How cigarette smoking could help save the planet... sort of...

Ok... not smoking per-se, but the 12V plug they built into every car to run the cigarette lighter.

Many of the electronic items in the home do not require 110V AC. If it has a big plug adapter or a power brick attached, the item is converting the 110V AC down to some smaller (something around 5 - 12V) DC current. Those conversions are not perfect and energy is lost.

What if you recovered those losses by having a single efficient conversion instead of many less efficient ones (or required that all converters be more efficient)?

Assuming that computer power supplies are representative of overall power supply conversion efficiencies, the average AC/DC converter is about 75% efficient. On the upper end, you can get ~90% efficient converters. So say a 15% difference in conversion efficiency between several average converters and one efficient one.

According to the DOE End Use Consumption of Electricity Survey (2001 - latest data), about 15% of a house hold's electricity usage goes to things that could, plausibly, run on 12V DC power. That was about 173.5 Million MWh of electricity. Improve that by 15% and you save 26 Million MWh of power.
The average output of a coal fired power plant is about 233 MW. So, for simplicity, if you assume 100% duty cycle, that 15% efficiency boost becomes the equivalent of 12.7 coal fired power plants.
While that is maybe not world saving (<1% of the 1445 coal fired plants in operation in 2008), that is ~5.5x the total Solar (thermal + PV) generating capacity in 2008... a bigger impact than solar has gotten us so far.

Where DC wiring, rather than just more efficient converters, might be more important is in Solar PV on homes. In that case, the DC output of the panel is converted to AC before being used in the home. That step has about a 90% efficiency.
However, many appliances then convert the AC right back to DC with all the losses noted above. Even with 90% efficient converters, that's a 19% efficiency loss over using the electricity directly from the panels.
Viewed another way, the AC wiring in your house is raising the price of Solar PV by almost 20%!

photo credit: David Baker (Flickr)

Solar Thermal in the Desert - A Design Optimization Problem

Solar thermal power is an efficient, well proven method for generating power from sunlight by concentrating sun light in order to heat up a working fluid that, in turn is used to generate steam to run turbines. You need to put these things where there is a lot of sun. Such places are usually hot and dry... e.g. deserts.

But, running a power plant typically requires significant amounts of water to run and cool the turbines.
This 2002 report summary puts the number for wet cooling at around 15,000 gal / MWh.
Fortunately, there are technologies to use much less water: 200 - 250 gal / MWh, a >90% drop. However these technologies are:

• Expensive: ~7x to 17x the wet cooling system cost 
• Less efficient: 
• power output must be reduced at times when the ambient temperature exceeds the design temperature (to avoid damaging the turbines) 
• Output of the heat cycle drops when the output temperature is high.

So, in this context, when you hear about a project like Ivanpah, a 400MW solar thermal plant to be put in the CA desert, near (~60miles from) Death Valley National Park, you should be wondering where the water will come from.


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Fortunately, BrightSource Energy is using dry cooling so that they "Will use 100 acre feet [~32 million gal] per year, the equivalent of 300 homes’ annual water usage."  If every power plant did this, following the logic of my previous post,  it would:

• easily beat the ~20% improvement you'd need to be equivalent to all buildings being LEED 3.0 for water efficiency
• would be enough to meet the ~46% improvement you'd need to replace all the water used domestically and industrially.

So at least you know where much of the $1.4B in loan guarantees from the DOE must be going and it's a good thing.