Heat transfer and mechanical stability analyses to determine the aspect ratio of rock caverns for thermal energy storage

Park, D.,Park, E.S.,Sunwoo, C. Association for Applied Solar Energy ; Elsevier Sc 2014 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.107 No.-

Thermal stratification in solar thermal storages is used to improve the efficiency of solar heating systems because a high degree of thermal stratification in the storages increases the thermal performance of the systems. It has been demonstrated that better thermal stratification can be achieved by increasing the aspect ratio (height-to-width ratio) of the heat storage containers. However, a high-aspect-ratio design may lead to mechanical (structural) instability of the storage space because of its tall, narrow shape. Therefore, heat storage containers should be designed to provide good thermal performance, while considering the mechanical stability of the storage space. This is an important issue in the design of thermal energy storage (TES) spaces, particularly the underground rock caverns used for TES, because the stability of rock caverns depends largely on geomechanical factors, such as rock properties and in-situ stresses. To address this issue, we present a numerical approach for determining the aspect ratio of underground TES caverns that considers both thermal performance and mechanical stability. This approach is based on a thermal performance evaluation in terms of thermal stratification using heat transfer analysis and a mechanical stability assessment that calculates the factor of safety using finite element analysis combined with a shear strength reduction (SSR) method. The applicability of our approach is demonstrated in the preliminary design of a silo-shaped rock cavern used to store hot water for district heating. The results of the numerical analyses under various design conditions are presented and discussed in detail, and we propose an aspect ratio for the rock cavern.

Influence of Al content on surface passivation properties of Al rich ZnO films for solar cell application

Khan, F.,Baek, S.H.,Singh, S.N.,Singh, P.K.,Husain, M.,Kim, J.H. Association for Applied Solar Energy ; Elsevier Sc 2014 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.110 No.-

A systematic study of hydrogen sintered aluminum rich zinc oxide (AZO) films is made from the view point of their application as surface passivation layer for silicon solar cell. AZO films of various Al and Zn molar concentration ratios (0<R<SUB>Al/Zn</SUB><40%) are made and their electrical and optical properties are studied. The lowest surface recombination velocity (~10cm/s) is realized in the film with R<SUB>Al/Zn</SUB>~30% as inferred from minority carrier lifetime measurements. The passivation effect of AZO layer may be attributed to presence of hydrogen whose concentration is maximum (~3.93x10<SUP>22</SUP>atoms/cm<SUP>3</SUP>) for R<SUB>Al/Zn</SUB>~30%. Effect of surface passivation is manifested in the efficiency of solar cell where an improvement>4% vis-a-vis the control cell (without AZO layer) is seen after AZO layer is applied on the rear surface of the cell.

Analysis of photovoltaic cell parameters of non-vacuum solution processed Cu(In, Ga)Se₂ thin film based solar cells

Khan, F.,Lee, H.J.,Oh, M.,Kim, J.H. Association for Applied Solar Energy ; Elsevier Sc 2014 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.108 No.-

The losses in Cu(In, Ga)Se<SUB>2</SUB> (CIGS) solar cells due to photovoltaic (PV) cell parameters, namely the shunt resistance R<SUB>sh</SUB>, series resistance R<SUB>s</SUB>, diode ideality factor n, and reverse saturation current density J<SUB>0</SUB>, were analyzed in this study. The PV cell parameters of the solar cells were analytically determined for various doping concentrations of Cu and In in CIGS films. The CIGS films were deposited using a low cost non-toxic solvent (deionized) by a non-vacuum process (spray pyrolysis technique). They were subsequently characterized using XRD, FE-SEM, I-V and UV-Vis techniques to correlate the structural, electrical, and optical properties of the films with solar cell performance. Maximum short circuit current density of 0.0218A/cm<SUP>2</SUP> is achieved for Cu/Se and In/Se molar ratios of 0.131 and 0.318, respectively. However, the maximum obtained V<SUB>oc</SUB> value is 0.431V for Cu/Se and In/Se molar ratios of 0.105 and 0.191, respectively. The maximum achieved efficiency was ~4.38% for Cu/Se and In/Se molar ratios of 0.105 and 0.191, respectively. Analytically predicted values of R<SUB>sh</SUB>, R<SUB>s</SUB>, n, and J<SUB>0</SUB> were 116.82Ωcm<SUP>2</SUP>, 4.64Ωcm<SUP>2</SUP>, 1.8016, and 1.4952x10<SUP>-6</SUP>A/cm<SUP>2</SUP>, respectively.

Design and installation of floating type photovoltaic energy generation system using FRP members

Lee, Y.G.,Joo, H.J.,Yoon, S.J. Association for Applied Solar Energy ; Elsevier Sc 2014 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.108 No.-

In this paper, we present the result of investigations pertaining to the development of floating type photovoltaic energy generation system. For the floating structure, durable and light-weight materials such as a pultruded FRP composite material are essential. Pultruded FRP has superior mechanical properties and excellent corrosion-resistance compared with those of conventional structural materials. In the paper, we discussed the development concepts of the floating type photovoltaic energy generation system briefly. The mechanical properties of the FRP structural member used in the structural system are investigated through the tensile and shear tests. Test results are used in the finite element analysis and design of the system. Link system which is composed of pultruded FRP, used tire, and PE rope is also analyzed by the finite element method. Finally, floating type photovoltaic energy generation system is designed, fabricated, and installed successfully at the sea site in Korea. In addition, the energy production of floating PV energy generation system is measured and compared with that of land type plant. Energy productions of floating type from June to August are significantly higher, but the energy productions from September to October are lower.

Efficient dye-sensitized solar cells with surface-modified photoelectrodes

Park, S.K.,Han, Y.S. Association for Applied Solar Energy ; Elsevier Sc 2014 Solar energy Vol.110 No.-

The influence of a mixed overlayer comprising titanium dioxide (TiO<SUB>2</SUB>) and niobium pentoxide (Nb<SUB>2</SUB>O<SUB>5</SUB>), which was coated on the surfaces of TiO<SUB>2</SUB> nanoporous photoelectrodes, on the performance of dye-sensitized solar cells (DSSCs) was studied. The DSSCs with the TiO<SUB>2</SUB>:Nb<SUB>2</SUB>O<SUB>5</SUB>-coated photoelectrodes showed an increase in short-circuit current (J<SUB>sc</SUB>) and open-circuit voltage (V<SUB>oc</SUB>), resulting in a power conversion efficiency of 10.37% compared to 7.90% for the reference cell with a TiO<SUB>2</SUB>-coated photoelectrode. The increase in J<SUB>sc</SUB> was due to an improvement in the light harvesting and electron collection efficiency. The suppression of the charge recombination between the injected electrons and the I<SUB>3</SUB><SUP>-</SUP> ions was found to increase the V<SUB>oc</SUB> value of the device with the TiO<SUB>2</SUB>:Nb<SUB>2</SUB>O<SUB>5</SUB>-coated photoelectrodes.

Evaluation of optimized PV power generation and electrical lighting energy savings from the PV blind-integrated daylight responsive dimming system using LED lighting

Kim, S.H.,Kim, I.T.,Choi, A.S.,Sung, M. Association for Applied Solar Energy ; Elsevier Sc 2014 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.107 No.-

There have been a variety of studies on the application of a Building Integrated Photovoltaic (BIPV) system, but very few on integration with other energy saving systems in buildings. Therefore, this study proposed a Photovoltaic (PV) blind-integrated daylight responsive dimming system using LED lighting. It integrates a PV blind system with a daylight responsive dimming system, which can generate electricity and save electrical lighting energy at the same time. In addition, using LED lighting resulted in greater dimming efficiency compared to conventional fluorescent lighting. The PV blind slat angle should also be controlled according to the profile angle of the sun to maximize power generation. However, it can reduce electrical lighting energy saving indoors. To evaluate the impact of the PV blind slat angle control on such reductions in energy savings, this study conducted experiments throughout the season to identify the amount of electricity generation of the PV blind systems and the amount of electrical lighting energy savings by implementing the slat angle of blinds control. Power generation and electrical energy savings in the test room were about 32% more and 35% less, respectively, than those in the reference room.

Color tunable nanopaper solar cells using hybrid CH₃NH₃PbI_3-xBr_x perovskite

Jung, M.H.,Park, N.M.,Lee, S.Y. Association for Applied Solar Energy ; Elsevier Sc 2016 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.139 No.-

We demonstrate a flexible solar cell by combining nanopaper and perovskite, which are used as the substrate and light absorber, respectively. Since nanopaper is made of the porous nanofiber, they should be is the high transmittance and haze property. Paper substrates have drawn interest for the application flexible devices due to high thermal expansion coefficient compared to conventional plastic substrates and can be disposed as environmental waste. The conductive electrode consisting of three alternating layers (TiOx/Ag/TiOx, DMD) on nanopaper was prepared by sputtering deposition. We introduced mixed halide perovskite (CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3-x</SUB>Br<SUB>x</SUB>) into DMD structure on nanopaper to change color of solar cell from dark brown to yellow. We expect that the presented solar cell would give not only a new candidate of the environmental solar system but also excellent durable and stable solar cells with low cost.

Effect of pre-annealing on the phase formation and efficiency of CZTS solar cell prepared by sulfurization of Zn/(Cu,Sn) precursor with H₂S gas

Lee, J.H.,Choi, H.J.,Kim, W.M.,Jeong, J.H.,Park, J.K. Association for Applied Solar Energy ; Elsevier Sc 2016 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.136 No.-

The effect of pre-annealing on the phase formation behavior and efficiency of CZTS thin film solar cell prepared by sulfurization of sputtered Zn/(Cu,Sn) metal precursor with H<SUB>2</SUB>S gas was investigated. Precursor with stacking structure of Zn/(Cu,Sn) was deposited by sputtering of Cu, Zn, and Sn metal targets. The depth profile of metal elements and cell efficiency of the sulfurized CZTS films with H<SUB>2</SUB>S were observed to be critically dependent on the pre-annealing conditions. For the CZTS film prepared by sulfurization in N<SUB>2</SUB>-5vol.% H<SUB>2</SUB>S at 550<SUP>o</SUP>C after pre-annealing at 350<SUP>o</SUP>C in Ar, segregation of SnS phase at the surface region was observed to be pronounced. When the pre-annealing was performed at 350<SUP>o</SUP>C in N<SUB>2</SUB>-5vol.% H<SUB>2</SUB>S, however, uniform depth profile of metal elements with a small amount of CuS phase was observed. The CuS phase was disappeared with increase in the pre-annealing temperature in N<SUB>2</SUB>-5vol.% H<SUB>2</SUB>S. The phase formation behavior influenced by pre-annealing condition was observed to affect solar cell performance of the CZTS thin film synthesized at 550<SUP>o</SUP>C in N<SUB>2</SUB>-5vol.% H<SUB>2</SUB>S. In contrast to the CZTS thin film prepared with pre-annealing at 350<SUP>o</SUP>C in Ar showing bad efficiency (~0.93%), the CZTS solar cells fabricated with pre-annealing at 450<SUP>o</SUP>C in H<SUB>2</SUB>S shows higher efficiency of 3.04%. By the optimization of Zn layer thickness, solar cell efficiency of 4.40% was obtained in the CZTS thin film prepared with pre-annealing at 450<SUP>o</SUP>C in N<SUB>2</SUB>-5vol.% H<SUB>2</SUB>S. This phenomenon was due to the change in the secondary phase formation behavior during sulfurization of the Zn/(Cu,Sn) metal precursor with various pre-annealing conditions.

Thermal performance comparison of line- and point-focus solar concentrating systems: Experimental and numerical analyses

Imtiaz Hussain, M.,Lee, G.H. Association for Applied Solar Energy ; Elsevier Sc 2016 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.133 No.-

The purpose of this study was to analyze and compare the thermal performances of a line-focus and a point-focus solar concentrators (LFSC and PFSC, respectively) under similar operating and weather conditions in South Korea. Both concentrator systems had Fresnel lenses of the same surface area. To reduce heat loss, absorber pipes of the LFSC and PFSC systems were covered with borosilicate glass tube and foamed polyethylene insulation, respectively. Collector temperature (inlet and outlet temperature differential), collected energy, and thermal efficiency over experimental test time were calculated while the solar radiation, volumetric flow rate, and inlet temperature of the thermal fluid were varied. Good harmony was observed between the experimental and predicted results. Calibration of the both models reduced the deviation between the experimental and predicted temperature results compared to the uncalibrated models. Although both solar collectors showed similar trends in the variations of the average efficiency and total collected energy, these parameters were about 7% and 8% higher, respectively, in the PFSC compared to the LFSC. Overall, the results indicate that the PFSC performs slightly better than the LFSC in terms of the thermal efficiency, total collected energy, and heat loss.

Novel approach for fabrication of buried contact silicon nanowire solar cells with improved performance

Khan, F.,Baek, S.H.,Kim, J.H. Association for Applied Solar Energy ; Elsevier Sc 2016 SOLAR ENERGY -PHOENIX ARIZONA THEN NEW YORK- Vol.137 No.-

Generally, a selective SiNW-type structure is used to avoid resistive loss in SiNW-based solar cells. However, the performance of these selective SiNW-based solar cells is lower than that of conventional Si solar cells, due to their low collection efficiency and high series resistance. Herein, a novel process is developed to enhance the collection efficiency of photogenerated charge carriers, and hence the performance of SiNW solar cells. Self-aligned single-step lithography is used to fabricate buried contact SiNW (SiNWBC) solar cells. The effectiveness of the SiNWBCs is manifested in the conversion efficiency (η~15.02%) of the solar cell, which is improved by ~7.82% compared to that of the control selective SiNW cell (η~13.93%). The performance and PV cell parameters of the SiNWBCs are analyzed and compared with those of this control cell. Losses due to the PV cell parameters of the SiNWBC solar cell are lower than those of the control cell. The reduced number of front surface recombinations lowers the n and J<SUB>0</SUB> values, resulting in enhanced SiNWBC cell performance.