The present invention relates to an organic Rankine cycle plant (ORC), the peculiar characteristics of which allow to obtain a high yield of the same cycle.
As is well known, a thermodynamic cycle is defined as a finite succession of thermodynamic transformations (for example isotherm, isochore, isobaric or adiabatic transformations) at the end of which the system returns to its initial state. In particular, an ideal Rankine cycle is a thermodynamic cycle consisting of two adiabatic transformations and two isobars, with two phase changes: from liquid to vapor and from vapor to liquid. Its purpose is to turn heat into work. This cycle is generally mainly adopted in thermoelectric power plants for the production of electric power and uses water as the motor fluid, both in liquid form and in the form of steam, with the so-called steam turbine.
Furthermore, plants are known based on an organic Rankine cycle (ORC) for the conversion of thermal energy into mechanical and/or electrical energy. In these plants, organic (high or medium molecular weight) working fluids are used instead of the traditional water/steam system, as an organic fluid is able to convert heat sources more efficiently at relatively low temperatures, generally of 100° C. and 300° C., but also at higher temperatures. The ORC conversion systems are therefore finding ever wider applications in different fields, for example in the geothermal field. A known type of plant for the conversion of thermal energy through an organic Rankine cycle (ORC) in general comprises: at least one heat exchanger that changes heat between a high-temperature hot source and an organic working fluid, so as to heat up, evaporate (and possibly overheat) the working fluid; at least one turbine powered by the working fluid in vapor phase exiting the heat exchanger, so realizing a conversion of a thermal energy present in the working fluid into a mechanical energy according to a Rankine cycle; at least one generator operatively connected to the turbine, in which the mechanical energy produced by the turbine is converted into electrical energy; at least one condenser in which the working fluid exiting the turbine is condensed and sent to at least one pump. From the pump the working fluid is sent to the heat exchanger for beginning a new thermal cycle.
For issues related to a correct sizing of the machines, to avoid high pressures, or in any case to exploit other favorable characteristics of organic fluids, it is often preferred to refer to a scheme with multiple levels of pressure and/or temperature.
In fact, cascade cycles are known, in which the fluid of the upper cycle transfers heat to the fluid of the lower cycle (where the two working fluids are different to better adapt to the different temperatures of the upper cycle compared to the lower one), or cycles at multiple pressure levels and/or temperature which have the purpose of better accompanying the cooling of the hot source (i.e. with small delta T between the heat transfer curve of the hot-source and that of heat reception of the organic fluid).
A widely adopted scheme since the 1980s is a dual level plant scheme, such as the one described, for example, in GB2162583A. The described cycle is called “cascade” with reference this time to the fact that 2 ORC systems are placed in series (or in cascade) with respect to the heat source. This scheme uses different levels of temperature (and pressure, in case the working fluid is the same in the 2 systems) allowing a better exploitation of the heat source. In other words, this cascade cycle uses a plurality of Rankine cycle modules, each having an associated heat exchanger, the source fluid being applied in series to the exchangers of each module in order to maximize the net power produced by the system. Typically, in case of two modules, they will be referred to as high temperature cycle and low temperature cycle.
With reference to the cited document, in a cascade cycle according to the known art, the hot source first feeds the vaporizer of the high temperature cycle. The high-temperature vaporizer performs both a preheating of the organic fluid and its vaporization (and possibly also its overheating) and can be made in a single container (as in document GB2162583A) or in two different containers (as in similar document EP2217793). The hot source then passes through the vaporizer of the low temperature cycle and subsequently it is divided into two streams that feed two partial pre-heaters of the high temperature cycles and low temperature.
The prior art documents reported above refer to a two-level cycle, but the same principle can be applied to a greater number of “levels”.
Therefore, a technique to increase power is to extract more heat from the source fluid by increasing the fall of overall temperature at the end of the thermal exchanges and at the same time trying to keep as high as possible the steam generation temperature that feeds the turbine/s, to keep high the conversion efficiency of heat into mechanical energy. A multi-level temperature system already performs this task better than a single-level subcritical cycle.
The problem still to be solved concerns the further optimization of the mechanical conversion efficiency in an ORC cycle in applications in which the flow rate and temperature characteristics of the thermal source in relation to the usable organic fluids do not find an ideal solution in the known art, for example in some geothermal applications or heat recovery. There is therefore the need to further optimize the efficiency of an organic Rankine cycle to improve the economic yield in particular of geothermal plants often heavily penalized by high costs for the construction of plant engineering works and for which therefore an increase in electricity production is of significant help, together, of course, with a simplification of the systems and a consequent reduction in constructive and management costs.
The object of the present invention is therefore an organic Rankine cycle plant with cascade cycles, capable of increasing the overall efficiency of the plant. More particularly, as will be seen in what follows, the present invention proposes to solve the drawbacks present in the embodiments according to the prior art, namely: to improve the thermodynamic efficiency, to simplify the system from a constructive point of view of the plant, to reduce the construction cost of the plant itself.
According to the present invention, therefore, is described, an organic Rankine cycle plant with cycles in cascade with respect to the thermal source, having the characteristics set out in the appended independent claim.
Additional preferred and/or particularly advantageous ways of implementing the above-mentioned plant are described according to the characteristics set out in the appended dependent claims.
The invention will now be described with reference to the attached drawings, which illustrate some non-limiting examples of embodiment, in which:
Referring now to
In the plant 10 the first ORC 20 cycle, at high temperature, comprises an evaporator 21 in which a first organic working fluid is brought to evaporation (and possibly to a subsequent superheating not shown in the figure), a turbine 22 in which the steam of the first organic fluid is expanded, being the turbine 22 operatively connected to an electric generator 27, a condenser 23 (e.g., a condenser whose cold source 50 is air) in which the working fluid is condensed and returns to the liquid state, a supply pump 24 which compresses the organic working fluid and sends it to a pre-heater 25 and then to the evaporator 21 for a new thermodynamic cycle.
The second ORC cycle 30, at low temperature or in any case at a temperature lower than the first ORC cycle 20, comprises an evaporator 31 in which a second organic working fluid is led to evaporation (and possibly to a subsequent superheating not indicated in figure), a turbine 32 in which the steam of the first organic fluid is expanded, being the turbine 32 operatively connected to a gene electric operator 37, a condenser 33 (for example, a condenser whose cold source 50 is air) in which the working fluid is condensed and returns to the liquid state, a supply pump 34 which compresses the organic working fluid and sends it to a pre-heater 35 and then to the evaporator 31 for a new thermodynamic cycle.
The fluid of the hot source, for example, a geothermal source, follows a path for heat exchange with both ORC cycles. After entering the plant 10 at the entry point 41, it crosses with the whole of its flow 40 the evaporator 21 of the first ORC cycle 20.
It should be remembered that by an evaporator a heat exchanger is meant that receives an organic working fluid in a liquid state and at a temperature close to that of evaporation. The difference between the evaporation temperature and the inlet temperature of the organic working fluid to be evaporated is defined with the term “approach”. Normally in an evaporator the thermal power to be supplied to evaporate the organic working fluid is strongly preponderant with respect to the thermal power to be supplied to complete the preheating of the fluid, being the approach only equal to few degrees centigrade.
At the exit from this first evaporator 21, the fluid of the hot source is divided into two flow rates: a first partial flow rate partial 43 is dependent from the second ORC cycle 30 and supplies in cascade the evaporator 31 and the pre-heater 35 of the second ORC cycle 30, whereas a second partial flow rate 42 remains dependent from the first ORC cycle 20 and supplies the pre-heater 25 of the first ORC cycle 20. Finally, the partial flow rate 43 of the cycle 30 and the partial flow rate 42 of the cycle 20 join together to form the full flow rate 40 which leaves the plant at the outlet point 44.
Preferably the working fluids of the two ORC cycles 20, 30 are different. A suitable choice of the two fluids allows to optimize the overall conversion efficiency, as it is possible to use fluids with different critical points (which are typically lower for the cycle at a lower temperature) and/or a more compressed fluid (i.e. with higher evaporation pressures) for the lowest temperature cycle. For this cycle, in fact, the adoption of the same fluid of the higher temperature cycle would lead to too low operating pressures and therefore, for example, to specific volumes and too large volumetric flow rates with consequent bad dimensioning of the turbine.
The choice of the quantity of source fluid to be divided between the two cycles is optimized on the basis of the temperature profile of the source in relation to the heat introduction curves in the two organic fluids.
In
The changes in slope (i.e. of the flow rates in a temperature−thermal power diagram) corresponding to the flow split of the reference diagram are highlighted as follows: the partial flow rate 42 at the exit of the evaporation phase has a greater slope (lower flow rate) than the flow rate 40 so as the partial flow rate 43 (i.e., its slope) is lower than flow rate 40.
A variant of the diagram of
In the plant 110 the first ORC cycle 120, at high temperature, corresponds to the previous first ORC cycle 20 examined in
As previously explained, by an evaporator a heat exchanger is meant which receives an organic working fluid in the liquid state and at a temperature close to that of evaporation. The difference between the evaporation temperature and the inlet temperature of the organic working fluid to be evaporated is defined “approach”.
Normally in an evaporator the thermal power to be supplied to evaporate the organic working fluid is strongly dependent with respect to the thermal power to be supplied to complete the preheating of the fluid, being approach only equal to few degrees centigrade. This definition applies to the evaporator 121 (as well as to the previous evaporator 21), whereas the second pre-heater 126 is a heat exchanger having a substantial function of an additional pre-heater, not being intended to evaporate the fluid but to preheat it with an increasing temperature greater than a few degrees centigrade (typically 2-5° C.) with respect to the “approach” described above made either for the evaporator 21 (or for the evaporator 121).
The second ORC cycle 130, at low temperature or in any case at a temperature lower than the first ORC cycle 120, comprises, as in the example of
The fluid of the hot source, for example a geothermal source, follows, as in the previous case, a path of thermal exchange with both ORC cycles. After being entered in the plant 110 at the entry point 141, it crosses with its whole flow rate 140 the evaporator 121 and the second pre-heater 126 of the first cycle ORC 120.
At the exit from the second pre-heater 126, the fluid of the hot source is divided into two flow rates: a first partial flow rate 143 is dependent from the second ORC cycle 130 and supplies, in cascade, the evaporator 131 and the pre-heater 135 of the second ORC cycle 130, whereas a second partial flow rate 142 is still dependent from the first ORC cycle 120 and supplies the first pre-heater 125 of the first ORC cycle 120. Finally, the partial flow rate 143 of the cycle 130 and the partial flow rate 142 of the cycle 120 join to form the full flow rate 140 leaving the plant at the exit point 144.
In
In this way, in relation to the working fluids used in the two cycles, the further optimized solution object of the present invention can be obtained.
As already mentioned, the invention also includes numerous other variants, among which, for purely illustrative purposes, some of them are highlighted.
First of all, the organic working fluids can be the same both for the first ORC cycle 20 at a high temperature and for the second ORC cycle 30 at a lower temperature, as in the scheme of
According to another embodiment, the electrical generator plant could be single and the two turbines could be both connected to the single electrical generator. Referring to
Indeed, instead of two turbines, have a single turbine 22a could be used with two pressure supply levels 22b, 22c (see
The solution of the double turbine entry has already been invented by the writer and is disclosed, for example, in document EP3455465. A further variant consists in providing a regeneration phase for the two cycles 20, 120 at high temperature and/or for the two cycles 30, 130 at a lower temperature. As is known, by a regeneration a heat exchange is meant which is carried out in a dedicated heat exchanger (regenerator) in which the expanded vapor of the organic working fluid coming from the turbine transfers heat to the same organic fluid in liquid phase coming from the supply pump to upstream of the pre heater or the pre-heaters.
In addition, should the plant needs require it, the schemes of
Similarly to what has been seen in the diagram of
In addition to the embodiments of the invention, as described above, it is to be understood that there are numerous further variants. It must also be understood that said embodiments are only examples and do not limit neither the object of the invention, nor its applications, nor its possible configurations. On the contrary, although the above description makes it possible for the skilled man to implement the present invention at least according to an exemplary configuration thereof, it must be understood that numerous variations of the components described are conceivable, without thereby departing from the object of the invention.
Number | Date | Country | Kind |
---|---|---|---|
102020000012907 | May 2020 | IT | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/IB2021/054564 | 5/26/2021 | WO |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2021/240379 | 12/2/2021 | WO | A |
Number | Date | Country |
---|---|---|
202007012871 | Dec 2007 | DE |
202007015236 | Feb 2008 | DE |
Number | Date | Country | |
---|---|---|---|
20230220789 A1 | Jul 2023 | US |