AR No. # Efficient Irrigation

Transcription

1 AR No. # Efficient Irrigation Recommendation Optimize the facilities irrigation pumping system by ensuring pumps are operating at their Best Efficiency Point (BEP). We estimate the system pumping efficiency can be improved from the current average of 62.7% to a reasonable average of 72.0%, reducing pump operation costs by 13.0% Assessment Recommendation Summary Energy Energy Cost Implementation Payback (MMBtu) 1 (kwh) 1 Savings Cost (Years) ,900 $970 $ MMBtu = 1,000,000 Btu, 1 kwh = 3,413 Btu Background The facility pump irrigation system features two pumps operating in parallel to serve a piping network with a configuration that varies from day to day depending on irrigation needs. These pumps are on their own dedicated electrical service. Based on the annual pump electrical draw, delivered flow and a calculated average pump head, the pump is delivering needed water at a 62.7% average system efficiency. According to the pump performance curve, the modeled average flow and head delivered to the irrigation system can be developed at an optimum efficiency of 81.0%. While it would be unrealistic to expect to consistently achieve this optimum efficiency, this pump should be able to achieve a much better efficiency. We estimate that an average efficiency of 72.0% is practically obtainable. In any irrigation system, opportunities for reduced energy consumption are typically available by: Reducing the required hydraulic horsepower: Hydraulic horsepower is defined as the power required to move a volume of liquid at a specified pressure and flow rate. There are two ways the required hydraulic horsepower can be reduced. Reduce pump outlet pressure: This can be accomplished by minimizing distribution line pressure losses though proper sizing of mainline to match the flow rate. When expanding or replacing mainlines we recommend consulting a professional to help analyze which sizes may be most cost effective. Pump outlet pressure can also be reduced by eliminating throttle valve control or by minimizing end of line pressure requirements such as installing low pressure irrigation systems. Low pressure irrigation systems can operate at pressures as low as psi without reducing flow rate. Reducing pump flow: Annual water consumption can be reduced by ensuring excess water is not delivered to any end points. This can be accomplished by using moisture

2 water is not delivered to any end points. This can be accomplished by using moisture sensing and irrigation scheduling practices to reduce over irrigating. Water consumption can also be reduced by sealing line leaks and eliminating controls that bypass flow from the source without being used. Increasing system efficiency: Efficiency is defined as the ratio of output to the input of any system, pumping system efficiency can be improved by maximizing the output by following these measures. Change operating point: All centrifugal pumps are designed to operate at one Best Efficiency Point (BEP) on its flow versus head performance curve. When system requirements move away from this point the efficiency of the pump deteriorates. This can be solved by either better matching the end use requirements to the pump or vice versa, by replacing the pump with one that matches that end use requirements. Pump maintenance costs also tend to increase at operating points away from the BEP. Overhaul pump: The impeller and casing can be worn down over time, particularly if the pump has ingested dirt or other foreign matter or if it runs in a cavitating condition. This will increase the gap between casing and impeller, reducing the pump efficiency. Testing pump efficiency every two to three years can help ensure that pumps perform properly and that no significant losses are occurring. Depending on required head and flow, pumps can feasibly reach efficiencies as high as 80%. Unfortunately they are able to get the job done at abysmal efficiencies (20% or lower) with little outward sign of inefficient operation. Overhaul motor: Using Premium Efficiency (PE) motors instead of standard efficiency motors can increase system efficiency. PE motors are between 2 and 10 percent more efficient than standard efficiency motors, and the savings commonly justify the greater initial cost. We suggest you consider purchasing a new PE motor instead of rewinding an older motor. If choosing to rewind, investigate the ability of your rewind shop to maintain motor efficiency in the rewind process. Consider Variable Speed Drive: Most irrigation pumps run at full speed no matter the load on the system. This can be very inefficient, particularly when a wide range of flow rates and pressures are needed. A more energy efficient system uses a variable speed drive (VSD) to slow the motor speed to match the end use requirements. In a multiple parallel pump system, banks of pumps are combined in order to handle a wide variation of flows. This arrangement is widely used in agricultural operations, where water being supplied to irrigation systems experiences large fluctuations in demand from one time of the day to another. The use of multiple pumps allows pumps to be switched on and off as required to meet the varying demand. In such systems, all the pumps take their suction from a common source and discharge into a common header. Each pump will operate at the same head, but share the flow demand with the other pumps.

3 When a single pump or a group of pumps run, they will settle into delivering a flow and head at the intersection of the pump operation and system curve. When pumps operate in parallel, their combined operating curve can be developed by adding the flow each pump can deliver at any particular head. The new point of operation for multiple pumps will be at the new intersection of the system and combined pump curve. It is worth noting that flow is clearly not doubled with two pumps running in parallel. The efficiency of each pump will however drop significantly as the operating point moves away from BEP and the electrical draw will increase. Figure 1) Note that the third pump is not adding much additional flow and causes all three to run at a poor operating point (low efficiency). The facility pump system uses two pumps rated to deliver 575 gpm each in parallel with an output pressure of approximately 60 psi. We observed the pumps providing a combined 842 gpm at the header with a relatively high outlet pressure of 75 psi. The pumps are running at a flow that is well below their design point, decreasing efficiency. By adding additional end uses to reduce the line resistance and outlet pressure to 60 psi, the pumps will be able to increase developed flow from 842 gpm to 1,150 gpm, and will operate closer to their BEP. This action will increase pump efficiency and reduce the hydraulic energy required per gallon of water pumped. Alternatively, subtracting end uses to increase line resistance and only operating one pump at 60 psi, decreasing developed flow from 842 gpm to 575 gpm, will increase pumping efficiency.

4 Proposal Based on the available electrical and flow data, your pumping system is only converting a small portion of electrical energy consumed into hydraulic energy (flow & head) delivered to the irrigation system. To correct this low system efficiency, we recommend taking the following actions: Have a professional ensure that the pumps are in good operating conditions. Take care to operate these pumps at a point close to their BEP, by ensuring the number of irrigation lines attached result in an outlet pressure near 60 psi. For our estimates we assume you will be able to achieve an conservative system efficiency of at least 72.0%. However; you may be able to achieve a higher efficiency depending on the amount of care taken in choosing the appropriate size end use requirement. If the previously mentioned actions are taken, they will save 15,868 kwh annually and result in an annual cost savings of $975. This will result in a net payback of 0.2 years and an implementation cost of $175. Source: Notes An added benefit of optimizing pump efficiency will be that pump maintenance requirements are statistically lower when pumps operate closer to their BEP. However, this additional benefit is not evaluated in this recommendation. See the Best Practices Section later in the report for more information on how to improve irrigation efficiency. Author Readability Review Engineering Review Math Review Mikhail Jones Carl Moen Nathan Keeley Nathan Keeley

5 Efficient Irrigation Data Collected Motor Data Rated Horsepower (P) 30 bhp (N. 1) Motor Speed (N) 3,520 rpm (N. 1) Total Live Power (P L ) 46.0 kw (N. 1) Pump Data Total Flow Rate (Q) 842 gpm (N. 2) Intake System Data Suction Pipe Diameter (D S ) 5.8 inches (N. 1) Suction Gauge Pressure (P S ) 0 psi (N. 1) Suction Gauge Elevation (E S ) -3 feet (N. 1) Suction Line Loss Coefficient (C S ) 0.5 (N. 3) Discharge System Data Discharge Pipe Diameter (D D ) 5.8 inches (N. 1) Discharge Gauge Pressure (P D ) 75 psi (N. 1) Discharge Gauge Elevation (E D ) 3 feet (N. 1) Discharge Line Loss Coefficient (C D ) 1.0 (N. 3) Energy Consumption Data Annual Energy Consumption (EC C ) 121,980 kwh (N. 4) Incremental Energy Data Incremental Energy Cost (IC E ) $ /kwh (N. 4) Notes References Assumptions Rf. 1) Specific gravity of water at 60 o F. Material Properties Rf. 2) Vapor Pressure of water at 60 o F. Specific Gravity (SG) 1.0 (Rf. 1) Rf. 3) Barometric pressure at sea level. Vapor Pressure of Fluid (V P ) 0.57 ft. abs. (Rf. 2) Environmental Conditions Barometric Pressure (P B ) 34.0 ft. abs. (Rf. 3) Acceleration of Gravity (g) 32.2 ft/sec 2 (Rf. 4) Efficiencies Percent of Achievable Efficiency (η E ) 95% (N. 5) Motor Efficiency (η M ) 93.6% (N. 6) Conversion Factors Pressure Conversion Factor (CF 1 ) 2.31 ft H 2 O/psi Volume Conversion Factor (CF 2 ) ft 3 /gal Time Conversion Factor (CF 3 ) 60 min/hr Time Conversion Factor (CF 4 ) 60 sec/min Power Conversion Factor (CF 5 ) Kw/hp Length Conversion Factor (CF 6 ) 12 in/ft N. 1) Data collected during the site assessment. N. 2) Flow rate was estimated based on number sprinklers, nozzle size, and pressure. N. 3) Line loss coefficients are estimated based on pipe length, diameter, and material as well as fitting type and quantity. N. 4) Data is from utility bills found in the Site Data section. N. 5) As a conservative estimate we assume that only a percentage of the average achievable efficiency provided by the pump performance curve will be reached. N. 6) Motor efficiency based on The National Electric Manufactures Association (NEMA) Standards Publication, MG 1. Rf. 4) Acceleration of gravity at sea level.

6 Efficient Irrigation Operating Conditions Equations Operating Hours (OH) 2,652 hours (Eq. 1) Eq. 1) Operating Hours (OH) Pump Head Development PL Differential Elevation Head (H E ) 6 feet (Eq. 2) Eq. 2) Differential Elevation Head (H E ) Differential Pressure Head (H P ) feet (Eq. 3) ED E S Suction Velocity (V S ) ft/sec (Eq. 4) Eq. 3) Differential Pressure Head (H P ) Discharge Velocity (V D ) ft/sec (Eq. 5) PD CF1 PS CF 1 Differential Velocity Head (H V ) 0.00 feet (Eq. 6) SG SG Suction Friction Head (H SF ) 0.82 feet (Eq. 7) Eq. 4) Suction Velocity (V S ) Discharge Friction Head (H DF ) 1.64 feet (Eq. 8) Q CF2 CF4 Total Pump Head (H) feet (Eq. 9) DS CF6 2 2 Eq. 5) Discharge Velocity (V D ) Cavitation Development Q CF2 CF4 Net Positive Suction Head Required (H R ) 15 feet (Rf. 5) DD CF6 2 2 Net Positive Suction Head Available (H A ) 29.6 feet (Eq. 10) Eq. 6) Differential Velocity Head (H V ) Pump Cavitation (C) No (N. 7) 2 2 Current System Efficiency Eq. 7) Suction Friction Head (H SF ) Required Hydraulic Horsepower (RP) 38.6 whp (Eq. 11) 2 V K S S Required Annual Energy (EC R ) 76,428 kwh (Eq. 12) 2 g Current System Efficiency (η C ) 62.7% (Eq. 13) Eq. 8) Discharge Friction Head (H DF ) References Rf. 5) Based on pump performance curve located on the following pages. 2 V K D D 2 g Eq. 9) Total Pump Head (H) Notes Eq. 10) NPSH Available (H A ) P E V H N. 7) If the NPSH A is less than the NSPH R the pump is most likely cavitating. VD V S 2 g 2 g HE HP HV HSF HDF B S EC Eq. 11) Required Hyd. Horsepower (RP) Q H SG 3960 Eq. 12) Required Annual Energy (RE) RP CF5 OH Eq. 13) Current System Efficiency (η C ) C P ECR ECC SF

7 Efficient Irrigation Proposed System Efficiency Equations Achievable Pump Efficiency (η P ) 81.0% (Rf. 6) Eq. 14) Achievable System Efficiency (η S ) Achievable System Efficiency (η S ) 72.0% (Eq. 14) P M E Eq. 15) Proposed Energy Usage (PE) Energy Savings Summary C C Proposed Energy Usage (PE) 106,112 kwh (Eq. 15) S EC Energy Savings (ES) 15,868 kwh (Eq. 16) Eq. 16) Energy Savings (ES) Implementation Costs Summary Eq. 17) Cost Savings (CS) Labor Costs Pump Specialist Wage (L R ) $35 /hr (N. 7) ES IC E Eq. 18) Implementation Cost (IC) Hours (L H ) 5 hrs (N. 7) Economic Results Cost Savings (CS) $975 /yr (Eq. 17) Implementation Costs (IC) $175 (Eq. 18) Payback (PB) 0.2 yrs References Rf. 6) Based on pump performance curve on the following page. EC C PE LR L H Notes N. 7) Estimated time and costs for a pump specialist to find the flow rate and pressure corresponding to the optimum operating efficiency.

8 Efficient Irrigation Berkeley B3ZPL Pump Performance Curve Source: