Let’s Clear the Air…Energy Content December 2012

Monday, December 31st, 2012 Technical Articles

Let’s Clear the Air…Energy Content December 2012 By Derek Johnson Alternative fuels have been around for decades, but have been increasing in popularity as energy security, the economy, and consumer awareness continue to grow. The goal of this article is to clear the air on the topic of energy content. Most concerned citizens may not necessarily be concerned directly with fuels’ energy content. Instead, they may be concerned about vehicle driving range and operating costs, which are most often associated with the total at the pump. However, energy content is an essential component of automotive study. Energy content was recently discussed at the Clean Cities Learning Program’s Petroleum Reduction Technologies regional pilot training in Charleston, WV. Energy content is the amount of chemical energy contained in a certain amount of a fuel. Most often, the energy content is represented by energy per unit mass. Examples include Btu/lb, kJ/kg, and others. However, it is also important to understand that energy content may be represented on a volume basis such as Btu/gallon or kJ/gallon. Yes, units are often mixed.

For anyone that may not know the exact definition of kJ or Btu, check out the following definitions: A kJ is a kilojoule or 1,000 joules. This is the energy required to raise the temperature of 239 grams of water by 1 °C. Btu is a British thermal unit. This is the energy required to raise the temperature of one pound of water by 1 °F. To convert a Btu to kJ, multiply by 1.055. To convert a kJ to Btu, multiply by 0.948. To convert from lbs to kg, multiply by 0.454. To convert kg to lbs, multiply by 2.2. This article will look at the energy content of six alternative fuels in comparison with conventional ‘gasoline’ and ‘diesel’. However, both gasoline and diesel have variations in energy content based on blends. Most gasoline sold in the U.S. may have up to 10% ethanol by volume. Pump diesel fuel may also contain a small concentration of biodiesel, usually a few percent. The values presented here are for ‘pure’ gasoline and biodiesel. Note: The energy contents presented here are based on the Lower Heating Value (LHV) for each fuel. The density of diesel is approximately 7.15 lbs/gallon. The density of gasoline was taken to be 6.3 lbs/gallon. The density of biodiesel was taken to be 7.34 lbs/gallon. The density of propane was taken to be 4.2 lbs/gallon in the liquid state at 60 °F. The density of natural gas was taken to be 1.07 lbs/gallon at 80 °F and ambient pressure. The density of hydrogen was taken to be 0.084 kg/cubic meter at 68 °F and ambient pressure. An average density for natural gas was taken to be 0.75 kg/cubic meter at 68 °F at ambient pressure. The density of E85 was taken to be 6.5 lbs/gallon.

Table 1: Energy Content of Fuels.

⁺Ethanol or E85 can be up to 85% ethanol with the remainder being conventional gasoline. However, when pure ethanol leaves a plant it is denatured typically with a few percent gasoline. This usually yields an upper concentration of 83% as opposed to the more obvious limit of 85%. The concentration can be as low as 51% and anywhere in between. The lower concentrations are used to improve cold weather properties.

* The energy content varies by what is used to produce electricity. However, electricity is energy and not necessarily a fuel. Most of us purchase energy based on the kW-hr or kilowatt-hour. This is sometimes also shown as kWh. This is the measure of using energy at a rate of 1kJ per second over the time period of one hour. This is equal to 3,600 kJ of energy. There are 3,414 Btu/kw-hr of electricity.

So from the Table 1 above we can create Figures 1 and 2. Figure 1 compares the energy content of the fuels by mass, while Figure 2 compares the energy content by volume.

Figure 1: Energy Content by Mass.

Figure 2: Energy Content by Volume.

As is seen from Figure 2, the energy content by gallon varies significantly between gaseous and liquid fuels. The propane in Figure 2 is a liquid; this is usually accomplished by low pressure tanks. The tank pressures are typically on the order of a few hundred pounds of force per square inch (psi). The pressures of diesel, gasoline, biodiesel, and ethanol are ambient pressures, as these fuels do not require onboard pressurization. The largest disparity is for natural gas and hydrogen, which are shown at near ambient temperatures and pressures. Natural gas is typically stored 3,600 psi in order to increase its volumetric energy density. This allows for a more comparable vehicle range compared to liquid fuels. Hydrogen is usually stored onboard at pressures of at least 5,000 psi to increase its volumetric energy density. The different fuels have various energy contents by mass and volume. These fuels are stored at various pressures and typically have varying prices. In order to better make a fair comparison between fuels, you may run across the term gasoline gallon equivalent (GGE). Table 2 lists each fuel and how many equivalent units of its fuel yields the energy content in 1 gallon of conventional gasoline.

Table 2: GGE for Various Fuels.

But for natural gas and hydrogen, how much volume will it take to hold 1 GGE? For hydrogen at 5,000 psi and near ambient temperatures, it will take a volume of about 9.3 gallons to have the same energy as a volume of 1 gallon of gasoline. For natural gas at 3,600 psi and near ambient temperatures, it will take a volume of about 3.5 gallons to have the same energy as a volume of 1 gallon of gasoline. GGEs of alternative fuels are handy for a few reasons. One, they can help to better compare the prices of the fuels based on energy content instead of units purchased. Two, they can help to explain why vehicles may travel different ranges when switching fuels. For the second reason, we will introduce the Miles Per Gallon Equivalent (MPGe) measurement. For the first reason, we can walk through an example on how to estimate the GGE cost of electricity to charge our new battery electric vehicle (BEV). If we were to purchase a gallon of gasoline, we would expect to pay an average price of $3.85 this week. If you look at your electric bill, you may find that you are currently paying an average price of $0.12 per kW-hr. To find out the price of a GGE of electricity, multiply 33.70 kW-hr/GGE by $0.12/kW-hr. This yields a GGE price of about $4.04. So in this case, electricity would cost about $0.19 more per GGE than gasoline. This may make many people shudder and say, why would we switch to a BEV? Well, remember, if we were ‘filling up’ our Nissan Leaf as an example, we would be getting a fuel economy of up to 106 MPGe in the city. That’s 3-4 times higher than most conventional vehicles. This example shows where the savings may occur. This understanding of energy can also help when understanding battery size and BEV range. Let’s stick with the Leaf as an example. Instead of having a gas tank with a volume like conventional vehicles, it has a battery with a capacity. The capacity of batteries is usually measured in kW-hr. The Leaf’s lithium ion battery is rated at 24 kW-hr. We know that a GGE of electricity is about 33.70 kW-hr. So if the math is correct, that means that a Nissan Leaf only has about the equivalent onboard energy as about 0.71 gallons of gasoline. It seems really low. But again, the fuel economy of this vehicle can be as high as 106 MPGe. Let’s multiply 106 by 0.71 and see what we get. Well, that number would be about 75.26 miles. If you look at Nissan’s website, they advertise a range of up to 73 miles. This number is slightly lower because of combined highway and city MPGe estimates. Now that we have touched on MPGe again in example one, let’s look at the second reason why everyone should understand GGE. This relates to the driving range of an alternative fueled vehicle. Let’s say you just purchased a new 2012 flexible fuel sport utility vehicle. When filling up on conventional gasoline, you were only getting about 22 miles per gallon, conventional MPG. However, you spot an E85 refueling station and decide to fill up your empty tank on ethanol. If the blend is around 83% ethanol, your driving range will decrease. If your FFV has a tank volume of 25 gallons you were previously able to drive about 550 miles. However, it takes 1.3 gallons of E85 to yield the same energy as 1 gallon of gasoline from the above GGE table. Since the energy content decreased but the fuel tank stayed the same size, you can estimate that you will only be able to drive around 420 miles or get about 17 MPG. So, why would a customer want to run E85 when there is a decrease in fuel economy and range? Well, remember that it has lower energy content by volume so the vehicle is not running any less efficiently or poorly. In fact, research may soon yield FFVs that are able to improve fuel economy when running on higher blends of ethanol. So if you decide to fill up on E85, don’t be alarmed if your ‘MPG’ drops. The MPGe should stay about the same. Remember from above⁺ that E85 can be a blend of anywhere from 51-83% ethanol, so real world values may vary if you do not know which blend you actually purchase. Hopefully, this article has shed some light on GGE and MPGe. Both of these terms are being seen more and more with the increasing numbers of available alternative fuel vehicles. It is better to have a basic understanding of these differences in order to understand how and why these various vehicles perform differently based on fuel type.