FIELD OF THE INVENTION

The invention relates to a free piston type compressor. The invention further relates to an air conditioning system. The invention further relates to a heat pump or liquid pump. BACKGROUND OF THE INVENTION

An embodiment of a free piston type compressor is known from US 5, 275, 542. In the known compressor the piston and a driving linear electromotor are connected by a shaft.

The known free piston type compressor has the following disadvantages:

piston movement is not controlled mechanically;

speed of the electromotor is limited to the speed of the compression piston. Accordingly, a relative large and thus expensive motor is required to deliver a sufficient force to the piston;

motor and compression piston move simultaneously in the same direction which may induce considerable vibration.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved free piston compressor. In particular, it is an object of the invention to provide a free piston compressor which is substantially vibration free. To this end the free piston type torsion drive compressor according to the invention comprises a first piston and a second piston interconnected with a torsion body and elastic bending elements, said first piston and said second piston being driven by a first motor and a second motor by applying force in opposite circumferential directions, so that in use the first piston and second piston displace along a displacement direction wherein the said motors drive the first piston and the second piston in the circumferential direction transversal to the displacement direction. It is found that when the free piston is driven by a torsion body the resulting construction is substantially vibration-free due to the fact that the compressor according to the invention comprises two opposite moving pistons which are adapted to move with substantially the same speed. More in particular, it is found that when the pistons are connected with elastic elements a rotational displacement of the pistons is translated into axial displacement accurately.

In an exemplary embodiments of a free piston type compressor/pump it may comprise the first piston and the second piston, positioned in opposite sides and both connected to a torsion body and elastic bending elements.

Accordingly, the torsion body may comprise a central torsion rod encircled by a set of elastic bending elements for controlling the axial displacement.

During operation the pistons are driven by the motor(s) which causes them to rotate with alternating direction with a frequency, which is equal to the natural frequency of the mechanical assembly of the pistons and the torsion body. The motor(s) drive the motion by applying driving force in circumference direction. This means the motor(s) takes care for continuous natural torsion vibration. The elastic elements, also connected to the piston convert this alternating rotational displacement in an alternating axial displacement of the piston, which may also be referred to as 'a piston stroke'. Accordingly, movement of the piston has a rotating component (the driving direction) and an axial component (the stroke). The speed of rotation is higher than the axial speed. Pistons move in axially opposite directions during operation. During operation the torsion body and the elastic bending elements undergo an elastic deformation. In Future embodiments the torsion bar and the elastic bending elements may be integrated.

There are two possibilities to drive the system. First embodiment is the use of two motors. Each piston is directly driven by its own motor and the piston is an integrated part of the electromotor, called the rotor. In the second embodiment of the drive system is the use of single central positioned motor. The motor is positioned in between the pistons.

In the first embodiment the first motor and the second motor may be adapted to rotate with the same frequency having the opposite phases. This results in an opposite axial movement of the pistons. This technical measure is based on the insight that when the first motor and the second motor are adapted to rotate with the same frequency having the opposite phases for both translation and rotation, the overall vibration of the compressor is very small.

In the second embodiment, the motor is positioned centrally between the pistons, whereby each motor side is provided with its own torsion bar and elastic bending elements connected to the piston. The first piston and the second piston are adapted to rotate with the same frequency having

synchronous phases. Both pistons move in opposite phase to the central motor. This results in an opposite axial movement of the pistons.

In addition, it is found that the stroke of the compressor is mechanically controlled, which is advantageous. More in particular, it is found that the speed of rotation of the compressor according to the invention is much higher that the speed of translation. Accordingly, a relatively small motor(s) may be used. This may substantially reduce the manufacturing costs of the compressor according to the invention. More in particular, the torsion based compressor may have the following additional advantages:

the motor power may be completely independent from the compression control; the energy delivered by the motor is used to control the amplitude of the torsional vibration. Increasing the amplitude gives an increased compressor power. Stroke of the piston increases, while the top dead centre remains unchanged;

the rotor of the electromotor may be integrated in the piston.

Accordingly, the motors of the compressor according to the invention may be adapted to deliver a substantially limited force or energy for

compensating for the energy loss of the compression process. Accordingly, the efficiency of the compressor of the invention is increased. An embodiment of the invention with an integrated motor will be discussed in more detail with reference to Figure 2, Figure 3 and Figure 4. In this embodiment the compressor comprises two opposite interconnected pistons, where each piston is also part of the electromotor, the rotor. A first connection element is a torsion body provided in the centre to control the torsion stiffness. An exemplar embodiment is given in Figure 2. A second connection element may comprise a set of flexible rods, required to control the axial movement of the pistons. An exemplary embodiment is discussed with reference to Figure 3. Flexible rods may be provided to transform the rotation to a controlled translation in accordance with the invention. Figure 4 shows the torsion drive system positioned in a cylinder liner with the static parts of the motors (windings).

It is found to be advantageous to implement the torsion body because the motion is not directly dependent on the characteristics of the electromotor.

The air conditioning system according to the invention comprises a free piston type compressor according to any one of the preceding claims.

These and other aspects of the invention will be discussed with reference to drawings wherein like reference signs correspond to like elements. It will be appreciated that the drawings are presented for illustrative purposes only and may not be used for limiting the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 presents in a schematic way an embodiment of the compressor according to the invention, having a central positioned motor.

Figure 2 presents in a schematic way an embodiment of the compressor with integrated motors, showing the torsion body.

Figure 3 presents in a schematic way an embodiment of the compressor with integrated motors, showing the movement of the piston controlled by a set of flexible rods.

Figure 4 presents schematically a complete design of the torsion drive compressor according to a further aspect of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 presents in a schematic way an embodiment of the compressor with a central positioned motor. The compressor 10 according to the invention comprises a first free piston 2a and a second free piston 2b interconnected by a torsion body 3a and 3b, and elastic bending element 4a and 4b to the central electromotor 5. In accordance with the invention the driving force is applied transversely to the direction of the expected

displacement D of the first free piston and the second free piston.

The capacity of such compressor may easily be adjusted by adjusting the vibration amplitude, by changing driving power/amplitude of the torsion vibration.

The movement of the pistons 2a, 2b in the direction D is enabled by opposite driving directions of the motor In a particular embodiment, the motor 5 rotates with a substantially the same frequency having the opposite phases of the pistons. The motors thus couple the driving force to the pistons via the torsion body 3a, 3b. The torsion rods are implemented from a material which undergoes elastic deformation when driven by the motor. The opposite phase of the motor with the piston enables a well-balanced design. It will be appreciated that in the known conventional free piston designs the piston is directly coupled to the translating electromotor. This has at least two disadvantages: the absence of mechanical piston control and the engine speed that equals the piston speed. For the compressor efficiency it is very important to have an accurate and fixed top dead centre position, to minimize the dead volume losses. Dead volume reduces the volumetric efficiency, especially at high compression ratios. Due to absence of transmission between the electromotor and the piston, the relative large compression forces of the piston are directly transmitted to the electromotor, so it has a large negative impact on the size of electromotor. Given the required electromotor power, the size can be reduced by increasing its speed.

A principal disadvantage of a linear motor known from the prior art is that it has to change direction instead of continued rotation. For a conventionally known free piston design the motor is directly coupled to the piston, so the translation speed equals the piston speed. The average speed of the piston of a refrigeration compressor is low, typically 2 m/s for a small commercially available compressor up to 5.5 m/s for a large industrial compressor. In accordance with the invention, an aspect of which has been illustrated with reference to Figure 1, a free piston type torsion drive compressor (or pump) is provided, wherein the dead volume is mechanically controlled and wherein the engine speed of the electromotor exceeds the piston speed.

The principle of the torsion drive is based on the reduction of the length of a straight flexible elements in X-direction, when it is loaded. When two pistons are connected by a straight flexible element, the pistons create a sliding guide which allows substantially no rotation. Loading causes bending of the flexible (elastic) element. Loading gives vertical displacement Ay and an axial displacement Δχ, which is a second order effect. This has a number of advantageous properties:

a high transmission ratio Ay/Δχ may be enabled;

the pistons mode symmetrically, therefore they are balanced; there is an accurate mechanical top dead position.

The torsion drive technology uses the elastic properties of dynamically loaded components. At maximum capacity the material stress is found to be well below the fatigue limit. This means that the elastic elements and the torsion bar have substantially unlimited lifetime. The technology can be combined with common valve plates. It is also mentioned that the lifetime of the compressor is not limited by the frequency of the start/stop cycles. More details on operation of the free piston type torsion drive compressor (or pump) are given in Figures 2 - 4.

Figure 2, 3 and 4 present in a schematic way an embodiment of the compressor having integrated electro motors. The pistons are part of the electromotor, the rotors. The pistons are connected by a torsion bar in the center, see 23, which may be encircled by a set of elastic rods 33 required to control the displacement of both pistons 21, 22 in Figure 2 or 31a, 31b in Figure 3, respectively. In a particular embodiment of Figure 2 depicting the compressor 20 according to the invention, the first free piston 21 and the second free piston 22 are interconnected by a torsion body 23, which may be implemented as an elastic torsion spring. It will be appreciated that other per se known embodiments of an elastic spring may be used. For the clarity reasons the complete motors driving the torsion body are not depicted.

In accordance with the invention when the alternating driving force is applied to the torsion body 23 the free pistons 21, 22 will rotate to and fro and translate back and forth along the translation direction D which is substantially parallel to the longitudinal axis L of the compressor 20. Figure 3 shows the sets of elastic rods controlling the movement of the pistons. In view 30a a shortening of the compressor is illustrated upon the elastic bending of the rods 33 driven by the torsion force by the electromotor on the pistons. The free pistons will rotate and translate in the opposite directions as is depicted by the respective arrows.

In view 30b the compressor returns to its original (rest) state when no driving force is applied. The pistons are now in their top dead centre position, approaching the valve plates 24A and 24B.

In view 30c a second shortened condition of the compressor is shown when the free pistons 31a, 31b are driven by the torsion force of the electro motors in opposite direction with respect to the direction shown in view 30a.

The weight of the piston, or more particularly, the torsion inertia of the pistons, and the stiffness of the torsion bar are selected so that the natural torsion vibrating frequency equals about 50 Hz. This substantially improves operating parameters of the device according to the invention.

By suitably alternating the driving directions for the free pistons 31a, 31b a pulse-like translational movement of the compressor may be obtained. Figure 4 presents schematically a complete design of the torsion drive compressor 40 according to a further aspect of the invention. The torsion bar 47 is cooperating with the elastic elements 45. Preferably, the compressor has a fully symmetrical design in both the static and the dynamic modes. In the present embodiment, the compressor comprises two integrated oscillating electromotors, wherein the piston is arranged as a rotor with integrated permanent magnets 48. The stator windings are schematically given by 41. The motors may be canned motors. The stator windings are physically separated from the refrigerant/liquid by a cylinder liner. This has an

advantage that in the case of application as compressor, it is suitable for operating with different refrigerants, including ammonia. The cylinder liner 43 encloses all moving components, except the valve plates 49.

The cylinder liner is part of the hermetically sealed outer shell of the compressor. It is found that the compressor according to the invention may operate with a refrigeration capacity of 0.5 to 5 kW. This corresponds to a compressor with an effective length of 500 mm and a piston diameter of 65mm. The compressor is hermetic, oil free, compatible with all refrigerants and has a step less capacity from 0 to 100%. This combination is very useful for application with natural refrigerants and enables new combinations of technologies, gas compression and sorption cycles.

The torsion drive compressor is able to increase the efficiency of the compressor itself and of an overall system using the compressor. The low internal friction, the step less capacity control, the absence of a frequency controller and other part load losses gives a significantly improved energetic efficiency of the compressor. The efficiency of the system will be increased by the used of a natural refrigerant like ammonia and the absence of oil. As oil free system has the following positive effects"

i) improved heat transfer in the heat exchangers, which is used to be the most problematic at low temperature applications of the known systems;

ii) reduced flow losses in the suction line;

iii) layout of the suction line may be less critical. An additional effect of the absence of oil is that hydrocarbon will not dissolve in the oil. Accordingly, the 150g refrigerant can be used more effective, increasing the refrigeration capacity. An important new possibility is the use of hybrid heat pimps, based on vapor compression and absorption technology. Sorption cycles do not function properly with oil circulating in the system, so the absence of oil is advantageous. The hybrid pump may be used to increase efficiency and application range of the compressor. As it may use two independent energy sources (electric and heat), it is easier for heat pumps of the type discussed above to handle peak loads.

It is found that the compressor according to the invention has the following additional advantages:

- well vibration balanced of construction;

high speed of the electromotor is easy transfer to a high driving force of the free pistons;

a continuous adjustment of the capacity is possible by varying the amplitude of the torsion movement;

- oil-free application is possible, which makes the compressor according to the invention particularly versatile in use;

high motor speed enables the use of small and cheap motors.

While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described. Moreover, specific items discussed with reference to any of the isolated drawings may freely be inter-changed supplementing each outer in any particular way. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described in the foregoing without departing from the scope of the claims set out below.