Batteries against Fossil Fuel

NOTE: This article has been archived. Please read our new "Cost of Power" for an updated version.

Lifting off in a large airplane is always exhilarating. At a full weight of almost 400 tons, the Boeing 747 requires 90 megawatts of power to get airborne. Takeoff is the most demanding part of the journey, and when the plane reaches cruising altitude the power consumption decreases to half.

Powerful engines were used also when the mighty Queen Mary was launched in 1934. The 81,000-ton ocean liner measuring 300 meters (1,000ft) in length was propelled by four steam turbines producing a total power of 160,000hp (120 megawatts). While in service, the ship carried 3,000 souls and traveled at a speed of 28.5 knots (52km/h). The Queen Mary is now retired in Long Beach, California.

Large propulsion systems are only practical with internal combustion engines, and fossil fuel serves as a cheap and readily available power source. Low energy-to-weight ratio in terms of net calorific value (NCV), as well as a relatively short life span, makes batteries unsuitable beyond a given application. While fossil fuel delivers a NCV of 12,000Wh/kg, a manganese type lithium-ion battery offers 120Wh/kg, which is one hundred times less per weight. Even at a low efficiency of 25 percent, the internal combustion engine outperforms the best battery in terms of energy-to-weight ratio. The capacity of a battery would need to increase twenty-fold before it could compete head-to-head with fossil fuel.

Another limitation of battery propulsion over fossil fuel is the fuel by weight. While the weight diminishes as it is being consumed, the battery has the same deadweight whether fully charged or empty. This puts limitations on EV driving distance and would make the electric airplane impractical. Furthermore, the combustion engine delivers full power at freezing temperatures and continues to perform well with advancing age, a trait that is not achievable with the battery. A battery that is a few years old may deliver only half of the rated capacity.

Power from Primary Batteries

Energy from non-rechargeable batteries is most expensive in terms of cost per kilowatt-hours (kWh). Primary batteries are used for low-power applications such as wristwatches, remote controls, electric keys and children’s toys. Military in combat, light beacons and remote repeater stations also use primaries because charging is not practical. Table 1 estimates the storage capability and cost per kWh of primary batteries.

Table 1: Energy and cost comparison of primary alkaline cells. Energy from primary batteries is most expensive; cost increases with smaller battery sizes.

Power from Secondary Batteries

Improved runtimes, lower unit price and the convenience of recharging have shifted many portable applications previously reserved for primary batteries to rechargeable batteries. Table 2 compares the cost of power with rechargeable batteries. The cost is based on battery price and the number of possible discharge/charge cycles. The analysis does not include electricity for charging or the cost of purchasing and maintaining charging equipment. The table compares commercial battery packs used for communications, computing or medical devices.

Table 2: Energy and cost comparison using rechargeable batteries. Older technologies have lower cost/kWh than newer systems; larger cells are most cost-effective. The costs are commercial packs at estimated over-the-counter prices.

* Cycle life is based on battery receiving maintenance.

Power from Other Sources

With dwindling fossil fuel supply, governments and the private sector are studying alternate energies. Table 3 compares the cost to generate 1kW of power by taking into account the initial investment, adding the consumption of fuel and including the eventual replacement of the system. Power from the electrical utility grid is most cost-effective; consumers in industrialized countries pay between $0.05 and $0.25US per kWh. (The typical daily energy consumption per household is 25kW.) Gasoline (and equivalent) is the most economical portable fuel.

Table 3: Cost of generating 1kW of energy
The table includes the initial investment, fuel consumption, maintenance and eventual replacement of the equipment. The figures are estimates at the time of writing.

The fuel cell is most effective in converting fuel to electricity, but high equipment cost makes this power source expensive in terms of cost per kWh. In virtually all applications, power from the fuel cell is considerably more expensive than from conventional methods.

We now look at the energy that our bodies consume. An active man requires 3,500 calories per day to stay fit, which relates to roughly 4,000 watts in a 24-hour day (1 food calorie = 1.16 watt-hour). Traveling on foot covers about 40km (25 miles) per day and a bicycle increases the distance by a factor of four to 160km (100 miles). Eating two potatoes and a sausage for lunch can propel a bicyclist for the entire afternoon, covering 40km (25 miles), as I have experienced myself. The human body is amazingly efficient in converting food to energy.

Table 4 compares the energy per passenger/kilometer for a loaded Boeing 747, the retired Queen Mary ocean liner, a gas-guzzling SUV, a fit person on a bicycle, and a person walking on foot. The figures are estimated.

Table 4: Power needs of different transportation modes. In terms of high-speed transportation, air travel consumes theleast amount of energy per passenger-km. The boat is efficient for slow and heavy freight. The absolute lowest energy consumption is the bicycle.

* 1 joule is the energy of 1A at 1V for 1 second, or 1 watt/s, or 0.238 calorie/s; 4.186 joules raise the temperature of 1g of water by 1°Celsius; 1,000 joules are 0.277Wh.

Bicycles are by far the most effective form of transportation. Comparing the energy consumption of a bicycle to that of a car, a cyclist would consume only 0.4 liter of fuel per 100km (630mpg). Walking is also efficient; it uses about one liter per 100km (228mpg). The problem with self-powered propulsion is the limited travel range before fatigue sets in.

In terms of energy usage, cars are one of the least efficient modes of transportation. The internal combustion engine utilizes only 25 percent of the net calorific value from the fuel for propulsion. The calculation looks even worse when taking into account the weight of the vehicle with a single passenger, the driver. The ratio of machine to man is typically ten-to-one. When accelerating a 1.5-ton vehicle, less than two percent of the energy moves the 75kg (165lb) driver, his briefcase and the lunch bag; 98 percent goes to heat and friction. Even a modern jet plane has better fuel efficiency than a car. A fully occupied Airbus 340 gets 3.4l/100km (70mpg), cruising at 950km/h (594mph).

Trains are one of the most efficient modes of transportation. The 36km Yamanote circle line connecting major urban centers in Tokyo carries 3.5 million passengers per day. During rush hour, the 11-car train runs every 150 seconds. Such a passenger volume would be unthinkable by private cars on city streets. Trains are also economical to move freight. Transporting one ton of freight consumes only 0.65 liters of fuel per 100km (362mpg).

Affluent societies want personal transportation, but with a large critical mass driving vehicles on government-funded highways with minimal contribution by the drivers and without mandated limits, this free-roaming lifestyle is taking a toll on our energy resources. Developing countries also desire personal transportation. As car become affordable to them, they will begin consuming fossil fuel too and this will increase the need for hydrocarbons further. According to the US Department of Energy, 71 percent of the oil consumed in the USA is for transportation. Out of this, 51 percent goes to passenger cars and light trucks. Smaller vehicles and the development of efficient rail systems could cut the energy for transportation in half. Read more about Free energy... Almost!

Alternate Fuels for Transportation

Governments are exploring ways to reduce the dependency on fossil fuel and to lower emissions. They do this by promoting the electric car. This is done in good faith, but looking at Figure 5 we may be facing an impossible task. Many readers will agree that the success of personal transportation was only made possible with the abundance of oil at very low price in terms of net calorific value. The notion of driving a large vehicle for long distances may not be transferable with battery propulsion, even with government subsidies. Today’s batteries are weak contenders against petroleum, and the chart below demonstrates this. Li-ion, the battery choice for the electric vehicle, is hardly visible; the 90 percent efficiency of the electric motor does not make up for the low net calorific value.