Advanced Energy Materials

Ashutosh Tiwari, Sergiy Valyukh, Ashutosh Tiwari, Sergiy Valyukh

• Publisher: Wiley-Scrivener
• Year: 2014
• Print ISBN: 9781118686294
• eBook ISBN: 9781118904848
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

An essential resource for scientists designing new energy materials for the vast landscape of solar energy conversion as well as materials processing and characterization

Based on the new and fundamental research on novel energy materials with tailor-made photonic properties, the role of materials engineering has been to provide much needed support in the development of photovoltaic devices. Advanced Energy Materials offers a unique, state-of-the-art look at the new world of novel energy materials science, shedding light on the subject's vast multi-disciplinary approach

The book focuses particularly on photovoltaics, efficient light sources, fuel cells, energy-saving technologies, energy storage technologies, nanostructured materials as well as innovating materials and techniques for future nanoscale electronics. Pathways to future development are also discussed.

Critical, cutting-edge subjects are addressed, including:

• Non-imaging focusing heliostat; state-of-the-art of nanostructures
• Metal oxide semiconductors and their nanocomposites
• Superionic solids; polymer nanocomposites; solid electrolytes; advanced electronics
• Electronic and optical properties of lead sulfide
• High-electron mobility transistors and light-emitting diodes
• Anti-ferroelectric liquid crystals; PEEK membrane for fuel cells
• Advanced phosphors for energy-efficient lighting
• Molecular computation photovoltaics and photocatalysts
• Photovoltaic device technology and non-conventional energy applications

Readership

The book is written for a large and broad readership including researchers and university graduate students from diverse backgrounds such as chemistry, materials science, physics, and engineering working in the fields of nanotechnology, photovoltaic device technology, and non-conventional energy.

Chapter 1

Non-imaging Focusing Heliostat

Kok-Keong Chong

Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kuala Lumpur, Malaysia

* Corresponding author. [email protected], [email protected]

Abstract

Overcoming astigmatism has always been a great challenge in designing a heliostat capable of focusing the sunlight on a small receiver throughout the year. In this chapter, a non-imaging focusing heliostat with dynamic adjustment of facet mirrors in a group manner is presented for optimizing the astigmatic correction in a wide range of incident angles. Non-imaging focusing heliostat that consists of m × n facet mirrors can carry out continuous astigmatic correction during sun-tracking with the use of only (m + n − 2) controllers. A further simplified astigmatic correction of non-imaging focusing heliostat is also discussed which reduces the number of controllers from (m + n − 2) to only two. A detailed optical analysis is carried out and the simulated result has shown that the two-controller system can perform comparably well in astigmatic correction with a much simpler and more cost effective design. The new heliostat is not only designed to serve the purpose of concentrating sunlight to several hundreds of suns, but also to significantly reduce the variation of solar flux distribution with incident angle.

Keywords: Non-imaging focusing heliostat, new heliostat, optical analysis, solar flux

1.1 Introduction

There are two fundamental designs of solar concentrator technologies for harnessing high concentration solar energy: on-axis and off-axis focusing techniques. The most popular devices for on-axis focusing include parabolic dish, parabolic trough, spherical bowl (or so-called Fixed Mirror Distributed Focus), Fresnel lens, etc. [1, 2]. The off-axis focusing device involves the use of heliostat to focus sunlight onto a fixed receiver in the systems such as the central power tower, the solar furnace, etc. [2–7]. The on-axis focusing devices are usually used for distributed and smaller scale power generation (in the range from several kW to tens of kW) compared to that of off-axis focusing devices in the application of a central receiver system. For the central power tower, the concave mirrors used for the heliostat encounter a serious deterioration in focused image due to the off-axis aberration.

Off-axis aberration or astigmatism is a key factor in limiting the solar concentration ratio, especially for the central tower system that consists of a stationary receiver located in a field of focusing heliostats [8]. Full correction of the astigmatism requires a continuous adjustment in the local curvature of the reflector in both space and time. Although this method has been implemented in extremely large telescopes, it is obviously impractical for solar energy application because it would impose a very expensive and complicated control system with a total of 2 × m × n motors to orient each facet to its own unique direction for the heliostat composed of m × n facets. As a result, a new non-imaging focusing heliostat, that employs a clever approach to maneuver the facets in group manner for astigmatic correction has been proposed. Many research works on non-imaging focusing heliostat have been carried out by Chen et al. [9–15], Chong et al. [16–22] and Lim et al. [23] to establish the principle and technology of the new heliostat. Overall, there are two major advancements achieved in the new heliostat compared to the conventional heliostat that has remained unchanged for many decades [24]. One is the first mathematical derivation of the new spinning-elevation tracking formula to replace the commonly used azimuth-elevation tracking. Even the principle of spinning-elevation or target-aligned tracking method was first discussed by Ries et al. [25] and Zaibel et al. [26], but they did not propose any method of implementation in their papers such as derivation of new sun-tracking formula or construction of a prototype to implement the new tracking method. The second advancement is the correction of the first order astigmatism with the innovative line movements of the facets instead of trivial individual movements that would lead to complex and expensive mechanics.

1.2 The Principle of Non-imaging Focusing Heliostat (NIFH)

In the design of the non-imaging focusing heliostat (NIFH), mirrors are arranged into rows and columns. The central column is maintained in the optical plane by rotating the frame. The master mirror is fixed at the center with slave mirrors surrounding it, they share the same frame but the slave mirrors have two extra moving freedoms about their pivot points. To focus all the mirror images into one fixed target, each slave mirror is angularly moved about its pivot point to reflect sunrays onto the same target as the master mirror. The result at the target is the superposition of individual mirror images. As the sunlight is not coherent, the result is the algebra sum of the energy of the beams without a specific optical image.

1.2.1 Primary Tracking (Global Movement for Heliostat Frame)

The purpose of primary tracking is to target the solar image of the master mirror into a stationary receiver. Then, this image acts as a reference for secondary tracking where all the slave mirror images will be projected on it. In Figure 1.1(a), we define

as the vector normal to the reflector surface;

as the vector that points to the sun;

as the vector that points to a fixed target. Figure 1.1(b) shows the rotation of the plane of reflection, that plane which contains the three vectors (

,

and

), during primary tracking. In Figure 1.1(b), the vector

points to the new position of the sun and the vector

’ is the reflector normal of the new orientation so that the sunlight is still reflected towards the target. The tracking movement can be studied by two independent components (Figure 1.2):

Figure 1.1 Rotation modes of non-imaging focusing heliostat. (a)

is defined as the normal vector of the heliostat surface;

is the vector that points to the sun;

is the vector that points to a fixed target. (b) The plane that contains the three vectors is rotated about the vector

during primary tracking. The new vectors

’ and

’ shown in the figure indicate the new position of the sun and the heliostat frame so that the sunlight is still reflected towards the target.

Figure 1.2 Diagram showing the mounting of non-imaging focusing heliostat. The heliostat has two tracking axes that are perpendicular to each other, as does the conventional mount. The first rotational axis is pointing toward the target and it is indicated by TT’ axis; the second axis is the elevation axis (attached parallel to the reflector) and it is shown as FF’ axis.

a. Spinning movement:

The heliostat has to rotate about the TT’ axis so that the plane of reflection can follow the rotation of the vector

. Therefore, as the sun moves through the sky from the morning to solar noon, the plane will rotate starting from horizontal and turning to vertical. The angular movement about this spinning axis is denoted as θ.

b. Elevation movement:

The rotation of the heliostat about the FF’ axis (perpendicular to the plane) will adjust the reflector normal position within the plane until it bisects the angle between

and

. As a result, the sunlight will be reflected onto the target. This angular movement depends on the incidence angle of the sun relative to the heliostat surface normal and it is denoted as θ.

The formulas for ρ and θ can be derived by transformation study of two different coordinate systems: one attached to the center of the earth and the other attached to the local heliostat.

In Figure 1.3(a), by defining a coordinate system with the origin, C, set at the center of the earth, the CM axis is a line from the origin to the intersection point between the equator and the meridian of the observer at Q. The CE (east) axis in the equatorial plane is perpendicular to the CM axis. The third orthogonal axis, CP, is the rotation axis of the earth. Vector

pointing to the sun can be described in terms of its direction cosines, S_m, S_e and S_p to ...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Preface
• Chapter 1: Non-imaging Focusing Heliostat
• Chapter 2: State-of-the-Art of Nanostructures in Solar Energy Research
• Chapter 3: Metal Oxide Semiconductors and Their Nanocomposites Application towards Photovoltaic and Photocatalytic
• Chapter 4: Superionic Solids in Energy Device Applications
• Chapter 5: Polymer Nanocomposites: New Advanced Dielectric Materials for Energy Storage Applications
• Chapter 6: Solid Electrolytes: Principles and Applications
• Chapter 7: Advanced Electronics: Looking beyond Silicon
• Chapter 8: Ab-Initio Determination of Pressure-Dependent Electronic and Optical Properties of Lead Sulfide for Energy Applications
• Chapter 9: Radiation Damage in GaN-Based Materials and Devices
• Chapter 10: Antiferroelectric Liquid Crystals: Smart Materials for Future Displays
• Chapter 11: Polyetheretherketone (PEEK) Membrane for Fuel Cell Applications
• Chapter 12: Vanadate Phosphors for Energy Efficient Lighting
• Chapter 13: Molecular Computation on Functionalized Solid Substrates
• Chapter 14: Ionic Liquid Stabilized Metal NPs and Their Role as Potent Catalyst
• Chapter 15: There’s Plenty of Room in the Field of Zeolite-Y Enslaved Nanohybrid Materials as Eco-Friendly Catalysts: Selected Catalytic Reactions
• Index