Direct-drive power conversion system for wind turbines compatible with energy storage

Abstract

A system suitable for extracting power directly from the main shafts of slow-moving mechanical systems. The system has a closed gas circuit having a lower-pressure (LP) side and a higher-pressure (HP) side. The LP side is at a pressure substantially greater than atmospheric pressure. The system includes primary compressors coupled to the wind turbines, thermal stores coupled to heat-exchangers on both the LP and HP sides of the closed gas circuit, a secondary motor-compressor set and an expander-generator set. The system allows some degree of independence between the input power resource and the output power. When substantial wind power is present and the demand for electrical power is weak, this system can export a fraction of the energy captured and store the rest. When the wind resource is low, the system can export more power than is being collected by drawing energy from the thermal stores.

Claims

1. A system comprising: a closed gas circuit; and an expander-generator set, wherein the system is configured to convert power from one or more slowly-rotating shafts into electrical power, wherein the system is configured such that, when in use, a working gas flows in the closed gas circuit, is compressed isentropically using power from the one or more slowly-rotating shafts and expanded isentropically in the expander-generator set and wherein the system is configured such that, when in use, low-pressure parts of the closed gas circuit are at a pressure at least 3 times greater than atmospheric pressure and wherein the system is configured such that, when in use, the ratio between pressures in high pressure parts and low pressure parts of the closed gas circuit is such that a ratio of absolute temperatures greater than 1.5 occurs across the compression and expansion processes.

2. The system as claimed in claim 1 which also includes thermal stores and heat-exchanger units on both the high-pressure and low-pressure parts of the closed circuit such that the system is configured such that, when in use, the power being supplied into the gas circuit by the one or more slowly-rotating shafts may be different from the power being extracted at the expander-generator set with the difference in exergy being transferred into the thermal stores or from the thermal stores.

3. The system as claimed in claim 2 comprising a motor-driven compressor such that the system is configured such that, when in use, the working gas can be circulated through the heat exchanger units and expander-generator set even when no power is available from the one or more slowly-rotating shafts.

4. The system as claimed in claim 1 comprising an expansion vessel on the closed gas circuit to allow pressures to remain stable even when the mean temperature of gas in the closed gas circuit changes.

5. The system as claimed in claim 1 wherein the expander-generator set comprises at least one of several different expanders and several different generators in order to allow performance at power levels smaller than the total rated power.

6. The system as claimed in claim 1 wherein one or more of the one or more slowly-rotating shafts are the main shafts of wind turbines.

7. The system as claimed in claim 1 wherein one or more of the one or more slowly-rotating shafts is coupled to or is configured to be coupled to more than one compressor.

8. The system as claimed in claim 1 wherein the system is configured such that, when in use, one or more of the one or more slowly-rotating shafts are driven from tidal power converters.

9. The system as claimed in claim 1 wherein the system is configured such that, when in use, one or more of the one or more slowly-rotating shafts are driven from wave energy converters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a power conversion system incorporating features of the invention;

(2) FIG. 2 is an enlarged view of a heat exchanger unit shown in FIG. 1;

(3) FIG. 3 is a schematic representation of one of the operational modes of the system shown in FIG. 1;

(4) FIG. 4 is a schematic representation of another one of the operational modes of the system shown in FIG. 1;

(5) FIG. 5 is a schematic representation of another one of the operational modes of the system shown in FIG. 1;

(6) FIG. 6 is a schematic representation of another one of the operational modes of the system shown in FIG. 1; and

(7) FIG. 7 is a schematic representation of another one of the operational modes of the system shown in FIG. 1.

(8) Operation of the System.

(9) The system has five main modes of operation:

(10) (a) Direct production of electrical power output with no deliberate heat transfer.

(11) (b) Reduced production of electrical power output with deliberate exergy transfer into the thermal stores.

(12) (c) Increased production of electrical power output with deliberate recovery of exergy from the thermal stores.

(13) (d) Further-reduced (or negative) electrical power output with deliberate exergy transfer into the thermal stores.

(14) (e) Further-increased electrical power output with deliberate recovery of exergy from the thermal stores.

(15) The term exergy used above is a formal thermodynamic term. Exergy describes the ability to extract work from a system by allowing it to return to equilibrium with its environment. Heat stored in a hot (hernial store has some associated exergy because one could use a heat-engine (like a Stirling Engine) to recover at least some mechanical work from it. Similarly, coldness stored in a cold thermal store has some exergy associated with it because one could use a heat-engine (like a Stirling Engine) to recover at least some mechanical work from it.

(16) In all modes of operation described above, gas flows within the closed-system in a clockwise direction as FIG. 1 suggestsincluding passage through the expander (6). The only exception to this is when the system is not operating. Positive electrical power is output in the case of modes (a), (b), (c) and (e). In mode (d), the net electrical output power is normally negativewith exergy being stored in the thermal stores. Mode (d) is expected to be used for only a small fraction of the time. In operational modes (a), (b) and (c), the gas flow is driven solely by the primary compressors. In operational modes (d) and (e), any gas flow through the primary compressors (which clearly depends on the availability of wind) is supplemented with gas flow through the secondary compressor (11).

(17) Expanded textual descriptions of the five operational modes are given below. FIGS. 3-7 provide schematic representations of these operational modes. In these figures, the block arrows represent flows of exergy. The labels {HHT}, {HT}, {MT}, {LT} and {LLT} indicate the temperature states of parts of the closed gas circuit. These correspond to very-high, high, medium, low and very-low temperature respectively.

(18) In operational mode (a), the working as is compressed in the primary compressors (5a), (5b), (5c) etc. from the LP side of the circuit and pushed into the HP side at a higher temperature. That gas passes directly to the expander (6) where it is then expanded to recover most of the work originally put in. The shaft-power from the expander is fed into the main generator (7) where most of it is converted to electrical power. If the electrical generator is an AC generator, it would normally run at synchronous speed such that the electrical power produced could be fed directly into an electrical transmission system without passing through power-electronics.

(19) If the system has been in operational for a period and then begins to operate in mode (a), the LP gas entering the primary compressors will initially be at ambient temperature and the HP gas emerging from these compressors will be very hot. Because of small thermal losses in the HP side (1) of the main gas circuit, the HP gas entering the expander (6) will be slightly less hot than that exiting the primary compressors and thus the LP gas emerging from the expander (6) will be below ambient temperature. When this slightly-cooled gas reaches the primary compressors again, these will produce cooler HP gas output than they had done originally. If operated in mode (a) for sufficiently long, the system would come to an equilibrium state in which the HP gas was significantly above ambient temperature and the LP gas was significantly below ambient temperature. These temperatures would be {HT} and {LT} shown in FIGS. 3-7.

(20) In operational mode (b), the working gas is compressed in the primary compressors (5a), (5b), (5c) etc. from the LP side of the circuit and discharged into the HP side at a higher temperature as before. The HP gas passes through the HP heat-exchanger (8a) where heat is extracted from it and put into the high-temperature store (9b). The HP gas then passes to the expander (6) with a temperature only slightly above ambient. There the gas is then expanded to recover only some of work originally put in. The shaft-power from the expander is fed into the main generator (7) where most of it is converted to electrical power. The LP gas emerging from the expander is at a temperature substantially below ambient and this LP gas is then passed through the LP heat-exchanger (9a) where the coldness is transferred from it into the low-temperature store. The LP gas leaving the LP heat-exchanger (9a) is at a temperature only slightly below ambient. This LP gas is returned to the primary compressors.

(21) In operational mode (c), the working gas again passes through the primary compressors (5a), (5b) etc. but the gas entering these primary compressors has been chilled using coolness stored in the cool thermal store, (9b). As the gas emerges at the HP side, it is at approximately ambient temperature and then it passes through the HP heat-exchanger unit (8a) which adds substantial temperature to the gas. The HP gas just before the main expander (6) is at a high temperature such that after the main expansion process, it is approximately at ambient temperature again. In this operational mode, some exergy is supplied to the gas in the closed-circuit by the primary compressors (5a), (5b), (5c) etc. and some is also supplied from each of the thermal stores (8b) and (9b).

(22) In operational modes (d) and (e) the secondary compressor (11) is driven by the electric motor (12) to cause a significant flow of gas even if the compression power available from the wind turbines is small or zero.

(23) In operational mode (d), significant exergy enters the system through the electric motor (12) and some of that exergy is put into storage in thermal stores (8b) and (9b). The remaining exergy emerges from the system again through generator set (7) but the net output of electrical power in this mode will often be negative. In other words, more electrical power is drawn from the grid to drive the motor (12) than is returned to the grid via the generator (7).

(24) In operational mode (e), significant exergy enters the system through the electric motor (12) and additional exergy is sourced from thermal stores (8b) and (9b). The accumulated exergy emerges from the system again through generator set (7) and in this case the output electrical power will be greater than the input electrical power by a factor.

(25) Features and Advantages of the System.

(26) Because the system described uses pressurised gas even on the LP side of the circuit, the primary compressor units (5a), (5b), (5c) etc, can be compact even when for high powers and low rotational speeds. Thus these primary compressor units are compatible with being used in direct-drive mode with large onshore wind turbines. This contrasts with other proposals where an extremely large inlet swept volume is required.

(27) If the system was to be used in operational mode (a) only, components (8a), (8b), (9a), (9b), (11) and (12) would not be present in the system and the system would be comparable with a straightforward wind farm which generates electricity directly. We employ the term core system to indicate all components which are required for the production of output electrical power. The term storage subsystem will be used to encompass components (8a), (8b), (9a), (9b), (11) and (12). Because a single expander-generator set may be used in conjunction with a number of wind turbines, and because the working stress in the primary compressors is high the total cost of the core system may be competitive with the total cost of a conventional wind farm producing the same net output electrical power.

(28) The marginal cost of the storage subsystem for a given quantity of energy storage is extremely low by comparison with alternative methods of storing energy. Thermal energy storage is known to be very cost-effective but it suffers from the drawback of relatively poor turnaround efficiency.

(29) The marginal energy losses associated with passing energy through storage are extremely low by comparison with alternative methods of storing energy. In most existing views of energy storage, electrical energy is drawn from the grid, converted into a form which is compatible with storage and then after the energy has been stored for some time, the energy is converted back into electricity. Thus all of the energy which passes through storage in the conventional systems undergoes two transformations and there is a loss (typically 7-15%) associated with each transformation. In the present case, most of the energy passing through storage does not undergo any additional transformations. There is some loss of efficiency associated with transferring heat but this can be relatively small.

(30) In the steady-state, the system is fully reversible (in a thermodynamic sense) in all of its operational modes. In other words, if all components of the system were completely ideal, the total electrical energy emerging from the main generator (7) over a long period of time would be identical to the sum of the total electrical energy input to the motor (11) and the total mechanical energy fed into the primary compressors by wind turbines. Of course all real components introduce some irreversibility. The motors, compressors and expander(s) will all have efficiencies lower than 100%, the heat-exchangers will have finite temperature-differences across them while they are operating and the pipes and thermal-stores will suffer small energy losses through heat-exchange with the environment. This system contrasts with others where irreversibility is an intrinsic element of the system.

(31) The Rationale for Elevated-Pressure LP side of the Gas Circuit.

(32) The energy absorbed by an ideal compressor in each cycle is

(33) $\begin{array}{cc}{E}_{\mathrm{cycle}}=p_{\mathrm{in}}{V}_{\mathrm{in},\mathrm{cycle}}\frac{\left(r^x-1\right)}{}& \left(1\right)\end{array}$

(34) where p.sub.in represents the inlet pressure, r represents the pressure ratio, V.sub.in,cycle, represents the volume of inlet air sucked into the rotary compressor in each individual rotation and is derived from the ratio of specific heats for the gas as

(35) $\begin{array}{cc}:=\frac{\left(-1\right)}{}& \left(2\right)\end{array}$

(36) To illustrate the above, consider a 5 MW wind turbine rotating at 1.2 rad/s in rated conditions. A single revolution of the turbine rotor takes 5.2 seconds to complete and therefore 26 MJ of energy is collected during one cyclei.e. E.sub.cycle=26 MJ. With r=50 and p.sub.in=5 MPa, the required swept inlet volume of the rotary compressor can be deduced to be 3.6 m.sup.3. For a positive displacement compressor, this provides a good indication of the overall volume of the machine. The actual volume occupied by the machine might be two or three times greater than the inlet swept volume.

(37) Note that equations (1) and (2) assume ideal gas behaviour for the working fluid. This is reasonably accurate for the fluids of most interest. Neither one of equations (1) or (2) depends on temperature. This is an important point. The input power from a 5 MW wind turbine can be absorbed by compressing 0.139 m.sup.3/s (input) of nitrogen from 5 MPa up to 250 MPa whether that input is at 100 C. or at ambient temperature.

(38) Extensions of the Fundamental Concept.

(39) There may be more than one compressor driven by any one wind turbine shaft in order to provide the capability for part-loading the wind turbine. With more than one compressor on a single wind turbine shaft, one might size the units differently (e.g. 2 MW and 1 MW) to allow for a wide spread of working powers.

(40) One or more of the primary compressors such as (5a) might be driven from some other renewable energy source such as a wave energy converter or tidal energy converter.

(41) The high temperature heat-exchanger (8a) and associated thermal store (8b) might be integrated into a single unit in the form of a thermocline. Similarly the low temperature heat-exchanger (9a) and associated thermal store (9b) might be z into a single unit. In some implementations, the low temperature thermal store might use liquid carbon-dioxide as the heat-transfer fluid.

(42) In some implementations the electrical drive (11) for the secondary compressor (10) could be replaced by a mechanical transmission from the expander unit(s) (6).

(43) The expander unit (7) might comprise a set of discrete expander units in order that good part-load performance can be achieved.

(44) The system might be implemented such that a single primary compressor (5a) is present.

(45) A contrived alternative to the present system might not use a closed system for the working fluid. Air could be used as the working fluid and a further electrically-driven compressor could be fitted to produce the LP feed to the primary compressors. Most of the power to run this compressor could be sourced from an expander fitted subsequent to (6) operating to drop the pressure of air further to atmospheric pressure. In this way, a close analog of the present system could be developed. This would introduce further exergy losses in the additional compression and expansion operations and in the heat-transfer which would be used to minimise the net power input for these operations.

(46) The electric generator (7) which is driven by the expander (6) would normally be placed on the same electrical circuit as the electric motor (12) which drives the secondary compressor (11). In this way only the net power from this combination would be exchanged with the grid and thus the ratings of the line can be minimised.