The HVAC sector is dominated primarily by the vapor compression cycle. However, there are two primary concerns: first, they are driven by electricity and secondly, the refrigerants used in the vapor compression cycles have the potential to cause harm to the environment. Hence, widespread efforts are being directed toward the investigation of refrigerants with low ozone depletion potential as well as global warming potential, and also with low flammability and toxicity. Adsorption heat pumps have the potential to utilize waste heat and provide cooling and heating with minimal electricity use and few moving parts. Conventional embodiments of adsorption heat pumps restrict their applicability due to some key design limitations. The adsorption and desorption cycle times for such systems can be up to a few hours depending on the system size and working pair, making them impractical for applications such as space conditioning. Rapid cycling of the adsorbent beds increases the adsorbent system viability for HVAC applications by yielding a competitive performance. This research thrust involves the development of adsorption heat pumps incorporating adsorbent-coated microchannels.
A variant of this pump can be used to store energy. Depending on requirements, adsorption and desorption processes in this adsorbent bed can be triggered. Furthermore, such a heat pump design can be easily implemented in automobiles, where the radiator coolant can become the source for actuation of the heat pump, thereby eliminating the need of a dedicated compressor.
Pahinkar, D. G., D. B. Boman and S. Garimella (2020). "High Performance Microchannel Adsorption Heat Pumps." International Journal of Refrigeration DOI: https://doi.org/10.1016/j.ijrefrig.2020.07.020.
The development of Negative Emission Technologies (NETs) has been deemed essential to mitigate the environmental concerns due to continued emission of CO and other greenhouse gases. While Direct Air Capture (DAC) of CO is a very attractive approach for this purpose, dilute concentrations (~400 ppm) of CO in the atmosphere makes the existing DAC systems highly energy intensive, when compared to post-combustion capture technologies. Among various DAC technologies, alkaline solutions (AS) have high affinity for CO in air, however, the regeneration of the solution requires temperatures up to 900℃. In contrast, the solid sorbent systems have lower regeneration energy requirement with temperatures typically less than 100°C. Yet, their conventional embodiments often struggle with the rate of CO capture for a given energy input, because their designs are bulky and not often scalable or modular leaving a great scope for improvement.
This research involves the development of a high performance modular metal organic framework (MOF solid sorbent)-based DAC system that will operate on combined pressure swing adsorption (PSA) and temperature swing adsorption (TSA) to minimize the energy required for DAC. This system will concentrate the ambient CO from 0.04% to 1% by volume. The DAC component shall employ minichannels with a porous MOF-adsorbent layer coated on the inner wall of each of them. The use of minichannels to fabricate the DAC component results in extremely small thermal mass of the system, quick species transport, small cycle times, and sharp thermal and species wavefronts, thereby improving the DAC design drastically, instead of relying on mesh, membranes or packed bed designs employed by conventional systems. Building on these attributes, this innovative DAC design will allow us to increase CO capture rate as high as the same with AS with a reduced regeneration energy .
Globally, there are only two companies in North America that are actively developing DAC systems, while there are five others in Europe. Their business models depend on using CO2 as a commodity for conversion into synthetic fuels, plastics, concrete mixes, carbon nanotubes to sustain the operating cost of capturing CO2, and as a result generally incur compression, storage and transportation costs of CO2 for such applications. The total sequestration cost is high and is expected to rise every year due to inflation, yet the state-of-the-art DAC systems are far from competitive in making CO2 as a profitable commodity. Therefore, even though the existing DAC companies are removing atmospheric CO2, these solutions are not truly carbon negative. Either a significant technological improvement is needed to lower the capital and operating cost of the DAC plants by a factor of at least 15 or CO2 from the DAC must be utilized to produce a high revenue generating commodity, which can in turn sustain the operating and maintenance cost of any DAC system. Therefore, a DAC technology that provides an integrated self-sustaining solution, wherein the captured CO2 is utilized onsite to generate and cultivate biomass and biofuel precursors, such as human and fish food, agar etc. has an immense potential to improve its market penetration and become competitive source of biofuels.
Microchannel-based temperature swing adsorption (TSA) processes, which involve alternating flow of gases and liquids have been shown to not only yield competitive or superior performance than the existing processes, but also to be environment-friendly as they use heat for their operation. To realize this potential, however, understanding of simultaneous interactions of gases and liquids with adsorbents on a fundamental level becomes critical. For example, when hot water heat transfer fluid (HTF) makes direct contact with the adsorbent that already has CO2 present in adsorbed phase; the CO2 is forced to desorb because of temperature rise and in a gaseous state encounters hot liquid water. Water also has some affinity for CO2 and therefore, CO2 from an adsorbed phase in the adsorbent is absorbed in bulk liquid water. Another factor adding to the complexity of the process is the competitive adsorption of water and CO2. In this case, relative kinetics of sensible heating and cooling of the adsorbent, adsorption and desorption of water and CO2 in the adsorbent and absorption of CO2 in water must be characterized, so that impurity removal using this approach can be understood and scaled up for commercial utility. Such a diverse characterization can be expanded to other working-coupling fluid pairs and adsorbent materials, and depending on gas separation applications, analogous processes can be designed.
While detailed fundamental multi-phase heat and mass transfer and fluid mechanics models can be developed using appropriate first principle approaches, experimental insights into the adsorption and absorption phenomena can be obtained using transient mass spectrometry. The overall research plan can include modeling and experimental research thrusts, wherein both activities would be complementary to each other.
Recently, wide band gap (WBG) semiconductor materials like silicon carbide (SiC) and gallium nitride (GaN) have been demonstrated to be excellent candidates for next generation electronic devices, which can deliver at least twice the power densities as compared to conventional silicon (Si) based semiconductors with a significantly better switching performance. However, existing electronic packaging and thermal management techniques need significant upgrades to handle higher voltages and greater heat dissipated, so that WBG semiconductor technology can be commercially viable. Conventional packaging techniques can be improved in terms of better materials that can handle the mechanical and thermal stress with improved reliability, better material stacking sequence so that heat spreading through the material layers can be optimized, and better cooling techniques that can dissipate high heat fluxes within a compact available volume. Integration of these approaches via experimental and computational analyses is the key to the development of high performance electronic systems.
Low Temperature Direct Bonding of Aluminum Nitride to AlSiC Substrates, Georgia Tech Research Corporation, 2017, Application No. 62/549,123 https://patents.google.com/patent/US20200262000A1/en
Pahinkar, D. G., L. Boteler, D. Ibitayo, S. Narumanchi, P. Paret, D. DeVoto, J. Major and S. Graham (2019). "Liquid-Cooled Aluminum Silicon Carbide Heat Sinks for Reliable Power Electronics Packages." Journal of Electronic Packaging 141(4) DOI: https://doi.org/10.1115/1.4043406.
Pahinkar, D. G., W. Puckett, S. Graham, L. Boteler, D. Ibitayo, S. Narumanchi, P. Paret, D. DeVoto and J. Major (2018). "Transient Liquid Phase Bonding of AlN to AlSiC for Durable Power Electronic Packages." Advanced Engineering Materials: 1800039 DOI: https://doi.org/10.1002/adem.201800039.