Energy Model

Each player in a Fields of Fuel game has an energy balance that calculates the energy used by a player for his or her choice in farming practices, along with the energy produced by crops that are used for ethanol production. Similar to the emissions calculations, the energy balance is broken down into two categories—those produced during the agricultural production phase (ag phase), and those produced during the conversion of the feedstock to ethanol (refinery phase). This provides players with a better sense of where in the production line the emissions are coming from, and how their decisions affect these sources.

Ag Phase

The ag phase energy calculations are derived from six different actions: tilling, planting, fertilizing, pesticide application, harvesting, and transportation of the crop to market. Pesticide and fertilizer have energy costs both for production and application. For simplicity, it is assumed that pesticides are applied equally across all management options (applied at ~1kg/ac). These are only applied when appropriate; ie. there are no tilling emissions if the player does not till. Planting and harvest phases are applied each year for corn, while they may not be necessary each year for switchgrass or alfalfa. The energy cost of these actions is as follows:

Action Energy Cost model parameter
Tilling 1816.23 MJ/ac Etill
Planting 466.31 MJ/ac Eplant
Fertilizer Application 204.77 MJ/ac Efert,app
Fertilizer Production 60.04 MJ/kg fertilizer Efert,production
Pesticide Application 63.74 MJ/ac Epest,app
Pesticide Production 345.76 MJ/ac Epest,production
Harvesting 232.29 MJ/ac Eharvest
Transportation 0.256 MJ/kg yield Etransport

The total energy input for the ag phase (Eag) can be calculated by summing all of the appropriate values listed in the table. A formulaic representation of this can be constructed using binary variable P that reflects if a particular action is executed that turn (note that pesticide and fertilizer actions are assumed to be activated every turn), and a variable λ that represents the quantity of fertilizer and pesticide produced:

Eag = Ptill * Etill + Pplant * Eplant + Pharvest * Eharvest + Efert,app + Epest,app + Efert,production * λfert + Epest,production * λpest

pesticides are assumed to be applied at ~1kg/ac, while fertilizer application rates are specified in the game as either high or low, with the specific quantity dependent on the crop being planted:


Crop High Fertilizer Application Rate Low Fertilizer Application rate
Corn 80.9 kg/ac 40.5 kg/ac
Switchgrass 24.3 kg/ac 12.1 kg/ac
Alfalfa 12.1 kg/ac 0 kg/ac



Refinery Phase

The refinery phase reflects the process of converting the raw materials from harvest to ethanol fuel. There are two types of feedstocks (or plant materials) that can be used for this process; the sugar rich corn grain (or kernels), or cellulosic sources, which are generally any plant matter that isn't corn grain. Only the corn and switchgrass crops go to producing ethanol, while the alfalfa is used entirely for feed. If dynamic markets are activated, a portion of the corn can also go to feed, and is therefore not used for fuel production (see the economy section for more info). The entirety of switchgrass yield is processed as a cellulosic feedstock, while the total corn biomass is 50% grain and 50% stover (cellulosic). Half of the stover is left on the field to resupply nutrients and prevent erosion, while the other half is sold at market. This is important as cellulosic- and grain-based feedstocks have different conversion rates and energy requirements. Conversion of feedstocks to ethanol can be calculated as

λfeedstock = Yfeedstock * Cfeedstock

where λfeedstock is the amount of ethanol produced by a farm for a given feedstock, Yfeedstock is the amount of feedstock converted (obtained from the yield function), and Cfeedstock is the feedstock-specific rate at which the feedstock is converted to ethanol (0.38 liters/kg for cellulose, 0.4 liters/kg for corn grain).

The refinery process requires a relatively large energy input, primarily for the distillation process. More fuel is produced per unit biomass for sugar-rich sources (such as corn grain), but production of cellulose-based ethanol minimizes the energy required from outside sources, as much of the byproduct can be burned to supply some of the necessary heat energy. The total amount of energy used during the refining process (EU,refinery) can be calculated as:

EU,refinery = Σ(EU,feedstock * λfeedstock)

where EU,feedstock is the energy usage rate for each litre of ethanol produced for the different feedstocks (12.58 MJ/l of corn ethanol, 3.1 MJ/l of cellulosic ethanol) and λfeedstock is the amount of ethanol (in liters) produced from each feedstock.

The amount of energy produced during the refining phase (EP,refinery) is a function of the total amount of ethanol produced summed across all feedstocks (λ), the energy content of the ethanol (Eethanol, we assume a standard energy content of 21.2 MJ/l), and a coproduct credit for each feedstock  (C, 4 MJ/l):

EP,refinery = λ * (Eethanol + C)

This provides an estimate of the gross energy gained. For the net energy gained from the refining process (Erefinery), we simply subtract the amount of energy used (EU,refinery) from the energy produced (EP,refinery):

Erefinery = EP,refinery - EU,refinery


Game Values

In the game, energy values are presented at both the farm and the field level. At the farm level, the total energy produced is graphed in the "Energy" window on the "Energy" tab, and reflects the total net energy produced by the farm (in GJ) summed across all fields, calculated as

E = Erefinery - Eag

At the field level, more detail is provided; total energy expended during the entire crop lifecycle (Ein) is calculated as

Ein = EU,refinery + Eag

The gross amount of energy produced at the field level (Eout) is also shown, and is calculated as

Eout = EP,refinery

The net energy gained (or lost) at the field level (Enet) is also displayed; this is just the combination of the refinery and ag phases:

Enet = Erefinery - Eag