# Category Physics of Solar Energy

## Energy Unit Conversion

Because energy is one of the most important quantities, there are many energy units which often creates confusion. Throughout this book, we used the SI units for all physical quantities. The SI unit of energy is joule, defined as the energy capable of pushing an object with one newton of force by one meter:

J = N • m. (A.1)

The basic SI unit of power, the watt, equals one joule per second, W = J/s.

The most frequently used units of energy and power are listed in Table A.1.

Electrical power is the product of the current in amperes and voltage in volts:

W = A • V. (A.3)

Utility companies often use an energy unit derived from electrical power, the kilowatt- hour, or kWh:

1 kWh = 3600 kJ = 3.6 MJ. (A.4)

Table A.1: Energy and Power Units

 Name Symbol Equals Kilojoule kJ 103 J

## Solar Energy and Electric Vehicles

According to the Energy Information Administration, in the United States, transporta­tion uses 26.5% of the total energy, or 67.6% of the petroleum. To reduce the use of fossil energy, the transition to electric cars using rechargeable batteries, especially using Li ion batteries as storage medium and solar energy as the source, is the best approach. Electric cars have many desirable features:

• The intrinsic efficiency of electric motors is very high, typically 90%.

• The round-trip efficiency of energy storage in rechargeable batteries, especially Li ion batteries, is very high, typically around 90%.

• The mechanical structure of electric cars is much simpler than either the Otto engine cars or the diesel engine cars.

• The regenerative brake can be implemented naturally...

## Mineral Resource of Lithium

As Li ion batteries become a major component of automobiles in the future, the problem of the mineral resource of lithium is currently of interest. First, let us estimate how much lithium is needed to equip all the automobiles in the world, and then compare it with the known mineral resources.   The atomic weight of lithium is 6.94 g/mol. Each mol of lithium has 96,490 coulombs of electrical charge. The working voltage of the Li ion battery is 3.5 V. Therefore, the specific capacity of lithium is

If each car needs a battery of 30 kWh capacity, then 2.2 kg of lithium is sufficient. Currently, there are 600 million cars in the world. The total amount of lithium required is then 1.32 million tons.

According to the 2009 U. S...

## Lithium-Ion Batteries

Currently the lithium ion battery is the most rapidly developing energy storage device. Soon after its invention in 1991, the CoO2-Li ion battery became the predominant power source for small portable electronics such as mobile phones, digital cameras, and laptop computers. It is widely believed to be the best candidate for powering automobiles because it has the highest specific energy and the longest lifetime; see Table 12.4.

In some sense, the electrochemistry of Li ion batteries is the simplest. The only ion involved is the lithium cation, Li+. It has the smallest radius and the highest standard potential, -3.01 eV. The negative electrode is made of graphite, where the small Li ion intercalates into the space between adjacent sheets of grapheme...

## Nickel Metal Hydride Batteries

In recent decades, nickel metal hydride rechargeable batteries have been widely used in automobiles and relatively large portable electronic devices. The positive electrode is nickel hydroxide, and the negative electrode is an intermetallic compound. The most common metal has the general form AB5, where A is a mixture of rare earth elements, lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and aluminum.

The electrochemistry is as follows. During discharging, at the positive electrode, NiOOH is reduced,

NiOOH + H2O + e – -^ Ni(OH)2 + OH-. (12.22)

At the negative electrode, metal hyride is oxidized,

MH + OH – -^ M + H2O + e-. (12.23)

During charging, at the positive electrode, Ni(OH)2 is oxidized,

Ni(OH)2 + OH – -^ NiOOH+H2O + e-. (12.24)

At the negative electro...

To date, the most widely used rechargeable battery is the lead-acid battery. Every automobile should have one with six cells. For a fully charged lead-acid battery, the positive electrode is made of PbO2 and the negative electrode is made of pure lead. The electrolyte is diluted sulfuric acid. After discharging, both the positive electrode and the negative electrode become PbSO4. Sulfuric acid is thus consumed. By measuring the specific gravity of the electrolyte, and thus the concentration of sulfuric acid, the state of discharging, thus the energy remaining, can be determined.

The electrochemistry is as follows. During discharging, at the positive electrode, PbO2 is reduced,  PbO2 + H2SO4 + 2e“ PbSO4 + 2OHA

At the negative electrode, lead is oxidized,

Pb + H2 SO4 -^ PbSO4 + 2H+ + 2eA

Du...

## Electrochemistry of Rechargeable Batteries

The basic structure and the charging-discharging processes of rechargeable batteries

are shown in Fig. 12.7. For reference, definitions are provided as below. For more

details, see, for example, Handbook of Batteries .

Cell The basic electrochemical unit converting electrochemical energy to electrical energy.

Battery One or more electrochemical cells connected in series or parallel to provide electrical power.

Primary cells or batteries One-time source of electricity, cannot be recharged after usage. Are discarded after usage.

Secondary (rechargeable) cells or batteries Can be recharged electrically after usage to their original condition.

Oxidation Loss of electron(s).

Reduction Gain of electron(s).

Redox Reduction and oxidation. Anion Negative ion — after gaining electron(s).

Ca...