the 1→2sv transformation in CT′.
As previously, the working fluid GT used is supposed to end up, after this isentropic transformation, in the superheated vapor domain.
From an energy standpoint, during this αβ phase, heat Q45 has been pumped into the evaporator at Tlo and work Wαβ has also been consumed by the pump HP. This work has been delivered by the outside at increasing power since the pressure upstream of the pump remains practically constant (=Plo), whereas the downstream pressure increases up to Phi.
βγ Phase (Between Times tβ and tγ):
At time tβ, that is to say when the level of LT has reached the predefined values (J in CT and I in CT′), SV1, SV3, SVlo and SVhi′ are left open and the solenoid valve SV, is opened. As a result, the GT vapor continues to be produced in the evaporator and to expand in CT (5→1 transformation), thereby again delivering the transfer liquid taken up by the Pump into the cylinder CT′, which this time is connected to the condenser. The GT vapor contained in CT′ is desuperheated (i.e. the 2sv→2g transformation partly in CT′) and condenses completely in the condenser (2sv→2g→3 transformation). The fluid GT in the saturated liquid state is expanded by EV and introduced into the evaporator.
From an energy standpoint, during this βγ phase, heat Q51 is pumped into the evaporator at Tlo, heat Q23 is released into the condenser at Thi (where Thi>Tlo), thereby requiring work Wγδ delivered by the outside. This work is at a virtually constant power since the pressures upstream and downstream of the pump are also practically constant (with nonlimiting heat exchangers at the condenser and the evaporator).
At time tγ, one half of the cycle has been completed. The other half is symmetric: the evaporator and the condenser are the sites for the same successive transformations, while the roles of the chambers CT and CT′ are reversed.
γδ Phase (Between Times tγ and tδ) and δα Phase (Between Times tδ and tα):
These phases are equivalent to the αβ phase and to the βγ phase respectively, but with the transfer chambers CT and CT′ reversed.
More particularly:
After several cycles, the plant operates in a steady state.
As regards refrigeration: in the initial state, GT is maintained in the condenser Cond at high temperature by heat exchange with the hot sink at Thi and in the evaporator Evap at a temperature equal to or below Thi by heat exchange with a medium external to the machine, said medium having initially a temperature equal to or below Thi; and in the steady state, net work is consumed by the hydraulic pump HP, the condenser Cond continuously removes heat to the hot sink at high temperature Thi and heat is continuously consumed by the evaporator Evap, that is to say heat is extracted from the external medium in contact with said evaporator Evap, the temperature Tlo of said external medium being strictly below Thi.
As regards heat production: in the initial state, GT is maintained in the evaporator Evap at low temperature by heat exchange with the cold source at Tlo, and in the condenser Cond at a temperature equal to or above Thi by heat exchange with a medium external to the plant at a temperature equal to or above Thi; and, in the steady state, net work is consumed by the hydraulic pump HP, the cold source at Tlo continuously supplies heat to Evap, and Cond continuously removes heat to the hot sink, that is to say there is heat production to the external medium in contact with Cond, the temperature Thi of said external medium being strictly above Tlo.
In this configuration (called the receiving configuration of the 1st type), equation (2) and inequality (2) linking the densities of GT in the various steps of the cycle are still valid.
The modified receiving Carnot machine of the 1st type is simpler in its operation and comprises fewer constituent elements. However, as in the case of a conventional mechanical vapor compression cycle, the 3→4 and 2sv→2g transformations generate a few irreversibilities, this having an unfavorable effect on the coefficient of performance or coefficient of amplification of the cycle. However, since this degradation is moderate, the configuration of the 1st type is preferred for the modified receiving Carnot machine. This is because, although the modified receiving Carnot machine of the 1st type is similar to conventional mechanical vapor compression machines, it still retains two key advantages:
The choice of one or other type of receiving machine will be made according to the means available, especially according to the temperature of the heat source and heat sink, the working fluid GT and the intended result.
The same modified Carnot machine may provide, alternately, depending on the user's choice, either the driving function or the receiving function. In such a case, said modified Carnot machine is termed a “multipurpose” machine. This possibility means that the machine possesses the constituent elements necessary for satisfying each of the two (driving or receiving) operating modes as described above and additional elements for switching from one mode to the other, the two modes not being able to operate simultaneously. Many constituent elements necessary for each mode may be the same, namely the elements Cond, Evap, CT, CT′, most of the controlled valves and certain portions of the GT and LT circuits. It is therefore unnecessary to duplicate these elements in the multipurpose modified Carnot machine. Other elements are specific to one particular mode. For example, the device PED, combining the chamber ABCD with the solenoid valves SV3 and SV4, as described in
In one particular embodiment, a modified Carnot machine may be coupled to a complementary device, by thermal coupling or by mechanical coupling.
A modified driving or receiving Carnot machine according to the invention may be thermally coupled at its condenser and/or its evaporator to a complementary device. The thermal coupling may be achieved by means of a heat-transfer fluid or a heat pipe, or by direct contact or by radiation.
The complementary device may be a driving or receiving thermodynamic machine. The two most advantageous cases relate to the coupling of a modified driving Carnot machine to a driving thermodynamic machine or the coupling of a modified receiving Carnot machine to a receiving thermodynamic machine. In both cases, the driving thermodynamic machine or the receiving thermodynamic machine receives heat from the condenser of the modified driving Carnot machine or the modified receiving Carnot machine respectively or gives heat to the evaporator of the modified driving Carnot machine or the modified receiving Carnot machine respectively. Said driving or receiving thermodynamic machines may be a second modified driving Carnot machine (of the 1st type or of the 2nd type) or a modified receiving Carnot machine different from the first one (of the 1st type or of the 2nd type).
One mode of thermally coupling two modified driving Carnot machines is illustrated schematically in
Optionally, the working fluids GT1 and GT2 may be identical. In parallel, the amounts of work W1 and W2 are delivered by the HT machine and the LT machine respectively. The overall efficiency ((W1+W2)/Qhi) of the cascaded combination of the two modified driving machines is not necessarily equal to, but in general somewhat lower than, that of a modified driving Carnot machine alone operating between the same extreme temperatures Thi and Tlo, as shown schematically in
The thermally cascaded combination of modified driving Carnot machines may involve machines of the same (1st or 2nd) type or machines of different types.
A first advantage of the cascaded combination of two modified driving Carnot machines of the 2nd type lies in the fact that the temperature difference Thi−Tlo is no longer limited as when a single modified driving Carnot machine of the 2nd type is used (due to the condition on the densities expressed by equation (1)). Thus, the overall efficiency of the cascaded combination may again become higher than that of the single machine when the difference (Thi−Tlo) of said combination becomes greater than the maximum difference permitted for said single machine.
A second advantage of the cascaded combination of two modified driving Carnot machines of the 1st or 2nd type is that the pressure of each of the working fluids GT1 and GT2 is lower than that of the working fluid of the single modified driving Carnot machine (of the 1st or 2nd type) operating between the same extreme temperatures Thi and Tlo.
Cascaded coupling may be achieved using more than two modified driving Carnot machines according to the same principle. The first machine is supplied with heat at the highest temperature Thi to evaporate a working fluid, and the last machine of the cascade releases the heat, generated by condensation at the lowest temperature Tlo, into the environment, Tlo nevertheless being above the temperature of said environment. Between these two extreme machines, each intermediate machine receives the heat released by the condensation of the working fluid of the preceding machine and transfers the heat released by the condensation of its own working fluid to the machine that follows it. Each machine delivers an amount of work to the environment.
Two modified receiving Carnot machines may be coupled in cascade in a manner similar to that described above in the case of the driving machines. The work flux and the heat flux are in the opposite directions to those shown in
The cascaded combination of two modified receiving Carnot machines has the not insignificant advantage of reducing the pressure of each of the working fluids GT1 and GT2 '2 relative to that of the working fluid found in the case of a single modified receiving Carnot machine, whether of the 1st type or the 2nd type, operating between the same extreme temperatures Tlo and Thi.
A modified Carnot machine according to the invention may be mechanically coupled to a complementary device at the hydraulic motor if the machine is a driving machine or at the hydraulic pump if the machine is a receiving machine. The mechanical coupling may be achieved for example by means of a belt, a universal joint, a magnetic or nonmagnetic clutch, or directly onto the shaft of the hydraulic motor or of the hydraulic pump.
The complementary device may be a driving device, for example an electric motor, a hydraulic turbine, a wind turbine, a petroleum-driven engine, a gas-driven engine, a diesel engine, or another modified driving Carnot machine.
The complementary device may be a receiving device, for example a hydraulic pump, a transport vehicle, an alternator, a mechanical vapor compression heat pump, an air compressor, or another modified receiving Carnot machine.
The complementary device may also be a driving/receiving device, such as a flywheel for example.
One particularly preferred method of implementing mechanical coupling consists in coupling a modified driving Carnot machine to a modified receiving Carnot machine.
A first embodiment of a plant comprising a modified driving Carnot machine mechanically coupled to a modified receiving Carnot machine is shown schematically in
The driving machine contains a working fluid GT1. It receives an amount of heat Qhi from a source at the temperature Thi, it releases an amount of heat QmD at a temperature TmD and work W. The temperature Thi of the source is necessarily above the temperature TmD of the heat sink.
The receiving machine contains a working fluid GT2. It releases an amount of heat QmR at a temperature TmR. It receives an amount of heat Qlo from a source at the temperature Tlo and the work W released by the driving machine. The temperature Tlo of the source is necessarily below the temperature TmR of the heat sink.
The two main applications intended by such a combination, which uses only heat at Thi as single energy source, are:
A second embodiment of a plant comprising a modified driving Carnot machine mechanically coupled to a modified receiving Carnot machine is shown schematically in
The driving machine contains a working fluid GT2. It receives an amount of heat QmD from a source at the temperature Tm, it releases an amount of heat Qlo at a temperature Tlo and work W. The temperature Tm of the source is necessarily above the temperature Tlo of the heat sink.
The receiving machine contains a working fluid GT1. It releases an amount of heat Qhi at a temperature Thi. It receives an amount of heat QmR from the source at the temperature Tm and work W released by the driving machine. The temperature Tm of the source is necessarily below the temperature Thi of the heat sink.
Such a plant according to the invention makes it possible to obtain an amount of heat at a higher temperature than the temperature of the available heat source without consuming work delivered by the environment. This application is particularly advantageous when there is discharge of unutilized heat and when heat is required at a higher temperature.
A plant according to the present invention may be used to produce, from a heat source, electricity, heat or refrigeration. Depending on the application in question, the plant comprises a modified driving Carnot machine or a modified receiving Carnot machine associated with an appropriate environment. The working fluid and the hydraulic transfer liquid are chosen according to the desired objective, the temperature of the available heat source and the temperature of the available heat sink.
A modified receiving Carnot machine may be used in the entire field of refrigerating machines and heat pumps: freezing, refrigeration, “reversible” air conditioning, that is to say cooling in summer and heating in winter.
Conventional MVC (mechanical vapor compression) refrigerating machines are reputed to have a good coefficient of performance COP (=Qlo/W) or a good coefficient of amplification COA (=Qm/W). In fact, these coefficients are much lower (by about 50%) than those of the Carnot machine and therefore of the modified receiving Carnot machine of the present invention, in particular of the 2nd type, and to a lesser extent of the 1st type. By replacing current MVC machines with modified receiving Carnot machines it is possible to reduce the electrical energy needed to meet the same requirements.
As in the case of conventional CMV heat pumps, the reasonable pressure range for the working fluid GT of a modified receiving Carnot machine lies between 0.7 bar and 10 bar approximately. At pressures below 0.7 bar, the size of the pipes between the transfer cylinder and the evaporator and, most particularly, the volume of the transfer cylinder itself would become too large. Conversely, at pressures above 10 bar, safety and material strength problems arise. The use of alkanes or HFCs is very suitable for these applications. For example, isobutane has already been used in current refrigerators or freezers (since isobutane has no effect on the ozone layer). The transfer liquid that may be associated with these alkanes in a modified receiving Carnot machine for refrigerating applications is water. For refrigerating below 0° C., it would however be necessary in this case to insert a membrane between GT and LT so as to prevent any icing from obstructing the interior of the evaporator or to envisage regular deicing operations and devices for returning LT to the transfer chambers. Instead of water as transfer liquid, it is also conceivable to use an oil in which the chosen working fluid GT is weakly miscible.
The modified driving Carnot machines may be used for centralized or dispersed electricity generation, work production for pumping water, seawater desalination, etc., or the production of work for a dithermal receiving machine, i.e. one for the purpose of heating or for refrigerating, and in particular a modified receiving Carnot machine.
The advantages of a modified driving Carnot machine and those of a modified receiving Carnot machine may be added together by combining the two machines. Indeed, the mechanical-electrical conversion is then no longer necessary, thereby obviating the slight loss of efficiency that such a conversion involves.
A plant according to the invention may be used for the centralized generation of electricity from a centralized high-temperature heat source, for example produced by a nuclear reaction. A nuclear reaction produces heat at 500° C. The use of this heat involves either the use of a driving fluid compatible with this high temperature or the implementation of an intermediate step using a steam turbine, the steam being superheated to between 500 and 300° C. and the heat at 300° C. then being delivered to a modified driving Carnot machine that operates between this heat source at 300° C. and the cold sink of the external environment. With such a temperature difference, it is necessary for at least two modified driving Carnot machines involving different working fluids to be thermally cascaded. For the machine at the highest temperature, water is best suited as working fluid. In this configuration, the advantage afforded by the invention is that the overall electrical generation efficiency is better than that of current nuclear power stations.
An installation according to the invention may be used for decentralized electricity generation, using solar energy as heat source, this being renewable and available everywhere, albeit intermittent and quite dilute (with a maximum of about 1 kW/m2 in fine weather). Current cylindro-parabolic solar collectors may bring the driving fluid to about 300° C. Compared with centralized generation, the work delivered by the turbine between 500 and 300° C. is lost but only a renewable energy source is used.
It is also possible to use thermal solar energy delivered at lower temperatures, such as about 130° C., with vacuum tube collectors or about 80° C. with flat collectors. Obviously the lower the temperature of the hot source, the lower the efficiency of the modified driving Carnot machine. However for the lowest temperature Thi, that delivered by flat solar collectors, a thermally cascaded combination is no longer necessary; the modified driving Carnot machine is then simpler and therefore less expensive. When the sun is not shining, an auxiliary boiler may supply the necessary heat.
A plant according to the invention may be used to convert heat into work, without necessarily converting it into electricity. The mechanical work may be used directly, for example for a hydraulic pump or for a heat pump, the compressor of which is not driven by an electric motor. In the latter case, the end results are:
The present invention is illustrated by the following eight examples to which the invention is not however limited.
In these examples, three working fluids GT are used, namely water (denoted by R718), n-butane (denoted by R600) and 1,1,1,2-tetrafluoroethane (denoted by R134a). The Mollier diagrams for these three fluids are shown in
The objective is to produce work (which can be converted to electricity) with the best efficiency possible. For a given cold sink temperature (Tlo=40° C.), the efficiency will be higher the higher the temperature Thi of the hot source and the closer the machine cycle is to the ideal Carnot cycle. The modified driving Carnot cycle of the 2nd type is therefore used in its preferred configuration, that is to say by satisfying the constraint whereby the density of the working fluid leaving the condenser is the same as that leaving the evaporator (as described in
With a heat source at Thi3 of 85° C., the working fluid used is 8600 and this describes the a-b-c-d-a cycle shown in
With a heat source at Thi2 of 175° C. and in thermal cascade with the preceding cycle, the working fluid used is R718 and this describes the e-f-g-h-e cycle shown in
Finally, with a heat source at Thi1 of 275° C. and in thermal cascade with the preceding cycle, the working fluid used is again R718, and this describes the a-b-c-d-a cycle shown in
The thermally cascaded combination of these three modified driving Carnot machines of the 2nd type (
η=(W1+W2+W3)/Qhi=η1+η2(1−η1)+η3(1−η2)(1−η1)
giving η=39.10%, i.e. 91% of the efficiency of the Carnot machine operating between the same extreme temperatures.
This efficiency is better than that of current nuclear power stations (≈34%) which nevertheless work with superheated steam at much higher temperatures (≈500° C.). Furthermore, the heat source at Thi1 (=275° C.) could be supplied by cylindro-parabolic solar collectors.
As for the previous example, the objective is to produce work (which can be converted to electricity) but with a simpler machine using combinations of modified driving Carnot machines of the 1st type. The temperature differences between the heat source and the heat sink are no longer limited by the constraint of the density of the working fluid leaving the condenser having to be the same as that leaving the evaporator. However, excessively large pressure differences generate other technological problems; thus, using the same extreme heat source and heat sink (275° C. and 40° C.), it is preferable for two machines to be thermally cascaded rather than to have a single machine operating over such a large pressure difference.
The thermal cascading (
Steps j→b and f→b of these two cycles cause additional irreversibilities, but the efficiencies of the two cycles nevertheless remain very satisfactory (in comparison with the Carnot efficiency): η1=27.47% for the cycle with R718 and 12=10.82% for the cycle with R600.
The overall efficiency of the thermally cascaded combination (
η=(W1+W2)/Qhi=η1+η2(1−η1)
i.e. η=35.32% (82% of the efficiency of the Carnot machine operating between the same extreme temperatures).
Compared with the previous example, for quite a small degradation in the efficiency (−3.78%), the simplification of the machine is relatively substantial: two combined machines instead of three, and most particularly those of the 1st type which are simpler than those of the 2nd type.
The intended objective in Example 3 is the heating of a dwelling by low-temperature emitters (radiators or underfloor heating). A modified receiving Carnot machine operating between 5 and 50° C. is very suitable for this application (
The two possible options, that the machines of the 2nd type or the machines of the 1st type constitute, using R600 as working fluid, are compared.
With a modified receiving Carnot machine of the 2nd type, the cycle described is the 1-2-3→4′-9-1 cycle shown in
The coefficient of amplification of this modified receiving Carnot machine describing this cycle is:
COA=Qhi/W=7.18.
This COA is virtually the same as that of the Carnot machine operating between the same extreme temperatures since the irreversibility caused by the 9→1 superheating is very small.
However, the machine of the 2nd type requires the chamber ABCD and the associated connections, incurring a cost and involving more complex management of the cycle. With a modified receiving Carnot machine of the 1st type, the cycle described is the 1-2-3-4-9-1 cycle shown in
The intended objective in Example 4 is to cool a dwelling in summer.
A modified receiving Carnot machine of the 1st type operating between 15 and 40° C. is very suitable for this application (
COP=Qlo/W=10.33, i.e. 90% of the COP of the Carnot machine and in particular much better than the COP values of current MVC machines operating between the same extreme temperatures.
The intended objective in Example 5 is low-temperature refrigeration (for freezing purposes). Even though the temperature difference between the heat source and heat sink is not limited by any constraint on the densities of the working fluid being equal, it is preferable for there not to be too high a pressure difference in the machine as this generates other technological problems. Thus with the cold source at −30° C. and the hot sink at 40° C., it is preferable for two machines to be thermally cascaded rather than providing a single machine operating over such a large temperature difference. The thermal cascading (see
The overall coefficient of performance of the thermally cascaded combination of these two modified receiving Carnot machines of the 1st type is:
COP=Qlo/(W1+W2)=1/[1/COP2+(1+1/COP2)/COA1].
This gives COP=2.85, i.e. 82% of the COP of the Carnot machine and above all much better than the COP values of current two-stage MVC machines operating between the same extreme temperatures.
The intended objective in Example 6 (
The coefficient of performance of this combination (
The intended objectives in Example 7 (
For these practical objectives, a first machine—the modified driving Carnot machine of the 1st type using the working fluid R718, which describes the l-m-g-n-1 cycle shown in FIG. 16—is coupled to a second machine, the modified receiving Carnot machine of the 1st type described in Example 3.
The efficiency η1 of the first machine is 25.34% (i.e. 91% of the Carnot efficiency), this being much higher than the current efficiency of photovoltaic solar collectors.
Although the electricity is not recovered for the receiving machine (
COA=COP+1=COP2×η1+1,
giving, respectively, COA=2.28 (84% of the Carnot COA) and COP=1.28 (74% of the Carnot COA).
The intended objective in Example 8 (
This thermotransformation objective between 85 and 120° C. (capable of generating vapor at 2 bar) may be carried out by mechanically coupling a first machine, namely the modified receiving Carnot machine of the 1st type, using R718, operating between 85 and 120° C. and describing the 1-2-3-4-1 cycle shown in
The coefficient of performance COP1 of the first (receiving) machine is 9.14 (89% of the COP of the dithermal Carnot machine). It should be noted that with water as working fluid, the steam at the end of the isentropic compression step is highly superheated (T2=208° C.>>120° C.).
The overall coefficient of performance of the combination of the two machines (
COP=Qhi/(Qm1+Qm2)=(COP1+1)/(COP1+1/η2),
giving, with these source and sink temperatures: COP=55.2% (89% of the COP of the trithermal Carnot machine).
The various examples described above confirm that one and the same working fluid may be used as driving fluid or as receiving fluid, depending on the plant and the intended objective.
The fluid n-butane (R600) describes a driving cycle of the 1st type in Example 2 (
| Number | Date | Country | Kind |
|---|---|---|---|
| 0801786 | Apr 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2009/000365 | 3/30/2009 | WO | 00 | 2/14/2011 |
