Fuels compared

Some introductory notes

With this proliferation has come a blurring of the lines between fuels, energy sources and technologies for storing energy. We are familiar with thinking of petroleum (and its derivatives, petrol [gasoline], diesel, etc.) as a fuel, and of a battery as a way of storing electricity, but both are convenient ways of storing readily available energy.

Coal is a fuel, a coal-fired power station is an energy source; wind is a natural resource, a wind farm is an energy source; a battery is a thing that stores energy for us; but we can get energy from all three and we need to be able to compare their advantages and disadvantages.

There is too much involved here to use just one table, so I have used three. The first table covers the use, portability, dispatchability, storability and abundance of energy sources other than biofuels, which are covered in the second table. The third table deals with environmental aspects and technological maturity.

On this page I have used oil and petroleum as synonyms.

Explanatory notes

Sustainability

For example, I consider the burning of wood as an energy source to be sustainable (so long as trees are grown at the same rate). While burning wood released CO₂ into the atmosphere, the same amount of CO₂ is absorbed again when a new crop of trees are grown to replace the burned wood.

Burning fossil fuels is unsustainable because the rate of natural production of fossil fuels is almost infinitely slower than the rate of use, resulting in a damaging build up of CO₂ in the atmosphere and oceans.

I have used the terms 'portability', 'dispatchability', 'storability' and 'abundance' in Tables 1 and 2. A few words need be said about my use of these terms.

Portability Some 'fuels' are readily portable (for example the various forms of petroleum), others are not (for example, wind). Of course wind can be used to move a sailing boat, but apart from that, if we are to use it as an energy source it is necessary to first use it to generate electricity which can then be delivered to where it is needed, or we can store the electricity (for example, in a battery) by such things as using it to produce hydrogen or ammonia that can be moved around and accessed later. Dispatchability This term is used in the electricity supply industry to describe how readily power generation is increased or decreased to follow changes in demand. Some forms of gas-fired generators are the most flexible, coal-fired generators are only slowly varied, while nuclear is usually run at a constant rate. Output from wind and solar PV installations are normally entirely dependent on how much wind there is or the brightness of the light. Many forms of generation (or forms of energy storage) can be varied, but there is an economic compromise between producing the maximum amount of power as often as posible or producing a smaller amount of power when prices are higher. Storability A power grid has a need for substantial amounts of 'top-up' power from time to time and this need will increase as the amount of renewable energy in power grids increases. But when the capital cost of an energy technology is high there is economic pressure to maximise total energy output rather than just generate for relatively short periods occasionally. The facility to store substantial amounts of energy will become steadily more valuable in power grids. Abundance All fossil fuels are finite; the rate at which they are being replenished by natural processes is thousands or millions of time slower than that rate at which we are consuming them. Some forms of energy – hot dry rock for example – are potentially very abundant, but they may only be economically viable in some areas or they may not prove to be economically viable at all. Biological forms of energy are limited by the amount of suitable space for growing the organisms that produce the fuels.

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Base load

Peak load

It has been found in South Australia, where one in four houses have solar PV power, that the peak demand for the day is tending to be lower and later in the day, because the solar power systems are operating in the earlier part of the peak demand period. (See Renew Economy)

Horizontal drilling

Fuels such as coal seam gas and shale oil are often found in geological formations having low permeability ('tight' formations). That is, gasses and liquids do not readily move through the rocks.

The greater the length of drill-hole in contact with the formation the more gas or oil can seep into it.

Hydraulic fracturing (Fracking)

There are a number of environmental concerns with fracking.

• It can lead to leakage of gas or liquids from the target formation into overlying aquifers, contaminating them;
• It can lead to the escape of methane from the formation into the atmosphere;
• The chemicals may leak to the surface, contaminating soil and waterways;
• Wastes from the operation are sometimes mishandled, leading to contamination of soils and waterways;
• When used to enhance the recovery of petroleum in an old field it may allow carbon dioxide previously injected to escape into the atmosphere.