Liu / Bashir Advanced Nanomaterials and Their Applications in Renewable Energy

1. Auflage 2015
ISBN: 978-0-12-801708-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 436 Seiten

Advanced Nanomaterials and Their Applications in Renewable Energy presents timely topics related to nanomaterials' feasible synthesis and characterization, and their application in the energy fields. In addition, the book provides insights and scientific discoveries in toxicity study, with information that is easily understood by a wide audience. Advanced energy materials are important in designing materials that have greater physical, electronic, and optical properties. This book emphasizes the fundamental physics and chemistry underlying the techniques used to develop solar and fuel cells with high charge densities and energy conversion efficiencies. New analytical techniques (synchronous X-ray) which probe the interactions of particles and radiation with matter are also explored, making this book an invaluable reference for practitioners and those interested in the science. - Provides a comprehensive review of solar energy, fuel cells, and gas storage from 2010 to the present - Reviews feasible synthesis and modern analytical techniques used in alternative energy - Explores examples of research in alternative energy, including current assessments of nanomaterials and safety - Contains a glossary of terms, units, and historical benchmarks - Presents a useful guide that will bring readers up to speed on historical developments in alternative fuel cells

Dr.Jingbo Louise Liu received her Ph.D. in Materials Science and Engineering from the University of Science and Technology Beijing in 2001. She was promoted to a tenured Associate Professor at Texas A&M University-Kingsville (TAMUK) due to her outstanding creativity and productivity for Nanostructured Materials Preparation and Characterization. Dr. Liu is also a long-term visiting scientist at Texas A&M University-College Station and Advanced Light Source, Lawrence Berkeley National Laboratory. Dr. Liu innovatively contributed to the synthesis, discovery, characterization and understanding of fundamental physical and chemical properties of nanoparticles, nanofilms and nanotubes, as well as applications of engineered nanomaterials in alternative energy and biological science. She established highest power density to advance performance of proton exchange membrane fuel cells and directed a new paradigm to apply metal-organic frameworks in the disinfection science. Dr. Liu has authored and co-authored textbooks (2), book chapters (3) and over 50 peer reviewed journal articles. Dr. Liu entire publications have been cited for about 1,000 times and the H-index of his publications is 8.7. She chaired and organized international conferences and reviewed dozens of journal articles and NSF proposals. She has been hosting and co-hosting four visiting scholars to conduct leading-edge research on biomedicine, hydrogen fuel cell and nanotechnology. During 6-year services in TAMUK, she trained about 60 undergraduate students, 6 master and 2 PhD (courtesy for CSC) students. Dr. Liu was awarded the '2012 and 2013 Annual foreign experts and talent from overseas project” supported by the State Administration of Foreign Experts Affairs, P.R. China (2012, and 2013); Japan Society for the Promotion of Science (JSPS) Invitation Fellow and worked at the Department of Materials Science, University of Tokyo (2010-2011). She has served as a 'Faculty and Student Team” fellow, collectively funded by the National Science Foundation and US Department of Energy, Office of Science and worked at the Argonne National Laboratory (2009). She also received Faculty Fellowship Summer Institute in Israel (2008) and outstanding research and teaching awards at the university level. She directed and participated in the projects (>16) supported by the NSF (USA, CHINA), NSERC (CANADA), R. Welch Foundation (since 2006), industrial and TAMUK as PI, Co-PI and senior personnel. She also received dozens of travel funds to attend QEM Workshops; NIH Faculty Grant Writing Workshop; Higher Education Consortium Workshop, Universities Space Research Association; and COACh (NSF women advancement) workshops.

Autoren/Hrsg.

Liu, Jingbo Louise

Bashir, Sajid

Leseproben

4.2. PV Cells

A photovoltaic cell (PV, also known as solar cell) is a device that spontaneously converts sunlight into electricity using the photoelectric effect. PVs can literally be translated as light electricity. PVs provide power for small electronic appliances, such as watches, and large amounts for the electric grid, and everything in between. The PV draws increasing attention due to its advantages [15].

1. It's highly reliable and needs little maintenance.

2. It costs little to build and operate.

3. It has virtually no environmental impact.

4. It's produced domestically, strengthening our economy and reducing our (US) trade deficit.

5. It's modular and thus flexible in terms of size and applications.

6. It meets the demand and capacity challenges facing energy service providers.

7. It helps energy service providers manage uncertainty and mitigate risk.

8. It serves both form and function in a building.

Use of nanomaterials as electrocatalyst for PV cells enhances energy conversion efficiency from sunlight into electricity. These materials are made of semiconductors such as crystalline silicon, organic materials, and metal oxides. The nanomaterials affect the energy conversion efficiency from light energy into electrical energy at the atomic level. The PV cells, producing about 1 or 2W of power (energy/time), are the basic building block of a PV system. To adjust the power output of PV cells, we connect single-cell units together to form larger units called modules. Furthermore, these modules can be connected to form even larger units called arrays. The arrays can be interconnected to produce high power to drive various appliances. In this way, we can build PV systems able to meet almost any electric power need, whether small or large. PV systems can be classified into two general categories: flat-plate systems or concentrator systems [16].

Optional principle of PVs: Upon photon absorption, an electron–hole (eh) or bound exciton pair is generated. The pair is separated as two (electron or hole) conductors, or the exciton pair is dissociated at the phase boundary into a free electron and a free hole, each in a separate material [17]. To understand how the PV cell operates, we select a typical silicon (Si) PV cell as an example. The Si-PV is composed of a thin wafer, which consists of a layer of negative (n)-type and positive (p)-type semiconductors. The thin layer of n-type semiconductor is made of phosphorus-doped silicon (electron-rich relative to silicon), while the p-type is made of boron-doped silicon (electron-deficient relative to silicon). The n-type thin layer is placed on the top of a thick layer of p-type silicon to compose the cathode and anode. A p-n junction is created by electrical field, where n-type and p-type are in contact [18]. When solar energy interacts with surface of a PV cell, photons are absorbed by the p-type silicon. The light-stimulated electrons are freed up, resulting in a flow of direct electrical current to the n-type. This current will then power an electrical load. The operational principle of PV is shown in Fig. 4.4 [19].

Figure 4.4 The schematic diagram of photovoltaic cell and its operational principle.

To improve the PV output power density, there are several challenges. It is important to “tune” the p-layer to absorb as many as possible according to the properties of incoming photons. Further, as many electrons as possible can be freed up. It is also critical to keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. In general, the key step is to design the material to free the electrons as close to the junction as possible. Therefore, the electric field can help send the free electrons through the conduction layer (the n-layer) and out into the electrical circuit. By optimizing all these characteristics, we improve the PV cell's conversion efficiency, which is how much of the light energy is converted into electrical energy by the cell [20].

Electrical contacts are essential to a PV cell because they bridge the connection between the semiconductor material and the external electrical load, such as a light bulb. The back contact of a cell (on the side away from the incoming sunlight) is relatively simple. It usually consists of a layer of aluminum or molybdenum metal. But the front contact—on the side facing the sun—is more complicated. When sunlight shines on the PV cell surface, electron ejection occurs, resulting in generation of a current of electrons that flows all over its surface [21]. If we attach contacts only at the edges of the cell, it will not work well because of the great electrical resistance of the top semiconductor layer. Only a small number of electrons would make it to the contact. To collect the most current, we must place contacts across the entire surface of a PV cell. This is normally done with a “grid” of metal strips or “fingers.” However, placing a large grid, which is opaque, on the top of the cell shades active parts of the cell from the sun. The cell's conversion efficiency is thus significantly reduced. To improve the conversion efficiency, we must minimize these shading effects [22].

Ideal current–voltage characteristics under dark: Under an external voltage (bias), the potential across the positive-to-negative (p-n region) depletion area is reduced Fig. 4.5(A) due to forward bias, resulting in a reduction of the drift current (I), and transport of major and minor carriers such as electrons (to p region) and holes (to n region) is increased by diffusion processes respectively. Under these conditions, electrons (to p region) and holes (to n region) would be injected to their respective sides, which for electrons population (at the n region) amount under thermal equilibrium conditions can be described by expression (4.7) [23]:

Figure 4.5 Energy band diagram under different bias, (A) forward bias and (B) reverse bias.

no=npoeqvB/kT

(4.7)

Since there are two, electron-rich and electron-deficient sides, under forward bias voltage (VF), when applied, the concentration of electrons at the junction, representing the interface between the depleted n region on the n-side us expanded to:

n=npeq(vB-vF)/kT

(4.8)

Conversely, the corresponding concentration at the interface of the p depletion region can also be expressed in a similar manner.

If electron injection is near zero (nn~nn0), the above expressions can be simplified to:

p=np0eqVF/kT

(4.9)

where kT is thermal energy (eV), VF is forward voltage (V), n is charge density (cm3), q is charge (C), and p is equilibrium hole density in n (cm3).

Under steady-state conditions, the differential becomes a continuity and can be expressed as for the n-region or layer:

pd2pndx2-pn-pn0tp=0

(4.10)

Factoring for a solution, gives:

n-Pn0=Pn0(eqVF/kT-1)e-(x-xn)/Lp

(4.11)

where the hole diffusion length is represented by Lp (nm) in the n-region or layer, with x=xn representing the density of the current, on the n-side:

p=-qDpdpndx|x=xn=qDpPnoLp(eqVF/kT-1)

(4.12)

In an analgous manner x=-xp the current density on the p-side is similarly expressed as:

n=-qDndnpdx|x=-xp=qDnPpoLn(eqVF/kT-1)

(4.13)

where the diffusion length is represented by Ln of electrons (on the p-side), with the sum current density being:

=Jn+Jp=(qDpPn0Lp+qDnnp0Ln)(eqVF/kT-1)

(4.14A)

=J0(eqVF/kT-1)

(4.14B)

where J is the current density (A/m2) and J0 is the saturated current density (A/m2) from which the open circuit voltage can be related to:

0=(qDpPn0Lp+qDnnp0Ln)=(qDpni2LpND+qDnni2LnNA)

(4.15)

The previous expressions were for forward bias, in a similar manner, the reverse bias voltage (VR) can also be determined by considering the bias to the p-to-n junction. Under these conditions, the electrostatic voltage is expected to increase across the depletion area as shown in Fig. 4.5(B) and unlike in forward bias, here the diffusion of electrons is inhibited and can be expressed in Eq. (4.16) for the current-voltage profile under reverse bias conditions [24]:

=J0(eqVF/kT-1)

(4.16)

PV properties: When incoming photons from the sun, have energies in excess of the band gap of the...