RU2162161C2 - Low-temperature heat energy-to-mechanical work conversion method - Google Patents

Abstract

FIELD: power engineering. SUBSTANCE: method includes expansion of subsonic operating medium flow with supply of heat from external source of energy at higher temperature level, expansion of flow with taking off of mechanical work, and compression of operating medium by changing flow velocity and exposure to action of energy from additional energy source at lower temperature level. When flow is acted upon by energy from additional source, additional energy or agent is supplied to operating medium flow. Method can be used at operating medium temperatures lower than those ambient medium. EFFECT: increased efficiency of heat-mechanical conversions, enlarged sphere of application. 8 cl, 6 dwg

Description

 The invention relates to a power system, in particular to methods using a working medium in a gaseous or liquid phase to obtain mechanical energy from the heat of an external source, preferably a low-temperature source.

 Widely known are methods of converting thermal energy into operation in steam turbine units operating on the Rankine cycle. In these methods, the working fluid is compressed, heated from an external heat source to form steam, the steam is expanded to produce mechanical energy (work), the steam is cooled and condensed to a liquid state during the thermal interaction of steam with an additional heat source, which is used as the surrounding space [1].

 There is a method of converting heat into work [2], in which mechanical energy is generated by expanding a two-phase working medium in the turbine (a mixture of steam and liquid), and to close the cycle, the working medium after the turbine is divided into steam and liquid components, the first of which is cooled and condensed during thermal contact with an additional source of energy, the environment, and the pressure of the total flow of the working medium is increased by mixing accelerated to supersonic speeds the flow of steam of the working medium and the flow of its heated idkogo component.

 The known method [3] converts the thermal energy of an external heat source into mechanical work, which allows to increase the efficiency of the thermal unit to values close to unity, i.e. until the complete conversion of the heat of the working medium into mechanical work. In this method, which includes heating the working medium, expanding it to obtain mechanical work and compressing, such a high efficiency is achieved by dividing the working medium in a centrifugal field of a supersonic swirling flow into “hot” and “cold” components, the first of which is used for heat recovery and reuse expansion, and the second before heating is pre-compressed in the compressor.

 A known method of converting the thermal energy of an external heat source into mechanical work, selected as an analogue closest to the proposed invention by the totality of features (prototype), which consists in obtaining mechanical work in an unpressor cycle of a closed gas turbine plant [4]. It consists of the following. In a heated subsonic channel, the gas (working medium) is accelerated to the speed of sound. The statistical gas temperature is maintained constant by increasing the flow rate with a change in the channel area. Then the gas enters the wheel of the active turbine, where it expands with decreasing speed and does the job. After the turbine, the gas flow is expanded in a supersonic thermally insulated nozzle. Then, to close the cycle, gas is supplied to a supersonic cooled diffuser, in which the gas pressure rises (gas is compressed) at a constant temperature and flow enthalpy due to a change in the nozzle geometry and transfer of a part of the flow thermal energy to an additional energy source - a refrigerator. After this, the cycle is closed by braking the gas (working medium) in a heat-insulated nozzle with the conversion of the kinetic energy of the flow into the potential energy of the working medium.

 The known method has the effectiveness of a Carnot cycle, and thanks to the regulation of the flow rate of the working medium as an additional process parameter in this cycle, it is possible to do without using a compressor to close the cycle.

The disadvantage of this method is the impossibility of a sufficiently effective conversion of heat into work, because and in this method, a significant part of the supplied heat energy is lost in the refrigerator (an additional source of energy). Especially large losses of thermal energy (up to 80%) occur in this case when using external heat sources with a temperature of 100-150 ^o C.

 The aim of the invention is to increase the efficiency of a thermal unit due to the complete conversion of the heat of the working medium received from an external source into mechanical work.

 This goal is achieved by the fact that the method of converting the thermal energy of an external heat source into mechanical work, including heating (increasing energy) of a subsonic flow of a working medium by transferring heat from an external source to it, expanding the flow with performing mechanical work, changing the speed of the flow by adjusting the geometry of the nozzles or channels and energy exchange with an additional energy source, as well as compression of the working medium by braking the flow with the conversion of its kinetic energy of motion potential energy exchange between the flow of the working medium and an additional source of energy, carried out by changing the flow rate, is carried out with the flow of energy and / or mass (substance) to the working medium.

 The technical result is enhanced if, after expanding the flow, its speed is increased to values greater than the speed of sound.

In addition, the features of the proposed method, leading to the achievement of a technical result, are:
- use as an additional source of energy a source of mechanical work (instead of a heat source in the prototype);
- the use as part of an additional energy source of a part of the total flow of the working medium of increased density, which is preliminarily isolated from the general flow of the working medium after its expansion using gravitational and / or centrifugal fields, and then injected (injected) into the accelerated flow of another part of the working medium of low density ;
- the use as a working medium of a mixture of components with different boiling points;
- use as working media mixtures of liquid and gaseous substances;
- conducting multi-stage heating and expansion of the working environment;
- conducting immediately before heating the working environment of its additional compression.

 In the claimed method, in contrast to the known method, the process of compressing the working medium in a supersonic diffuser, carried out by reducing the speed of the flow with the selection of thermal energy, is replaced by the process of compressing the working medium in a channel of variable cross-section with the supply of matter or energy, for example mechanical.

 In the proposed method for closing the cycle, the compression of the flow of the working medium when performing work is carried out with a simultaneous and energy equivalent increase in the speed of flow. In this case, the process of increasing the pressure of the working medium can occur at constant values of enthalpy and flow temperature.

This position is explained using the expression of the law of conservation of energy for the flow
ΔH = Q + A + W, (1)
determining the dependence of the change in the enthalpy of the substance DN both on the values of heat fluxes Q and work A, and on the value of the kinetic energy of the medium W. From (1) it follows that for the traditional static ideal Carnot gas cycle proceeding infinitely slowly (W = 0), compression or expansion of the working medium in isothermal (T = const) and isoenthalpic (H = const) processes is possible only with the simultaneous exchange of the working medium and energy sources with equivalent amounts of thermal Q, and mechanical A _i energy at each temperature level T _i i.e. at constant temperature, the equality
ΔH _i = Q _i + A _i = 0 and ± Q _i = ∓ A _i .
In the known method (prototype) isothermal compression (and expansion) of the working medium is carried out only with one form of interaction of the working medium with energy sources, namely, thermal interaction, while there is no exchange of mechanical energy (work) (A = 0) (i.e. uncompressed cycle). The constancy of the enthalpy and temperature of the working medium in this case is achieved by taking away heat energy from the flow (cooling the flow) and decreasing its speed during compression of the working medium and heating the flow with increasing its speed during expansion, or otherwise, at A = 0, expression (1) is transformed to
ΔH _i = Q _i + W _i = 0 and ± Q _i = ∓W _i .
In the claimed method, it is proposed (in the case of a gas cycle) to carry out isothermal (and isoenthalpic) compression of the working medium after the turbine when transferring energy to the flow, for example, by performing mechanical work, and increasing its speed by changing the geometric dimensions of the nozzles (channels), i.e. . in this case
ΔH _i = A _i + W _i = 0 and -A _i = W _i .
Features of the energy balance of the proposed method are
- the possibility of almost complete conversion of thermal energy of the working medium Q _out received from an external heat source into mechanical energy A _out transmitted to an external consumer;
- the use of two sources of thermal energy for transmitting to the expanding flow of the working medium heat energy Q ₁ at the upper temperature level T _{1 of the} cycle, one of which is an external heat source of heat with a heat flux Q _out , and the other is an internal heat source Q _in , distinguished by conversion part of the flow velocity head into the friction work (internal work) and equal to the amount of energy A ₂ expended during compression of the flow at the lower temperature level T _{2 of the} cycle (Q ₁ = Q _out + Q _in = Q _out + A ₂ ).

формально похожим на температурную зависимость коэффициента полезного действия цикла Карно.In relation to an ideal gas cycle with isothermal compression and expansion processes, the quantitative relations between the work A _out transferred to an external consumer, the thermal energy Q _out received by the stream from an external heat source at a temperature T ₁ , the amount of energy A ₂ received by the stream under compression at a temperature level T ₂ and the total amount of heat Q ₁ = (Q _out + A ₂ ), supplied to the flow at a temperature T ₁ , is determined by the expression

formally similar to the temperature dependence of the efficiency of the Carnot cycle.

However, unlike the Carnot cycle and the prototype method, in the inventive method, the amount of consumed thermal energy Q _{out is} almost completely converted into the extracted mechanical energy A _out , i.e. A _out = Q _out regardless of the temperature or the value of the temperature range ΔT = T ₁ -T ₂ . The temperature range determines only the productivity of the cycle, i.e., the quantitative values of the flows Q _out and A _out . Moreover, as follows from (2), the efficiency of the cycle increases with an increase in its temperature range ΔT = T ₁ -T ₂ , which, as in the prototype method, can be achieved by increasing the flow rate to supersonic values.

The total amount of mechanical energy A ₁ generated in such a cycle is determined, as usual, by the algebraic sum of the energy flows of the upper and lower temperature (energy) levels of the cycle, i.e.

A ₁ = Q ₁ = Q _out + A ₂ > A _out .

A part of the generated mechanical energy equal to A ₂ is used as an internal, regenerative additional energy source, and the rest of A _out = Q _{out is} allocated to an external consumer.

 According to the degree of conversion of the heat flux of an external source into mechanical energy, the claimed method, significantly superior to the Carnot cycle and, accordingly, the prototype method, is actually equivalent to the similar method [3] with the regeneration of thermal energy of a low-energy working medium flow using centrifugal fields. However, in contrast to the known analogue [3], the proposed method may be easier to implement due to the lack of a multi-stage regeneration system and allows the use of low-temperature heat sources due to the absence of restrictions on the temperature range of the cycle to the ambient level.

 The inventive method can also be implemented using working media in the form of complex multiphase vapor-gas-liquid mixtures. In this case, an increase in the pressure of the working mixture after the turbine, necessary to close the cycle, is carried out using a denser liquid phase as an additional source of energy and substance. In this case, an increase in the pressure of the working medium occurs due to a decrease in its internal energy. The process is as follows. The flow of the working medium after the turbine is separated in the separator into liquid (more dense) and gaseous (less dense) components (phases). The gaseous component of the total flow is accelerated to supersonic speeds using a Laval nozzle. In this case, the energy of the thermal (chaotic) motion of gas molecules is significantly converted into the kinetic energy of the flow, and the internal energy (and the statistical temperature of the medium) decreases. Next, a stream of liquid separated in the separator is introduced into the supersonic gas stream. When the flows are mixed, the isoentropic (adiabatic) index of the mixture changes, the local speed of sound decreases, and a shock wave occurs with the conversion of the kinetic energy of the stream into potential compression energy, accompanied by condensation of the vapor phase. In their physical sense, the effects arising in this case are inverse to the well-known Joule-Thomson throttling process.

 In addition, in the proposed method, along with the above processes (operations), it is also possible to use traditional techniques to further improve the efficiency of the cycle, for example, additional compression of the working medium in the compressor immediately before its heating, and the use of multi-stage heating and expansion of the working environment.

 In FIG. 1, 2 and 3 are depicted in T-S coordinates diagrams of ideal gas thermotechnical cycles: respectively, the Carnot machine (Fig. 1), an aggregate operating according to the prototype method (Fig. 2) and an aggregate operating according to the claimed method (Fig. 3). In FIG. 4 shows in T-S coordinates a diagram of the ideal heat engineering cycle of a unit operating according to the claimed method using a vapor-liquid mixture as a working medium. In FIG. 5 and 6 are schematic diagrams of installations for implementing the inventive method, respectively, in the embodiment of the use of a gaseous substance and a vapor-liquid mixture as a working medium.

The diagrams in FIG. 1, 2 and 3 explain the features of the proposed method compared to the Carnot cycle (Fig. 1) and the prototype method (Fig. 2). Here, all these cycles are depicted in the form of identical rectangles 1-2-3-4. The horizontal sections of diagrams 1-2 and 3-4 show, respectively, isothermal (T = const) and isoenthalic (H = const) processes of expansion and contraction of an ideal gas. These processes occur during the exchange of energy of the working medium (gas) with an external energy source (level T ₁ and H ₁ ) and an additional energy source (level T ₂ and H ₂ ). In the case of a quasistatic Carnot cycle, for which the velocity and kinetic energy of the flow are equal to zero, the processes of isoenthalpic expansion and compression of the gas proceed in the presence of both thermal and equivalent mechanical interaction of the working medium and energy sources at each level T _i and H _i , i.e. . if thermal energy is supplied (removed), then at the same time mechanical energy of equal magnitude is removed (supplied) and vice versa.

 In the prototype method (Fig. 2), taking into account the influence of speed, ideal gas processes of isoenthalpic compression (3-4, Fig. 2) and expansion (1-2, Fig. 2) are carried out only with one (namely thermal) form of interaction with energy sources, cooling the supersonic stream during compression and heating the subsonic stream during expansion. Moreover, the obligatory mechanical interaction characteristic of the Carnot cycle is absent in these processes.

In the proposed method, as shown in FIG. 3, the process of iso-enthalpic compression 3-4 is carried out with the transfer of additional energy to the stream, the shape of which corresponds to the compression of the stream in subsonic or supersonic areas of motion, for example, mechanical work. In this case, the nozzle geometry (flow cross section) is controlled (selected) so that the velocity (and kinetic energy) of the flow and the pressure of the working medium increase, and the enthalpy (temperature) remains constant. A feature of the process is 1-2 of FIG. 3 of the isentalpic expansion of the flow is that only part of the thermal energy Q _out necessary for expansion (heating) is supplied from the external heat source, and another part of this energy Q _{in is} extracted from the internal source (regeneratively) by converting the excess velocity head of the flow into its thermal energy by friction (throttling).

 Processes 2-3 and 4-1 in the diagrams of FIG. 1, 2 and 3 characterize the transition of the working medium (gas) from one isentalpic level to another. Moreover, if in the Carnot cycle such a transition is accompanied by mandatory selection (during expansion) and consumption (during compression) of mechanical energy, then in the methods of the prototype and the proposed method, the compression of the working medium (processes 4-1) can be carried out in heat and mechanically isolated nozzles the conversion of kinetic energy of the flow into potential, leading to an increase in the enthalpy and temperature of the substance with a constant total energy of the flow of the working medium.

 In addition, in the inventive method, it is possible to heat the working medium stream not only isothermally, but also under other conditions, for example, isobarically P = const, which is depicted by the curve 1-2 'of FIG. 3. Segments 1-5 of the isotherm 1-2 and 1-5 'of the isobar 1-2' of FIG. 3 characterize the area of internal (regenerative) heating of the flow of the working medium.

FIG. 4 illustrates a variant of the proposed method using a vapor-liquid mixture as a working medium. In this case, the precompressed working medium in the liquid state is expanded and evaporated in an isobaric process 1-2 with the supply of heat Q _out from an external heat source. The resulting wet or dry steam is further expanded by adiabat 2-3 in the turbine (expander) with selection of external work. In the process of this expansion, the temperature of the working fluid stream decreases and a condensed (liquid) phase forms. After the turbine, wet steam is separated in a separator, for example, using gravitational or centrifugal fields, into steam and liquid components. This process in FIG. 4 is represented by a dashed line 7-3-4-, points 7 and 4 of which are located respectively on the branches of the liquid and the vapor of the saturation curve. Then the vapor component of the flow of the working medium is accelerated in the Laval nozzle (process 4-5 in Fig. 4) to supersonic speeds, for example, to Mach numbers M = 2-5. With this increase in speed, the energy of the thermal motion of the molecules of the medium is converted into kinetic energy of the flow, the temperature and internal energy of the working medium decrease. The process 5-6 of the diagram of FIG. 4 illustrates the compression of a working medium by converting the kinetic energy of a stream into its potential energy. This transformation is carried out by braking the supersonic steam stream by introducing a liquid (denser) component into it, leading to a sharp decrease in the local sound velocity in the vapor-liquid mixture and the occurrence of shock waves with a multiple increase in the pressure of the working medium. More detailed information on shock waves is given in [2, 5]. In addition, in FIG. 4 shows the following processes:
6-1 - additional compression of the vapor-liquid working medium in the compressor with the completion of mechanical work equal to A _in ;
7-8 and 8-6 - initial cooling and subsequent heating of the liquid component of the working medium when it is mixed with a supersonic steam stream.

In this embodiment of the proposed method, a perfect conversion of the heat flux Q _out received by the working medium from an external heat source to the work flow A _out transmitted to an external load is also typical for an ideal cycle, i.e. Q _out = A _out . The total amount of mechanical energy A generated by the turbine is determined by the energy balance A = Q _out + A _in .

 The inventive method of converting thermal energy into mechanical energy can be carried out using a closed gas turbine installation, the scheme of which is presented in FIG. 5. It consists of a heated nozzle 1, a turbine 2 with an electric generator, a supersonic thermally insulated nozzle 3, a diffuser 4 provided with a mechanism (impeller) 5 for compressing the flow, and a thermally insulated diffuser 6. The arrows in FIG. 5 shows the flows of thermal and mechanical energy.

 The method can also be implemented using another device, presented in the form of a diagram of FIG. 6. This device is a closed vapor condensation cycle, including a heater 1, a turbine with an electric generator 2, a separator 3, a Laval nozzle 4, a pump 5, a mixer 6, and a compressor 7 driven by a turbine 2.

 From the diagrams shown in FIG. 5 and 6 it follows that the installation for implementing the proposed method is relatively simple and can be performed with the current level of technology.

 The proposed method in comparison with the prototype has several advantages: 1) increases the efficiency of thermomechanical transformations to values close to unity; 2) allows the use of low-temperature sources of thermal energy and a decrease in the temperature range of the cycle beyond the limits of the environment.

 Achieving positive effects in the proposed method becomes possible due to the use of a special system for the regeneration of thermal energy of the working environment, expanded after mechanical work. This regeneration system is carried out initially by converting the internal (thermal) energy of the working medium into kinetic energy of the flow at relatively low temperatures, and then converting the kinetic energy into other forms of energy (potential, thermal and mechanical) at elevated temperatures. Such regeneration practically does not require external energy consumption and allows to achieve a high degree of conversion of the thermal energy of an external source into mechanical energy.

Used sources
1. Heat power engineering / A.B. Baskakov, B.V. Berg, O.K. Witt et al. - M .: Energoizdat, 1991 .-- 224 p., Ill. 61.

 2. Fisenko V.V. The compressibility of the coolant and the efficiency of the circuits of nuclear power plants. - M: Energoatomizdat, 1987, p. 108.

 3. A method of converting thermal energy from an external heat source into mechanical work. Pat. 2078253 RU MKI 6 F 03 G 7/06. / Smirnov L.N. Claim 07/28/94; Publ. 04/27/97.

 4. Leontiev A.I., Schmidt K.L. Uncompressed ideal cycle of a closed gas turbine plant // Bulletin of the Russian Academy of Sciences. Energy - 1997. - N 3. - S. 132-141.

 5. Fisenko VV Critical two-phase flows. - M.: Atomizdat, 1978, 160 p.

Claims (8)

 1. A method of converting low-temperature thermal energy into mechanical work, including expanding the flow of the working medium by supplying heat from an external energy source, expanding the flow with the selection of mechanical work, compressing the working medium by changing the flow rate and exposing it to an additional energy source, characterized in that when the flow is affected by an additional energy source, additional energy and / or substance is supplied to the flow of the working medium.  2. The method according to claim 1, characterized in that after expanding the flow of the working medium with the selection of work, the flow rate is increased to values of high speed of sound.  3. The method according to PP.1 and 2, characterized in that when exposed to an additional energy source, the stream is compressed, performing mechanical work.  4. The method according to claims 1 and 2, characterized in that as an additional source of energy use part of the total flow of the working medium of high density, which is preliminarily isolated using gravitational and / or centrifugal fields from the total flow of the working medium after its expansion, and then return back after accelerating another part of the working environment.  5. The method according to claims 1 to 4, characterized in that a mixture of components with different critical temperatures is used as the working medium.  6. The method according to claims 1 to 5, characterized in that multiphase mixtures, for example mixtures of liquid and gaseous substances, are used as the working medium.  7. The method according to PP. 1 to 6, characterized in that immediately before heating, the working medium is additionally compressed.  8. The method according to claims 1 to 7, characterized in that multistage heating and expansion of the working medium are carried out.

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