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How to turn waste heat into electrical energy

Posted: 08 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:energy harvesting? thermoelectric generators? heat sinks?

In this feature, we will tackle Marlow Industries' EverGen PowerStrap. It is a thermoelectric-based energy harvesting solution that produces multiple watts of power by conversion of waste heat from industrial piping directly into electrical energy. This product provides remote power for wireless sensors, wireless transmitters, actuators, and controls in large industrial, chemical, oil and gas infrastructures. This energy harvesting solution can be customized to fit any pipe diameter and pipe orientation, without modification to existing pipeline infrastructure.

Power output is proportional to the temperature difference from the pipe surface to ambient, and the number of straps employed in the application. The EverGen PowerStrap is composed of three main components: Bi2Te3 thermoelectric generators (TEGs), anodized aluminum clamping straps and natural convection heat sinks.

The TEG modules produce power from the temperature difference between the pipe wall and ambient air. They have a maximum operating temperature of 230C and are sealed for environmental protection. The clamping straps provide a geometrical transition from the round exterior pipe wall to the flat TEG surface. The clamp attaches with a compression technique that requires no modifications to the pipe wall. Straps are custom sized based on pipe diameter. The heat sinks dissipate heat to the ambient environment; they are typically made of aluminum with anodized coatings.

Figure 1: A 10-inch diameter implementation of the EverGen PowerStrap energy harvester from Marlow.

Design methodology
Maximising power in the EverGen PowerStrap system requires a balance between the thermal and electrical system design. Thermal optimization starts by defining a thermal load resistance ratio (m).

Where RTEG, thermal is the thermal resistance of the thermoelectric elements, HSR is the thermal resistance from the hot source to the hot side of the thermoelectric elements and CSR is the thermal resistance from the cold source to the cold side of the thermoelectric elements.

Figure 2: Impact of thermal load matching on the EverGen PowerStrap design.

Figure 2 represents the impact on performance that different thermal load ratios have on the power output. For most thermoelectric applications, designing for a thermal load resistance ratio of one ensures the best performance possible. In the case of the EverGen PowerStrap, a natural convection heat sink was chosen, based on orientation, size, cost and manufacturing constraints. Computational fluid dynamics (CFD) software was used to aid in the heat sink design optimization. Next, the TEG devices were designed using Marlow's proprietary TEG software to match the thermal resistivity of the natural convection heat sinks under pure natural convection conditions.

The electrical system optimization is analogous to the thermal system. For maximum power transfer, the internal electrical resistance of the power source must match the electrical resistance of the load being powered. In this case, the electrical load ratio (n) is defined as

Where Rload is the electrical resistance of the load being powered and RTEG, electrical is the electrical resistance of the TEG module under operating conditions.

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