Optical properties of liquids for direct absorption solar thermal energy systems

Available online at www.sciencedirect.com

Solar Energy 83 (2009) 969–977 www.elsevier.com/locate/solener

Optical properties of liquids for direct absorption solar thermal energy systems Todd P. Otanicar a, Patrick E. Phelan a,*, Jay S. Golden b,c a

Department of Mechanical and Aerospace Engineering, National Center of Excellence on SMART Innovations, Arizona State University, P.O. Box 876106, Tempe, AZ 85287-6106, USA b Department of Civil and Environmental Engineering, National Center of Excellence on SMART Innovations, Arizona State University, P.O. Box 876106, Tempe, AZ 85287-6106, USA c School of Sustainability, National Center of Excellence on SMART Innovations, Arizona State University, P.O. Box 876106, Tempe, AZ 85287-6106, USA Received 2 June 2008; received in revised form 25 November 2008; accepted 21 December 2008 Available online 9 January 2009 Communicated by: Associate Editor Darren Bagnall

Abstract A method for experimentally determining the extinction index of four liquids (water, ethylene glycol, propylene glycol, and Therminol VP-1) commonly used in solar thermal energy applications was developed. In addition to the extinction index, we report the refractive indices available within the literature for these four fluids. The final value reported is the solar-weighted absorption coefficient for the fluids demonstrating each fluid’s baseline capacity for absorbing solar energy. Water is shown to be the best absorber of solar energy of the four fluids, but it is still a weak absorber, only absorbing 13% of the energy. These values represent the baseline potential for a fluid to be utilized in a direct absorption solar thermal collector. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Optical properties; Fluids; Solar thermal energy

1. Introduction Typical solar collector configurations absorb solar energy through the use of a black-surface absorber. Various limitations have been noted with this configuration and alternative concepts have been addressed. Among these alternative concepts is the approach of allowing the fluid to directly absorb the incident radiation, including the use of black liquids (Minardi and Chuang, 1975) and gases filled with particles (Bertocchi et al., 2004; Abdelrahman et al., 1979). The addition of small particles causes scattering of the incident radiation allowing higher levels of absorption within the fluid. A recent development in solar thermal collectors, proposed by Tyagi et al. (in press), is the use of nanofluids to directly absorb solar radiation. *

Corresponding author. E-mail address: [email protected] (P.E. Phelan).

0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.12.009

The term ‘nanofluids’ applies to colloidal suspensions of nanoparticles, which have been the subject of much recent study for their potentially enhanced thermal transport characteristics (see Wang and Majumdar, 2007; Trisaksri and Wongwises, 2007 for recent reviews). Adding nanoparticles in suspension to a base liquid can drastically alter the optical properties of the fluid (Prasher and Phelan, 2005; Chicea, 2008). The optical properties of the effective fluid are highly dependent on the particle shape, particle size, and the optical properties of the base fluid and particles themselves (Khlebtsov et al., 2005). In order to fully understand the effects of suspending nanoparticles within a base fluid the complete optical properties of the base fluid are needed. For most fluids the real part of the refractive index (n) is known and quantified within the broad wavelength ranges that encompass the majority of the solar wavelengths (Wohlfarth and Wohlfarth, 1996). However the imaginary

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part of the refractive index (k), often referred to as the extinction index, is not as widely known for many fluids. Experimentally determining the optical constants usually requires the measurement of the transmittance, reflectance or angles of refraction as the complex refractive index, n  ki, is not directly measurable. Typical methods for evaluating the optical constants are (Bohren and Huffman, 1998): 1. Measurement of refraction angles and utilization of Snell’s law to determine n (requires k = 0); 2. Measurement of transmittance and reflectance for a slab at near-normal incidence; 3. Measurement of the reflectance over a wide spectral range and then application of the Kramers–Kronig analysis (requires extrapolation and extended measurements); 4. Ellipsometry; 5. Reflectance measurements at various polarizations and incidence angles (requires large angles and sample sizes). Many of these techniques require specialty equipment to handle the reflectance measurements or are limited to a small spectral region or in some cases only to a single wavelength. In this paper we investigate the optical properties of four fluids (water, propylene glycol (PG), ethylene glycol (EG), and TherminolÒ VP-1), commonly used in solar energy applications, for their potential use as a base fluid for a direct absorption receiver by measurements with a transmission spectrophotometer. 2. Experimental procedure Measurements were made using a Shimadzu UV-3600 UV–Visible Spectrophotometer with an operating range from 0.170 to 3.300 lm encompassing 98% of the incident solar energy at air mass 1.5. The fluids were enclosed in

SpectrosilÒ (synthetic fused silica) cuvettes with pathlengths of 0.1, 2 and 10 mm. All the measurements were made at ambient temperature, 25 °C. The cuvettes limit the operating wavelength range to 0.170–2.700 lm. Because of the extremely strong infrared absorption bands in water and the two glycols the wavelength range of interest was limited to 200–1500 nm. The solar energy in this wavelength range is still nearly 85% of the total energy (ASTM G173-03e1, 2003). The spectrophotometer produces values of transmittance, for a given pathlength cell, as a function of wavelength. Tuntomo et al. (1992) proposed a method for iteratively calculating the optical constants of organic fuels using transmittance measurements of two pathlength cells via Fourier Transfer Infrared Spectroscopy (FTIR). An important difference between Tuntomo et al.’s and the present work is Tuntomo et al.’s use of a cell with a refractive index close to 1, allowing the cell to be treated as a single slab since this eliminates any reflected or absorbed energy from the air to the window. Because the cuvettes used in this analysis are quartz, with a real refractive index near 1.5 for the majority of the spectrum, the multiple reflections at the interfaces between the air and the window and the window and the fluid sample can’t be ignored. The system used here is a 3-slab system (Fig. 1) which complicates the analysis by requiring the inclusion of the multiple reflections. Chen (2005) notes that three primary methods exist for analyzing radiative transport in layered systems; ray tracing, resultant wave, and the transfer matrix method. The transfer matrix method is effective for multi-slab systems but applies to thin films in the coherent limit. The appropriateness of when incoherent or coherent superposition applies is defined by Stenzel (2005). For incoherent superposition (no interference) the following condition must be met: k2 ð1Þ ds > 2pnDk

Fig. 1. Three slab system.

T.P. Otanicar et al. / Solar Energy 83 (2009) 969–977 2000

Slab Dimension (micrometers)

1800 Incoherent

1600

Coherent

1400

Δλ = 0.5 nm

1200 1000 Incoherent

where T is the transmittance of the element defined by the subscript, R is the reflectance from the side of incidence for the element defined by the subscript, and R0 is the reflectance from the non-incidence side for the element defined by the subscript. It is then noted in Large et al. (1996) that this method can be used for combining three elements (slabs) based on the formulation below. ½RT

800

R0T

T T  ¼ ½R1

Coherent

 ½R3

Δλ = 1.0 nm

600

Incoherent Δλ = 5.0 nm

Coherent

0 0

0.5

1

1.5 Wavelength (μm)

2

2.5

3

R0i ¼

and for coherent superposition: k2 2pnDk

ð2Þ

where ds is the thickness of the slab, n the index of refraction, k the wavelength and Dk the spectral bandwidth of the instrument. Fig. 2 demonstrates where these conditions will be met based on an assumption of n1 = 1.54 (approximately valid for quartz) for varying spectral bandwidths. Fig. 2 illustrates the importance of the spectral bandwidth of the measurement device in determining whether interference will be detected. Here, a bandwidth of 5 nm was used to ensure incoherent superposition. Because the overall thickness of the 3-slab system is well over 1000 lm no interference pattern is expected to be measured. Because of this the ray tracing (geometric optics) approach is taken to model the 3-slab system. This method is easily done for 1 slab, but by adding more slabs accounting for all the possible reflections at the interfaces and transmissions becomes a very tedious and time consuming task. Because of this Large et al. (1996) proposed a method for evaluating the reflection and transmission of beams through discrete layers. The method works by combining all backward traveling fluxes into a single flux, independent of direction or phase. The same holds for the forward traveling fluxes. The overall medium is then broken into discrete elements, in this case 3, with the fluxes simplified as shown in Fig. 1. In essence two elements (for example, slab 1 and slab 2 in Fig. 1) are combined to give the following relations for the transmittance and reflectance of both sides (Large et al., 1996): T ¼ T 1T 2 þ R ¼ R1 þ

R01 R2 T 1 T 2 1  R2 R01

ð3Þ

T 21 R2 1  R2 R01

ð4Þ

T 22 R01 1  R2 R01

ð5Þ

R0 ¼ R02 þ

T 1   ½R2 R03

2

Ri ¼

Fig. 2. Conditions for coherent and incoherent superposition.

ds 

R01

T 3

R02

T 2 ð6Þ

where  denotes the combination of two elements according to Eqs. (3)–(5). The reflectance for each surface can be calculated based on Fresnel’s relations (Stenzel, 2005):

400 200

971

ðnj  ni Þ þ ðk j  k i Þ

2

ðnj þ ni Þ2 þ ðk j þ k i Þ2 ðni  nl Þ2 þ ðk i  k l Þ2 2

ðni þ nl Þ þ ðk i þ k l Þ

2

ð7Þ ð8Þ

where j represents the properties of the medium to the left and i the medium to the right in Eq. (7), and i is the medium to the left and l is the medium to the right in Eq. (8). To determine the transmittance of each slab, the Fresnel reflection and transmission coefficients are needed. The transmittance through one slab can be found as follows (Stenzel, 2005): Ti ¼

ð1  Ri R0i Þe4pkL=k 1  Ri R0i e8pkL=k

ð9Þ

The equations above show the dependence on the optical constants n and k, wavelength, as well as the thickness of the sample (L). By measuring the transmittance at two different pathlengths one can solve, by iteration, the system of equations above to determine n and k for the fluid. Due to the complexity of the resulting equations n and k are found through an iterative process by minimizing the error between the calculated and measured transmittance for the two pathlengths at each wavelength. The values of n and k are both determined numerically based on the transmission curves. There is no attempt to constrain n based on known values. Unfortunately, a common problem in the determination of optical constants is the existence of multiple solutions that satisfy the measured transmittance or reflectance values (Lamprecht et al., 1997). This can also be seen by observing the higher order polynomials of n and k that appear in the reflection and transmission equations. The extinction index k, although appearing in the polynomials, also appears in the exponential term in the transmission equation resulting in a single solution for k. Figs. 3 and 4 demonstrate the duality of solutions for the real part of the refractive index, and the unique solution obtained for the imaginary part respectively, by comparing the published values of the complex refractive index of water to the numerical solutions obtained by iteration from theoretical transmission curves. The theoretical transmission curves represent calculated values of transmission obtained

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using Eqs. (3)–(9) from published values of n and k. In addition to the dual solutions, the refractive index n is extremely sensitive to any experimental error; this led here to experimentally focusing on determining k and just reporting the available literature values for n for the four fluids. We also report on the solar-weighted absorption coefficient (Am). This quantity represents the percentage of solar energy that is absorbed across a fluid layer of selected thickness (Drotning, 1978) and is defined by:  R  4pkx Ek 1  e k dk R Am ¼ ð10Þ Ek dk

where Ek is the amount of solar irradiance per unit wavelength at the given wavelength for a given solar air mass m, and x is the thickness of the fluid layer (Duffie and Beckman, 1991). 3. Experimental validation Using the experimental and numerical procedure outlined in the previous section the extinction index of water as a function of wavelength is presented in Fig. 5. The experimental results are shown in relation to the published values (Brewster, 1992; Hale and Querry, 1973). The experimental method provides accurate results over a majority

Fig. 3. Calculated and published values of n for water.

Fig. 4. Calculated and published values of k for water.

T.P. Otanicar et al. / Solar Energy 83 (2009) 969–977

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−3

10

Brewster, 1992 Hale and Querry, 1973 Experimental −4

10

−5

Extinction Index, k

10

−6

10

−7

10

−8

10

−9

10

−10

10

0.2

0.4

0.6

0.8 1 Wavelength (μm)

1.2

1.4

Fig. 5. Experimental values of the extinction index (k) of water.

of the spectrum. The exception is in the 250–700 nm range where the fluid is clear and no variations are observed in the transmission spectrum. Evaluating k at points where the transmission is at an extreme (0% or 100%) has been noted to be challenging (Irvine and Pollack, 1968). In order

b

100

100

90

90

80

80 Transmittance (%)

Transmittance (%)

a

to evaluate the value of k at these points a more accurate machine, different technique or cells with greater pathlength would be needed. This provides a minimum value for which k is experimentally valid, in this case limited to kmin = 2  108. The value for the solar-weighted absorp-

70 60 50 40 30

70 60 50 40 30

20

20

10

10

0

0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

0.2

0.3

0.4

0.5

0.6

Wavelength (µm)

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.1

1.2

1.3

1.4

1.5

Wavelength (µm)

d

100

100

90

90

80

80 Transmittance (%)

Transmittance (%)

c

0.7

70 60 50 40 30

70 60 50 40 30

20

20

10

10

0

0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

1.1

1.2

1.3

1.4

1.5

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (µm)

Fig. 6. Transmittance spectra for four common solar fluids ((a) water, (b) ethylene glycol, (c) propylene glycol and (d) Therminol VP-1, sample thickness = 10 mm).

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tion coefficient of water was also calculated using the published values of Brewster (1992) and was found to differ from the experimental value (13.57%) by 1.7%. We therefore estimate the uncertainty of our measurements to be ±2  108 for k, and ±1.7% for Am. 4. Results The experimental transmittance spectra measured for a 10-mm pathlength cell for all 4 fluids are presented in Fig. 6. Fig. 6 shows that strong absorption bands exist for water, EG and PG at 950–1000 nm and again at 1200 nm. In addition at nearly 1400 nm these fluids are essentially opaque to incoming radiation. The Therminol

VP-1Ò heat transfer fluid has 2 strong bands at 1050 nm and again at 1400 nm. It is also opaque in the UV region for wavelengths less than 300 nm. Observing Figs. 7 and 8 the extinction indices of ethylene glycol and propylene glycol are very similar to each other and to that of water. Again a large portion of the spectrum, 250–700 nm, has an extremely small value as expected with clear fluids. Finally in Fig. 9 the extinction index of Therminol VP-1 is presented. For all three fluids the spectral extinction index is not readily available in the literature. As shown in Fig. 5 Therminol is highly transmissive over parts of the measured spectrum, and because of this spectral extinction index is very small. In addition, measured transmittances at the two pathlengths were both

−3

10

1.52 k, Experimental n, Wohlfarth and Wohlfarth, 1996

−4

1.51

10

1.5 −5

10

−6

10

1.48

1.47

−7

10

Refractive Index, n

Extinction Index, k

1.49

1.46 −8

10

1.45 −9

10

1.44 −10

10

0.2

0.3

0.4

0.5

0.6

0.7

0.8 0.9 Wavelength (μm)

1

1.1

1.2

1.3

1.4

1.43 1.5

Fig. 7. Experimental values of the extinction index (k) and refractive index (n) of ethylene glycol.

−3

10

1.45 k, Experimental n, Wohlfarth and Wohlfarth, 1996

−4

10

1.448

−5

1.446

−6

10

1.444

−7

10

1.442

−8

10

1.44

−9

10

1.438

−10

10

0.2

Refractive Index, n

Extinction Index, k

10

0.4

0.6

0.8 Wavelength (μm)

1

1.2

1.4

1.436

Fig. 8. Experimental values of the extinction index (k) and refractive index (n) of propylene glycol.

T.P. Otanicar et al. / Solar Energy 83 (2009) 969–977

975

−3

1.69

10

k, Experimental n, Wohlfarth and Wohlfarth, 1996 1.68

−4

10

1.67

−5

Extinction Index, k

1.66 −6

10

1.65 −7

10

1.64

Refractive Index, n

10

−8

10

1.63 −9

10

1.62

−10

10

0.2

0.3

0.4

0.5

0.6

0.7

0.8 0.9 Wavelength (μm)

1

1.1

1.2

1.3

1.4

1.61 1.5

Fig. 9. Experimental values of the extinction index (k) and refractive index (n) of Therminol.

Table 1 Experimental values of extinction index (k) for the four tested liquids. k (lm)

Water

Ethylene glycol

Propylene glycol

0.200 0.220 0.240 0.260 0.280 0.300 0.320 0.340 0.360 0.380 0.400 0.420 0.440 0.460 0.480 0.500 0.520 0.540 0.560 0.580 0.600 0.620 0.640 0.660 0.680 0.700 0.720 0.740 0.760 0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920

2.51E–07 1.23E–07 6.34E–08 6.89E–08 7.02E–08 6.73E–08 6.91E–08 7.18E–08 6.30E–08 6.26E–08 6.91E–08 6.54E–08 7.14E–08 7.11E–08 7.09E–08 6.88E–08 7.25E–08 4.17E–08 7.37E–08 7.37E–08 8.25E–08 8.43E–08 9.15E–08 9.90E–08 1.07E–07 1.18E–07 1.43E–07 2.11E–07 2.37E–07 2.40E–07 2.38E–07 2.64E–07 3.10E–07 3.57E–07 4.36E–07 5.21E–07 6.91E–07

1.56E–05 6.99E–07 2.88E–07 2.05E–07 9.13E–08 4.73E–08 4.01E–08 3.33E–08 2.82E–08 2.40E–08 2.06E–08 2.05E–08 1.97E–08 2.12E–08 2.25E–08 2.35E–08 2.42E–08 2.52E–08 2.55E–08 2.56E–08 2.72E–08 3.13E–08 3.70E–08 3.42E–08 3.65E–08 4.05E–08 5.39E–08 1.03E–07 1.65E–07 1.60E–07 1.60E–07 1.65E–07 7.63E–08 8.79E–08 1.54E–07 3.98E–07 8.48E–07

3.68E–05 1.29E–06 5.41E–07 2.46E–07 1.82E–07 1.18E–07 9.67E–08 8.67E–08 7.97E–08 7.33E–08 6.83E–08 6.59E–08 6.32E–08 6.27E–08 6.28E–08 6.40E–08 6.40E–08 6.42E–08 6.41E–08 6.40E–08 6.54E–08 6.98E–08 7.12E–08 6.88E–08 7.05E–08 7.20E–08 8.30E–08 1.47E–07 1.73E–07 1.79E–07 1.86E–07 1.88E–07 8.80E–08 1.08E–07 1.82E–07 5.51E–07 6.13E–07

Therminol VP-1 1.69E–04 2.45E–04 2.63E–04 2.83E–04 3.05E–04 7.38E–05 6.05E–06 2.41E–06 3.60E–07 2.09E–07 1.39E–07 8.23E–08 3.86E–08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.21E–07 1.53E–07 0.00 0.00 (continued on next page)

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Table 1 (continued) k (lm)

Water

Ethylene glycol

Propylene glycol

Therminol VP-1

0.940 0.960 0.980 1.000 1.020 1.040 1.060 1.080 1.100 1.120 1.140 1.160 1.180 1.200 1.220 1.240 1.260 1.280 1.300 1.320 1.340 1.360 1.380 1.400 1.420 1.440 1.460 1.480 1.500

1.42E–06 3.33E–06 3.77E–06 3.25E–06 2.39E–06 1.63E–06 1.25E–06 1.27E–06 1.66E–06 2.29E–06 5.41E–06 1.06E–05 1.15E–05 1.20E–05 1.17E–05 1.12E–05 1.08E–05 1.13E–05 1.36E–05 1.89E–05 2.84E–05 3.87E–05 6.25E–05 1.74E–04 2.95E–04 3.52E–04 3.60E–04 3.13E–04 2.53E–04

7.17E–07 5.55E–07 1.30E–06 1.72E–06 1.82E–06 1.73E–06 1.62E–06 1.42E–06 1.19E–06 9.96E–07 1.34E–06 3.16E–06 6.57E–06 1.20E–05 1.13E–05 7.46E–06 6.39E–06 5.64E–06 4.66E–06 4.77E–06 6.57E–06 1.12E–05 1.64E–05 2.05E–05 4.68E–05 7.80E–05 9.67E–05 1.09E–04 1.15E–04

5.56E–07 5.24E–07 1.20E–06 1.52E–06 1.56E–06 1.32E–06 1.24E–06 1.08E–06 9.00E–07 8.49E–07 1.92E–06 3.60E–06 8.72E–06 8.22E–06 8.77E–06 6.75E–06 5.67E–06 4.58E–06 3.70E–06 3.71E–06 5.26E–06 1.21E–05 1.61E–05 1.84E–05 4.05E–05 6.73E–05 7.86E–05 8.65E–05 8.83E–05

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.46E–07 2.08E–06 1.50E–05 2.02E–06 5.10E–07 2.03E–07 1.13E–07 0.00 0.00 0.00 0.00 6.32E–07 1.20E–06 1.34E–06 1.92E–06 2.52E–06 2.44E–06 2.58E-06 1.74E–06 1.49E–06 1.34E–06

Table 2 Solar weighted absorption coefficient (m = 1.5, x = 1.0 cm).

k = 200–1500 nm,

Fluid

Am (%)

Water Ethylene glycol Propylene glycol Therminol VP-1Ò

13.57 9.250 9.062 2.098

at maximum value and no measurable change of the transmittance was measured between the two pathlengths. This yields three spectral ranges where the extinction index drops below the minimum resolution for the technique. Table 1 lists the experimental spectral values of k determined here for water, ethylene glycol, propylene glycol and Therminol. Using the spectral extinction indices for all the fluids, as shown in Figs. 6–9, and Eq. (10) the solar-weighted absorbance for each fluid can be determined. The values are tabulated in Table 2 for air mass 1.5 and fluid depth of 1 cm, the same used in the study by Drotning (1978) and for a proposed experimental collector being developed by the authors. To achieve a solar-weighted absorbance of 90% or greater the fluid depth needs to be increased to 1.0 m or larger. As seen in the table water is the strongest solar absorber, with the two glycols absorbing less energy but at approximately the same level. The Therminol heat transfer fluid has the lowest solar-weighted absorption coeffi-

Table 3 Published refractive index (n) values for the four fluids. Fluid

k (lm)

n (published)

T (°C)

Ref.

Water

0.200 0.300 0.400 0.600 0.800 1.000 1.200 1.500 0.220 0.240 0.260 0.300 0.420 0.580 0.434 0.486 0.589 0.656 0.4308 0.4861 0.589 0.6563

1.424 1.359 1.343 1.332 1.328 1.326 1.323 1.318 1.5152 1.4940 1.4805 1.4623 1.4412 1.43178 1.44900 1.44507 1.43983 1.43775 1.61978 1.62868 1.65321 1.67734

20

Brewster (1992)

20

Wohlfarth and Wohlfarth (1996)

20

Wohlfarth and Wohlfarth (1996)

20

Wohlfarth and Wohlfarth (1996)

Ethylene glycol

Propylene glycol

Diphenyl oxide

cient. The solar-weighted absorbance of these three fluids was expected to be low as all of them are visibly clear, and therefore they absorb little energy in the visible band where a large portion of solar energy is concentrated.

T.P. Otanicar et al. / Solar Energy 83 (2009) 969–977

In addition to the experimentally determined portion of the optical properties of the fluids, for convenience we report on the more widely published data available for the refractive index n. Table 3 lists published values for the refractive indices of the four fluids. Because Therminol VP-1 is a special formulation of diphenyl oxide and biphenyl developed by the manufacturer the refractive index is not publicly available, but since it is a formulation based on diphenyl oxide, the refractive index for diphenyl oxide is reported. 5. Discussion Using a standard scanning wavelength spectrophotometer we are able to retrieve the extinction index for four fluids commonly used in solar thermal energy applications. If these fluids are to be used as the media for directly absorbing solar energy the optical properties of these fluids are highly important. Using the values determined for the extinction index one can determine the solar-weighted absorption coefficient. From these results water is shown to be the best absorber of solar energy of the four fluids, but it is still a weak absorber, only absorbing 13% of the energy while the Therminol VP-1 heat transfer fluid is the weakest absorber examined – only absorbing 2%. These values represent the baseline potential for a fluid to directly absorb solar energy, but this potential can be modified by suspending nanoparticles within the fluid. In a future paper we will look to demonstrate how one can best modify these four fluids through the addition of nanoparticles in conjunction with the optical properties just reported to increase the performance of direct absorption solar thermal systems. Acknowledgments This material was based in part on work supported by the National Science Foundation, while one of the authors (P.E.P.) was working at the Foundation. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. In addition a portion of this work was supported by the National Center of Excellence on SMART Innovations. References Abdelrahman, M., Fumeaux, P., Suter, P., 1979. Study of solid-gassuspensions used for direct absorption of concentrated solar radiation. Solar Energy 22, 45–48.

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