FIELD OF THE INVENTION 
     The application generally relates to variable capacity screw compressors and systems having variable capacity screw compressors and more specifically to infinitely variable capacity screw compressors. 
     BACKGROUND OF THE INVENTION 
     In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Positive displacement compressors include, for example, reciprocating compressors, rotary compressors, scroll compressors and screw compressors. Screw compressors, also known as helical lobe rotary compressors, are well-known in the air compressor refrigeration, water chiller and natural gas processing industries. 
     Reciprocating compressors utilize a movable piston in a cylinder. The piston is attached to a connecting rod which is attached to a crank. An electric motor drives the crank which causes the piston to reciprocate within the cylinder, increasing and decreasing the volume within the cylinder. Fluid is introduced into the cylinder through a valve when the piston is at the top of its stroke. The fluid is compressed and removed from the cylinder through a valve when the piston is at the bottom of the its stroke. 
     Scroll compressors generate a series of crescent-shaped pockets between two scrolls, the crescent-shaped pockets receiving fluid for compression. Typically, one scroll is fixed and the other orbits around the fixed scroll. As the motion occurs, the pockets between the two forms are slowly pushed to the center of the two scrolls. This reduces the fluid volume. 
     Rotary compressors are of two general types: stationary blade and rotating blade compressors. The blades or vanes on a rotating blade rotary compressor rotate with the shaft within a cylindrical housing. In a stationary blade compressor, the stationary blade has a blade that remains stationary and is part of the housing assembly, while a cylinder rotates within the housing assembly, via a roller on an eccentric shaft within the cylinder. In both types, the blade provides a continuous seal for the fluid. Low pressure fluid from a suction line is drawn into an opening. The fluid fills the space behind the blade as it revolves. The trapped fluid in the vapor space ahead of the blade is compressed until it can be pushed into the compressor exhaust. 
     Screw compressors generally include two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing. 
     Common to each type of compressor is an inlet, an outlet and a working chamber. A compressor inlet is sometimes also referred to as the “suction” or “low pressure side,” while the discharge is referred to as the “outlet” or “high pressure side.” Refrigerant gas, after passing through the inlet, is compressed to a higher pressure in the working chamber. A mechanical means acts on the refrigerant gas to compress it from a first pressure to a second chamber. The mechanical means for compressing the refrigerant gas differs among the various positive displacement compressors. The compressed refrigerant gas then passes from the compressor through an outlet or discharge port to the remainder of the refrigeration system. 
     Screw compressor rotors intermesh with one another and rotate in opposite directions in synchronization within a housing. The rotors operate to sweep a gas through the housing from an intake manifold at one end of the housing to an output manifold at the other end of the housing. Commercially available screw compressors most commonly include threaded shafts or helical rotors having four lobes, however, others have been designed to have five or more lobes; however, it may be possible to use rotors having 2-5 lobes. The rotor shafts are typically supported at the end walls of the casing by lubricated bearings. 
     Capacity control for such compressors can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In a refrigeration system, capacity also can be regulated based upon a temperature set point for the space being cooled. In other systems where the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor. However, all of the currently available methods are expensive and add to the initial cost of investment in the equipment. 
     In chiller applications where economy is desired both in the initial cost of the system and in operation of the system, a variable volume ratio application is desired. In a screw compressor, the volume, or compression ratio Vi, is the ratio of the volume of a groove at the start of compression to the volume of the same groove when the discharge port begins to open. Hence, the volume ratio in a screw compressor is determined by the size and shape of the discharge port. 
     For maximum efficiency, the pressure generated within the grooves during compression should exactly equal the pressure in the discharge line when the volume begins to open to it. If this is not the case, either over-compression or under-compression occurs, both resulting in internal losses in efficiency. Such losses in efficiency increase power consumption and/or noise, while reducing efficiency. 
     If the operating conditions of the system seldom change, it is possible to specify a fixed-volume ratio compressor that will provide good efficiency. But since over-compression can cause damage to a compressor, compressors are designed to limit over-compression, so they do not frequently operate in an over-compression mode. Compressors designed to limit over-compression are often designed to run at a maximum or substantially maximum compression under the most severe operating conditions. When not under the most severe operating conditions, the fixed-volume ratio compressor designed to limit over-compression will run in under-compression mode, which results in at least reduced efficiency. 
     What is needed is a system that permits adjustments to the volume ratio depending on the conditions that the compressor experiences. This will allow the compressor discharge volume to be adjusted to change the discharge volume, and hence the volume ratio, as operating conditions change resulting in a change in refrigeration demand, allowing the compressor to operate at increased an improved efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a positive displacement, variable efficiency compressor in which the volume of the discharge port includes means for adjusting the discharge port volume in response to a change in demand so that the compressor can operate at or near maximum efficiency in response to demand. 
     In an exemplary embodiment, a variable-efficiency screw compressor includes an inlet port to draw refrigerant into the variable-efficiency screw compressor, at least one rotating screw, in fluid communication with the inlet port to compress the refrigerant, forming a compressed refrigerant gas, a discharge port having a volume in fluid communication with the rotating screws to receive the compressed refrigerant gas and discharge the compressed refrigerant gas, wherein the discharge port includes an adjustable piston movable within the discharge port from a first position in which volume is higher to a second position in which volume is reduced or lowered, the adjustable piston arranged and disposed to adjust volume of the discharge port in response to a change in demand. Screw compressors may include a plurality of rotating screws synchronized to rotate together. 
     In another exemplary embodiment, a variable-efficiency refrigeration system includes a compressor that compresses a refrigerant gas, to produce a compressed refrigerant gas, a power source powering the compressor, a control panel modulating the power source, a condenser in fluid communication with the compressor that condenses the compressed refrigerant gas to a high pressure compressed liquid, an evaporator in fluid communication with the condenser and with the compressor, an expansion valve positioned between the condenser and the evaporator, wherein the expansion valve receives condensed, high pressure refrigerant liquid and expands the condensed refrigerant, reducing the pressure, to form a mist of gas and liquid for the evaporator, and wherein the compressor is a variable-efficiency screw compressor. The variable-efficiency screw compressor further includes an inlet port to draw refrigerant gas into the variable-efficiency screw compressor, one or more rotating screws in fluid communication with the inlet port to compress the refrigerant, forming a compressed refrigerant, a discharge port in fluid communication with the rotating screws to receive the compressed refrigerant gas and discharge the compressed refrigerant gas, wherein the discharge port includes an adjustable piston movable within the discharge port from a first position in which volume is higher, to a second position in which volume is lower, the and to any intermediate position between the first position and the second position, the adjustable piston arranged and disposed to adjust volume of the discharge port in response to a change in demand. 
     In another exemplary embodiment, a variable-efficiency screw compressor system includes an inlet port to draw refrigerant into the variable-efficiency screw compressor, one or more rotating screws in fluid communication with the inlet port to compress the refrigerant, compressing the refrigerant gas, a discharge port in fluid communication with the rotating screws to receive the compressed refrigerant gas and discharge the refrigerant, wherein the discharge port includes an adjustable piston movable from a first position that provides the discharge port with a maximum volume and a second position providing the discharge port with a minimum volume, and to any intermediate position between the first position and the second position, the intermediate position providing an intermediate volume in response to a change in demand. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a closed loop system for refrigeration. 
         FIG. 2  schematically illustrates a screw compressor configured for use as the closed loop system of  FIG. 1 . 
         FIG. 3  depicts a section view of the screw compressor of  FIG. 2  showing the interior components of the screw compressor through the housing, the view further showing the discharge port with a piston in the discharge port. 
         FIG. 4  depicts a cross-section view of a piston in the screw compressor of  FIG. 2  and  FIG. 3  retracted in a discharge port. 
         FIG. 5  depicts a cross-section view of a piston in the screw compressor of  FIG. 2  and  FIG. 3  extended in a discharge port. 
         FIG. 6  schematically depicts a piston variation control process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  and  FIG. 2 , a schematic for a closed loop refrigeration system (refrigeration system)  21  is shown. Refrigeration system  21  includes refrigerant  28  as the working fluid. Refrigerant  28  is compressed by a compressor  23 , such as a screw compressor  38 , forming a compressed refrigerant gas  24 . Compressor  23  is powered by a power source  10 , and power source  10  is modulated by a control panel  22 . Compressed refrigerant gas  24  from compressor  23  is discharged through a discharge port  48  ( FIG. 3 ) which is in fluid communication with a condenser  25 . Condenser  25  condenses compressed refrigerant gas  24  into a liquid refrigerant  26 . Condenser  25  is a heat exchanger that provides heat exchange communication of the refrigerant with a heat transfer medium that removes heat of condensation resulting from compressed refrigerant gas  24  undergoing a change of state as it is condensed into liquid refrigerant  26 . This heat transfer medium includes, but is not limited to, atmospheric air (air or forced air), a liquid (preferably water), or a combination thereof. Liquid refrigerant  26  is in fluid communication with an expansion valve  31  that expands at least a portion of liquid refrigerant  26  into refrigerant  28  as it flows to an evaporator  27 . The refrigeration system  21  from discharge port  48  of compressor  23  to expansion valve  31  is termed the high-pressure side of refrigeration system  21 . 
     Expansion valve  31  decreases the pressure of liquid refrigerant  26  having a higher pressure, converting it into a mist of gas and liquid droplets having a lower pressure as the gas traverses it, while evaporator  27  receives the mist from expansion valve  31 . Evaporator  27  is in heat exchange communication with a heat transfer medium. Heat is absorbed from the heat transfer medium as refrigerant mist changes state to refrigerant gas in evaporator  27 , cooling the heat transfer medium. The cooled heat transfer medium may be used directly to cool or refrigerate an area, for example, when the heat transfer medium is air circulating from the area to be cooled passing over the evaporator. Alternatively, the heat transfer medium may be liquid, such as water in heat exchange relationship with the evaporator that is sent to a chiller. Refrigerant  28  from the evaporator, now a low pressure gas, is then returned to an inlet port  44  on a suction side of compressor  23  to complete the closed loop of refrigeration system  21 . The refrigeration system  21  immediately after expansion valve  31  to the suction side of compressor  23  is termed the low-pressure side of refrigeration system  21 . 
     Referring to  FIG. 2  and  FIG. 3 , in one embodiment, positive displacement compressor  23  in refrigeration system  21  of  FIG. 1  may be a screw compressor  38 .  FIG. 3  depicts, in cross-section through a compressor housing, some of the interior components of screw compressor  38 . The compressor housing encloses one or more rotating screws  52  of screw compressor working within an operating chamber. Operating chamber varies in length based on a position of rotating screws  52 . Operating chamber has an increased length when rotating screws  52  are not aligned with one another. Operating chamber has a decreased length when the rotating screws  52  are in meshing alignment with one another. Screw compressor  38  includes control panel  22  connected to power source  10 , which powers a motor  43  that drives one or more rotating screws  52 . Rotating screws  52  include helical-grooves, each groove decreasing in volume between inlet port  44  and discharge port  48 . The decreasing volume of the helical-grooves across the compressor compresses refrigerant gas  28  entering screw compressor  38  through inlet port  44 , providing high pressure compressed refrigerant gas  24  at discharge port  48 . 
     In one embodiment, screw compressor  38  includes a lubrication system as is known in the art. Lubrication systems include lubricating oil  32  (usually specially formulated mineral oils which are completely dehydrated, wax-free and non-foaming), an oil pump to deliver oil under pressure to all bearing surfaces, and an oil separator  29 , which is an optional component in  FIG. 1 , being present when compressor  23  is a screw compressor  38 . Lubricating oil  32  is separated from compressed refrigerant gas  24  exiting screw compressor  38 . Lubricating oil  32  is then returned to the low pressure side of screw compressor  38  to seal a clearance between rotating screws  52 , and between rotating screws  52  and a cylinder. 
     Screw compressor  38  is in fluid communication with oil separator  29 . Low pressure refrigerant  28  from evaporator  27  and lubricating oil  32  are introduced into the suction side of screw compressor  38  at inlet port  44  to lubricate rotating screws  52  of screw compressor  38 . Once compressed within screw compressor  38 , the mixture of compressed refrigerant gas  24  and lubricating oil  32  is discharged from discharge port  48  of screw compressor into oil separator  29  where a mist of lubricating oil  32  in the form of finely divided particles entrained in compressed refrigerant gas  24  is separated from compressed refrigerant gas  24 . Oil separator is maintained at or near the gas pressure of the compressor discharge. After separation, compressed refrigerant gas  24  exits oil separator  29  and is provided to condenser  25  in refrigeration system  21 . The exit of oil separator  29  may also be termed the oil separator discharge port. For simplicity, it shall be referred to herein as the exit of oil separator  29  or oil separator exit. 
     Referring to  FIG. 3 , in one embodiment, the internal mechanisms of screw compressor  38  can be seen. A shaft  50  extending from motor  43  is connected to at least one rotating screw  52  of a pair of screws  52 . One rotating screw  52  of the pair of screws  52  may be stationary, or both screws  52  of the pair may rotate, driven by the use of rotor-synchronized timing gears that synchronize rotating screw  52  rotation. Refrigerant  28  enters screw compressor  38  through inlet port  44  and is compressed within the helical-grooves of screws  52 . Compressed refrigerant gas  24  is discharged into discharge port  48 , which is in fluid communication with downstream condenser  25  and optional oil separator  29  in refrigeration system  21 . As seen in  FIG. 3 , a piston  54  is positioned within discharge port  48 . Piston  54  is urged to move by pressure fed through a proportional valve  56  coupled to discharge port  48 . The pressure through proportional valve is balanced by a biasing means. The position of piston  54  within discharge port  48  covers or uncovers by-pass holes  58  between a rotor bore  60  and discharge port  48 . 
       FIG. 4  provides another view of piston  54  positioned within discharge port  48 . Referring to  FIG. 4 , in one embodiment, a partial horizontal cross-section view of screw compressor  38  through its center viewed from above is shown, providing a detailed view of discharge port  48 . In  FIG. 4 , rotating screws  52  are not visible, as the view is taken below rotating screws  52 . However, this view shows the path taken by compressed refrigerant gas  24  into discharge port  48 . Piston  54  is secured within discharge port  48  using a spring  46  to bias piston  54  within discharge port  48 , although any other deformable securing device or biasing means which selectively urges piston  54  to a position within discharge port  48  may be used. Piston  54  further includes at least one o-ring groove  62  for insertion of an o-ring. O-rings may be made of materials including, but not limited to, neoprene, chloroprene, other refrigerant fluid-resistant elastomeric compounds, or a combination thereof. Positioning an o-ring in o-ring groove  62  of piston  54  eliminates leakage of compressed refrigerant gas  24  around piston  54  within discharge port  48 . Additionally, seals preventing leakage of compressed refrigerant gas  24  for use in combination with piston  54  include compression seals, mechanical seals, and the like. In  FIG. 4 , piston  54  is shown in a first position, fully retracted into discharge port  48 . The first position of piston  54  provides discharge port  48  with a higher volume as compared to a second position of piston  54 , shown in  FIG. 5 . This means that the pressure of fluid on the side of piston  54  opposite discharge port  48 , assisted by fluid flow through proportional valve  56 , is less than the force provided by biasing means, here spring  46 , causing biasing means to move to a relaxed position while pulling piston to a position that provides a maximum volume to discharge port  48 . 
       FIG. 5  is a partial horizontal cross-section view of screw compressor  38  through its center viewed from above, providing a detailed view of discharge port  48 .  FIG. 5  is identical to  FIG. 4 , except that piston  54  is fully extended within discharge port  48  to a second position that minimizes the volume of the discharge port. In the second position, piston  54  covers apertures or by-pass holes  58 , and discharge port  48  has a lower volume as compared to piston  54  in its first position, as shown in  FIG. 4 . Spring  46  is elongated as pressure behind piston  54  from proportional valve  56  increases, overcoming the force from biasing means, here spring  46  urging piston  54  to move within discharge port  48 , downward in  FIG. 5 . Piston  54  is retracted by spring  46  upon decreasing and/or removing pressure from proportional valve  56 , the spring urging the piston to return to the position depicted in  FIG. 4  or to an intermediate position. The spring  46  or biasing means will provide a force that balances the pressure applied by fluid from proportional valve  56 , so that any intermediate position of piston  54  within discharge port  48  can be achieved between the maximum volume position shown in  FIG. 4  and the minimum volume position shown in  FIG. 5  by controlling the fluid pressure applied to the piston through proportional valve  56 . 
       FIG. 4  and  FIG. 5  depict piston  54  in two extreme positions within discharge port  48 , a first position in which the discharge port  48  has a higher volume ( FIG. 4 —maximum volume) and a second position in which the discharge port  48  has a lower volume ( FIG. 5 —minimum volume), respectively. It will be understood by those skilled in the art that piston  54  may be positioned within discharge port  48  at any position between the first position (maximum volume) depicted in  FIG. 4  and the second position (minimum volume) depicted in  FIG. 5  to provide a discharge port volume dependent on the location of piston  54  in port  48 , the discharge port volume being variable with the position of the piston in the discharge port. Piston  54  generally may be fabricated of any suitable material for sealing by-pass holes  58 , while also slidable within discharge port  48 . The pressure fed from proportional valve  56  maintains piston  54  in a predetermined position, the predetermined position established by monitored parameters discussed below. 
       FIG. 6  depicts an exemplary control process  61 . In control process  61 , a value of a voltage signal  64  is adjusted based upon reference pressure  65  and oil pressure  67  monitored by a controller, such as may be located in control panel  22 . Proportional valve  56  receives voltage signal  64  from control panel and adjusts pressure provided to discharge port  48  in response to the value of voltage signal  64 . The pressure from proportional valve  56  in turn controls the position of piston  54  within discharge port  48  as discussed above. 
     Volume ratio V i  is the ratio of a suction volume to a discharge volume and represents a measure of the efficiency of operation of screw compressor  38 . The volume ratio is determined by a size and shape of discharge port  48 . The volume associated with discharge port  48  is referred to as a discharge port volume. The suction volume is a volume within the helical-grooves of rotating screws  52  before compression. In one embodiment, the pair of rotating screws  52  has male helical-grooves and female helical-grooves. The male helical-grooves mesh with the female helical-grooves to compress refrigerant  28 . The discharge volume is a volume of rotating screws  52  meshing just prior to an opening to discharge port  48 . More specifically, the volume ratio is provided as: V i =             1/κ , where V i  is the volume ratio,           is compression ratio, and κ is a refrigerant constant. For refrigerant  134 A, κ is 1.18.
     Referring to  FIG. 4 , in one embodiment, piston  54  in the first position provides discharge port  48  with the higher volume as compared to piston  54  in the second position. Compressed refrigerant gas  24  from the screws is discharged into discharge port  48  with the higher volume achieves a decreased volume ratio, since for a fixed suction volume, an increase in discharge volume results in a smaller volume ratio. The decreased volume ratio increases efficiency of screw compressor  38  and refrigeration system  21  during periods of decreased demand, such as when ambient temperature is low, such as during winter months and/or during maintenance periods. As used herein, ambient temperature refers to an environmental temperature at a time of measurement. Thus, screw compressor efficiency is improved during periods of decreased demand be increasing the volume of the discharge port, which decreases the volume ratio. 
     Referring to  FIG. 5 , in one embodiment, piston  54  in the second position provides discharge port  48  with the lower volume as compared to piston  54  in the first position of  FIG. 4 . Compressed refrigerant gas  24  discharged into discharge port  48  with the lower volume provides an increased or higher volume ratio. The increased volume ratio increases efficiency of screw compressor  38  and refrigeration system  21  during periods of increased demand, such as during a start-up and/or when the ambient temperature is high, such as during summer months. Thus, screw compressor efficiency is improved during periods of increased demand be decreasing the volume of the discharge port, which increases the volume ratio. 
     It will also be recognized by those skilled in the art that the volume ratio V i  may be adjusted, if desired, to intermediate positions between the extremes shown in  FIGS. 4 and 5  during periods of intermediate demand. An intermediate adjustment is desirable for conditions between higher demand and lower demand, such as during conditions that may occur during spring and autumn. To increase efficiency of refrigeration system  21  throughout various operating conditions, a continuously variable volume ratio V i  is desirable. 
     In one embodiment, a higher volume ratio V i  is desired for higher ambient temperatures. The ambient temperature is the current or present environmental temperature of a geographical region during a season. Higher operating pressures are desirable under higher ambient temperatures, such as may occur during summer months as well as late spring or early autumn, and the lower volume of discharge port  48 , produced by piston  54  biased toward the second position ( FIG. 5 ), provides such higher pressures. A higher pressure of compressed refrigerant gas  24  at discharge port  48  increases a downstream pressure at evaporator  27 , which in turn increases cooling capacity of the system. An increase in compressed refrigerant gas pressure represents an increase in work performed by screw compressor  38 . The increase in work represents an increase in energy usage by screw compressor  38 , but screw compressor  38  is operated in a more efficient manner at higher pressure when demand is high. 
     In one embodiment, a lower volume ratio V i  is desired for lower ambient temperatures such as may occur during the winter season or during early spring and late fall. Lower ambient temperatures permit lower operating pressures, and the larger volume of discharge port  48 , produced by piston  54  biased toward the first position ( FIG. 4 ), provides lower pressures. A lower pressure of compressed refrigerant gas  24  at discharge port  48  decreases the downstream pressure at evaporator  27 , which in turn decreases the cooling capacity of the system, desirable when the ambient is cooler. In one embodiment, the reduction in pressure represents a decrease in work performed by screw compressor  38 , which results in improved screw compressor efficiency at lower ambient temperature conditions. In one embodiment, by matching the volume ratio V i  to the current demand on refrigeration system  21 , screw compressor  38  is operated more efficiently, and noise from screw compressor  38  operations is also reduced. 
     In one embodiment, voltage signal  64  is varied in value based upon reference pressure  65  and/or oil pressure  67 . Reference pressure  65  includes, but is not limited to, head pressure, condenser pressure, volume ratio, or a combination thereof. Changes in oil pressure  67  follow changes in discharge pressure. As reference pressure  65  and/or oil pressure  67  increase or decrease, the value of voltage signal  64  is adjusted accordingly. In response to adjustments in the value of voltage signal  64 , proportional valve  56  increases or decreases pressure to discharge port  48 . As demand changes, the adjustments in pressure from proportional valve  56  to discharge port  48  move piston  54 , which adjusts the discharge port volume to increase efficiency. The position of piston  54  within discharge port  48  is determined by any convenient method. 
     The proportional valve  56  may be in communication with a controller located at or in communication with control panel  22 , which also monitors a reference pressure such as oil pressure, head pressure, condenser pressure or a combination thereof. Controller may also monitor ambient temperature, temperature of the space being cooled or other relevant measurable parameter of the refrigeration or cooling system, as are well known to those skilled in the art. The controller may then generate a voltage signal based on one or more of the values monitored, which signal is provided to proportional valve  56  to vary the position of piston  54  within discharge port  48 . The controller may generate the voltage based on an algorithm that includes one or more of these monitored values or it may generate the voltage based on a predetermined table, the controller using the table to determine the desired voltage value based on the values of the monitored conditions, and providing the voltage to proportional valve  56  move piston  54  in response to the monitored conditions. 
     By using discharge port  48  with piston  54  to provide a variable discharge volume, screw compressor  38  may be fabricated for uninterrupted use and increased efficiency in any climate. The volume ratio V i  of screw compressor  38  can be adjusted by continually monitoring operational or environmental conditions, or both, without stopping or disassembling screw compressor  38 , thereby providing increased efficiency of refrigeration system  21 . Additionally, screw compressor  38  having a continuously variable volume ratio V i  can be continuously adjusted during operation to match demand on refrigeration system  21 , providing increased efficiency. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
A variable-efficiency screw compressor for use in a closed-loop system configured to perform refrigeration is provided. The variable-efficiency screw compressor includes an inlet port to draw refrigerant into the variable-efficiency screw compressor, one or more rotating screws in fluid communication with the inlet port to compress the refrigerant, forming a compressed refrigerant, a discharge port in fluid communication with the rotating screws to receive the compressed refrigerant and discharge the refrigerant, wherein the discharge port includes an adjustable piston movable within the discharge port from a first position in which volume is higher to a second position in which volume is lower, the adjustable piston arranged and disposed to adjust volume of the discharge port in response to a change in demand.