Euro plan to harness waste heat
Researchers at NTNU have been looking at
the possibilities for doing something about this.
“Surplus heat from industrial processes is
a huge resource,” says Kim Kristiansen.
He has just completed his PhD on a
technology that can harness some of the surplus heat that currently goes to
waste. Almost all the heat generated by industrial processes is currently
released directly into the air or the ocean, and we are not talking about small
amounts. In Norway alone, industry produces around 20 TWh of waste heat each
year.
That number might not mean much to you, but according to the Norwegian Water Resources and Energy Directorate (NVE), this amount of energy corresponds to half of the electricity consumption of all Norwegian households combined. In other words, approximately the entire heating demand.
Kristiansen is part of the thermodynamics
research group at PoreLab in the Department of Chemistry. Academic supervisor
Signe Kjelstrup and research group manager Øivind Wilhelmsen are co-authors of
the articles in question.
Drinking water as an added bonus
The technology also has another effect that
may not be as relevant in Norway, but which might be a game changer in
countries with limited drinking water.
“The technology doesn’t just recycle the
waste heat energy, it can also purify the waste water produced by industry,”
says Kristiansen.
In many parts of the world, drinking water
is becoming an increasingly scarce resource.
“According to UNICEF, four billion people
are already experiencing severe drinking water shortages for at least one month
of the year, and there is a high demand for technology that can meet these
challenges,” says Kristiansen.
A lack of drinking water is therefore a
problem for approximately half of the world’s eight billion people.
Producing clean water
So what is this new technology?
“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,” says Kristiansen.
This method is best suited for purifying
water with so-called non-volatile impurities, such as salt. This is in contrast
to alcohols and a number of other organic substances that can evaporate along
with the water through the membrane.
“The most important area of application for
this technology is therefore desalination of seawater. The treatment of process
water is not being ruled out, but it involves additional challenges depending
on its content,” says Kristiansen.
So, the technology can produce drinking
water, but what about exploiting the waste energy?
Exploiting temperature differences to pump
up water
When water is heated on one side of the
membrane, it evaporates and releases heat on the other side through
condensation. A pressure difference may then arise between the two sides of the
membrane
“The temperature difference is used to pump
the water up, and the pressure difference represents mechanical energy that can
be used to power a turbine,” says Kristiansen. The phenomenon is called thermal
osmosis.
Seemingly simple, but ingenious.
“We have investigated the interactions
between water and the pores in the membrane, and what happens when the water
evaporates, is transported through the pores, and condenses,” says Kristiansen
about the doctoral research.
He has designed theories on membrane
properties and the effect they have on the entire process. He has also
systematically measured this effect in the laboratory.
“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,” says Kristiansen.
A Dutch idea
Kristin Syverud at the RISE PFI research
institute is interested in improving the membranes used in this technology.
“The starting point for the work was an
idea that TNO
in the Netherlands gets the credit for,” says Kristiansen’s
academic supervisor Signe Kjelstrup.
She is Professor Emerita and former Head
Researcher at PoreLab – Centre of Excellence. TNO is an independent institute
that works to translate research findings into real-life applications.
TNO experimented with the concept called
‘MemPower’ (simultaneous production of water and power) and the prototype was
made at their facilities. The researchers wanted to collaborate but had no
funding. The solution was to continue the project as open research at NTNU.
“Leen van der Ham from TU Delft got in
touch with me and I introduced the idea to the group I then had at the
Department of Chemistry, and later at PoreLab.”
Van der Ham took his PhD in Chemistry at
NTNU a few years ago, which shows just how important it is to have contacts.
They worked with Luuk Keulen, a student at TU Delft, and the research was
continued by Kristiansen and Michael Rauter from PoreLab.
Practical challenges
“Industry is showing interest in the
concept of membrane distillation, but so far, there are only a few pilot plants
worldwide,” says Kristiansen.
The main reason industry is lagging behind
academia is related to practical challenges associated with membrane
technology, he explains. For example, this applies to the lifespan of membranes
under harsh industrial conditions.
“A lot of work is being done
internationally in both academia and industry to meet these challenges and
commercialize the technology,” says Kristiansen.
The MemPower concept involves converting
waste heat into mechanical energy based on differences in temperature.
“My impression is that industry is not yet
fully aware of this concept and the opportunity it offers,” says Kristiansen.
One of the conclusions in the latest
article is that the potential for energy production is competitive in relation
to more established membrane-based energy processes. He believes this could
help increase commercial interest.
Reference: Kristiansen, Kim and Wilhelmsen,
Oivind and Kjelstrup, Signe, Thermo-Osmotic Coefficients in Membrane Distillation
experiments and Theory for Three Types of Membranes. Desalination,
Volume 586, 2024, 117785, ISSN 0011-9164, https://doi.org/10.1016/j.desal.2024.117785