TCASE 2: Energy primer

Before getting entangled in the thorny bramble of sustainable energy options, I thought it helpful to arm you with a set of terminological secateurs. So TCASE #2 (recalling that TCASE = the Thinking Critically About Sustainable Energy series) is a brief primer and glossary on energy terms. This is not meant to be anything comprehensive, but it’s enough to get your technical feet wet and to understand some of the units and concepts that are liberally thrown around by those who are used to talking in the energy jargon. (If readers feel I have missed something important [no doubt], please feel free to add this to the comments, and I will also update this post to reflect the important suggestions.)

Anyway, first up, we need to understand the difference between power and energy. Let’s say you have a jug of water. It has some volume, which is the amount of water the jug holds. Now, let’s say you gradually tip out the water — the flow of water (the amount of water being poured per unit time) is a rate. Well, in caricature, the volume of water is like energy, and the flow of water is like power. Not a perfect analogy, but they never are…

Now, when measuring anything, you could use any manner of units. I’m going to consistently stick to SI (Système Internationale) units. If you want to translate back and forth (imperial, metric, nonsensic, etc.), look up the tables here. The basic SI unit of energy is the Joule. The basic unit of power is the Watt (W), which has units of Joules per second (J/s). So, a 60 W incandescent light globe uses up energy at a rate of 60 J/s, or 216,000 J per hour (60 x 3,600 = 216 kilojoules, kJ). Or, to express it another way, in one hour (h) that light would use up 60 Wh worth of energy, and in a day, it’d use 60 x 24 = 1,440 Wh, or 1.44 kWh. So, kWh are a unit of energy.

Energy comes in various forms, such as heat and electricity (the ones that are relevant to TCASE — there are also forms such as ionising radiation, light etc.). Heat (hereafter thermal) energy is considered lower quality than electrical energy — it’s less flexible and difficult to transport — but thermal energy is easier to store. Also, many power production methods, such as coal- or gas-fired, nuclear, geothermal and solar thermal power stations, generate thermal energy and then convert it to electrical energy, in a process that necessarily must throw away waste heat (roughly 2/3 of it) — first used in a practical way by Thomas Newcomen and later improved upon by James Watt. This is commonly done via a steam generator and condenser, although gas turbines are also used. Indeed, combined cycle gas turbines use both a gas turbine (Brayton cycle) and then use the waste heat to power a steam turbine (Rankine or Sterling cycle), which increases their conversion efficiency. Efficiency is strongly affected by the temperature differential, so if (for instance) your steam goes in really hot and then is water cooled, this will be more efficient than if your steam goes in at a lower temperature and then is air cooled. So air cooling saves water, but lowers your efficiency.

Wind turbines are connected (via gearing) to an electrical generator directly, and so avoid the need to first produce thermal energy. Solar photovoltaics also generate electricity without any thermal step, via the photoelectric effect. A hydro or tidal power device will generally use the flow of water to turn a turbine, rather than expanding steam or gas, and an ocean wave generator might pump water to shore at high pressure to turn a turbine. You get the idea.

An important thing to distinguish is the difference between conversion efficiency and capacity factor. You might, for instance, have a nuclear power station that has a conversion efficiency of 38%, but a capacity factor of 92%. What’s the difference? The conversion efficiency is (roughly) the efficiency with which thermal energy is converted into electrical energy through one or more steps. The capacity factor is the amount of energy a power station generates over a given length of time compared to the energy it might have produced if it had been running at full power for the whole period. There is a good explanation of capacity factor on Wiki.

Here, let’s take an example of wind turbines to better explain capacity factor. One of the largest wind turbines yet built is the Enercon E-126 (see picture), which produces a peak power of 6 MWe (that’s 6,000 kWe, where the “e” distinguishes this as electrical energy as opposed to “MWt” for thermal energy). This impressive structure has rotor (blade) diameter of 126 m, and a hub height of 198 m. Let’s say you stuck this on the west coast of the Eyre Peninsula, where it sometimes got strong wind speeds that allowed it to generate its full rating of 6 MW. Other times, the wind would be modest, weak, or calm, at which times it would be generating at less than its peak (nameplate) power. It would also shut off it the wind got too strong in a gale. Now, let’s say you tallied up the energy this turbine had generated over the course of one year at this site, and found it to be 16,820 MWh. If the turbine had generated at full power the whole time, you would have expected it to have produced 6 x 24 x 365 = 52,560 MWh. So, in this case, it’s capacity factor for the year was 16,820/52,560 x 100/1 = 32 %.

Alternatively, let’s say an AP-1000 nuclear power station was rated at 1,154 MWe, and for 11 months it was run at this power output. Then, for one month (say December) it was offline being refueled. It would generate 1154 x 24 x (365-31) = 9,250 GWh for 11 months and for December it would generate 0 GWh. It’s capacity factor would, in this example, be 9,250/10,109 x 100/1 = 91.5 %. And so on, for all the other technologies we’ll be discussing in TCASE.

So, 1 gigawatt (GW) = 1,000 megawatts (MW) = 1,000,000 kilowatts (kW) = 1 billion Watts (W). Solar panels are usually described in terms of their peak kW power. Wind turbines are (these days) usually rated in MW. Nuclear power stations are expressed in MW or GW. Almost universally, their peak (nameplate) electrical power, rather than thermal power or average power (after accounting for capacity factor), is what is reported. So watch out when converting to energy.

Finally, recall I said a W was in units of J/s? A J is a unit of energy. But why then did I start to talk about energy in kilowatt hours (kWh) etc.? Well, this is often a convenient way to express energy (David Mackay chose to use this as his standard), as it’s easy to mentally switch back and forth between power and energy (though there is also the potential to get confused!). Also, J is too small to be of much practical value. But the megajoule (MJ) is a useful value for expressing the energy content of a litre of liquid fuel (for instance), and the petajoule (PJ) and exajoule (EJ) are sufficient for expressing the energy use of nations and civilisations. For instance, the primary energy use (thermal and electrical) of the global human enterprise in 2007 was (very approximately) 500 EJ, which is 138,890 TWh (terawatt hours) — where 1 TW = 1,000 GW. I’m sure by now you’re getting the hang of this!

I like to use EJ and TW when expressing really large energy budgets and power demands — which, incidentally, is the topic of TCASE #3.