Thermoelectrics

Could there be more untapped alternative energy sources at our fingertips? It is a common occurrence in our everyday lives, the sensations of hot and cold that make some of the most popular drinks and foods enjoyable to our sensory palate. The sensation, of course, is thermodynamic in nature, and we often associate the hot and cold with “heat.” The term is ubiquitous in our weather reports, food descriptions and oddly in the clinics of Chinese alternative medicine, but never do we commonly associate “heat” with the production of energy. Ironically, in the world of engineering, heat is often termed “waste energy,” as many engines lose energy from fuel as a transfer of heat to the environment. But the avid science reader would know that using “heat” as a form of energy is nothing interesting considering that renewable energy  possible. Welcome to the world of thermoelectrics brought to you by the Peltier and Seebeck effects.

Obviously, the physics of thermoelectrics can get quite frustrating to most non-doctorate students, but the basic premise of thermoelectrics goes back to the work of German physicist Thomas Seebeck,¹ and French watchmaker and part time physicist Jean Peltier.¹ In 1821, Thomas Seebeck observed that a circuit made from two dissimilar metals at junctions of different temperatures deflected compass needles. This deflection, of course, was caused by a magnetic field, and one of the tell tale signs of the existence of an electric current is a magnetic field. Seebeck realized the existence of this induced current produced from two different temperature gradients, and established a relationship between the voltage of a circuit and the temperature difference between two conductors in the circuit to describe what is known today as the Seebeck effect.

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In 1834, Jean Peltier made observations regarding the converse of Seebeck’s phenomena by finding that an electric current would produce heating and cooling at the junction of two dissimilar metals in what became known as the Peltier effect.³ Then twenty years later, William Thompson⁴ was able to relate the Peltier and Seebeck effects by postulating that heat is produced or absorbed when current flows in a material that has a temperature gradient. Thompson provided a way to better quantify Peltier’s findings, which is essentially like the converse of the Seebeck effect. While Seebeck found that a thermodynamic gradient can produce an electric current, Thompson and Peltier proved that current flows absorbed or released heat.

In English, then, all three findings by three independent physicists established the foundations of thermoelectrics. For the purposes of renewable energy , however, we are especially interested in Seebeck’s findings that a heat difference can create electricity, and this is the principle function of modern thermo-electric generators. So why is this important for alternative energies? Well, heat is one of the most wasted forms of energy in every mechanical device that we use in our daily lives.⁵ Rather than waste it, why not couple thermoelectrics with current renewable power generation techniques? Let’s look at some current and proposed applications of thermoelectric power units.

Solar Panels: The knowledge derived from developing thermoelectrics can actually be used in many facets of green energy technologies. In 2011, a team of researchers from MIT and Boston College were able to create a new type of solar panel that acted as a solar thermoelectric generator with a peak efficiency of 4.6% in converting heat from the sun directly into elctricity.⁶ Although 4.6% may sound ridiculous, it is actually quite impressive for a solar panel of this nature, in fact it is 7-8 times higher than previous efforts. Additionally, there has also been work in creating hybrid solar panels that utilize current photovoltaic technologies in solar panels coupled with thermoelectrics. A team of researchers from Columbia who came up with this concept predict an increase to 12% in solar conversion efficiency compared to the current 5-10%.⁷

Thermoelectric Power Generation Module

Space Flight: NASA is perhaps the world’s hot bed of new and outlandish propulsion technologies for unmanned space exploration vehicles. They certainly left no stone unturned, for NASA has turned to thermoelectrics to create what they call a radio isotope thermoelectric generator (RTG).⁹ The fundamental principle of an RTG is the same as traditional space exploration vehicles except that a heat difference in the circuit is created by radioactive heating caused by decaying radio isotopes. RTGs are not a new technology by any means, since the usage of RTGs goes back to pioneer,¹⁰ a research vehicle launched in 1972.10 The technology, however, has been improved over time and it continues to be used even in the Mars Science Laboratory—a robot launched to Mars in November of 2011 for soil sampling and observatory purposes. ¹¹

Industrial Processes Coupling: One of the largest investigations into thermoelectric power relates to coupling thermoelectric generators with industrial processes to recover “waste heat.” A study conducted by faculty at Cardiff University assessed the feasibility of using thermoelectric coupling with industrial applications such as that in steel plants, utilities’ power production, and nuclear power plants.¹² Like various other studies, the Cardiff study underscored the potential for energy recovery using thermoelectrics in industry. Additionally, thermoelectrics can be coupled with modern alternative energy technologies such as hydroelectric dam turbines  to recover wasted heat. The laws of physics make it impossible to achieve extremely high energy yields from mechanical turbines,¹³ therefore such coupling mechanisms could be a much needed accessory to future power generation techniques.

Refrigeration: It seems that until this point we have kept the spotlight on the electricity generation aspect of thermoelectric, with much due gratitude to the important findings of Thomas Seebeck. In the world of refrigeration, however, we begin to realize the specific importance of Peltier’s and Thompson’s contributions to the field. New types of refrigerators are currently being developed in which14″> If thermoelectric power is seemingly such an obvious and easy choice, why do we rarely ever hear about this form of energy production? Well, the simple answer is that modern thermoelectric generators can rarely achieve an energy output beyond the single digits. Although the output of this energy producing and capturing mechanism is seemingly dismal—4.6%¹⁵—the coupling of thermoelectric generators with industrial processes would contribute to energy savings on a macroscopic scale. Although this technology for renewable energy purposes is still infantile, strides are being made to develop thermoelectric technologies that boast the energy efficiency numbers of modern gasoline powered generators. We should hope to hear much more about thermoelectric power in the near future!