Turning waste heat into electrical energy
Marlow Industries’ EverGen PowerStrap 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 aluminium 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 230°C 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 aluminium with anodized coatings.
Maximizing 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 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, the best performing 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.
Figure 3 highlights why this is a particularly important consideration when designing thermoelectric power generation systems. In reality, both electrical and thermal characteristics of the TEG are interrelated with the thermal resistance of the TEG being affected by the electrical load connected to the TEG. In real world applications, where operating conditions and loads vary, it would be very difficult to always ensure proper load matching across all operating points due to temperature dependent properties of the TEG. Fortunately, commercially available maximum power point tracking (MPPT) controllers originally designed for the solar industry can also perform this function for thermoelectric systems. In cases where hybrid solar/thermoelectric systems are employed, a single MPPT controller accommodates both. The only design requirements are that the TEG system voltage and current outputs for the operating range meet the input requirements of the MPPT controller.
Figure 1 shows a photograph of a 10-inch diameter EverGen PowerStrap. The unit was designed for outdoor use, 120°C operation in a vertical orientation for an industrial exhaust pipe. Twelve identical TEG and heat sink assembly sections were spaced evenly around the perimeter of the strap base. Each thermoelectric generator module was held in compression between the heat sink and the strap base with two stainless steel bolts fitted with insulating phenolic washers. Thermal pads on both sides of the TEG were used to improve thermal resistance at the hot (strap) and cold (heat sink) interfaces. The clamping strap base was divided into three identical sections that form a compression fit around the pipe when bolted together. A series of lab tests were conducted with this design that mimicked varying operating conditions throughout the year. The test assembly was made from a section of 10” diameter steel pipe that was capped at one end and filled with oil. Submersible heaters, attached to an electronic temperature controller, were used to control test assembly wall temperature. The PowerStrap was clamped to the exterior of the test assembly, with non-setting thermal mastic applied between the pipe wall and the strap base to aid in heat transfer. During testing, ambient temperature around the test assembly was altered to reflect seasonal changes.
Both natural convection and forced convection up to 6.5 mph were studied. Omega OM-420 data acquisition equipment was used to collect temperature, voltage and current measurements during testing. Figure 4 is an expanded view sketch that depicts thermocouple placement on the test assembly. Readings were collected and recorded in two second intervals.
The results of this testing, compared against model predictions, are shown in Figure 5 for two different pipe temperatures covering a wide range in ambient conditions. From the data, it is obvious that the EverGen PowerStrap performance is maximized when ambient temperatures are the coldest. This is to be expected since thermoelectric efficiency is greater for larger temperature differentials. In real world operation, this means that the PowerStrap performance will be maximized during the colder months of the year. Such performance makes this product a natural complement to solar cells, which usually perform poorly during the winter months. Another key point is that there is significant performance increase, by as much as 40 percent, when typical outdoor wind conditions are accounted for. For applications requiring higher power levels, multiple units can be employed. The data also shows that the model predictions close well with experimental data. By expanding the model to include different pipe temperatures and diameters, performance under different operating scenarios can be predicted.
About the author:
Josh Moczygemba is Power Generation Product Engineering Manager at Marlow Industries – www.marlow.com