Elsevier

Applied Energy

Volume 146, 15 May 2015, Pages 383-396
Applied Energy

Heat transfer fluids for concentrating solar power systems – A review

https://doi.org/10.1016/j.apenergy.2015.01.125Get rights and content

Highlights

Abstract

There is a strong motivation to explore the possibility of harnessing solar thermal energy around the world, especially in locations with temperate weather. This review discusses the current status of heat transfer fluid, which is one of the critical components for storing and transferring thermal energy in concentrating solar power systems. Various types of heat transfer fluids including air, water/steam, thermal oils, organic fluids, molten-salts and liquid metals are reviewed in detail, particularly regarding the melting temperature, thermal stability limit and corrosion issues. Stainless steels and nickel based alloys are the typical piping and container materials for heat transfer fluids. Stability of the stainless steels and alloys while in contact with heat transfer fluids is very important for the longevity of concentrating solar power systems. Corrosion properties of stainless steels and nickel based alloys in different heat transfer fluids are discussed in terms of corrosion rates.

Introduction

As part of the intensive global search for alternative clean and renewable energy sources, concentrating solar power (CSP) is being considered as one of the key technologies due to its potential to meet base load applications. The International Energy Agency (IEA) has set an electricity generation target of 630 GWe for CSP technology by 2050 [1]. Fig. 1 shows the most common CSP technologies including (a) parabolic dish systems (PDS), (b) parabolic trough collector (PTC), (c) solar power tower (SPT) and (d) linear Fresnel reflector (LFR). Among these, the parabolic trough collectors are currently the most utilized technology with >95% of the CSP installations. Although the CSP technologies date back to 1970s, most of the commercial plants have been developed in the last decade [2], [3], [4]. To date, Spain (∼60%) and the United States (∼40%) are the two largest markets for the CSP technologies, and the world’s largest CSP plant was commissioned in the USA in 2014 (Ivanpah Dry Lake, CA). This installation is a collaborative venture of NRG Energy, Bright Source Energy and Google. The Ivanpah plant is capable of producing 392 MWe of electricity that can power roughly 100,000 homes [5]. A comprehensive list of CSP stations in operation and under construction around the world is available in a recent publication [4].

Among these four different CSP technologies, PDS and LFR are quite uncommon in current installations, except a recently commissioned LFR in India. This Indian LFR is the world’s largest LFR, known as ‘Dhursar’ (Dhursar, Rajasthan – 125 MWe) [6], [7]. LFR technology is still in the experimental stages and the main advantage with this technology is the low amount of required land area, but the efficiency of LFR seems to be less compared to other CSP technologies [6]. Even though PTC systems occupy greater than 95% installation among the currently installed CSP plants, SPT systems are more preferred than PTC in future trends. Most of the recent CSP installations around the world as well as in the United States are SPT systems; the world’s largest ‘Ivanpah Solar Power Tower’ is also an SPT system. The main reason for the present increasing trend of installing SPT systems is that they are more suitable for achieving very high temperatures [8] compared to other CSP technologies and thereby enhancing the efficiency of converting heat into electricity.

CSP systems are based on a simple operating principle; solar irradiation is concentrated by using programmed mirrors (heliostats) onto a receiver, where the heat is collected by a thermal energy carrier called heat transfer fluid (HTF). Such is the configuration of a solar tower CSP system shown in Fig. 2 which tracks the sun across the sky. The heliostat mechanism can capture sunlight efficiently during winter, when the sun is typically angled lower than the horizon [9]. The HTF can be used to directly drive a turbine to produce power or, more commonly, be combined with a heat exchanger and a secondary cycle to generate steam [2], [10], [11].

HTF is one of the most important components for overall performance and efficiency of CSP systems. Since a large amount of HTF is required to operate a CSP plant, it is necessary to minimize the cost of HTF while maximizing its performance. Besides transferring heat from the receiver to steam generator, hot HTF can also be stored in an insulated tank for power generation when sunlight is not available. Desired characteristics of a HTF include: low melting point, high boiling point and thermal stability, low vapor pressure (<1 atm) at high temperature, low corrosion with metal alloys used to contain the HTF, low viscosity, high thermal conductivity, high heat capacity for energy storage, and low cost [3], [10]. The HTFs can be classified into six main groups based on the type of materials; (1) air and other gases, (2) water/steam, (3) thermal oils, (4) organics, (5) molten-salts and (6) liquid metals [3], [4]. Fig. 3 provides a comprehensive list of working temperatures of various HTFs. As seen in Fig. 3, the working temperature range for organics and thermal oils are 12-393 and (−20) – 400 °C, respectively. Molten-salts have been the most widely studied HTF due to their high working temperature (more than 500 °C) and heat capacity, low vapor pressure and corrosive property, and good thermal and physical properties at elevated temperatures [12]. Liquid metals are also promising candidates for high temperature solar plants.

One important concern in CSP designs is the capability for thermal energy storage for night-time power generation. Spain has pioneered in thermal energy storage technologies and thermal energy storage capability of CSP systems employing molten-salts has been commercially proven after the launch of ‘Andasol-1’ trough plant in Spain at the end of 2008 [9]. Presently, almost half of the CSP plants in Spain are equipped with thermal energy storage capability; molten-salts are used in almost all the thermal storage systems. Not only these molten-salts can withstand high temperatures and are suitable for thermal energy storage, but also they are relatively cheaper compared to other types of HTFs such as organics or thermal oils.

Corrosion of container and piping alloys is an important problem in CSP systems. HTFs act as the electrolyte in a corrosive system that attacks the metal containers [13]. High operating temperature is necessary to improve efficiency in the CSP system and molten-salts are the most promising HTF candidates at high temperatures up to 800 °C, but the corrosion issues are more significant in CSP plants operated with molten-salts compared to other HTFs, mainly because of the high operating temperatures. The corrosion issues of piping/container alloys in contact with commercially used HTFs as well as recently proposed and tested molten-salts HTFs are discussed and summarized in detail in the following sections. The thermal stability range, thermal conductivity, viscosity, heat capacity, cost and corrosion rate for piping/container alloys are summarized in the comprehensive Table 1 for all the possible HTFs.

Section snippets

Air and other gases

Air is a relatively uncommon HTF in large CSP plants. Only one commercial scale system has been constructed, a 1.5 MWe pre-commercial plant built in Jülich, Germany (Jülich solar tower) which began operation in 2009. With air as HTF, extensive temperature range is possible [3], [14]. In the Jülich solar tower, air at atmospheric pressure is heated up to about 700 °C [4] and then the hot air is used to generate steam. It is both a research facility and a model project for future power plants in

Conclusion

The IEA’s target of 630 GWe power generation by CSP technology by 2050 has triggered the development of various technologies such as (a) parabolic trough collector, (b) solar power tower, (c) linear Fresnel reflector and (d) parabolic dish systems. The central theme in all these technologies is harnessing solar thermal energy through heat transfer fluids for storing and transferring thermal energy in concentrating solar power systems. Solar power tower technology is the current and future trend

References (72)

  • G. Meier

    A review of advances in high-temperature corrosion

    Mater Sci Eng A-Struct Mater Prop Microstruct Process

    (1989)
  • J. Birnbaum et al.

    Steam temperature stability in a direct steam generation solar power plant

    Sol Energy

    (2011)
  • A. Modi et al.

    Performance analysis of a Kalina cycle for a central receiver solar thermal power plant with direct steam generation

    Appl Therm Eng

    (2014)
  • L. Tan et al.

    Corrosion behavior of Ni-base alloys for advanced high temperature water-cooled nuclear plants

    Corros Sci

    (2008)
  • J. Robertson

    The mechanism of high-temperature aqueous corrosion of stainless-steels

    Corros Sci

    (1991)
  • A. Gil et al.

    State of the art on high temperature thermal energy storage for power generation. Part 1 – Concepts, materials and modellization

    Renew Sustain Energy Rev

    (2010)
  • M. Ouagued et al.

    Estimation of the temperature, heat gain and heat loss by solar parabolic trough collector under Algerian climate using different thermal oils

    Energy Convers Manage

    (2013)
  • D. Cabaleiro et al.

    Thermophysical properties of (diphenyl ether + biphenyl) mixtures for their use as heat transfer fluids

    J Chem Thermodyn

    (2012)
  • Q. Peng et al.

    Design of new molten salt thermal energy storage material for solar thermal power plant

    Appl Energy

    (2013)
  • S. Kuravi et al.

    Thermal energy storage technologies and systems for concentrating solar power plants

    Prog Energy Combust Sci

    (2013)
  • N. Boerema et al.

    Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

    Sol Energy

    (2012)
  • R.I. Olivares et al.

    LiNO3–NaNO3–KNO3 salt for thermal energy storage: thermal stability evaluation in different atmospheres

    Thermochim Acta

    (2013)
  • A.G. Fernandez et al.

    Development of new molten salts with LiNO3 and Ca(NO3)(2) for energy storage in CSP plants

    Appl Energy

    (2014)
  • Y. Wu et al.

    Experimental study on optimized composition of mixed carbonate salt for sensible heat storage in solar thermal power plant

    Sol Energy

    (2011)
  • C.Y. Zhao et al.

    Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems

    Sol Energy Mater Sol Cells

    (2011)
  • T. Wang et al.

    Novel low melting point quaternary eutectic system for solar thermal energy storage

    Appl Energy

    (2013)
  • K. Vignarooban et al.

    Corrosion resistance of hastelloys in molten metal-chloride heat-transfer fluids for concentrating solar power applications

    Sol Energy

    (2014)
  • J. Pacio et al.

    Thermodynamic evaluation of liquid metals as heat transfer fluids in concentrated solar power plants

    Appl Therm Eng

    (2013)
  • A.K. Rivai et al.

    Compatibility of surface-coated steels, refractory metals and ceramics to high temperature lead-bismuth eutectic

    Prog Nuclear Energy

    (2008)
  • World’s largest solar power plant opens in California. Is it the Future, or a Dead End? Retrieved on May 23, 2014 from:...
  • Dhursar. Retrieved on Jan 4, 2015 from:...
  • M. Moser et al.

    Potential of concentrating solar power plants for the combined production of water and electricity in MENA countries

    J Sustain Dev Energy Water Environ Syst

    (2013)
  • R.I. Dunn et al.

    Molten-salt power towers: newly commercial concentrating solar storage

    Proc IEEE

    (2012)
  • J.G. Cordaro et al.

    Multicomponent molten salt mixtures based on nitrate/nitrite anions

    J Sol Energy Eng-Trans ASME

    (2011)
  • S. Zunft et al.

    Julich solar power tower-experimental evaluation of the storage subsystem and performance calculation

    J Sol Energy Eng-Trans ASME

    (2011)
  • J. Klower

    High temperature corrosion behaviour of iron aluminides and iron–aluminium–chromium alloys

    Mater Corros-Werkstoffe Und Korrosion

    (1996)
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