The
challenge in solar energy today is not the cost of photovoltaic
(PV) electricity generation, already competing with fossil fuel prices,
but rather utility-scale energy storage and flexibility in supply.
Low-cost thermal energy storage (TES) exists but relies on expensive
heat engines. Here, we introduce the concept of luminescent solar
power (LSP), where sunlight is absorbed in a photoluminescent (PL)
absorber, followed by red-shifted PL emission matched to an adjacent
PV cell’s band edge. This way the PV cell operates nearly as
efficiently as under direct illumination but with minimal excessive
heat. The PL absorber temperature rises because of thermalization,
allowing it to store the excessive heat, which can later be converted
into electricity. Tailored luminescent materials that support an additional
1.5 kW h PV electricity for every 1 kW h of (virtual) heat engine
electricity with a dynamic shift between the two sources are experimentally
demonstrated. Such an ideal hybrid system may lead to a potential
reduction in the cost of electricity for a base-load solution.
Subcooled flow boiling has been investigated for horizontal mini and micro channels of which hydraulic diameters are 1mm and 150μm, respectively for high heat flux cooling in electronics. The heating surface is 1mm in width and 10mm in length for the mini channel. Eleven micro grooving are made on the copper heating block of 5.25mm×5.25mm. Aqueous solutions of ethanol, 10% and 50% in mass concentration, are used as boiling liquid for the micro channel. Microbubble emission boiling (MEB) of water is generated at liquid subcooling of 40K in the mini channel as same cases of conventional macro channels and the maximum heat flux obtained is a 10MW/m2 at liquid velocity of 1m/s (1000kg/m2s). However, the boiling turns to film boiling at low liquid velocity, 0.3m/s (300kg/m2s) for an example. In subcooled boiling of aqueous solutions, the heat flux becomes small for the lower ethanol concentration. The critical heat fluxes are well agreed with the existing theories and the maximum heat fluxes are higher than CHF. However, no micro bubble emission boiling is observed in subcooled flow boiling of mini channels and the CHF is considerably smaller than the existing theories. It is difficult to generate MEB for micro channels with heating surface of large thermal capacity because the coalescing bubbles formed on the heating surface are filled up in the channel and the liquid vapor exchange is disturbed.
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