This laboratory setup was used to test the principles behind Wang and Miljkovic’s concept for a hybrid solar thermoelectric system that could deliver both electricity and heat. Photo: Dominick Reuter |
Systems
to harness the sun’s energy typically generate either electricity or heat in
the form of steam or hot water. But a new analysis by researchers at the
Massachusetts Institute of Technology (MIT) shows that there could be
significant advantages to systems that produce both electricity and heat
simultaneously.
The
new study incorporates thermoelectrics into a concentrating solar thermal
system, also called a parabolic trough. Such systems use long, curved mirrors
(the trough) to focus sunlight onto a glass tube running along the centerline
of the trough. A liquid pumped through that tube gets heated by the sun, and
then can be used to produce steam to drive a turbine, or used directly for
space heating or industrial processes that require heat.
The
new MIT study “shows a unique opportunity for thermoelectrics integrated within
solar thermal systems,” says Evelyn Wang, associate professor of mechanical
engineering at MIT, who was coauthor of a paper describing the potential for such hybrid systems in
Solar Energy.
The
novel arrangement proposed by Wang and graduate student Nenad Miljkovic embeds
a thermoelectric system in the central tube of a parabolic-trough system so
that it produces both hot water and electricity at the same time. The key to
making this work is a device called a thermosiphon that draws heat away from
the “cold” part of a thermoelectric system, maintaining its temperature
gradient.
Wang
and Miljkovic’s system would modify a parabolic-trough system’s central tube
into a series of concentric tubes: A narrower tube inside the first would
contain the thermoelectric material, with an even narrower tube at the center
of the apparatus housing the thermosiphon, passively transferring heat from the
thermoelectric cold side and alleviating the need to pump cooling fluid as in a
conventional parabolic-trough system. The heat carried away by the thermosiphon
could then be used to heat water for space heating, industrial processes, or
hot water.
One
advantage such a system has over traditional photovoltaics, Wang says, is that “thermoelectrics can be much cheaper than photovoltaics.” Also, conventional
solar cells do not operate well at high temperatures. But, she explains,
thermoelectrics thrive in hot conditions, which allow them to build up a
greater temperature gradient.
“There
really is no solar system now to do combined electricity and heat production at
high temperature,” Miljkovic says. But he adds, “there are companies actively
trying to pursue this.”
“There’s
an opportunity for bringing together different technologies,” Wang says. The
thermosiphon, which draws heat from one place to another just as a siphon draws
liquid, is “a passive way to transfer heat … and can be low cost as well,” she
says.
Thermosiphons
are typically filled with materials that undergo a phase change (usually from
liquid to vapor) as they heat up, and can achieve a thermal conductivity “much
higher than any solid material,” Wang says. “It’s an efficient way to carry
away the heat, to whatever you want to deliver it to.”
Wang
and Miljkovic devised a computer model to search for optimal combinations of
existing materials for the thermoelectrics and the thermosiphon. This model
allows different combinations to be tested at varying operating conditions to
make the overall system as efficient as possible.
A
system for a single house could provide both heat and electricity, Wang says. “In a house, you need a lot of heat, but you only need so much electricity,”
she says. While the thermoelectric efficiency of such a system is relatively
low, “in a household system you don’t need that much power” relative to heat,
she says.
Abraham
Kribus, a professor of mechanical engineering at Tel
Aviv University
in Israel
who was not involved in this research, says this paper “describes a fresh
approach to solar energy conversion, with optimistic results showing high
theoretical conversion efficiency.”
Kribus
adds that because this is still an early stage of analysis, it’s not yet clear
how such a system would stack up to traditional solar systems on cost and
reliability. But that’s not a criticism, he says: “This is the situation at
early stage with every nonconventional idea. … Overall, the paper shows a nice
start and a very capable team behind it.”
Wang
agrees that it is likely to take a few years to develop a practical
implementation of these ideas. She and Miljkovic are going ahead with “working
on building a system to demonstrate” how the combination could work, she says.
