In analyzing and discussing the energy balance of alternative fuel produc­tion, a term called net energy value (NEV) is often used. The net energy value is the difference between the energy content of product ethanol and the total energy used/consumed in producing and distributing ethanol. Higher corn yields of modern agricultural industry, lower energy consumption per unit of output in the fertilizer industry, and recent advances in fuel conver­sion technologies have significantly enhanced the economic and technical feasibility of producing ethanol from corn, when compared with that of just a decade ago. Therefore, studies based on the older data may tend to overestimate energy use (input), because the efficiency of growing corn as well as converting it to fuel ethanol has improved significantly over the past decade [5].

A large number of studies have been conducted to estimate the NEV of ethanol production. However, variations in data and model assumptions resulted in a widely differing range of estimated values (conclusions), rang­ing from a very positive to a negative value. A negative net energy value would mean that it takes more energy to produce the energy content of prod­uct ethanol. According to the study by Shapouri, Duffield, and Graboski [5] of the USDA, the net energy value of corn ethanol was calculated as +16,193 BTU/gal, assuming that fertilizers were produced by modern (1995 or so) processing plants, corn was converted in modern (also about 1995) ethanol facilities, farmers achieved normal corn yields, and energy credits were allocated to coproducts. Updated values for the NEV of corn ethanol by Shapouri et al. show +21,205 BTU/gal (July 2002) [6] and +30,528 BTU/gal (October 2004), respectively [7]. The first value of 21,205 BTU/gal was based on the higher heating value of ethanol, whereas the second value of 30,528 BTU/gal was based on the lower heating value of ethanol. However, another study conducted by Pimentel and Patzek (2005) showed that the NEV of eth­anol was -1,467 kcal/liter (equivalent to -16,152 BTU/gal), which was based on the LHV [8]. A recent study thoroughly conducted by Argonne National Laboratory (2005) shows that ethanol generates 35% more energy than it takes to generate [9].

As shown, sharp differences in the calculated NEV of ethanol produc­tion among the studies still existed and they stemmed from several factors, which were comprehensively identified and directly compared in a report by MathPro Inc. [10]. According to MathPro’s analysis, the differences in the NEV reflected sharp differences in four energy usage categories and they are [10]

1. Energy used in corn production: The USDA estimates (20.2 K BTU/ gal in 2002 and 18.7 BTU/gal in 2004) are about half of Pimentel — Patzek’s (37.9 K BTU/gal).

2. Energy used in corn transport: The USDA estimates (2.1 K BTU/gal in 2002 and 2004) are less than half of Pimentel-Patzek’s (4.8 K BTU/gal).

3. Energy used in ethanol production: The USDA estimates (46.7 K BTU/gal in 2002 and 49.7K BTU/gal in 2004) are substantially lower than that of Pimentel-Patzek (56.4 K BTU/gal).

4. Coproduct energy credit: The 2002 USDA estimate (-13.5 K BTU/gal) is twice that of Pimentel-Patzek (-6.7 K BTU/gal). The 2004 USDA estimate (-26.3 K BTU/gal) is twice the 2002 USDA estimate and four times Pimentel-Patzek’s estimate [10].

The "true" and "actual" value of ethanol’s NEV would depend on vari­ous factors that involve the geographical region, agricultural productivity, efficiency of the ethanol production process, energy efficiency of fertil­izer manufacture, and much more. It has been observed that the ethanol proponents have claimed positive NEVs, whereas the ethanol critics have referred to negative NEVs. As such, this subject has been controversial, from analytical and technoeconomical standpoints. However, it is cer­tain that modern corn ethanol plants use substantially less energy and produce more ethanol per bushel of corn than older plants, and it also appears certain that the claims of negative NEVs have been based on obso­lete material and energy balance data of the corn ethanol industry [11]. In 2008, Mueller conducted a very extensive milestone survey of the nation’s ethanol plants in terms of new energy use and coproduct data as well as land use and his conclusions published in 2010 clearly showed significant improvements over the 2001 data [12].

The 2001 survey by BBI International [42] found that dry mill plants use, on average, 36,000 BTU of thermal energy and 1.09 kWh of electrical energy per gallon of ethanol produced, while producing an average of 2.64 gallons of ethanol per bushel of corn. However, ethanol plants in 2008 used an aver­age of 25,859 BTU of thermal energy and 0.74 kWh of electricity per gallon of ethanol produced, which is 28.2 and 32.1% lower than the values of 2001, respectively. Ethanol produced per bushel of corn, meanwhile, increased by 5.3% to 2.78 gallons per bushel in 2008 [12]. This survey clearly supports that the NEV of ethanol production based on modern technology data is on the positive side.