# Category Physics of Solar Energy

## Shockley—Queisser Efficiency Limit

With a load of matched impedance, the output power of a solar cell can be maximized. As shown in Section 9.1.3, the maximum power is related to the nominal power by a form factor nf approximately

## Statistics of Particles

In the microscopic treatment of matter, the particles, for example, molecules and elec­trons, are distributed in a system of energy levels. How the particles distribute among the energy levels determines the behavior of the system in a significant way. Regarding to applications with solar energy, an understanding of Maxwell-Boltzmann statistics and Fermi-Dirac statistics is essential. Maxwell-Boltzmann statistics is valid for a system of distinguishable particles in a system of energy levels allowing unlimited occu­pancy; whereas Fermi-Dirac statistics is valid for a system of indistinguishable particles in a system of energy levels allowing limited occupancy, that is, systems satisfying the Pauli exclusion principle.

The starting point of the derivation is the Boltzmann expression of en...

## 1 Harmonic Oscillator

The Hamiltonian of a one-dimensional harmonic oscillator is – 1 a2 mj2

ff = 2mp + ~q •

where the momentum p and coordinate q satisfy the commutation relation

[p, q] = pq — qp = ih.

We introduce a pair of operators, the annihilation operator,

Physics of Solar Energy C. Julian Chen Copyright © 2011 John Wiley & Sons, Inc. (C.3)

 a

and the creation operator

„ + 1 „ . [шш

a тшш”- V-2ira (C4)

the meanings of these terms well be clarified soon. Using the commutation relation C.2, we have H = НшаР a +— Нш,

and the commutation relation

[a, af] = aaP — aPa = 1. (C.6)

To find the eigenstates and energy levels of the Hamiltonian C.1,

Hn) = Enn), (C.7)

it is sufficient to find the eigenstates and eigenvalues of the operator apa,

ap an) = unn), ...

## Quantum Mechanics Primer

Usually, introductory quantum mechanics starts with Schrodinger’s equation and using partial differential equations as the mathematical tools. For example, the hydrogen atom problem is resolved with spherical harmonics and Laguerre polynomials. A possi­ble shortcoming with this approach is that the readers become submerged in pages and pages of mathematical formulas and lose the conceptual understanding of the physics. Historically, before Erwin Schrodinger discovered the partial differential equation for­mat, Heisenberg and Pauli developed the algebraic approach of quantum mechanics, and resolved several basic problems in quantum mechanics, including harmonic oscil­lator, angular momentum, and the hydrogen atom...

## Sine Formula

In planar trigonometry, there is the sine formula

abc
sin A sin B sin C

In spherical trigonometry, a similar formula exists,

sin a sin b sin c

sin A sin B sin C

Obviously, for small arcs, Eq. B.15 reduces to Eq. B.14.

To prove Eq. B.15, we rewrite Eq. B.2 as

sin b sin c cos A = cos a — cos b cos c. (B.16)

Squaring, we obtain

sin2 b sin2 c cos2 A = cos2 a — 2 cos a cos b cos c + cos2 b cos2 c. (B.17)

The left-hand side can be written as  sin2 b sin2 c — sin2 b sin2 c sin2 A, (B.18)

By definition, in a spherical triangle, the sides and the vertical angles are always smaller than 180°. Therefore, in Eq. B.22, only a positive sign is admissible. Because Z is symmetric to A, B and C, we obtain

sin a sin b sin c sin A sin B sin C

B. 4 Formula C

We rewrite the cosine formula B...

## Cosine Formula

In planar trigonometry, there is a cosine formula

a2 = b2 + c2 – 2bc cos A. (B.1)

In spherical trigonometry, a similar formula exists:

cos a = cos b cos c + sin b sin c cos A. (B.2)

When the arcs are short and the spherical triangle approaches a planar triangle, Eq. B.2 reduces to Eq. B.1. In fact, for small arcs,

cos b & 1 – 1 b2, (B.3)

2

and

sin b & b, (B.4)

and so on. Substituting Eqs B.3 and B.4 into Eq. B.2 reduces it to Eq. B.1.

Here we give a simple proof of the cosine formula in spherical trigonometry by an analogy to that in planar trigonometry. To simplify notation, we set the radius of the sphere OA = OB = OC = 1. By extending line OB to intersect a line tangential to AB at a point D, we have AD = tan c; OD = sec c.

Similarly, by extending line OC to intersect a line tengential ...

## Spherical Triangle

A plane passing through the center of a sphere O cuts the surface in a circle, which is called a great circle. For any two points A and B on the sphere, if the line AB does not pass the center O, there is one and only one great circle which passes both points. The angle AOB, chosen as the one smaller than 180° or n in radians, is defined as the length of the arc AB. Given three points A, B, and C on the sphere, three great circles can be defined. The three arcs AB, BC, and CA, each less than 180° or n in radians, form a spherical triangle; see Fig. B.1.

Following standard notation, we denote the sides BC, CA, and AB by c, b, and a, respectively...