EP0161297A1

EP0161297A1 - Energy conversion system

Info

Publication number: EP0161297A1
Application number: EP84904127A
Authority: EP
Inventors: Herman Branover; Michael Petrick
Original assignee: Solmecs Corp NV
Current assignee: Solmecs Corp NV
Priority date: 1983-11-10
Filing date: 1984-11-09
Publication date: 1985-11-21
Also published as: ZA848762B; IL70205A0; AU3615084A; WO1985002226A1; JPS61500622A

Abstract

Un système de conversion d'énergie thermique (en 18) en énergie électrique ou en d'autres formes d'énergie comprend un circuit hydraulique fermé pour un liquide (4) tel que du mercure, comprenant un tube de montée (8), un réservoir (2) et un tube de descente (6) associé à un générateur de puissance tel qu'un générateur magnéto-hydrodynamique (12). Un fluide ayant un point d'ébullition bas est injecté (14) dans le tube de montée (8) et fait circuler le premier liquide (4). La section du tube de montée (8) augmente vers le haut pour réduire les pertes de friction et d'accélération. Dans une variante (non illustrée), plusieurs de ces étages sont disposés en série, les tubes de montée (8) étant successivement de plus en plus grands d'étage en étage.A system for converting thermal energy (at 18) into electrical energy or other forms of energy comprises a closed hydraulic circuit for a liquid (4) such as mercury, comprising a riser tube (8), a tank (2) and a downpipe (6) associated with a power generator such as a magneto-hydrodynamic generator (12). A fluid with a low boiling point is injected (14) into the riser (8) and circulates the first liquid (4). The cross-section of the riser (8) increases upwards to reduce friction and acceleration losses. In a variant (not shown), several of these stages are arranged in series, the riser tubes (8) being successively larger from stage to stage.

Description

ENERGY CONVERSION SYSTEM

The present invention relates to a system for conversion of thermal energy into electrical energy, particularly of thermal energy from such relatively little exploited sources as solar radiation, geothermal brine and industrial waste heat, but also nuclear and conventional fuels.

A conversion system using a vapour-liquid cycle is proposed in U.S. Patent 3,443,129 (A.G.Hammit).

This known system, however, suffers from several disadvantages which severely impair its practical usefulness. The system has a single, constant-diameter "common leg" or "bubble tube" 14, in which the liquid/vapour bubble mixture moves against a force field (gravitational or electromagnetic). This makes the system not only very tall (with all the structural problems involved) but, more important, very inefficient, for the following reasons. Assuming that the system works between reasonably different boiling and condensation temperatures, the rate of expansion of the vapour will be high. Thus, even if in vicinity of the mixer 16 the void fraction (the ratio between vapour volume and the total liquid-plus-vapour volume) is low, it is bound to become very high in the middle and upper portion of the bubble tube 14 after the vapour expands to a large multiple of its original volume at the mixer. Now it can be shown that in such two-phase flows a high void fraction results in high slip (slip being the ratio between the flow velocity of the vapour bubbles and that of the liquid). In a system in which the extraction of power is based on the pressure difference between the bubble tube 14 and the downflow tube 10, high slip inevitably results in low efficiency. Although, in principle, slip could be reduced by increasing flow velocity using a relatively narrow bubble tube, this would inevitably increase two other types of losses, namely acceleration losses (losses due to the energy expended in acclerating the heavy liquid metal along the bubble tube) and friction losses which are also a function of flow velocity. With this known system, the situation could be improved only by operating at very low two- phase-fluid quality (quality being the ratio between the mass flow rate of the gaseous phase and the mass flow rate of the liquid phase plus that of the gaseous phase). While this could be achieved by using a bubble tube of a substantially increased diameter, this would obviously cause the required amount of liquid-metal to become very large too, as a consequence of which specific power output, i.e., power produced per unit mass of the expensive liquid metal would be very low, increasing the cost, per unit power, of the system to a point where it would no longer be economically justifiable.

It is one of the objectives of the present invention to overcome the disadvantages and drawbacks of the prior art systems and to provide a system for energy conversion that is thermodynamically efficient, structurally flexible in that it can be relatively tall where height poses no problems, e.g., where it can be attached to already existing tall structures or relatively short wherever greater height would create difficulties, that for a comparable output uses much less liquid-metal than in prior-art systems, and is eminently cost-effective.

This the invention achieves by providing a system for conversion of thermal energy into another type of energy, comprising a reservoir, for a first, relatively high-boiling-point fluid, a downcomer communicating at its upper end with said reservoir, an upcomer communicating at its upper end with said reservoir, said upcomer communicating at its lower end with the downcomer, a mixing chamber located at the lower end of the upcomer, through which mixing chamber a second, relatively low-boiling-point fluid is introduced into the upcomer for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power-generating means associated with the downcomer, by which means power is generated by said first fluid moving down said downcomer, characterised in that the cross-sectional area of said upcomer increases with increasing height above the mixing chamber in such a way as to minimize slip, acceleration and friction losses.

According to a further aspect the invention provides a system for conversion of thermal energy into other types of energy, comprising a reservoir for a first, relatively high-boiling-point fluid, a downcomer communicating at its upper end with said reservoir, an upcomer, communicating at its upper end with said reservoir, said upcomer communicating at its lower end with a downcomer, a mixing chamber located at the lower end of said upcomer, through which mixing chamber a second, relatively low-boiling-point fluid is

^K introduced into the upcomer for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power- generating means associated with said downcomer, by which means power is generated by said first fluid moving down said downcomer, characterised in that the system comprises a plurality of stages, each stage having one such downcomer, one such upcomer, one such reservoir and one such mixing chamber connected to drive a power-generating means; in that the second and any further stage is coupled to the preceding stage whereby the said second, low-boiling-point fluid operates at successively greater temperatures from stage to stage; and in that the cross-sectional areas of successive ones of said upcomers increase from stage to stage, in such a way as to minimize slip, acceleration and friction losses.

According to a third aspect, the invention provides apparatus for converting thermal energy into another type of energy, comprising at least one reservoir, for a first, relatively high-boiling-point, fluid, at least one downcomer communicating at its upper end with the or each respective reservoir, at least one upcomer communicating at its upper end with the or each respective reservoir, the or each upcomer communicating via its lower end with the or each respective downcomer, at least one mixing chamber located at the lower end of the or each respective upcomer, through which mixing chamber a second, relatively low-boiling- point, fluid is introduced into the or each respective upcomer for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power-generating means associated with the or each downcomer, by which means power is generated by said first fluid moving down the or each downcomer, characterised in that the second fluid is arranged to pass in a closed circuit through the sole upcomer or through at least two successive upcomers, and in that the cross-sectional area of the upcomer or upcomers defining a path through the said circuit increases in the direction of flow of the second fluid.

In order that the invention may be better understood, several preferred embodiments will now be described, by way of illustration only, with reference to the accompanying schematic drawings, in which:-

Figure 1 is a view of a single-stage system embodying the invention,

Figure 2 shows a cycle diagram in temperature - entropy coordinates of a system embodying the invention (bold solid curve) and of a prior-art system (broken-line curve);

Figure 3 is a view of a multi-stage system embodying the invention;

Figure 4 shows two adjacent stages of another multi- stage embodiment of the system, in which each upcomer tube is conical;

Figure 5 represents another embodiment in which the upcomer has several stages or sections arranged as a

column ;

Figure 6 shows the last two stages of a multi-stage system, in which the last stage uses a different thermodynamic fluid;

Figure 7 is a view of a system in which the upcomer is in two stages, and in which the thermodynamic fluid is a gas;

Figure 8 represents an embodiment of the system suitable for use with geothermal brine, and

Figure 9 is a view of an embodiment using turbine means for power generation.

Referring now to the drawings, there is seen in Figure 1 a reservoir 2 for a liquid, electrically conductive substance having a relatively high boiling point, such as a liquid metal 4, e.g., mercury. Communicating at its upper end with the reservoir 2, there is provided a descending tube, in the following termed downcomer, 6. Via its lower end, the downcomer communicates with an ascending tube, in the following termed upcomer, 8, which at its upper end, communicates with the reservoir 2. For reasons to be explained further below, the upcomer 8 in this particular embodiment is conical, being narrow at its lower end and wider at its upper end. At some point along the downcomer 6 there is arranged a magneto-hydrodynamic (MHD) generator 10, which produces an electric current when the conductive liquid 4 flows along the downcomer 6 and crosses the magnetic field 12. -For such flow to take place it is obviously necessary to cause the liquid metal 4 to

( Ci.ir. circulate, moving down from the reservoir 2 through the downcomer 6 and up again into the reservoir through the upcomer 8. This is accomplished by the so-called bubble lift, whereby a low-boiling-point fluid, which may be a liquid or a gas when at standard temperature or pressure, is introduced via a nozzle 14 and a mixing chamber 16 into the upcomer 8, producing upon contact with the liquid metal - heated with the aid of a heater 18 - vapour bubbles that form together with the liquid metal a two-phase fluid having a lower density than the liquid metal. The pressure difference now obtaining between the downcomer 6 containing the pure liquid metal and the upcomer 8 containing the much lighter two-phase fluid will cause the latter to rise, to be displaced by the liquid metal which, once it has passed through the mixing chamber 16 becomes itself the buoyant two-phase foam.

In the reservoir 2, the thermodynamic fluid separates and leaves through a duct 20, passing through a regenerator 24, where its "sensible heat" is abstracted. (The term "sensible heat" is defined herein as heat associated with a change of temperature without a change of phase). From the regenerator 24, the vapour passes to a condenser 22, where it gives off its latent heat and returns to its liquid state. A pump 26 conveys the now liquid thermodynamic fluid back into the regenerator 24 where it is preheated by the sensible heat recovered from the vapour prior to its condensation. Via another duct 28, the preheated thermodynamic fluid, driven by the pump 26, is returned to the nozzle 14, after being vapourized by a boiler

30. The thermal energy required to heat the liquid metal to a temperature well above the boiling point of

O-v-ϊ-i the thermodynamic fluid (heater 18) and to vapourize the thermodynamic fluid prior to its injection into the mixing chamber 16 (boiler 30) is advantageously supplied by any of the already mentioned thermal-energy sources. With some low-boiling-point thermodynamic fluids as Freon (an application of which will be discussed further below), the boiler 30 can be dispensed with.

As can be seen in the drawing, the mixing chamber 16 is much wider than the lower end portion of the upcomer 8. By reducing the flow velocity of the liquid metal, this ensures full vaporization of the thermodynamic fluid already upstream of the entrance to the upcomer 8, by providing a longer contact between the thermodynamic fluid and the heated liquid metal, even if non- vaporized fluid is injected.

The conical shape of the upcomer 6 takes care of the above-discussed problem of high slip, by providing increasing space for the progressively expanding vapour bubbles. If conical tubes of greater length are difficult to produce, the same effect can also be obtained by using a stepped tube fabricated from a plurality of cylindrical sections of gradually increasing diameter.

The thermodynamic cycle of the system according to preferred embodiment of the invention is shown in temperature-entropy coordinates in Figure 2. This is a modified Rankine cycle, with the bold, solid curve 32 indicating the complete cycle as performed by the system embodying the invention, and the broken curve 34 representing an incomplete cycle such as that performed

Gϊ-άi in the above-discussed prior art. Curve 36 is the saturated-vapour curve. Arrow A designates the sensible heat recovered in the regenerator and returned to preheat the thermodynamic liquid, while arrow B indicates the latent heat liberated in the condenser.

It was found to be advantageous to arrange two or more of the above-described systems in "stages", as shown in Figure 3. It is seen that the vapour from stage I, having separated in the reservoir 2 from the two-phase foam, is introduced into the mixing chamber 16^» of stage II, and the vapour from stage II is introduced into the mixing chamber 16" of stage III. The three stages are served by a single condenser 22, regenerator 24, pump 26 and boiler 30, as is clear from the drawing. The three MHD-generators are advantageously connected in series, which increases the output voltage.

Since heat is introduced to the vapour at each successive stage, its temperature rises from stage to stage. In this particular embodiment, the upcomers 8 are cylindrical, and their diameter increases from stage to stage. This arrangement will solve the slip problem as long as the upcomers 8 are relatively short. For longer upcomers, conical upcomers 8 are required, such as those shown in Figure 4. Here it should be noted that the lower-end diameter of the upcomer 8 of the (n⁺¹)^st stage is advantageously at least as large as the upper-end diameter of the upcomer 8' of the n^th stage.

Whenever the height of the structure poses no problem, e.g., when the system is intended for utilization of solar energy collected by a solar tower system where a tall structure already exists and to which structure the system is easily attached, a different type of "staging" can be used, as illustrated in Figure 5, where the increasingly larger diameter upcomer cylindrical sections 8 are mounted one on top of another in a column, connected by conical transition pieces. It should be noted that in this embodiment proper continuity of flow is ensured by by-pass tubes 38 which connect each stage separately to the downcomer 6. Flow through these by-pass tubes 38 is adjustable by means of valves 39.

Using the "staging" principle, even greater efficiency is obtainable when, as illustrated in Figure 6, a different thermodynamic fluid is used in the last stage (or the second of two stages), which has a boiling point lower than that of the preceding stage or stages. This avoids high vacuum in the upper part of the last upcomer and thus greatly reduces slip in the last stage. If, for instance, water is used as thermodynamic fluid (water being in general one of the best thermodynamic fluids), and condensation takes place at 35°C, the pressure in the upper part of the last stage would be as low as 0.05 bar (5000 Pa), and bubble expansion very large. If, however, Freon 113 were used as thermodynamic fluid in the last stage, the absolute pressure at 35°C would be just below 1 bar (100000 Pa) and the high rate of expansion with the concomitant high slip would be avoided.

The drawing shows the last two stages, stage n-1 which works, say, on water, and stage n, which uses Freon as the thermodynamic fluid. Stage n-1 is fed steam coming from stage n-2 and entering at arrow A. The separated steam from stage n-1 enters a condenser/heat exchanger 40, where the heat liberated from the condensing steam is used to vaporize the liquid Freon driven through the heat exchanger coil by the Freon pump 42. The Freon vapour is subsequently introduced into the mixing chamber 16' of the last stage n and leaves the latter via a condenser 44, to return, as liquid, to the Freon pump 42. (It should be noted that water and Freon are just one of many possible combinations).

The embodiment illustrated in Figure 7 is a two-stage system, with just one upcomer in two sections, using an inert gas (e.g., nitrogen) as thermodynamic fluid and performing a Brayton cycle instead of the modified Rankine cycle characteristic of the previous embodiments.

As seen in the drawing, the upcomers 8 are mounted on top of one another, with a by-pass tube 38, controllable by a valve 39, connecting the upper upcomer section 8^» to the downcomer section 6. After preheating in a regenerator (not shown), the gas is injected into the mixing chamber 16. Instead of the condenser, there is provided a heat exchanger 46; instead of a pump, a compressor 48. Using a gas, there is.no limit to the temperature of the cycle. The lower temperature (sink temperature), too, does not present any problems, since no vacuum would occur in the heat sink at any temperature. The Brayton cycle could be of advantage for use with solar towers, nuclear heat sources as well as with conventional fuel.

A system for use with geothermal brine is shown in

Til .

—_£_.___- IO Figure 8. This is an open-cycle system, in which the raw brine is directly injected at A, its flow controllable by a valve 50. After separation in the reservoir 2, the steam is released into the atmosphere, while the dirt, salts, etc., are left floating on the liquid-metal surface in the reservoir 2, from which they have to be periodically removed, together with a small quantity of liquid metal 4 that has to be replaced. It should be appreciated that up to now use of geothermal brine was limited by the fact that the raw brine has to be cleaned before it goes to heat exchangers, turbines, etc. The present system, on the other hand, produces electricity from raw brine, cleaning it at the same time. The steam released into the atmosphere poses no environmental problems.

Figure 9, finally, is a schematic representation of an embodiment, in which the MHD-generator 10 is replaced by turbine means 52, either in the form of a turbo- generator producing electrical power, or a simple turbine, producing mechanical shaft power, e.g., for pumping. The turbo-generator version would be particularly advantageous, as it generates an a.c. current of higher voltage directly usable or at least easily transformable. Turbine means 52 can of course be used instead of, or in combination with, the MHD- generators also in any of the previously discussed embodiments.

Claims

CLAIM?

1. A system for conversion of thermal energy into another type of energy, comprising a reservoir (2), for a first, relatively high-boiling-point fluid, a downcomer (6) communicating at its upper end with said reservoir, an upcomer (8) communicating at its upper end with said reservoir, said upcomer communicating at its lower end with the downcomer (6), a mixing chamber (16) located at the lower end of the upcomer (8), through which mixing chamber a second, relatively low- boiling-point fluid is introduced into the upcomer (8) for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power-generating means (12) associated with the downcomer (6), by which means power is generated by said first fluid moving down said downcomer, characterised in that the cross-sectional area of said upcomer (8) increases with increasing height above the mixing chamber in such a way as to minimize slip, acceleration and friction losses.

2. A system as claimed in claim 1, wherein the upcomer (8 to 8", Fig.5) comprises a plurality of substantially cylindrical tube sections in series in a column, the diameters of successive sections increasing from the lowermost to the uppermost section.

3. A system for conversion of thermal energy into another type of energy, comprising a reservoir (2) for a first, relatively high-boiling-point fluid, a downcomer (6) communicating at its upper end with said reservoir, an upcomer (8), communicating at its upper end with said reservoir, said upcomer communicating at its lower end with a downcomer (6), a mixing chamber (16) located at the lower end of said upcomer (8), through which mixing chamber a second, relatively low- boiling-point fluid is introduced into the upcomer (8) for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power-generating means (12) associated with said downcomer (6), by which means power is generated by said first fluid moving down said downcomer, characterised in that the system comprises a plurality of stages, each stage having one such downcomer (6), one such upcomer (8,8^f,8"), one such reservoir (2) and one such mixing chamber (16,16',16") connected to drive a power-generating means (12); in that the second and any further stage is coupled to the preceding stage whereby the said second, low-boiling- point fluid operates at successively greater temperatures from stage to stage; and in that the cross-sectional areas of successive ones of said upcomers (8,8',8") increase from stage to stage, in such a way as to minimize slip, acceleration and friction losses.

4. A system as claimed in claim 3, characterised in that the cross-sectional area of each upcomer (8,8', Fig.4) increases with increasing height above the mixing chamber in such a way as to minimize slip, acceleration and friction losses.

5. A system as claimed in claim 3 or 4, further comprising ducts (20) connecting the reservoir of at least one stage (I,II) to the mixing chamber (16',16»)

,. -_-_2___H of the next stage (II,III).

6. A system as claimed in claim 3 4 or 5, comprising at least one heat exchanger (40,Fig.6) by means of which heat from the second, low-boiling-point fluid of one stage (n-1) is transferred to the second, low- boiling-point fluid of the next stage (n), the said second fluid of the said next stage (n) being restricted to circulation within that stage.

7. A system as claimed in any preceding claim, wherein at least at the point (14) of introduction of said second fluid, the cross-sectional area of the or each mixing chamber (16) is substantially larger than the cross-sectional area of at least the lower portion of the contiguous upcomer tube (8).

8. A system as claimed in any preceding claim, comprising heater means (18) for heating said first fluid to a temperature higher than the boiling point of said second fluid.

9. A system as claimed in any preceding claim, comprising a regenerator (24) for recovering the sensible heat of said second fluid associated with the or each upcomer (8).

10. Apparatus for converting thermal energy into another type of energy, comprising at least one reservoir (2), for a first relatively high-boiling- point fluid, at least one downcomer (6) communicating at its upper end with the or each respective reservoir, at least one upcomer (8) communicating at its upper end with the or each respective reservoir, the or each upcomer communicating via its lower end with the or each respective reservoir (6), at least one mixing chamber (16) located at the lower end of the or each respective upcomer (8), through which mixing chamber a second, relatively low-boiling-point, fluid is introduced into the or each respective upcomer (8) for producing, upon contact with said first fluid, a plurality of bubbles forming together with said first fluid a two-phase fluid having a lower density than said first fluid, and power-generating means (12) associated with the or each downcomer (6), by which means power is generated by said first fluid moving down the or each downcomer, characterised in that the second fluid is arranged to pass in a closed circuit through the sole upcomer or through at least two successive upcomers, and in that the cross-sectional area of the upcomer or upcomers defining a path through the said circuit increases in the direction of flow of the second fluid.