ES2439619A2 - Device for the generation of mechanical energy according to a regenerative and balanced brayton-rankine hybrid cycle and procedure of use (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Device for power generation according to a regenerative and balanced brayton-rankine hybrid cycle and its method of use, comprising a heat supply equipment (8), an expander (1), a regenerator (2) and a compressor (6) configuring a brayton cycle; a capacitor (11) and a drive pump (14) forming a rankine cycle; so that by the heat supply equipment and the regenerator, in its secondary circuit, cooling circulates a mass flow of fluid that is divided in two at its output, one called the main flow that follows the rankine cycle and another called flow balancer that runs through the brayton cycle, regenerating the liquid of the main flow after passing through the condenser and the pump, and that joins with the balancer after passing through the compressor, to complete the cycles, selecting the amount of flow balancer so that the regenerator is thermally balanced. (Machine-translation by Google Translate, not legally binding)

Description

Device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle and use procedure.

Technical sector

The invention falls within the field of thermodynamic cycles for the production of mechanical power, in turn convertible into electrical energy.

Due to the evolution of the working fluid and the components used, the proposed cycle is considered as a hybrid between the Brayton type cycles and the Rankine type cycles. Its use is of relevant interest in the energy industry, particularly when the calorific focus is of reduced temperatures compared to usual in chemical combustion plants. This makes it especially applicable to solar thermal or geothermal energy. It may also be applicable as a low temperature cycle that collects excess heat from a high temperature cycle.

Technical problem to be solved and Background of the invention

One of the objectives of Thermodynamics has been, since its inception, the study of machines for the production of mechanical energy from thermal energy. Not surprisingly, the first thermodynamic bases were established with the study of steam engines, which constituted the first successful thermal engines used at the industrial level.

Since then and until now, numerous thermodynamic cycles have been proposed that have been the core of the various thermal engines that are known. All of them have in common the following characteristics: a fluid volume is evolved cyclically by one or more mechanical equipment, thus the working fluid undergoes various thermodynamic processes among which, inevitably, at least one heat input process is found from a thermal source and at least one process of heat transfer to a thermal sink, which is usually ultimately the environment.

The fields of application of the most common thermal motors are the propulsion of vehicles and the generation of electrical energy. Although there are predominant cycles in each application, devices or engines that materialize Otto, Diesel, Joule / Brayton and Rankine cycles have been used with great success. Minority other devices have been used that follow other types of cycles, such as Stirling, Ericsson and Kalina among others.

In the case of electricity generation, the use of combined cycle facilities is very frequent, which effectively combine two cycles, one of high temperature Brayton type that works with air and another of low temperature Rankine type that works with water. Therefore, these are two different cycles working with different working fluids and thermally coupled, by a heat recovery boiler.

One of the main factors to measure the performance of a cycle is thermodynamic performance, which is the ratio between the mechanical energy produced divided by the thermal energy supplied from the heat source.

It is well known that the maximum thermal efficiency attainable by a thermodynamic power cycle is limited. This maximum achievable performance depends on the hot and cold focus temperatures, and coincides with the performance of an absolutely theoretical Carnot cycle, working between said hot or maximum focus (Tmax) and cold or minimum focus (Tmin) temperatures, according to the expression:

being:

-

ηCN the Carnot cycle performance;

-

Qf and Qc the heat ceded to the thermal sink and that received from the thermal source respectively;

-

Tf and Tc the absolute temperatures of the sump and source, which are the minimum and maximum attainable in the cycle;

-

Δsf and Δsc the absolute values of the entropy variations during the transfer and the heat input;

-

and CN the ratio between the maximum and minimum temperatures, hereinafter Carnot factor.

The dependence of the maximum yield achievable with the ratio of maximum and minimum temperatures is also followed by the yields of the cycles materialized with real machines. This is the reason why, in various applications, historically the highest mechanically permissible working temperatures have been sought by materials for heat input and lower heat transfer temperatures, limited by ambient or cooling medium temperature .

The natural application of the invention is the generation of mechanical energy for the production of electricity. In this field, Brayton cycles are absolute dominant today, thanks to gas turbines, and Rankine cycles based on steam turbines.

Brayton-type cycles reach a very high maximum temperature, exceeding 1700 K, although the average temperature is lower because the heat input starts from the end of compression. The minimum temperature is very low, since they take ambient air, but they also give heat to a medium high temperature because the exhaust gases are expelled in volume at 800 or 900 K. With this technology yields greater than 40% are obtained .

The Rankine type cycles reach more moderate maximum temperatures, up to 850 K, but yield heat at an average temperature very close to the ambient temperature since condensation occurs at a very low constant temperature. In this case, the yields may exceed 50% in some cases. The success of the combined cycles of gas and steam turbines, which are individual cycles with different fluids but thermally coupled, is that the heat transfer of the Brayton cycle is used to thermally feed the Rankine cycle, so that it is done use of the high heat input temperature of the Brayton cycles and the low heat transfer temperature of the Rankine cycle, increasing the Carnot factor. The combined cycle performance may exceed 60%.

However, the Carnot factor is not the only determining or limiting factor of the performance achieved by a thermodynamic cycle. By developing the previous expression you can reach the following:

where it is clear that there are several reasons why Carnot's performance cannot be achieved: when the average heat input temperature is not the maximum or the minimum assignment temperature; or when there are irreversibilities or entropy generation in any of the processes, which makes the absolute value of the entropy variation in the heat transfer (numerator) greater than during the heat input (denominator). In addition to the ratio between maximum and minimum temperatures or Carnot factor, three coefficients can be defined that can be called respectively average heat input temperature factor (Fmc), average heat transfer temperature factor (Fmf) and irreversibility factor (FΔS), which measure the effect of not being able to always be working in extreme temperatures and the effect of irreversibilities. All three have a lower value than the unit in general and unit for a Carnot cycle.

At present, mainly due to the desire to use renewable sources of energy such as solar or geothermal, there are applications where there is a moderate heat input temperature, since it is difficult to reach the usual temperatures after a combustion process. In these cases, the Carnot factor is significantly reduced and the achievable yields are much lower than those previously exposed. For example, with regenerative Rankine cycles with organic fluids it is difficult to exceed 25-27% for a Carnot factor less than

2.

The low performance, indeed, is essentially due to the low Carnot factor. But, for this reason, it is the other factors that are important when trying to improve the performance of cycles and thermal motors fed at low temperatures.

Specifically, Brayton-type cycles suffer from a low average heat input temperature - compared to the maximum - and a high average heat transfer temperature - compared to the minimum -, since the contribution and transfer of heat Heat is made at constant pressure and by sensible heat, so the temperature varies almost proportionally with the transfer and contribution of heat.

This problem can be partially alleviated if a regenerative exchanger is introduced, which uses part of the exhaust heat to preheat the fluid before the main heat supply with the thermal source. This solution improves the average temperature and heat transfer factors, and the performance increases. Another solution, which can be combined with the previous one, is the inclusion of stepped compressions with intermediate cooling and stepped expansion with intermediate reheating, which further improve these factors and allow, in addition, greater regeneration. This solution is close to the regenerative Ericsson cycle, which theoretically achieves Carnot's performance. However, the average temperature factors remain far from the unit as the contribution and transfer of thermal energy by sensible heat. In addition, if the heat of regeneration is very high, the irreversibility factor begins to gain relevance.

The performance of Brayton cycles fed with low temperature thermal source is usually very low. Solutions have been proposed such as the one described in the Spanish patent application, P20120034, whose applicants are the same as the present one; ES 0390877 A1 which describes improvements for compression cooling and for the regeneration phase; or US 7401475 B2 and WO 0006876 A1, with which almost isothermal compressions and expansions are proposed.

Rankine type cycles employ condensable fluids, meaning that they make use of the phase change in their processes. In this way, the heat transfer to the sump is done at a very low and constant temperature, due to the phase change, and the average heat transfer temperature factor is very close to one. In the heat input there is also a phase change, so during part of the heating there may be a constant temperature (if the pressure is lower than the critical point), although it is usually far from the maximum. For this reason and because much of the energy is obtained by sensible heat (before and after boiling if it exists and always if the pressure is supercritical), the average heat input temperature factor is less than unit.

To increase the average heat input temperature, the cycle is also regenerative in this case. The way to regenerate depends on the type of fluid. In this sense, it is possible to classify fluids according to three types:

-

negative or humid: understanding as such those fluids for which the entropy of saturated steam always decreases with saturation pressure;

-

isentropics: understood as such in the current state of the art to those whose saturated steam entropy practically does not vary with pressure except in the vicinity of the critical point and except at pressures several orders of magnitude below that of the critical point, which decreases;

-

positive or dry: understanding as such those whose entropy of saturated steam grows with pressure except in the vicinity of the critical point and except at pressures several orders of magnitude below that of the critical point, which decreases.

The growth or decrease of the entropy of saturated steam of the different fluids is very directly related to the value of the R / Cp ratio of the gas, R being the constant characteristic of the gas and Cp the specific heat at constant pressure under ideal gas conditions, behavior achievable at very low pressure and preferably high temperature, so that for this purpose the Cp of the fluid can be taken at one thousandth of its critical pressure and twice its critical temperature. Characterizing the fluids according to this parameter, there is the equivalence that those whose R / Cp value is greater than 1/12 in the above condition are negative fluids; approximately isentropic when the R / Cp value is between 1/16 and 1/12 in the above condition; and positive when the R / Cp parameter is less than 1/16.

For negative fluids, the thermodynamic state of the fluid at the outlet of the turbine or the expander is wet steam. As the temperature is very low, exactly the minimum, the regeneration cannot be carried out with this thermal state and is therefore materialized with a partial extraction in the turbine or the expander, at a higher temperature, and its mission is to preheat the liquid before evaporation.

For isentropic or positive fluids, the thermodynamic state of the fluid at the outlet of the turbine or the expander is dry steam, at a higher temperature than the minimum. The regeneration is carried out in a similar way to the Brayton cycles, taking advantage of the sensible heat of the steam exiting the expander. The objective is to preheat the liquid before the phase change.

The regenerative Rankine cycles, subcritical or supercritical, have been very studied and are being used frequently in low temperature applications. For example, EP 1801364 A1 proposes a device for materializing transcritical cycles; ES 237 487 4 T3 describes a device for the use of waste heat by means of Rankine cycles of organic fluids (ORC). On the other hand, Chen et al. in "A review of thermodynamic cycles and working fluids for the conversion of low-grade heat" (Renewable and Sustainable Energy Reviews, 14 (2010), pp. 3059-3067, Elsevier) review Rankine, subcritical and supercritical cycles, with different organic work fluids.

Already regenerate in one way or another, with regeneration the average heat input temperature factor is improved and performance improves. However, the irreversibility factor worsens markedly due to the high irreversibilities in regeneration, which are greater than in Brayton cycles. In fact, if the regeneration is carried out with a mixing exchanger, the process is highly irreversible due to the mixing. If it is with a contact exchanger there are two possibilities: the condensation of a certain fraction of steam extracted from the turbine or the expander to heat the liquid (for negative fluids) or the loss of sensible heat of the steam to heat the liquid (for fluids isentropic or positive). In both cases, there are high temperature differences between the two currents, hot and cold, that cause irreversibility. In one case due to the constant temperature condensation of one of the currents and in the other due to the thermal imbalance of the regenerator as the Cp of the steam is significantly lower than that of the liquid.

The yields achieved are higher than with the Brayton cycles but are far from satisfactory values.

In summary, it can be verified that there is no cycle in the current state of the art that simultaneously presents the three factors close to one, of irreversibilities (FΔS). of average heat input (FTf) and heat transfer (FTc) temperatures. This fact, together with the fact that applications with low temperature sources imply a low Carnot factor, results in low thermal efficiency.

The object of the invention is, therefore, to jointly improve the irreversibility and average temperature factors, in order to obtain satisfactory thermal yields despite the low Carnot factor. The goal is achieved through a device that hybridizes two Rankine and Brayton thermodynamic cycles, sharing fluid and in a way that has never been proposed. In this regard, documents WO 2009125103 A1 and US 8006496 B2 are helpful, in which an interesting review of the industrial property literature about thermodynamic cycles and the devices for materializing them is carried out. It is also possible to highlight patent WO 2007037599 A2 in which a device is proposed to work interchangeably according to Brayton, Rankine cycles

or Stirling, but independently and not jointly or hybridized, and for purposes other than those of the present invention. Finally, US 6880344 combines a Brayton cycle with a Rankine ORC but working with different fluids, as a conventional combined cycle, completely different from the device now proposed.

Description of the invention

As just mentioned, the objective of the invention is to jointly improve the irreversibility and average temperature factors in order to obtain satisfactory thermal yields.

The value of the average heat transfer temperature factor is left close to the unit by heat transfer to the cold source with a condensable fluid. The solution adopted to improve the average heat input temperature factor is to introduce the regeneration of the fluid and, optionally, the two-stage expansion with intermediate reheating. Finally, to improve the irreversibility factor, even using regeneration, the regenerator will be contact and not mixing. Likewise and for the same reason, the cycle is designed so that the regenerator is thermally balanced and that both the heat transfer and the contribution are by sensible heat. Since the Cp of the subcritical vapor is less than that of the liquid, the mass flow of steam must necessarily be greater than that of the liquid.

To achieve the above objectives, an installation is implemented that includes:

-

a condensable working fluid, for which the pseudo-critical point of a supercritical isobar is defined as the point of highest specific heat at constant pressure, Cp, of the isobar, this value of Cp being infinite at the critical point, and descending its value as the pressure increases;

-

a gas phase work fluid expansion machine;

-

a heat exchanger, regenerative type, with two circuits, the primary one that receives heat and the secondary one that yields it;

-

a flow division element that divides the total inflow into two flows or streams, called main flow and balancing flow;

-

a machine for compression of the gas phase fluid whose axis can be mechanically or jointly attached to the expansion machine or not;

-

a condenser for the condensation of the fluid, cooled by a cooling fluid such as air, water or any other, whose temperature at the entrance of the equipment is marked by the environment and is called Tamb;

-

a drive pump to circulate and pressurize the fluid in the liquid phase leaving the condenser;

-

a main heat input device to the fluid thermally fed by a heating fluid that comes from a thermal source;

-

an element of union of flows;

-

and a transmitter or transformer element of the mechanical power produced such as an axis or an electric generator, among others.

The previous components are connected as follows:

-

the duct or outlet manifold of the main heat input equipment is connected to the inlet of the expansion machine;

-

the output of the expansion machine is connected to the regenerator by the input of its secondary circuit;

-

The output of the secondary circuit of the regenerator is connected to the flow division element, which divides the flow into two: the main flow and the balancer;

-

the main flow outlet of the flow division element is connected to the condenser inlet;

-

the output of the condenser is connected to the input to the drive pump;

-

the outlet of the impulse pump is connected to the regenerator by the input of its primary circuit;

-

the outflow of the balancing flow of the flow dividing element is connected to the input of the compression machine;

-

the output of the compression machine is connected to the output of the primary circuit of the regenerator in the flow joint element;

-

The output of the flow joint element is connected to the input or input manifold of the main heat input equipment.

There are two thermohydraulic circuits that make the working fluid evolve according to a thermodynamic cycle called the regenerative and balanced Rankine-Brayton hybrid cycle, in which the following processes take place:

-

the working fluid is heated in the gas phase and at supercritical pressure in the main heat supply equipment up to the temperature called Tmax, from the outlet of the flow joint element of the main and balancer flows;

-

immediately afterwards it expands in the expansion machine, in which it performs the mechanical work on the outside;

-

then it gives heat circulating through the secondary circuit of the regenerator and consequently cools;

-

and at the output of the secondary circuit of the regenerator, the flow is divided into two flows, the main and the balancer.

The balancing flow is directed to the compression machine, which is connected at its outlet with the main heat input equipment, thus closing a Brayton cycle, although not of constant flow along it.

The main flow is directed to the condenser, where it condenses at setpoint temperature Tmin, giving heat to the environment, the condensed liquid being subsequently pumped into the discharge pump. Subsequently, it is regeneratively heated in the regenerator's primary circuit to the regeneration temperature, Treg, with the heat given by all the gas that is cooled in the secondary circuit of said regenerator, and finally reaches the junction element with the equilibrium flow, configuring a Rankine cycle, although not constant flow.

In said connecting element said main flow is joined with the equilibrium flow that has been compressed in the gas phase, both entering as a single flow in the main heat supply equipment.

The essential thermodynamic specifications of the invention are:

-

the condensation temperature Tmin corresponds to a saturation state with a pressure Pmin lower than the critical pressure of the fluid and whose specific entropy values of liquid and saturated steam are called sliq and svap respectively and the specific enthalpies of liquid and saturated steam hliq and hvap respectively, said temperature Tmin being greater than the temperature of the external refrigerant of the condenser or ambient heat sink Tamb;

-

the absolute pressure in the pump drive, Pimp, is supercritical and is greater than the pressure of the isobar whose pseudo-critical point has a value of Cp three times greater than the Cp of the liquid measured under pump output conditions;

-

the absolute temperature of the main fluid at the primary regeneration circuit outlet, Treg, is equal to or greater than 10% at the absolute temperature corresponding to the thermodynamic state with Pimp pressure and svap specific entropy;

-

in the regenerator the rate of passage of heat capacity, or product of the specific average heat per mass flow rate, of each of the two currents equal to each other, so that it is balanced, specifying that the proportion of balancing mass flow rate is prescribed by

mequilibrador / mtolal = (1 - cp, secondary / cp, primary)

 the balancing mass flow being mechalancing, total total mass flow, cp, secondary the average value of the specific heat at constant pressure during the total flow cooling and cp, primary the specific heat at medium constant pressure during the heating of the main flow in the primary circuit, both in the regenerator, the main flow being determined with the equation

 mprincipal = mtotal - mequilibrador

 the main mass flow being primary;

-

the terminal temperature difference between the secondary and primary currents of the regenerator is set to a value greater than 0 K and less than 35 K. When the primary flow outlet temperature, Treg, the terminal temperature difference in the regenerator is defined It determines the temperature of the gas in the secondary circuit at the outlet of the expansion machine and the secondary input of the regenerator, this temperature being the sum of Treg plus the selected terminal difference. The entropy corresponding to that thermodynamic state is called smax because it is the maximum of the installation, and the entropy generation in the expansion is called Δs. The srnax -Δs difference is the entropy of the steam at the entrance to the turbine or expander, the pressure at this point being equal to that of the pump drive minus the pressure losses in the thermohydraulic circuit. Both magnitudes, entropy and pressure, uniquely determine the Tmax turbine inlet temperature, which is selected in the main heat input equipment.

In this way, with all the previous prescriptions, the thermodynamic states of all the entry and exit points of the equipment and, consequently, the enthalpies corresponding to each of them are determined.

As a mounting variant, when the working fluid has a value of R / Cp greater than 1/16, R being the characteristic constant of the gas and Cp the specific heat at constant pressure measured at twice the critical absolute temperature of the fluid and at a thousandth of its critical absolute pressure, the flow dividing element is integrated in the condenser itself, which has three ways to have:

-

a steam inlet or manifold prior to the condenser's own heat exchanger,

-

a steam collection or aspiration system at the top,

-

and the conventional condensate collection system at the bottom that collects said condensate or main flow and directs it to the discharge pump.

For any of the variants, it is also prescribed that the specific enthalpies of the equilibrium flow at the outlet of the compressor and the main flow at the output of the primary circuit of the regenerator differ by less than 2% from the value given by the product of R and the critical temperature in absolute value in coherent physical units.

As a mounting variant, the installation incorporates a rolling valve at the outlet of the primary regenerator circuit, which reduces the pressure of the main fluid flow after regeneration and before it is mixed with the balancing flow, for which the output of the primary circuit with the inlet of said rolling valve and the output of the rolling valve with the primary fluid inlet of the current joining element. Thus, the discharge pressure, Pimp, and the main heat input pressure, now called Pq, are independent. The degree of freedom generated is restricted because it is prescribed that the main heat input pressure, Pq, is less than the discharge pressure, Pimp, and greater than the geometric average of Pimp and Pmin.

Finally, the main heat input is by sensible heat. This implies that the average heat input temperature is practically the arithmetic average between the temperature of the heat input and the output temperature. This, similar to the Brayton cycles, can be improved by making the stepwise expansion with intermediate reheating. In this way the output temperature of the expander equipment is higher and the regeneration capacity is higher.

For this, it is prescribed that the expansion is carried out as a two-stage mounting variant, with an intermediate reheating up to the same Tmax at a pmed intermediate pressure, for which:

-

 The expansion machine includes two steam expansion bodies and there is also a second heating circuit in the main heat input equipment;

-

 the output of the first main heating circuit is connected to the input of the first expander body, the output of the first expander body to the input of the second main heating circuit, the output of this second circuit to the input of the second expander body, and the output of the latter to the input of the secondary circuit of the regenerator;

-

 the intermediate pressure pmed is selected so that the powers provided by both bodies of the expander element do not differ by more than 20% from each other.

With all this, both the installation and the cycle, with any of the proposed variations, are completely defined. The application is also characterized by the type of thermal source. The heat provided in main heat input equipment comes from one of the following sources:

-

a solar thermal energy collection facility,

-

of geothermal origin,

-

of any type of combustion,

-

of nuclear reactions and radiation,

-

of the heat yielded from a higher temperature Brayton cycle, in its cold branch, than the main heat input temperature of this cycle.

Brief description of the figures

Figure 1 shows an example of the realization of the Brayton-Rankine hybrid cycle indicating the required components, while Figures 2 and 3 present the thermodynamic temperature-specific entropy and pressure-enthalpy diagrams specific to a thermodynamic definition of the cycle using isobutane as the flow fluid. job.

Figure 4 shows a configuration with variant in the design of the three-way condenser that allows access to saturated steam.

Figure 5 shows a configuration that introduces a rolling valve that allows a new degree of freedom to be introduced into the thermodynamic design of the cycle. Figure 6 shows the pressure-enthalpy diagram associated with this materialization variant.

Finally, Figure 7 shows another configuration that allows to increase the average heat input temperature. Figure 8 shows the temperature-entropy diagram associated with the variant.

To facilitate the understanding of the figures and of the preferred embodiments of the invention, the relevant elements thereof, which appear in the figures, and the characteristic thermodynamic points are listed below:

one.

Expander machine, which can be a turbine.

2.

Regenerative heat exchanger or regenerator.

3.

Primary regenerator circuit (2).

Four.

Secondary regenerator circuit (2).

5.

Flow dividing element, which divides the total flow of fluid into two, the main and the balancer.

6.

Compressor or compressor machine.

7.

Flow joint element, which mixes the main and balancer flows to obtain the total.

8.

Main heat input equipment to the fluid thermally fed by a heating fluid that comes from a thermal source and which can be the cycle's own working fluid.

9.

Heat input circuit of the main heat input equipment (8), in which the working fluid is heated thanks to the thermal energy supplied by a heating fluid in the secondary circuit (10), which comes from a thermal source , or by the thermal source itself in case the heating fluid coincides with the working fluid.

10.

Secondary circuit of the main heat input equipment (8), which transfers the thermal energy from the thermal source to the working fluid.

eleven.

Condenser, cooled by a cooling fluid such as air, water or any other, whose temperature at the entrance of the equipment is marked by the environment and is also called Tamb ·

12.

Primary condenser circuit (11), where condensation of the working fluid occurs.

13.

Secondary condenser circuit (11), through which the refrigerant that absorbs the heat of the condensation circulates.

14.

Drive pump that circulates and pressurizes the working fluid in liquid phase.

fifteen.

Shaft of compressor, expander and electric generator (16).

16.

Electric generator.

17.

Condenser variant so that the flow divider is integrated into the condenser itself by configuring a three-way condenser.

18.

Condensate collection system.

19.

Saturated steam collection system.

twenty.

Rolling valve at the outlet of the primary circuit (3) of the regenerator (2) that reduces the pressure of the main flow of fluid after regeneration and before it is mixed with the equilibrium flow.

twenty-one.

First expander body of the expander machine.

22

Second expander body of the expander machine.

2. 3.

Second main heat input circuit of the main heat input equipment (8).

In addition to the previous identifiers, which refer to physical elements of the circuit to execute the cycle, and their auxiliary systems, the following alphabetic-numerical labels are used in the drawings to identify different thermodynamic states of the working fluid:

f1. Thermodynamic state of the working fluid at the exit of the main heat input equipment (8) and input to the expander (1).

f2. Thermodynamic state of the working fluid at the outlet of the expander (1) and input to the regenerator (2) by its secondary circuit (4).

f3. Thermodynamic state of the working fluid at the output of the secondary circuit (4) of the regenerator (2). The entry point to the compressor equipment is called f3 '.

f4. Thermodynamic state of the working fluid at the outlet of the compressor (6) and input to the flow joint element (7).

f5. Thermodynamic state of the working fluid corresponding to the saturated condensed liquid leaving the condensing equipment (11) and entering the discharge pump (14).

f6. Thermodynamic state of the working fluid at the outlet of the discharge pump (14) and input to the regenerator

(2) by its primary circuit (3).

f7. Thermodynamic state of the working fluid at the outlet of the primary circuit (3) of the regenerator (2) and input either to the flow joint element (7) or to the rolling valve (20).

f8. Thermodynamic state of the working fluid at the outlet of the flow joint element (7) and input to the main heat input equipment (8).

f9. Thermodynamic state of the working fluid at the outlet of the rolling valve (20) and inlet to the flow joint element (7).

f10. Thermodynamic state of the working fluid at the exit of the first expander body (21) and input to the second main heat input circuit (23) when the expansion is staggered with intermediate heat input.

f11. Thermodynamic state of the working fluid at the exit of the second main heat input circuit (23) and input to the second expander body (22) when the expansion is staggered with intermediate heat input.

Each of the above points is completely defined if two of its thermodynamic variables are specified (between temperature, entropy, enthalpy, pressure and specific volume) except point f5 for which it is only necessary to specify the temperature or the condensation pressure .

Description of an embodiment of the invention

The invention requires identifying fluids that satisfactorily meet the requirements set forth in the prescriptions of the invention. Although the prescriptions allow the use of any fluid, both negative and isentropic or positive, the very positive ones, with R / Cp values (measured at one thousandth of the critical absolute pressure and twice the critical absolute temperature) less than 1 / 20 are not advisable, since compression implies a liquefaction of the fluid. To choose between the negative and the isentropic, both being valid, the isentropic are preferable, since the difference between the value of the Cp in the liquid state and in the gaseous state is smaller, which allows a lower equilibrium flow rate and a lower consumption per part. of compression equipment. For solar applications with maximum temperatures in volume at 400ºC they are the most advisable, but not limiting:

-

isopropanol;

-

acetone;

-

and R141b.

They are also advisable, although they are worse, isobutane and sulfur hexafluoride, among others.

A preferred embodiment of the invention is described below, although not unique or limiting, and then slight variations are introduced that allow the different options mentioned above to be introduced, neither limiting.

In order to achieve the described invention and the proposed objectives, the following cycle is implemented (Figures 1 to 3):

-

the working fluid is heated in the gas phase in the main heat supply equipment (8);

-

the steam at the outlet of the main heat input expands in the expander machine (1);

-

the steam at the exit of the expander machine (1) is introduced into the regenerator (2) where heat is circulating circulating through the secondary circuit (4) and consequently it is cooled;

-

at the output of the secondary circuit (4) of the regenerator (3) the flow is divided into two flows, the main and the balancer;

-

the equilibrium flow is directed to the compressor (6), which is connected at its outlet with the main heat input equipment (8) to close a Brayton cycle, although not constant flow along it;

-

the main flow is directed to the condenser (11) where it condenses at a constant temperature giving heat to the environment;

-

subsequently the condensed liquid is pumped into the discharge pump (14), after which it is preheated or regenerated in the primary circuit (3) of the regenerator (2) and finally reaches the junction element (7) with the equilibrium flow, with which is directed to the main heat input equipment (8), configuring a Rankine cycle, although not constant flow.

The main flow is that of the Rankine cycle and that is regenerated. The equilibrium flow rate is that of the Brayton cycle and has the mission of being the extra mass flow which, together with the main one, yields heat in the regenerator to increase the heat capacity of the hot fluid, of Cp less than the liquid. Both flows receive the main heat input from the thermal source (10) simultaneously, and also share an expander or turbine (1).

Figure 1 presents a mechanical scheme of the invention. Figures 1 and 2 complete the example showing the thermodynamic diagrams specific temperature-entropy and pressure-specific enthalpy of the cycle. Both are exemplified with isobutane, practically isentropic or slightly positive fluid with an R / Cp factor approximately equal to 1/15 measured in the above conditions.

From this initial configuration a series of aspects must be qualified. In the first place, the commencement of compression according to the scheme in Figure 1 takes place after the steam comes out of the regenerator. However, the initial point of compression may be the condition of saturated steam, accessible from the top of the three-way condenser (19) if it has a tank or boiler phase separator after the heat exchanger. This is advisable since compression consumes less work if it is done at a lower temperature. This variant, which has already been introduced in Figures 2 and 3, is possible if the fluid is isentropic or negative, but not when it is markedly positive since compression would involve liquefaction with a huge variation in volume.

On the other hand, the regeneration of the liquid is by sensible heat, without evaporation and up to a high temperature. The absolute working pressure is very high, at least twice the criticism, to avoid not only that evaporation does not occur at a constant temperature, but that the Cp value of the liquid hardly varies during the regeneration process and also comply with the prescription that the Cp of the pseudo-critical point of the isobar is at most three times the Cp of the liquid at the pump outlet. At subcritical pressures, the liquid Cp is almost constant but becomes infinite during evaporation and at the critical point. As mentioned, with the invention the phase change in an internal heat transfer is avoided, since it implies high temperature differences between the currents. If the pressure is moderately supercritical, there is no discontinuity in the phase change but the Cp grows very rapidly in the area near the critical point and reaches a maximum in the so-called pseudo-critical point, then descends, so that the behavior regarding of irreversibilities is similar to the case of evaporation, although somewhat less unfavorable. At very high pressure, however, each time the Cp at the pseudo-critical point is lower. What the invention seeks is to make use of that property, achieving a preheating practically with constant Cp in the liquid (and also in the steam that yields heat). The phase change, very continuous, takes place during the main heat input. The variation of the Cp in that phase change process is very smooth, and consists more of a transition from the value of the liquid Cp, greater, to the value of the vapor Cp, smaller, without going through a very marked intermediate maximum. This behavior is checked by observing the isobutane isobars in Figure 2 corresponding to 40 and 100 bar, moderately supercritical and very supercritical respectively. The isobar of 40 bar varies its slope until it is almost zero (maximum Cp), while the high pressure is practically a straight line with no marked transition between liquid and steam.

Another important issue is related to the mixture of the main fluids (which comes from the regenerator) and balancer (which comes from the compressor equipment). To avoid irreversibilities, both currents must be mixed with the same thermodynamic state, or very approximate. This feature is also prescribed.

By incorporating all the previous restrictions, the cycle is very constrained, with hardly any degrees of freedom. For this reason, a variant is introduced that incorporates a valve to release the supply and preheating pressure in the regenerator of the main heating pressure by the thermal source.

Finally, the main heat input is by sensible heat. This implies that the average heat input temperature is practically the arithmetic average between the temperature of the heat input and the output temperature. This, similar to the Brayton cycles, can be improved by making the stepwise expansion with intermediate reheating. In this way the output temperature of the expander equipment is higher and the regeneration capacity is higher. This feature, optional, is also prescribed.

With all this, you can define the processes and the main variables of the design that works in the optimal area in an idealized and approximate way:

one.

A fluid with a virtually isentropic saturated vapor curve is selected. For solar applications up to 400 ° C, isopropanol, acetone and R141b are suitable. So can isobutane and sulfur hexafluoride.

2.

It is based on a condensate that works at a saturation temperature Tmin from which saturated liquid and saturated steam can be extracted. In a solar application Tmin would take values in volume at 350 K to be cooled by air.

3.

A fraction of saturated liquid (most of it) is collected at the condensation pressure and pumped almost isentropically to a supercritical working pressure, taking twice the critical absolute pressure.

Four.

A certain fraction of saturated steam (a smaller fraction) is collected and compressed to the previous supercritical working pressure.

5.

The compressed liquid is preheated in the regenerator with a practically constant Cp until its entropy reaches the value of compressed steam. At that time you have a temperature Treg ·

6.

After regeneration, it is mixed with the compressor outlet and the main heating is carried out until the setpoint temperature Tmax (about 670 K in solar applications).

7.

The steam expands to the condensation pressure. The turbine outlet temperature will be slightly higher than Treg in order to be able to regenerate.

8.

The steam is directed to the regenerator to preheat the liquid from the pump. The mass flow rate of the fluid that is heated is less than that of the one that is cooled in such a way that the heat capacity of both streams (m · Cp) is the same.

9.

The steam that cools, at the outlet of the regenerator, goes to the condenser, where it cools.

In order for it to be fulfilled that, in fact, the temperature of the compressor's output and the turbine's output is the same, it must be adhered to that Treg2 = Tmax · Tmin. While equality is not satisfied because the steam gamma is not the same during compression than during expansion. Likewise, the pressure ratio must comply approximately with Tmax / Treg = (pmax / pmin) ^ (R / Cp).

The approximate estimate of the maximum performance assuming all processes without irreversibility can be calculated considering the average heat transfer temperatures equal to Tmin and the average heat input temperature such as the arithmetic mean of Treg and Tmax · Thus:

that in the case of a Carnot factor of 1.92 it takes a value of 39.5%, while the Carnot yield would be 47.9% and that of other conventional cycles does not exceed 28%. This value of 39.5% must be reduced by the irreversibility factor, which has been taken as the unit as all the ideal processes. With a value of FΔs in volume of 90%, 35% can be exceeded operating between 400ºC and 75ºC, notably better than the current cycles.

Claims (8)

1. Device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle comprising:

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a condensable working fluid, for which the pseudo-critical point of a supercritical isobar is defined as the point of highest specific heat at constant pressure, Cp, of the isobar, this value of Cp being infinite at the critical point, and descending its value as the pressure increases;

-

an expansion unit (1) of the working fluid in the gas phase;

-

a heat exchanger, regenerator type (2), with two circuits, the primary (3) that receives heat and the secondary (4) that yields it;

-

a flow division element (5) that divides the total inlet flow into two, called main flow and equilibrium flow;

-

an equipment for compression (6) of the gas phase fluid whose axis (15) can be mechanically or jointly attached to the expander equipment (1) or not;

-

a device for the condensation (11) of the fluid, cooled by a cooling fluid such as air, water

or any other, whose temperature at the entrance of the equipment is marked by the environment and is called Tamb;

-

a drive pump (14) to circulate and pressurize the fluid in liquid phase;

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a main heat input equipment (8) to the fluid thermally fed by a heating fluid that comes from a thermal source and which can be the cycle's own working fluid;

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a flow joint element (7);

-

and a transmitter or transformer element of the mechanical power produced such as an axis or an electric generator (16), among others;

and in which the previous components are connected as follows:

-

the duct or outlet manifold of the main heat input equipment (8) is connected to the input of the expansion equipment (1);

-

the output of the expansion equipment (1) is connected to the regenerator (2) by the input of its secondary circuit (4);

-

the output of the condensing equipment (11) is connected to the input to the impulsion pump (14);

-

the outlet of the impulse pump (14) is connected to the regenerator (2) by the input of its primary circuit (3);

 characterized by:

-

The output of the secondary circuit (4) of the regenerator (2) is connected to the flow division element (5), which divides the flow into two: the main flow and the balancer;

-

and the main flow outlet of the flow splitting element is connected to the condensing equipment (11);

- and the output of the balancing flow of the flow division element (5) is connected to the input of the compression equipment (6);

-

and the output of the compression equipment (6) is connected to the output of the primary circuit (3) of the regenerator (2) through the flow joint element (7);

-

and the output of the flow joint element (7) is connected to the input or inlet manifold of the main heat input element (8);

2. Procedure for using the device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle, characterized in that the thermodynamic specifications to which its operation conforms are that:

-

the setpoint condensation temperature Tmin is that corresponding to a saturation state with a pressure Pmin lower than the critical pressure of the fluid and whose specific entropy values of liquid and saturated steam are called sliq and svap respectively and the specific enthalpies of liquid and saturated steam hliq and hvap respectively and is selected such that the temperature Tmin is higher than the temperature of the condenser outer refrigerant, marked by the environment Tamb;

-

the absolute pressure in the pump drive, Pimp, is supercritical and is greater than the pressure of the isobar whose pseudo-critical point has a value of Cp three times greater than the Cp of the liquid measured under pump output conditions, the pseudo-critical point of a supercritical isóbara being defined as the point of greatest Cp of the isóbara;

-

the absolute temperature of the main fluid at the primary regeneration circuit outlet, Treg, is equal to or greater than 10% at the absolute temperature corresponding to the thermodynamic state with Pimp pressure and svap specific entropy;

-

in the regenerator the rate of passage of heat capacity, or product of the average specific heat by the mass flow rate of each of the two currents, is equal to each other, so that it is balanced, specifying that the proportion of mass flow balancing is prescribed by

mequilibrador / mtotal = (1 - cp, secondary / cp, primary)  the balancing mass flow being a balancing device, total total flow rate, cp, secondary specific heat at medium constant pressure during total flow cooling and cp, primary specific heat at medium constant pressure during heating of main flow in the primary circuit , both in the regenerator, being determined the main flow with the equation  mprincipal = mtotal - mequilibrador  the main mass flow being primary;

-

establishing the terminal temperature difference in the regenerator at a value greater than 0 K and less than 35 K, which determines the temperature of the gas at the exit of the expander equipment (1) and input to the secondary

(4) of the regenerator (2), the temperature of the fluid at that point being the Treg value plus the selected terminal difference, and whose entropy, corresponding to that thermodynamic state, is called smax, and the entropy generation in the expansion is denominated Δs, the difference being smax -Δs being the entropy of the steam at the entrance to the turbine and the pressure at this point of entry to the turbine being equal to that of the pump drive minus the pressure losses in the thermohydraulic circuit, for what both magnitudes, entropy and pressure, uniquely determine the turbine inlet temperature, Tmax. which is selected in the main heat input equipment;

-

and being determined, with all the previous prescnpc1ones, the thermodynamic states of all the entry and exit points of the equipment and, consequently, the enthalpies corresponding to each of them.

3. Device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to claim one and whose working fluid has a value of R / Cp greater than 1/16, R being the constant characteristic of gas and Cp heat specific at constant pressure measured at twice the critical absolute temperature of the fluid and one thousandth of its critical absolute pressure, characterized in that the flow divider element is integrated in the condenser itself, which is arranged with three ways (17) for having :

-

a steam inlet or manifold prior to the condenser's own heat exchanger,

-

a steam collection or aspiration system (19) at the top,

-

the conventional condensate collection system (18) in the lower part that collects said condensate or main flow and directs it to the discharge pump (14).

Four.

 Method of using the device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to claim two and with integrated or non-integrated flow divider element

integrated in the condenser, characterized in that the specific enthalpies of the balancing flow at the outlet of the compressor (6) and of the main flow at the output of the primary circuit (3) of the regenerator differ by less than 2% from the value given by the product of R and the critical temperature in absolute value in coherent physical units.

5.

 Device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to first or third claims, characterized in that in a mounting variant a rolling valve (20) is incorporated at the output of the primary circuit (3) of the regenerator (2) which reduces the pressure of the main fluid flow after regeneration and before it is mixed with the balancing flow, for which the primary circuit outlet (3) is connected with the inlet of the rolling valve (20) and the outlet of the rolling valve (20) with the primary fluid inlet of the current connection element (7).

6.

 Procedure for using the device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to claim two or four, and incorporating a rolling valve at the outlet of the primary circuit of the regenerator, characterized in that the supply pressure of main heat, now called Pq, less than the discharge pressure, Pimp, and greater than the geometric mean of Pimp and Pmin ·

7.

 Device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to first, third or fifth claims characterized in that, in a mounting variant, the expansion is carried out in two stages, with an intermediate reheating up to the same Tmax at an intermediate pressure pmed, for which:

-

 the expansion equipment includes two expansion bodies (21 and 22) of the steam and a second heating circuit (23) in the main heat input equipment (8);

-

 the output of the first main heating circuit (9) is connected to the input of the first expander body (21), the output of the first expander body (21) to the input of the second main heating circuit (23), the output of this second circuit (23) at the input of the second expander body (22), and the latter's output at the input of the secondary circuit (4) of the regenerator (2).

8. Method of using the device for generating mechanical energy according to a regenerative and balanced Brayton-Rankine hybrid cycle according to claims second, fourth or sixth, and with two-stage expansion with intermediate heating characterized in that the intermediate pressure pmed is selected so that the power provided by both bodies of the expander element does not differ by more than 20%.