altpower

This page is for the purpose of exploring alternate power sources- either for emergency power, to use where an electric outlet is not available, or as alternative to a stash of bunny batteries. So, if you like to experiment, here are a number of different technologies for consideration.

Chemical Battery

This is the source of electrical power that has been around the longest, and the simplest to make. A battery is basically two dissimilar metals immersed in an electrolyte (a substance that ionizes). The positive terminal is the anode; the negative is the cathode.

Electromotive Series

The electromotive series consists of a list of metals arranged according to the potential between the surface of the metal and an electrolyte in which they are immersed. Some (with respect to hydrogen, whose potential is zero) will gain a positive potential, while others will gain a negative potential. In the case of a battery, the more dissimilar the metals (i.e., farther apart on the list), the greater the potential voltage. Below is a list of the standard potentials:

Electrode

Element

Potential (Volts)

=======

=========

Themoelectric Generator

The thermoelectric cooler (TEC), also known as a Peltier cooler, is a semiconductor-based electronic component that functions as a small heat pump. These wafer-like devices are made from two elements of of semiconductor (primarily bismuth telluride), heavily doped to create either an excess (n-type) or deficiency (p-type) of electrons. At the hot junction, energy (heat) is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element to a higher energy level in the n-type semiconductor element. When you apply low voltage DC across a TEC, heat will be moved through the module from one side to the other. One face will be cooled while the opposite face is simultaneously heated. And this phenomenon may be reversed by changing the polarity (plus and minus) of the DC voltage.

But this device can also be used as a thermoelectric generator (TEG), producing electrical power (see Figure 2.1). By furnishing a source of heat, and creating a temperature differential at the opposing faces, a DC voltage will be produced at the terminals and current will flow.

The first important discovery relating to the TEC/TEG occurred in 1821 when a German scientist, Thomas Seebeck, found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals (see Figure 2.2), provided that the junctions of the metals were maintained at two different temperatures. In 1834, a French watchmaker and part time physicist, Jean Peltier, while investigating the "Seebeck Effect," found that there was an opposite phenomenon where thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit (see Figure 2.3). Twenty years later, William Thomson (a.k.a. Lord Kelvin) explained how the Seebeck and Peltier Effects relate to each other.

Any TEC can work as a TEG. The main difference is that the TEG has been ruggedized to withstand temperatures high enough to generate a practical amount of power. Ignore this, and you will have a worthless pile of bismuth telluride cubes. TECs are fabricated using solder with a melting point around 125~144 ºC. TEGs can handle 200~250 ºC (see manufacturer specs). See the YouTube demonstration of thermoelectric power generation.

Seebeck Effect: The simple thermocouple circuit in Figure 2.2 consists of two dissimilar metals denoted as Material X and Material Y. Typically, thermocouple A is used as a "reference" and is maintained at a relatively cool temperature when compared to thermocouple B, which is used to measure the temperature of interest.

When heat is applied to thermocouple B, a voltage (Vo), the Seebeck emf, will appear across terminals T1 and T2. This voltage, can be expressed mathematically as:

Vo = axy * (Th - Tc)

where:

Vo is the output voltage in volts.

axy is the differential Seebeck coefficient between the two materials, x and y, in volts/ºK.

Th and Tc are the hot and cold thermocouple temperatures, respectively, in degrees Kelvin (ºK).

If you are not familiar with degrees Kelvin (ºK), it is an absolute scale using the same divisions as Celcius (ºC), but with values no lower than zero ('Zero' being absolute zero, the coldest possible temperature. 0 ºK equals -273.16 ºC.).

Peltier Effect: Modifying the thermocouple circuit of Figure 2.2 to that of Figure 2.3 will make it possible to observe an opposite phenomenon known as the Peltier Effect.

If a voltage (Vin) is applied to terminals Tl and T2, an electrical current (I) will flow in the circuit. As a result of the current flow, a slight cooling effect (Qc) will occur at thermocouple junction A where heat is absorbed and a heating effect (Qh) will occur at junction B where heat is expelled. Reversing the direction of electric current flow will reverse the direction of heat flow. The Peltier effect can be expressed mathematically as:

Qc or Qh = pxy * I

where:

pxy is the differential Peltier coefficient between the two materials, x and y, in volts.

I is electric current flow in amperes.

Qc and Qh is the rate of cooling and heating, respectively, in watts.

Also note the phenomenom of Joule heating (also measured in watts), having a magnitude of I²R (where R is the electrical resistance in ohms). This occurs in the conductors as a result of current flow. Joule heating acts in opposition to the Peltier effect and causes a net reduction of the available cooling.

It's obvious from the chart above why lithium makes a great choice for battery construction. But a workable battery does not require such an exotic metal. Aluminum and copper can work well, and are easy to find. Immerse the two in an electrolyte (just about any liquid besides pure water), and you have a battery. See the YouTube clip of a battery made using water and Miracle Gro fertilizer.

Mechanical Generator

A mechanical generator works by moving a coil through a magnetic field. Almost any DC motor can work as a generator. But something better is the stepper motor, particularly at lower RPM. A stepper motor, though, unlike the others, generates a polyphase AC voltage, which must be rectified into DC. Below is a prototype using a stepper motor. I used it to charge a bank of series-parallel 1F (yes, 1 Farad) capacitors (11F total), storing the energy to keep a red LED lit for some time.