The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower

Gülden Adıyaman, Levent Çolak and İlhami Horuz

#### Abstract

The solar power tower is one of the most promising technologies in concentrating solar systems for electricity generation from solar energy. Since the incident radiation from the heliostats is absorbed directly by the heat transfer fluid in the receiver, central receiver is important component of solar tower. Heat transfer fluids directly affect the efficiency of solar tower systems. Recent studies have shown that, in addition to conventional fluids, various fluids can be used in solar tower central receivers and thus the efficiency of the receiver can be increased. On the other hand, resources need to be used efficiently for sustainable development. The sustainability of a system is not only related to its energy efficiency but also closely its exergetic efficiency. In this study, solar two receiver was modeled for verification. Different heat transfer fluids such as helium, lithium, sodium, air, neon and sodium-potassium were carried out in ANSYS fluent. Temperature changes on the receiver surface, receiver thermal, and exergetic efficiency were obtained and discussed. According to the results obtained, the thermal efficiency varied between 84.54 and 90.98%. The highest thermal efficiency was obtained from helium and the lowest thermal efficiency was obtained from the sodiumpotassium eutectic, which had the highest exergetic efficiency.

Keywords: sustainability, solar energy, solar power tower, heat transfer fluid, computational fluid dynamics, exergetic efficiency, solar receiver

#### 1. Introduction

The development of technology and industry has led to an increase in energy demands in the last century. Using fossil fuels have imposed negative effects on the economy and the society. Fossil fuels with limited resources and environmental problems require new sustainable electricity generation options. For solving the problems, an important alternative for increasing energy demand is solar energy. Electricity generation from solar energy which is one of the renewable energy sources has increased dramatically in recent years and the use of optically concentrated solar technologies, which provide particularly high thermal power, has gained importance. The solar power tower (SPT), one of the optical concentrating solar systems, is one of the most promising technologies for electricity generation from solar energy.

The solar power plant consists of four main subsystems: a heliostat field, a central receiver, a steam generation subsystem, and a power block. In SPT system, the sun rays fall on the surface of the heliostat, which consists of mirror. The solar beam is concentrated by the heliostat field and is reflected to receiver at the top of a tower. The incident energy is then transferred to a heat transfer fluid (HTF) from receiver tube walls. The heated HTF passes through the steam generation subsystem which consists of heat exchangers and transfers the thermal energy to the water. Water is heated from subcooled liquid to superheated steam and then is fed to the steam turbine for electricity generation.

Since the incident radiation from the heliostats is absorbed directly by the HTF, which flows through the receiver, receiver is the important component of SPT. Heat transfer fluids used in solar receivers directly affect the efficiency of solar tower systems. Today, central receivers use water and molten salts (solar salt or nitrate salt) that allow thermal storage easily. Recent studies have shown that, in addition to conventional fluids, various types of fluids can be used in receivers.

The extreme non-uniformity of solar flux distribution on the surface of the receiver will lead to a non-uniform surface temperature. Temperature fluctuations on the receiver surface and thermal losses can be easily analyzed by using computational fluid dynamics (CFD) software. In this way, thermal efficiency can be examined easily. The sustainability of a system is closely related to exergy. Exergy is defined as the maximum work which can be produced by a flow of matter or energy as it comes to equilibrium with a reference environment. In other words, exergy is related to the quality of energy. Sustainable development requires both the resources be used efficiently and sustainable energy resources be used. Rosen et al. suggested a measure of the relation between sustainable developments and exergetic efficiency and developed a sustainability index. The benefits of considering exergy for productivity evaluation and sustainability were indicated. It was resulted that the exergy concept played an important role in evaluating and increasing the use of sustainable energy and technologies. As exergetic efficiency increased, they showed that sustainability increased and environmental impacts decreased [1].

Colomer et al. presented a method for detailed modeling of flow dynamics and heat transfer in solar tower receivers. They have made numerical solutions by separating the receiver model into four submodels as two-phase flow, heat conduction, natural convection, and radiation [2]. Rodriguez-Sanchez et al. presented a thermal model for the central receiver. The solar flux of a tubular external type operating with molten salt was modeled and receiver was drawn as a polygon in the form of flat panels. Thermal, mechanical, and hydrodynamic analyses of the receiver were performed. Thermal analysis showed that thermal efficiency is lower than the literature because radiative losses were higher than in the literature [3]. Kribus et al. studied various receiver designs that could be adapted to systems designed for power generation with the solar tower, and showed how temperature and power output changed [4]. Yang et al. studied experimentally about heat transfer performance and thermal efficiency. They investigated how heat transfer affects thermal efficiency of a receiver used molten salt as HTF [5]. Christian and Ho used ANSYS fluent to characterize and evaluate convection and radiation losses in the solar two power plant receiver. They presented a model that could be used for receiver design and demonstrated whether convection correlations in the literature were suitable for analytical evaluation of external tubular receivers [6]. Zanino et al. investigated the effects of the Reynolds Average Navier-Stokes (RANS) type turbulence model selection available in Fluent in the case of convective thermal losses occurring in the solar two central receiver [7].

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

Pacio and Wetzel focused on liquid metal technology. Three main liquid metals such as molten tin (Sn), lead-bismuth eutectic alloy (LBE or PbBi), and sodium (Na) are proposed [8]. Boerema et al. investigated liquid sodium and Hitec salt as heat transfer fluid in solar power tower receivers. They showed that liquid sodium was potential for solar thermal power systems for their wide operating temperature range [9]. Bellos et al. examined the potential fluids that could be used in parabolic trough collectors in the large temperature range of 300–1300 K. Examined working fluids are water, solar salt, Therminol VP-1, air, carbon dioxide, liquid sodium, and helium. As a result of the study, it was proven that liquid sodium exhibits maximum exergetic efficiency (47.48%). The maximum exergetic efficiency of helium, air, and carbon dioxide were 42.21, 40.12, and 42.06%, respectively [10].

#### 2. Modeling and solution

The central receiver of solar two power plant was arranged in a tubular external cylinder. It consisted of 24 panels. Each panel consisted of 32 thin-walled tubes with end-bends connected to manifolds on each end of the panel. The external surfaces of the receiver tubes were coated with a black Pyromark paint that was absorbed 95% of the incident sunlight. Table 1 shows the technical properties of the receiver [11].

#### 2.1 Geometry and mesh

Solar two solar power tower receiver had a roughly cylindrical shape. It was the external tubular cylindrical type and its shape approximated by a polyhedron made of 24 heat absorbing panels. The receiver model geometry was simplified and


#### Table 1.

Technical properties of the solar two receiver.

modeled in ANSYS Design Modeler. The receiver model was shown in Figure 1. Simplified panels were arranged as a polygon with inner diameter of 5.1 m. Receiver model consisted of 24 rectangular panels. The height of each panel was 6.2 m.

Since the outside diameter of the panel tubes was 2.1 cm and the thickness was 1.2 mm, the flow area of the molten salt was calculated as 18.6 mm, only flow areas were modeled. The computational domain was shown in Figure 2. It was a hexahedron with a height of 36 m and a 30 m wide square base.

The grid structure of the model was shown in Figure 3. More frequent mesh structure was used in the mid-region of the air domain in contact with the receiver. This was because the turbulence in this region and the heat transfer between the receiver and the air domain can be calculated with high accuracy.

The grid structure element number was approximately 600,000. This number was selected by resolving in many elements counts, until the independence from the mesh element number in the results, and the appropriate number of elements had been determined. The skewness of the mesh elements was 0.58 and it was acceptable for fluent analysis.

Figure 1. Model of the solar two receiver with simplified absorbing panels.

Figure 2. Receiver and air domain model.

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

Figure 3. Half section of model with mesh structure.

#### 2.2 Boundary conditions

ANSYS Fluent 18.1 version which is CFD software was used for numerical solutions. CFD solutions were made in steady-state regime. Discrete ordinates (DO) radiation model and RNG k-ε turbulence model were used.

Molten salt enters through the two northernmost panels (panels E1 and W1) of the receiver and flows in a serpentine pattern through the adjacent six panels. It crosses over from one side of the receiver to the other along the east-west centreline. Then it completes the path through the remaining six panels and exits the receiver through panels E12 and W12 [11].

As seen in the simplified model of the receiver, the connection pipes between the panels were not modeled. In order to provide the serpentine flow without such a pipe model, a boundary condition called 'recirculation inlet-outlet' was used in fluent. Thermal losses and pressure drops due to connection tubes were neglected.

The surface heat load (W/m<sup>2</sup> ) on the receiver with a total incident power of 40 MW was taken from the radiation map published in [6]. Each receiver panel was divided vertically into 10 sections. Each value taken from the radiation map was divided into tube wall thickness 1.2 mm and in ANSYS fluent, a volumetric heat load (W/m<sup>3</sup> ) was applied. Adiabatic conditions were assumed on the internal surface of the receiver. The radiative condition is the DO model with phi and theta discretizations of 1 and divisions of 3. The opaque surface was defined by the emissivity ratio of 0.83 on the receiver walls [12].

Thermophysical properties of molten salt, air (wind), and 316H stainless steel were given in Table 2. The density of air at atmospheric pressure, which is necessary for natural convection calculation around the receiver, was determined from the polynomial, which is given in Eq. (1) [13].

$$\rho = 3.766 - 0.0154(T) + 3 \times 10^{-7}(T^2) - 3 \times 10^{-8}(T^3) + 1 \times 10^{-11}(T^4) \tag{1}$$


#### Table 2.

Thermophysical properties of molten salt, air (wind), and 316H stainless steel [12–14].

#### Figure 4.

Temperature distribution on the receiver using molten salt.


#### Table 3.

Comparison of reference results with study results.

In each half of the receiver, mass flow rate of molten salt was 45 kg/s and inlet temperature (Tin,ms) is 563 K. It was assumed that wind flowed from the west onto the receiver at speed Vair = 8.98 m/s and temperature Tair = 300 K.

#### 2.3 Model validation and results

The study of Zanino et al. [6] was taken as a reference for verification of the model. Table 3 showed the study results and reference results of the convection losses and two panel (W12 and E12) average outlet temperature (Tout,ms). The results were in accordance with the reference study and the error rates were acceptable. The differences in the results were due to mistakes made in reading the values on the radiation map.

The temperature distribution on the receiver was shown in Figure 4. Although the average temperature of the receiver was 792 K, temperature distributions on the receiver reach up to 872 K. A uniform temperature distribution has not been observed due to non-uniform heat load. The values above the mean temperature were generally on the outlet panels of the molten salt. The sum of radiation and convection losses was determined as 2.78 MW. This loss was of 1.88 MW of radiation, 0.89 MW of convective losses. The radiation loss was more than twice the loss of convection.

#### 3. Energy and exergy analysis

Since this study considers the exergy and energy performance of the central receiver system at steady state, only the receiver was taken into.

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

The receiver efficiency, ηth, is defined as the ratio of the average power absorbed by the working fluid, Qabs, to the average power incident on the receiver, Qinc, evaluated over a defined period under steady-state conditions [11].

$$
\eta\_{th} = \frac{Q\_{abs}}{Q\_{inc}}\tag{2}
$$

The absorbed power is:

$$\underline{Q}\_{abs} = \dot{m}c\_p \left( T\_{out,hf} - T\_{in,hf} \right) \tag{3}$$

where ṁ is mass flow of HTF, and unit is kg/s. cp is the average specific heat between HTF inlet and outlet temperature (J/kgK), Tin,htf and Tout,htf are the inlet and outlet temperatures of HTF in K units.

An important assumption was made by Pacheco [11]: "Under steady-state conditions with constant inlet and outlet salt temperatures and wind velocities, the temperature distributions on the receiver surface and throughout the receiver are independent of power level. Therefore, the thermal losses are independent of the incident power".

With constant thermal losses, the energy efficiency can be expressed in terms of the absorbed power, thermal losses, and absorptivity:

$$\eta\_{th} = \frac{\underline{Q}\_{abs}}{\underline{Q}\_{inc}} = \frac{\underline{Q}\_{abs}}{\underline{Q}\_{abs} + \underline{Q}\_{loss}} = \frac{\alpha}{1 + \frac{\underline{Q}\_{loss}}{\underline{Q}\_{abs}}} \tag{4}$$

where α is the absorption ratio (0.95) of the Pyromark paint on the receiver surface. Qloss is the total thermal losses (radiation, convection, and conduction).

The quality of the energy is directly related to the temperature. Since this can be explained by the exergetic efficiency (ηex) which accounts for the rate of utilization of the energy. The exergetic efficiency, ηex, for concentrated solar system is solar field area as the ratio of gain exergy (Exu) to available solar radiation exergy (Exs) [15]:

$$
\eta\_{\rm ex} = \frac{E \mathbf{x}\_{\nu}}{E \mathbf{x}\_{s}} \tag{5}
$$

The net exergy gained from the solar field is calculated as.

$$E\mathbf{x}\_u = \mathcal{Q}\_{abs}\left(1 - \frac{T\_{amb}}{T\_{rec}}\right) \tag{6}$$

The exergy of the solar heat radiation absorbed by the receiver can be evaluated as [16].

$$E\mathbf{x}\_s = \underline{Q}\_s \left[ 1 - \frac{4}{3} \left( \frac{T\_{amb}}{T\_s} \right) + \frac{1}{3} \left( \frac{T\_{amb}}{T\_s} \right)^4 \right] \tag{7}$$

where Ts is the sun temperature (5777 K), Tamb is the ambient temperature (300 K), and Trec is the average surface temperature of the receiver. Qs is the concentrated beam radiation absorbed by the absorber (40 MW).

#### 4. Results and discussion

The heat transfer fluid used in the receiver affects the thermal efficiency of the receiver. The thermophysical properties of the fluid used in the SPT receiver influence the thermal power absorbed by that fluid. This is an important parameter for receiver efficiency. In this study, three different liquid metals such as sodium, lithium, and sodium-potassium eutectic and three different gases such as helium, air, and neon were analyzed as HTF. All thermophysical properties of the HTFs are given in Table 4 with their references. All analyses were carried out in the solar two flow path at 45 kg/s and 563 K inlet temperature for E1 and W1 panels.

According to CFD results, the thermal efficiency (ηth) of the receiver using molten salt was calculated according to Eq. (4) and the result is 87.67%. Conduction losses were taken 0.3 MW in calculations since this numerical value was obtained from solar two receiver efficiency experiments [11].

When air was used as HTF in receiver, the thermal losses were determined as 3.68 MW. The thermal efficiency of the receiver was calculated as 85.49%. The exit temperature of air was 929 K. The average temperature of the receiver was 859 K. The highest temperature seen on the receiver surface was 971 K, as shown in Figure 5(a). As expected, the lowest temperatures were observed in the North direction, the temperature increased in the South direction and reached the highest value in the South side panels.

When helium (He) was used as HTF in receiver, a significant decrease in thermal losses was observed. Radiative and convective losses were determined as only 1.38 MW. The thermal efficiency of the receiver was calculated as 90.98%. The reason for the high efficiency was the low receiver temperature as shown in Figure 5(b). The average temperature of the receiver was 639 K, while the highest temperature was only 663 K. The outlet temperature of the He gas was 644 K.

The average receiver temperature in the receiver using neon (Ne) gas was found to be 869 K. Figure 6(a) showed the temperature distribution of the receiver. The outlet temperature from the W12 and E12 panels of neon gas was 947 K on average. The thermal efficiency of the receiver with a total loss of 4.14 MW was calculated as 85.12%.

The temperature distribution of the receiver using liquid sodium (Na) was shown in Figure 6(b). When the inlet and outlet temperatures of sodium were 782 and 884 K, respectively, the radiation losses on the receiver were determined as 1.85 MW and the convective losses were determined as 0.87 MW. The thermal efficiency was calculated as 87.78%.

In the receiver using sodium-potassium (NaK) eutectic, the thermal losses were 4.38 MW. The thermal efficiency was 84.58%. As seen in Figure 7(a), temperatures


#### Table 4.

Thermophysical properties of HTFs [8, 10, 13, 17].

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

#### Figure 5.

Temperature distribution on the receiver using air (a) and helium (b).

Figure 6. Temperature distribution on the receiver using neon (a) and sodium (b).

above 1000 K appear on the receiving surface. The average receiving temperature was 877 K.

The results obtained from the receiver using lithium (Li) as heat transfer fluid were similar to those obtained from helium gas. Figure 7(b) shows the temperature distribution of the receiver. The total thermal losses from the receiver with an average temperature of 644 K were 1.72 MW. In these conditions, the thermal efficiency was calculated as 90.89%. However, the temperature of the liquid lithium increased only in the receiver to 665 K.

The receiver thermal efficiency varied between 84.54 and 90.98%. Fluids with higher efficiency than solar two were helium, lithium, and sodium. Apart from sodium, the panel outlet temperatures were less than the molten salt, although the thermal efficiencies of helium and lithium were higher than the molten salt.

Table 5 shows the average surface temperature, thermal efficiency, outlet temperature and exergetic efficiency results for different HTF's. Although the inlet

#### Figure 7.

Temperature distribution on the receiver using sodium-potassium eutectic (a) and lithium (b).


#### Table 5.

Results for solar receiver for different HTF's.

temperatures are the same, lithium and helium with the highest thermal efficiencies were found to have the lowest outlet temperatures. On the other hand, Table 5 shows that the sodium-potassium fluid, which has the highest output temperature, provides the highest quality output energy.

Table 5 shows that although the highest thermal efficiency is obtained from helium gas, the exergetic efficiency is the lowest among other heat transfer fluids. That means the quality of energy for obtaining electricity is low for that fluid. Due to quality of energy output of solar receiver, the best result can be obtained from sodium-potassium which has the highest exergetic efficiency.

#### 5. Conclusions

Since the radiation from the heliostats is absorbed directly by the heat transfer fluid of the receiver, the central receiver is an important component of the solar power tower systems. In this study, solar two central receiver was modeled and a reference study [6] was used for model validation. The temperature changes on the receiver, thermal and exergetic efficiency were obtained for different gas and liquid metal HTFs by using ANSYS fluent.

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

According to the results, the thermal efficiency of the receiver values varied between 84.54 and 90.98%. The highest thermal efficiency was obtained from helium and the lowest thermal efficiency was obtained from the sodium-potassium eutectic. On the other hand, exergetic efficiency varied between 53.97 and 63.20%. Although helium had the highest thermal energy efficiency, when it is evaluated in terms of the energy quality depending on the outlet temperature, sodiumpotassium eutectic liquid metal had the highest exergetic efficiency.

The exergy plays an important role in evaluating the use of sustainable technologies. Since sustainable development means less exergy destruction, the sustainability of a system is closely related to exergetic efficiency. In this context, when the exergetic efficiency was taken into consideration for sustainability analysis of the receiver, it was concluded that the most suitable fluid was sodium-potassium which had the highest exergetic efficiency.

In future, the corrosion effects and their degradation times will be studied. Also, lifetime and temperature resistance of the materials used in the receiver system for heat transfer fluids discussed in this study will be investigated. The economic analysis for these fluids will also be taken into consideration as future work.

#### Nomenclature


#### Greek letters


#### Author details

Gülden Adıyaman1 \*, Levent Çolak1 and İlhami Horuz<sup>2</sup>


\*Address all correspondence to: gulden@baskent.edu.tr

© 2019 The Author(s). Licensee IntechOpen. This chapteris distributed underthe terms oftheCreative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Impact of Heat Transfer Fluids on the Sustainable Solutions for Solar Power Tower DOI: http://dx.doi.org/10.5772/intechopen.87836

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#### **Chapter 53**
