Autor: José González-Aguilar-IMDEA Energía
Current commercial concentrating solar power (CSP) plants are still largely based on mineral oil parabolic trough technology, developed nearly 30 years ago, and molten-salt and direct steam generation towers . At the same time, Fresnel technology has not fully developed its complete potential achieving a limited deployment and Stirling dish technology has not reached the required degree of development and cost-reduction in order to be competitive with respect to the other STE technologies. Parabolic trough power plants employ Rankine-cycle power blocks with low temperature (< 400ºC) steam turbines which operate with relatively low efficiencies (~35% when dry-cooled ), whilst central receiver CSP plants achieve higher temperatures, which have cycle efficiencies in region of 40%, leading to reduced costs. Despite this, both technologies have their associated drawbacks.
Molten-salt systems are limited to operating temperatures below 565 °C by the thermal stability of the salt itself, preventing the use of even more efficient, higher temperature power conversion cycles. Molten-salt systems also suffer from freezing problems if the salt temperature drops too low, resulting in high parasitic power consumption for heat-tracing. Direct steam systems are not limited in the temperatures they can achieve, as no intermediary heat transfer fluid is used. However, they typically operate with steam temperatures in the region of 535 °C and no cost-effective large-scale storage system has been developed for live steam. Use of this technology therefore negates the key advantages of solar thermal power: the ability to store energy . As such, if the true potential of CSP technology is to be unlocked, new advances on heat transfer media (HTM) are needed that can both reach higher temperatures and easily be stored. Reaching higher temperatures is seen as key to future cost reductions, as higher temperatures lead to both higher power conversion efficiencies and increased storage densities, directly reducing the total cost of the solar collector field and the specific cost of the storage units. Recent reports points out that improvement in heat transfer fluids (HTFs) and storage solutions result in an expected LCOE reduction varying from 2.3% for Central Receiver to 5.6% for Parabolic Troughs CSP plants .
A wide range of alternative high-temperature HTM are being studied for use in CSP plants, including improved molten salts, liquid metals, gases, and solid particles . In principle, the simplest solution would appear to be to develop new molten salt materials that are capable of resisting higher temperatures and/or have lower melting temperatures; in this way existing receiver technology could be used, reducing the required investment. However, in order to overcome the temperature limitations imposed by the nitrate salts currently used in CSP plants  it is necessary to switch to ternary, quaternary or even quinary mixtures based on nitrate, carbonates and chloride salts, which suffer from corrosion issues at high temperatures, significantly increasing maintenance costs . Liquid metals, mainly based on sodium and lead and their alloys (NaK, lead-bismuth eutectic) are also being explored as HTF due to their high thermal conductivity and low viscosity. However, these fluids have important safety hazards. Alkali metals react with both air and water, thus leading to the risk for accidental fires. Lead-containing liquid metals require specific measures in order to avoid their toxicity by ingestion (proper ventilation, isolation and hygiene facilities). In addition, liquid metals have higher costs that molten salts currently used in CSP plants and they have lower heat capacities, which conduct to lower performance than molten salts as storage media .
Inert gases (e.g. air, helium, sCO2, etc.) are other alternative as HTF, eliminating the thermal decomposition and corrosion problems and even use them directly as working fluid in appropriate turbines or thermal engines. Thus intermediate heat exchangers are avoided increasing the energy available for electricity production. The use of air as the working fluid in solar tower power plants has been demonstrated since the early 1980s. Main advantages of using air are its availability from the ambient, environmentally-friendly characteristics, no troublesome phase change, higher working temperatures, easy operation and maintenance and high dispatchability. It is a suitable heat transfer fluid in desert areas, where water availability is scare. However its low heat transfer poses challenges for receiver design, while their low densities complicate the integration of energy storage . Supercritical CO2 has recently attracted the solar community attention as HTF since it can operate at very high temperatures, provides suitable thermophysical properties related to the supercritical state and can be directly used as working fluid in sCO2 turbines .
The use of solid particles as the HTM is another option, capable of reaching temperatures of 1000 °C when ceramic particles are used . Solid particle HTM are also ideally suited for storage applications, which can be easily implemented through simple bulk storage of hot particles. The solid particles are typically directly irradiated by the concentrated sunlight, allowing for very high heat fluxes as there is no interposing material to limit heat transfer. However, this approach leads to high heat losses (thermal efficiencies < 50% under real conditions ) and significant difficulties in controlling the flow of loose particles within the receiver. Within this approach, the dense particle suspension (DPS) is an alternative to the classical solid particle HTM, combining the good heat transfer properties of liquids and the ease of handling of gases with the high temperature properties of solid particles. The DPS consists of very small (μm-scale) particles which can be fluidized at low gas velocities and then be easily transported in a similar manner to a gas .
Inert gases (Air and sCO2) and solid particles (including DPS), which have the highest potential to operate at very high-temperatures between the aforementioned HTFs, are currently investigated in the High Temperature Processes Unit in the framework of national and international research projects (CM Alccones, PN SOlarO2, FP7 CSP2 and IRP STAGE-STE). The research focuses on the development of innovative solar receivers and reactors capable to handle these heat transfer fluids including testing at 15 kWth scale using high-flux solar simulators as well as the design of plant layouts in order to analyze the integration of these HTF (including specific components) and its impact on the CSP plant performances.
Figure 1. Scheme of a CSP plant using dense particles suspension as Heat Transfer Fluid 
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