Energy audit of irrigation networks

The relationship between water and energy in water distribution systems (WDS) has been a growing concern among energy and water experts. Among the different strategies to improve water–energy efficiency in water distribution networks, energy audits are of paramount importance as they quantify water flow requirements, the amount of energy consumed to meet demand and leakage and friction losses. Previous work has presented the energy audit process for urban WDS and this energy audit is extended to irrigation networks here. This work analyses the most common types of irrigation emitters (sprinklers and pressure compensating and non-pressure compensating drippers), hydrant specifications, irrigation management systems (on-demand or rigid scheduled), and energy losses due to friction in pipes, control valves and irrigation hydrants. The energy audit does not assess whether management of the network is optimal, but analyses the energy consumption. Some of the performance indicators have already been defined for agricultural water networks, some are identical to those of urban WDS, but in addition, a new one is presented that disaggregates the energy dissipated into three terms, energy losses in pipelines, in hydraulic valves and in irrigation hydrants. These indicators show information necessary to better understand the performance of the irrigation network under study, to carry out a deep analysis of energy consumption and to allow for comparison with similar systems. The paper presents the analysis of a real case study conducted on the irrigation network of the garden of the Universidad Politécnica de Valencia.

Introduction

The headline “more crop per drop” perfectly reflects the need for more efficient irrigation, a direct consequence of the substantial increase in irrigated areas in recent decades. To achieve this goal, the strategy has been largely based on converting traditional gravity-fed irrigation into pressurised irrigation systems. And indeed, this has resulted in larger areas being irrigated with the same amount of water. But these water savings have entailed much greater energy consumption, energy itself being a scarce and valuable resource. Table 1 (Corominas, 2010) details water and energy consumption in Spain in the last century and clearly reflects how the situation has changed in a country with a long agricultural tradition.

Table 1 shows that energy consumption becomes relevant from 1950. The initial increase in energy use cannot be attributed to drip irrigation but the silent revolution (Llamas & Martinez Santos, 2005) which supported the intensive use of groundwater. A couple of decades later, in the 1970s, a progressive transformation of irrigation took place from gravity-fed to pressurised irrigation. Table 1 shows that between 1950 and 2007 the irrigated area grew by a factor of 2.5, while water consumption doubled and energy expenditure became 19 times greater.

The energy price has been increasing slowly but progressively. This has resulted in a reduction of benefits for farmers. However now prices have risen so much that farmers can no longer sustain this situation and the relationship between water and energy has become a key point on the agenda of developed countries (Department of Energy, 2006). Moreover, the first detailed analysis that quantifies this link between water and energy (CEC, 2005) showed that 19% of the electricity consumption of the State of California was related to water use, a significant amount.

On the other hand, although most of this energy consumption occurs in urban and industrial areas, agriculture is also energy hungry. The electricity consumed by agriculture reached more than 4% of the total energy consumed in the state of California (while the water use in agriculture represents 22% of the water consumption of the State). This energy use was divided between water supply (groundwater pumping consumption represented 30% of the total energy consumption in irrigation) and distribution (the remaining 70% was related to water distribution in pressurised irrigation networks).

The interest in reducing the energy bill can be addressed using two different and complementary policies. The first (and most natural) strategy deals with the reduction of water consumption, as water savings result in energy savings. This strategy involves a set of actions covered by the term “water demand management”. The first step is not to use more water than necessary (in short, to optimise the water delivered to the crop). These needs are directly linked to climatology and to soil moisture. Traditionally, great efforts to quantify the proper amount of water required in scheduled irrigation have been made. Studies in this area include those related to climate prediction (WMO, 2010, chap. 5), the use of soil moisture sensors (Greenwood, Zhang, Hilton, & Thompson, 2010), deficit irrigation strategies (Geerts & Raes, 2009) and remote sensing and agro-climatic water balance models (Bastiaanssen, Allen, Droogers, D'Urso, & Steduto, 2007; Droogers, Immerzeel, & Lorite, 2010).

The second is linked to the optimisation of the design and operation of irrigation networks from an energy-related point of view. This has been an active research area since pressure irrigation began (Allen & Brockway, 1984), and in recent years, for the aforementioned reasons, it has been attracting increased attention. Irrigation networks have to be dimensioned (Daccache, Lamaddalena, & Fratino, 2009; Farmani, Abadia, & Savic, 2007; González-Cebollada, Macarulla, & Sallán, 2011) taking into account energetic implications. Furthermore they require pumping stations (Moradi-Jalal & Karney, 2008; Moradi-Jalal, Rodin, & Mariño, 2004; Moreno, Córcoles, Moraleda, Martinez, & Tarjuelo, 2010) and complementary elements (Armindo, Botrel, & Garzella, 2011; Kale, Singh, & Mahar, 2008) to be implemented to minimise energy expenditure. And once the system is working, its management should also be optimised from the energy perspective (Jimenez-Bello, Martínez, Bou, & Ayala, 2010; Lamaddalena & Khila, 2012).

It should be highlighted that the delivery scheduling method in an irrigation system demonstrates different levels of energy consumption. These schedule types may be classified (Replogle & Gordon, 2007), in order of increasing flexibility, as rigid (rotation, predetermined), central control, intermediate control (arranged) or flexible (on-demand, modifiable). Several studies have shown that between these two extremes, the more flexible the schedule is, the more energy hungry the system becomes (Moreno, Córcoles, Tarjuelo, & Ortega, 2010; Rodriguez, López, Carrillo, Montesinos, & Camacho, 2009). Moreover, other approaches have been carried out to show the influence of management systems on energy consumption in farm systems, considering the life cycle assessment of a crop (Rodrigues, Carvalho, Paredes, Silva, & Pereira, 2010), and the energy gain of crops and water productivity (Chen & Baile, 2009; Guzman & Alonso, 2008). The use of pressurised (or not) irrigation networks is shown to be a key factor in these analyses.

Apart from the initial concern over irrigating during the hours at which the electricity tariffs are cheapest (Pulido-Calvo, Roldán, López, & Gutiérrez, 2003), the requirement of energy optimisation is also considered regarding the design and operation processes of irrigation networks. Moreover, performance indicators of irrigation systems have been defined (Calejo, Lamaddalena, Teixeira, & Pereira, 2008; Luc, Tarhouni, Calvez, Messaoud, & Sablayrolles, 2006; Moreno, Ortega, Córcoles, Martínez, & Tarjuelo, 2010; Pérez, Rodríguez, Camacho, & López, 2009; Rodríguez, Camacho, & Blanco, 2011). And even, in a clear attempt to consider all the possibilities for improvement, the comparison of different systems using benchmarking strategies (Córcoles, de Juan, Ortega, Tarjuelo, & Moreno, 2012; Makin, Burton, & Malano, 2004; Malano & Burton, 2001) allows the regulator to identify the networks whose practices should be followed.

When a decision maker deals with the reduction of energy consumption in irrigation networks, the first step is to properly calculate the amount of water required by crops. The second stage is to quantify the water and energy losses through the network in order to have all relevant information. The two last stages are closely linked as they include showing actions to reduce energy consumption and performing a cost–benefit analysis to select the most convenient option.

This work deals with the second stage of this process, the quantification of the water and energy consumption in irrigation networks. It includes the use of the energy audit (Cabrera, Pardo, Cobacho, Arregui, & Cabrera, 2010) in agricultural water networks (new terms such as the energy lost in hydraulic valves and hydrants have been added) and the definition of new performance indicators (necessary information to carry out an analysis of energy consumption throughout the system) that consider the key features of irrigation networks.

This energy audit is more comprehensive than those that have gone before, including the identification and quantification of all elements that either supply energy to (which can be of two kinds, potential energy supplied by reservoirs, which depends on the height of the header tank or reservoir, and shaft work supplied by the pumps) or draw energy from the irrigation network. The energy output is broken down into energy delivered to users (in irrigation networks, this term refers to energy delivered to crops); energy dissipated due to friction and energy losses through leaks (energy lost when water is depressurised and is lost). This last term is not negligible in irrigation networks and its calculation is one of the key objectives of this work. Water losses have always existed in irrigation ditches, although in pressurised water networks they involve energy losses as well.

In order to complete the energy audit, two premises should be met. The first is to have calculated the water audit, an easy task if the network has proper metering devices (a flow meter at the head of the network and water meters installed in every irrigated area); while the second is to obtain a calibrated hydraulic model that adheres as closely as possible to reality (unfortunately, all water distribution systems (WDS) are leaky and the model should consider leaks as pressure-dependent demand when the hydraulic calculations are first done). Once these stages are completed, the energy balance quantifies the amount of energy used for the delivery of water in any network.

As commented before, some performance indicators have been defined for agricultural water networks (while those used in urban networks also apply here). These indicators show the information necessary to carry out an analysis of energy consumption throughout the system. The current energy analyses (Moreno, Ortega, et al., 2010; Rodríguez et al., 2011) are summarised in just one indicator, shaft energy per volume (injected or consumed, kWh m⁻³). The fact that these studies do not disaggregate energy expenditure means that they do not effectively identify or diagnose the weaknesses of the systems they consider. The results obtained with the new performance indicators show where the head losses are produced.

In conclusion, this work applies the energy audit to a real landscape irrigation network (real case study). And according to the values of the indicators, actions to improve water and energy management are proposed, the energy benefits are quantified and a cost analysis is performed.