Low-cost zinc-oxide nanoparticles for solar-powered steam production: Superficial and volumetric approaches

Abstract

Using renewable energy, especially solar energy, is an essential requisite for preventing global warming and climate change. Solar-powered steam generation is one of the recent solutions to the problem of environmental pollution and can be a promising way to generate clean energy. In this study, the superficial and volumetric approaches of the zinc oxide nanoparticles (ZnO) in solar vapor generation have been compared experimentally. For the volumetric approach, the nanofluid performance at different mass concentrations (0.001%, 0.002%, and 0.004%) on the solar vapor generation was examined. For the superficial approach, the evaporative performance of delignified mulberry wood coated by ZnO nanoparticles as an absorbent (ZnO-Wood) was investigated. Moreover, the effect of nanoparticle size on optical and evaporative properties was evaluated. Finally, the economic performance of the ZnO nanofluid was compared to the other nanofluids. The results indicated that the evaporative efficiency of the nanofluids containing ZnO(50 nm) nanoparticles at 3 suns (3 kW/m²) was approximately two times greater than that of water (18%). However, the evaporative efficiency of ZnO-Wood was 3.15 (56%) times greater than that of water. In addition, the results of the nanoparticle size effect showed that increasing the size of nanoparticles from 20 nm to 50 nm in the superficial and volumetric approaches increases the evaporative efficiency by roughly 10% and 3%, respectively. Finally, the cost analysis of nanofluid for solar vapor generation illustrated that ZnO nanoparticles are cost-effective compared to other nanoparticles presented in the literature.

Introduction

During the past few decades, solar energy has attracted increasing attention partly because worldwide gas reserves are projected to be depleted dramatically (Li et al., 2020). However, solar energy is an abundant and accessible source of clean alternative energy (Sahota and Tiwari, 2016) and does not require many costly and advanced technologies (Vafaie et al., 2018). The literature shows that the commercialization of traditional solar energy harvesting systems is still a significant obstacle because of their low efficiency. Fundamentally, a great deal of energy is lost in these systems due to light scattering and energy transference (Mahian et al., 2013).

Many efforts have been devoted to the application of nanoparticles dispersed in water for vapor generation by considering the direct absorption of solar energy and reducing energy losses. For example, researchers have recently reported an enhanced photothermal conversion efficiency due to the Au (Wang et al., 2017), Ag (Wang, H. et al., 2017) and Ag@TiO₂core–shell (Li, H. et al., 2017) nanoparticles. Attempts have experimentally (Amjad et al., 2017) and numerically (Jin et al., 2016) been devoted to the effect of nanoparticles concentration along with the solar power intensity on the solar steam generation efficiency. The results showed that these nanostructures have great potential to produce clean steam generation (as a volumetric approach) in desalination (Liu and Li, 2016) and sterilization (Neumann et al., 2013a), distillation or separation (Neumann et al., 2013b) systems as well as other industrial processes (Zhang et al., 2015). Although early studies in this field illustrated that the highest efficiencies were allocated to the noble metals, they faced commercialization cost as a challenge. To tackle this issue, researchers have focused on inexpensive nanoparticles such as multi-wall carbon nanotubes (Ghafurian et al., 2019a), graphene nanoplate (Ghafurian et al., 2019b), graphene oxide (Ghafurian et al., 2019c), fabricated reduced graphene oxide (Liu et al., 2018), and Fe₃O₄(Shi et al., 2017). In these studies, the highest evaporation efficiencies of 39% (Ghafurian et al., 2019a), 55% (Ghafurian et al., 2019b), 22% (Ghafurian et al., 2019c), 39% (Liu et al., 2018), and 23% (Shi et al., 2017) were achieved (see Table 1). Therefore, it can be inferred that their clean steam generation is more cost-effective than that of noble metals because there is no limitation on the occupied space and time.

During this time, Ghasemi et al. (Ghasemi et al., 2014) presented an exfoliated graphite layer supported by a carbon foam, where solar energy effectively localizes via the absorber surface (called the superficial approach). They achieved evaporation efficiency of 69% at 3 suns by a solar simulator. Next, a variety of light-absorbing material was studied as solar steam generators. The materials included polymeric materials (Zhang et al., 2015), black-gold-coated alumina nanowire layers (Bae et al., 2015), cermet absorber with polystyrene thermal insulation (Ni et al., 2016), thin structures coated with alumina (Zhou et al., 2016), 3D-printed carbon nanotube/graphene oxide composite layer (Li, Y. et al., 2017), graphene sheets with polystyrene (Zhang et al., 2017), paper-based reduced graphene oxide layer (Guo et al., 2017), paper-based reduced graphene oxide with a silicone-based porous insulation layer (Wang, Z. et al., 2017), structures coated with Ti₂O₃(Wang et al., 2017), and CNT modified paper filter (Yang et al., 2017). In these attempts, evaporation efficiencies of 86.5% (Zhang et al., 2015), 49.5% (Bae et al., 2015), 70% (Ni et al., 2016), 58% (Zhou et al., 2016), 85.6% (Li, Y. et al., 2017), 85% (Zhang et al., 2017), 89.2% (Guo et al., 2017), 89.7% (Wang, Z. et al., 2017), 82% (Wanget al., 2017), and 75% (Yang et al., 2017) were achieved (see Table 1). Surveying the literature shows vapor generation by clean solar energy in this approach requires 3 main features, including absorption ability, excellent thermal insulation, and strong capillary force. Owing to the costly and non-renewable fabrication of materials with these attributes, wood was introduced as another candidate in the superficial approach (Xue et al., 2017). Carbonized wood with a hot plate at a temperature of 500 °C (Jia et al., 2017), plasmonic wood (Zhu et al., 2018) and carbonized wood with laser treatment (Ghafurian et al., 2020a) demonstrated that wood is an environmentally friendly, hydrophilic, broadly available, self-floating, low-cost, renewable material that has good thermal insulation, which can be suitable for clean steam production.

In this respect, it should be highlighted that clean steam generation using low-cost nanomaterials and wood is still a novel topic with many gaps. For instance, cost-effective, stable, and long-term clean steam production needs further investigation. Hence, in the present study, the performance of delignified mulberry wood coated with ZnO nanoparticles and the ZnO nanofluid in clean steam production are compared experimentally. ZnO nanoparticles and delignified mulberry wood were chosen because of the (1) decrease in the expense of generating solar steam, (2) increase in wood stability leading to long-term stability at the surface, and (3) increase in light absorption of wood, as well as (4) to compare superficial and volumetric approaches for ZnO nanoparticle (5) and evaluate nanoparticle sizes (20 nm and 50 nm) in the superficial and volumetric approaches.