原创 Does TEG have a place in your power future?

Figure 1: Go camping with a TEG: with the Biolite CampStove, you don't have to worry about your phone running out of power (though it may be out of range of a tower), and your cooking fire may also burn hotter due to the small internal fan.

 

[If you are not familiar with TEGs, they've been around a long time in both glamorous and non-glamorous roles. A TEG uses one or more thermocouples to generate power from a heat source (not to measure temperature, another common use of thermocouples). That heat source can be an open flame as in the stove, or a lower-temperature source such as from radioactive decay -- that's what is used for power on spacecraft that are too far from the sun to use solar panels. Even a standard gas-fired water heater has a TEG -- although they don't call it that -- where a thermocouple assembly in the pilot-light flame generates enough power to hold the gas-flow valve open: thus, no pilot light, and the valve stays shut -- a very simple and effective fail-safe shut-off mechanism.]

In this stove, it's used for two purposes. First, it can provide charging for a cellphone (of course, depending where you are camping, that may actually not be a beneficial function, and there are solar-powered chargers available for the same purpose). Second, it powers a small internal fan, which is designed to improve the air flow (draft) and thus enhance combustion performance.

It's actually that second role that really caught my attention. There's no doubt that better draft means a hotter fire, and with rescued polluting exhaust. But in this case, can it capture enough heat from the fire to make it worthwhile? After all, there is no "free lunch" in most energy gain/loss balance equations -- is the energy lost in heating up the thermocouples of the TEG module more than the gain from the added airflow? Keep in mind that TEGs, as with most low-heat harvesters, are fairly inefficient, usually running around 10% heat-to-electric energy-conversion efficiency, and that's without considering any losses in the harvesting-related electronics.

Going through the numbers of any energy-conversion approach is always revealing. Look at the attractively packaged GoalZero Generator (Figure 2), which uses modest-sized solar panels to charge some fairly hefty batteries (they can also be recharged from a car as a 12-V source or a standard AC line). The batteries provide 1200 Whr energy storage, which is certainly enough to do some real work, such as running a small refrigerator in a remote location.

Figure 2: The GoalZero Yeti can be charged by solar panels, 12V, or AC line, and provides various output-voltage options, with 1200 Whr capacity.

 

But looking closely at the charging times with those panels you'll see the drawback: charging time is 40 to 80 hours. (The vendor is very clear about charging times with various sources, there's no attempt to deceive.) Do the math and you'll find that even for modest loads, the charging time is much greater than the run time; for a 100-W load, operating time will be around 12 hours. Of course, that may be more than OK in some applications.

The underlying problem is, again, conversion efficiency. Maximum solar radiation reaching the Earth's atmosphere is about 1000 W/m² but the amount at the surface is less; then you have solar/electric conversion, which is about 10% to 15% efficient and losses in the various power-conversion stages. When you go through the numbers, you'll be lucky to have total conversion efficiency of more than a few percent, corresponding to perhaps 10 W/m² output per solar panel. You can look at it this way: there's two orders of magnitude between the source maximum and your end result.

Obviously, whether the cost of these losses is worth it depends on your situation. For some, it's a deal breaker; for others, there may be no viable alternative. Still, it's a reminder that when it comes to energy and power, you have to look carefully at the reality and not just the glamorous "something for almost nothing" aspects.

Have you seen any energy-source or -conversion products that you thought were especially clever or innovative? Were there any that you felt were nearly useless?