Heat transfer fluids for concentrating solar power systems – A review
Graphical abstract
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
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