November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
Concentrating Solar Power (CSP) technologies use mirrors to collect and focus sunlight onto a heat exchanger that conveys the solar-derived energy to a heat transfer fluid and ultimately to a thermal power cycle. CSP technologies have been around for decades, originally developed in parallel with photovoltaic (PV)-based systems as a lower cost alternative at the utility scale. Today, PV and CSP are nearly equivalent on a cost basis, and the value of CSP is closely tied to its ability to incorporate low-cost thermal energy storage at the utility scale, allowing solar power plants to produce electricity day and night. The use of thermal energy storage to decouple the solar resource from power generation greatly improves power generation flexibility, allowing CSP plants to complement PV facilities by increasing output in the early evening hours as PV plants, which generally don&’t include storage, come offline.Many thermal energy storage configurations have been envisioned for CSP generation. These are generally categorized by the manner in which the energy is stored (e.g. sensible, latent, or thermochemical) and the temperature at which the storage is required, which is tied to the operating point of the thermal power cycle. Today, the research direction in CSP is toward higher temperature power cycles (> 600#730;C to increase overall efficiency) and toward lower cost storage systems, which typically requires using inexpensive materials or increasing the overall storage capacity per mass of storage media. Many of the challenges, and opportunities, related to thermal energy storage for CSP exist at the intersection of materials science and systems engineering. That is, in order for CSP technologies to meet future techno-economic goals, thermal energy storage media having favorable properties (storage capacity, reactivity, and cost) must be developed and matched with systems engineered to efficiently transfer thermal energy in and out of storage. This may, in principle, seem straightforward and in fact solutions exist for storage up to about 565#730;C; however, moving beyond this temperature range leads rapidly to challenges related to the stability of the storage media, and to its reactivity with other system components. Combine this consideration with scale (gigawatt-hours per facility) and a typical thirty year plant design lifetime, and the development of thermal energy storage for utility-scale CSP becomes an interesting, and challenging endeavor.
Thermal energy storage enables low-cost dispatchable power production from renewable energy sources. Latent heat thermal energy storage (LHTES) is particularly attractive for its high energy density and nearly isothermal operation, and is ideally suited for systems such as Solar Thermoelectricity via Advanced Latent Heat Storage (STEALS), which integrates LHTES near 600 C with thermoelectric power generation from concentrated solar power. Dispatchable power generation is achieved in STEALS by use of a thermal valve (currently under development) that is capable of controlling heat flow between the LHTES and the thermoelectric generator subsystems. A strong connection between materials science and mechanical engineering have been used to address the challenges of integrating LHTES into STEALS, and solutions to these challenges are given in this present work.In selecting a LHTES material, three primary issues must be addressed: material thermal performance, corrosion mitigation, and overall system cost. Thermal performance properties of energy density (heat of fusion) and thermal conductivity have been determined through literature review and differential scanning calorimetry measurements. 200 hour corrosion experiments have been used to inform initial down-selection of LHTES materials, and further 3000 hour experiments have been used to demonstrate long-term stability with containment and heat transfer materials. Finally, both the cost of the LHTES material itself as well as the impact of the material thermal performance and corrosivity on the costs of other system components have been investigated. Using these results, a time domain system-level thermal model has been developed to consider the system configurations necessary to achieve high performance with different LHTES materials, with several insightful results. Salt materials with low thermal conductivity require extensive and costly heat transfer enhancements for exergetically efficient heat transfer, but may be compatible with low-cost standard engineering materials. Metal alloys with higher thermal conductivity may be used with much simpler heat transfer design, but corrosion experiments demonstrate that they require careful selection of containment materials or use of costly barrier coatings. By considering the impact of LHTES materials on the full STEALS system cost, this study has been able to evaluate the true merit of several candidate materials and demonstrate their viability when integrated into a full system design.
This study evaluates the performance of passive thermal management system of high power Li-ion batteries in a cold environment. Two different phase change materials (PCM) consisting of pure paraffin and paraffin-graphite composites are considered and compared with a battery module in absence of PCM material. Battery modules are subjected to different cold time periods representing short and long vehicle stops during winter. Battery performance parameters such as capacity, power, and temperature along with thermophysical properties of each module are recorded, calculated and compared for different scenarios.Results show that in spite of low thermal conductivity of paraffin wax as PCM, after short period stops, it improves battery module performance. Battery module will stay two times warmer than the module without PCM and retains its nominal capacity. On the other hand paraffin wax becomes detrimental after long cold soak. Warming rate is slower by 29% which cause 16% decrease in capacity retention compared to the module with no PCM. Increasing thermal conductivity of PCM based module using a commercial graphite/paraffin composite was shown to be disadvantages at both short and cold stops. After short stops battery warm up is as fast as unsupported module. Furthermore after long cold soak it keeps battery cold enough to lower its capacity by 17%. The unexpectedly poor performance of commercial paraffin-graphite composite PCM is thought to come from excess pressure in a battery cell. It is anticipated that by removing excess pressure, commercial PCM module resemble a module with no PCM with ability to warm up slightly faster. Lumped capacitance analysis showed that thermal diffusivity of commercial PCM module is identical to the module with no PCM and is 33