Nitrate salts doped with CuO nanoparticles for thermal energy storage with improved heat transfer Philip D. Myers Jr. a,b , Tanvir E. Alam b,c , Rajeev Kamal a,b , D.Y. Goswami a,b,⇑ , E. Stefanakos b,d a Department of Chemical and Biomedical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA b Clean Energy Research Center, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA c Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA d Department of Electrical Engineering University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA highlights  We propose a CuO-doped nitrate salts for thermal energy storage.  Doped salts show increased thermal diffusivity/conductivity.  No evidence of thermal instability was observed in the CuO doped nitrate salts. article info Article history: Received 23 June 2015 Received in revised form 27 November 2015 Accepted 28 November 2015 Keywords: Thermal energy storage PCM HTF Thermal conductivity abstract Molten salts possess significant potential for use as heat transfer fluids (HTFs) and/or thermal storage media in advanced high-temperature concentrating solar power (CSP) plants. However, the thermal per- formance of these materials is hindered by typically low thermal conductivity—on the order of 1 W/m-K in the solid phase and less in the liquid phase. Much work has been done to improve the thermal conductivity of HTFs through the addition of nanoparticles of higher conductivity, such as metallic nanoparticles or nanoparticulate graphite. These nanofluids display improved thermal conductivity and otherwise behave similarly to the pure HTFs. This investigation proposes such a system, focusing on the nitrate salts: potas- sium nitrate, sodium nitrate, and the potassium–sodium nitrate eutectic (54 weight percent potassium nitrate), with melting points of 334 °C, 306 °C, and 222 °C, respectively. Attention is also paid to use of these materials as latent heat thermal energy storage (TES) materials—i.e., phase change materials (PCMs). The nitrate salt melt is a highly oxidative environment, so the use of carbonaceous materials or elemental metals may be hampered by degradative effects. Hence, this investigation specifically examines cupric oxide (CuO) nanoparticle-enhanced nitrate salts systems. The thermophysical properties (e.g., ther- mal diffusivity, latent heat) of the salt-nanoparticle systems are measured. Further, temperature-variant FTIR spectroscopy is used to determine any potential degradation of the salt after thermal cycling. The suitability of the enhanced nitrate salts systems, in regard to the chemical stability of the additive and the improvement of the thermal performance of the system relative to the pure salts, is demonstrated. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction In the search for viable sustainable energy sources, solar ther- mal power stands out in its comparatively mature state of develop- ment. Like many renewable energy technologies, though, standalone solar energy conversion suffers a lack of dispatchability, due to the intermittency of the solar resource [1]. It is clear that reliable, efficient, and cost effective storage systems are needed to raise the level of competitiveness of solar power to that of exist- ing and inherently dispatchable fossil fuel conversion systems. Fur- thermore, improved efficiencies in the thermodynamic cycles of solar thermal power require elevated operating temperatures; at such temperatures organic HTFs are unacceptably unstable. Molten salts present a promising potential solution to these two problems. High temperature systems that utilize molten salts have proven effective [2]. As far as thermal energy storage (TES) is concerned, molten salts may be deployed in two ways: first, as sensible heat storage media (i.e., dual-tank storage) [2]; second, as latent heat http://dx.doi.org/10.1016/j.apenergy.2015.11.045 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Clean Energy Research Center, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA. Tel.: +1 8139740956; fax: +1 8139742050. E-mail address: goswami@usf.edu (D.Y. Goswami). Applied Energy 165 (2016) 225–233 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy