Abstract:
A machine for near-adiabatic compression/expansion of gas comprises two or more stages. Each stage utilises the oscillatory motion of liquid through a set of tubes to transfer power between a displacer part and a converter part. In the converter part, each of several vertical gas compression/expansion cylinders admits an oscillating column of liquid which rises and falls to act as a liquid piston. The surface of said liquid piston is thermally isolated from the gas above it by a cover-block floating atop the liquid. The cover-block also acts to minimise the transport of gas molecules into the liquid or liquid molecules into the gas. The displacer part transforms power between the oscillatory motion of the liquid and mechanical shaft power. The machine is suited to converting slow speed mechanical power into the form of (near-) adiabatically compressed gas and has potential applications in wind wave and tidal generation.

Description:
BACKGROUND 
       [0001]    The present invention is a slow-speed machine which may be operated either as a compressor or an expander. The compressor function is motivated largely by the requirement to draw energy from renewable energy harvesters such as large wind turbines, tidal turbines or wave-energy machines in a form which is compatible with the storage of at least a part of that energy. 
         [0002]    The expander function is motivated primarily by the requirement to convert energy back from a thermo-fluidic form to mechanical form so that either electricity may be generated or some other working function may be achieved (elevation, conveying, stirring for water treatment plant etc). The compressor function is expected to be the more important one since converting from energy form from the thermo-fluidic form of a compressed gas to electrical form is most economically done with a high-speed expander-generator set. 
         [0003]    The machine described here has the following features:
       It is a positive-displacement machine   It will normally comprise several positive-displacement stages of similar nature in series   Each of several positive-displacement stages includes a displacer part which converts the primary mechanical motion into oscillatory flow displacement of a liquid   Each of several positive-displacement stages includes a converter part in which the the displacement of a liquid drives (or is driven by) the process gas   It will normally operate between two steady gas pressures which are both substantially higher than atmospheric pressure   It is engineered to be suitable for low rotational/linear velocities such that it can be deployed as a direct-drive power converter in wind/wave/tidal power contexts   It includes design measures aimed to minimise the net heat-transfer between the gas passing through and the machine itself when operating in the steady state   The machine must be capable of operating with a range of input temperatures ranging from cryogenic temperatures (˜100 K) up to ambient temperature range   The machine must be capable of operating with a range of exhaust temperatures ranging from ambient temperature range up to high temperatures (˜4000 K)   It has a very high isentropic efficiency   It is capable of operating at part load       
 
         [0015]    This machine contrasts with the vast majority of existing compressors and expanders due to the above features. 
         [0016]    The application which has initially driven the design of this machine is described in patent application GB 1310717.2 (Jun. 16, 2013) which describes a complete system whereby a collection of wind turbines would each use an adiabatic compressor to raise the pressure of gas in a closed circuit. An expander-generator elsewhere in the closed circuit of gas would extract power from the gas and thermal stores located on both the high and low pressure sides of the circuit would provide the capability to store exergy. 
         [0017]    Some other potential applications exist for adiabatic compression and expansion. These applications include numerous heat-pumping applications. The high efficiency of the present machine increases its potential applicability in these areas but the intrinsic low-speed natural of this machine renders it likely that other solutions provide a more cost-competitive route except where very high isentropic efficiency is required. 
       BRIEF SUMMARY OF THE INVENTION 
       [0018]    The invention comprises a multi-stage machine where pressure is raised (or dropped) in to or more different stages and heat transfer between the gas and the machine itself is deliberately minimised. Each stage comprises two main parts—(i) a displacer part which directly couples shaft motion to alternating flow of liquid through several ports and (ii) a converter part which couples the alternating flow of liquid to volume changes in the gas-filled parts of several different cylinders. The displacer part is coupled to the converter part by a set of tubes filled with the same liquid that is found in both the displacer part and the converter part. Within the converter part, each cylinder contains some of the liquid (acting as a liquid piston) above which floats a cover-block serving as a thermal isolator between the liquid and the gas in the volume above the cover block. Within the displacer part, shaft rotation is coupled positively to the motion of the liquid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  depicts the machine as a whole indicating multiple stages with one common mechanical input/output shaft and one gas path between low pressure gas (on the left) and high pressure gas (on the right).  FIG. 2  shows the main elements of a single stage of the machine with a displacer part at the bottom of the figure and the converter part at the top.  FIG. 3  is an expanded view of the displacer part of a single stage allowing individual components to he identified.  FIG. 4  is an expanded view of the converter part of a single stage allowing individual components to be identified.  FIG. 5  provides  6  views of a single stage at different instants through a single shaft rotation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The invention is described here in terms of a compressor. If all valving is actively controlled (rather than simply comprising one-way valves) the same hardware can serve perfectly well as either expander or compressor and any person skilled in the art can easily deduce how the function could be switched between compression and expansion. 
         [0021]      FIG. 1  shows an elementary overview of the invention. In this overview, an input component  1  referred to here as a shaft supplies mechanical power simultaneously to several discrete devices,  201 ,  202 ,  203 ,  204  and  205  called displacers. Each displacer device,  201 - 205 , is connected via a respective tube-set,  301 - 305 , to a corresponding convener device,  401 - 405 . Each converter device,  401 - 405 , draws gas in at one pressure and discharges gas at a higher pressure. Plenum chambers,  501 - 504  are located in the gas path to minimise pulsations in pressure since the net flow rate of gas either into or out from one converter device will not be a constant. The block arrows on  FIG. 1  indicate the direction of power flow. 
         [0022]    Power flows from the shaft  1  into the displacers  201 - 205 . Each displacer (e.g.  201 ) transforms power into the oscillatory flow of a liquid within the associated tube-set (e.g.  301 ). This oscillatory flow is communicated into the internals of the associated converter device (e.g.  401 ) within which the rising and falling surface(s) of the liquid inside one or more cylinders controls the intake of process gas at one pressure, compression of that gas to a higher pressure and subsequent exhaust of the higher pressure gas. In effect, the liquid with oscillatory flow within each tube of one tube-set (e.g.  301 ) forms a moving liquid piston within the converter device. 
         [0023]    Each displacer unit (e.g.  201 ) combined with its corresponding tube-set (e.g.  301 ) and associated converter device (e.g.  401 ) forms a separate compressor stage (e.g.  601 ). The machine of interest here comprises several compressor stages.  601 - 605  in series. Each individual compressor stage has a well-defined swept volume per cycle. The swept volume per cycle of successive compressor stages reduces from the lower-pressure stages to the higher pressure stages to reflect that the gas density is raised in each stage. The gas density rises with pressure but not in direct proportion to pressure because the gas temperature also rises. The number of series compressor stages used in the machine is a trade-off between overall performance and complexity/cost and this depends on the overall pressure ratio intended for the machine. Typically, the pressure ratio will be between 10 and 100 and the number of compressor stages will be between 3 and 7. 
         [0024]      FIG. 2  shows a simple representation of one possible compressor stage  601 . In this figure, the displacer part  201  is coupled to the converter part  401  via the tubeset  301  and the input power shaft  1  is seen at the centre. 
         [0025]      FIG. 3  shows a more detailed view of the displacer part  201  of a single stage. This contains a cam  11  which rotates with the drive-shaft  1 . A cam-follower ring  12  encircles the cam and this cam-follower ring  12  undergoes an orbiting motion as the cam  11  and shaft  1  rotate together. The cam-follower ring itself  12  does not undergo net rotation. 
         [0026]    Conn-rods (connecting rods)  13  are joined to the cam-follower ring  12  through pin-joints so that one end of each one of those conn-rods  13  must move with the cam-follower ring  12  but the other end need not be on the line defined by the centre of the cam-follower ring and the pin-joint at any one time. Each one of the conn-rods  13  is joined to a respective piston  14  through another pin-joint. The pistons run within cylinders  15  and the head of each cylinder has a single simple port  16  leading to one tube of the tube-set  301 . The port at the top of the cylinder is shaped to provide a reasonably smooth transition of flow between the liquid within the cylinder and the liquid in the tube, 
         [0027]    At any one instant of operation of the compressor stage  501 , the liquid pressures will be different in the different cylinders above the pistons  14 . Within the displacer unit, the cavity inboard of all of the pistons at any one time which contains the cam  11 , the cam-follower ring  12  and all of the conn-rods  13  is filled with the same liquid that is present above the pistons. The pressure of that liquid in the cavity remains at approximately the average pressure in all the cylinders  15  above the pistons  14 . The effect of this is that the net rate of fluid leakage around any one piston (averaged over one cycle) is zero and the peak pressure drop across any one piston driving leakage flow is much lower than it would have been if the central cavity was not pressurised. 
         [0028]    The liquid driven back and forth from any one cylinder  15  of the displacer unit  201  passes through one tube of the tube-set  301  and into a corresponding cylinder within the converter part  401  of the corresponding compressor stage  601 . 
         [0029]      FIG. 4  shows a more detailed view of the converter part  201  of a single stage. Here, each individual tube from tube-set  301  is connected directly to a corresponding gas compression cylinder  20  which is oriented in a (substantially) vertical orientation. Another smoothly-shaped port  21  on each gas compression cylinder allows the liquid oscillating backwards and forwards within the tube-set to transition its flow smoothly into the compression cylinder  20  without substantial loss of energy in eddies. As a result of the continuous liquid connection between the motion of one piston  14  of the displacer unit and motion of the liquid surface within the gas compression cylinder  20 , the liquid effectively provides a very efficient mechanical transmission between the pistons  14  of the displacer unit  301  and equivalent liquid pistons (with the same swept volume) in the converter unit. That mechanical transmission achieves two things: (a) the conversion of motion to the vertical direction in the converter unit from non-vertical in the displacer unit and (b) the elongation of the cylinder. The gas compression cylinder  20  is necessarily (substantially) vertical in orientation because the process gas must remain at the top of this cylinder and the liquid must remain at the bottom. 
         [0030]    Each gas compression cylinder  20  is elongated relative to the corresponding cylinder  15  of the displacer unit  301  for two main reasons—both connected with the inevitable existence of temperature gradients. The first of these reasons is thermal isolation. The upper parts of the compression cylinder will (at least sometimes) be at temperatures either well above or well below ambient temperature. The lower parts of that cylinder are intended to remain fairly close to ambient temperature. Heat will naturally flow axially along the compression cylinder from the warmer end towards the cooler and this heat flow is undesired if the compression is to be as reversible as possible. A longer gas compression cylinder  20  assists in minimising this flow. The second motivation for a long compression cylinder is to do with thermal stresses and is closely related to the thermal isolation point. Thermal stresses will arise in proportion to the magnitude of a temperature gradient between the upper part of the cylinder and the lower, Given that there may be several hundred degrees of temperature difference between the upper and lower parts, there are lower limits (depending on the material of the compression cylinder) to the length of the cylinder in order that stresses arising from thermal gradients can be maintained tolerably low, 
         [0031]    A cover-block  22  floats above the liquid piston in the gas compression cylinder  20 . The cross-section of this cover-block  22  is very similar to the cross-section of the inside of the gas compression cylinder  20  but there must be a small clearance between the two at all times to prevent the cover-block  22  from sticking within the gas compression cylinder  20 . The volume of this cover-block  22  is deliberately chosen to be greater than the swept-volume of the gas compression cylinder  20 . A typical starting point in the design of the converter unit  401  would be to set the volume of the cover-block  22  equal to twice the swept volume within the corresponding gas-compression cylinder. Note that the swept-volume of the compression cylinder  21  is identical to the volume swept by the piston  15  within the displacer unit  301 . 
         [0032]    The cover-block  22  serves three critically-important functions: (a) it maintains a substantial thermal barrier between the working gas and the surface of the liquid forming the liquid piston (h) it reduces the amount of surface area of liquid exposed to the gas so that direct diffusion of the gas into the liquid when the gas pressure is high (and the converse effusion of gas from the liquid when the pressure reduces again) is minimised and (c) it ensures that the portion of the compression cylinder wall which comes into direct contact with the liquid is far away from the upper part of the compression cylinder  20  where wall temperatures will deviate substantially from the temperature of the liquid. 
         [0033]    The external surface of the cover-block  22  must be impermeable to prevent either the liquid or the gas phases from penetrating into it. The cover-block  22  should exhibit a high volumetric stiffness—in that it should not undergo significant changes in total volume when exposed to the expected fluctuations in temperature. The requirement that the cover-block  22  should float above the liquid couples with the requirement that it should be a poor conductor of heat to indicate that this component should be largely hollow internally. 
         [0034]    At the top end of stroke of the liquid piston within the compression cylinder, the total residence volume for gas remaining within the cylinder should be very small—typically less than 1% of the total swept volume. Each cylinder has an inlet valve port  23  which can connect the internal volume of the gas compression cylinder  20  to a manifold (or in the case of the first stage of the machine, to the primary inlet to the compressor). Similarly each cylinder has an outlet valve port  24  which can connect the internal volume of the gas compression cylinder  20  to a manifold  501  (or in the case of the final stage of the machine, to the primary outlet of the compressor). 
         [0035]      FIG. 4  does not show any details of the ducting which conveys the working gas into or out of the gas compression cylinders and neither does it show any details of the movable components of the valves which close the valve ports. These details are easily completed by a suitably-trained mechanical engineer who will appreciate that the valves must be capable of opening wide to minimise any throttling of gas flow and of opening/closing relatively quickly so that no significant pressure drops are in place across any valve whilst that valve is partially open. Aspects of the valve control are also omitted from this description on the grounds that these will be understood naturally by a suitably trained person who will appreciate that the opening and closing of each valve should be controlled to take place when the pressure difference across that valve is close to zero. In this way, irreversibility arising from throttling the gas is avoided. 
         [0036]      FIG. 5  shows one complete compression stage  6  of the machine in each one of 6 positions throughout its cycle. The wave of motion of the cover-blocks  22  in the displacer unit  401  is clearly visible from this illustration. 
         [0037]    Numerous minor practical issues concerning the implementation of this invention are not spelled out explicitly here but will be obvious to a person skilled in the art. One particular instance of such an issue is that some provision must be put in place to prevent the cam-ring  12  from undergoing any (significant) net rotation which might cause the connecting rods  13  to foul the edges of the cylinders  15 . Simple measures using parallel-bar linkages may be used to achieve this or an alternative is to incorporate some elastic deformation at the joint between conn-rods  13  and pistons  14  such that each such joint is always attempting to maintain alignment between its respective conn-rod  13  and piston  14 . 
         [0038]    Another instance concerns the possible imbalanced leakage of fluid. Any one stage of the compressor comprises a number of discrete volumes containing liquid and comprising the space above one piston  14  of the displacer unit, the space within a single tube of the tube-set  301  with the space beneath one cover-block  22  of the converter unit. In operation, the intention is that this volume remains quite constant. Over a period of time, imbalanced leakage of fluid could occur such that one volume either increased or decreased. It is straightforward to recognise that by measuring how close a target on the cover-block  22  came to the top and bottom of its gas compression cylinder  20  in the converter unit  401 , this volume could be monitored and measures taken to make minor adjustments to the liquid content in each discrete volume. 
         [0039]    A further instance of such an issue concerns the rigidity of the tubes in the tube-set  301 . The design intention of this machine is that there is a stiff one-to-one connection between the volume swept by the pistons  14  of the displacer unit and the corresponding surfaces of the liquid within the gas compression cylinders  20 . If any one of the tubes has excessive ability to distort such that the volume of liquid contained within that tube can vary with the contained pressure, the function of the machine will be compromised. 
         [0040]    The design of sealing around the pistons  14  of the displacer unit is yet another matter where established engineering skill is needed in trading-off leakage of fluid against friction losses. Clearance between piston and cylinder would be minimised as a first measure (possibly requiring some careful temperature control of the entire displacer unit) and then the designer would weigh up the benefits of incorporating circumferential grooves in the piston outer wall (to exploit the “half-rho-v-squared” component of pressure drop) against loss of pressure drop due to pure viscous flow through the narrow gap. The viscosity of the liquid itself is also a matter of some optimisation in this context. 
         [0041]    A final instance of an issue of standard engineering is the possible use of a barrier liquid at the top of the liquid piston to minimise the dispersion of the working gas into the liquid forming the mechanical connection between the displacer unit and the convertor unit. Such barrier fluids have been used in so-called liquid-ring compressors. 
       COMPARISON WITH PRIOR ART AND ADVANTAGES OF THIS INVENTION 
       [0042]    The normal objectives in design of a gas compressor or expander machine differ very strongly from the intention here and this fact already sets the present design apart from most practice in the design of compressors and expanders. The background section outlined the primary differences. 
         [0043]    Most commonly, compressors and expanders are designed to operate at relatively high speeds so that the costs of the machine itself and the associated prime-mover or power-consumer machine are minimised. The present invention is specifically engineered for low-speed applications where high torques (or high linear forces) must be developed. This is the first advantage of this invention: that mechanical power can be absorbed or transmitted at low speeds without the use of intermediate gearing or equivalent means of achieving mechanical speed transformation. 
         [0044]    The present design falls into the category of multi-stage positive displacement machine in that (apart from very small leakage flows) the gas compression/expansion occurs as a result of discrete volumes within the machine increasing and decreasing in size cyclically. In the present invention, these volumes are prismatic in geometry and the movable boundary of each volume comprises a covered liquid piston. Each cover-block is a light but stiff block of material which floats above the liquid piston and serves to provide thermal isolation between the upper part of the cylinder which may be at thermal extremes and the lower part which will tend to remain close to ambient temperature. The cover-blocks are essential in delivering the second advantage of this invention: very high isentropic. efficiency of adiabatic compression. 
         [0045]    The present invention relies on the concept of liquid pistons to achieve zero leakage of the gas being compressed. The concept of compression using liquid pistons is very well established and dates back, at the very least, to U.S. Pat. No. 1,880,241 by DeRemer which described a Liquid-Piston Type Compressor System. The very well known Oscillating Water Column type of wave energy convertor is also a liquid-piston compressor of a type and has been an established concept for some decades. The displacer unit elements of this invention are closely related to the radial-piston engines of early powered aircraft. 
         [0046]    Aspects of this invention appear common to the compressor discussed in [Van der Van JD &amp; Li PY. Liquid Piston Gas Compression, Applied Energy 2009, doi:10.1016/j.apenergy.2008.12.001]. Like the present invention, the system described in that paper does aim for high isentropic efficiency through the use of liquid pistons. However, unlike the present invention, that system aims at isothermal compression and seeks to maximise heat transfer between the working gas and the liquid. The cover blocks of this present invention are present precisely to minimise this heat transfer. The use of multiple stages in the present invention to achieve the overall pressure ratio is motivated by minimising heat transfer so that by contrast. Van der Van and Li utilise a single stage. 
         [0047]    U.S. Pat. No. 7,748,219 discloses the use of cover-blocks floating (described there as floating pistons) above liquid pistons in a heat engine involving the compression and expansion of gas. Their purpose in U.S. Pat. No. 7,748,219 is similar to their purpose in the present invention—minimising heat transfer. U.S. Pat. No. 7,748,219 does not describe a multi-stage (near-) adiabatic compressor or expander suitable for high pressure gas. Such cover-blocks are also disclosed by several publications relating to Stirling engines (referred to as floating pistons) including [West AD, Liquid Piston Stirling Engines, Van Nostrand Reinhold, New York, 1983] and their purpose is again similar in these contexts. Again, these do not describe multi-stage machines specifically for gas compression/expansion. 
       EXTENSIONS AND ALTERNATIVE EMBODIMENTS OF THIS INVENTION 
       [0048]    The description accompanying the Figures has been delivered using the implicit assumption that the machine is acting as an adiabatic compressor. This machine is reversible—it can serve perfectly well as an adiabatic expander. Aside from the arrows indicating the direction of power-flow in  FIG. 1 , nothing about the Figures is specific to compression. It is the action of valves at the top of the gas compression cylinders  20  that distinguishes whether this machine will act to compress or expand gas. A compressor could be implemented using one-way valves that would activate passively. However, this would not be a preferred implementation of the present invention because of the pressure-drops always present in one-way valves. The preferred implementation is that the valves would be controlled actively and in this case, exactly the same hardware could serve as either compressor or expander subject only to a change in the opening and closing sequence of the inlet and outlet valves. 
         [0049]    The number of cylinders in the displacer unit  301  is a matter of choice. All of the figures show a displacer unit with 12 cylinders and this has the attraction that one can maintain symmetry and still achieve part-loading of the device by having 2,3,4 or 6 pistons “active” at one time. One extremely straightforward of making a given piston inactive at a given time is to keep either its inlet port or outlet port open throughout the complete cycle so that there is no net flow of the working gas through any one of the gas compression cylinders. 
         [0050]    In the displacer unit, either the cylinders  15  or the pistons  14  may be caused to move. Moving the cylinders  15  is mechanically more complex but still a relatively obvious extension of the present concept. 
         [0051]    In the displacer unit, the piston-and-cylinder format may be replaced by a format wherein elastically deformable volumes (such as could be formed with axisymmetric bellows) are used to cause liquid to be driven back and forth through the tube-set  301 , 
         [0052]      FIG. 1  showed a single machine comprising 5 discrete compressor stages  601 - 605 . Each one of these compressor stages could have the format of the compressor stage  601  illustrated in  FIG. 2  but one might validly choose to different liquids in each stage. Although the same gas passes through all of the stages, the range of temperature conditions for the gas is differerent for the different stages. 
         [0053]    The inner walls of the gas compressor cylinders  20  might be coated partly or fully with a coating which repelled the liquid of the liquid piston such that the liquid naturally sought to minimise the area of contact. This could help to minimise the extent to which droplets of the liquid might migrate up towards the upper reaches of the cylinder. 
         [0054]    The gas compressor cylinders  20  might not be circular in cross-section. There are clear attractions to a circular cross-section in terms of minimising the surface area of the wall for a given swept volume but mechanical design considerations might behove alternative cross-sections in other cases. 
         [0055]    Although  FIGS. 2 to 5  indicate that each compressor stage has cylinders whose centrelines all lie in a single plane, this is not necessary. There are clear design advantages to grouping the gas compression cylinders  20  into a bundle and the extreme versatility of the liquid connection between the displacer cylinders  15  and the gas compression cylinders  20  makes this entirely feasible. Similarly, although the use of a cam  11  fixed to the shaft  1  which drives a cam-ring  12  makes it possible for all of the displacer cylinders  15  to have their centrelines in one plane, it is quite feasible to use a crank in place of a cam and to stagger the cylinders axially. Doing this is an obvious alternative format—albeit one which is probably inferior in numerous respects.