Plumbing for power: Energy harvesting turns temperature differences into electricity

The engineering team at Nextreme Thermal Solutions, Durham, N.C., have developed a new way to convert small temperature differences into electricity using sold-state-thin-film technology. This new technology, dubbed Thermobility, will let engineers develop wireless and battery-free sensors powered by electricity harvested from plumbing and HVAC systems. These sensors could monitor temperature, humidity, and airflow, while taking advantage of the hot and cold temperature differences commonly found in air plenums. Or they could be attached to hot water pipes and generate power from otherwise wasted heat.

One of the first devices in the Thermobility product line is the WPG-1, a wireless power generator. It can provide a constant voltage output of 2.2, 4.1, or 5.0 Vdc to electrical loads of at least 15 kΩ. The generator is about the size of a golf ball and contains a pin-fin heat sink, a circuit board, a metal attachment plate that gets applied to the heat source, and a Nextreme eTEG HV56 thermoelectric power generator module. It also uses a Linear Technology LTC 3108 Ultralow Voltage Step-up Converter and Power Manager chip that puts out up to 1 mW of electrical power. It can do all this based on a temperature difference of 15 to 20°K. Larger temperature differences can generate significantly more power.

The RoHS-compliant HV56 power generating module measures 3.1×3.3 mm and 0.6 mm thick and can generates 1.5 mW on a temperature difference of 10°K and 36.5 mW with a 50°K difference. The solid-state device uses the Seebeck Effect in which the temperature difference between the hot and cold sources creates a difference in potential and this difference drives a current. The module puts out more power as the temperature difference increases, up to about 215°C. Performance declines if the cold side goes below 0°C, so the maximum temperature difference is about 215°C.

The WPG-1 costs about $500, and works with any flat-surfaced heat source. And the circuit board includes DIP switches for setting the output voltage. Electrical connections can be made using available 2 or 6-pin connectors. The 6-pin version is a Texas Instrument connector that mates to the eZ430 wireless target board, making the WPG-1 a good wireless power source for eZ430 development platform.

Another Nextreme Thermobility device is the WPG-1S, a wireless power thermoelectric generator that is similar to the WPG-1 but incorporates an Infinergy D-MPM101, a power module from Infinite Power Systems that includes ultra-thin, rechargeable Thinergy sold-state batteries. These batteries can store up to 2.0 mAh of power. The module efficiently manages battery power, along with recharging, and has low-loss energy storage.

The onboard batteries let the $600 WPG-1S supply power even when the heat source is unavailable or intermittent. The unit can generate 200 mW at 25°C at a regulated 3.3 V.