Energy audits in irrigation communities
Angel Sánchez de Vera Quintero. Head of the Department of Services and Agriculture
Agricultura. Detall...

Agricultura. Detalle rama de limonero con frutos

Irrigated land is a fundamental component of the Spanish food and agriculture system. It delivers more than 50% of the final agricultural production, while occupying only 20% of our country’s useful agricultural land. A hectare of irrigated land produces about 6 times more than a hectare of rainfed land and generates an income four times greater. This income is also more secure, allows diversification of production and brings increased flexibility to farming. This is why transforming rainfed land to irrigated land might be every farmer’s ambition. 

But this transformation must take place with due consideration for energy efficiency criteria which will allow future operating costs to be reduced to the required minimum. Irrigation techniques have evolved from irrigation by flood or gravity (through the supply of water through ditches or channels) to sprinkler irrigation through the supply of water by pressure pipes. This is possible through the use of pumping systems, that is, through the energy that is demanded and consumed by the motors which drive the pumps. This modernisation process has meant that in the past 30 years water consumption has reduced by more than 20% while energy consumption has increased by 650%.

The critical/sensitive points which have an impact on energy consumption are not only the type of crop and the surface to be irrigated, but also the design of the system: where the water is brought from (underground or surface), the dimensions of the pressure pipes (length and diameter) and their layout (topography of the land and elevations), and the final pressurization of the selected irrigation system (sprinkler, drip or gravity), not forgetting the pumping station. 

The design of the network, based on the topography of the land, must be divided into segments by plots with hydrants at the same level and plots with the same type of irrigation system (sprinkler, drip or gravity) in such a way that the use of pressure-reducing valves is reduced. These valves are extremely wasteful of energy when irrigation is indiscriminate over the entire surface of the irrigation community. If the network cannot be divided into these sub-circuits, the implementation of irrigation “shifts” should be analysed, so that different zones that demand the same pressure (according to irrigation levels or irrigation types) can be fed from the same pumping installation at different times.

This will allow the regulation of pumping flow rates, for which it is preferable to implement variable frequency drives (on-demand variable flow rates and pressures) rather than regulation reservoirs, which require raising water up and then having to lose pressure in the hydrant regulator, which is not a correct use of energy. For new works, or reform of existing installations, the implementation of multiple pumps, connected in parallel, should be analysed. This alternative offers potential savings because one or more pumps can be taken out of service at times of low demand, making the pumps in service operate at high efficiency. A system with multiple pumps should be considered in cases where the demand is remains below half capacity of the required or installed pump for prolonged periods.

And while we’re talking about pumps, this equipment offers the greatest potential for energy savings in many installations. It is a typical error to have selected the wrong pump by oversizing it through the application of excessive safety factors, or by foreseeing a future increase in activity with new partners or new crops that in reality doesn’t happen, or to anticipate a possible reduction in performance due to the aging of the installations. This oversizing will cause the pump to run outside its optimum power point and the power absorbed over time (the energy paid for in the electricity bill) is much higher than the power actually required. The relationship between the two powers defines the energy efficiency of the pump. Measurements at multiple facilities have given values of only 40% or 50% efficiency, when this value should approach 80% or 85%. This is a lot of money wasted on the electricity bill and easily justifies the proposal that all irrigation communities should have network analysers that measure this absorbed power and allow the necessary adjustments to be made based on its reading. 

Today, it’s difficult to achieve advances in pump technology to improve performance, as the technology is mature. What’s now important is to ensure that the pumps work at the right point of their curve. The solution is a good choice of pump size, based on the performance required, and a high efficiency. For example, once the water flow rate and the pressure have been established, a pump will be chosen which puts this flow rate between 75% and 110% of the point of best performance.

To summarise the critical points in the energy consumption of an irrigation system, the following checklist should serve to determine the potentials for improvement:

  1. Energy efficiency in pumping: introduction of variable frequency drives in pumps, improvement of performance in the drive equipment, automation of the control and switching systems, and design of energy optimized networks.
  2. Focus on sectorization when designing irrigation networks, by irrigation type and by land height; analysis of timings. We must also analyse the water control system with individual meters and a system to compare individual and general water consumption. Billing for the water used should be made in accordance with the Ordinances of the Irrigation Community, with a two-part tariff and penalties for excesses.
  3. Irrigation unit and equipment in the fields: Migrate from sprinkler systems to drip; internal design of any plots of land using pressure irrigation to minimize energy requirements; analyse pressure losses in hydrants; locate the water supply points or hydrants at high points where possible, migrate gravity irrigation systems to drip irrigation based on pumps powered by photovoltaic solar energy (this has already been very well applied)

With all this in mind, the energy auditor’s work plan should be 

First phase: Measurement of the electrical and hydraulic parameters of the pumps
Measurements will be carried out sequentially pump by pump, taking continuous data for at least one complete cycle of operation of the pump, where a cycle is the period between the starting and re-starting of the pump. In general, this period shouldn’t be less than a week, in order to take into account the variations in operation from one day to the next.

Independently but concurrently, measurements will be taken of the hydraulic variables (flow rate and pressure at the input and output of the pump), synchronizing these measurements with those made by the network analyser, so that for each moment that data is recorded, both electrical and hydraulic data are available.

Second phase: Calculation of the energy efficiency of the equipment
The energy efficiency will be determined as the ratio between the hydraulic power supplied and the electrical power consumed. 

Third phase: Analysis of the hydraulic operation of the water distribution network
Take topological and operational data from the network and study various water management and distribution alternatives that are more energy efficient.

Fourth phase: Proposal and economic evaluation of corrective measures
Based on all the data analysed, study possible alternatives to improve energy efficiency, assessing in each case both the potential savings of the measure and the costs of adopting it.

The increase in the cost of electricity in recent years has significantly increased the operating cost of these installations. This should encourage us to analyse them and their mechanical and hydraulic operation and determine the potential for improvements that will directly result in significant energy savings.