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ILP Institute Insider

June 18, 2018

Riding the thermals for solar power, thermal batteries, and water harvesting

Evelyn Wang's photovoltaic and thermal technologies aim to provide units with greater reliability, reduced cost, and lower emissive losses than current market products.

Eric Brown

Increasingly efficient solar photovoltaic (PV) cells that convert light into electricity represent a growing percentage of our energy production. Yet most solar technologies miss out on a secondary, and potentially far more efficient, source of energy: heat.

“Typical solar photovoltaic technologies take advantage of only a partial spectrum of the Sun’s energy,” says Evelyn Wang, Gail E. Kendall Professor at MIT’s Department of Mechanical Engineering. “With solar thermophotovoltaics, we’re using the entire spectrum by converting thermal energy to electricity. We can target thermal emission properties at high temperatures to efficiently generate electricity.”

Evelyn Wang,
Gail E. Kendall (1978) Associate
Professor of Mechanical Engineering

Wang is the director of the Device Research Lab (DRL), which uses macro- to nanoscale materials engineering to enhance thermal transfer processes for a variety of applications, mostly concerning solar thermophotovoltaics (TPV). Recent projects include a low-cost cost receiver unit coated with specialized aerogels that could replace more expensive parabolic troughs. The lab is also developing solid-state solar TPV cells with potentially greater reliability, reduced cost, and lower emissive losses than today’s fluid-based solar-thermal power generation technologies.

Non-solar technologies emerging from the DRL include a thermal battery that can more efficiently drive automotive HVAC systems by offloading work from an electric battery. Wang is also working a low-power, low-cost water harvesting system that can produce drinking water from air even in arid climates.

Aerogel-based Solar TPV receivers
Large scale solar power generation usually involves one of two approaches. There are solar farms of standard solar PV panels, which require large tracts of land, as well as solar-thermophotovoltaic concentrator facilities. The latter deploy mirrors in parabolic trough designs that concentrate the heat from solar energy onto a pipe or a tower containing a fluid such as synthetic oil. This conversion process in turn drives a steam engine.

Solar TPV plants have the advantage of requiring minimal real estate, but they still play a minor role in power generation, primarily due to cost. The technology also suffers from high emissive losses that reduce efficiency.

Wang’s DRL has developed new receiver technology that could provide a more affordable solar TPV solution. “The idea is to create a new type of receiver design that costs less than a typical parabolic trough, and is better at mitigating radiant losses,” says Wang.

Developed in collaboration with Gang Chen, the Head of MIT’s Department of Mechanical Engineering, the aerogel-based receiver could potentially “mitigate the need for high concentrations of sunlight,” says Wang. “Because the mirrors are typically the key economic bottlenecks in these systems, our receiver could potentially reduce the price per kilowatt hour and enable broader deployment.”

The receiver’s key innovation is the development of a new type of insulating material called optically transparent, thermally insulating aerogels. “The aerogels are good insulators, and allow us to optically transmit all of the sunlight onto the receiver,” says Wang. “The design minimizes and suppresses the radiant losses by internally keeping the pipe very hot with a non-selective, black absorbing surface. You don’t need to use a vacuum or a selective surface so it’s also simpler and costs less.”

The biggest challenge was the process of improving the aerogels’ transparency. “Aerogels already have very good insulating properties, but due to the scattering of materials, they have a bluish tint that reduces optical transparency,” says Wang. “We’ve been able to control the pore sizes of the aerogels to mitigate scattering and reduce the bluish tinge.”

With the help of Chen’s lab, Wang has successfully synthesized the aerogels and created a one meter-square prototype with a field of concentrating optics that beams sunlight onto the receiver. “We’ve demonstrated an efficient aerogel receiver in a lab,” says Wang. “Now, the key is to scale up to mass production of the aerogels.”

Solid-state solar TPVs
In the past, Wang’s lab has worked on improving the thermal fluid conversion of solar TPV systems by using micro- and nano-particles that decrease emissive losses at high temperatures. More recently, Wang has focused on developing a solid-state energy conversion process that avoids liquids altogether by concentrating sunlight on a specialized PV cell. Working in collaboration with MIT Department of Physics professor Marin Soljacic, Wang has developed a prototype of a solar TPV cell that directly converts solar thermal energy into electricity without requiring a steam engine.

The solid-state technology could eventually improve the efficiency of solar TPV. It more certainly offers improved simplicity and scalability. In this case, the goal is to scale down instead of up.

“A typical solar TPV powered steam power plant must operate at a scale of hundreds of megawatts,” says Wang. “It’s not efficient at compact sizes due to the significant radiant losses that occur at high temperatures. With our technology, we can mitigate some of the losses of the photons that are below the bandgap of a photovoltaic cell, which leads to heat generation. This gives us the spectral control such that we can get the highest efficiencies possible.”

The simplicity derives from removing the pipes, heat exchangers, and steam engine from the equation. “Instead of needing a lot of pumps to move the fluid through pipes, there are no moving parts, so we have improved reliability,” says Wang. The simplicity of the design “makes it easier to build different form factors that vary from compact to larger scales.”

The biggest challenge in solid-state solar TPV is determining how to best tailor the solar spectrum, says Wang. “One way to do this is to take advantage of photonic crystals, which can be tailored to minimize emissive losses. We have created structural components at the micron and nanoscale levels to target the spectral characteristics favorable for energy conversion. The sunlight comes in at broad spectrum at various wavelengths, so we try to manipulate that to focus on a specific wavelength that is targeted for the PV cell.”

The photonic crystals are patterned on the silicon surface of the PV cell. “You could use layers of materials such as silicon-silicon dioxide that create constructive and destructive interference such that you can tailor the spectral characteristics,” says Wang. “Alternatively, you could use two-dimensional photonic crystals – cavity structures that are etched into the material.”

Standard, single-function PVs have evolved dramatically in recent years, but are still capped at about 30 percent efficiency. “By contrast, our thermal PV cells have a theoretical limit of over 80 percent,” says Wang. “It’s the difference between capturing part of the spectrum vs. the entire spectrum.”

One additional advantage over conventional solar PV is storage. “Solar TPV is more dispatchable to storage than solar PV because batteries are significantly more expensive than thermal storage,” says Wang.

Thermal batteries for automotive HVAC
Wang’s lab is not currently working on thermal storage for solar TPV, but it is exploring a smaller scale thermal battery for automotive HVAC systems. Wang’s absorption based thermal battery is designed to offload cooling and heating duties from the electric battery bank in an electric vehicle. “Our batteries save on electricity usage, extending the driving range,” says Wang.

Like an electric battery, the thermal battery can be charged electrically, and it similarly requires high energy densities per weight and size, as well as high power densities. To achieve this, Wang has turned to absorbent materials.

“Our absorption based thermal battery uses a refrigerant that is absorbed onto material that allows us to release heat during absorption,” says Wang. “To store this thermal energy, we exploit the chemical potential difference between a wet refrigerant and a dry absorbent, separated by a valve. Opening the valve enables evaporation, which gives you cooling. You get heating from the absorption process when the refrigerant interacts with the absorbent materials.”

The lab has experimented with different types of absorbent materials. One promising candidate is a microporous, aluminosilicate mineral called Zeolite, which is widely used in industry and easily scalable. “We’ve tailored Zeolite to improve its capacity for energy storage,” says Wang. “We’ve also tried using metal organic frameworks, or MOFs, which you can tailor a little better since you can modify the structures to capture more water to further improve capacity.”

The problem with absorbents is that they typically have very poor thermal conductivity, which slows the heat and mass transfer properties. Wang is using thermal binders to compensate and achieve the required power densities. “We’ve been creating and tailoring carbon-based binders and different types of metallic foam binders to enhance the thermal conductivity.”

Several prototypes have been developed, and the battery technology is now ready for use in electrical vehicles and hybrids, says Wang. “It might even be adaptable to internal combustion engine vehicles or home HVAC,” she adds. “In the home, solar power could give you the heat to recharge the thermal battery, and you could take advantage of other waste heat streams.”

Water harvesting in the desert
The DRL has extended its thermal expertise beyond power generation and storage to address a range of water processing technologies from desalination to atmospheric water generators (AWGs), better known as water harvesting systems. There are two main types of AWGs: fog-based systems that require very high humidity, and the more common dewing systems, which typically use refrigeration to condense below the dewpoint to convert water vapor to liquid.

Wang has recently developed a more affordable and efficient alternative to dewing-based AWGs that does not rely on dewing or require refrigeration, and can even operate in dry climates. Like the thermal battery, the device makes use of absorbents. “We’re using absorbents to capture water from the humidity in the air by soaking it up like a sponge,” says Wang. “We can then release it easily at relatively low temperatures to condense the humidity as drinking water.”

The concept is well established, but has been stymied by the lack of an efficient absorbent material. Working with Professor Omar Yaghi at UC Berkeley, Wang has developed a type of MOF that can soak up moisture even in low humidity. “We have demonstrated a device that uses these MOFs to capture water at as low as 20 percent relative humidity,” says Wang.

Because the technology does not require refrigeration, it can be powered using solar panels, which heat up and release the MOF-based absorbents. “Our device is intended for remote areas with dry conditions and very limited power and infrastructure,” says Wang. “Each unit might serve a family of four. Eventually, we should be able to scale up to a system that can provide drinking water for a whole village.”

Considering that approximately 2.1 billion people lack access to safe drinking water at home, the technology could have a significant impact. The breakthrough inspired Foreign Policy to select Wang as a 2017 Global Rethinker.