Heat transfer fluid is one of the main component of a typical CSP system. It is
responsible to transfer the absorbed heat from the receiver to the power block section where power is generated and in some cases, where heat storage is applied, it is used to store heat for later use when sun rays are not available. For any CSP plant to operate, a large amount of heat transfer fluid is required. Thus, it is necessary to minimize it is cost and maximize its performance. The preferred characteristics of a typical heat transfer fluid include: low melting point, high boiling point and thermal stability, low vapor pressure (less than atmospheric pressure) at high temperature, low corrosion with the metal alloys that contains the heat transfer fluid, high heat capacity for energy storage, low viscosity, high thermal conductivity, and low cost [36]. In Figure 2-20, the working temperature ranges for thermal oils, molten-salts and liquid metals are shown.
39 Figure 2-20: Operating temperature range for various heat transfer fluids [37].
Based on the material of the heat transfer fluid, it can be classified into six main groups:
(1) air and other gases, (2) water/steam, (3) thermal oils, (4) organics, (5) molten-salts and (6) liquid metals [36]. As the liquid metals are still under study for the concentrated solar applications, it will not be considered.
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2.3.3.1 Air
Air usage as a heat transfer fluid is still limited in large CSP plants. In 2009, a 1.5MWe plant was built in Jülich, Germany that utilizes air as the heat transfer fluid in the open volumetric receiver. The air is heated up to a temperature of 700°C at atmospheric pressure to generate steam in the power block section [38]. As the air is abundant and cost free, this technology is cost effective and has high efficiencies. Moreover, due to the very low dynamic viscosity related to other liquid metals heat transfer fluids, air has decent flow properties inside the pipelines in a CSP system [39] . One of the draw backs of using air is that it requires large volume of air.
2.3.3.2 Water/ Steam
Usually water/steam fluid is used as both heat transfer fluid and working fluid in plants where steam Rankine cycle is used to produce the electricity. This means that plant operated with less losses and costs associated with heat exchangers. The use of
water/steam as both heat transfer fluid and working fluid in the power cycle simplifies the system and end up with improved efficiency and cost reduction of electricity production [40].
This heat transfer fluid is used currently in one of the world’s largest CSP plant – the Ivanpah solar power facility that was launched in February 2014. Moreover, there are
41 seven commercial CSP plants in the world working with water/steam as the single fluid.
Four plants are in Spain (Puerto Errado 1, PS10 solar power tower, PS20 solar power tower and Puerto Errado 2) and the other three are in California, USA (Kimberlina solar thermal energy plant, Bakersfield, Sierra sun tower, Lancaster and Ivanpah solar power facility, Ivanpah dry lake) [38].
Besides all of the less losses and cost reduction with using water/steam as both heat transfer fluid and working fluid, the system will require extra effort to control due to the phase change phenomena of the water and steam (evaporation) in the receiver [13].
Moreover, one of the major problem with using water/steam as a heat transfer fluid is the lack of water in desert regions CSP plants where large land area and high direct solar radiation intensity are available [41].
2.3.3.3 Thermal oils
Synthetic oils, silicone oil and mineral oil have been used as heat transfer fluids in CSP plants along time ago. Examples of such plants are the Andasol-3, Helioenergy, Aste, Solacor and Solnova plant located in Spain using parabolic trough collector [38]. They have the advantage of delivering predictable and stable receiver operation. Most of these oils have the same thermal conductivity and can be thermally stable only up to 400 °C and that is why they are not usually used for high temperature applications and very efficient solar thermal systems [38]. From Figure 2-20, it can be noticed that thermal oils
42 have limited operating temperatures compared to the molten salt and liquid metals.
Moreover, they show a decomposing affect when operated at high temperature with fire hazards if leaking outside the pipes. Cost wise, these thermal oils are highly expensive [42].
2.3.3.4 Organics
Organic materials are also heat transfer fluids that are used in CSP systems.
Biphenyl/Diphenyl, for example, is an oxide pair (also known as Therminol VP-1) that is usually used in total of eight commercial CSP systems, especially in thermal plants located in Spain [37]. Operating temperature range of this Biphenyl/ Diphenyl oxide is very narrow within 12–393 °C [38]. As the thermal oils, the operating temperatures are limited compared to molten salts and liquid metals.
2.3.3.5 Molten salts
Molten salt heat transfer fluid is a fluid that has the advantages of being a single phase fluid in the receiver, has a high specific heat, and has a thermal stability at high
temperatures. Using the molten salt in the thermal energy storage sector has proven the its effectiveness with the use of insulated tanks. Moreover, molten salts also have
properties similar to water at high temperature including similar viscosity and low vapor pressure [43]. Most of the used salts solidify at temperatures below 220 °C and this means that external heating is required to keep the salt away from solidification during
43 cols starts [13]. In addition, molten salts are corrosive and it reacts with air and water if leaks occur.
Molten salts are used in modern CSP systems with the earliest molten salt power tower systems operated back in 1984. These innovative systems were the THEMIS tower (2.5MWe) in France and Molten-salt Electric Experiment (1MWe) in the United States [37].
Most of the currently used salts are based on nitrates/nitrites among various heat transfer fluids. Solar salt, NaNO3 (60 wt%)–KNO3 (40 wt%), is a common used salt in many modern CSP systems. It melts at 223 °C and remains in thermally stable liquid phase at temperatures up to 600 °C [44]. The second commonly used salt is the Hitec salt. It consists of NaNO3 (7 wt%)–KNO3 (53 wt%)–NaNO2 (40 wt%) and it is mixture of alkali-nitrates/nitrites. The major advantage of Hitec salt is that its melting point (142°C) is much lower than that of Solar Salt [45]. This advantage will reduce the amount of energy required for heating to keep the salt from solidification.