Transforming industrial waste heat into energy and clean drinking water
As the manufacturing sector strives to become more sustainable, researchers throughout industry and academia are working on new ways to analyze and utilize byproducts generated during the production process. Whether this waste is being repurposed, recycled, or reused, manufacturers are constantly on the search for cost-effective (or even profitable) ways to go green, and it looks like reclaiming industrial waste heat might be the next frontier.
According to Steinar Brandslet for Norwegian SciTech News, Norwegian industry produces 20 TWh of waste heat each year, which is equivalent to half of all the energy consumed by Norwegian households. Most of this heat is released into the air or the ocean, but there might be a new way to utilize this resource.
Research on reclaiming waste heat began at TNO in the Netherlands. The institute, which strives to convert academic research into real-life applications, developed a prototype called ‘MemPower’ (simultaneous production of water and power), but due to a lack of funding, the project was transferred to researchers at the Norwegian University of Science and Technology (NTNU).
The NTNU team recently published a paper titled "Thermo-osmotic coefficients in membrane distillation: Experiments and theory for three types of membranes," that outlines how waste heat can be converted into mechanical energy based on differences in temperature. Basically, the process utilizes thermal osmosis, which occurs when water on one side of a membrane is heated, evaporates, and releases heat on the other side through condensation. A pressure difference is then created, which represents mechanical energy that can be used to generate power.
In an excerpt from the paper’s abstract, the team writes: “The thermo-osmotic coefficient of a porous medium is the ratio of the mass flow rate and the temperature difference that drives the flow. Thermo-osmotic coefficients of the Millipore DuraPore HVHP, GVHP and VVHP membranes have been measured using a new apparatus designed to provide accurate mass flux measurements with tightly controlled temperatures, pressures, and compositions on the feed and distillate side. To properly analyze the experimental data, recommendations on data reduction procedures anchored in non-equilibrium thermodynamics are discussed. An expression is presented for the apparent energy of activation of the transport process in terms of the derivative of the thermo-osmotic coefficient with respect to the inverse mean temperature. The expression is shown to accurately predict the temperature dependence of the thermo-osmotic coefficients from experiments. The expression is next used to explain observations in similar porous systems from the literature. Based on this, an optimization scheme to find the optimal mean pore radius to maximize the power density of a pressure-retarded membrane distillation (PRMD) process is presented.”
Reclaiming waste heat is not the only benefit of this technological advancement. It can also purify industrial waste water. In a recent quote, NTNU researcher Kim Kristiansen said, “The waste water produced by industry is often contaminated. If we evaporate this impure water through small pores in a water-repellent membrane, the condensed water that emerges on the other side is drinkable.” According to the team, this advanced membrane technology is best used to remove non-volatile impurities like salt from water, making it an ideal option to desalinize seawater.
Kristiansen added, “The conclusion is that the technology has great potential. Through modification of the membranes, we can help address both the increasing challenges associated with energy efficiency requirements and the lack of clean drinking water.”
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