BU-1007: Net Calorific Value

Since the beginning of time, wood was a readily available fuel to mankind; however during the medieval period, King Henry VIII (1491–1547) was concerned that England could not produce enough wood for heating, cooking and building houses and he urged citizens to conserve. Coal mining in the 1700s lifted this apparent shortage and the abundant new energy source became the nucleus for the Industrial Revolution. But burning large amounts of coal soon began to darken the skies over cities and caused health problems.

In 1859, explorers discovered oil, first in Pennsylvania and then in Texas. By 1900, the Middle East became a key supplier of oil, and after World War I, Mexico, Venezuela and Iran began pumping liquid energy. Oil was cheap, plentiful, easy to transport, safe to use and relatively clean to burn; it soon became the preferred energy resource.

As wood led to coal and coal to oil, scientists turned to nuclear power to generate what was seen as an unlimited pool of energy at low cost. The common nuclear fuels are uranium-235 and plutonium-239, of which plutonium-239 is so powerful that 1kg can produce nearly 10 million kWh of electricity. Science writer David Dietz (1897–1984) wrote, “Instead of filling the gas tank of your car two or three times a week, you will travel for a year on a pellet of atomic energy the size of a vitamin pill.”

In the 1950s, nuclear plants began generating electricity and nuclear-powered submarines and aircraft carriers became common. Amendments were written and the Atomic Energy Act invited the private sector to harness nuclear energy. This was met with a sharp learning curve that led to accidents and meltdowns. The most serious nuclear accidents were Three Mile Island in the USA, Chernobyl in the Ukraine and Fukushima in Japan. The enormity of damage led to slowing nuclear growth and to this day, radiation and disposal of spent fuel remains a problem.

Scientists pointed to hydrogen as the next energy miracle as it has an unlimited supply and is clean. Cars powered by the hydrogen fuel cells would run so clean that the hot water from the tailpipe could be used to serve tea. But hydrogen is expensive to produce because it takes as much energy to create as it delivers. After much anticipation, hydrogen became a pipedream.

Much of the global energy comes by burning hydrocarbons in the form of petroleum, natural gas and coal that are leftovers of living matter from past geological times. The sun, the source of all life, provided these canned energies but they are non-renewable. Figure 1 illustrates the fuels used to generate electricity. Coal, the most common fuel, produces the highest amount of CO2; natural gas is about half that of the coal equivalent, and oil sits somewhere in between.

Coal is cheap but emits about twice the CO2 of natural gas. The CO2 emmission of oil is in between coal and natural gas.

Table 1 lists the net calorific value (NCV) and efficiency of various energy sources in Wh per liter. Diesel and gasoline overshadow hydrogen and the Li-ion battery in terms of NCV. Any departure from a simple combustion process to harvest energy is met with higher energy costs, but the gain must be offset with the benefit of generating less greenhouse gas (CO2).

Diesel and gasoline surpass hydrogen and Li-ion. The conversion efficiency is thermal output and does not include friction and drag.

* CNG (compressed natural gas) is 250 bars (3,625psi)
** Hydrogen is at 350 bar (5,000psi)

Table 2 provides a summary of the net calorific values of ancient and modern fuels by mass (kg) and volume (liter). With the exception of hydrogen by mass, hydrocarbons offer the highest energy by weight.

* Hydrogen has the highest energy to mass ratio (Wh/kg), but energy by volume (Wh/l) reveals a truer picture in terms of storage and delivery. Diesel has almost 14 times the specific energy of pure hydrogen by volume (750Wh/l at 350 bar or 5,000psi)

Oil and natural gas can be drawn from the earth cheaply and with little preparation. Hydrogen, in comparison, needs energy to be produced and it is hard to store. Economics are a deciding factor when choosing a fuel for heating and mobility. This puts environmental issues on the backburner. Fossil fuel is among the cheapest, most efficient and readily available fuels, but the ecological harm when consumed in large scale is beginning to get everyone’s attention.

References

[1] Courtesy: Internal Energy Agency