Cooling systems today collectively account for 17 percent of the electricity used worldwide. All together, that’s 8 percent of global greenhouse gas emissions.
“What keeps me up at night is that our energy use for cooling might grow sixfold by the year 2050, primarily driven by increasing usage in Asian and African countries,” said Aaswath Raman, a professor of materials science and engineering at UCLA. “That is, emphatically, a good thing for the health, well-being, and productivity of people living in warmer climates. However, one of the most alarming things about climate change is that the warmer our planet gets, the more we’re going to need cooling systems — systems that are themselves large emitters of greenhouse gas emissions. This then has the potential to cause a feedback loop, where cooling systems alone could become one of our biggest sources of greenhouse gases later this century.
“But this also points us to an amazing opportunity,” he continued. “A 10 or 20 percent improvement in the efficiency of every cooling system could actually have an enormous impact on our greenhouse gas emissions, both today and later this century.”
What might that look like? Below is a sampling of some of the up-and-coming technologies that could change the way HVAC looks for the next generation.
A teapot full of boiling water on the kitchen table will gradually cool down because heat is flowing from the hot water to the colder table. However, its temperature is not expected to fall below that of the table. By itself, heat can only flow from a warmer object to a colder one, not the other way around. To cool the water further, it would have to be placed in a refrigerator, which consumes energy from outside to make it work. That’s the second law of thermodynamics — one of the fundamentals of physics.
At the University of Zurich, Andreas Schilling, a professor in the department of physics, created a device that, at first glance, seems to challenge that. Researchers were able to to cool a 9-gram piece of copper from over 100°C to significantly below room temperature without an external power supply, using the Peltier element — a component often used to cool hotel minibars or CPUs in computers. The Peltier element can transform electric currents into heat currents.
“The more electric current you feed into a Peltier element, the more heat is transported from one end to the other end of the element, with the result that one end becomes cold and the other one hot,” Schilling said. “The inverse process is also possible: If there is a temperature difference between both ends of a Peltier element, an electric voltage results, and an electric current starts to flow as soon as the electric circuit is closed.”
Schilling’s team is using both effects, in combination with an electric inductance. The temperature difference (the water is initially hotter than its surroundings) creates a voltage across the ends of the Peltier element, and a current starts to flow. The resulting electric current does not stop flowing when the temperatures are equal and, therefore, continues to cool the water further.
Schilling said this research is actually consistent with the second law of thermodynamics as it was originally stated; “heat can only flow from hot to cold” is a paraphrase, he said.
“The original version by [German physicist Rudolf] Clausius was more cautious, stating that the reverse process is possible if there is some ‘other change’ occurring at the same time. This ‘other change’ here is the changing electric current that is flowing at the same time.”
In principle, any material could be cooled well below room temperature in this manner, Schilling said; the passive thermal circuit could also be used as often as desired, without the need to connect it to a power supply. However, air would need a heat exchanger connected to the Peltier element.
“Although the circuit cannot run in a strictly continuous mode like a refrigerator, one could repeat the process as often as desired if the ‘heat bath’ used is large enough — our ground can be such a heat bath, similar to ground-source heat pumps,” he said. “In my view, if ever commercialized, it would be a tremendous device in regions or situations where you simply have no source of energy — or just for the sake to save energy for cooling.”
Schilling admitted that large-scale application is still a long way off. The Peltier element that is commercially available today is not efficient or affordable enough, and the current setup requires the use of superconducting inductors to minimize electric losses.
“So I have to say that one has to wait until technology has developed better thermoelectric materials … [widespread adoption] probably has to wait for a new material discovery, rather than for incremental improvements.”
THE SOLAR THERMAL COLLECTOR
Therm-X is a solar-thermal assisted air-conditioning, heating, and refrigeration system, and it’s already on the market.
“We firmly believe that what the LED light did to the lighting industry, this product is going to offer the same in the HVAC industry,” said Mark Crabtree, CEO of SolXenergy LLC. “It’s a game changer, as far as energy consumption is concerned.”
Crabtree said the problem in the industry today is the general perception that mass flow is in line with the system’s running capacities.
“For example, if you have a four-stage compressor system, the perception is that if one compressor is running then only 25 percent of the mass flow is flowing. But that’s not the case,” he said. “In all the new systems today, the majority of the mass flow is delivered much earlier in the process. Take inverter compressors … 100 percent of the mass flow is flowing at 75 percent of the capacity of the compressor, which is why they say that the inverter compressor is at its most efficient when it is running at 75 percent. Essentially, all it needs to ramp up further than that for is for heat gain.”
The Therm-X adds a solar thermal collector, following the compressor and before the condenser. That allows the sun to provide free energy, so the compressor doesn’t have to work as hard to produce the thermal energy required to meet the delta T requirements to produce credible subcooling. The collector has a copper or stainless steel pipe that runs through an evacuated glass tube, so the glass doesn’t come into contact with the refrigerant. The sun’s radiation creates heat inside the evacuator tube, a bit like a thermal flask.
“We passed that heat into the refrigerant, and it passes through the pipe that runs through the evacuator tube,” he said. “You would still need the electrical source. It allows the system to run on a much lower pull of electricity, and as such, makes creating or developing a system … much more cost-effective.”
The Therm-X has been installed in residential applications in California, Texas, and Florida, and sold commercially for just over five years outside the U.S. One client saw in excess of a 50 percent reduction in cooling costs since the system was installed.
“Over the period of a year, on a system running 24 hours a day, it would save between 35 and 40 percent reduction on an average system,” Crabtree said. “When the sun’s in the sky ... during the day, we can see savings in excess of 60 percent.
“To me, because I’m a commercial guy, it’s the cost savings,” he said. “Obviously, there are other benefits, like the switch to green applications, the fact that this means the system is producing significantly less CO2 into the atmosphere, [and requiring less] production of energy to run the system in the first place. And it’s essentially maintenance-free. It’s just not rocket science. It’s a very simple technology to install.”
The return on investment on traditional solar in the U.S. is between seven and nine years; this product promises a return on investment to the end user between two and five years, he said. It’s about 30 to 35 percent more in upfront cost, compared to a traditional off-the-shelf product of a similar SEER rating.
The SKYCOOL SYSTEM BLASTS HEAT INTO SPACE
Most people see space as a source of heat from the sun. But away from the sun, outer space is really a cold place. Shanhui Fan, professor of electrical engineering and director of the Edward L. Ginzton Laboratory at Stanford University, and his team have developed a material that turns the cold of outer space into a renewable resource for heating and cooling. They’re using a solar material — an optical film 1.8 microns thick, made of quartz and silicon carbide, which not only reflects heat from the sun but also uses radiative cooling — to reflect as much as 97 percent of sunlight while emitting the thermal energy of the building it is placed on.
Thermal radiation is not a new concept, said Raman.
“[There was] an ice house, also called a Yakhchal, located in the southwest of Iran,” he said during a TED talk. “The people who operated this ice house, many centuries ago, would pour water in the pool in the early evening hours, as the sun set. Even though the air temperature might be above freezing, say 5°C or 41°F, the water would freeze … all the way through the summer months. As implausible as it may sound, for that pool of water, its heat is actually flowing to the cold of space.”
That pool, like most natural materials, sends out its heat as light; the output, called thermal radiation, can be visualized with thermal cameras. The atmosphere absorbs some of that heat and sends it back, but at certain wavelengths — particularly between 8 and 13 microns — the atmosphere’s transmission window allows some of the heat that goes up as infrared light to escape, carrying the pool’s heat all the way out to outer space, which can be as cold as minus 454°F.
“So that pool of water is able to send out more heat to the sky than the sky sends back to it,” Raman said. “And because of that, the pool will cool down below its surroundings’ temperature.”
The surface that’s doing the cooling needs to be able to face the sky, not the sun. Fan and his team applied nanophotonics or metamaterials research to make this possible during the day.
The multilayer optical material they designed is more than 40 times thinner than a human hair. It simultaneously sends heat out precisely at the transmission window, and avoids getting heated up by the sun.
While the top layer, made of traditional solar panel material, gets hot, the bottom layer is cold to the touch, demonstrating that heat radiates up from the bottom layer, through the top layer, and into space.
The team launched a startup, SkyCool Systems, in 2017 and built fluid cooling panels that can be integrated with a condenser to improve a system’s efficiency. Per a field trial in Davis, California, efficiency was demonstrated to improve as much as 12 percent.
Raman believes the cold of space could potentially be used to reduce a building’s energy usage by two-thirds, build a cooling system that requires no electricity input, power a heat engine that generates electricity, and help with water conservation and off-grid scenarios.
“We’re constantly bathed in infrared light; if we could bend it to our will, we could profoundly change the flows of heat and energy that permeate around us every single day,” he said. “This ability, coupled with the cold darkness of space, points us to a future where we, as a civilization, might be able to more intelligently manage our thermal energy footprint at the very largest scales.”
‘SMART MUSCLE’ RELEASES AND ABSORBS HEAT
Biomedical applications, like stints for clogged arteries, make use of the super-elastic properties of nickel-titanium: It’s elastic, and it has shape-memory.
“You have a very large straining capability of the wire; you can pull it by 6 or 8 percent of its original length without damaging it,” said Prof. Stefan Seelecke, chair in intelligent material systems at Saarland University.
Seelecke and Prof. Andreas Schütze, also at Saarland University, are making use of those properties with a highly efficient, environmentally friendly heating and cooling device.
“It’s a completely novel way of making use of a smart material, and it brings the potential of coming up with a very highly energy-efficient solution for the future,” Seelecke said. “If you stretch it, it releases heat. When you then unload it, it absorbs heat. Basically, you can use the absorption quantity to extract heat at a low temperature, and when you quickly pull it, you transport the heat to a high temperature level. You unload it again, and it cools way below ambient, and you can repeat the process. You have a cycle that’s similar to the vapor compression cycle in a refrigerant.”
The device requires an external energy source to do the stretching and unloading, but it eliminates the need for refrigerants.
It has been termed the most promising alternative to vapor compression technology by the U.S. Department of Energy and the EU Commission.
“It has the potential of a very large coefficient of performance, and … it doesn’t require any greenhouse gases as refrigerant,” Seelecke said. “It’s all solid state – you don’t have any greenhouse-harming constituents like R-134.”
While the basic science is pretty well understood and the first demonstrator has been built, Seelecke said it’s still a ways from hitting stores.
“We’re at the step of looking at more technology-oriented research that tries to build machines that can be used for … climatization of buildings,” he said. “We need to look at the specific applications and apply it to the technology and the specific requirements.”
RESPONDING WINDOW PANEL
As associate professor at the University of Southern California’s school of architecture, Doris Sung is intimately familiar with concrete and glass and steel walls and floor-to-ceiling windows — and the complex HVAC systems and massive amounts of energy they require.
“When we lose power, we can’t open a window here [because it’s hot], and so the buildings are uninhabitable … until that air conditioning system can start up again,” she said in a TED talk. “Even worse, with our intention of trying to make buildings move toward a net-zero energy state, we can’t do it just by making mechanical systems more and more efficient. What I propose is that our building skins should be more similar to human skin, and by doing so can be much more dynamic, responsive, and differentiated, depending on where it is.”
Sung works with smart materials — in particular, a smart thermo-bimetal made by laminating two different metals together, that requires no controls and no energy.
When heated, one side will expand faster than the other and make the metal curl. Using this technology, she designed a responsive window system. As the sun moves across the shade, each tile moves individually.
“One, it’s a sun-shading device, so that when the sun hits the surface, it constricts the amount of sun passing through, and in other areas, it’s a ventilating system, so that hot, trapped air underneath can actually move through and out when necessary,” she explained. “[One prototype] is made out of about 14,000 pieces, and there’s no two pieces alike at all. And the great thing with that is the fact that we can calibrate each one to be very, very specific to its location, to the angle of the sun, and also how the thing actually curls.”
As an example, Sung said the thermo-bimetal layer could be used to sheath a four-story glass building.
“It’s a screen that goes around it, and that layer can actually open and close as that sun moves around on that surface,” she said. “In addition to that, it can also screen areas for privacy, so that it can differentiate from some of the public areas in the space during different times of day.
“And what it basically implies is that, in houses now, we don’t need drapes or shutters or blinds anymore,” she continued, “because we can sheath the building with these things, as well as control the amount of air conditioning you need inside that building.”
She’s developing building components for the market, like double-glazed window panels with a bimetal pattern between the panes.
Those could be used in a high-rise building where the panel systems go from floor to floor, up to 30 or 40 floors. She’s also looking at how to bring air through holes in the sides of a building.
“On the left, it’s when it’s cold … the thermo-bimetal is flat so it will constrict air from passing through the blocks, and on the right, the thermo-bimetal curls and allows that air to pass through,” Sung said. “Air could potentially be coming through the walls instead of opening windows.
“When you’re tired of opening and closing those blinds day after day, when you’re on vacation and there’s no one there on the weekends to be turning off and on the controls, or when there’s a power outage, and you have no electricity to rely on, these thermo-bimetals will still be working tirelessly, efficiently, and endlessly,” she said.
Publication date: 5/27/2019