The present invention relates to an autonomous system for generating electricity, intended to be installed directly in the buildings where the energy is used, such as for example detached houses. It can make it possible to dispense with a link to the electrical network, to create less pollution and to be more economical in use than the systems of the same type which are already known, especially (but not exclusively) if it is used in cogeneration mode.
Cogeneration systems are intended to produce electricity and heat simultaneously. They have both an economic and ecological benefit, since they make it possible to save energy and to limit the emissions of carbon dioxide which generates a greenhouse effect. They allow autonomous energy production, precisely where networks are not accessible.
However, cogeneration is used especially on a large scale, in units of several hundred kilowatts which are more adapted to industry or to dense housing. When the housing becomes less dense, and particularly in respect of detached houses, the benefit of centralized cogeneration of high power diminishes, since an expensive heat distribution network has to be constructed, giving rise to considerable energy losses. It would be much more beneficial to install the cogeneration system in the building where the energy is used. Numerous inventors have therefore devised cogeneration systems for small buildings, such as detached houses. The basic idea, consisting of using an electric generating set while recovering the heat of the heat engine in order to heat the building and the sanitary water, is described for example in patent U.S. Pat. No. 2,130,606. However, this simple system is not satisfactory. At the end user, the demand for electricity fluctuates greatly, with a maximum demand which is much greater than the average demand, and long periods of very low demand. The electric generating set must be dimensioned to provide the maximum electrical power, but it usually operates at very low load, this giving rise to very poor efficiency and premature wear. This system is not therefore used in practice.
U.S. Pat. No. 3,944,837 proposes that this drawback be alleviated by connecting the cogeneration system to the public electrical network, in order to smooth the electricity demand. The electric generating set provides the average power continuously, and the network furnishes the difference between the average power and the instantaneous power. The company Senertec, in Germany, is proposing a cogeneration system based on a similar principle. However, this technique requires on the one hand that the network be accessible, thereby losing the advantage of autonomy, and on the other hand that the operator of the network purchase electricity under good conditions, this not always being guaranteed.
U.S. Pat. No. 4,150,300 proposes that the electricity produced by the electric generating set be stored in electrochemical batteries. This solution currently represents the state of the art, since it affords numerous advantages relative to the previous solutions. It is found in particular on boats since the system is thus completely autonomous. The engine can rotate at its state of maximum efficiency long enough to charge the batteries, and stop completely between two charges. It can be dimensioned for an appreciably lower power than the maximum power. However, drawbacks still remain. The overall efficiency is fairly low on account of the poor storage efficiency of the batteries: of the order of 50% for lead batteries. Batteries have a lifetime limited to a few thousand charging and discharging cycles: they have to be renewed frequently, this being very expensive. Moreover, the batteries used for these applications contain heavy metals such as lead or cadmium: this is therefore a polluting solution.
Numerous patents, for example U.S. Pat. No. 4,444,444, propose that the electrochemical batteries be replaced by an inertial flywheel, also referred to as an electromechanical battery. The flywheel has the advantage over the battery of wearing out less rapidly, of being nonpolluting and of restoring the major part of the energy transmitted to it. To do this, the flywheel is placed in an evacuated enclosure so that the friction of the air at its periphery does not dissipate too much energy. Specifically, the peripheral speed of a storage flywheel is extremely high: of the order of 300 m/s for a steel flywheel, much more for a flywheel made of composite materials. The flywheel is then linked to the outside of the enclosure by a purely electrical link, the electric machine serving both as motor and as generator. This solution is beneficial if the electricity comes from the network, but for an autonomous system, it is not at all beneficial, since the mechanical energy of the heat engine must be converted into electrical energy before again being converted into mechanical energy bound for the flywheel, this giving rise to numerous losses.
Numerous patents, for example the document DE-A-40 30 134, propose that the flywheel be coupled mechanically to the heat engine, with the aid of a clutch, thereby eliminating the above problem. When the flywheel reaches its maximum speed, the clutch is opened, thereby enabling the motor to be stopped while leaving the flywheel and the electric generator rotating. This system has excellent storage efficiency, but still has drawbacks. In particular, the heat engine, coupled directly to the flywheel, must rotate at extremely variable duty regimes. The efficiency and the power of heat engines varying extremely greatly with the duty regime, the consequences of this are on the one hand that the efficiency is not very good and on the other hand that the motor must be overdimensioned in order to have enough power at the lowest duty regime.
Numerous patents relating to the transmissions of motor vehicles propose that this problem be solved by interposing between a heat engine and a storage flywheel a mechanical transmission allowing speed discrepancies: for example in the document U.S. Pat. No. 3,886,810, this transmission comprises a hydraulic torque converter, a mechanical speed variator and a clutch. This arrangement, which seems never to have been proposed for an electricity generating system, makes it possible to render the duty regime of the engine independent of the state of charge of the flywheel.
However, the above two patents suffer from a major drawback inherent to mechanical transmission between the flywheel and the engine: the flywheel rotates in the air, thereby bringing about very considerable losses which decrease the efficiency, hence the benefit of the system. It would certainly be possible to place the flywheel in an evacuated enclosure with a dynamic seal on the transmission shaft, but this seal would in turn give rise to undesirable friction, let alone the risks of failure linked with the wearing thereof, since it is invoked continuously.
An objective of the present invention is to solve all or some of these problems, by improving the known systems of the same type. These systems comprise a heat engine producing heat and mechanical power from a fuel, a flywheel for storing kinetic energy, a clutch between the heat engine and the flywheel and an electric generator coupled rigidly to the flywheel. The system according to the invention is essentially characterized in that the clutch, the flywheel and the electric generator are placed inside an enclosure where the pressure is appreciably lower than atmospheric pressure.
In preferred embodiments of the invention, recourse may moreover be had to one and/or to the other of the following arrangements:
the clutch is controlled by electromagnet;
a power transmission member is placed between the heat engine and the clutch;
the power transmission member comprises at least one element chosen from: a hydraulic coupler, a hydraulic torque converter, a step-down gear, a step-up gear, a mechanical speed variator;
the power transmission member is a mechanical speed variator controlled in such a way that the heat engine rotates at a substantially constant duty regime during operation thereof;
the mechanical speed variator comprises a plate driven by the motor and an idler driving the flywheel, the power being transmitted from the plate to the idler by friction, it being possible for the idler to be moved on the plate in such a way as to modify the transmission ratio;
the mechanical speed variator is of the type with trapezoidal belt and variable-diameter pulleys;
the mechanical speed variator is of the toroidal type.
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the system furthermore comprises a heat-bearing fluid circuit adapted to cool at least one element chosen from: the heat engine, the power transmission member, the electric generator, at least one bearing supporting the flywheel: the heat-bearing fluid circuit thus recovers the heat dissipated by all or some of these elements;
the heat-bearing fluid circuit comprises a heat exchanger adapted to cool exhaust gases emitted by the heat engine by heat exchange with the heat-bearing fluid circuit, thereby recovering the heat of these exhaust gases;
the system furthermore comprises a burner adapted to produce heat from the fuel, the heat exchanger being adapted to at least partially transfer this heat to the heat-bearing fluid circuit;
the system furthermore comprising a fan adapted to blow air across the heat exchanger so as to cool the heat-bearing fluid.
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the heat engine is of a type chosen from: a controlled-ignition internal-combustion piston engine, a spontaneous-ignition internal-combustion piston engine (diesel), an external-combustion piston engine (Stirling), a gas turbine;
the heat engine is of the internal-combustion piston type and comprises a longer expansion stroke than the compression stroke (Miller cycle).
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the electric generator is a synchronous generator comprising a permanent-magnet rotor;
the electric generator is a homopolar generator, comprising a rotor made of a ferromagnetic material and a stator comprising on the one hand, a first coil making it possible to create a substantially constant magnetic flux in the rotor, and on the other hand, a plurality of second coils whose flux varies periodically with the rotation of the rotor, thus inducing the alternating output voltage;
a part of the flywheel serves the electric generator as magnetic circuit.
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the flywheel exhibits a shape chosen from the following list: a torus of substantially rectangular cross section, a side of the rectangle being oriented parallel to the axis of rotation, a solid disc of substantially constant thickness;
the flywheel is monoblock, axisymmetric, and it has a thickness that exhibits two bulges, respectively at the center and at the periphery, and an intermediate narrowing between said bulges;
the generator comprises a rotor fixed on one of the two bulges of the flywheel and a stator disposed opposite the narrowing of the flywheel;
the flywheel is made of steel;
the axial cylindrical surface or surfaces of the flywheel are prestressed by shot-peening;
the outer axial cylindrical surface of the flywheel is prestressed by fibers wound under tension;
the fibers wound under tension are made of a material chosen from the following list: glass, carbon, aramid.
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the flywheel is fixed elastically to its hub;
the flywheel comprises fixing tabs extending radially inward, the hub comprises arms extending radially outward, and the tabs are linked to the arms by blocks of elastic material.
Recourse may also be had to one and/or to the other of the following arrangements, which may possibly be used independently of the other arrangements described in the present patent application:
the heat engine and the flywheel each rotate about a horizontal axis;
the heat engine and the flywheel each rotate about a vertical axis;
the heat engine rotates about a horizontal axis and the flywheel rotates about a vertical axis;
the flywheel rotates about a vertical axis and is and is situated entirely in a dish made in a rigid base supporting the engine and the generator;
the flywheel is installed in a cavity under the level of the ground so as to hold back any eruption in the event of bursting.