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Technological Advances to Maximize Solar Collector Energy Output

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Technological Advances to Maximize Solar Collector Energy Output Swapnil S. Salvi School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India Technological Advances to Vishal Bhalla1 School of Mechanical, Maximize Solar Collector Energy Materials and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India Output: A Review Robert A. Taylor Since it is highly correlated with quality of life, the demand for energy continues to School of Mechanical and increase as the global population grows and modernizes. Although there has been signifi- Manufacturing Engineering; cant impetus to move away from reliance on fossil fuels for decades (e.g., localized pollu- School of Photovoltaics and tion and climate change), solar energy has only recently taken on a non-negligible role Renewable Energy Engineering, in the global production of energy. The photovoltaics (PV) industry has many of the same The University of New South Wales, electronics packaging challenges as the semiconductor industry, because in both cases, Sydney 2052, Australia high temperatures lead to lowering of the system performance. Also, there are several technologies, which can harvest solar energy solely as heat. Advances in these technolo- Vikrant Khullar gies (e.g., solar selective coatings, design optimizations, and improvement in materials) Mechanical Engineering Department, have also kept the solar thermal market growing in recent years (albeit not nearly as rap- Thapar University, idly as PV). This paper presents a review on how heat is managed in solar thermal and Patiala 147004, Punjab, India PV systems, with a focus on the recent developments for technologies, which can harvest heat to meet global energy demands. It also briefs about possible ways to resolve the Todd P. Otanicar challenges or difficulties existing in solar collectors like solar selectivity, thermal stabil- ity, etc. As a key enabling technology for reducing radiation heat losses in these devices, Department of Mechanical Engineering, the focus of this paper is to discuss the ongoing advances in solar selective coatings and The University of Tulsa, working fluids, which could potentially be used in tandem to filter out or recover the heat Tulsa 74104, OK that is wasted from PVs. Among the reviewed solar selective coatings, recent advances in selective coating categories like dielectric-metal-dielectric (DMD), multilayered, and Patrick E. Phelan cermet-based coatings are considered. In addition, the effects of characteristic changes School for Engineering of Matter, in glazing, absorber geometry, and solar tracking systems on the performance of solar Transport & Energy, collectors are also reviewed. A discussion of how these fundamental technological advan- Arizona State University, ces could be incorporated with PVs is included as well. [DOI: 10.1115/1.4041219] Tempe, AZ 85287 2 Keywords: solar thermal collector, solar energy, solar selective coating, glazing, Himanshu Tyagi absorber, heat transfer, nanofluid, concentrated photovoltaic, photovoltaic cooling School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India e-mail: [email protected] 1 Introduction incident on the surface of the earth for 30 min, it can fulfill the world’s total energy demand for one year [1]. Solar energy is The demand for a clean and sustainable environment is of top abundant, and widely available throughout the world, but it only priority due to the impact fossil fuels have on the environment. To represents a small slice of the “energy consumption pie” relative achieve a clean and sustainable environment, the usage of renew- to oil, coal, and natural gas. able energy resources should be increased because they have the While delivery of solar energy is free, the collectors needed to potential to provide significant amounts of energy. There are a convert it to useful energy have historically been expensive (rela- variety of renewable energy resources available like solar energy, tive to fossil fuels). However, with improvements in conversion wind energy, biomass, geothermal, ocean thermal, hydroelectric- efficiency and with mass production in China, the cost of solar ity, and nuclear energy on the earth’s surface. Solar energy is a collectors ($/W) has been falling rapidly. Photovoltaics (PVs) prime source of energy and is widely available. The sun’s total have enjoyed very rapid year-on-year growth rates (with a dou- power output is 3.8 Â 1023 MW, out of which 1.7 Â 1014 kW is bling of installed capacity every 2–3 years). In fact, 7–10% of incident on the surface of the earth. If this amount of power is the 7–8 Â 106 metric tons of silicon produced each year is now devoted to the production of silicon photovoltaics (even with con- 1Present address: Centre for Energy and Environmental Engineering, National tinual trend toward using thinner wafers). In fact, compared to Institute of Technology, Hamirpur, 177005, Himachal Pradesh, India integrated circuits, photovoltaics now represent a 100Â larger 2Corresponding author. silicon wafer area than electronic packaging (1100 Â 109 m2 of Contributed by the Electronic and Photonic Packaging Division of ASME for PV wafer versus 7 Â 106 m2 of semiconductor wafer shipments publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received May 4, 2018; final manuscript received August 19, 2018; published online October 1, 2018. in 2017) [2,3]. Silicon PV modules convert only 12–18% of the Assoc. Editor: Ankur Jain. incoming radiation into electricity, so more than 80% of solar Journal of Electronic Packaging Copyright VC 2018 by ASME DECEMBER 2018, Vol. 140 / 040802-1 Downloaded From: https://electronicpackaging.asmedigitalcollection.asme.org on 08/18/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use irradiation either gets converted into heat or reflected back [4]. and GaInAsP/GaInAs—use less than half of the solar spectrum While this conversion rate does depend on the material of the cell and the rest of the incident solar energy (other than the band gap and the working conditions, most of the incoming solar energy must of semiconductor used) is dissipated as heat [28]. Thus, most of be dissipated, either through passive or active mechanisms. In order the incident solar energy ends up as heat. This heat can either be to reduce the reflective losses of desired wavelengths (those above considered a waste product (dumped to the environment) or, the bandgap of the cell), antireflective layers [5] can be added. To potentially, as a useful source of energy for thermal applications prefilter the incoming spectrum (e.g., ensure only short wavelengths (Heating, ventilation and air conditioning, process heat, and even reach the cells), decoupled photovoltaic/thermal hybrid solar collec- further electricity generation), depending on its thermodynamic tors (PV/T) have been proposed. These use spectrally selective mir- quality. Ultimately, the design of a PV cooling system is analo- rors or absorption filters [6,7] to utilize the wasted infrared (IR) part gous to a typical electronics package, in that the goal is to keep a of the spectrum and to avoid heating the PV cell. thin semiconductor from exceeding a set junction temperature. Although there are a variety of semiconductor materials that For example, to ensure a loss of less than 10% from its rated per- work as photovoltaic materials—e.g., cadmium telluride (CdTe), formance (at 25 C), the cells would have to stay below 45 C for copper-indium-gallium-selenide (CIGS), cadmium sulfide (CdS), a module with a degradation of 0.5% per C. It should be noted, amorphous silicon (a-Si), cuprous sulphide (Cu2S) and gallium however, that this may not be possible with passive air cooling in arsenide (GaAs) and others [8], mono- and multicrystalline silicon hot climates where the ambient temperature can exceed 40 C. technology seems to have won the commercial battle [9], with In conventional photovoltaic systems, this heat has almost 95.5% share of the global PV production in 2017 (the other 4.5% exclusively been perceived as a harmful waste product, which is thin-film technology, namely CdTe and CI(G)S, but also amor- must be removed as efficiently as possible (at low temperature). phous silicon) [10]. To highlight how far this technology has This perspective comes from focusing on minimizing the damage come in the last 6 decades, in 1958, the solar-to-electrical energy done by the loss coefficient. However, with the addition of a good conversion efficiency of a silicon cell was 11% and the cost thermal package, a significant portion of the >80% heat can be was $1000/W [1]. Now, monocrystalline cells have achieved extracted (rather than dissipated) for use in air and water heating 26.7% efficiency in the lab and crystalline Si modules cost less for buildings [30]. Further, with advanced photovoltaic materials than $1/W [10]. Due to the high initial cost of photovoltaics, the and architectures, it may even be possible for this unused heat to technology was originally limited to space applications (with very be harvested at high temperature for industrial process heat or high efficiency cells still limited to space applications), where even as a means to generate additional electricity [31,32]. Regard- cost is not as much of a constraint as weight or reliability [11–13]. less of its intended use, heat removal from PV cells can be done As costs came down, photovoltaics gained traction in terrestrial either actively or passively, depending on the heat fluxes and rela- applications, such as remote monitoring [14–16], and off-grid tive temperatures involved. lighting [12,13], water pumping [17,18], and battery charging [19–23]. In recent years, however, photovoltaics have achieved 1.1.1 Passive Photovoltaic Cooling. Whether they have been grid parity in many parts of the world and several >100 MWe designed for it or not, all PV modules exhibit some amount of pas- solar farms have been constructed [10].
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