Parametric design and performance validation of low-cost, low-pressure drip emitters and irrigation systems

Abstract

This thesis proposes and validates methods to reduce the cost and energy use of drip irrigation systems, with the aim of increasing their adoption among smallholder farmers. By 2050, the growing world population will require a 55% increase in food production above 2010 levels. Yet, agriculture already places a large strain on the earth's resources, occupying 47% of habitable land area and comprising 70% of freshwater withdrawals. Thus, agricultural intensification needs to occur through increased efficiency, rather than increased resource consumption. While irrigation is an effective means to increase food production over rainfed land, traditional surface and overhead irrigation systems--such as flood, furrow, and sprinkler--have low water use efficiencies. Drip irrigation, which distributes water through a pressurized pipe network and slowly releases it through emitters in the immediate root zone of each crop, has been shown to increase water efficiency by 25-65% over flood or furrow irrigation. However, adoption of drip irrigation is limited by several factors, including high initial cost compared to conventional practices. To address the cost barrier to drip irrigation adoption, this work focuses on modeling, designing, and validating drip components and systems that operate at low pressures, reducing energy consumption and the costs of pumps and power systems. These savings are enabled by pressure-compensating (PC) emitters--which maintain a constant flow rate with variations in pressure--specifically designed for low-pressure operation. The first part of this thesis experimentally validates the ability of low-pressure PC online emitters (used for tree crops) designed by the MIT Global Engineering and Research Lab to reduce pumping power and energy in a series of field trials in the Middle East and North Africa. With a minimum operating pressure of 0.15 bar, these online emitters are shown to reduce pumping energy by at least 43% compared to commercial emitters with higher operating pressures, without compromising water distribution uniformity. The next section focuses on the design of low-pressure PC inline emitters (used for vegetable crops), which are bonded to the interior of irrigation tubing. While inline emitters are manufactured widely, their design in industry occurs largely by trial-and-error, which may limit product performance. To address this gap, this section presents a new, fully-analytical, parametric model for predicting the activation pressure and flow rate of typical inline PC emitters from their geometry and material properties of the membrane. The model's utility is demonstrated by systematically redesigning a commercial emitter to reduce its minimum compensating pressure from 0.4 bar to 0.15-0.25 bar, depending on the membrane used, while maintaining a similar flow rate. The last section of this thesis places low-pressure emitter designs in a system-level context to evaluate their impact and suggest further research directions. Concurrently, it presents a flexible, parametric model for designing cost-optimal drip irrigation systems with grid and off-grid power sources for any farm location, size, and crop. When applied to case studies representative of typical farms in Morocco, the model shows potential reductions of up to 20% in initial cost and up to 9% in lifetime system cost with optimized low-pressure drip systems, compared to conventional system designs. The results are used to identify and recommend opportunities for further system cost reduction.