Friday, August 01, 2014

Intermittent and Unreliable

Intermittent energy is not necessarily such a bad idea.

This first graph shows the output of a half-wave rectifier that converts alternating current into pulsating direct current. Although it is intermittent, it is reliable: you know that every second 60 (or 50) pulses of electricity will be produced.

Electronic equipment requires constant direct current (not a pulsating one) but it is easy to convert the pulsating electricity into a constant one by the addition of storage.

In the case of power supplies for electronic equipment, this "storage" is usually one or more capacitors.

So, the output, once the "storage" is added looks much more appealing:

In real life, the "ripple" can be of very small amplitude.

The amount of storage needed can be very easily calculated (and implemented) because the energy output of a half wave rectifier is intermittent but completely predictable and reliable.

On the other hand, if the output of a system is not only intermittent, but also unreliable things begin to look more complicated.

Here, as an example, we can see the electrical output of a wind turbine:

We could classify this output as intermittent and unreliable. In this circumstance the amount of storage required is not as easily calculated and unless we pretty much decide to store weeks of power we will end up short at several instances during the year.

Storing vast amounts of electrical power for weeks is a very expensive proposition and that is the reason most renewable energy in the world has back-up, usually fossil fuel plants. Some people actually prefer to call these plants "primaries" because they end up supplying the required power most of the year.

Solar photo-voltaic (PV) is not inherently an intermittent / unreliable technology. For example, in geosynchronous orbit PV provides constant power most of the year (except near the equinoxes) and even satellites in LEO (low Earth orbit) produce intermittent but reliable power (and thus the size of the batteries required for storage is modest as they need to store only a few hours of energy).

The "problem" is that on Earth we have cloud cover and seasons (not to mention variable wind patterns with their respective seasonality in the case of wind turbines).

Conclusion: intermittent / reliable power, with a modicum of storage, can easily supply power continually.  On the other hand intermittent / unreliable power can hardly provide a constant supply even with storage.

Feel free to add to the conversation on Twitter: @luisbaram

Thank you.

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