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.

Solar is Cheaper than Ever

Several CEO's of solar companies were interviewed today in the WSJ to discuss the amazing price decreases of solar technology in the past few years.

First they began by stating that solar went from being among the most expensive energy sources to one of the least expensive.

At the beginning of their presentation they mention prices below one dollar per watt but then one of them corrects this information by stating that once installation, inverters, etc., are included the total price for a residential system is in the range of 4 to 5 US dollars per watt.

What was completely overlooked, however, was that on AVERAGE a solar installation produces power only 20% of the time (in a good and sunny place on Earth).  Thus for 80% of the time OTHER energy sources have to generate the needed electricity.

The above means that solar is always a surplus investment.  No matter how much solar is installed, we cannot remove any conventional generating capacity because aside from the obvious fact that at night there is no sun, a cloudy day can easily reduce the output of a solar installation by 90%.

Sure "if solar could be stored" the situation would be different, but here we have two issues:

1. Storage is extremely expensive and thus would drastically change the economic equation of solar.

2. We have to decide for how long we want to store it?  For a day? For a week? For three months? (Germany, the country with the most installed solar capacity has experienced its darkest winter in 43 years**).  Obviously, storage costs grow exponentially with the number of hours / days required to be stored.

If solar is somewhat viable today it is only because it almost fully depends on the conventional electrical grid to mask its intermittent nature. Furthermore, the costs quoted by the CEO's today do NOT include the additional costs the grid must incur to support the fluctuating nature of solar.

Conclusion: the money invested in solar could be better used, for example, in efficiency improvements and nuclear power.

** Spiegel: February 26, 2013.