ENERGY RECOVERY METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102017000151139 filed on 29/12/2017, the entire disclosure of which is incorporated herein by reference .

TECHNICAL FIELD

The present invention relates to an energy recovery circuit from a thermal source using a vane expander and an energy recovery method achievable by means of such circuit .

BACKGROUND ART

Vane expanders are known comprising a stator provided with an inlet port and an outlet port, and a rotor eccentrically housed in the stator, internally tangent to a side wall of the stator and provided with a plurality of vanes sliding radially relative to the rotor and cooperating in a sealed manner with the stator.

The stator and rotor delimit between them a circumferential chamber with width varying from zero in the tangent area to a maximum value near one end of the outlet port; the vanes divide the chamber into a plurality of compartments with volume varying with the rotation of the rotor, each of which has a minimum volume at the inlet port and a maximum volume near the beginning of the outlet port.

The operating fluid entering a compartment communicating with the inlet port is thus expanded and discharged to the outlet port; the expansion makes mechanical energy available to an outlet shaft rotationally connected to the rotor.

It is known to use vane volumetric expanders in energy recovery systems (e.g. based on a Rankine cycle using a high molecular weight organic operating fluid, known as ORC) from waste heat sources in the industrial, automotive or civil sectors, etc.

Volumetric vane expanders offer important advantages in terms of efficiency and power produced compared to other available technologies, especially when the enthalpic content of thermal sources is very low, for example when the temperature of the thermal source is between 80° and 120 °C.

Other advantages of vane volumetric expanders compared to other technologies available for energy recovery systems are the low-rotation speed operating capacity, automatic starting without the need to be driven by an electric motor, reduced noise emission, low vibration, and a structure based on simple components and absence of flow control valves.

Despite the advantages described above, the performance of vane volumetric expanders is conditioned by the fluid dynamic losses occurring during the expansion phase due to the unavoidable clearance between the vanes, the rotor and the stator.

In addition, in small ORC systems, to avoid significant system design complication, direct lubrication systems are not used but a small amount of lubricant mixed with the operating fluid is used.

The fraction of lubricant that can be used to ensure the correct lubrication of the expander without causing an excessive reduction in the efficiency of heat exchange in the heat exchangers is insufficient to properly fill the geometric clearance and thus to ensure a good seal between the compartments of the expander: this generates a clear reduction in pressure during the filling of the compartments before closing the inlet port.

Fluid dynamic losses due to leaks are also the cause of a deviation of the real pressure/volume curve from the (ideal) adiabatic curve.

This condition is inherent to vane technology and cannot be improved merely with correct geometric design.

Considering a typical ORC thermodynamic cycle associated with a recovery system equipped with a vane expander, there is a difference between the flow rate supplied by the pump and the flow rate of operating fluid that actually produces a useful effect during expansion: in fact in the absence of a perfect seal between the compartments of the expander, a part of the flow rate supplied by the pump does not participate in the expansion process and leaks between the compartments in the direction of the lowest pressure. This phenomenon can be assimilated to rolling or expansion through a nozzle (therefore without useful production) .

The higher the leaks in the expander, the greater the difference between the flow rate that actually participates in the expansion and the total flow rate supplied by the pump. This difference can be up to 20%.

This rolling also results in a reduction of temperature and pressure inside the expander, which also results in an inefficient and incomplete filling phase and a less efficient subsequent expansion, further away from the ideal adiabatic curve. This penalises the work indicated and the volumetric yield .

DISCLOSURE OF INVENTION

The purpose of the present invention is the construction of an energy recovery circuit comprising a vane expander, which overcomes the drawbacks described above.

The above purpose is achieved by an energy recovery circuit according to claim 1.

The present invention also relates to an energy recovery method according to claim 8.

Thanks to the injection of operating fluid in the liquid state, the sealing of the compartments is improved and consequently the work and the volumetric efficiency of the expander are optimized.

According to a preferred embodiment of the invention, the fluid may be preheated prior to injection into the expander, so as to avoid lowering the temperature in the compartment. In a non-recovery ORC circuit, this can be achieved by means of the residual heat of the thermal source or by the hot fluid in output from the expander; in a recovery ORC circuit, the fluid can be drawn downstream of the recovery heat exchanger placed between the pump and the evaporator where it is heated by the hot fluid in output from the expander.

Conveniently, the flow rate of liquid injected into the expander is less than 5% of the total flow rate, and preferably less than 3%. This optimises the seal without causing substantial and unwanted temperature reductions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention some preferred embodiments are described below with reference to the appended drawings, wherein:

figure 1 is a cross-section of a first embodiment of a vane expander that can be used in an energy recovery circuit according to the invention;

figure 2 is a cross-section of a second embodiment of a vane expander that can be used in an energy recovery circuit according to the invention;

figure 3 is a diagram of a first embodiment of an energy recovery circuit according to the present invention; and figure 4 is a diagram of a second embodiment of an energy recovery circuit according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to figure 1, reference numeral 1 globally denotes a vane expander comprising a stator 2 and a rotor 3 eccentrically housed rotationally free inside the stator 2 and provided with at least one output shaft not shown.

The stator 2 comprises a substantially hollow cylindrical stator barrel 4 and a pair of axial closing heads 5, of which only one is visible in figure 1. The stator barrel 4 has an inner cylindrical surface 6 of axis A.

The rotor 3 comprises a cylindrical body 7 of axis B tangent to the inner surface 6 of the stator barrel 4 and a plurality of vanes 9 sliding in respective radial seats 10 of the body 7. The vanes 9 are pushed in a centrifugal direction, e.g. by means of springs or ejection and motion control rings, fluid pressure or a combination thereof, to cooperate in a sealed manner with the inner surface 6 of the stator barrel 4.

The stator barrel 4 and the body 7 of the rotor 3 delimit between them a circumferential chamber 14 of radial width varying from a substantially null value at the tangent generatrix T to a maximum value at a diametrically opposite generatrix S. The chamber 14 is divided by the vanes 9 into a plurality of compartments 15.

The chamber 14 communicates with an inlet port 16 and an outlet port 17 for an operating fluid made in the stator 2. In particular, the inlet port 16 is placed at one end of the chamber 9 near the tangent zone between the body 7 of the rotor 3 and the stator barrel 4, the outlet port extends from the area of maximum width of the chamber 14 to an opposite end of said chamber, near the tangent zone.

The chamber 14 therefore has an expansion zone (with increasing radial width) and an outlet zone (with decreasing radial width) ; depending on the number of vanes 9, in the example shown seven, a certain number of compartments 15 are located in the expansion zone and a certain number of compartments are located in the outlet zone.

According to the present invention, the expander comprises at least one supplementary inlet for a supplementary introduction of operating fluid in the liquid state into an expansion portion 18 of the chamber isolated from the inlet port and the outlet port.

This portion 18, indicated by a lined section in figure 1, is angularly distant from each of the inlet 16 and outlet ports 17 by an angle equal to the angular width of a compartment 15, and therefore extends along an angle equal to the angular distance between the inlet port 16 and the outlet port 17 minus twice the angular width of a compartment 15.

In figure 1, the rotor 3 is shown by a solid line in a first position where a first compartment 15 of the expansion zone begins to be isolated from the inlet port 16 by a vane 9. The vanes 9 are also illustrated (lined) in a second position in which a last compartment 15 of the expansion zone is in an incipient outlet position, i.e. a position in which a vane 9 still isolates such compartment 15 from the outlet port 17 but wherein a minimum rotation of the rotor 3 would bring it into communication therewith.

From this representation it can be seen clearly that the portion 18 is always isolated from both the inlet port 16 and the outlet port 17, at any angular position of the rotor 3. Thus, the fluid injected into such portion does not find low resistance pathways to the ports 16, 17 and tends to infiltrate the axial and radial clearance between the vanes 9, the body 7 of the rotor 3 and the stator 2, significantly improving the seal.

In the embodiment illustrated in Figure 1, the injection of fluid takes place through a calibrated radial hole 20 made in the stator barrel 4, and provided with a threaded fitting 21 for connection to a supply pipe. Conveniently, the fitting 21 is arranged in an initial area of the portion 18.

In the embodiment of figure 2, the injection takes place through a pair of axial holes 22 arranged one after the other in a circumferential direction and made in one of the heads 5. Conveniently, the holes 22 are shifted towards the rotor relative to a longitudinal centreline of the chamber 14 so as to lubricate the interlocking zone of vanes 9 in the seats 10.

The operation of the expander 1 is described below, with reference to its application to an energy recovery circuit, of which figures 3 and 4 illustrate two embodiments.

Figure 3 illustrates a simple (non-recovery) ORC-type energy recovery circuit 25. The thermal source the residual energy of which is to be recovered is the lubricating oil of a compressor C. The circuit 25 uses an organic operating fluid, conveniently consisting of a hydrofluorocarbon (HFC) or a hydrofluoroolefin (HFO) to which a compatible lubricant is added, e.g. a polyester-based oil (POE), e.g. in a 5% by mass percentage.

The ORC circuit comprises:

- a circulation pump P pressurising the operating fluid in the liquid state,

- a first high temperature heat exchanger HTHX, or evaporator, wherein the operating fluid exchanges heat with the fluid of which the thermal energy is to be recovered, in the example illustrated the lubricating oil of the compressor, and passes to the vapour state;

- the expander 1, wherein the operating fluid expands producing mechanical energy available to the rotor output shaft, to which an electric generator G is conveniently connected; and

- a second low temperature heat exchanger LTHX, or condenser, wherein the operating fluid exchanges heat with a coolant fluid, conveniently water, for cooling and condensing the operating fluid.

According to an embodiment example of the present invention, the operating fluid in the liquid state is drawn from the high pressure branch of the circuit 25, i.e. downstream of the circulation pump, and is injected into the expander 1 as described above. To such purpose, a duct 26 connects a node of the circuit downstream of the pump P to the fitting 21 or holes 22.

Preferably, in order to avoid temperature reductions inside the compartment 15 which the liquid is injected into, this is preheated, e.g., by a third heat exchanger PXH in which it exchanges heat with the vapour at the outlet of the expander 1.

According to an alternative not shown, the operating fluid in the liquid state may be preheated by the residual heat of the thermal source, in this case the compressor oil rather than by the operating fluid in output from the expander .

The circuit of figure 4, denoted by reference numeral 27, differs from that of figure 3 described in that it is a recovery circuit, i.e. in which all the operating fluid in output from the pump P is subjected to a heat exchange with the vapour at the outlet of the expander 1 in a heat exchanger RHX or recovery heat exchanger. In this case, the operating fluid in liquid phase is drawn downstream of the RHX heat exchanger, and injected into the expander 1.

The amount of operating fluid drawn and injected in the liquid state into the evaporator is conveniently less than 5% of the flow rate of the circulation pump P, and preferably less than 3%, in order to avoid unwanted temperature drops in the expander compartments. Within these limits, the effects of the liquid injection are neutral or even favourable from a purely thermodynamic point of view, and very favourable from the point of view of the work indicated and the volumetric efficiency thanks to the improvement of the seal in the compartments.