The idea for this book came from the experience. Our team is in thermoelectric business for over 20 years. Firstly, for many years it was thermoelectric cooling solutions for miniature applications. In recent years, thermoelectrics moves in microgenerators trend. Green energy, energy harvesting…
Promising directions — applications of thermoelectric microgenerators for tasks of low energy — recycling waste heat surrounding thermal sources, of human body energy harvesting and so on.
This direction only began its development. Of course, there are many ideas for development come from inventors and scientists. Often an absence of sufficient knowledge and understanding of the subject of thermoelectricity is compensated by enthusiasm, the broadest fields of promising applications, interesting tasks.
Numerous appeals of such potential consumers to assist them in thermoelectric solutions usually are accompanied by neediness of detailed technical consulting from fundamental physical base of the thermoelectricity phenomenon till fine nuances in their applications.
It has given rise to the idea of writing of the reference book FAQ (Frequently Asking Questions) on the most important questions. Gradually this idea has developed into the separate book on the volume of the reference material.
The structure of this book contains detailed explanations addressed to a wide range of readers, which for the most part are not specialists in the field of thermoelectricity, of the basic ideas, important aspects of the practical application of thermoelectric microgenerators in the tasks in energy harvesting. I will be glad, if this book will serve as a reference tool in developing appropriate solutions.
Every Chapter was considered as a separate small manual, easy for an inexperienced user to find answers to their particular questions without resorting to detailed studying of the all material of the book. Therefore, the repetition takes place in formulas and expressions from Chapter to Chapter, for example.
Professionals may also say about excessively detailed clarification of “simple” basics of thermoelectricity. Please, these moments can be simply ignored.
But exactly simple and detailed explanations of important aspects of the application of thermoelectric microgenerators and was the purpose of this book. Many formulas, but their detailed explanations, I hope, will help users understand the issues of interest to them.
Chapter 1. Introduction
Creation of alternative, first of all renewable, power sources is the perspective direction of development of power engineering in the world now. The whole direction under definition “green power” (green energy) was appead.
It is the power engineering based on renewable natural sources of heat, causing the minimum damage to the environment, safe and eco-friendly. The big section of new power engineering is the direction of utilization of waste heat of the sources surrounding the person. Including, very interestingly, — thermal emission of a human body.
It should be noted that due to the nature of these heat sources, this is small power engineering. It received a general definition- energy harvesting.
To transform the energy of such energy sources it is possible to use devices that are based on different physical principles: electrodynamic, photovoltaic (small solar panels), piezoelectric and thermoelectric (microgenerators).
Every of the above types of energy converters has its advantages and disadvantages at the same time. And mostly they still compete for the tasks of energy harvesting at the level of concepts and laboratory projects.
Thermoelectric (TE) converters, thermoelectric generators (TEG) have several advantages over other mentioned:
— “omnivore” — convert any weak heat flow, all sources, including the heat of the human body.
— all-weather is not dependent on the daylight cycle (as solar panels).
— latent installations, due to the very small size, which is important for masking.
— highest reliability and long service life (over 25 years).
— no moving parts, no noise, do not need periodic maintenance.
In accordance with recent forecasts , , the direction of thermoelectric converters of thermal energy in the coming years will be developed into a large segment of the world market with a volume of about 1 billion dollars to 2024 (see Chapter 17).
Thermoelectric generators have, as we know, very low conversion efficiency, which is no more than 15–20% of the Carnot cycle efficiency. It is only few percent of conversion of energy from a heat source.
In energy harvesting applications, where natural heat sources have a slight temperature, generator is working at very low temperature differences and therefore, a fortiori, with very low levels of efficiency.
Although it is most commonly “waste” energy and low efficiency seems to be tolerated, but efficiency remains a key challenge of thermoelectric generators
In this regard, optimization of all aspects of device design and materials of thermoelectric generator is a very pressing task of extracting maximum effectiveness in their application.
The primary task is to develop an efficient thermoelectric material. This is a key component of the device as a whole, determining its main parameters. It is important to obtain material with not only high thermoelectric efficiency, but also sufficiently high mechanical properties. It is the task of the semiconductor materials science where there are their approaches, achievements and expectations.
The second task is the optimum design parameters to ensure the effectiveness of the device in general. It includes development of methods of constructing of optimal thermoelectric microgenerators with available thermoelectric materials.
This book discusses a range of aspects of optimization of thermoelectric microgenerators, their applications in directions of energy harvesting. Interrelations of various parameters are demonstrated, various nuances of operation of thermoelectric microgenerators are discussed.
We will discuss thermoelectric generators in whole, but meaning their miniature (micro — is a slang) types suitable for energy harvesting, small-scale power engineering.
Chapter 2. Physical basics
Preface. Key aspects of applications of thermoelectric microgenerators derived from fundamental physical principles underlying their work. These are effects of Seebeck, Peltier, Thompson, and Joule. Accordingly, we start with the physical basics of thermoelectric modules. This Chapter introduces key concepts and parameters of thermoelectric generators.
Heat balance equations
To date, the widest use of thermoelectric micromodules is connected with cooling of miniature objects — thermoelectric cooling applications.
For comparison, let us give the equations of thermal balance (2.1) — (2.2) of thermoelectric module in cooling mode (Fig. 2.1).
Heat balance equations of thermoelectric module in generating mode (Fig. 2.2) are the following.
If to neglect temperature dependences of physical parameters, then heat balance equations in the generator mode can be written as the following:
Here and below average values α, k, R are used — average values of paired thermoelements (pellets) of n-and p-type. For example, α= (αn+αp) /2. And these parameters refer to average operating temperature of pellets, in situations where the temperature dependency properties can generally be neglected.
Thermoelectromotive force (ThermoEMF) E of a thermoelectric generator depends on the temperature difference ΔT, number of thermoelements N and Seebeck coefficient α as the following
We introduce the notation for internal resistance of thermoelectric generator ACR=2NR, and for external load resistance — Rload.
Electric current through the generator is determined by thermoEMF E and total resistance of closed circuit (Fig. 2.2):
Here we introduce important parameter m — the ratio of the external load Rload to internal resistance of generator module ACR.
From the balanced equation (2.5) and (2.6) one can find that:
First member EI of the difference (2.10) is the total heat that is converted into electric current by the thermoelectric generator. The second member I2ACR is what part which comes back into heat due to the Joule effect.
— When the difference (2.10) is equally to zero (short circuit mode, external load resistance equal to zero) then all the converted energy comes back into heat.
— When the difference is positive (there is non-zero external load), i.e. the generator converts heat into electricity.
With given ratios (2.7) — (2.9) the balance equations (2.3) and (2.4) can be rewritten as the follows:
Here the heat transferred through the Peltier effect QP
Joule heat Qλ
Voltage U in external circuit, taking into account (2.8), is the following.
Power P in the external circuit (converted power).
Here an important dimensionless factor F is appeared.