KR20070116106A - Cascaded Organic Rankine Cycle to Use Waste Heat - Google Patents

Abstract

The pair of organic Rankine cycle systems 20 and 25 are combined and their respective organic working fluids have a condensation temperature at which the organic working fluid of the first organic Rankine cycle is sufficiently higher than the boiling point of the organic working fluid of the second organic Rankine cycle system. Condensate in, a single common heat exchanger 23 is selected to be used as the condenser of the first organic Rankine cycle system and as the evaporator of the second organic Rankine cycle system. The preferred organic working fluid of the first system is toluene and the preferred organic working fluid of the second system is R245fa.

Description

Cascaded Organic Rankine Cycles for Waste heat Utilization

The U.S. federal government has certain rights in the present invention under the DE-FC02-00CH11060 contract between the Department of Energy and United Technologies Corporation.

Power generation systems that provide low-cost energy with minimal impact on the environment and can be immediately integrated into existing power grids or quickly installed as stand-alone units help address critical power demands in many areas. Combustion engines or reciprocating engines, such as microturbines, generate power with low efficiency of 25% to 40% using generally available fuels such as gasoline, natural gas and diesel fuel. However, atmospheric emissions such as nitric oxides (NOx) and particulates can be problematic for reciprocating engines.

One way to generate electricity from the waste heat of a combustion engine without increasing emissions production is to apply a bottoming cycle. The lower cycle uses waste heat from such an engine and converts its thermal energy into electricity. Rankine cycles are often applied as bottom cycles for combustion engines. The basic organic Rankine cycle consists of a turbogenerator, preheater / boiler, condenser, and liquid pump. This cycle may receive waste heat at a temperature slightly above the boiling point of the selected organic working fluid, and typically releases heat to ambient air or water at a temperature slightly below the boiling point of the selected organic working fluid. The choice of working fluid determines the temperature range / thermal efficiency characteristics of the cycle.

A simple ORC system using one fluid is efficient and cost effective when converting a low temperature waste heat source into power and using hardware and working fluids similar to those used in the air conditioning / refrigeration industry. An example is an ORC system using a radial turbine derived from a conventional centrifugal compressor and a working fluid such as the refrigerant R245fa.

For hot waste heat streams, the most cost-effective ORC systems still operate at relatively low working fluid temperatures, allowing still use of equipment and general refrigerants derived from HVAC. However, this system, although very cost effective, does not fully utilize the thermodynamic potential energy of the waste heat stream.

In summary, according to one aspect of the present invention, a pair of organic Rankine cycle (ORC) systems are combined and one common heat exchanger is used as the condenser in the first ORC system and as the evaporator in the second ORC system.

According to another aspect of the present invention, the refrigerant of the two systems is selected such that the condensation temperature of the first high temperature system is a temperature that can be used to boil the refrigerant of the second low temperature system. In this way, greater efficiency can be achieved and waste heat losses to the atmosphere are substantially reduced.

According to another aspect of the invention, a single common heat exchanger may be used to overheat and condense the working fluid of the first ORC system.

According to another aspect of the present invention, if the second heat exchanger is provided to the first ORC system with the common heat exchanger acting to overheat the working fluid of the first ORC system, the second condenser may draw the working fluid in the first ORC system. Act to condense.

According to another aspect of the invention, a preheater using waste heat is provided to heat the working fluid in the second ORC system before it enters the common heat exchanger.

In the drawings described hereafter, while the preferred and modified embodiments are shown, various other modifications and alternative arrangements of the embodiments can be made without departing from the true spirit and scope of the invention.

1 is a schematic diagram of an organic Rankine cycle system according to the prior art.

2 is a TS diagram of the organic Rankine cycle system.

3 is a schematic representation of a pair of organic Rankine cycle systems coupled in accordance with the present invention.

4 is a TS diagram of the organic Rankine cycle system.

5 is an alternative embodiment of the present invention.

Fig. 6 is a TS diagram of the embodiment.

Figure 7 is another alternative embodiment of the present invention.

8 is a TS diagram of the embodiment.

Referring now to FIG. 1, a conventional form of organic Rankine cycle is shown to include an evaporator / boiler 11 that receives waste heat from the sources described above. The heated working fluid travels to the turbine 12 which is converted into motive force for driving the generator 13. As a result, the low temperature low pressure working fluid is transferred to the condenser 14, where it is converted into a liquid and then pumped by the pump 16 to the evaporator / boiler 11.

In such conventional systems, the common working fluid is toluene. In the steam generator 11, the working fluid is raised to about 525 ° F. (274 ° C.) and then moved to the turbine 12. After passing through the turbine 12, the temperature of the steam drops to about 300 ° F. before it condenses and is pumped back to the evaporator / boiler 11.

The TS diagram of the organic Rankine cycle system shown in FIG. 1, using toluene as the working fluid, is shown in FIG. As can be seen, because of the relatively high critical temperature, toluene is thermodynamically more efficient than a system of working fluids having a low critical temperature. However, this is less cost effective and still lacks. There are two reasons for the high cost of such high temperature ORC systems. First, working fluids such as toluene with a high critical temperature allow to operate at high evaporation temperatures, which is relatively beneficial for efficiency, but requires very large and expensive condensation equipment as they exhibit very low concentrations at ambient conditions. Secondly, the nature of these high critical temperature organic fluids is that the higher the turbine pressure ratio (typically 25: 1 or higher in such a system), the more superheated steam leaving the turbine. Thus, the thermal energy represented by the overheating of the steam leaving the turbine is not used for power generation and requires additional condenser surfaces to be discharged to the environment. Thus, there is a substantial amount of low temperature waste heat (i.e., heat of superheated low pressure steam leaving the turbine) that does not convert to power to limit the efficiency of the turbine.

Referring now to FIG. 3, there is shown a modified device including a pair of organic Rankine cycle systems 20, 25 coupled in a manner to be described now. Evaporator boiler or steam generator 17 receives heat from heat source 18 and produces a relatively high pressure hot steam that travels to turbine 19 to drive generator 21. After passing through the turbine 19, the low pressure low temperature steam moves to the condenser / evaporator 23, where the steam is condensed into liquid and then back to the steam generator 17 by the pump 24 to become steam. Pumped into.

Generally, unrecovered microturbines have an exit temperature of about 1200 ° F. (649 ° C.) exhaust gas. Such hot gases can be used to boil hot organic fluids such as pentane, toluene, or acetone in the ORC. If toluene is the working fluid, the temperature leaving steam generator 17 will be about 500 ° F. (260 ° C.), and the temperature of the steam leaving turbine 19 and entering condenser 23 will be about 300 ° F. (149 ° C.). Will be. After the liquid toluene has condensed, the temperature is about 275 ° F. (135 ° C.) when it leaves the condenser 23 and is moved by the pump 24 to the steam generator 17. This temperature and associated entropy are shown in the TS diagram of FIG.

In such cascaded ORC devices, the first ORC system (i.e., toluene loops) is characterized by sensible heat from hot gases or hot liquids or latent heat from condensation of fluids such as steam in a refrigerant boiler / evaporator. It is a high temperature system that extracts all the same heat, producing high pressure and high temperature steam. This high pressure steam expands through the turbine 19 to a low pressure with a saturation temperature corresponding to the level at which a low cost / low temperature ORC system can be used to efficiently and cost effectively convert low temperature waste heat into power. By doing so, the hot refrigerant still has a positive pressure and corresponding large density in the condenser 23. This results in less pressure drop, better heat transfer and smaller size condenser, all of which result in a more cost effective ORC system. The high pressure and large density of steam present in the turbine 19 also allows for a compact turbine design. Substantial reductions in costs can be achieved by this modification. In addition, a low pressure ratio (ie 5: 1) in the turbine 19 allows for high turbine efficiency.

Considering now that the temperature of the toluene vapor entering the condenser / evaporator 23 is relatively high, the condenser / evaporator 23 acts as a condenser in the first ORC system 20 and of the second ORC system 25. In the state acting as an evaporator or boiler, the energy can now be used as a heat source for the steam generator of the second ORC system 25. Thus, the second ORC system has a turbine 26, a generator 27, a condenser 28 and a pump 29. The organic working fluid for the second ORC should have a relatively low boiling and condensation temperature. Examples of stable organic working fluids in this cycle are R245fa or isobutane.

In the second ORC system 25 having R245fa as the organic working fluid, the temperature of the working fluid moving to the turbine 26 is about 250 ° F., and the temperature of the steam moving to the condenser is about 90 ° F. After condensing the vapor, the refrigerant is pumped to the condenser / evaporator 23 by the pump 29.

Referring to FIG. 5, an alternative analogous embodiment is shown in which the working fluid moves from the boiler or steam generator 17 to the turbine and back to the common heat exchanger 31 in the toluene circuit. Again, the heat exchanger 31 is an evaporator in the R245fa circuit in a state in which the R245fa refrigerant moves from the boiler 31 to the turbine 26, the condenser 28, the pump 29 and the boiler 31 again. It acts as a boiler. However, unlike the condenser / evaporator 23 of the embodiment of FIG. 3, the heat exchanger 31 has a condenser 28 completing the condensation process before the working fluid moves to the boiler 17 via the pump 24. In the state of being applied to act as a desuperheater only in the toluene circuit. The TS diagram of such a similar ORC cycle system is shown in FIG.

In this similar device, cost reduction is achieved by adding a low temperature R245fa ORC system in such a way that the overall system efficiency is increased. The main irreversibility (thermodynamic losses) of a simple cycle high temperature ORC system is the so-called overheating loss in the condenser. The organic fluid is desuperheated more than it enters the turbine and leaves the turbine. The greater the pressure ratio in the turbine, the stronger this effect. The high temperature simple cycle ORC system is thermodynamically more efficient than the low temperature ORC system, which is a simple cycle, but generates a large amount of mesothelial waste heat that must be discharged from the superheat reducer / condenser. As a result, a relatively large condenser is required. In a similar ORC system, overheating reduction takes place in a low temperature ORC evaporator 31. This increases the overall power output since the heat was previously released to ambient and is now used to generate power in low temperature ORC systems. Another advantage is that the size of the high temperature ORC condenser 32 can be reduced.

Thus, the overall result of a similar ORC system is a more cost effective overall ORC system for hot waste heat sources. Increased cost efficiency is achieved by reducing the power output and reducing the size of the original superheater / condenser unit.

Although the embodiment of FIG. 5 has been described using two different refrigerants, it should be appreciated that the same refrigerant may be used in two circuits.

Another embodiment of the present invention is shown in FIG. 7, in which the embodiment of FIG. 5 is adapted by adding a preheater 33 in the illustrated R245fa cycle. Here, the working fluid passes through condenser 28 and pump 29 and then through liquid preheater 23 using a waste heat source at low temperature (400 ° F. to 200 ° F.). The corresponding TS diagram is shown in FIG.

Although the invention has been particularly shown and described with reference to the preferred and alternative embodiments as shown in the drawings, various changes may be made in the details without departing from the spirit and scope of the invention as defined by the claims. Will be understood by those skilled in the art.

Claims (23)

An organic Rankine cycle system comprising a turbo generator for receiving a first organic fluid from a steam generator, a first condenser, and a first pump for returning a refrigerant to the steam generator in a continuous flow relationship, the generating additional energy. How to, A second organic generator having a second turbogenerator for receiving a second organic working fluid from the condenser, a second condenser, and a second pump for returning the second organic working fluid to the condenser in a continuous flow relationship; Providing a cycle system, And the first and second organic working fluids flow through the first condenser in a heat exchange relationship. The method of claim 1 wherein the first organic working fluid is toluene. The method of claim 1 wherein the second organic working fluid is R245fa. The method of claim 1, comprising overheating and condensing the first organic fluid in a first condenser. The method of claim 1 including providing a third condenser between the first condenser and the first pump. 6. The method of claim 5, comprising overheating the first organic fluid in a first condenser and condensing the first organic fluid in a third condenser. 2. The method of claim 1 including providing a preheater between the second pump and the first condenser. Is a combination of organic Rankine cycle systems, A first organic Rankine cycle comprising a first turbogenerator for receiving a first organic working fluid from a steam generator, a first condenser and a first pump for returning the first organic working fluid to a steam generator in a continuous flow relationship System, A second turbogenerator for receiving a second organic working fluid from the first condenser, a second condenser, and a second pump for returning the second organic working fluid to the first condenser in a continuous flow relationship; A second organic Rankine cycle system, Wherein said first and second organic working fluids are circulated in a heat exchange relationship within said first condenser. 9. The combination of organic Rankine cycle system according to claim 8, wherein said first organic working fluid is toluene. 9. The combination of organic Rankine cycle system as recited in claim 8, wherein said second organic working fluid is R245fa. 10. The combination of claim 8, wherein said first condenser operates to overheat and condense said first organic working fluid. The combination of claim 8 comprising a third condenser between the first condenser and the first pump. 13. The combination of claim 12, wherein the first condenser is only applied to overheat the first organic working fluid and the third condenser is adapted to condense the first organic working fluid. 10. The combination of claim 8, comprising a preheater between the second pump and the first condenser. Is a system for converting waste heat into energy, A steam generator in heat exchange relationship with the waste heat, a first turbo generator for receiving a first organic working fluid from the steam generator, a first condenser, and a first for returning the first organic working fluid to the steam generator. A first organic Rankine cycle system having one pump in a continuous flow relationship, A second turbogenerator for receiving a second organic working fluid from the first condenser, a second condenser, and a second pump for returning the second organic working fluid to the first condenser in a continuous flow relationship; A second organic Rankine cycle system, And the first organic working fluid moves from the first condensation temperature to the first condenser, the condensation temperature being substantially above the boiling temperature of the second organic working fluid. The system of claim 15, wherein the first organic working fluid is toluene. The system of claim 15, wherein the second organic working fluid is R245fa. 16. The system of claim 15, wherein the first condenser operates to overheat and condense the first organic working fluid. The system of claim 15, comprising a third condenser between the first condenser and the first pump. 20. The system of claim 19, wherein the first condenser is only applied to overheat the first organic working fluid and the third condenser is applied to condense the first organic working fluid. . The system of claim 15 comprising a preheater between the second pump and the first condenser. The method of claim 1, wherein the first and second organic working fluids are different kinds of fluids. The combination of claim 8, wherein the first and second organic working fluids are different kinds of fluids.

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