Energy Efficiency Measures

AGRICULTURAL: IRRIGATION

Minimize frictional losses in irrigation pipes by using large diameter pipe on the mainline and minimizing turns. The installation of a 12-inch versus a 10-inch mainline will save three horsepower when the flow rate is 1,200 gallons per minute. Assuming a quarter-mile mainline and a five-month irrigation season, the extra pipe cost of about $1 per foot will be paid back in reduced energy cost in two to three years. (See “Reducing pumping costs through optimal pipe sizing,” a DOE Energy Tip, at www.oit.doe.gov/bestpractices/pdfs/motor1.pdf.)

Convert from high-pressure top impact sprinklers to low-pressure systems. When using a low-pressure system, lower spray packages to match crop growth. Low-pressure systems using drop tubes deliver about one percent more water for every foot of drop tube used because evaporative losses are reduced. The lower the spray package, the more efficient the system becomes.

When replacing or installing a new motor, select a high- or premium-efficiency pump motor over a standard-efficiency model. More efficient motors run cooler, last longer, and save 3 – 5% a month on electric consumption. Motors that run under load at a substantial duty cycle typically use electricity that costs 10 to 20 times their purchase price each year. So investing in premium-efficiency motors can have a very fast payback. The Consortium for Energy Efficiency (CEE) maintains model specifications and a data base on premium-efficiency motors; see www.cee1.org/ind/motrs/motrs-main.php3.

Match the pump and motor combination to flow and pressure requirements. Over-sizing makes for inefficiencies that waste energy and cost money. Installing variable speed drives (VSD) on existing motors allows for modulating motor speed to achieve the desired flow. When irrigation requirements change, flow can be adjusted quickly and accurately.

Replace worn nozzles and regulators. A useful rule of thumb is that nozzle wear of 10 percent results in a 20 percent increase in flow. The average life expectancy of a nozzle and a regulator under optimum conditions is around 8 to 10 years. However, during low-water conditions, irrigation pumps must pull water from lower depths. This pushes more sand and silt through nozzles and regulators, increasing wear. When such sediment is present, life expectancy decreases to 3 to 5 years for nozzles and 5 years for regulators.

For example, if an irrigation system has a typical quarter-mile pivot, it will have perhaps 140 nozzles and 140 regulators. If the irrigation system was set for 1,200 gallons per minute five years ago and the nozzles are 20% worn, the pump is now trying to push more water through the system, but it is working overtime and providing less head. This results in too much flow at the start of the pivot and too little at the end. As a result, the system wastes energy, wastes water, and produces uneven crop growth. New nozzles cost around $1.50 and regulators $6.00. For $210 in nozzles and $840 for regulators (not counting labor) the system can be brought back to its original efficiency.

Combining efficiency measures with measuring soil moisture

The water source for over 2,000 irrigation wells in Yuma and Washington counties in Colorado is the Ogallala aquifer, a valuable resource that farmers and various state and federal agencies are interested in preserving. In a recent demonstration¹ aimed at saving both water and energy, ten irrigation systems were upgraded. Measures undertaken were:

• Replacing old well pumps and motors with high-efficiency units matched to anticipated loads (100 hp motors were installed in most cases);
• Refitting center pivot units with drop nozzles that apply water near ground level; and
• Adopting watering schedules based on soil moisture measurements.

The measuring system plays a particularly important role. It consists of gypsum blocks buried at regular intervals in holes 4.5 feet (for corn fields) to 5 feet (alfalfa) deep. Sampling their moisture content at regular intervals produces data useful in adjusting watering intervals and quantities to optimize growth while avoiding both over- and under-watering. The results of the demonstration projects are not yet available, but the energy efficiency improvements alone are estimated at 30%.

¹ Energy Services Bulletin, October 2003, Vol. 22 No. 5 available at www.es.wapa.gov/pubs/ESB/03oct/esb103.htm.

© 2003 Southwest Energy Efficiency Project
2260 Baseline Road, Suite 212, Boulder, CO 80302
(303) 447-0078 fax: (303) 786-8054 info@swenergy.org