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Research Papers

Mechanism of Upgrading Low-Grade Solar Thermal Energy and Experimental Validation

[+] Author and Article Information
Hui Hong, Jun Sui, Jun Ji

 Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080, P.R.C.

Hongguang Jin1

 Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080, P.R.C.hgjin@mail.etp.ac.cn

1

Corresponding author.

J. Sol. Energy Eng 130(2), 021014 (Mar 25, 2008) (8 pages) doi:10.1115/1.2840609 History: Received December 04, 2006; Revised June 29, 2007; Published March 25, 2008

Solar thermochemical processes inherently included the conversion of solar thermal energy into chemical energy. In this paper, a new mechanism of upgrading the energy level of solar thermal energy at around 200°C was revealed based on the second law thermodynamics and was then experimentally proven. An expression was derived to describe the upgrading of the energy level from low-grade solar thermal energy to high-grade chemical energy. The resulting equation explicitly reveals the interrelations of energy levels between middle-temperature solar thermal energy and methanol fuel, and identifies the interactions of mean solar flux and the reactivity of methanol decomposition. The proposed mechanism was experimentally verified by using the fabricated 5kW prototype of the receiver∕reactor. The agreement between the theoretical and the experimental results proves the validity of the mechanism for upgrading the energy level of low-grade solar thermal energy by integrating clean synthetic fuel. Moreover, the application of this new middle-temperature solar∕methanol hybrid thermochemical process into a combined cycle is expected to have a net solar-to-electric efficiency of about 27.8%, which is competitive with other solar-hybrid thermal power plants using high-temperature solar thermal energy. The results obtained here indicate the possibility of utilizing solar thermal energy at around 200°C for electricity generation with high efficiency by upgrading the energy level of solar thermal energy, and provide an enhancement to solar thermal power plants with the development of this low-grade solar thermochemical technology in the near future.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 4

(a) Variation of the theoretical exergy efficiency in the solar-driven methanol decomposition process with the increase of the mean solar flux and (b) variation of the theoretical exergy efficiency in the solar-driven methanol decomposition process with the increase of the methanol feeding rate

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Figure 5

(a) Comparison of experimental and theoretical values for the relative upgrade of energy level of solar thermal energy at a different mean solar flux and (b) comparison of experimental and theoretical values for the relative upgrade of energy level of solar thermal energy at different methanol feeding rates

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Figure 6

(a) Experimental results for exergy efficiency at a different mean solar flux and comparison to theoretical results and (b) experimental results for exergy efficiency at different methanol feeding rates and comparison to theoretical results

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Figure 7

Schematic of the new solar∕methanol combined cycle hybrid plant

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Figure 8

(a) Profiles of the net solar-to-electric efficiency and the thermal solar share for the proposed system with the increase of the mean solar flux and (b) behavior of the net solar-to-electric efficiency and the thermal solar share with the increase of the methanol feeding rate

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Figure 1

Schematic of the mechanism of thermochemical energy conversion integrating middle-temperature solar thermal energy and methanol fuel energy conversion process

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Figure 2

Energy and exergy flow of the proposed middle-temperature solar thermochemical process using methanol decomposition

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Figure 3

(a) Influence of the mean solar flux on the theoretical energy level upgrade of solar thermal energy and (b) influence of the methanol feeding rate on the theoretical energy level upgrade of solar thermal energy

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