University of Texas at Dallas researchers have developed a generator prototype that uses liquid metal to convert waste heat from sources such as electric cars or data centers into clean electricity.
The researchers detailed the project in the October print edition of Sustainable Energy Technologies and Assessments.
“Heat is an abundant renewable energy source,” said Dr. Babak Fahimi, Distinguished Chair in Engineering and professor of electrical engineering in the Erik Jonsson School of Engineering and Computer Science.
“In data centers, for example, we spend a lot of time getting rid of the heat by using chillers and air conditioning. Our work focuses on recycling that heat back to electricity.”
Data centers, electric car batteries and appliances such as air conditioners generate a largely untapped potential energy source, Fahimi said.
Most efforts to harvest energy from waste heat have focused on the heat byproducts from manufacturing, refineries and steel mills, which generate high temperatures.
The UT Dallas project zeroes in on sources that generate lower-temperature heat, between 80 and 115 degrees Fahrenheit, which have been more challenging to convert to electricity.
The researchers started with a magnetohydrodynamic power (MHD) generator, a device that generates electricity by moving fluid through a magnetic field.
MHD generators were developed in the 1960s, but the technology has not been widely used.
As a UT Dallas doctoral student, Eva Cosoroaba Ph.D.”17 proposed an MHD generator that uses gallium for the working fluid.
Gallium becomes liquid at temperatures greater than about 85 degrees – a sweet spot for harvesting lower-temperature heat sources.
While any conductive fluid could be used in the generator, gallium has the advantage of being a better conductor of electricity.
The project became her doctoral thesis, with Fahimi serving as her advisor. Cosoroaba, the lead author of the study, is now a lecturer in electrical and biomedical engineering at the University of Vermont.
“Dr. Cosoroaba demonstrated that we can harvest energy from low-temperature sources of heat and use that heat to melt and maintain the liquid metal that in turn drives the generator,” Fahimi said.
The technology has many potential applications.
For example, data centers – the computers that store massive amounts of information users access through cloud computing – produce substantial waste heat while also consuming a lot of electricity.
The technology also could improve the efficiency of electric vehicles by converting heat from the cars’ batteries and exhaust into electricity.
“Renewable sources of energy such as electric cars all have fantastic merits.
One thing we don’t do anything about is how to recycle the heat they generate,” said Fahimi, founding director of the Renewable Energy and Vehicular Technology Laboratory at UT Dallas. “If there is a way to recycle that heat back to electricity, that would be fantastic.”
Viscous flows in channels and pipes possess large amounts of mechanical applications which may incorporate cooling frameworks, petrochemical transport (oil and petroleum gas) and biotechnology.
Regularly such flows are going with heat transfer and a representative example is the removal of thermal energy from hydronic space heating framework  by means of circling water in the heater, after which it is transported to the individual areas through pipes.
In ongoing decades, engineers have additionally explored the change of viscous flows by means of porosity of the pipe or channel.
Injection or evacuation of fluid by means of pores is a powerful instrument for flow control . This innovation finds vital potential in biomedical sciences (e.g. counterfeit dialysis, blood flows) and in other topics, for example, rocket transpiration cooling and food preservation.
Scientific demonstrating of flows in channels/pipes with wall mass flux has, in this manner, invigorated some enthusiasm for the examination network.
Berman  was the first to examine the steady Newtonian flow in a permeable (porous) straight channel with uniform suction/injection impacts.
Later Bansal  stretched out this work to a steady viscous flow through a porous circular pipe with the suction and axial pressure gradient. He found that the velocity accomplishes the greatest incentive along the axis of the pipe.
Tsangaris and Kondaxakis  examined unsteady viscous laminar flow in a straight pipe.
They got an exact solution for time-changing infusion/suction at the permeable wall. Cox and Hill  have inspected the Newtonian fluid through carbon nanotubes with a Navier slip at the boundary. Ramana Murthy et. al . investigated micropolar flow generated by a porous cylinder showing rotatory motions. They found that drag diminishes numerically when the suction parameter increments. Srinivas and Ramana Murthy  have examined wall suction effects in immiscible couple stress flow between two homogeneous permeable walls. They recognized that fluid velocity increments with Darcy number.
Magnetohydrodynamic (MHD) is additionally a functioning zone of present day engineering applications and includes the interaction between the applied magnetic fields and electrically conducting fluids.
MHD pipe flows emerge in ionized accelators, MHD flow control in atomic reactors, MHD bypass energy generators, fluid metal manufacture forms, bubble levitation and so on .
MHD flows including suction/injection wall impacts have accumulated significant consideration. Terrill and Shrestha  presented an analytical study of laminar flow between two parallel porous plates with a connected magnetic field.
They had demonstrated that the surface friction increments with the rise in magnetic number. Attia  examined the unsteady flow through a circular pipe with axial pressure gradient applied for two-stage MHD non-Newtonian fluids. He found that with the stronger magnetic field, the velocity and temperature components for the two phases decrease.
Attia and Ahmed  studied the unsteady flow through magnetized viscoplastic (Bingham) fluid in a circular pipe. They identified that skin friction enhances with particle-phase viscosity.
El-Shahed  investigated the impacts of a transverse magnetic field and porous medium in second-grade fluid flow through a circular pipe. He obtained velocity solutions in terms of Fox’s H-function. Ramana Murthy and Bahali have examined the impacts of periodic suction/injection through magneto-micropolar flow in a permeable circular pipe.
They identified that wall shear stress rises with the magnetic parameter. Ramana Murthy et al.  studied the effects of wall suction/blowing in hydromagnetic micropolar fluid through a rectangular channel.
They found that the magnetic field reduces the flow rate sizes extensively. Mabood et al. studied the impacts of MHD and radiation in chemically reactive nanofluid through a stretching surface. Mabood et al  investigated double-diffusive impacts in MHD, non-Darcian stretching sheet flow. Shateyi and Mabood  inspected mixed convection and stagnation point flow with radiation and viscous heating through the non-linear MHD stretching surface.
The above examinations overlooked viscous heating impacts which can apply a significant effect in numerous applications. It is realized that viscous dissipation adjusts the temperature distributions and acts as an energy source.
This thusly particularly impacts heat transfer rates. The effect of viscous heating is firmly needy additionally on whether the pipe is being heated or cooled i.e. thermal boundary conditions at the pipe surface apply a important work in how much viscous heating changes the distribution of heat in viscous flows , In addition, viscous heating has been shown to play an enhanced role in fluids with low thermal conductivity and high viscosity.
Ou and Cheng  considered the impact of viscous dissipation on heat transfer at the inlet of a pipe with steady heat flux. Béget al.  obtained numerical solutions for the nonlinear flow and heat transfer of MHD Hartmann–Couette in a Darcian channel with porous medium and Ohmic dissipation. Other interesting and recent research into the thermofluid dynamics of pipes has been reported in [29, 30, 31, 32, 33, 34, 35, 36, 37].
Again, their results are not analytical and are obtained through FLUENT software or numerical methods. Therefore, in this case, analytical results are necessary and we aim to examine the analytical solution for the two-dimensional heat transfer effects in a circular pipe under externally applied uniform suction and magnetic field on the surface of the circular pipe.
The transformed, non-dimensional boundary value problem is solved by the powerful Homotopy Analysis Method (HAM) , which offers excellent flexibility and convergence features for non-linear ODE resolution. The method was successfully applied to a number of interesting problems [39, 40, 41, 42, 43, 44]. The present study relates to MHD energy systems.
Analytical solutions for the thermal transfer of viscous magneto-hydrodynamic pipe flow using the homotopy analysis method (HAM) have been presented. Viscous heating, Mhd and suction/injection effects of the wall have been included. A HAM convergence study was also carried out to guarantee the robustness of the series solutions. The calculations showed that:
- ➢ Increasing magnetic body force parameter quickens the radial flow while it will in general decelerate axial flow.
- ➢ Increasing the magnetic parameter improves temperatures since it produces thermal energy dissemination attributable to the additional work required to drag the liquid against the magnetic field the pivotal way.
- ➢ Increasing suction Reynolds number decelerates the axial flow and upgrades the radial flow.
- ➢ With expanding Eckert number, the temperature along the outspread course is diminishing while with expanding Prandtl number it is raised.
- ➢ Skin friction is almost constant after a certain range of suction Reynolds number for all magnetic parameter values i.e. for low Reynolds numbers, the effect of magnetic parameter is significant.
More information: Eva Cosoroaba et al. 3D multiphysics simulation and analysis of a low temperature liquid metal magnetohydrodynamic power generator prototype, Sustainable Energy Technologies and Assessments (2019). DOI: 10.1016/j.seta.2019.05.012