Comparison of selective transmitters for solar thermal applications

Robert A. Taylor; Yasitha Hewakuruppu; Drew DeJarnette; Todd P. Otanicar

doi:10.1364/AO.55.003829

Applied Optics
Vol. 55,
Issue 14,
pp. 3829-3839
(2016)
•https://doi.org/10.1364/AO.55.003829

Comparison of selective transmitters for solar thermal applications

Robert A. Taylor, Yasitha Hewakuruppu, Drew DeJarnette, and Todd P. Otanicar

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Robert A. Taylor, Yasitha Hewakuruppu, Drew DeJarnette, and Todd P. Otanicar, "Comparison of selective transmitters for solar thermal applications," Appl. Opt. 55, 3829-3839 (2016)

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Abstract

Solar thermal collectors are radiative heat exchangers. Their efficacy is dictated predominantly by their absorption of short wavelength solar radiation and, importantly, by their emission of long wavelength thermal radiation. In conventional collector designs, the receiver is coated with a selectively absorbing surface (Black Chrome, ${TiNO}_{x}$ , etc.), which serves both of these aims. As the leading commercial absorber, ${TiNO}_{x}$ consists of several thin, vapor deposited layers (of metals and ceramics) on a metal substrate. In this technology, the solar absorption to thermal emission ratio can exceed 20. If a solar system requires an analogous transparent component—one which transmits the full AM1.5 solar spectrum, but reflects long wavelength thermal emission—the technology is much less developed. Bespoke “heat mirrors” are available from optics suppliers at high cost, but the closest mass-produced commercial technology is low-e glass. Low-e glasses are designed for visible light transmission and, as such, they reflect up to 50% of available solar energy. To address this technical gap, this study investigated selected combinations of thin films that could be deposited to serve as transparent, selective solar covers. A comparative numerical analysis of feasible materials and configurations was investigated using a nondimensional metric termed the efficiency factor for selectivity (EFS). This metric is dependent on the operation temperature and solar concentration ratio of the system, so our analysis covered the practical range for these parameters. It was found that thin films of indium tin oxide (ITO) and ZnS-Ag-ZnS provided the highest EFS. Of these, ITO represents the more commercially viable solution for large-scale development. Based on these optimized designs, proof-of-concept ITO depositions were fabricated and compared to commercial depositions. Overall, this study presents a systematic guide for creating a new class of selective, transparent optics for solar thermal collectors.

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Parameter	Value
Substrate (thickness)	Low-iron glass (2 mm) [36]
Surrounding medium	Air, vacuum ( $n = 1$ , $k = 0$ )
Operating temperature, $T_{ABS} = T_{cover}$	100°C–600°C (100°C increments)
Concentration ratio, $C_{r}$	10 and 100

Film No.	Materials	Film No.	Materials
1	${TiO}_{2} - Ag - {TiO}_{2}$	5	ZnS-Ag-ZnS
2	${TiO}_{2} - Au - {TiO}_{2}$	6	ZnS-Au-ZnS
3	${TiO}_{2} - Cu - {TiO}_{2}$	7	ZnS-Cu-ZnS
4	${TiO}_{2} - Al - {TiO}_{2}$	8	ZnS-Al-ZnS

Film No.	Type	Doping Range	Temp. (°C)	$m^{*} / m$	$ε (\infty)$
1-Fluorine doped tin oxide (FTO)	SP	0–180 F/Sn (at. %)	490–525 ^a [45]	0.45	3.7
2-Sb doped tin oxide (ATO)	SP	0–12 Sb/Sn (at. %)	400 ^a [46]	See [47]	4 [47]
3-Cadmium tin oxide (CTO)	DCRS	4–30 $O_{2}$ (%)	20–200 [48]	0.23 [49]	5.82 [49]
4-Tin doped indium oxide (ITO)	PLD	0–15 ${SnO}_{2}$ (wt. %)	250 [52]	0.4 [54]	3.57 [54]
5-Boron doped zinc oxide (BZO)	SGSC	0–1.0 B (at. %)	Anneal at 450 [42]	0.28 [52,54]	3.85 [52,54]
6-Al-doped zinc oxide (AZO)	AP-CVD	0.2–0.7 Al (at. %)	400 [44]
7-Gallium doped zinc oxide (GZO)	AP-CVD	0.5–10 Ga (at. %)	370 [43]
8-Fluorine doped zinc oxide (FZO)	SP	0.2–1.0 F/Zn	425 ^a [53]

	$C_{r} = 10$
Rank	100°C	200°C	300°C	400°C	500°C	600°C
1	#5, ZnS-Ag-ZnS, 0.79 (4/1)	#5, ZnS-Ag-ZnS, 0.71 (10/3)	#5, ZnS-Ag-ZnS, 0.56 (18/5)	#5, ZnS-Ag-ZnS, 0.47 (26/8)	#5, ZnS-Ag-ZnS, 0.31 (28/11)	#5, ZnS-Ag-ZnS, 0.13 (28/16)
2	#7, ZnS-Cu-ZnS, 0.78 (4/1)	#1, ${TiO}_{2} - Ag - {TiO}_{2}$ , 0.69 (2/2)	#1, ${TiO}_{2} - Ag - {TiO}_{2}$ , 0.56 (4/4)	#6, ZnS-Au-ZnS, 0.42 (32/8)	#6, ZnS-Au-ZnS, 0.26 (34/12)	#6, ZnS-Au-ZnS, 0.10 (34/19)
	$C_{r} = 100$
Rank	100°C	200°C	300°C	400°C	500°C	600°C
1	Bare glass 0.85	Bare glass 0.84	Bare glass 0.81	#5, ZnS-Ag-ZnS, 0.77 (4/1)	#5, ZnS-Ag-ZnS, 0.71 (6/2)	#5, ZnS-Ag-ZnS, 0.64 (10/3)
2	#5, ZnS-Ag-ZnS, 0.84 (4/1)	#5, ZnS-Ag-ZnS, 0.83 (4/1/4)	#5, ZnS-Ag-ZnS, 0.81 (4/1)	Bare glass 0.76	#1, ${TiO}_{2} - Ag - {TiO}_{2}$ , 0.69 (2/2)	#1, ${TiO}_{2} - Ag - {TiO}_{2}$ , 0.61 (2/3)

	$C_{r} = 10$
Rank	100°C	200°C	300°C	400°C	500°C	600°C
1	Bare glass 0.77	# 4, ITO, 0.70 (150/4.06)	# 4, ITO, 0.63 (200/4.06)	# 4, ITO, 0.52 (250/4.06)	# 4, ITO, 0.39 (300/5.94)	# 4, ITO, 0.20 (350/5.94)
2	# 40.76 (50/5.94)	# 6, AZO, 0.67 (250/0.2)	# 1, FTO, 0.57 (200/180)	# 1, FTO, 0.44 (300/180)	# 1, FTO, 0.24 (350/180)	# 1, FTO, −0.06 (450/180)
	$C_{r} = 100$
Rank	100°C	200°C	300°C	400°C	500°C	600°C
1	Bare glass 0.85	Bare glass 0.84	Bare glass 0.81	Bare glass 0.76	# 4, ITO, 0.71 (100/4.06)	# 4, ITO, 0.66 (150/4.06)
2	# 4, ITO, 0.80 (50/4.06)	# 4, ITO, 0.79 (50/4.06)	# 4, ITO, 0.77 (50/4.06)	# 4, ITO, 0.74 (50/5.94)	# 1, FTO, 0.68 (150/180)	# 1, FTO, 0.63 (150/180)

	Operating Conditions	Film Material	$τ_{SOL}$	$T_{BB}$	$E_{BB}$
Case 1	$C_{r} = 10$ $T_{abs} = 100 ° C$	ZnS-Ag-ZnS ^a	0.845	0.007	0.530
Case 1	$C_{r} = 10$ $T_{abs} = 100 ° C$	ITO	0.803	0.004	0.408
Case 2	$C_{r} = 10$ $T_{abs} = 400 ° C$	ZnS-Ag-ZnS	0.655	0.024	0.145
Case 2	$C_{r} = 10$ $T_{abs} = 400 ° C$	ITO ^a	0.710	0.008	0.156
Case 3	$C_{r} = 100$ $T_{abs} = 400 ° C$	ZnS-Ag-ZnS ^a	0.845	0.100	0.604
Case 3	$C_{r} = 100$ $T_{abs} = 400 ° C$	ITO	0.803	0.064	0.448
Case 4	$C_{r} = 100$ $T_{abs} = 600 ° C$	ZnS-Ag-ZnS	0.793	0.138	0.347
Case 4	$C_{r} = 100$ $T_{abs} = 600 ° C$	ITO ^a	0.761	0.063	0.237

Film Material	$τ_{SOL}$	$T_{BB}$	$E_{BB}$	EFS, $C_{r} = 10$	EFS, $C_{r} = 100$
ITO MTI	0.69	0.005	0.17	0.49	0.67
ITO Geomatec	0.8	0.007	0.25	0.50	0.77
ITO Dip Coat	0.87	0.01	0.91	−0.19	0.76
ITO Dip w/Silica	0.88	0.01	0.91	−0.18	0.78

Film Material	$τ_{SOL}$	$T_{BB}$	$E_{BB}$	EFS, $C_{r} = 10$	EFS, $C_{r} = 100$
ITO MTI	0.69	0.005	0.17	0.49	0.67
ITO Geomatec	0.8	0.007	0.25	0.50	0.77
ITO Dip Coat	0.87	0.01	0.91	−0.19	0.76
ITO Dip w/Silica	0.88	0.01	0.91	−0.18	0.78

Abstract

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Applied Optics