Patent Publication Number: US-2015078926-A1

Title: Regenerative hydraulic lift system

Description:
This application claims priority to and through and is a Divisional application of U.S. patent application Ser. No. 13/317,139 filed on Oct. 11, 2011, which application claims priority to and is a Divisional application of U.S. patent application Ser. No. 11/548,256 filed on Oct. 10, 2006, which was a Continuation In Part (CIP) of U.S. patent application Ser. No. 11/001,679 filed on Nov. 30, 2004 which claims priority to Provisional Application 60/526,350 filed on Dec. 1, 2003. The disclosures of the Ser. No. 13/317,139, 11/548,256, 11/001,679 and 60/526,350 applications are herein incorporated by reference. Claims 8, 10, 11 and 12 of application Ser. No. 11/548,256, which claims were elected pursuant to a restriction requirement, have been allowed and are the subject of U.S. Pat. No. 8,083,499 B1. Claims 13-16 and 18-20 of application Ser. No. 13/317,139, which claims were elected pursuant to a restriction requirement, have also now been allowed. The claims presented for this divisional application, namely claims 1-7, were non-elected without traverse in application Ser. No. 13/317,139, and are original claims of application Ser. No. 11/548,256. No amendments have been made to any of the claims 1-7 as originally presented. The specification presented with this divisional application is identical to the specification of application Ser. No. 11/548,256, except that claims 8, 10-12, 13-16, and 18-20 are presented in the form allowed and claims 9 and 17 were canceled during the prosecution of application Ser. Nos. 11/548,256 and 13/317,139 respectively. The drawings are identical to the drawings originally presented for application Ser. No. 11/548,256, except for FIG. 5, which was corrected during the prosecution of application Ser. No. 11/548,256. No new matter has been added. 
    
    
     BACKGROUND OF THE INVENTION 
     Disclosed herein are a system, apparatus and method for recapturing energy in lift systems. 
     Many lift systems produce a substantial amount of non-useful energy. These lift systems can be of various configurations such as of a reciprocating type. More particularly, in the case of certain reciprocating lift systems, these reciprocating loads/actions are performed by reciprocating rod-type lift systems. When these lift system produce a substantial amount of non-useful energy it can be dissipated, for example, in the form of heat due to a great extent to the pressure differential of certain fluid regulating devices. This lifting equipment typically has, for instance, elements that move up and/or move down, or which speed up and/or slowdown. 
     For example, a reciprocating rod lift system can be provided for artificially lifting of down well fluid production systems from a subterranean reservoir or stratus layer(s) for purposes of raising or lowering same to desired positions, and for speeding up or slowing down same. In these systems, much of the total energy used to lift fluid and gas from the well is directed toward operating a sucker rod string and down hole pump. 
     There is some useful, non-recoverable energy expended in the pumping process, consisting of friction from pivot bearings, mechanical non-continuously lubricated bearings, cables/sheaves, gear box friction, and gear contact friction. In some conventional systems, high pressure nitrogen gas leakage along with heat of compression of said gas results in loss of non-recoverable energy required to counterbalance the weight of the down hole component while lowering the sucker rod string into the well. Still other energy loss occurs for certain non-recoverable inefficiencies such as friction or windage. 
     Some conventional lift systems provide for a means of recapturing energy by means of storing energy in a physical counterweight or flywheel during a downward stroke of the down hole component. A large mechanical crank mounted counterbalance is used to counter the effect of the down hole component weight and provide resistance to movement as the down hole component is lowered into the well. 
     Other systems store energy by compressing a gas, such as nitrogen, during the downward stroke. These systems similarly oppose movement of the down hole component and store the energy while lowering the load. A minimum and maximum pressure level is fluctuated based upon an initial precharge ambient temperature and a rate of pressure change. 
     In yet other conventional lift systems, the fluid flow is restricted over a metering or throttling valve, thereby wasting all the energy contained in the elevation by merely heating the hydraulic fluid. Heat from these throttling devices must then be dispelled employing coolers that use even more energy. 
     The inherent inefficiencies of these and other conventional systems, in addition to the other non-recoverable energy expended during operation of down well fluid production systems, increase the cost of materials extraction. 
     The present invention addresses these and other problems associated with the prior art. 
     SUMMARY OF THE INVENTION 
     A method is herein disclosed for pumping a subterranean fluid to the surface of the earth. The method includes increasing a hydraulic pressure at a first control rate during a pumping operation and decreasing the hydraulic pressure at a second control rate during a lowering operation. The method further includes controlling an amount of down hole fluid being pumped during the pumping operation by metering the first control rate and controlling a lowering speed of a down hole pump by metering the second control rate. The first and second control rates may be metered according to a hydraulic pressure being provided by a pump, wherein electricity is generated during the lowering operation. 
     A system for pumping the fluid may include a hydraulic pump, a down hole pump, and a rod and cylinder assembly. The rod is configured to reciprocate up and down with respect to the cylinder according to a hydraulic pressure supplied by the pump to control an operation of the down hole pump. 
     A hydraulic cylinder assembly for a fluid pump may include a cylinder, a bearing attached to an approximate first end of the cylinder, a rod slideably mounted within the bearing, and a piston located about an end of the rod in the cylinder opposite the bearing. A central axis of the rod is offset from, and parallel to, a centerline of the cylinder to impede a rotation of the piston about the rod. 
     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example hydraulic lift system including a linear actuator. 
         FIG. 2  illustrates the hydraulic lift system of  FIG. 1  with the linear actuator in an extended position. 
         FIG. 3  illustrates a cross sectional view of an example linear actuator. 
         FIG. 4  illustrates a top view of the linear actuator illustrated in  FIG. 1 . 
         FIG. 5  illustrates an example hydraulic system schematic of a lift system. 
         FIG. 6  illustrates an example energy grid connected to a lift system. 
         FIG. 7  is a flow chart illustrating an example method of recapturing energy in a lift system. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     A lift system may be used for pumping down hole fluids to the surface to obtain natural gas or petroleum that is contained therein. Similarly a lift system may be used to raise other fluid from a down hole well to above ground. A reciprocating rod lift system is one such system. 
     In one application, a lift system is used to dewater coal bed methane gas wells. The methane gas found in coal beds tends to adhere to a local surface while under pressure. When the coal beds are submerged in water, the hydraulic pressure causes the methane gas to adhere to the coal itself according to the principle of adsorption. When the lift system removes and raises the water, the hydraulic pressure acting on the methane gas is temporarily decreased, which allows the methane gas to desorb off the coal and flow through coal seams to the surface. The methane gas is then removed from the raised water by conventional means. 
     As the water is removed from the coal bed, the existing ground water will tend to refill the coal bed back to at or near its previous level over time. When the water reaches its equilibrium level due to the inflow of the ground water, the hydraulic pressure tends to retain the existing methane gas as described above. However, if the lift system continues to remove the water at a rate that exceeds the ability of the ground water to refill the coal bed, then the hydraulic pressure will continue to decrease, causing more of the methane gas to desorb and flow to the surface. 
     If the lift system continues to remove water, at some point the coal bed may be effectively pumped dry, if at least temporarily. Operation of the lift system without sufficient amounts of down hole water may cause serious damage to the lift system and its components. In some conventional systems, the lift system operates for some set period of time, and then rests idle while the coal bed refills with water. The system will therefore cycle on and off to remove the water and then allow the water level to refill. 
     In one embodiment, the lift system monitors a hydraulic pressure associated with the removal of the down hole water so that it can control the rate at which the water is removed. By controlling the rate of water removal to avoid the down hole well from being pumped dry, the lift system can continuously operate without having to cycle between the on and off operating modes. As such, a smaller lift system may be used as compared to conventional pumps when removing an equivalent amount of methane gas over time. Smaller lift systems use less electricity to operate and have lower operating and up front purchasing costs. 
       FIG. 1  illustrates an example hydraulic lift system including a linear actuator  15  that may be used to pump fluid from a down hole well. The lift system includes a hydraulic pump  40  and motor  42 , a fluid pressure transducer  44 , a conventional down hole pump  55  and the linear actuator  15 . The linear actuator  15  includes a rod  20  and cylinder  10 , and is shown mounted to a base unit  66  which is placed on the ground  100 . The pump  40 , motor  42 , and transducer  44 , represented as simple operational blocks, may be contained within the base unit  66 . 
     A sheave  58 , or wheel, is rotatably mounted about a pinion  16  connected to the rod  20  near a first end  12  of the cylinder  10 . The sheave  58  may rotate in either a clockwise or counterclockwise direction of rotation about the pinion  16 . In one embodiment, two or more sheaves, similar to sheave  58 , may be rotatably mounted about the pinion  16  to provide for additional mechanical advantage, as is known in conventional pulley systems. A cable  60  is connected at one end to an equalizer sheave or idler pulley  62  which may be mounted to the base unit  66 . The cable  60  engages an upper radial section of the sheave  58 . A second end of the cable  60  is shown connected to a carrier bar  56 , hanging suspended from the sheave  58 . A sucker rod string or sucker rod  50  is connected to the carrier bar  56  and inserted into a well head  54 . The well head  54  directs the sucker rod  50  down beneath the ground  100  into the down hole well, where the sucker rod  50  is further connected to the down hole pump  55 . 
     The rod  20  is slideably mounted to the cylinder  10  in a radially offset position from a centerline of the cylinder  10 , and configured to reciprocate up and down according to a hydraulic pressure supplied by the pump  40  to control an operation of the down hole pump  55 . A sensor  30  is mounted within the cylinder  10  and spaced apart from the rod  20 . An exposed portion of the sensor  30  is visible from the first end  12  of the cylinder, and includes electronics that are accessible for maintenance. The sensor  30  is configured to measure a rod position within the cylinder  10  which is transmitted as a sensor input. The pump  40  controls the hydraulic pressure within the cylinder  10  during both up and down reciprocating motions of the rod  20  to control a pumping rate of the down hole pump  55 . 
       FIG. 2  illustrates the hydraulic lift system of  FIG. 1  in an extended position. As the pump  40  increases the hydraulic pressure within the cylinder  10 , a hydraulic force exerted on the rod  20  causes the rod  20  to raise to the extended position. Because one end of the cable  60  is connected to the idler pulley  62 , as the rod  20  is raised, the sheave  58  rotates in a clockwise rotational direction due to a friction force with the cable  60 . As the sheave  58  raises and rotates, it lifts the sucker rod  50  and the down hole pump  55  located beneath the well head  54  ( FIG. 1 ). At the end of the upstroke of the rod  20 , the pump  40  decreases the hydraulic pressure in the cylinder  10 , allowing the rod  20 , and down hole pump  55 , to lower. As the down hole pump  55  is raised and lowered successively, water, or other fluid, located in the down hole well is pumped and raised to the surface. 
       FIG. 3  illustrates a cross sectional view of an example linear actuator  15  shown with reference to the hydraulic lift system of  FIG. 1  and identified as reference number  3 - 3 . The rad  20  and cylinder  10  are shown in partial view, where the middle section of the assembly has been removed for convenience. A bearing  22  is shown attached to an approximate first end  12  of the cylinder  10 , wherein the rod  20  is slideably mounted within the bearing  22 . The bearing  22  may include a rod seal  27 . The rod seal  27  may include one or more seals as well as a wiper mechanism to keep the rod seal  27  and hydraulic fluid clean. A piston  24  is located in the cylinder  10  about an end of the rod  20  opposite the bearing  22 . The piston  24  extends through an inner diameter of the cylinder  10 . The piston  24  includes a channel  35  which allows hydraulic fluid in cavity  36  to be released through the piston  24  in either an upwards or downwards direction as the rod  20  reciprocates within the cylinder  10 . In one embodiment, channel  35  includes two through holes. 
     The lower end  29  of the rod  20  is shown supported within a stop tube  13 , which may be mounted to the piston  24 . The stop tube  13  provides additional support for the rod  20  particularly when the rod  20  is in the extended position, shown in  FIG. 2 . A length of the stop tube  13  may be approximately one half the length of the rod  20 , such that the distance of the upstroke of the rod  20  would be nearly equal to the length of the stop tube  13 . 
     The sensor  30  includes a sensor probe  32  attached to the first end  12  of the cylinder  10  and extending through the piston  24  towards a second end of the cylinder  14 . Sensor probe  32  may include a magnetostrictive position monitoring transducer having a pressure tube assembly with a magnetostrictive strip, for example. The first end  12  of the cylinder  10  may be referred to as a rod end cap. The second end  14  of the cylinder  10  may be referred to as a mounting base or cap end. A proximity device  34  is attached to the piston  24 , the sensor probe  32  also extending through the proximity device  34 . The proximity device  34  may be a magnet or magnetic device that provides a relative position of the piston  24  with respect to the sensor probe  32 . For example, the sensor  30  and sensor probe  32  may include a feedback transducer that measures a relative position of the piston  24  within the cylinder  10 . 
     The hydraulic pump  40  is fluidly connected to the cylinder  10  by a hydraulic port  37 . The pump  40  is configured to provide a hydraulic pressure to cavity  36  in the second end  14  of the cylinder  10 . Hydraulic fluid in cavity  36  flows down through the channel  35  as the rod  20  is raised, and hydraulic fluid flows up through the channel  35  as the rod  20  is lowered. Because the hydraulic pressure in cavity  36  is approximately equalized on either side of the piston  24 , the hydraulic force does not act directly against the piston  24 . The hydraulic pressure in cavity  36  acts against the lower end  29  of rod  20 , causing the rod  20  to raise or lower within the cylinder  10  as the pressure is modulated by the pump  40 . The bearing  22  and sensor probe  32  do not move vertically up and down while the piston  24  and rod  20  reciprocate. By determining a position of the piston  24 , the sensor  30  is also able to determine a position of the rod  20  within the cylinder  10 . 
     The position of the bearing  22  is fixed with respect to the first end  12  or rod end cap of the cylinder  10 , whereas the piston  24  is constrained and guided by the inner diameter of the cylinder  10  as the rod  20  and piston  24  reciprocate up and down. As the rod  20  is raised and lowered within the cylinder  10 , its lateral or rotational movement is therefore constrained by the bearing  22  and the piston  24 . 
     The linear actuator  15  of  FIG. 3  may be incorporated into the hydraulic lift system of  FIG. 1 . The pump  40  and motor  42  of  FIG. 1  may therefore be configured to pump a down hole fluid during an up-stroke of the piston  24  and rod  20 . The pump  40  and motor  42  may be further configured to generate electricity on the down-stroke of the piston  24  and rod  20 . 
       FIG. 4  illustrates a top view of the linear actuator  15  illustrated in  FIG. 3 , showing the first end  12  of the cylinder  10 . The rod  20  includes a central axis  21  that is offset from, and parallel to, a centerline  11  of the cylinder  10 . A central axis  31  of the sensor probe  30  is shown offset from the central axis  21  of the rod  20 . During the reciprocating motion of the rod  20  within the cylinder  10 , the hydraulic force of the pressurized fluid in cavity  36  tends to impart a rotational force to the piston  24  about the rod  20 . 
     By offsetting the rod  20  from the centerline  11  of the cylinder  10 , and furthermore slideably mounting the rod  20  through the bearing  22 , the rotational force acting on the piston  24  about the rod  20  is impeded. The bearing  22  maintains the rod  20  in a substantially fixed vertical orientation within the cylinder  10 , and acts through the rod  20  to maintain a similar orientation of the piston  24 . By impeding this rotation of the piston  24 , the sensor  30  and sensor probe  32  are protected from damage that might otherwise occur due to the rotational force acting on the piston  24 . 
       FIG. 5  illustrates an example hydraulic schematic of a regenerative hydraulic lift system. The hydraulic schematic in  FIG. 5  includes an electronic closed loop control system. A closed loop controller  514  is included in a hydraulic transformer shown as functional block  550 . The hydraulic transformer  550  may include the controller  514 , the motor  42  and the pump  40 , as well as other components shown in  FIG. 5 . 
     A control valve  507  may be remotely controlled by the controller  514  to increase pressure in the system according to a predetermined rate of change and the maximum amplitude in a closed loop (PID) control algorithm. Controller  514  is able to provide a command signal to control valve  507  to increase a hydraulic pressure at a predetermined rate of change and amplitude. Control valve  507  is able to command the pump  40  to produce a flow rate to the linear actuator  15  of  FIG. 3 . The pump  40  may therefore be remotely controlled as a variable axial piston pump. The output signal of the control valve  507  may be modified by the controller  514  based upon a previous cycle of linear actuator  15 . If the pressure transducer  44  measures a pressure which is not consistent with the previous cycle, the controller  514  may suspend repressurization of the hydraulic system for a period of time, or dwell time, in order for the cycle to correct itself. After the dwell time has elapsed, control valve  507  may again be commanded by the controller  514  to increase the pressure signal to the pump  40 . 
     When the sensor  30  of  FIGS. 1-3  determines that the piston  24  is approaching a predetermined upper position with respect to the first end  12  of the cylinder  10 , the controller  514  commands the control valve  507  to decrease the pressure signal to the pump  40 . Discharging the fluid rate of the pump  40  in a controlled manner also results in less system shock. The control valve  507  then further decreases the pressure signal to the pump  40 , which allows the rod  20  to lower. 
     Hydraulic fluid lines  521  and  522  may be connected to the rod seal  27 , providing both a seal flush supply and a seal flush drain, respectively, for the hydraulic fluid. The hydraulic system of  FIG. 3  may also include a recirculation pump  502  to filter and cool the hydraulic fluid, a thermostatic bypass valve  508  and an air to oil heat exchanger  509 . 
     Fluid line  525  is connected to hydraulic port  37  of  FIG. 3 . When a fluid pressure in fluid line  525  is equal to a fluid pressure in the cavity  36  of  FIG. 3 , the rod  20  is stationary. When the fluid pressure in the fluid line  525  is higher than the pressure in the cavity  36 , then the rod  20  is raised or elevated. When the fluid pressure in the fluid line  525  is lower than the pressure in the cavity  36 , then the rod  20  is lowered. Alternately increasing and decreasing the pressure in fluid line  525  therefore results in the reciprocating motion of the rod  20  within the cylinder  10 . Fluid line  525  may include a fluid connection between the pressure transducer  44  and the hydraulic port  37 . The pressure transducer  44  may be included in a manifold (not shown) which is mounted directly to the rod base at the second end  14  of the cylinder  10  in  FIG. 3 . The manifold may include both the transducer  44  and a solenoid valve  511  or emergency lock valve of  FIG. 5 . 
     The pressures in the fluid line  525  are monitored by the pressure transducer  44  and controlled by the pump  40 . The pressure transducer  44  converts fluid pressure into a feedback signal that monitors load amounts. The pump  40  may be included in, or referred to as a hydraulic transformer. The pump  40  controls the rate at which hydraulic fluid is pumped from a port  530  when a load, including the sucker rod  50  of  FIG. 1 , is being lifted. The pump  40  further is able to control the rate at which the hydraulic fluid is reclaimed during a downstroke of the rod  20 . The pump  40  is connected to the motor  42  by a shaft coupling set  505 . As the pump  40  consumes the pressurized hydraulic fluid through port  530  during the downstroke, it produces an increase of shaft torque to motor  42  which causes it to rotate above a synchronous speed that was used to drive the pump  40  when the load was being lifted. The elevated rotational speed of the motor  42  generates electrical energy at a rate that is determined by the efficiencies of the lift system as well as the amount of load being supported by the lift system. The generated electrical energy may be output on electrical line  527 . 
     Port  530  may therefore serve as both a supply port and an inlet port to pump  40 . The port  530  is configured to function as an inlet port of the pump  40  during a down stroke of the rod  20 , and as a supply port during an upstroke of the rod  20 . This allows the system to alternatively function as a generator of energy and then as a consumer of energy during an upstroke and downstroke of the rod  20 . 
     When the linear actuator  15  is lowering the sucker rod  50  and down hole pump  55 , as shown in  FIG. 1 , the hydraulic fluid is returned from the cavity  36  to the hydraulic reservoir  520 . Pressured hydraulic fluid trapped underneath the rod  20  is swallowed by the pump  40  as described above. When the linear actuator  15  is raising sucker rod  50  and the down hole pump  55 , as shown in  FIG. 2 , the hydraulic fluid from a hydraulic reservoir  520  is pumped into the cavity  36  when the rod  20  is being raised. Energy is required to pump the hydraulic fluid into the cavity  36 . The closed loop control system described in  FIG. 5  provides for a method of controlling the speed and force of the lift system according to changing down hole conditions and work load. By controlling the flow rate and pressure of the hydraulic fluid, the pump  40  is able to control both the raising and lowering of the rod  20  without the use of a throttle. The hydraulic system as described produces significantly less heat compared to conventional systems which operate with throttles in which the heat and potential energy in the lift system are wasted. 
       FIG. 6  illustrates an example of a simplified energy grid  90  connected to a lift system. The lift system could be either of the users  82 ,  84 ,  86  or  88 . The users  82 - 88  are connected to the power grid  90 . The power grid  90  may further be connected to a substation  80 . The substation  80  may serve to allocate or control a flow of electricity from and between the users  82 - 88 . The substation  80  may further include a means to store electricity. The substation  80  may be local or remote from the users, and may further be connected to or part of a public utility or remote power station  85  by multiple power lines. 
     A regenerative hydraulic lift system is connected to the power grid  90  as one of the users  82 - 88 , for example user  82 . When the hydraulic lift system is acting as a consumer of energy, user  82  draws electricity from the power grid  90 . Similarly, other users  84 - 88  may be acting as consumers of energy and draw additional electricity from the power grid  90 . At some point, user  82  may become a generator of electricity, and user  82  may be able to transfer the generated electricity to the power grid  90 . The additional electricity generated by user  82  may be transferred to the substation  80  and routed to one or more of the users  84 - 88  for consumption. Similarly, the electricity generated by user  82  may be placed on the power grid  90  and transferred to remote power stations or power grids for use by other systems or devices, for example, in a public utility. One or more of the users  82 - 88  could include regenerative hydraulic lift systems, such that electricity generated by any one of them could be distributed or reused between them, thereby increasing the efficiencies of a fleet of lift systems. 
     The regenerative hydraulic lift system therefore does not require a local external means of storing this energy, but rather it is able to create a voltage supply which is transferred to the main electric power grid that originally powered the lift system. Instead of using a mechanical or pressurized gas to counterbalance the lowering of the down hole components, the regenerative hydraulic lift system uses an electric counterbalanced system. Electricity is generated at a rate that is proportional to the rate that the down hole components are being lowered. In this manner, the energy recovered from lowering the down hole component including the sucker rod  50  is recaptured and transformed into electrical energy fed back into the power grid  90  via the electrical line  527  of the motor  42 . 
     In one embodiment the pump  40  comprises a variable displacement pump. The pump  40  may include a mooring pump or a swallowing pump, or other hydraulic pump. The pump  40  recaptures the operational potential energy of the lift system by providing a controlled rate of resistance. This can be implemented without wasting the operational potential energy as heat that may otherwise occur as a result of throttling the hydraulic fluid, such as in conventional systems which include a throttle. The recapturing of the operational potential energy is transformed into electric energy by spinning the motor  42  faster than its synchronous speed, causing the motor  42  to become a generator which in turn produces clean linear voltage potential/current supply to be fed back onto the power grid  90 . 
     In a further embodiment of the invention, the rod  20  is lowered using the pump  40  to backdrive the electric motor  42 . This backdriving action increases the speed of the electric motor  42  from zero, and when an appropriate speed is reached, the power can be reconnected smoothly without any surges. Then, during the remainder of the lowering operation, the electric motor  42  will act as a generator as described above. In this manner the hydraulic system provides inherent soft-starting capabilities. 
       FIG. 7  is a flow chart illustrating an example method of recapturing energy in a lift system. The lift system may provide a method for pumping a subterranean fluid to the surface of the earth. 
     In operation  710 , a hydraulic pressure within a lift cylinder, such as cylinder  10  of  FIG. 3 , is increased at a first control rate during a pumping operation, when the rod  20  is being raised. The pumping operation may be performed by the down hole pump  55  shown in  FIG. 1 . 
     In operation  720 , the hydraulic pressure within the lift cylinder  10  is decreased at a second control rate during a lowering operation, for example a lowering of the down hole pump  55  and a sucker rod  50 . 
     In operation  730 , an amount of down hole fluid being pumped is controlled during the pumping operation by metering the first control rate. A pump, such as pump  40  of  FIG. 1 , may be used to meter the first control rate of the hydraulic pressure. 
     In operation  740 , a lowering speed of a down hole pump  55  is controlled by metering the second control rate. Both the first and second control rates may be metered according to a hydraulic pressure being provided by the pump  40 . A sensor, such as sensor  30 , may provide input to a controller  514 , which is used to control the first and second control rates provided by the pump  40 . The sensor input may include a position input. For example, the sensor  30  may measure a relative position of the rod  20  that reciprocates within the hydraulic cylinder  10 . 
     In operation  750 , electricity is generated during the lowering operation. The electricity may be generated by spinning the motor  42  faster than a synchronous speed during the lowering operation such that the motor  42  operates as a generator. A rotational torque may act on the motor  42  when hydraulic fluid is swallowed by the pump  40 , such that a supply port of the pump  40 , such as port  530 , operates as an inlet port during the lowering operation. 
     In operation  760 , the electricity is transmitted to a power grid. The power grid may include a local power station or be part of a public utility. The electricity generated by the motor  42  may then be redistributed for use by other devices or systems connected to the power grid. 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.