- freely available
Energies 2017, 10(1), 29; doi:10.3390/en10010029
2. Materials and Methods
2.1. Energy Efficiency Indicators
2.2. Comparison of the High-Intensity Industrialized and Low-Intensity Eco-Agricultural Systems
2.3. Estimating the Potential Energy Efficiency of High-Intensity Industrialized and Low-Intensity Eco-Agricultural System
2.4. Data, Conversion Factors and Assumptions
2.4.2. Conversion Factors
2.4.3. Other Assumptions
- For alternative tractor power options
- While the energy consumed by four wheel drive > 50 HP tractors for all farm operations was obtained from detailed data provided from tractor test runs by Grisso et al. , for other smaller tractor implementations (two wheel drive 20–49 HP, single axle riding type 10–19 HP, ordinary single axle < 9HP), energy for ploughing was assumed to be essentially the same and/or not significantly different from those for all the other seven farm operations undertaken (assuming the required detachable implements needed to achieve the other tasks are available). This assumption was made due to lack of data for fuel consumption for all other operations (aside ploughing) by the smaller tractor implementations. As with animal labour, the assumption and resulting estimates were regarded as conservative because ploughing is the most tedious of all the farm operations. Based on the data obtained, two wheel drive 20–49 HP tractor, single axle riding type 10–19 HP tractor, ordinary single axle < 9HP tractor consume 22.5–28.0 L of diesel, 5.0–6.3 L of diesel and 16.7–25.1 L of gasoline per hectare during ploughing operations respectively. Based on the assumption, they are expected to consume approximately 180.0–224.0 L of diesel, 40.0–50.4 L of diesel, and 133.6–200.8 L of gasoline for all farm operations per hectare annually [39,55,57,58].
- For alternative tillage options
- It was assumed that there were zero energy requirements for natural, rain-fed irrigation. The zero energy consumption for rain-fed irrigation was however substituted with various amounts of energy (mostly electricity) used for powering artificial irrigation options: surface—0.2–6.5 GJ·ha−1, sprinkler—3.9–10.4 GJ·ha−1, drip—3.1–9.5 GJ·ha−1 [60,61]. Note that water is assumed to be pumped directly from surface and ground water. Energy for storage in tanks is not included.
- While the substitution of other agronomic factors was assumed not to result in significant change in maize yield, artificial irrigation was assumed to cause an increase as shown in Table 2.
- Energy for production of N, P and K fertilizers was zero if obtained from animal manure and biogas digestate. This is because they are freely delivered to the field from farm or bio-refinery plant. Only energy for transport and manure spreading was considered as fertilization costs.
- For biogas digestate, energy for lime (CaO) production and application was also zero because it is freely delivered to the field with the biogas digestate.
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|Agronomic Factor||High-Intensity Industrialized Agriculture||Low Intensity Eco-Agricultural Systems|
|Farm power||Mostly large tractor driven (four wheel drive > 50 HP and two wheel drive 20–49 HP tractors)||Smaller tractor implementations encouraged (single axle riding type 10–19 HP and ordinary single axle < 9 HP tractors). Substitution of tractors with human and animal (mostly ox, buffalo, horse, donkey, mule and camel) labour.|
|Tillage||Conventional mouldboard tillage with and without pesticide application||Reduced tillage options (e.g., chisel, disk, ridge plant, and stubble and mulch) and no tillage options (e.g., strip-till and no-till)|
|Fertilizer||Expensive, synthetic (inorganic) fertilizers for maximum productivity and higher profit margins||Cheap, renewable (naturally available and/or organic) waste based fertilizer options such as animal manure, biogas digestates, decaying straws etc.|
|Seed-sown||Hybrid and genetically modified(GMO) seeds for maximum productivity and higher profit margins||Native seeds are encouraged|
|Irrigation||Drip and sprinkler irrigation systems for precision farming||Rain-fed and surface irrigation systems|
|Co-product reintegration||Reintegration of co-products as fertilizers and pesticides not prioritized||Reintegration of co-products as fertilizers and pesticides encouraged|
|Transport distances||21–800 km between input market, farm and plant||10–20 km between input market, farm and plant|
|Agro-Climatic Zones||Tropics—Brazil, Thailand, Philippines, Indonesia||Sub-Tropics—India, South Africa||Temperate—France, United States|
|N-Fertilization rates (kg·ha−1)||8.0–23.9||25.2–75.6||30.0–90.0||5.3–45.1||16.8–142.8||20.0–170.0||13.3–45.1||42.0–142.8||50.0–170.0|
|P-Fertilization rates (kg·ha−1)||30.0–50.0||45.0–80.0||45.0–90.0||45.0–60.0||60.0–100.0||100.0–300.0||53.0–55.0||56.0–62.5||62.5–84.0|
|K-Fertilization rates (kg·ha−1)||0.0–30.0||15.0–60.0||30.0–60.0||30.0–45.0||45.0–120.0||82.5–120.0||56.0–57.0||66.0–85.0||85.0–93.5|
|Maximum attainable harvested grain yield under rain-fed irrigation (t·ha−1·a−1)||1.1–5.1||2.7–8.5||4.6–12.5||1.8–5.8||4.0–8.9||6.3–12.3||1.8–5.3||3.8–8.7||6.1–12.1|
|Maximum attainable harvested grain yield under artificial irrigation (t·ha−1·a−1)||-||3.5–10.5||6–15.6||-||5.3–12.2||6.1–12.1||-||4.9–11.3||6.1–12.1|
|Energy from Co-Products of Ethanol Production (Maize Gluten Meal) [37,38,52,53]|
|Proportion of maize gluten meal per ton of maize (kg·t−1)||36.3–57.0|
|Energy saved by use of maize gluten meal as herbicide replacement (MJ·kg−1)||2.1–4.7|
|Energy saved by use of maize gluten meal as herbicide replacement (MJ·t−1)||76.2– 267.9|
|Energy saved by use of maize gluten meal as N-fertilizer replacement (MJ·kg−1)||43.0–65.3|
|Percentage of N-fertilizer in maize gluten meal (%)||10.0|
|Energy saved by use of maize gluten meal as N-fertilizer replacement (MJ·t−1)||325.1–1749.4|
|Total energy from co-products (ethanol) (MJ·t−1)||401.3–2017.3|
|Energy from Co-Products of Biogas Production (Biogas Digestate) [54,55]|
|Ratio of digestate to biomass (%)||96.0–98.0|
|Energy for producing lime saved by use of biogas digestate (MJ·kg−1)||0.6–1.8|
|Energy for producing N-fertilizer saved by use of biogas digestate (MJ·kg−1)||43.0–65.3|
|Energy for producing P-fertilizer saved by use of biogas digestate (MJ·kg−1)||4.8–32.0|
|Energy for producing K-fertilizer saved by use of biogas digestate (MJ·kg−1)||5.3–13.8|
|Quantity of Lime in biogas digestate (kg·t−1)||0.8|
|Quantity of N-fertilizer in biogas digestate (kg·t−1)||3.7–16.1|
|Quantity of P-fertilizer in biogas digestate (kg·t−1)||1.8–19.8|
|Quantity of K-fertilizer in biogas digestate (kg·t−1)||4.5–32.0|
|Energy from co-products (Lime energy replacement) (MJ·t−1)||0.5–1.4|
|Energy from co-products (N-fertilizer energy replacement (MJ·t−1)||152.7–1030.3|
|Energy from co-products (P-fertilizer energy replacement) (MJ·t−1)||8.3–620.9|
|Energy from co-products (K-fertilizer energy replacement) (MJ·t−1)||22.9–432.8|
|Total energy from co-products (biogas) (MJ·t−1)||184.4–2085.4|
|Agronomic Factors||Effects on NEG||Effects on EROEI|
|Farm power (humans, animal and 10–20 HP tractors)||12.5%–229.4%||7.7%–27.4%|
|Tillage (reduced and no-till)||4.2%–9.0%||7.7%–9.4%|
|Fertilizer (animal manure and biogas digestate)||3.1%–51.2%||7.7%–31.6%|
|Co-product reintegration (biogas digestate, maize gluten meal)||2.1%–724.2%||2.5%–188.9%|
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