| Facing severe water shortages,great efforts were made in obtaining fresh water from salt water.With abundant solar energy reserve,photothermal evaporation becomes a strong candidate.However,traditional photothermal evaporation system is limited by low efficiency.Therefore,rational designs of solar absorber materials and their arrangement have drawn plentiful attention.After fully discussions,the low-cost and recoverable Fe3O4 nanoparticles were selected as solar absorber material.The absorption and heat conduction were additionally analyzed.Based on the position of heat source,the arrangements of evaporation systems were divided into two types,namely interfacial heating and bulk heating.Therein,interfacial heating reached high efficiency,with poor stability and economy as a price.In contrast to interfacial heating,bulk heating is observably stable and economical,with limited evaporation efficiency instead.As a result,rational designs of heat arrangements in evaporation systems still remain challenging.In order to solve this problem,improving the evaporation efficiency of bulk heating and realizing the steadiness of interfacial heating arrangements were explored and verified.Specific works respectively aimed at pool evaporation and droplet evaporation as follows:1.Based on the theory of thermal boundary layer,a nanoparticle self-assembly model at bubble interface was proposed in the evaporation of liquid pool.In this form,nanoparticles attached to the bubble surface generate heat and directly conduct in the thermal boundary layer,forming interfacial heating.The remaining nanoparticles maintain bulk heating in the water.Particularly when the bubble size is controlled in millimeter level,the superposition effect of multi-particle would generate more heat at the phase interface,thus significantly boost the evaporation.The experimental results show that,the bubbly nanofluid with 0.1%volume fraction can absorb more than 94%of the incident sunlight,becoming a good absorber material.Under the synergistic action of the bubbles and the nanoparticles,the evaporation rate is 2.5 times than that of the base liquid,which is better than the respective effect of nanoparticles and bubbles.2.The concept of liquid marbles is introduced innovatively in droplet evaporation.Liquid marbles are droplets coated with hydrophobic powder,principally applied in dynamic studies due to its stable properties.Inspired by the convenient manufacture of liquid marbles,solar absorber materials were treated with hydrophobic and stably wrapped on the droplet surface.By this way,economical interfacial heating arrangement was realized.The experimental results show that the evaporation of the liquid marbles is far stronger than that of nanofluids,realizing more than 3 times enhancement than that of water.Surface temperature of liquid marbles is significantly higher than that of nanofluids,while the internal temperature is significantly lower.Therefore,heat generated by nanoparticles directly utilized for evaporation without heating bulk water.As the properties of liquid marbles may be effected by the thermocouple used for hanging droplets,a plate evaporation experiment was designed as the control.The experimental results show that when the liquid marble is placed on the glass plate,the thermal conductivity of the liquid marble is lower than that of the thermocouple,so that the water droplets have a larger surface temperature gradient,which leads to the improvement of the evaporation intensity.In a word,liquid marbles are promising in photothermal evaporation.This work explores economic and efficient heat source designs of photothermal evaporation,which provides a new way to take advantages of available raw materials and renewable energy for direct contact solar evaporator.Such concepts and methods are of great significance in the design of nanoscale heat sources and solar-powered seawater desalination. |
