WO2008125827A2

WO2008125827A2 - Organic rankine cycle apparatus and method

Info

Publication number: WO2008125827A2
Application number: PCT/GB2008/001286
Authority: WO
Inventors: Ian Kenneth Smith; Nikola Rudi Stosic; Herman Leibowitz
Original assignee: Electratherm Inc
Current assignee: Electratherm Inc
Priority date: 2007-04-13
Filing date: 2008-04-11
Publication date: 2008-10-23
Anticipated expiration: 2009-10-13
Also published as: WO2008125827A3; GB0919961D0

Abstract

An organic Rankine cycle apparatus and method for generating power from two different sources of heat (60,110) at different temperatures comprising a lower pressure organic working fluid circuit (10) and a higher pressure organic working fluid circuit (20) in fluid communication with one another. A portion of the organic working fluid is heated by a higher temperature heat source and is expanded at higher pressure in a higher pressure expander (120) to generate mechanical power before being combined with a portion of the working fluid that is partially evaporated at a lower temperature by heat from a lower temperature heat source (60), such that the total mass of the organic working fluid is expanded in a lower pressure expander (140) to generate additional mechanical power.

Description

ORGANtC RANKINE CYCLE APPARATUS AND METHOD

FIELD OF THE INVENTION

The invention relates to an organic Rankine cycle apparatus and method for generating mechanical power from two different heat sources operating at different temperatures using a single apparatus.

BACKGROUND OF THE INVENTION

A number of processes which require the combustion of fuel reject heat that can be recovered by an apparatus for generating power. In many cases, such heat may be rejected in two streams at very different temperatures. Typically, one source of heat can be directly from the actual combustion products which, after supplying heat to a process, may still be at temperatures of several hundred degrees centigrade. The other source of heat can be from the process for which the original combustion_ofiuel is required. This can be at temperatures of the order of only 100°C, or even less.

An example of two such sources is the heat rejected from an internal combustion (IC) engine, where the exhaust gases leave the engine at a temperature of 350-500°C while the cooling jacket surrounding the engine block rejects heat from the jacket coolant to the atmosphere at temperatures of the order of 70-90^°C. In such cases, it is known to provide a separate system for recovering heat from each heat source to generate power.

Organic Rankine Cycle Technology

Known systems for power recovery from low grade heat sources operate on a sequence of processes known as an Organic Rankine cycle (ORC). An

ORC is in essence identical to a steam power plant but uses organic fluids such as light hydrocarbons or common refrigerants as the working fluid instead of water. These fluids have unique properties and much of the art of getting the best result from them is based on the choice of the most suitable fluid for each application. Commonly used or considered organic fluids are either refrigerants, such as R124 (Chlorotetrafluorethane), R134a (Tetrafluoroethane) or R245fa (1 ,1,1 ,3,3-Pentafluoropropane), or light hydrocarbons such as isoButane, n- Butane, isoPentane and n-Pentane. Some systems incorporate highly stable thermal fluids, such as the Dowtherms and Therminols but the very high critical temperatures of these fluids create a number of problems in system design which lead to high cost solutions.

The essential features of an ORC system and the mechanical components needed to implement the cycle are shown in Fig 1. There are five main components: a feed pump 20, a feed heater 30, an evaporator 50, an expander 120 and a condenser 150. These are all connected in a closed loop, which must be well sealed in order to prevent any significant loss of the working fluid to the atmosphere, and any inward leakage of air into the circuit. In this simple system, the working fluid is pressurised, heated to its boiling point, evaporated, expanded from the saturated vapour condition, condensed and then repressurised.

A major difference between organic working fluids and steam is that when organic working fluids are expanded from the saturated vapour point, they leave the expander with some superheat which needs to be removed in the condenser before condensation begins, as indicated in the temperature-entropy diagram in

Fig 1. This can produce some benefits in the design of the expander, since steam expanding from the saturated vapour state leaves the expander as wet vapour and this reduces the expansion efficiency when a turbine is used for this process. There are a number of other advantages to the use of organic fluids in place of steam but their description is not relevant to the background to this invention.

Compared to water, the critical temperatures of the commonly used fluids are low, being generally in the range of approximately 100^°C- 200^°C. Thus, should the heat source be at a comparatively higher temperature, such as engine exhaust gases (typically 350-500°C), then there are some variants possible to the cycle and a commonly considered alternative is shown in Fig 2.

In this case, the vapour entering a boiler is heated in a feed heater 30, evaporated in an evaporator 50 and then superheated in a superheater 160 in order to maximise the working fluid temperature. Significantly higher cycle efficiencies can thereby be achieved but the system requires a superheater 160 in addition to the feed heater and the evaporator, and a recuperative heat exchanger 200 recaptures heat from the superheated vapour leaving the expander which is then used to preheat the working fluid entering the feed heater. Without this latter feature, gains in efficiency resulting from raising the maximum fluid temperature may only be low.

ORC Operation with Screw Expanders

In a large majority of known ORC systems, the expander is a turbine and usually one of the radial inflow type. However for power outputs of up to approximately 1 MW, screw expanders have some advantages.

First, unlike turbines, screw expanders can admit wet vapour from the boiler without the risk of incurring mechanical damage or of loss in adiabatic efficiency. This characteristic produces significant advantages in systems where the cycle is of the simple evaporative type, such as that shown in Fig 1. Thus, with a screw expander, this cycle can operate in the mode shown in Fig 3 where the working fluid entering the expander can be sufficiently wet to eliminate the need to desuperheat it after expansion. This also reduces the proportion of heat transferred in the evaporator at constant temperature and thereby the evaporation temperature using the same heat source can be higher than it would otherwise be if complete evaporation took place. When combined, these effects lead to increased work per unit mass flow in the expander and, taken overall result in gains in cycle efficiency of the order of 5%. Also, if the fluid leaves the evaporator as wet vapour, it can be heated in a single pass heat exchanger that combines feed heating and partial evaporation in one unit. The net result is higher power output and lower cost heat exchangers.

Another advantage to wet admission of the working fluid is, that provided some 1-5% oil is dissolved in the liquid, the oil deposits itself on the rotors as the expansion proceeds and the working fluid evaporates. This, together with a supply of the unheated liquid to the bearings, enables the conventional lubrication systems normally required for both screw expanders and turbines to be dispensed with and thereby results in a significant reduction in cost. Because screw expander machines operate at relatively low tip speeds, even small units can be linked to a standard 50/60Hz generator, without the need for a reduction gearbox.

Finally, provided that the operating pressures do not exceed about 30 bar, standard, low cost, mass produced screw compressors, used for compressing air, can be readily and cheaply adapted to act as expanders in such ORC systems.

SUMMARY OF THE INVENTION

The invention provides an organic Rankine cycle apparatus for generating power from two different sources of heat at different temperatures as defined by independent claim 1 to which reference should now be made.

The invention also provides a method of generating power from two different sources of heat at different temperatures using an organic Rankine cycle as defied by independent claim 10 to which reference should now be made.

Some preferred features of the invention are set out in the dependent claims to which reference should also now be made.

It was previously thought that converting heat from a high temperature heat source to mechanical power was achievable with approximately twice the efficiency of converting heat from lower temperature heat sources. However, we have appreciated that there are potential operational and cost advantages to be gained from capturing heat from both a higher and a lower temperature heat source in a single power recovery system.

Preferred embodiments of the invention provide an organic Rankine cycle apparatus and a method of generating power based on an organic Rankine cycle system in which power is generated from two different heat sources at different temperatures in a single apparatus. This is achieved with negligible performance penalties compared to a system in which a separate power recovery system is used to recover heat from each of the heat sources. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a known type of organic Rankine cycle system in which saturated dry vapour is admitted to an expander, together with an associated temperature-entropy diagram;

Figure 2 is a schematic illustration of a known type of superheated recuperative organic Rankine cycle system in which superheated vapour is admitted to an expander, together with an associated temperature-entropy diagram;

Figure 3 is a schematic illustration of a known type of organic Rankine cycle system having a screw expander capable of admitting wet vapour (a "wet organic Rankine cycle system"), together with an associated temperature-entropy diagram;

Figure 4 is a schematic illustration of a first embodiment of an organic Rankine cycle system of the invention in which dry saturated vapour is delivered to a higher pressure and a lower pressure expander;

Figure 5 is a temperature-entropy diagram for the organic Rankine cycle system of Figure 4;

Figure 6 is a temperature-entropy diagram of a second embodiment of the invention in which wet vapour is admitted to the lower pressure and higher pressure expanders;

Figure 7 is a schematic illustration of an organic Rankine cycle system of a third embodiment of the invention which includes a superheater between the second evaporator and the higher pressure expander for supplying superheated steam to a higher pressure evaporator.

Figure 8 is a temperature-entropy diagram for the organic Rankine cycle system of Figure 7; Figure 9 is a temperature-entropy diagram of a fourth embodiment of the invention in which superheated vapour is admitted to the higher pressure expander and wet vapour is admitted to the lower pressure expander;

Figure 10 is a schematic illustration of a fifth embodiment of the invention in which the higher pressure expansion is carried out in two expander stages;

Figure 11 is a schematic illustration of a sixth embodiment of the invention in which the lower pressure expansion is carried out in two expander stages;

Figure 12 is a schematic illustration of the output shafts of the expanders of any of the first to the sixth embodiments being mechanically coupled with a drive belt to drive a generator;

Figure 13 is a schematic illustration of a known simple organic Rankine cycle system used to recover heat both from the cooling jacket and exhaust gas stream of an internal combustion engine, in order to provide a comparison with the performance of the Rankine cycle system of embodiments of the invention.

DETAILED DESCRIPTION

A first embodiment of the invention is shown in Figure 4, and Figure 5 shows the corresponding temperature-entropy diagram. In a first organic working fluid circuit 10, an organic working fluid is pressurised to a lower evaporative pressure by a lower pressure pump 20, leaving the lower pressure pump at state 1 and entering a first, lower pressure feed heater 30. The organic working fluid leaves the first feed heater as a saturated liquid at the evaporative temperature (state 2), this being dependent upon the lower pressure feed pump discharge pressure.

The flow of the organic working fluid is then divided at a junction 40 so that a portion of the saturated liquid enters a first, lower pressure evaporator 50, in which it is partially evaporated to state 3. The heat required for heating the organic working fluid in the first feed heater 30 and evaporating the organic working fluid into a vapour in the first evaporator is supplied by a lower temperature heat source 60, entering the first evaporator at point C and leaving the first feed heater at state D. The remaining portion of the organic working fluid leaving the first feed heater 30 enters a second organic working fluid circuit 70 under the action of a higher pressure feed pump 80, the working fluid leaving the higher pressure pump at a higher pressure in state 4. This portion of the organic working fluid then passes through a second feed heater 90, the fluid leaving the second feed heater as a saturated liquid at state 5. The saturated liquid then enters a second, higher pressure evaporator 100, where it is evaporated to a dry vapour state (state 6). The heat required for heating the organic working fluid in the second feed heater and evaporating the organic working fluid into a vapour in the second evaporator is supplied by a higher temperature heat source 110 entering the second evaporator 100 at point A and leaving the second feed heater 90 at state B.

The higher pressure vapour leaving the second evaporator is expanded in a higher pressure expander 120, leaving as superheated vapour at an intermediate pressure at state 8. This superheated vapour then mixes at a junction 130 with the partially evaporated vapour leaving the first evaporator at state 3, such that the superheated vapour delivers heat to the partially evaporated vapour, thereby having a similar effect to that of a recouperator. The total mass of the organic working fluid then enters a lower pressure expander 140 as dry saturated vapour at state 9. The organic working fluid is expanded to state 10, where it enters a desuperheater-condenser 150 which desuperheats and condenses the working fluid to a saturated liquid at state 11. Finally, the working fluid is returned by the lower pressure feed pump to the first feed heater to complete a working fluid cycle.

As dry vapour is admitted to each of the lower pressure and higher pressure expanders, the expanders may be turbines such as those of the radial inflow type.

A second embodiment is shown in Figure 6, in which the expanders 120,140 are of the positive displacement type, more specifically of the twin screw or scroll type. In this embodiment, the organic working fluid leaves the higher pressure evaporator 100 as wet vapour (state 6), typically with a dryness fraction of approximately 75%, so that the working fluid leaves the higher pressure expander 120 substantially in the dry saturated vapour phase (state 8). The precise state of the working fluid leaving the expansion stage is dependent upon on the range of temperatures and pressures at which the Rankine cycle system operates, hence the dryness fraction may be varied to control the state of the working fluid leaving the higher pressure expander 120.

A third embodiment is shown in Figures 7 and 8 in which the higher temperature heat source delivers heat to a superheater 110 which superheats the dry saturated vapour leaving the second evaporator 100, so that the working fluid is delivered to the higher pressure expander 120 as superheated vapour (state 7). The vapour expanded in the higher pressure expander (at state 8) then combines with the partially evaporated vapour leaving the first evaporator 50 so that the organic working fluid is delivered to the lower pressure expander 140 as dry saturated vapour (state 9).

A fourth embodiment is shown in Figure 9, this being a modification of the third embodiment. In this embodiment, the organic working fluid enters the higher pressure expander as superheated vapour as in the third embodiment however, when the portion of the organic working fluid from the second working fluid circuit combines with that leaving the first evaporator, the total mass of the working fluid enters the lower pressure expander 140 as wet vapour (state 9) which is expanded to dry saturated vapour in the lower pressure expander (state 10).

Depending on the nature of the two heat sources 60,110 and their respective temperatures, it is usually more advantageous for the fluid to enter the lower pressure expander 140 in this wet vapour state. There are several thermodynamic advantages to the cycle arrangement shown in Fig 9, some of the most significant of which are as follows:

i) unlike the feed heating requirements for two separate Rankine cycle systems, heat being delivered to each system from a separate heat source, none of the heat from the higher temperature source 110 is required for feed heating of the working fluid below an intermediate working fluid temperature (i.e. following expansion in the higher pressure expander);

ii) using the superheated vapour leaving the higher temperature expander 120 to evaporate the partially evaporated vapour leaving the first evaporator 50, recovers heat which would otherwise be wasted since it is now partially converted to work as the organic working fluid is expanded in the lower pressure expander 140;

iii) using the superheated vapour leaving the higher temperature expander 120 to partially evaporate the working fluid at an intermediate pressure (Figure 11) reduces the proportion of the heat supplied from the lower temperature source 60 required for evaporation of the working fluid. This enables the intermediate evaporation temperature to be increased while still recovering the same amount of heat and thereby:

a) increases the mass flow of fluid in the higher pressure working fluid circuit 70, thereby raising the power recovery from the higher pressure expander 120;

b) increases the mass flow in the lower pressure circuit 10, thereby raising the power recovered in the lower pressure expander 140;

^■ c) raises the thermodynamic efficiency of the lower pressure working fluid circuit 10.

The cumulative effect of these advantages is to make the overall efficiency of the Rankine cycle system of the invention higher than the use of two independent Rankine system cycles without recuperation.

In a fifth embodiment shown in Figure 10, the higher pressure expansion stage takes place in two discrete expander stages 120,121. This is because, the pressure ratio across a twin screw machine at which expansion can take place efficiently, is more limited than in a turbine and hence it may be advantageous, depending on the pressure ratio, to carry out this stage of the expansion in two expanders 120,121 in series. Thus, state 7 denotes an intermediate pressure condition on leaving the first stage higher pressure expander 120, while the higher pressure expansion is completed in a second stage higher pressure expander, leaving that expander in state 8. Because these units are not intended for large power outputs, if this stage of expansion is divided into two, the size of the higher pressure machine is relatively small and the additional cost is not very significant. The expanded fluid at state 8 then mixes with the partially evaporated vapour at state 3 at constant temperature, so that the total mass of the working fluid enters the lower pressure expander as wet vapour at state 9, the vapour being approximately 90% dry. The total flow then expands to condensing conditions at state 10, which, as in state 8, is approximately dry saturated vapour.

In a sixth embodiment as shown in Figure 11 , depending on the temperatures and heat available from each heat source, it may be more advantageous for the higher pressure expansion to be carried out in a single expander stage 120, and the lower pressure expansion to be carried out in two expanders 140,141 in series.

As shown in Figure 12, whether the higher pressure and/or lower pressure expansion is carried out in two discrete expander stage, the three expanders can be mechanically coupled by a drive belt 170, or other suitable equivalent to drive an electrical generator 180 to generate electrical power.

Example

To give an example of the advantages that are attainable from the organic Rankine cycle apparatus and method of the invention when it incorporates screw expanders, a study was made of the recovery of power from the waste heat of a typical stationary gas engine used for electrical power generation.

The engine considered was a GE Jenbacher J320GS-LL. This engine has a rated electrical power output of 1065kW. The recoverable heat from the exhaust gases in cooling from 450^°C to 150^°C is 543kW, while the heat that has to be rejected from the coolant to the surroundings is 604kW to return it at 70^°C, after leaving the jacket at 90^°C.

Four cases were analysed. In all of them, it was assumed that the heat finally rejected from the waste heat power recovery system to the surrounding atmospheric air is at a temperature corresponding to annual average ambient conditions in the UK. In all four cases, the working fluid was taken to be R245fa. Cost estimates were made for all four systems, based on vendors¹ price quotations for the key components and common agreed costing procedures, as used in the power generation industry. Regardless of the absolute accuracy of the cost and power estimates, the reliability of the estimates, for comparative purposes, is high.

The first case considered (Figure 13) was one intended to minimise the system cost by using the engine coolant circuit (210) of an engine (220) to recover using a heat exchanger (230) the heat from the exhaust gases (240), in addition to the jacket heat, and then to transfer the entire heat to a single wet organic Rankine cycle system boiler (250) with a single screw expander (120) to recover the power. The disadvantage of this is that the maximum temperature of the coolant would only be raised from 90^°C to approximately 110^°C and this would result in a lower cycle efficiency and hence less power recovery than from two separate organic Rankine cycle systems, one for each source of heat.

The second case considered was to use a simple wet organic Rankine cycle system, as shown in Fig 3, to recover power from the jacket heat and a separate superheated organic Rankine cycle system, without a recuperative heat exchanger, to recover heat from the exhaust gases.

The third case considered was to use a simple wet organic Rankine cycle system, as shown in Fig 3, to recover power from the jacket heat and a separate superheated recuperative cycle, as shown in Fig 2, to recover heat from the exhaust gases.

The fourth case considered was a dual pressure cycle system as shown in Figs 4 and 6, where the higher pressure and lower pressure expanders are of the twin screw or scroll type such that they can admit wet vapour. The results of the study are contained in the following table.

The most important criterion for the power plant owner is the cost per unit output, since this determines the economic viability of installing a waste heat power recovery system.

It is clear that the dual pressure system (i.e. one having a lower pressure and higher pressure organic working fluid circuit according to the invention - case

4) is the most preferred system as it produces 30% more power than the simple low cost system (case 1) at a saving of 18% in cost per unit output, it produces

9% more power than two separate simple systems (case 2) with a saving in cost per unit output of 24% and, for a sacrifice of only 1% in net power produced from two separate systems with recuperative heating (case 3), the cost per unit output

- the most important criterion - is reduced by 19%.

Claims

1. An organic Rankine cycle apparatus for generating power from two different sources of heat at different temperatures, comprising: a lower pressure organic working fluid circuit (10) comprising a first feed heater (30) for supplying heat to the organic working fluid, a first evaporator (50) for evaporating the organic working fluid into a vapour, heat being delivered to the first feed heater (30) and first evaporator (50) from a lower temperature heat source (60), a lower pressure expander (60) for expanding the vapour to generate mechanical power, a condenser (150) for condensing the expanded vapour, and a lower pressure pump (20) for returning the organic working fluid to the first feed heater (30); and a higher pressure organic working fluid circuit (70) comprising a second feed heater (90) and second evaporator (100) for heating and evaporating the organic working fluid using heat delivered from a higher temperature heat source (110), a higher pressure expander (120) for expanding the vapour to generate additional mechanical power, and a higher pressure pump (80) for returning the organic working fluid to the second feed heater (90), wherein the lower pressure (10) and higher pressure (70) circuits are in fluid communication with one another, and the apparatus is arranged so that the lower pressure pump (20) pumps only a portion of the organic working fluid leaving the first feed heater (30) through the first evaporator (50), and the higher pressure pump (80) pumps the remaining portion of the organic working fluid leaving the first feed heater (30) through the second feed heater (90) and second evaporator (100) to the higher pressure expander (120), and wherein the portion of the organic working fluid leaving the higher pressure expander (120) is combined with the portion of the organic working fluid leaving the first evaporator (50), so that the total mass of the organic working fluid is expanded in the lower pressure expander (140) and condensed in the condenser (150) before being returned to the first feed heater (30).

2. An apparatus according to claim 1, comprising a superheater (160) for superheating the organic working fluid leaving the second evaporator (100).

3. An apparatus according to claim 1 or 2, wherein the condenser (150) is a desuperheater-condenser.

4. An apparatus according to any of claims 1 to 3, wherein the higher pressure and lower pressure expanders (120,140) are of the positive displacement type.

5. An apparatus according to claim 4, wherein the expanders (120,140) are of the twin-screw or scroll type.

6. An apparatus according to any of the preceding claims, wherein the higher pressure expander (120) comprises a first (120) and a second (121) higher pressure expander connected in series.

7. An apparatus according to any of the preceding claims, wherein the lower pressure expander (140) comprises a first (140) and a second (141) lower pressure expander connected in series.

8. An apparatus according to any of the preceding claims, wherein the lower temperature heat source (60) is the coolant and the higher temperature heat source (110) is the exhaust gases of an internal combustion engine.

9. An apparatus according to any of the preceding claims, wherein the expanders (120,140) are mechanically coupled to drive a generator (170).

10. A method of generating power from two different sources of heat (60,110) at different temperatures using an organic Rankine cycle comprising the steps of:

(i) pumping an organic working fluid through a first feed heater (30) at a lower pressure;

(ii) delivering heat to the organic working fluid in the first feed heater (30) from a lower temperature heat source (60);

(iii) pumping a portion of the organic working fluid leaving the first feed heater (30) to a first evaporator (50);

(iv) evaporating the portion of the organic working fluid from the first feed heater (30) using heat from the lower temperature heat source (60); (v) pumping a remaining portion of the organic working fluid leaving the first feed heater (30) to a second feed heater (90) at a higher pressure; (vi) delivering heat to the organic working fluid in the second feed heater (90) from the higher temperature heat source (110);

(vii) evaporating the organic working fluid leaving the second feed heater (90) in a second evaporator (100) using heat from the higher temperature heat source (110);

(viii) expanding the vapour from the second evaporator (100) in a higher pressure expander (120) to generate mechanical power;

(ix) combining the expanded vapour from the higher pressure expander (120) with the portion of the working fluid leaving the first evaporator (50);

(x) expanding the total mass of the working fluid in a lower pressure expander (140) to generate additional mechanical power;

(xi) condensing the expanded vapour in a condenser (150);

(xii) pumping the condensed organic working fluid to the first feed heater (30) at a lower pressure.

11. A method according to claim 10, wherein the second evaporator (100) delivers dry saturated vapour to the higher pressure expander (120).

12. A method according to claim 10 or 11 , wherein the first evaporator (50) delivers dry saturated vapour to the lower pressure expander (140).

13. A method according to any of claims 10 to 12, including the step of superheating the evaporated organic working fluid leaving the second evaporator (100) with heat form the higher temperature heat source (110).

14. A method according to claim 10 wherein the second evaporator (100) supplies the higher pressure expander (120) with wet vapour.

15. A method according to claim 14 wherein the wet vapour has a dryness fraction of approximately 75 percent.

16. A method according to claim 10, 13, 14 or 15, wherein the first evaporator supplies the lower pressure expander with wet vapour.

17. A method according to claim 16, wherein the wet vapour has a dryness fraction of approximately 90 percent.

18. A method according to any of claims 10 to 17, wherein the higher pressure expander (120) expands the vapour in two separate expansion stages (120,121).

19. A method according to any of claims 10 to 18, wherein the lower pressure expander (140) expands the vapour in two separate expansion stages (140,141).

20. A method according to any of claims 10 to 19, wherein the step of condensingjhe expanded vapour includes desuperheating the expanded vapour.