Environmental studies and Forestry/food energy
This small project entails students estimating the edible food energy produced by a certified organic farm that operates in Chittenden County, Vermont, and the total energy required in that farm’s operation. This information will be used to calculate the farm’s energy efficiency (edible energy output divided by energy input). Students will turn in a short report (roughly 500 words) that includes:
Their estimate of the total edible kilocalories the farm produced during the year of operation represented by the data
Their estimate of the total energy input required for farm operation as derived from the provided data
Their calculation of the farm’s energy efficiency, which divides edible output energy by input energy
A short reflection on the process of doing these calculations and what, if anything, surprised them about the results
Data for this assignment can be found below. Below you will also find some basic instructions to get you started on this assignment. You can work in small groups of up to three people for this assignment, although you do not have to. Please feel free to ask questions before or during class, or via email if you need additional assistance. If you would like to read the original references where the conversion factors supplied came from, let me know and I will provide them.
To translate the marketed products into energy values, you will need to look up the calorie density of these vegetables and convert their weight (given in pounds) to kilocalories (kcal). Realize that when you look up the calorie contents of most foods in the Internet, the value given will say ‘calories’ but it is actually kilocalories, which are 1,000 calories. All nutritional calories are actually kilocalories. Real calories are a very small unit of heat that is only used by chemists and physicists.
To translate the gallons of gasoline and diesel used into energy, you will need to convert their volumes to an energy value using their volumetric energy densities. The volumetric energy density of gasoline is 29,000 kcal/gal, and for diesel it is 32,000 kcal/gal. In addition to having energy stored in their chemical bonds, these fuels also require energy to manufacture. To account for the energy required to manufacture gasoline and diesel, multiply the gasoline energy you calculated by 0.2 and add this result to the gasoline energy to get the total energy associated with gasoline. For diesel, multiply your initial figure by 0.3 and add that result to get the total energy associated with diesel.
Convert the provided electricity usage in kilowatthours (kWh) into kilocalories using the conversion factor 860 kcal/kWh. Like gasoline and diesel, electricity requires energy to make and we must account for this indirect energy in our calculations. To do this, multiply the energy value of the electricity used by 1.3 and add the result to the initial value to get the total energy associated with electricity.
To convert hours of human labor into an energy value, assume that the average farm laborer burns 110 kcal per hour of labor. Like gasoline, diesel and electricity, there is also indirect energy associated with providing human labor, but these calculations are very complicated and we will ignore them for this assignment.
Vehicles and machinery require energy in their manufacture and maintenance. We will estimate the indirect energy associated with farm vehicles and machinery in a very simplified way, by using a conversion factor that relates their mass to an energy value. For this project, assume that all vehicles and machinery require 6,500 kcal/lb in their manufacture and maintenance. It is not reasonable to count all of the indirect energy use associated with a vehicle or a piece of machinery’s manufacture against one year of a farm’s output because that vehicle or machinery will be used for multiple years. To spread out the indirect energy over multiple years, please divide the estimate you get by 20 years (an average lifetime for well cared for vehicles and machinery) and use this smaller number in your calculations.
Lastly, you will need to include estimates of energy used indirectly in the manufacture of farm inputs. For nitrogen fertilizer, use the conversion factor 5,500 kcal/lb to convert the weight of fertilizer used into an estimate of the energy required to manufacture it. For potassium fertilizer, use the conversion factor 750 kcal/lb. For phosphorus fertilizer, use the conversion factor 490 kcal/lb. Finally, assume that it takes 1,100 kcal of energy to manufacture 1 pound of industrially-produced compost.
Marketed Products (pounds)
Bok Choi (2,395)
Cayenne peppers (46)
Mustard greens (965)
Nappa cabbage (520)
Sweet potatoes (359)
Diesel: 990 gallons
Gasoline: 45 gallons
Electricity: 67,000 kWh
Human labor: 5,794 hours
Transport Vehicles and Farm Machinery (assume 20 year lifetime)
Delivery van (5,040 pounds)
Pickup truck (3,200 pounds)
Pickup truck (3,000 pounds)
Tractor with mower (2,500 pounds)
Tractor (4,800 pounds)
Large tractor (11,300 pounds)
Potassium fertilizer (500 pounds)
Phosphorus fertilizer (200 pounds)
Nitrogen fertilizer (200 pounds)
Compost (109,000 pounds)