Increasing the efficiency of the thermoelectric system

27.10.2023

An increase in the efficiency of any device always means an increase in the share of useful work in total energy consumption. The thermoelectric heat pump pumps heat and performs cooling and heating functions. Having certain advantages and disadvantages, a thermoelectric pump has a small cooling COP compared to pumps operating on a phase transition of the working substance.

COP_cool = Q_cool / W_el:

Q_cool - is the cooling capacity,
W_el - is the total electrical power.

As a rule, the COP cool of the most advanced thermoelectric devices is no more than 1. While gas-compression heat pumps have a COP of 2 or more.

Continuing attempts by developers and manufacturers of thermoelectric coolers to increase COP cool by optimizing the design of devices do not lead to improvements that are significant for industrial use. A significant improvement in COP cool can only be achieved by increasing the efficiency of the thermoelectric material.

However, increasing the thermoelectric efficiency of materials is in itself a non-trivial task. Looking at the formula, we will see a direct dependence of the thermoelectric figure of merit of a material on its electrical and thermophysical properties:

Z = α2σ/κ,

α - is the Seebeck coefficient (thermopower),
σ - is the specific electrical conductivity,
κ- is the total thermal conductivity of the material.

α2σ - is called the power factor, which essentially determines the efficiency of charge carrier transport. Thus, an effective thermoelectric material must have a high Seebeck coefficient and electrical conductivity, while its thermal conductivity must be minimal. However, due to the electron-phonon interaction, these parameters of the material are highly interdependent and a change in one of them entails the opposed change in the others.

New applications of thermoelectric cooling are constantly emerging, usually focused on niche, highly specialized devices. Over the past 15-20 years, the search for a solution to increase the efficiency of thermoelectric material in order to expand the use of Peltier coolers has been particularly intensive.

Materials science research and development were carried out in the following areas:

• materials with naturally low thermal conductivity, such as chalcogenides of heavy and transition metals, layered metal oxides, borides, including halide perovskites.
• increase in thermoelectric efficiency in nanostructured thermoelectric from traditional materials due to a decrease in lattice thermal conductivity as a result of an increase in phonon scattering at the boundaries of nanograins and structural defects inside the grains.

Crystal Ltd. is one of the key manufacturers for the advanced thermoelectric devices. Over the past 25 years, we have significantly modernized our unique technology for the production of thermoelectric material - Bismuth Telluride, including through the implementation of many research and development projects aimed at increasing the figure of merit - Z. Right now, we can say that the goal is to significantly increase the thermoelectric figure of merit for mass-produced products by global manufacturers not achieved. The best commercial thermoelectric materials have a quality factor (efficiency) of about 3.15 K¯¹ at a temperature of 300K, measured by the Harman method.

Let's return again to the overall efficiency of the device. We analyzed the cooling function in thermoelectric heat pumps as the main useful work, assuming by default that heating is a parasitic effect and does not perform useful work.

Efficiency = A_cool / A_el x 100%

A_cool is the cooling work,
A_el is the electrical work expended.

Obviously, this is true. There are few devices that use both thermoelectric heating and thermoelectric cooling in one cycle of electricity consumption. The ambient temperature in everyday life does not change simultaneously. However, such changes take place throughout the day and seasons throughout the year. The implemented task of heat accumulation when a thermoelectric heat pump operates in cooling mode and the use of this heat will SIGNIFICANTLY increase the efficiency of Peltier heat pumps.

Efficiency = (A_cool + A_heat)/A_el x 100%

A_heat is the heating work.

Here it would be appropriate to recall that heating coefficient for thermoelectric heating К = Q_heat/A_el at temperature differences of up to 10 ÷ 12 K reaches a value of 2 or more.

The R&D team of materials scientists at Crystal is currently studying and analyzing materials capable of accumulating significant amounts of thermal energy. Obviously, these are phase change materials. A phase-change material (PCM phase-change material) is capable of storing and releasing large amounts of energy compared to storing heat in a single physical state. Heat is absorbed or released when a material changes from solid to liquid and vice versa, or when the internal structure of the material changes (LHS latent heat storage).

There are two main classes of phase change materials: organic (carbon-containing) materials and salt hydrates. The third class is the phase transition from solid to solid. The accumulation of latent heat can be achieved by changing the state of a substance: liquid → solid, solid → liquid, solid → gas and liquid → gas. However, for the heating-cooling-heating cycle, only the phase transitions solid → liquid and liquid → solid are practical. Although liquid-gas transitions have a higher heat of transformation (as evidenced by the operation of compression coolers) than solid-liquid transitions, liquid → gas phase transitions are impractical for heat storage because large volumes or high pressures are required to store materials in the gas phase. Transitions between solid phases usually occur very slowly and have a relatively low heat of transformation. Initially, solid-liquid RSMs behave like materials with sensible heat storage (SHS). Their temperature rises as they absorb heat. However, unlike conventional SHS materials, when PCMs reach their phase transition temperature (melting point), they absorb large amounts of heat at a nearly constant temperature until the entire material is melted. When the ambient temperature around the liquid material drops, the PCM solidifies, releasing stored latent heat.

Already today, PCMs are used in many different commercial applications, and as the cost of renewable electricity gradually falls, such applications will become more common. By far the largest potential market is in building heating and cooling.

Thermoelectric converters in this segment can see huge growth in applications as climate control systems. Moreover, the structure for transporting and storing thermal energy implemented at the design stage can be connected not only to building coolers, but also to numerous outdoor telecom equipment, electronic cabinets and charging stations for electric vehicles, where high-quality cooling is also necessary for trouble-free operation.

The transition between solid phases has a number of advantages that may be of interest for the research and development of thermoelectric climate systems with PCM heat accumulators. These materials change their crystal structure from one lattice configuration to another at a fixed and well-defined temperature, and the transformation can involve latent heat comparable to most efficient solid-liquid PCM. Such materials are useful because, unlike solid/liquid PCM, they do not require nucleation to prevent supercooling. Additionally, since this is a solid/solid phase change, there is no visible change in the appearance of the PCM and no fluid handling issues such as containment, potential leakage, etc. Currently, the solid-solid temperature range PCM solutions range from -50 °C (-58 °F) to +175 °C (347 °F). Hybrid heat storage cells, where the material with a transition between solid phases is enclosed in a shell of solid-liquid RSM, may also turn out to be promising. In this case, of course, the materials must have the close heat capacity.

Summary:

The use of heat accumulators with a phase change material can give impetus to the use of thermoelectric climate control units in applications where this was previously unprofitable.

Thermoelectric converters do not use refrigerants that destroy the ozone layer of the earth and are environmentally friendly devices. The use of thermoelectric heating and cooling as useful work in one cycle will make an additional significant contribution to the reduction of CO₂ emissions.

Aleksandr Koriakin