Solar Powered Pumping System

Abstract

A solar powered pump that maximizes available energy usage under variable insolation conditions is disclosed. It also permits the integration of parameters such as minimum flow control, set point operation, etc., without the use of additional sensors, thereby reducing the overall cost of the system.

Claims

1. A pumping system utilizing a direct current (dc) solar power source for pumping a fluid in a flow circuit comprising, an ac motor coupled to a fluid prime mover to pump the fluid through said flow circuit, a dc-dc voltage converter that receives power from said power source and provides output power at a constant voltage, a dc to variable-frequency-ac voltage converter that receives power from said dc-dc converter and supplies it to said ac motor with an output voltage frequency, at least one sensor in said fluid circuit, said sensor is one of a flow sensor for measuring the rate of flow in said flow circuit, a pressure sensor for measuring the pressure corresponding to said rate of flow, or a differential pressure sensor for measuring the differential pressure corresponding to said rate of flow, said dc to variable-frequency-ac voltage converter comprising a comparator/controller and circuitry for providing output at voltage with adjustable frequency, said comparator/controller in communication with said sensor, wherein said comparator/controller utilizes data from said sensor and implements a perturb-and-observe control algorithm adjusting the output voltage frequency of said dc-variable-frequency-ac voltage converter to maximize the magnitude of the said sensor measured quantity.

2. The pumping system in claim 1, wherein the dc to variable-frequency-ac converter may be split into multiple modules.

3. The pumping system in claim 1, wherein the dc-dc converter and dc to variable-frequency-ac converter are combined into a single module.

4. The pumping system in claim 1, wherein the sensor and the comparator/controller communicate using one of electronic, wireless or optical means.

6. The pumping system in claim 1, wherein said fluid prime mover is one of a pump, a compressor, a blower or a fan.

7. The pumping system in claim 1, wherein said magnitude of said sensor measured quantity and perturb-and-control algorithm is constrained by a set-point.

8. A pumping system utilizing a direct current (dc) solar power source for pumping a fluid in a flow circuit comprising, a dc motor coupled to a fluid prime mover to pump the fluid through said flow circuit, a variable output voltage dc-dc converter that receives power from said power source and supplies output power to said dc motor at an output voltage, at least one sensor in said fluid circuit, said sensor is one of a flow sensor for measuring the rate of flow in said flow circuit, a pressure sensor for measuring the pressure corresponding to said rate of flow, or a differential pressure sensor for measuring the differential pressure corresponding to said rate of flow. said variable output voltage dc-dc converter comprising a comparator/controller and a reference voltage and circuitry for providing output with adjustable dc voltage, said comparator/controller in communication with said sensor, wherein said comparator/controller utilizes a voltage divider circuit with said reference voltage and data from said sensor, and implements a perturb-and-observe control algorithm adjusting the output voltage of said dc-dc converter to maximize the magnitude of the said sensor measured quantity.

9. The pumping system in claim 8, wherein the variable output voltage dc-dc converter may be split into multiple modules.

10. The pumping system in claim 8, wherein the sensor and the comparator/controller communicate using one of electronic, wireless or optical means.

11. The pumping system in claim 8, wherein said fluid prime mover is one of a pump, a compressor, a blower or a fan.

12. The pumping system in claim 8, wherein said magnitude of said sensor measured quantity and perturb-and-control algorithm is constrained by a set-point.

13. A variable-frequency-ac voltage drive for use with an ac electric motor in a fluid flow circuit, said flow circuit comprising at least one sensor and a fluid prime mover driven by said ac electric motor, wherein said sensor is one of a flow sensor for measuring the rate of flow in said flow circuit, a pressure sensor for measuring the pressure corresponding to said rate of flow, or a differential pressure sensor for measuring the differential pressure corresponding to said rate of flow, said variable-frequency-ac voltage drive comprising a comparator/controller and circuitry for providing output at voltage with adjustable frequency, said comparator/controller in communication with said sensor, wherein said comparator/controller utilizes data from said sensor and implements a perturb-and-observe control algorithm adjusting the output voltage frequency of said variable-frequency-ac voltage drive to change the speed of said ac electric motor to maximize the magnitude of the said sensor measured quantity.

14. The variable-frequency-ac voltage drive in claim 13, wherein the sensor and the comparator/controller communicate using one of electronic, wireless or optical means.

15. The variable-frequency-ac voltage drive in claim 13, wherein said magnitude of said sensor measured quantity and perturb-and-control algorithm is constrained by a set-point.

16. A variable voltage dc motor drive for use with a dc electric motor in a fluid flow circuit, said flow circuit comprising at least one sensor and a fluid prime mover driven by said dc electric motor, wherein said sensor is one of a flow sensor for measuring the rate of flow in said flow circuit, a pressure sensor for measuring the pressure corresponding to said rate of flow, or a differential pressure sensor for measuring the differential pressure corresponding to said rate of flow, said variable voltage dc motor drive comprising a comparator/controller and circuitry for providing output with adjustable dc voltage, said comparator/controller in communication with said sensor, wherein said comparator/controller utilizes a voltage divider/compensation circuit with said reference voltage and data from said sensor and implements a perturb-and-observe control algorithm adjusting the output voltage of said variable voltage dc motor drive to change the speed of said dc electric motor to maximize the magnitude of the said sensor measured quantity.

17. The variable voltage dc motor drive in claim 16, wherein the sensor and the comparator/controller communicate using one of electronic, wireless or optical means.

18. The variable voltage dc motor drive in claim 16, wherein said magnitude of said sensor measured quantity and perturb-and-control algorithm is constrained by a set-point.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic diagram of the first preferred embodiment of the invention incorporating an ac motor.

[0018] FIG. 2 is a schematic diagram of the second preferred embodiment of the invention incorporating a dc motor.

DETAILED DESCRIPTION

[0019] The invention as summarized can be implemented using either ac (1- or 3-phase induction) or dc motors. The lower cost and longer life of an ac motor based system makes it preferable as long the cost (and complexity) of the power conversion electronics associated with the ac drive are not excessive.

[0020] By using an integrated approach to the power conversion and control system, this incremental cost can be minimized, and both approaches will be practical.

First Preferred Embodiment

[0021] FIG. 1 shows a schematic diagram of a first preferred embodiment of the overall system. It is based on an ac drive and comprises of the following:

i) A solar photovoltaic (PV) panel(s)/array (100) that is the primary power source.
ii) A flow circuit/loop (500) incorporating a fluid prime mover ((400), i.e. a pump for a liquid, though it may be a compressor, blower or fan for a gas) for driving a fluid (typically water, but it may be any liquid or gas), one or more sensors (700) that provide flow rate data (directly or indirectly), as well as other fittings, valves (600), etc. that are typical of standard flow systems. Note that only a small portion of the flow circuit/loop (500) is shown in the figure: it may be an open circuit for transferring fluid from one location to another or may be a closed loop for circulating a fluid within.
iii) A fixed output dc-dc converter (200) that takes solar power at varying voltages as the input and provides output power at a fixed dc voltage suitable for driving a motor.
iv) A variable frequency dc-ac converter (250) that converts the dc voltage output from the dc-dc converter to an ac voltage whose frequency can be adjusted based on a control parameter.
v) An ac motor (300) that receives power from the dc-ac converter (200) and drives the fluid in the flow circuit/loop (500) using the fluid prime mover (400).
vi) A comparator/control module (800, shown here as part of the dc-ac converter (250)) that implements a perturb-and-observe algorithm to control the frequency of the output of the dc-ac converter (250) by using flow data from sensor(s) (700) in the flow circuit/loop.
vii) A communication link (900) between the sensor(s) (700) and the comparator/control module (800) that allows them to communicate as required per (vi) above. This may be done via wired, wireless, optical or other means.

[0022] The sensor(s) (700) used for the control function are particularly important in ensuring maximum power usage. In this invention, these are selected by noting that the pumping power ultimately determines the power demand from the source (the solar array (100)). This pumping power is related to the product of the flow rate and the pressure drop between the pump outlet and the pump inlet. However, since the flow rate is determined by the pumping pressure, maximum power usage can be attained by maximizing this pressure or the flow rate. Thus the comparator can be designed to work on the output one or more of the following sensor/transducer types: [0023] a flow transducer/sensor that directly measures the flow rate [0024] a pressure transducer(s)/sensor(s) that measures a differential pressure that can be related to the flow rate [0025] a pressure transducer/sensor that measures the maximum system pressure at the pump outlet

[0026] The configuration of the variable frequency dc-ac converter (250) is also important in ensuring that it meets the goals of the invention (low cost, high reliability/life and good efficiency). Dc-ac converters or inverters are of three types, square wave, modified sine wave and pure sine wave. The first two are less expensive but provide ac power with high frequency harmonics that cannot be adequately filtered out over the entire speed range. These harmonics have an adverse effects on motor life (as well as other electronics in the vicinity), and a pure sine-wave inverter is therefore used in this preferred embodiment.

[0027] FIG. 1b shows a overview/block diagram of the variable frequency dc-ac converter (250) incorporating a pure sine-wave inverter architecture. These inverters comprise high frequency switching networks (260) that change a constant voltage dc input to a time-varying output by controlling the timing and polarity via multiple switching processes. The switching networks are controlled via driver(s) (270) using output signals from pulse-wave-modulation (PWM) circuits (275). The output of the PWM modules are obtained by combining a reference sinusoidal signal (i.e. the modulating waveform at the required frequency, e.g. 60 Hz) together with one or more high frequency carrier waves (typically triangular pulses) which are generated in a separate block(s) (280). Suitable low pass filters (265) are used to remove unwanted high frequency harmonics from the output of the switching networks so that it corresponds closely to a pure-sine wave.

[0028] All inverters incorporate a feedback loop (290) that utilize the reference/carrier wave generating circuits (280) to ensure that the frequency remains stable regardless of load (and supply) changes. This is done by comparing the output frequency with the frequency of the modulating signal and correcting for any error via suitable compensating signals sent to the PWM modules. In the preferred embodiment, an important modification is made to this feedback-compensation loop, i.e. it is coupled with the reference/carrier signals and the comparator/control module (800) so that the frequency can be adjusted as required during operation (instead of it being maintained constant).

[0029] By combining frequency (i.e. speed) control with the dc-ac conversion, a separate variable speed ac drive is not required in the preferred embodiment. Such drives are a significant cost adder so that ac motor powered systems are often more expensive than dc motor driven systems even though dc motors typically cost much more than comparable ac motors. This use of a direct dc-ac variable frequency (1 phase or 3 phase) inverter is a particularly important feature of this preferred embodiment. An additional advantage of this approach is that the maximum allowable frequency for the reference signal (and the corresponding carrier wave) can be preset to match the maximum allowable speed for a specific design in order to provide a hard operating limit for the motor/pump if necessary.

[0030] It is also important to note that (iii) and (iv) above, i.e. the dc-dc converter (200) and dc-ac converter (250) can be interchanged in an equivalent design. The alternative would be to implement the dc-ac conversion in the first step after which the ac voltage is changed (using a transformer for example) so that it is suitable for use with an ac motor. Cost and conversion efficiency will determine the preferred approach.

[0031] Based on the above, the overall system functions using the “perturb and observe algorithm” to adjust the frequency of the output voltage of the dc-ac converter (and thereby speed of the ac motor) to maximize the flow rate, i.e. the sensor output. This is done as follows (note that there are many variations of the P&O algorithm—this a typical approach that may be replaced by other equivalent implementations):

a. Power from a solar PV array is supplied via the dc-dc converter to the dc-ac inverter. Power from the inverter is supplied to the ac-motor/pump with the initial predetermined frequency (corresponding to a low speed in the preferred embodiment).
b. Sensor data related to flow (i.e. flow rate or pressure or differential pressure as described above) is supplied by the sensor(s), after suitable processing/conditioning (e.g. averaging, noise-filtering) if necessary, to the comparator/controller module.
c) The converter output frequency is adjusted by an small incremental value (which will typically be a preset amount) in a pre-selected “positive” direction.
d) Sensor data is once again sent to an electronic comparator/controller module and compared with the previously sent value.
e) Two possible outcomes are possible here: [0032] (i) If the previously sent value is less than (or “equal to”) the flow rate in (d), step (c) is repeated. [0033] (ii) If the previously sent value is greater than the flow rate in (d), this shows a lower power usage. In this case, step (c) is repeated, but in the reverse direction by a small incremental value, and this reverse direction is now set as the “positive” direction.

[0034] Steps (c-e) are now repeated on a continuous basis at preset time intervals with the latest value being replaced as the previous value at the end of every step. This will ensure that operating condition corresponds to the maximum flow and maximum power point.

[0035] It is important to note that the same algorithm with minor changes can be used if the goal is to use a flow related set-point (e.g. fixed flow rate, pressure, etc.) instead of the maximum flow condition. In this case, the set-point is used as a constraint for the maximum, and the set point (instead of previous value) is compared to measured value in step (d) above.

Second Preferred Embodiment

[0036] FIG. 2 shows a schematic diagram of a second preferred embodiment of the overall system. It is similar to the first preferred embodiment but is based on a dc drive, and comprises of the following:

i) A solar photovoltaic (PV) panel(s)/array (100) that is the primary power source.
ii) A flow circuit/loop (500) incorporating a fluid prime mover ((400), i.e. a pump for a liquid, though it may be a compressor, blower or fan for a gas) for driving a fluid (typically water, but it may be any liquid or gas), one or more sensors (700) that provide flow rate data (directly or indirectly), as well as other fittings, valves (600), etc. that are typical of standard flow systems. Note that only a small portion of the flow circuit/loop (500) is shown in the figure: it may be an open circuit for transferring fluid from one location to another or may be a closed loop for circulating a fluid within.
iii) A variable output dc-dc converter (225) that takes solar power as the input and provides output power over a wide voltage range suitable for driving a dc motor.
iv) A dc motor (325) that receives power from the dc-dc converter (225) and drives the fluid in the flow circuit/loop (500) using the fluid prime mover (400).
v) A comparator/control module (800) that implements a perturb-and-observe algorithm to control the output voltage of the variable output dc-dc converter (225) by using flow data from sensor(s) (700) in the flow circuit/loop.
vi) A communication link (900) between the sensor(s) (700) and the comparator/control module (800) that allows them to communicate as required per (vi) above. This may be done via wired, wireless, optical or other means.

[0037] The sensor(s) (700) used for the control function are similar to those used in the first preferred embodiment. However, the variable output dc-dc converter is different so that it is suited to dc-dc converter circuit designs. The focus here is on the a specific aspect of the dc-dc converter, viz. a feedback loop that is used to ensure that the converter provides a stable output voltage regardless of load (and supply) changes. In conventional designs, this loop is used to compare the output voltage of the converter with a reference (constant) voltage source, so that any deviations between the two can be corrected.

[0038] In the preferred embodiment (FIG. 2b), an important modification is made to the feedback loop (235) used with the dc-dc converter block (230). In this case, the loop is coupled with (or replaced by) the comparator/control module (800) so that the reference voltage (240) can be adjusted as required during operation instead of it being maintained constant. More specifically, the output of the sensor is used together with a voltage divider/compensation circuit (245) to change the reference voltage, and thereby the output voltage and motor speed as required. The upper limit of the reference voltage can be set such that it corresponds to the upper operating limit for the pump, so that the overall design is simplified considerably. Thus, a separate variable speed dc drive is not required and system costs are minimized.

[0039] As in the first embodiment, the overall system functions using the “perturb and observe algorithm” to adjust the output voltage of the dc-dc converter (i.e. the input voltage and speed of the dc motor) to maximize the flow rate, i.e. the sensor output. This is done as follows:

a. Power from a solar PV array is supplied to the dc-dc converter. Power from the dc-dc converter is supplied to the dc-motor/pump with the initial output voltage.
b. Sensor data related to flow (i.e. flow rate or pressure or differential pressure as described above), after suitable processing/conditioning if necessary, is supplied by the sensor(s) to the comparator/controller module.
c) The converter output is adjusted by an small incremental value (which will typically be a preset amount) in a pre-selected “positive” direction.
d) Sensor data is once again sent to an electronic comparator/controller module and compared with the previously sent value.
e) Two possible outcomes are possible here: [0040] (i) If the flow rate in (d) is greater than (or “equal to”) the previously sent value, step (c) is repeated. [0041] (ii) If the flow rate in (d) is less than the previously sent value, this shows a lower power usage. In this case, step (c) is repeated, but in the reverse direction by a small incremental value, and this reverse direction is now set as the “positive” direction.

[0042] Steps (c-e) are now repeated on a continuous basis at preset time intervals. This will ensure that operating condition corresponds to the maximum flow and maximum power point. As in the first embodiment, note that other similar/equivalent P&O algorithms and flow related set-points can be used.

[0043] It is important to note that in both embodiments above, the controller can readily have additional functions for special operating conditions. For example,

1. When a control valve is closed (e.g. if a tank is full), the flow rate (or differential pressure in the flow line) will become zero/very small. The comparator/controller can have the additional function to shut down the pump below a (preset) low flow rate/differential pressure. Alternatively, since the pressure in the flow circuit will become high (beyond the value for normal operating conditions), the comparator/controller can have the additional function that shuts down the pump above a (preset) high pressure.
2. If there is a flow line break leading to a major leak, the flow rate may become very high (higher than values under normal operating conditions). For this case, the controller can have an additional function set to shut of the motor/pump above a (preset) high flow rate.

[0044] A number of other variations are to above embodiments are also possible. Some examples are as follows:

a. Instead of solar power, the power source can be another similar source where the available power varies with time in an unpredictable (or partially predictable, e.g. a renewable source such as wind, etc. without adequate storage) manner. At the same time, the prime mover can be a compressor (for a refrigerant or other gas) used in a flow loop of a cooling, thermal energy storage or other system (e.g. refrigeration, air conditioning, etc.) instead of a pump in a solar pumping system.
b. In an ac motor/pump based system (the first preferred embodiment), the fixed voltage output dc-dc converter and the variable frequency dc-ac converter may be combined into a single dc-ac converter module together with the comparator/controller. Alternatively, the different subsystems (dc-dc conversion, dc-ac conversion, variable frequency ac-ac conversion) may be combined/split in different modules if that is advantageous from a design, cost and use perspective.
c. A similar approach to (b) is also be possible for the second preferred embodiment. The single variable output dc-dc converter can be replaced by two separate modules—a fixed output dc-dc converter (as in present systems) coupled with a secondary variable output dc-dc converter that works on the fixed output of the first. This is equivalent to the preferred embodiment, but may make for a more manufacturable and modular package. The comparator/controller may also be split from the variable output module.
d. Instead of using data from a single measurement for comparison at every stage, data from multiple measurements can be used after suitable averaging using an appropriate signal conditioning circuit. This can help achieve more smooth system operation.
e. The incremental adjustment can be time-varying. Various options are possible here, e.g. larger increments can be used when the relative change in flow rate is small, whereas smaller increments can be used when the measured change in flow rate is high. This should also lead to quicker and smoother system operations.
f. The incremental adjustments can be done at varying time intervals instead of fixed time intervals. For example, when the relative change in flow rate is high, the time interval can be small; in contrast, when the relative change in flow is small, the time interval can be high. This too should result in more smooth operation as above.
g. Periodic and/or flow dependent re-initializations to preset controller settings may be incorporated in the control algorithm to ensure that the system does not get constrained at a local minima condition. This can help ensure that maximum flow (and power usage) conditions are achieved even under non-design conditions, e.g. when a portion of the array becomes shaded.
h. Other parameters can also be used to supplement/complement the flow data, e.g the voltage input to the motor, motor temperature, etc. may be used as additional control parameters. This may be particularly useful as a secondary or backup control mode (e.g. in case of primary sensor failure, motor protection, etc.)—note that voltage (or motor speed) control by itself, will not lead to maximum power usage conditions under all circumstances, and would not meet the goals of this invention.
i. Finally, it is important to reiterate that there are a number of variations to the P&O algorithm that has been described in the previous sections. These may be based on differences between consecutive measurements (the difference between consecutive measurements will change from a positive to a negative value or vice-versa as one crosses the maxima—the sign change (+ve/−ve) can be used for control), the slope of the flow rate v/s converter output function (which will also change signs across a maxima), etc. Similarly, when a set point or constraint condition (fixed flow, maximum allowable pressure, etc.) is imposed, an equivalent method would be to minimize the difference between the output and the constraint. Thus, any of the alternative implementations of the P&O algorithms can be used in this invention as equivalent embodiments.

[0045] Details of the power source, the motor drive system/variable output power module, the sensor/transducer data collection, transmission and conditioning hardware and software, the control logic, flow loop architecture, etc. have not been described above since many variations are feasible based on prior art. Thus, while the invention has been described and disclosed in various terms or certain embodiments, the scope of the invention is not intended to be, nor should it be deemed to be limited thereby, and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.

Definition: Sensor is used interchangeably with transducer.