EP3732351B1

EP3732351B1 - Energy recovery circuit

Info

Publication number: EP3732351B1
Application number: EP18839733.5A
Authority: EP
Inventors: Giulio Contaldi; Stefano MURGIA
Original assignee: Ing Enea Mattei SpA
Current assignee: Ing Enea Mattei SpA
Priority date: 2017-12-29
Filing date: 2018-12-28
Publication date: 2023-05-24
Anticipated expiration: 2038-12-28
Also published as: WO2019130268A1; EP3732351A1

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102017000151144 filed on 29/12/2017 .

TECHNICAL FIELD

The present invention relates to a vane expander and an energy recovery circuit using such an expander.

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 tangency 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 at the outlet port; the expansion makes mechanical energy available at 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.
Known vane volumetric expanders are usually sized and optimized to operate in a predetermined range of operating conditions, and have a limited capacity to adapt to any variations in the thermal load imposed by the hot source. In particular, an excess of thermal power made available by the hot source causes an increase in the flow rate of operating fluid circulating in the ORC system which must therefore be disposed of by the expander.
The excess flow rate can be managed in three ways:

by oversizing the machine for the most critical operating conditions. This condition results in worse machine performance under nominal operating conditions, which are the most frequent;
by leaving the rotation speed of the expander unchanged: under the same conditions, the excess flow rate causes the expansion start pressure to increase. This condition, which is in itself positive, is sometimes incompatible with the size of the ducts and the mechanical strength characteristics of the machine components. In particular, increased pressure may adversely affect the reliability of components (seals, supports, vanes, etc.) that are usually designed for nominal operating conditions and may result in the need to oversize them; or
by introducing a speed control of the expander: with the same geometric dimensions of the machine, increasing the flow rate increases the rotation speed of the machine to avoid abnormal pressure increases. This option allows an increase of the operating range of the expander but involves operation at high rotation speeds. The increase in rotation speed results in two negative effects: it reduces the filling time of the compartments and therefore negatively affects volumetric efficiency; it increases the friction losses that vary with the square of the expander speed and therefore determines a reduction in the mechanical efficiency of the machine.

All these conditions may result in either a negative effect on the overall efficiency of the machine or an excessive increase in the overall dimensions of the machine (design complications).
US6595024 B1 discloses a closed refrigeration system including an expressor (expander-compressor) to recover energy from the expansion of the operating fluid in the expander section of the expressor and use the recovered energy in the compressor section of the expressor. Operating fluid drawn from an outlet of the compressor section is injected into a supplementary inlet of the expander section.
US 2008/0141705 A1 discloses an air conditioning system having an expander for recuperation of mechanical energy from the expansion of the operating fluid. The expander has an auxiliary inlet for the injection of fluid from downstream the condenser into the expansion chamber.

DISCLOSURE OF INVENTION

The present invention relates to an energy recovery circuit using an expander, as claimed in claim 1.
Thanks to the supplementary introduction of operating fluid in the vapour state, an increase in flow rate, pressure and temperature is obtained. This allows for an improvement in thermo-fluodynamic conditions in an expansion phase in which the fluid, under nominal operating conditions, has already almost completely yielded its energy content to the machine.
The supplementary introduction of operating fluid may be made via one or more axial or radial holes communicating with a closed compartment of the expander.
According to a preferred embodiment of the invention, the expander is used in an ORC circuit and the operating fluid can be tapped downstream of the evaporator by means of an on-off control valve; in this way, by appropriately calibrating the diameter of the additional operating fluid inlet hole(s), the inlet flow rate is determined by the difference between the filling pressure of a compartment and the pressure in the compartment during expansion, at the time of introduction of the supplementary flow rate.
Conveniently, the flow rate of liquid injected into the expander is less than 50% of the total flow rate processed by the circulation pump of the circuit.

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 according to the invention;
Figure 2 is a cross-section of a second embodiment of a vane expander according to the invention;
Figure 3 is a diagram of a first embodiment of an energy recovery circuit according to the present invention;
Figure 4 is a diagram of a second embodiment of an energy recovery circuit according to the present invention; and
Figure 5 is a graph showing the pressure in the expander compartments as a function of the rotation angle of the rotor, with the varying of the supplementary flow rate.

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 a supplementary injection system 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 α 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. As a result, the operating fluid injected into such portion does not find low resistance pathways towards the ports 16, 17 and remains confined in the compartment 15, increasing the pressure therein.
In the embodiment illustrated in Figure 1, the injection of operating fluid in the vapour state occurs 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 via a pair of axial holes 22 arranged one after the other in a circumferential direction and made in one of the heads 5.
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) type ORC 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 hydrofluorocarbide (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, in a known manner:

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 the thermal energy of which 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 tapped downstream of the HTHX evaporator and is injected into the expander 1 as described above. To such purpose, a duct 26 connects a node 27 of the circuit downstream of the HTHX evaporator to the fitting 21 or holes 22. The supplementary flow rate that is introduced during the expansion phase is controlled by a valve 28 located on the duct 26.
Preferably, the valve 28 is of the on-off type. When opened, the fluid flow rate through the duct 26 is determined, once the diameter of the inlet holes 20 or 22 is fixed, by the pressure difference between the node 27 (which coincides with the filling pressure of the compartments 15) and the pressure in the compartments 15 in the portion of the expansion phase in which the introduction occurs.
The circuit in Figure 4, denoted by reference numeral 29, differs from that of Figure 3 described in that it is a recovery circuit, in which that is, 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.
The tapping and supplementary introduction of operating fluid are made in a similar manner to that described above.
Figure 5 is a graph showing the pressure trend in a compartment 15 as a function of the rotation angle of the rotor, as the flow rate of the supplementary operating fluid introduced varies.
Curve A refers to a comparative example, relative to a vane expander without a supplementary injection of operating fluid, supplied at the nominal flow rate.
Curves B, C and D refer to three embodiments of the invention in which, with the same machine and nominal flow rate, a supplementary flow rate of 20%, 30% and 50% of the nominal flow rate respectively is introduced.
As is easy to observe, the increase in flow rate in the terminal part of the expansion results in an increase in pressure (and temperature) that entails the increase of the useful area under the expansion curve which is directly proportional to the power developed.
With the use of the supplementary flow rate inlets, the first part of the expansion remains unchanged and takes place in compliance with the nominal design conditions of the machine, while the second part of the expansion is characterized by thermo-fluidodynamic conditions favourable to the development of greater power, which can be obtained at the same size and speed of rotation of the machine.

Table 1 below shows the mechanical power, volumetric efficiency and percentage power increase values as a function of the percentage of supplementary flow rate, under the same conditions as the graph in Figure 5.

Table 1

	A	B	C	D
	A	*+20% supplementary flow rate*	*+30% supplementary flow rate*	*+50% supplementary flow rate*
Mechanical power, W	827	953	1032	1151
Volumetric efficiency	0.82	0.89	0.89	0.89
Power increase		15.2%	24.8%	39.2%

Claims

A circuit for recovering energy from a thermal source comprising an operating fluid circulation pump (P), a high-temperature heat exchanger (HTHX) in which the operating fluid exchanges heat with the thermal source, an expander (1) and a low-temperature heat exchanger (LTHX) in which the operating fluid exchanges heat with a cooling fluid,
the expander (1) comprising:
- a stator (2) provided with an inlet port (16) and an outlet port (17) for an operating fluid,

- a rotor (3) eccentrically housed in the stator (2) and provided with a body (7) internally tangent to an internal surface (6) of the stator (2) and with a plurality of vanes (9) sliding in respective seats (10) provided in the body (7) of the rotor (3) and pushed in a centrifugal direction for cooperating in a sealed manner with the internal surface (6),

the stator (2) and the body (7) of the rotor (3) delimiting between one another a circumferential chamber (14) with varying radial width, communicating with the inlet port (16) and the outlet port (17) and divided by the vanes (9) into a plurality of compartments (15);

characterized in that the expander comprises at least one supplementary inlet (20, 21; 22) for the introduction of a supplementary flow rate of operating fluid in the vapour state into an expansion portion (18) of the chamber (14) isolated from the inlet port (16) and from the outlet port (17), the circuit

comprising a supply duct (26) of operating fluid in the vapour state from a point downstream of the high-temperature heat exchanger (HTHX) to the supplementary inlet (20; 22) of the expander (1).
The circuit according to claim 1, characterized by comprising a valve (28) for controlling the supplementary flow rate of operating fluid in the supply duct (26).
The circuit according to any of claims 1 or 2, characterised by comprising a recovery heat exchanger (RHX) for exchanging heat between the operating fluid flowing out of the expander (1) and the operating fluid in the liquid state downstream of the circulation pump (P).
The circuit according to one of the preceding claims, characterized in that the operating fluid is an organic fluid with the addition of a lubricant.
The circuit according to claim 4, characterised in that the organic fluid is a hydrofluorocarbon or a hydrofluoroolefin, and in that the lubricant is a polyester-based oil.