Thermal harvesting and storage: a heavenly match

Thermal harvesting and storage: a heavenly match

Thermoelectric power generators combined with batteries and energy cells offer an ideal solution for self-powered applications

BY PAUL A. MAGILL
Nextreme Thermal Solutions
Durham, NC
http://www.nextreme.com

Energy harvesting is the process whereby a portion of energy is removed, captured, and stored from an existing source of unused but available energy. The use of thermoelectric power generators (TEGs), in which a temperature difference creates an electrical potential, can convert waste heat from thermal sources into electricity as an alternative source of energy.

Energy harvesting offers the opportunity to directly power devices such as wireless sensors, but the stability of the heat source must be considered if thermal energy is to be viewed as a reliable electrical energy source. Combining thin-film thermoelectric power generators with energy storage devices offers an ideal solution to manage variability in the energy source.

Thermoelectric generators

The core component of a thermoelectric device is a thermocouple, which consists of an n-type and a p-type semiconductor connected by a metal plate. Electrical connections at the opposing ends of the p- and n-type material complete an electric circuit.

Fig. 1. Thermal-to-electric conversion with thermoelectrics.

Thermoelectric generation (TEG) occurs when the couple is subjected to a thermal gradient (that is, the top is hotter than the bottom), in which case the device generates a voltage and causes a current to flow, converting heat into electrical power by what is known as the Seebeck effect.

A thermoelectric module is then formed from arrays of these thermocouples connected in series. If heat is flowing between the top and bottom of the module (forming a temperature gradient), a voltage will be produced and an electric current will flow.

Thin-film thermoelectrics

TEGs made with thin-film technology may improve the energy conversion and hence the ability to use them as an energy source. Thin-film thermoelectric generators are smaller and thinner than conventional TEGs and show promise for direct integration using industry-standard manufacturing methods.

Fig. 2. An example of an eTEG HV14 thermoelectric power generator.

Thin films are material layers ranging from fractions of a nanometer to several microns in thickness. Thin-film thermoelectric materials may be grown in a variety of ways, but usually involve a vacuum deposition technology such as a metalorganic chemical vapor deposition (MOCVD) reactor. Devices are then made using conventional semiconductor fabrication processes.

Power generation

A thermoelectric generator converts heat (Q) into electrical power (P) with efficiency η.

P =ηQ (1)

Larger devices that use more heat, Q, will produce more power, P. Similarly, the use of twice as many power converters will naturally produce twice the power given that they can capture twice the heat. Without a specific constraint on heat flux and system geometry, it is convenient to focus on power per unit area (P/A) produced and heat flux density (Q/A) rather than absolute power and heat consumed (see Eq. 2). This is particularly convenient for thermoelectric power generation because the devices are so easily scalable: A large device can simply be an array of smaller modules.

Coupling TEGs and energy storage devices

Depending on the stability of the heat source, thermoelectric generators may be coupled to an application as a power source in one of two ways: either directly if the heat source is sufficiently large and stable or by charging a battery or other energy storage device.

The simplest way to charge a battery with a TEG is to just apply a constant voltage or constant current to the battery. Of course, if the voltage or current is too large, damage to the battery can occur.

If the charging current or charging voltage is chosen to match the discharge rate of the battery, then the battery can be charged indefinitely without damaging it. This type of charging is referred to as trickle charging the battery. This would maintain a full-capacity battery. It is the slowest method for charging the battery, but also the cheapest and safest. Most rechargeable batteries, particularly nickel-cadmium batteries or nickel metal hydride batteries, have a moderate rate of self-discharge, meaning they gradually lose their charge even if they are not used in a device.

A host of other methods are available, including timer based, intelligent, inductive, and pulsed. Trickle charging is the most likely scenario when using an integrated TEG/battery device since it does not require any additional regulatory circuitry to monitor the battery and modify the charging rate.

Thermoelectric power generation is an attractive means of power conversion. That these devices are not mechanical implies they will be very reliable, but there are some limitations on the use of the devices.

A heat flux must be present to use a TEG for power conversion. This heat flux must have an inflow and outflow through the TEG. This implies that there must be some type of heat rejection or heat-sinking path.

One common misconception about TEGs is that the heat flux is provided simply by putting them into a hot environment. Initially a current will flow, but very shortly the entire TEG will reach thermal equilibrium (same temperature everywhere) and the heat flow through the TEG will cease as will the electrical current.

Another point of interest is that converting the heat flow out of a device will affect the thermodynamic properties of the systems in the near area. This is because TEGs have a high resistance to the flow of heat.

Such high thermal resistance will lead to a slowing of the heat flux in the direction of the TEG and in turn lead to a rise in the temperature of the device used as a heat source. This is due to the increased thermal resistance from the device to ambient environment. For this reason, TEGs for power generation are best employed where the device has some temperature headroom, that is, it is not already operating near its temperature maximum.

Since the performance of the module can be improved by providing a good thermal path for the rejected heat, it is beneficial to provide high thermally conductive pathways. For small packages, this is typically accomplished through the electrical connections themselves, and depending on the operating characteristics, this level of thermal management might be sufficient. For packages with higher heat densities, thermally conductive feed-throughs or posts may need to be employed to manage the heat.

Thermoelectric power generators when combined with batteries and energy cells offer an ideal energy solution for a variety of autonomous, self-powered applications. These solutions can reduce the total cost of ownership by eliminating the prohibitive cost of battery replacement. This approach leads to “instant on” power solutions based on energy harvesting, dramatically reducing the size of the power supply required and enhancing the performance of the embedded devices for maintenance-free operation. ■