Surface heat transfer due to sliding bubble motion

International audienc

Thermal performance predictions and tests of a novel type of flat plate solar thermal collectors by integrating with a freeze tolerance solution

A novel design concept of Flat Plate Solar Collectors (FPSCs) is conceived and developed by integrating with a freeze-tolerant (so-called 'ice immune') solution using flexible silicone tubing. It is intended to directly run water in the solar thermal systems with the FPSCs instead of using expensive anti-freeze fluids, and to remove secondary heat transfer facilities (e.g. an extra tank with a buried heat exchanger). Successful development of such kind of solar thermal collectors will enable a reduction of installed cost of conventional solar thermal systems without needing secondary heat transfer facilities. In the prophase design, thermal performances of FPSCs with two configurations, i.e. the serpentine tube type and the header riser type, were predicted based on the collector lumped thermal capacitance model alongside CFD (Computational Fluid Dynamics) calculations. Then two prototypes of FPSCs with the ice-immune silicone tubing (one AES serpentine tube type, one modified Chinese micro-heat-pipe-array panel) were made to determine the collector performance and compared to an original AES solar keymark reference panel via experimental tests. The results show that the Chinese micro-heat-pipe-array panel performs better than the AES header riser solar keymark panel in terms of flow rate per m2 collector aperture area, while the AES serpentine tube panel with silicone tubing performs somewhat lower than the solar keymark with T_m^*≤0.035 and better than the solar keymark when T_m^*>0.035. The serpentine tube panel and the Chinese micro-heat-pipe-array panel both integrated with silicone tubing for freeze tolerance are proven to be effective as the modification doesn’t compromise the collector thermal performance markedly

Experimental investigation of lithium-ion cells ageing under isothermal conditions for optimal lifetime performance

Lithium-Ion cell ageing is sensitive to cell temperature. Previous studies have investigated ageing under adiabatic or controlled environmental temperature (i.e., isoperibolic) conditions. Notably, these conditions do not impose a uniform cell surface temperature (i.e., isothermal condition) or a controlled cooling rate, as an active Thermal Management System (TMS) would. This leads to a clear interdependence between charge/discharge rates and cell temperature, due to the uncontrolled cell temperature history. Consequently, the separate influence of these variables on the cell performance cannot be investigated. In this study, the ageing of a 300 mAh Lithium Cobalt Oxide (LCO Li-Ion) pouch cell under isoperibolic and isothermal conditions in the range of 0 °C - 40 °C is investigated. Each cycle comprises a CC-CV (constant current-constant voltage) charge of 1C and a CC discharge of 2C. Similar average ageing rates for isoperibolic and for isothermal conditions but at different reference temperatures were found. For example, an isoperibolic temperature condition of 25 °C yielded a similar degrading rate as an isothermal condition at 30 °C. This is mainly due to the effect of the cell self-heating (Joule heating) which increases the median operating temperature above that of the surroundings. These findings emphasise that uncontrolled cell thermal conditions lead to overall performance strongly dissimilar and randomly dependent on the transient heat transfer coefficient of isoperibolic TMS. Finally, an optimal isothermal condition that maximises the cell electrochemical efficiency and minimises its ageing is identified in the range of 25 °C-35 °C