Energy Metrics and Analysis Tools

There are several types of energy quantities that come up frequently. How do you decide which of these quantities is relevant to your estimation and decision?

Here is a list of types of quantities and their uses

• Density
  • converts volumes to masses
• Energy Density
  • calculates the volume or mass required to generate an amount of energy
  • A gravimetric energy density relates energy and mass
  • A volumetric energy density relates energy and volume
• Efficiency
  • determine how much of one type of energy can be converted to another
• Carbon Intensity
  • calculates the carbon released from a given amount of energy
  • Be aware that the number may be for the mass of carbon dioxide or the mass of carbon.
• Power and Energy Density per Unit Area
  • Often used for renewable energy
  • Power available per area of land or rooftop
  • Energy available per area of land or rooftop (in a given time period)
• Others
  • MPGe the equivalent miles per gallon
  • “charging velocity” miles of range added per hour
  • Equivalent cost of gasoline

Costs

Electricity Costs

Material/Technology	Cost USD per kWh
Solar PV	0.038 – 0.078
Geothermal	0.066 – 0.109
Onshore Wind	0.037 – 0.086
Natural Gas	0.048 – 0.109
Coal	0.071 – 0.173

Source: Lazard LCOE

Mass Density

A density converts a mass to a volume or a volume to a mass.

Material	Density(g/cubic centimeter)
Crude Oil	~0.9
Water	1.0
Air	0.0012
Gasoline	0.740

Gravimetric Energy Density

This is the quantity of energy is released by the conversion (often combustion) of a given mass of the material. Here is a table of the gravimetric (mass) energy densities for a few popular energy storage sources.

Material	Energy Density (MJ/kg)
Gasoline	45
Crude oil	42–44
Natural gas	33–37
Butter	30 – 37
Coal	12–31
Sucrose	17
Wood	14–16
Lithium Battery	0.5

An energy density has different dimensions and different units on the top and bottom. The dimensions of a mass energy density are energy over mass.

This means multiplying by an energy density changes the dimension of a quantity unlike a unit conversion.

Volumetric Energy Density

This is the quantity of energy that is released by a given volume of the material.

Efficiencies

Energy Efficiency

Efficiency

• Whenever we convert energy, we are not able to convert all of it
• A measure of how well a resource is converted
• Defined as useful energy out divided by total energy in

\eta = \frac{E_{out}}{E_{in}}

Example Efficiencies

Device	Efficiency (%)
Electrical generators	70–99
Electric motors	50–90
Gas furnace	70–95
Wind turbine	35–50
Fossil fuel power plant	30–40
Nuclear power plant	30–35
Automobile engine	20–30
Solar cell	5–28
Fuel cell	40–60

Multiplication of efficiencies

• When we want to know the efficiency of a process with many steps, we multiply the efficiencies at each step to get the total.

\eta_{total} = \eta_1 \cdot \eta_2 \cdot \eta_3 \cdots

Energy Efficacy

Efficiencies are usually expressing energy to energy conversions and are dimensionless. Sometimes, we want to express a quantity like the miles traveled per gallon of fuel or the amount of light provided per unit of electrical power. These are called efficacies.

Usually they are a fraction with the quantity of service provided on the top and the amount of energy, power, or fuel input on the bottom.

Power Per Unit Area

something about the DOE use of this metric

Miles Per Gallon Equivalent

is this a unit conversion between electricity and gas?

Equivalent Cost of Gasoline for EV

If we set the cost per mile of an EV and an internal combustion engine vehicle (ICEV) equal, we can solve for cost of gasoline and create an “equivalent” cost of gasoline.

The equation answers the question, at what price of gasoline is the cost per mile of the ICEV the same as that of an EV?

Set equal cost per mile for EV and ICV

• p_e is the price of electricity
• e_{ev} is kWh per mile
• p_g is cost of gas
• e_{ICV} is gal per mile

p_e e_{ev} = p_g e_{ICV} \frac{\$0.13}{kWh} \frac{kWh}{4 mi} = p_g \frac{gal}{30 mi} p_g = \frac{\$0.13}{kWh} \frac{kWh}{4 mi} \frac{30 mi}{gal} p_g = \$0.98\; \textrm{per gal}