Nanoscale Thermophysics and Transport in Liquid-Vapor Phase Change Processes
Funded by NSF, the Hass Sustainable Products and Solutions Program, UC Berkeley Blum Center, CITRIS, the endowment for the A. Richard Newton Chair, and DOE through the U.S.-China Clean Energy Research Center (CERC).
Molecular-level modeling of interfacial region thermophysics for pure water and ionic solutions
A hybrid methodology combining Molecular Dynamics simulations and statistical thermodynamics was developed to explore the stability of free liquid films formed during incipient bubble merging in boiling processes. The resulting model was used to predict how dissolved ionic solids (salts) affected the interfacial region thermophysics and the stability of free liquid water films that form between adjacent bubbles during boiling processes. Subsequent experimental studies demonstrated that dissolved ionic solids (salts) in water suppresses bubble merging, which is consistent with the effects of dissolved salts on free liquid film stability predicted by our molecular dynamic simulation studies. Results of these studies clarify the mechanisms by which dissolved mineral salts affect quenching of aluminum forgings and casting in water. This research also included statistical thermodynamics modeling of the effects of near wall intermolecular attractive forces on bubble nucleation in liquids. The resulting model predicted that wall attractive forces tend to suppress nucleation, which explains the observed first appearance of bubbles away from the wall during experiments that simulate rapid transient heating of liquids in inkjet printing devices. Results of this research have been recognized as providing an enhanced nanoscale-level understanding of mechanisms associated with boiling and condensation processes. These results were summarized in a 2012 Keynote Lecture at the 8th International Conference on Boiling and Condensation Heat Transfer, which is the leading international conference on boiling and condensation heat transfer.
Nano-engineered enhanced condensing surfaces for sustainable fresh water technology
This research explored the physics of condensation on nanostructured surfaces and identified strategies for design of nanostructured surfaces that will maximize the surface heat transfer coefficient for dropwise condensation processes. Numerous investigators in this field are fabricating wide varieties of nanostructured surfaces and experimentally exploring how the surface affects dropwise condensation of water. Our work has focused on developing accurate models of dropwise condensation that can be used to explore the mechanisms at the nansocale, and provide guidance for optimizing high performance condenser surfaces. As an outcome of this research, a Provisional US Patent application was filed for the invention “Patterned Dual Filmwise-Dropwise Condensation on Metal Substrates”, Sara S. Al-Beaini and Van P. Carey, inventors (UC case number BK-2013-096). The invention consists of patterning, with a scalable and practical technique, a mirror-finish metal surface that has alternating regions of filmwise and dropwise condensation modes. A start-up company, NBD Nanotechnologies in Boston, is interested in licensing this technology.
Heat transfer in high performance inward swirl flow heat exchanger designs
Supported by funding for NSF Fellowships
This research explored the use of a novel swirl-flow heat exchanger as a means of achieving high heat transfer performance in electronics cooling or hybrid solar power applications. Use of this device with a single-phase liquid coolant and with a vaporizing coolant were explored. This work is the PhD dissertation research of Maritza Ruiz, who has been supported by a National Science Foundation Research Fellowship. Results have shown that this type of heat exchanger design can achieve very high heat flux and transfer heat very efficiently, making it an attractive option for next-generation electronics cooling and solar applications. The high performance has been shown to be a consequence of secondary-flow mixing within the channel that is induced by the swirling inward flow. Exploration of water flow boiling heat transfer in the inward swirl flow heat exchanger has demonstrated that this device can absorb heat fluxes in excess of 300 W/cm2. This is about three times the heat flux level attainable for flow boiling in straight passages under similar conditions. This high level of heat flux removal is critically important to development of the next generation of high power electronics for communications and aerospace applications, and high powered lasers for manufacturing applications. Our studies have shown that enhancing the wetting of the surface in the swirl flow device by using a nanostructured surface enhances the heat transfer and maximum heat flux capability of this device.
High Temperature Heat Collection and Storage Using Molten Salt or Molten Glass
Supported by NSF Fellowship and by funding from ARPA-E through a subcontract from Alcoa Aluminum.
Development of heat exchanger technology for aluminum production with high temperature waste recovery
In this 2 year project, research was done collaboratively with researchers at Alcoa Aluminum and Halotechnics, Inc. under the ARPA-E funded grant: “Energy Efficient, High Productivity Aluminum Electrolytic Cell with Integrated Power Modulation and Heat Recovery.” The UC Berkeley research developed heat exchangers for a waste heat recovery system for a new energy-efficient electrolytic cell design for aluminum production being developed by Alcoa Aluminum. Approximately 30% of power input is lost as waste heat from the sidewalls of conventional cells. A system design is being developed that will collect waste heat from the cell walls and deliver it to a Rankine cycle or gas turbine power generation system which will feed power back into the grid. The UC Berkeley team has developed detailed computational models of heat flow in existing and new electrolytic cell designs, and are using them to model how a heat exchanger interacts with the thermal field of the cell. With this type of modeling, we analyzed performance of heat exchanger designs to develop an optimized heat exchanger for this application. We worked with Alcoa to explore how to best integrate the heat exchanger elements into the new cell design and evaluate the sustainability and economics of this technology. This research has the potential to create next-generation aluminum production technologies that are much more energy efficient and can produce aluminum at lower cost.
Molten glass falling film heat exchangers for high temperature solar thermal and waste heat collection
Since May of 2012, we have been collaborating with researchers at Halotechnics, Inc. on modeling and design analysis of a falling film central receiver for concentrating solar thermal power generation using molten glass as the working fluid to facilitate high temperature heat collection. Use of molten glass mixtures developed by Halotechnics for heat collection at temperatures up to 1300 °C would have a transformational effect on solar thermal power generation technologies, significantly increasing their efficiency and economic competitiveness. As her Masters project, Ruth Reed completed development of a solar receiver performance model. In her PhD research Ruth is developing a more sophisticated model of transport in a molten glass falling film solar central receiver that includes the effects of strongly temperature dependent viscosity and wavelength-dependent radiation absorption and emission within the high temperature glass. Ruth has support from a National Science Foundation Research Fellowship to continue her work in this area. Her work will provide a deeper understanding of the physics of heat transfer in high temperature molten glass in solar thermal and waste heat recovery technologies, and will provide the means to optimize the designs of such systems.
Efficiency & Sustainable Information Technologies
Supported by Hewlett Packard and UC Discovery Grant Program
Fast Compact models for analysis of data center thermal management and energy efficiency:
The COMPACT fast modeling software for modeling data center thermal performance developed in this research has been found to agree well with full CFD software predictions, and the COMPACT software can compute flow and temperature fields in minutes that would take hours or days with full CFD software. Predictions of the COMPACT software have also been shown to agree well with measured thermal management data for operation of a Hewlett Packard data center and the LBNL Scientific Data Center in Oakland. COMPACT software has also been used to explore strategies for improving airflow distribution in data centers by adding partitions to alter airflow patterns. Results of this research have demonstrated that fast compact models of energy transport in data centers can be a very valuable tool for optimizing initial data center designs and for maximizing the energy efficiency when updating data centers with new equipment.
Life cycle exergy analysis of data center components and systems
The life-cycle exergy analysis (LCEA) computational software tool developed in this research has two levels of usefulness. By using it to evaluate the exergy consumption over the lifetime of a device or system during the design processes, the design can be optimized to minimize life cycle exergy consumption. Life cycle exergy consumption is used as a metric of sustainability, and the proposed strategy is to maximize sustainability by minimizing life cycle exergy consumption. For Hewlett Packard data center components and systems, the LCEA analysis software was used to explore the relative life cycle exergy costs of different data center components, and to explore the total exergy cost of information processing as a metric of its sustainability. This type of study indicated the component design features that result in the most exergy consumption, and hence have the most negative effect on sustainability. This work resulting in contribution of a chapter on exergy analysis that was include in a handbook on Energy Efficient Thermal Management of Data Centers.
Advanced Component Technologies for High Performance Renewable Energy Systems
Supported by UC Berkeley’s Center for Information Technology Research in the Interest of Society (CITRIS)
Innovative expander technologies for solar Rankine power and waste energy recovery
Small scale solar Rankine combined heat and power systems for residential buildings and small businesses have the potential to make very efficient use of solar input by using waste heat from a Rankine cycle for hot water or space heating. Because most off-the-shelf turbine expanders are optimized for gas cycles, our research explored how designs of several types of expander technologies can be optimized for combined heat and power systems using steam or a hydrocarbon working fluid. We collaborated with researchers at United Technologies Research Center in Connecticut on this research. These investigations indicated that optimized inflow radial turbines are substantially different than off-the-shelf turbines designs for air breathing applications like turbochargers. We also developed and tested improved Tesla turbine designs to evaluate their potential use in solar Rankine power systems. As an offshoot of this work, we also collaborated with Professor Michel Maharbiz’s research group in the EECS Department at Berkeley on development of a miniature liquid water Tesla turbine for power production in an energy scavenging system. Our modeling helped guide the development of the turbine design, and assess the performance data obtained for turbine prototypes fabricated by Professor Maharbiz’s research group. This technology shows promise as a means of producing mechanical power in waste heat energy scavenging systems.
Working fluid selection and high performance boiler design for solar Rankine power generation
This work included an exploration of a similarity framework for representing near-saturation properties of solar Rankine working fluids. This framework provides a very insightful way of evaluating the performance of a wide spectrum of working fluids for optimized solar thermal Rankine power systems. In a subsequent research project we also developed an innovative two-pass counter-current absorber-boiler design for solar Rankine power generation that achieves high performance through a design that minimizes heat losses to the surroundings at high thermal energy collection temperatures. A detailed performance model for the design was developed and used to optimize it for solar thermal power applications. A prototype was fabricated and tested to explore its performance characteristics. The heat transfer performance of the u-flow design was shown to be substantially more efficient than a conventional single tube concentrating absorber design. An invention disclosure for the design has been filed.
Higher performance heat pipes for concentrating solar PV power
This work was motivated by the need for heat pipes with high heat flux capabilities for thermal management of concentrating photovoltaic (PV) solar power generation systems. Our fundamental studies indicated that use of water/2-propanol mixtures as a heat pipe working fluid produces strong Marangoni effects that substantially enhance the dryout (critical) heat flux in the heat pipe evaporator. Based on our fundamental work, we subsequently developed a prototype heat pipe design that was optimized for high heat flux heat removal from a PV cell subjected to highly concentrated solar radiation (about 1000 times normal solar flux). A prototype of the optimized heat pipe design was fabricated and tested to explore its performance characteristics. Experimental data for the prototype demonstrated the high heat flux capability compared to conventional heat pipe designs, and the high effective conductivity of the design.