WO2005031123A1

WO2005031123A1 - Deriving power from a low temperature heat source

Info

Publication number: WO2005031123A1
Application number: PCT/GB2004/004089
Authority: WO
Inventors: Ian Kenneth Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-09-25
Filing date: 2004-09-24
Publication date: 2005-04-07
Anticipated expiration: 2006-03-25
Also published as: GB0322507D0

Abstract

A fluid having a positive-slope for a vapour part of its temperature-entropy diagram, such as n-pentane, is heated as a liquid under pressure in a heat exchanger (13) by heat from the source (12) and expanded to generate power in a two phase expander (17) to form a homogenous mixture of liquid and vapour which is delivered to a separator (20). The separated vapour is expanded in a turbine (25) to generate further power before being condensed in a condenser (28), the condensate being supplied to a first feed pump (41). The output of the pump (41) and the relatively small amount of liquid from the separator (20) are returned by a second feed pump (42) to the heat exchanger (13). In a modified arrangement, the liquid from the separator is returned by a further feed pump to an intermediate point of the heat exchanger.

Description

DERIVING POWER FROM A LOW TEMPERATURE HEAT SOURCE

This invention relates to power production from low temperature heat sources, where the available temperature is too low for a steam cycle, for example, from a flow of a single phase fluid, such as pressurised hot water. Examples of this are found occasionally in industrial processes, and are more widely available from geothermal brines obtained from natural aquifers or possibly, from artificially created aquifers obtained from the deep drilling and fracturing of rock formations in what is known as Hot Dry Rock (HDR) .

One method of recovering power from such resources, has been described previously by I . K. Smith et al in the Proceedings of the Institution of Mechanical Engineers, Part A at 207 (A3) pp. 179-194, 1993; 208 (A2) pp. 135-144, 1994; and 210 (A2) 75-93, 1996. It is an essentially simple arrangement of components in which a volatile fluid, preferably a light hydrocarbon, such as isobutane, receives heat, at constant pressure, from the brine in a counter flow heat exchanger. Thereby, its temperature is raised from a highly subcooled condition at normal power plant condensing temperatures of, say 20-40°C, up to its saturation point, without evaporation. The hot liquid is then expanded in a two-phase expander and then condensed in an air or water cooled condenser. It is then repressurised in a feed pump and returned to the heater to complete the cycle. It has been shown that because the single-phase heating of the working fluid closely matches the cooling of the heating medium by this means more heat can be recovered while simultaneously the working fluid can attain higher maximum temperatures than in other systems used or recommended for this purpose, namely, Organic Rankine (ORC) and Kalina type cycles. For brine temperatures of up to approximately 120°C, it is possible to perform the entire expansion in a single stage expander by use of a twin screw expander or a turbo- expander of the radial inflow type, as manufactured by specialist companies for the petrochemical industry.

At higher brine temperatures, when the volume ratios of the expansion process exceed about 25:1, it is not possible to obtain efficient expansion in a single stage and a two- stage expansion process is then required. The difficulty then arises that for substantial power outputs, in excess of say 1 MW, the size of screw expander required becomes very large while the radial inflow turbine is not suited to the ingestion of wet vapours.

According to one aspect of the invention, there is provided apparatus for deriving power from a low temperature heat source comprising a heat exchanger for heating a liquefied volatile fluid under pressure with heat from the source, a primary two-phase expander connected to receive heated liquid from the heat exchanger and thereby generate power, a liquid/vapour separator connected to receive the fluid from the primary expander, means to direct the separated vapour stream to a further power generating expander and thence to a condenser, and feed pump means to return both the separated liquid phase stream and the condensed vapour phase stream to the heat exchanger under the said pressure

According to a further aspect of the invention there is provided a closed cycle method of generating power from a low temperature source of heat, including the steps of heating a liquefied volatile fluid under pressure in a heater with heat from the source, expanding the liquid to flash a portion thereof into vapour and thereby generate power, separating remaining liquid from the vapour and further expanding the vapour, condensing the vapour into liquid and pumping the condensed fluid back into the heater. The separated dry vapour may then be expanded at very high efficiency in a conventional vapour turbine of the axial or radial flow type while the liquid may be further expanded in a parallel screw or turboexpander. However, depending on the nature of the working fluid used and the maximum temperature attained by the working fluid in the heater, it is possible that the contribution of the additional liquid expansion to the total power generated by the system, may not be large, because the relative mass of separated liquid is small, or the volume ratio of expansion is too high or both. In that case, the separated liquid may be reinjected into the heat exchanger at an intermediate entry point with the aid of an additional feed pump. This has two potential advantages, namely: i) It reduces the total feed pump work, since the flow through the main feed pump is reduced while the additional feed pump does not have to operate over such a high pressure difference. ii) Returning the unexpanded liquid at the intermediate temperature has the effect of improving the cycle efficiency by raising the average temperature at which the heat is supplied in the cycle.

A further possibility is that where the gains achievable by returning the separated liquid to the heater by means of a second feed pump are not significant, this liquid may be throttled and returned via the condenser to the main feed pump. The advantage of this is in the saving of the total plant cost, since the additional costs of either the additional feed pump or turbine are not justified by the power gain thereby achieved. The invention will now be further described by way of example with reference to the accompanying drawings, in which: - Figs . 1 , 3 , 5,7 and 9 are diagrams showing the connection of the components of three embodiments of the invention; and Figs. 2, 4, 6, 8 and 10 are graphs showing temperature plotted against entropy for the embodiments shown respectively in figs. 1, 3, 5, 7 and 9.

Fig. 1 shows an installation for generating power from a source of relatively low temperature heat, for example, a geothermal installation in which brine is injected from a line 11 into a rock formation and thereby heated. Heated brine is pumped along a line 12 to a heat exchanger 13 before being returned to the line 11.

A pump 14 delivers a volatile fluid under pressure in • liquid form along a line 15 to the heat exchanger 13 where the liquid is heated in counter-flow to the brine. The volatile liquid enters the heat exchanger 13 at point 1 and leaves at point 2 by way of a line 16, the pressure being sufficient to maintain the fluid in its liquid phase.

The heated volatile liquid is supplied by the line 16 to a two-phase expander 17 of the twin screw or radial inflow turbine type (e.g., a Francis turbine) where some of the liquid during expansion flashes into vapour. It is not practical to make a single expander capable of expanding all liquid into vapour so that the fluid leaving the primary expander 17 (at point 3) is a homogenous mixture of liquid and vapour phases .

This homogenous mixture of liquid and vapour is conveyed by a line 19 to a separator 20 of the gravitational or centrifugal type, in which the liquid and vapour phases are substantially separated and delivered respectively to lines 21 and 22. The liquid phase (at point 3') is delivered to a further expander of the twin screw or radial inflow turbine type where some of the liquid flashes into vapour while generating power delivered to a shaft 24.

The separated vapour from line 22 (at point 3") is fed to the inlet of a high efficiency turbine 25 of the radial or axial flow type, where it generates further power which may be delivered to the shaft 24 to drive a further generator or other mode G.

The exhaust vapour from the expander 23 and turbine 25 is delivered (at point 4' and 4") by lines 26 and 27 to a condenser 28 where the vapour is condensed by heat exchange with a cooling system 29 and the resulting liquid is delivered by a line 30 (at point 5) to the inlet of the pump 14.

The cycles shown in the graphs in Figs . 2 , 4, 6, 8 and 10 result from using a working fluid having a high number of atoms per molecule, such as n-pentane. For such fluids, the temperature-entropy diagram for the vapour phase has a positive slope as opposed to water / steam where the corresponding slope for the vapour phase is negative.

As a result of the positive slope of the vapour phase part for n-pentane, the length of the line 3" -3 (which represents the portion of the mixture still in liquid form) is much less than the length of the line 3-3' (which represents the portion now in vapour phase) . Accordingly, the dryness of the homogenous mixture leaving the expander 17 is at least 70%. In contrast in the case of water, the mixture would only comprise about 10-20% vapour.

By expanding the liquid in the expander 17 as a homogenous mixture, a higher efficiency of 75% or more is obtainable as compared with impulse-type turbines previously proposed, in which the fluid is discharged through nozzles.

Fig. 3 shows a modification of the installation shown in Fig. 1, for use where the contribution of the liquid expansion in the expander 23 to the total power generated by the system is not large, either because the relative mass of separated liquid is small or the volume ratio of expansion is too high or both. In this modification, the relatively small liquid phase leaving the separator 20 is fed by a line 31 through a further feed pump 32, which delivers the liquid to an intermediate point of the heat exchanger 13. This has three potential advantages: i) It reduces the total feed pump work, since the flow through the main feed pump is reduced while the additional feed pump does not have to operate over such a high pressure difference. This effectively recovers some 12% of the power lost by omitting the second two-phase expander . ii) Returning the unexpanded liquid at the intermediate temperature has the effect of improving the cycle efficiency by raising the average temperature at which the heat is supplied in the cycle. iii) As a result of the improvement in the cycle efficiency, less external heat has to be removed in the condenser and supplied in the primary fluid heater and hence both these heat exchangers will be smaller and less expensive.

An alternative arrangement to that shown in Fig. 3 is shown in Fig. 5. This takes advantage of the fact that at the high pressure ratios normally associated with large volume ratio expansion, a two-stage feed pump is normally required formed by first and second pump stages 41 and 42 connected in series (or by two pumps by a line 43) . In that case, rather than use an additional feed pump (such as the pump 32 in Fig. 3) and admit the separated liquid at an intermediate point in the primary fluid heater, the fluid is admitted into the second feed pump stage by way of the line 43. This then raises the liquid admission temperature to the heater and hence results in less recovery of heat from the brine but with no other performance penalty and it still results in a correspondingly reduced size condenser.

Although the intention is primarily to maximise heat transfer to the power generation system and hence the power output from it, if the second stage two-phase expander 23 of Fig. 1 is omitted, then the same power output is possible when combined with a limited amount of external heating, for example, for space heating. Thus, in Fig. 3 external heat may be drawn from the brine by inserting an external heater 135 in parallel with the section of the primary heater which heats the working fluid between stages 1 and 3' . In Fig. 7, a similar, though not wholly identical, result is achievable by supplying the external heat by cooling the separated liquid prior to its injection into the second stage of the feed pump by means of a heat exchanger 51.

In the further modified system shown in Fig. 9, the liquid phase from the separator is delivered by a line 121 to a throttle valve 34 from which the liquid phase is returned to the condenser 28 with a drop in pressure to that of the vapour leaving the expander 25. This arrangement would be appropriate where the gain achievable by returning the separated liquid to the heat exchanger by means of a second feed pump (32, Fig. 3) would not be significant.

EXAMPLE

To illustrate the concepts described, consider a flow of 75 kg/s of brine, initially at a temperature of 190°C, as is currently envisaged for the hot dry rock programme in Soultz, in Alsace Lorraine. For this application n-Pentane is the most suitable fluid, since it has a critical temperature of 196°C and can therefore be heated to a high temperature while remaining in the liquid phase. Under these circumstances, two-phase expansion throughout the process would result in the working fluid expanding through a volume ratio of approximately 165:1 and the working fluid leaving the expander as dry vapour. Although, the conditions assumed here are not the optimum, they are typical of what is achievable and illustrate the relative contribution of the various components to the total system output . The estimated values of recoverable power outputs from each expander are based on expansion efficiencies of 80% for the first stage two-phase expander, 40%-60%, depending on the entry conditions, for the second stage two- phase expander and 85% for the dry vapour expander and are given in the following table. These component efficiencies represent experimentally verified values for the first stage liquid and dry vapour expanders and estimates for the second stage liquid expander. The lower values assumed for this latter component are due to the fact that very high volume ratios are incurred in this case and hence only partial recovery of the power is possible within it.

Assumed First Stage Inlet Temperature : 182°C Assumed Condensing Temperature: 28°C

In this case the first stage two-phase expansion has a volume ratio of approximately 9:1, when separating at 125°C increasing to approximately 25:1 at a separation temperature of 90C. In contrast, the second stage two-phase expansion is of the order of 100:1 and varies far less with the separation temperature. The dry vapour expander requires a volume ratio of 14:1 at 125°C admission temperature falling to only 6:1 at 90°C admission. The very high volume ratio of the second stage two-phase expansion implies that its efficiency is low, but since the available energy from this is relatively small, the penalty for this in terms of overall expansion efficiency is not so significant.

As the intermediate temperature decreases the amount of dry vapour admitted to the second stage increases, while the contribution of the second stage liquid expander decreases. 90°C was taken as the lower limit because the volume ratio of expansion in the first stage then starts to exceed 25:1 and hence the two-phase expander efficiency would decrease. However, at this stage, there is little to be gained from the second stage liquid expander and at this point the throttling of the separated liquid and elimination of this component, as shown in Fig 9, may be a more cost effective arrangemen .

Overall, it can be seen that expansion efficiencies of the order of 80% can be obtained by this means which represents a much larger value than was previously considered possible for expansion of two-phase fluids over such large pressure and volume ratios. This makes the systems of the invention superior in performance to Rankine cycle systems, even at higher resource temperatures.

Claims

1. Closed-cycle apparatus for deriving power from a low temperature heat source comprising a heat exchanger for heating a liquefied volatile fluid under pressure with heat from the source, a primary two-phase expander connected to receive heated liquid from the heat exchanger and thereby generate power, a liquid/vapour separator connected to receive the fluid from the primary expander, means to direct the separated vapour stream to a further power generating expander and thence to a condenser, and feed pump means to return both the separated liquid phase stream and the condensed vapour phase stream to the heat exchanger under the said pressure.

2. Closed-cycle apparatus according to claim 1 wherein the vapour part of the temperature-entropy diagram for the fluid has a positive slope.

3. Closed-cycle apparatus according to claim 2 wherein the fluid is n-pentane.

4. Closed-cycle apparatus according to any of the preceding claims, wherein the primary expander is constructed to expand the liquid into a homogenous mixture of liquid and vapour.

5. Closed-cycle apparatus according to claim 4, wherein the primary expander is a radial inflow or screw expander.

6. Closed-cycle apparatus according to any of the preceding claims, and including a further two-phase expander for expanding the separated liquid phase stream to generate further power prior to condensation.

7. closed-cycle apparatus according to any of claims 1 to 5 , wherein the feed pump means includes a primary feed pump for returning the fluid from the condenser to the heat exchanger and a further feed pump for delivering the separated liquid phase stream to an intermediate portion of the heat exchanger.

8. Closed-cycle apparatus according to any of claims 1 to 5, wherein the feed pump means includes first and second pump stages connected in series between the condenser and the heat exchanger and the liquid phase outlet of the separator is connected to the input of the second pump stage.

9. Closed-cycle apparatus according to claim 8, wherein the connection between the liquid phase outlet of the separator and the input of the second pump includes a heat exchanger for providing external heat .

10. Closed-cycle apparatus according to any of claims 1 to 5, wherein the liquid phase outlet of the separator is connected to the condenser input through a throttle.

11. A closed-cycle method of generating power from a low temperature source of heat, including the steps of heating a liquefied volatile fluid under pressure in a heater with heat from the source, expanding the liquid to flash a portion thereof into vapour and thereby generate power, separating remaining liquid from the vapour and further expanding the vapour, condensing the vapour into liquid and pumping the condensed fluid back into the heater.

12. A closed-cycle method according to claim 11 wherein the vapour part of the temperature-entropy diagram for the fluid has a positive slope.

13. A closed-cycle method according to claim 12 wherein the fluid is n-pentane.