Diesel or Electric? The Results are Surprising

Overview

A Queensland cotton farm with a large-scale irrigation operation required an energy audit to evaluate the performance of a tail water return pump station and identify opportunities for reducing energy costs. The station, consisting of four 26HBC-40 China pumps—three powered by electric motors and one by a diesel engine—was responsible for lifting irrigation water into a storage dam or directly into irrigation channels. A comprehensive Type 3 Energy Audit was conducted to explore solutions for improving system efficiency and reducing energy expenses.

Challenge

The pump station consumed 70,598 kWh of electricity in the 2022/23 financial year, incurring a total cost of $56,104.90 under SAC Large Tariff 44. However, the highest electricity consumption occurred in 2022, with a total usage of 214,539 kWh, reflecting elevated operational demand during that period. In 2022, the electric pumps operated for 7,305 hours, while in 2023, their usage dropped significantly to 1,552 hours. Despite being operationally sound, inefficiencies were observed, including high water velocities in the suction pipes, wear and tear on pump components, and elevated energy costs. The farm sought strategies to optimise energy use while maintaining system performance.

Proposed Solutions

  • A cost-benefit analysis determined that diesel-driven pumps were more economical for short-term use (less than 256.8 hours per month, as illustrated in the chart). This is primarily due to the structure of electricity tariffs under SAC Large Tariff 44, which includes fixed supply charges and peak demand charges that must be paid regardless of how much electricity is consumed. For instance, even minimal monthly usage triggers a peak demand charge, which significantly inflates costs for low electricity consumption months. In contrast, diesel engines avoid these fixed costs and only incur fuel and maintenance expenses, making them a more cost-effective option when operational hours are low
  • However, for longer operational periods, electric motors proved more cost-effective due to lower perhour running costs compared to diesel fuel. With an existing diesel-driven pump on-site, adding another diesel engine was deemed unnecessary and financially unjustifiable.
  • Increasing the diameter of the suction pipes was evaluated to reduce friction losses and improve efficiency. However, this modification would move the pump’s duty point away from its optimal efficiency range, ultimately decreasing performance. Regular maintenance to mitigate wear and tear was recommended as a more practical solution.
  • A 100kW solar system was assessed, with the potential to reduce grid electricity consumption by 40,009 kWh annually. However, the high capital cost of $100,743 and low self-consumption rates (22%) resulted in a lengthy payback period of 13.9 years and a modest return on investment of 7.2%, making it an impractical option for the farm.

Results

The audit revealed that the pump station was operating within acceptable efficiency limits, achieving an energy usage rate of 4.15 kWh/ML/m head. No major capital investments were recommended, as the existing system was already well-suited to the farm’s irrigation requirements. Key strategies identified included: Using the diesel-driven pump for operations under 256.8 hours per month to minimise electricity demand charges. Operating the electric motors for longer periods to benefit from their lower running costs. Implementing regular maintenance to sustain pump performance and reduce energy losses caused by wear

Conclusion

Through operational adjustments and targeted maintenance, the farm was able to maintain efficient irrigation performance while minimising energy costs. The recommendations provided a flexible, cost-effective framework for managing energy use, balancing the advantages of diesel-driven pumps for short-term needs with the efficiency of electric motors for extended use.

Key Takeaway

This case highlights the importance of tailored energy management strategies for agricultural operations. By thoroughly evaluating options and prioritising maintenance and operational improvements over costly upgrades, significant savings can be achieved. The farm’s approach demonstrates that strategic decision-making can optimise energy use without unnecessary capital expenditure. Saturn Engineering Group remains committed to supporting farms in achieving sustainable and cost-effective energy solutions.