Thermal Energy Storage for Decarbonizing Building Heat
The building sector accounts for a third of the total energy consumption in the United States, of which 60% is utilized in space and water heating. Thermal energy storage (TES) is one of the “five thermal energy grand challenges for decarbonization” as it has the potential to be low cost, can be implemented at large scales (kW – GW), and can store energy across different time scales (hourly to seasonal) to match the supply and demand. Thermochemical materials (TCMs) in the form of salt hydrates that undergo an endothermic dehydration reaction (charging) and an exothermic hydration reaction (discharging) are promising materials for a reversible solid-gas thermal battery. To address the hygrothermal and structural stability of these materials under thermal cycling, we are developing inorganic-organic composite materials and binary salt mixtures that combine stable reaction enthalpies with fast reaction kinetics. We are also developing coupled heat and mass transfer models across different length- and time scales to predict the performance (storage capacity and thermal power output) of packed bed TCM reactors.
Thermal Brine Concentration and Zero Liquid Discharge Desalination
Desalination technologies hold great promise to sustainably provide reliable and climate-independent freshwater on a global scale, especially by treating nontraditional water sources that are inland. However, the issue with current desalination technologies is the large volumes of brine (by-product) generated during the process that has an adverse environmental and economic cost. Further concentration of the brine not only address this, but also enables zero liquid discharge (ZLD) with a water recovery >90%. We are focused on developing a thermal brine concentrator based on Air Gap Diffusion Distillation (AGDD) that operates at ambient pressure and uses modified plastic heat transfer surfaces to manage salt precipitation at high salinities. We are also developing a combined heat and mass transfer model to optimize the performance of the AGDD operating parameters, while integrating latent heat recovery for a high thermal efficiency. Overall, this AGDD-based brine concentration process has the potential to desalinate high concentrated streams and lower the levelized cost of water (LCOW) to enable beneficial reuse.
Related publications: MRS Communications (2022)
Another emerging desalination technology is based on forward osmosis (FO), which uses a draw solution to drive water flux across a membrane. However, separation of the draw from the permeate water requires significant amounts of energy, and this has limited the applicability of FO thus far. To address this, we are developing thermally responsive draw solutes based on ionic liquids (IL) and photo-thermal converters to efficiently harness the solar spectrum. We are also exploring IL mixtures and thermal regeneration pathways with fast kinetics for this liquid-liquid phase separation. This in turn enables the use of solar heat for the water-draw separation process via direct radiative heating to yield fresh water.
Related publications: Environmental Science & Technology (2021)
Photo-Thermal Converters for Enhanced Evaporation (Water)
The water-energy nexus necessitates the use of renewable energy sources for wastewater treatment, such as evaporation ponds that rely on solar energy to passively evaporate water from waste streams to achieve zero liquid discharge. However, efficient utilization of solar energy for evaporation is limited by the transparency of water in the visible and near-infrared. To address this, we convert sunlight into mid-infrared radiation where water is strongly absorbing using a photo-thermal converter. This results in radiative heat localization at the water surface, which in turn enhances the evaporation rate by over 100%. Furthermore, the non-contact nature of the device eliminates issues such as fouling and scaling with high salinity streams, thus making it suitable for brine management.
Related publications: Nature Sustainability (2020)
Air conditioning systems currently account for approximately 4% of global greenhouse gas emissions, due to the refrigerants and electricity needed to provide dehumidification and cooling. These emissions are projected to increase significantly as countries begin to adopt air conditioning more heavily. To address this, we are researching a new air conditioning cycle that uses phase separation in lower critical solution temperature (LCST) mixtures to provide both dehumidification and cooling. The cycle is powered by low-grade heat, allowing for the use of sustainable energy sources like solar or waste heat, and the cycle uses no greenhouse gas refrigerants. Our work on this “LCST cycle” spans all the way from fundamental thermodynamic analyses to experimental demonstrations.
Related publications: Energy Conversion and Management (2022)
Carbon-Negative Engineered Wood for Structural & Thermal Insulation Applications
Building materials (embodied carbon) and energy use (operational carbon) account for 47% of annual carbon dioxide (CO2) emissions. These building-related emissions are projected to rise unless significant efforts to decarbonize buildings occur. To address this problem, I am developing carbon-sequestering engineered wood composites to replace traditional carbon-intensive building materials like steel and concrete. Specifically, I am designing structural insulated panels (SIPs), which yield higher strength and insulating performance than traditional building materials. SIPs are typically manufactured using oriented strand boards (OSBs) comprised of wood chips bonded with carcinogenic adhesives (formaldehyde-based) and petroleum-derived insulation in their foam core. I am developing alternate bio-based adhesives for the OSB and natural fiber-based thermal insulation foam core that poses no health risks and sequesters CO2.
Thermoelectric energy conversion (Energy)
Over 60% of energy rejected to the environment is at temperatures below 250 °C. Thermoelectric generators based on polymers can be used to economically capture this heat and convert it into electricity at a low cost and on a large scale. Their inherently low thermal conductivity and solution processability enables new device architectures: for waste heat recovery from pipes, I developed a radial design based on characteristic thermal length scales for polymers ~1 cm. This design enables a 10x improvement in power density compared to flat plate TEGs. By spreading the heat outward, the need for active cooling is eliminated. I have also developed a close-packed printing layout for high fill factor TE devices. Fractal space-filling curves are used as interconnect patterns, enabling tessellation of the device into sub-modules for load matching to a variety of applications, and eliminating the need for power convertors. I have also developed textiled-integrated polymer TE devices. These developments enable new applications of TEs in self-powered sensors, internet-of-things, and wearable electronics.
Polymer and Hybrid Organic-Inorganic Materials (Energy)
A practical thermoelectric device requires both a p-type and an n-type material with reasonably large power factors. Although p-type organic materials with high performance have been reported and are commercially available, their n-type counterparts have not benefited from the same level of development due to their propensity to react in air. To address this, I have synthesized and characterized metal-organic polymers as electrically conducting and air stable materials, and studied their structure-property relationship. I have also developed a probe station to measure temperature-dependent thermoelectric properties of thin films.
Related publications: Journal of Applied Physics (2019); Advanced Energy Materials (2019); Advanced Electronic Materials (2019) – 1; Advanced Electronic Materials 2029) – 2; Advanced Energy Materials (2018); Advanced Functional Materials (2018) – 1; Advanced Functional Materials (2018) – 2
Renewable Hydrogen Production (Energy)
Hydrogen serves as an attractive alternative to fossil fuels for the production of steam, power, and some major chemical commodities with zero or near to zero emissions. However, hydrogen production is currently via the steam reforming of methane, which has a large carbon footprint. To address this, solar-thermal cracking of natural gas has been investigated which yields high-grade carbon as the by-product. Concentrated solar energy is used to maintain reactor temperatures over 1000 °C using a variable aperture mechanism. I have performed optical ray tracing and Monte-Carlo simulations to simulate the reactor temperature and heat loss mechanisms under different operating conditions.