RANKINE CYCLE DESCRIPTION

The present invention relates to a plant for the production of energy based upon the organic Rankine cycle (ORC), in particular a plant for the production of energy comprising a plurality of cascaded ORC systems that use particular turbines.

As is known, ORC plants are systems generally used for simultaneous production of electrical and thermal energy, the latter being made available in the form of water at the temperature of 60 - 90°C. Organic Rankine cycles are similar to the cycles used by traditional steam turbines, except for the operating fluid, which, normally, is an organic fluid with high molecular mass.

A typical ORC plant is substantially made up of a pump, a turbine, and some heat exchangers. The organic operating fluid is vaporized by using a heat source in the evaporator. The steam of the organic fluid expands in the turbine and is then condensed generally using a flow of water in a heat exchanger. The condensed liquid is finally sent via a pump into the evaporator thus closing the cycle. In order to increase the yield of the plant it is possible to envisage the use of a regenerator. In this case, the fluid leaving the turbine traverses a regenerator before being condensed and, once condensed, is pumped into the regenerator, where it is pre-heated by the fluid leaving the turbine, before being sent to the evaporator.

Generally, these plants are used for the production of energy with waste fluids coming from a wide range of industrial and energy-generating processes (cogeneration engines and turbines, furnaces of all types, chimneys of petrochemical plants, sources of geothermal nature, etc.), characterized by a temperature jump that is potentially high but by flows that are on average limited or in any case variable in time or, otherwise, by high flows associated, however, to a low temperature level.

The energy vector used for vaporization of the organic fluid is in general diathermic oil (mineral oil, or synthetic oil for temperatures above 300°C) or water, whereas for condensation water is used. The use of diathermic oil moreover prevents the need to use high- pressure boilers.

The operating fluid is generally constituted by an organic compound or by a mixture of organic compounds, characterized by a high molecular weight. The choice of the organic fluid to be used for optimizing the yield of the thermodynamic cycle is made according to the temperature of the heat source available. In addition, also the turbine must normally be designed according to the characteristics of the organic fluid and to the operating conditions. Consequently, it is evident that the application of ORC systems is subject to limits and critical aspects deriving from the thermal power available as primary source, from the temperature level/levels (quality of the source), and from the stability and/or relative variability in time of the thermal load.

It is moreover evident that current ORC systems are far from flexible and far from adaptable to different operating conditions given that, as previously mentioned, both the choice of the organic operating fluid and the characteristics of the turbine are markedly dependent upon the operating conditions.

The purpose of the present invention is consequently to provide a plant for the production of electrical energy based upon the organic Rankine cycle (ORC) that will be able to eliminate or reduce the aforementioned drawbacks.

In particular, a purpose of the present invention is to provide a plant of an ORC type for the production of electrical energy that will enable maximization of the yield in terms of electrical energy produced.

Another purpose of the present invention is to provide a plant of an ORC type for the production of electrical energy that will be readily adaptable to different operating conditions. Yet another purpose of the present invention is to provide a plant of an ORC type for the production of electrical energy that will enable optimal characteristics of yield to be maintained even during a drop inefficiency of the primary flow.

A further purpose of the present invention is to provide a plant of an ORC type for the production of electrical energy that will present a small number of parts, and will be easy to produce at competitive costs.

In accordance with the present invention, the above-mentioned purposes are achieved by a plant for the production of energy based upon the organic Rankine cycle (ORC), which is characterized in that it comprises a first ORC system comprising a first organic operating fluid circulating, in sequence, between a first evaporator in conditions of heat exchange with a heat source, a first expansion stage in a turbine operatively connected to a generator, a first evaporator/condenser, and a first pump for recirculating said first organic operating fluid to said first evaporator. A further peculiar characteristic of the plant according to the invention is represented by the fact that said turbine is a partializable turbine comprising means for partializing the incoming flowrate of said organic operating fluids, said means being designed to partialize said incoming flowrate to maintain the r.p.m. of said turbine constant.

Preferably, the plant for the production of energy according to the invention comprises a second ORC system, which comprises a second organic operating fluid circulating, in sequence, between said first evaporator/condenser, a second expansion stage in a turbine operative ly connected to a generator, a second evaporator/condenser, and a second pump for recirculating said second organic operating fluid to said first evaporator/condenser, said turbine comprising, for each of said stages, means for partializing the incoming flowrate of said organic fluids.

The plant according to the invention enables a considerable series of advantages to be achieved as compared to the ORC systems of a known type.

In practice, in the case of the multistage organic Rankine cycle, the system is characterized by a series of successive enthalpic jumps performed not by a single fluid within various rotor- stator assemblies of a turbine (e.g., the water vapour that expands at various levels of pressure in the stages of a turbine), but by a number of fluids, each operating on a number of pressure and temperature levels, within their own turbine, which is coupled, axially aligned or in parallel, to the turbine for the other fluids that together with it constitute the system. The plant hence comprises a so-called "primary" fluid, which interfaces with the heat source, and an appropriate number of secondary fluids, ordered in such a way that the condensation of the previous one causes evaporation of the next one, in order to recover the maximum possible amount of energy available to the source, yielding the minimum fraction thereof into the environment.

The limit of quality of the source (e.g., low enthalpic level) obviously dictates the limits of application of the technology. The availability and the temperature of the cooling fluid define, instead, the lower-limit stage, enabling, with the use of organic fluids and binary mixtures for low-temperature applications, extension of the discharge from the last turbine to levels theoretically lower than thermal zero.

By way of example, for the most frequent cases of high temperature and low flowrate of the power source it is preferable for the operating fluid of the primary circuit to have a rather high molecular weight so as to exploit to the full the high temperatures that can present at discharge (typically, 500-900°C). The flowrate of said fluid is, however, in this case limited by the thermal power effectively available, and this circumstance is a first limiting factor on the power produced by the primary fluid.

A second factor, which is no less important, is the molecular weight itself of the fluid. A high molecular weight, which advantageously enables the high temperatures of the heat source to be pursued, proves a penalizing factor in terms of enthalpic jump in the turbine and condensation temperature. For example, if this fluid at the evaporator is able, with not excessively high pressures (20-40 bar), to vaporize at quite high temperatures (250-350°C), at the condenser, albeit with pressures far higher than 1 bar, still comes out at rather high temperatures, in the region of 160-250°C. Hence, with the first operating fluid, with a low flowrate, and a limited enthalpic jump in the turbine, a low power is normally obtained, in the region of 15% of the one available at discharge.

However, the still significantly high temperatures of the primary fluid undergoing condensation, and the phase thereof selected for heat exchange with very high coefficients of transmission, enable use of a secondary operating fluid that will recover entirely or partially the heat of condensation of the first fluid, and will perform an altogether independent Rankine cycle, producing a further, significant amount of electric power. In fact, assuming the value of 15% referred to previously as yield of the primary cycle means that of 100 kW at input for the first fluid there remain 85 kW available for the second organic operating fluid. Choosing the latter with characteristics such as to be optimal for working at lower operating temperatures and with a range of pressures similar to those of the first fluid, said fluid will operate on lower isotherms, guaranteeing in any case a similar yield. Assuming then again a 15% recovery means that, out of 85 kW available, a further 12.75 kW are obtained, for a total of 27.75 kW, which is the theoretical yield of a plant that works with just two "cascaded" fluids.

Hence, extrapolating the concept, there may be readily envisaged the possibility of using a third fluid, or in general "n" fluids, which, by working in cascaded fashion, i.e., as has been said, with the evaporation on the condensation of the previous one, optimize the entire, process limiting to a minimum the heat yielded by the last condensation, which is the one that is then discharged into the environment, and thus maximizing the total yield.

In the plant for the production of energy according to the invention, preferably said means for partializing the incoming flowrate of said organic operating fluids comprise, for each of said stages, a hydraulic device driven by the corresponding organic operating fluid.

In addition, advantageously, said partializable turbine is a multistage turbine, with single shaft or separate shafts, axially aligned or with parallel axes.

A preferred embodiment of the plant for the production of energy according to the present invention envisages that said heat source will comprise heat-accumulation means.

For example, said heat-accumulation means can comprise a refractory-mass accumulation system or a molten-salt closed-circuit battery.

A further preferred embodiment of the plant for the production of energy according to the present invention envisages that said heat source comprises means for integration of the available energy.

For example, said means for integration of the available energy comprise a thermodynamic solar-energy system.

A particular embodiment of the plant for the production of energy according to the present invention envisages the presence of a device for mechanical coupling between said turbine and said generator.

Said device for mechanical coupling between said turbine and said generator can, for example, comprise a reducer, a flywheel governor, and a brake set between the shaft of said turbine and the shaft of said generator.

An alternative embodiment of the plant for the production of energy according to the present invention envisages, instead, that said turbine is directly coupled to said generator, said plant further comprising electronic means for conversion of the output voltage of said generator. Further characteristics and advantages of the present invention will emerge from the description of preferred, though not exclusive, embodiments of a plant for the production of energy based upon the organic Rankine cycle (ORC), illustrated by way of non-limiting example in the annexed drawings, in which:

Figure 1 shows a diagram of a general embodiment of a plant according to the present invention;

Figure 2 shows a diagram of a first particular embodiment of a plant according to the present invention;

Figure 3 shows a diagram of a second particular embodiment of a plant according to the present invention;

Figure 4 is a schematic representation of a first embodiment of the mechanical coupling between a turbine and a generator in a plant according to the present invention; and

Figure 5 is a schematic representation of an embodiment of the coupling to the electrical mains of a plant according to the present invention.

With reference to the attached figures, a plant for the production of energy based upon the organic Rankine cycle (ORC) according to the present invention, designated as a whole by the reference number 1 , in its more general embodiment illustrated in Figure 1 , comprises at least one first ORC system 10.

Said first ORC system 10 in turn comprises a first organic operating fluid that circulates, in sequence, between a first evaporator 11 in conditions of heat exchange with a heat source 2, a first expansion stage 12 in a turbine operative ly connected to a generator 4, a first evaporator/condenser 13 and a first pump 14 for recirculating said first organic operating fluid to said first evaporator 11.

Preferably, as illustrated in Figure 2, the plant 1 for the production of energy according to the present invention, comprises a second ORC system 20, which in turn comprises a second organic operating fluid circulating, in sequence, between said first evaporator/condenser 13, a second expansion stage 22 in a turbine operatively connected to a generator 5, a second evaporator/condenser 23, and a second pump 24 for recirculating said second organic operating fluid to said first evaporator/condenser 13, said turbine comprising, for each of said stages 12 and 22, means for partializing the incoming flowrate of said organic fluids

Basically, as has already been mentioned, the plant comprises at least two organic operating fluids, which, working in cascaded fashion (namely, with the evaporation of the second fluid on the condensation of the first fluid), optimize the entire process, limiting to a minimum the heat yielded by the last condensation.

In other words, using this scheme, the closed-circuit organic Rankine cycle uses the primary source of energy for converting the first fluid into steam, the expansion in the turbine converts this heat accumulated by the steam into kinetic energy, which in turn will become electrical energy. In the cascade the condenser becomes the primary source for the second fluid, and so forth for a possible third stage and subsequent stages.

By applying the principle illustrated in the attached Figure 2, it is in fact possible to extend said concepts to a plant that comprises a third ORC system, which includes a third organic operating fluid that circulates, in sequence, between said second evaporator/condenser, a third expansion stage in a turbine operatively connected to a corresponding generator, a third evaporator/condenser, and a third pump for recirculating said first organic operating fluid to said second evaporator/condenser. In general, this principle can then be extended to a plant that comprises "n" further ORC systems.

One of the peculiar characteristics of the plant 1 according to the present invention is represented by the fact that said turbine is a partializable turbine and comprises means for partializing the incoming flowrate of said organic operating fluids. In particular, said partialization means are designed to partialize said inlet flowrate to maintain the r.p.m. of said turbine constant.

It has in fact been seen that, using a turbine of the type described above, it is possible to maintain the optimal characteristics of yield even during a drop of efficiency of the primary flow. Thanks to the automatic partialization of the flowrate, the r.p.m. of the turbine is, in fact, kept constant, which is an extremely important aspect for achieving maximum yield. In practice, partialization of the turbine through the control of incoming flowrate in order to keep the r.p.m. of the shaft constant, enables the level of electrical yield to be kept constant irrespective of the load, thus contributing to the maximum exploitation of the source of thermal power in every condition that arises upstream.

The application of a process of a cascaded type finds justification in the very nature of the heat exchange and of the characteristics of the power fluid. In fact, it is precisely the first exchanger of the system, the evaporator of the primary fluid with highest enthalpy, the one that is most critical given that it is interfaced with the waste fluids, which each time present low coefficients of heat exchange, high corrosiveness, and non-constancy of the flowrate. Using a cascaded system, it is possible to concentrate the process of heat recovery in this first exchanger, which constitutes the central element of the entire cycle both in terms of efficiency and in terms of costs.

The subsequent condensers-evaporators are favoured by the cascaded functional logic and by the nature of the fluids adopted, both in the efficiency of interchange and in the nature of the design and manufacturing materials required, thus involving small surfaces and hence low costs.

Preferably, said means for partializing the incoming flowrate of said organic operating fluids comprise a hydraulic device driven by the corresponding organic operating fluid. In this way, the flowrate at inlet to said organic operating fluids is adjusted exploiting the variations of pressure of the organic fluids themselves.

In addition, advantageously, said partializable turbine can be a multistage turbine, with single shaft or with separate shafts, axially aligned or with parallel axes.

A particular embodiment of the plant 1 for the production of energy according to the present invention, illustrated in Figure 3, is characterized in that said heat source 2 comprises heat- accumulation means 6.

In the case where the plant of the present invention is applied to industrial processes with production of intermittent waste in the short period, there may arise in fact the need to stabilize the thermal flow at levels compatible with the stepwise regulation of the plant.

There may consequently arise the following situations:

expedience of limiting the load with re-use of part thereof for optimization of the consumption correlated to the basic production processes;

expedience, if not strict need for regulation of the ORC unit downstream, of proceeding to a partial accumulation of the heat available to stabilize the thermal flow at homogeneous levels.

By using the heat-accumulation means 6, which comprise, for example, a refractory-mass accumulation system or a molten-salt closed-circuit battery, it is possible to maintain an average thermal level guaranteed of the heat source 2, managing the situations described above in an optimal way.

For low thermal powers a refractory-mass accumulation system with low impact from the economic and maintenance standpoints is preferable. In this case, the hot process fluid traverses the refractory material until its own thermal load is reduced to the value at input to the high-temperature evaporator. When the flowrate of the waste fluid is reduced and/or is about to cease, a closed-circuit circulation of hot air (or recirculation of the gas itself) is activated between the accumulation chamber and the evaporator. For applications of this type, the optimal accumulation temperature range is between 200°C and 400°C.

For high powers, the accumulation system may conveniently be based upon a closed-circuit molten-salt battery, also according to the economic aspects of the investment required for the accumulation tank in the light of the effective thermal capacity that can be recovered and hence of the corresponding electrical production.

The aggressiveness of the molten salts requires a limitation of the temperature of the mixtures (nitrates and nitrides of sodium, potassium and calcium) to values of 400-450°C to enable use of low-cost materials for heat exchangers and tanks. Furthermore, to prevent combination of the effects of aggressiveness of the salts with the source fluid it is expedient to use an intermediate vector fluid of a diathermic type. Finally, the choice of technologies of exchangers that ensure low loss of head for viscous fluids ('EM-Baffle®' technology, and the like) completes the requirements of the system.

A further particular embodiment of the plant 1 for the production of energy according to the present invention, illustrated once again in Figure 3, envisages that said heat source 2 comprises means 7 for integration of the available energy.

Both using the refractory-mass system and the molten-salt system, as likewise other accumulation systems, it is in fact possible to integrate the power available so as to ensure the enthalpic jump within the organic Rankine cycle. Said integration can be obtained through reuse of part of the electrical power produced for stabilizing the cycle and/or achieving maximum thermodynamic yield thereof. It is likewise possible to increase the absolute power thereof through the use of parallel sources according to the time of day and the demand. Specifically, technologies applied to the thermodynamic solar sector can, for example, be used to increase the available flowrate adequately during periods of peak demand.

Coupling of the energy- integration means to the power-accumulation means ensures maximum flexibility of the entire cycle, duly exploiting the stepwise operation of the turbine, as illustrated previously in the case of cascaded multistage organic Rankine cycles. Both the accumulation means 6 and the integration means 7 can in fact be conveniently applied to plants comprising a plurality of organic Rankine cycles of the type illustrated in Figure 2. The plant 1 for the production of energy according to the present invention can moreover comprise means for electrical transduction, namely, for the transformation of the kinetic energy developed by the turbine into electrical energy.

For example, with reference to the attached Figure 4, the plant 1 for the production of energy according to the present invention can advantageously comprise a mechanical-coupling device 8 between said turbine and said generator 4, 5.

Said mechanical-coupling device 8 can, for example, comprise a reducer 81 set between the shaft 84 of said turbine and the shaft 85 of said generator 4, 5. Said reducer 81 can, for example, be a reducer of an epicyclic type in such a way as to guarantee reduction of r.p.m. from the turbine to the generator (alternator), thus maintaining the right frequency for output the energy produced to the mains.

Furthermore, the reducer 81 conveniently comprises all the elements (for example, a flywheel governor 82 and a brake 83) that will enable maintenance of the correct working points of the entire system. The connection with the mains can be made via a transformer 86 and/or other means for synchronization with said mains.

Alternatively, as illustrated in Figure 5, said turbine is directly coupled to said generator 4, 5. For example, the turbine can be directly coupled to a (synchronous or asynchronous) motor, eliminating the entire mechanical block described previously. In this case, said device can be used in the four quadrants (Vxl > 0; Vxl < 0) both as motor for entering the transients of the turbine and exiting therefrom and as electric generator.

In this case, electronic means 9 are present, for example an AC/DC converter for continuous conversion of the high-frequency voltage (frequencies much higher than 50 Hz) given by the direct coupling with the turbine, as well as a DC/ AC converter to obtain the right output voltage with the appropriate synchronism for output into the mains. Transformer means 91 may likewise be conveniently present.

It is clear from the foregoing description that the plant for the production of energy according to the present invention fully achieves the pre-set task and purposes.

The partialization of the turbine responds in fact to the scenarios of thermal load that can vary according to pre-defined steps, where alternative options of use of the power are available or, more directly, for adapting to reduction of the demand, whatever the origin of said reduction. Said application consequently constitutes an effective response, for example in the cases of cogeneration coupled to partializable generators, the cycle of which finds itself pursuing the load in steps (down to a minimum of less than 50% of the nominal size), and/or enabling an increase in the global production of electricity to destine the entire thermal load available to said production (e.g., absence of use or reduced use of hot water/steam in non-winter periods and non-primary interest in trigeneration).

Coupling with a secondary heat source (e.g., solar concentration source) moreover enables exploitation of the turbine at its maximum level of power, reducing the primary consumption or simply increasing the global production of electricity in the periods of increasing demand. Finally, the accumulation of power enables balancing of the production in time and, once again, maximization of the production within the limit of the power effectively available, during periods of increasing demand.

From the foregoing it may hence be stated that the plant for the production of energy according to the present invention, in particular when it combines cascaded organic Rankine cycles, downstream of an accumulation-integration plant, with partializable turbines, presents as ideal from the technical and economic standpoints for maximization of the yield of production of electrical energy from waste and/or sources of heat of medium-to-low absolute power of any nature.

On the basis of the foregoing description, other characteristics, modifications, or improvements are possible and evident to the average person skilled in the sector. Said characteristics, modifications, and improvements are hence to be considered as forming part of the present invention. In practice, the materials, used as well as the contingent dimensions and shapes, may be any according to the requirements and the state of the art.