Enhancement of thermoelectric energy harvesting of thermal fluctuations with thermocapillary flows in phase-change materials embedded in metallic foams
Santiago Madruga
Abstract
Abstract We present a complete model of a thermoelectric micro-energy harvester to transform ambient thermal fluctuations into electricity that couples a thermoelectric generator (TEG) to a heat storage unit (HSU) filled by a phase-change material embedded in a metallic foam subjected to convective flows due to the Marangoni effect. Convective heat transfer at the HSU is simulated with the Darcy–Brinkman–Forchheimer bulk equations, a shear stress boundary depending on the porosity, and the phase change with the enthalpy-porosity method. The porous matrix weakens the Marangoni effect by decreasing the surface shear stress while increasing the effective thermal conductivity. Introducing the metallic foam multiplies the efficiency in the transformation between thermal spatial gradients and electric energy with respect to only Marangoni driving. For a heat storage unit of 8 cm $$\times$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>×</mml:mo> </mml:math> 1 cm, Darcy number Da $$=10^{-2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>=</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> coupled to a TEG with Seebeck coefficient 0.027, we find that the harvested energy multiplies with respect to a base PCM driving by Marangoni: 3.2 times (23.6 J) for porosity $$\epsilon =0.95$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>ϵ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.95</mml:mn> </mml:mrow> </mml:math> , 3.9 times (28.4 J) for $$\epsilon =0.9$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>ϵ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.9</mml:mn> </mml:mrow> </mml:math> , 4.3 times (31 J) for $$\epsilon =0.85$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>ϵ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.85</mml:mn> </mml:mrow> </mml:math> . Decreasing the permeability augments the resistance to the convective flows and slightly reduces the electric energy generated.