Glück Auf is loosely translated as "good luck to us all".

Saturday, August 18, 2012

Water shortages hit US power supply - New Scientist

Here's another reason to get off the grid and find more ways to become self-sufficient. I had no idea how much water goes into energy production!

Source: New Scientist

As the United States' extended heat wave and drought threaten to raise global food prices, energy production is also feeling the pressure. Across the nation, power plants are becoming overheated and shutting down or running at lower capacity; drilling operations struggle to get the water they need, and crops that would become biofuel are withering.

While analysts say the US should survive this year without major blackouts, more frequent droughts and increased population size will continue to strain power generation in the future.

Tuesday, August 14, 2012

Design and Sizing Decisions for Solar Power Systems

I have reached a critical point in my Airstream renovation where it is necessary to make some very important design decisions for my electrical systems. I am trying to consider as many factors as reasonably possible but I have to prioritize safety, efficiency and cost.

Design Decision #1: What voltages will my system support?
Options: 12/24/48 Volts DC and 110/220 Volts AC

    Here are some estimates I have found for typical AC inverter cutoff voltage:
  • 12 volt battery, 11.5 volt minimum operation voltage, 10.5 volt cutoff;
  • 24 volt battery, 23 volt minimum operation voltage, 21 volt cutoff;
  • 12 volt battery, 46 volt minimum operation voltage, 42 volt cutoff

Lets assume that we want 100 amps at nominal voltage.

    Calculate the most optimal watts for each voltage at 100 amps:
  • 12v * 100 amp = 1,200 watts;
  • 24v * 100 amp = 2,400 watts;
  • 12v * 100 amp = 4,800 watts

AC inverters are "constant power" devices. Ohm's Law says power = voltage * current. If you run the inverter at minimum voltage it will draw more current. So the wiring fuses and all electrical components must be designed 125% of the minimums for NEC safety rating. You also have to remember that inverters are ~85% efficient.

    Calculate 125% of the Minimum Amps based upon Volt Cutoffs and Watts, at 85% efficiency:
  • 1,200 watts * 1/0.85 * 1/10.5v cutoff * 1.25 = 168 Amps minimum
  • 2,400 watts * 1/0.85 * 1/21v cutoff * 1.25 = 168 Amps minimum
  • 4,800 watts * 1/0.85 * 1/42v cutoff * 1.25 = 168 Amps minimum

Now notice the difference between Vbatt-minimum and Vcut-off of the Inverter.

    Calculate Voltage Drop:
  • 11.5 volts - 10.5 volts = 1 volt drop
  • 23 volts - 21 volts = 2 volt drop
  • 46 volts - 42 volts = 4 volt drop

    Larger DC voltage supports higher voltage drops and smaller gauge wire.
  • 48 volts can carry 4 times the power 4 times the distance as a 12 volt system using the same gauge wire.
  • Inversely, a 12 volt system requires 4 times the gauge of wire than a 48 volt system.
  • A 12 volt system carrying 240 watts would require 20 amps and a 1 volt drop.
  • A 48 volt system carrying 240 watts would require 5 amps and a 4 volt drop.
  • A 120 volt system carrying 240 watts would require 2 amps at a 3.6 volt drop.

All of these factors translate to a significant increase in cost of a 12 volt system verses a 48 volt system.

Design Decision #2: Which appliances will I use?
Options: 120VAC vs 12/24/48VDC

Most 12 volt devices are designed for cars and usually support 12 volts when the engine is off and ~14.2 volts when the engine is running. So, quite a few of these 12 volt adapters may not work well on a solar power battery bank and may get toasted during charging. A well designed AC inverter will be designed for 10.5-16 volts and output a consistent 120 VAC to appliances.

Another major factor to consider is energy conservation verses price. Some of the Energy Star rated 120 VAC appliances actually approach the efficiency of the off-grid DC appliances at a lower price and with longer life and more features. This leads me to believe that it might often make more sense to use 120 Volt AC appliances even though the inverters may cost you ~15% loss in efficiency.

Design Decision #3: What type and how many batteries will I use?
Options 12/24/48VDC

    Amp Hours(AH) vs Watt Hours (WH) vs Kilowatt Hours(kWH)
  • A 100 watt bulb running for 10 hours is 1,000 WH or 1 kWH
  • 1,000 WH / 240 VAC = 4.2 AH
  • 1,000 WH / 120 VAC = 8.3 AH
  • 1,000 WH / 48 VDC = 21 AH
  • 1,000 WH / 12 VDC = 83 AH

So, I think I have concluded that I should try to design my system for 48 VDC batteries and 120 VAC appliances.

Sunday, August 12, 2012

Thermoelectric Power Generation

It's been terribly hot in Texas these last few weeks, so I haven't been willing to work in the Airstream. So, I've been doing some research and fiddling with various power generation techniques. As I mentioned in a previous post, thermoelectric generators (TEG) seem like a very efficient way to produce a great deal of electricity from heat. However, the more I researched the use of TEG to charge batteries, the more I became discouraged about the price and complexity of thermoelectric power generation.

The manufacturer of the F2F200 TEG claims it can generate over 200 watts if the heat source is over 270° C and the cooling water flow is 1500cm3/minute at 30° C. Those are some pretty impressive specifications, but I'm having trouble imagining a practical design that could safely deliver that much heat to the TEG to maximize the power output.

The retail price for this unit is $1,919.99 and if I was to invest that kind of money I would need to be pretty sure that I can push the temperatures to the optimal 270° C. If I was to attempt to generate power with this type of TEG, I would probably build some sort of parabolic solar collector that could be easily deployed with the TEG whenever I setup camp.

Due to the extreme temperature and pressure required by this TEG, I would want to keep it far enough away from my Airstream to minimize the risk of injury in case of a leak or break in the pipes. I would also want to include some sort of a pressure release valve that would automatically safely bleed the pressure out of the system automatically and by a safety valve that can be activated from a safe distance.

I've found at least one copper pipe manufacturer (Mexflow) that discuss "maximum working pressures at temperature up to 650° C". I've also read some discussions about "brazed joints" that can withstand very high temperatures because the alloys used melt above 600°C. Unfortunately all of the brazing alloys I've found seem to recommend a maximum working temperature of about 200° C. So I might have to exceed the safety recommendations to achieve the temperature of the TEG.

I ran these ideas by some of my family who are engineers and my cousin pointed out that if the fluid is not taking the heat away from the solar collection point excessive heat can occur and would probably quickly destroy several of the components. So he suggested that I would need something that could handle the heat as a buffer.

When I started researching heat buffers, I stumbled across discussions about using 1000-2000 ton water tanks with layered heat chambers for this exact application. Now I am beginning to think that it would be difficult for a TEG to compete with PV solar at twice the cost and at least four times the complexity. I still really like the idea of using a TEG for generating electricity because I could use the TEG with a wood fired stove to generate electricity at night and then use a solar collector during the day.

As I was researching, I found a similar device called a thermoelectric (peltier) cooler (TEC). The TEC modules are mostly used in portable coolers and refrigerators and they are much cheaper then the TEG modules that are designed for power generation. The TEC modules also seem to have a much lower heat requirement for generating electricity. So, I started to wonder if a large quantity of the TEC modules could possibly be used at about 100° C instead of trying to deliver 270° C to these very expensive TEG modules.

A nice fellow named Jack posted some very detailed videos about his power generation experiment with TEC modules. Below is the last of the videos which shows the end result of his project.

I actually ordered about twenty thermoelectric cooling modules that are very similar to the ones Jack used in his experiment. I ordered the TEC modules in the hopes that I can cool and heat my Airstream and maybe even build a custom refrigerator with them. I was planning to try to generate some power with the TECs, but it seems that these modules just don't produce nearly enough energy to justify the cost of a large-scale implementation. From what I can tell, these TEC modules are designed to use minimal power for heating and cooling. So I guess I would not be using the TEC modules for the purpose they were intended and it would take a large temperature difference to create a relatively small amount of power.

So, I don't think I will attempting thermoelectric power generation until I find a more practical and cost effective TEG module.