Solar Power

(Technology and Economics)

 

The earth receives more energy from the Sun in just one hour than the world's population uses in a whole year.

The total solar energy flux intercepted by the earth on any particular day is 4.2 X 10¹⁸ Watthours or 1.5 X 10²² Joules (or 6.26 X 10²⁰ Joules per hour ). This is equivalent to burning 360 billion tons of oil ( toe ) per day or 15 Billion toe per hour.

In fact the world's total energy consumption of all forms in the year 2000 was only 4.24 X 10²⁰ Joules. In year 2005 it was 10,537 Mtoe (Source BP Statistical Review of World Energy 2006)

Solar Radiation

Sunlight comes in many colours, combining low-energy infrared photons (1.1 eV) with high-energy ultraviolet photons (3.5 eV) and all the visible-light photons between.

The graph below shows the spectrum of the solar energy impinging on a plane, directly facing the sun, outside the Earth's atmosphere at the Earth's mean distance from the Sun. The area under the curve represents the total energy in the spectrum. Known as the "Solar Constant" G₀, it is equal to 1367 Watts per square metre (W/m²).

The radiant energy falling within the visible spectrum is about 43% of the total with about 52% in the infra red region and 5% in the ultra violet region.

The graph below shows the energy at sea level.

Direct energy is the energy received directly from the sun.

Global energy includes energy diffused, scattered or reflected from clouds and energy re-radiated by the earth itself.

Energy received at sea level is about 1kW/m² at noon near the equator

Irradiance and Insolation

Nuclear Energy - The Theory

Nuclear Reactions

In the chemical reactions associated with combustion, the atoms in the molecules of the active materials rearrange themselves into new, more stable, molecules in which they are more tightly bound and in the process, releasing surplus energy in the form of heat.

In nuclear reactions it is the sub-atomic particles in the atomic nucleus, the protons and neutrons, which rearrange themselves to form new elements or isotopes with more stable nuclei. In this case the energy released by the reaction in the form of kinetic energy (manifest as heat) and electromagnetic energy (gamma radiation) is millions of times greater. See Energy Content

Note: The reactions discussed on this page are all nuclear reactions not chemical reactions.

Practical applications of the use of nuclear energy to generate electricity are given on the Nuclear Energy - The Practice page

Atomic Structure

Definitions

The diagram above shows a representation of the constituents of an atom using Lithium as an example.

• Notation
  • Atomic Number Z - is the number of protons in the nucleus
  The Lithium atom has three electrons occupying 2 energy levels and three protons giving it an atomic number Z = 3 .
  • Atomic Mass A - is the number of nucleons in the nucleus.
  The nucleus also includes four neutrons making up its seven nucleons and thus a mass number A = 7
  • Atomic Structure
  The structure may be indicated by appending the mass number A after the name of the element or by indicating it as a superscript preceding the chemical symbol. Thus Lithium-7 or ⁷Li.
  The symbol may also indicate the full atomic structure ( ^ALi_Z) by adding a subscript representing the atomic number Z preferably before, but alternatively after, the chemical symbol (depending on the capability of your word processor). Thus  ⁷Li₃
  
  
  An isotope of Uranium with 143 neutrons is called Uranium-235 and may be represented as  ²³⁵U  or ²³⁵U₉₂
  
  

Lithium Battery Manufacturing

 

The processes used for manufacturing Lithium batteries are very similar to those used in the production of Nickel Cadmium cells and Nickel Metal Hydride cells with some key differences associated with the higher reactivity of the chemicals used in the Lithium cells.

Electrode Coating

The anodes and cathodes in Lithium cells are of similar form and are made by similar processes on similar or identical equipment. The active electrode materials are coated on both sides of metallic foils which act as the current collectors conducting the current in and out of the cell. The anode material is a form of Carbon and the cathode is a Lithium metal oxide. Both of these materials are delivered to the factory in the form of black powder and to the untrained eye they are almost indistinguishable from eachother. Since contamination between the anode and cathode materials will ruin the battery, great care must be taken to prevent these materials from coming into contact with eachother. For this reason the anodes and cathodes are usually processed in different rooms.

Particle size must be kept to a minimum in order to achieve the maximum effective surface area of the electrodes needed for high current cells. Particle shape is also important. Smooth spherical shapes with rounded edges are desirable since sharp edges or flaky surfaces are susceptible to higher electrical stress and decomposition of the anode passivating SEI layer, which can lead to very large heat generation and possible thermal runaway when the cells are in use.

The metal electrode foils are delivered on large reels, typically about 500 mm wide, with copper for the anode and aluminium for the cathode, and these reels are mounted directly on the coating machines where the foil is unreeled as it is fed into the machine through precision rollers.

The coating process is shown in the diagram below

The first stage is to mix the electrode materials with a conductive binder to form a slurry which is spread on the surface of the foil as it passes into the machine. A knife edge is located just above the foil and the thickness of the electrode coating is controlled by adjusting the gap between the knife edge and the foil. Since it is not unusual for the gravimetric or volumetric energy storage capacity of the anode material to be different from that of the cathode material, the thickess of the coating layers must be set to allow the energy storage per unit area of the anode and cathode electrodes to be matched.

From the coater, the coated foil is fed directly into a long drying oven to bake the electrode material onto the foil. As the coated foil exits the oven it is re-reeled.

The coated foils are subsequently fed into slitting machines to cut the foil into narrower strips suitable for different sizes of electrodes. Later they are cut to length. Any burrs on the edges of the foil strips could give rise to internal short circuits in the cells so the slitting machine must be very precisely manufactured and maintained.

Cell Assembly

Geothermal Energy

The Earth as an Energy Source

The geothermal energy available from the Earth is potentially enormous. A United States Government energy agency estimates that the total energy available from global geothermal resources is approximately 15,000 times the energy contained in all the known oil and gas reserves in the world. Unlike solar and wind energy, the supply of geothermal energy is constant and doesn't vary with the time of day or change with the weather. Although geothermal energy may always be available when it is needed, like the other two sources it is not always availablewhere it is needed.

The Earth's core maintains temperatures in excess of 6000°K due to the heat generated by the gradual radioactive decay of the elements it contains. Modern estimates (Sclater 1981) for the total present rate of radioactive heat generation within the Earth are about 2 × 10¹³ W. This heat energy continuously flows outwards from the hot core due to conductive and convective flows of the molten mantle beneath the crust.

Estimates of the mean heat flux through the Earth's surface resulting from its radioactive core vary between 0.04 and 0.08 Watts per square meter. At the surface the heat dissipates into the atmosphere and space. This geothermal heat flow is trivial compared with the 1000 W/m² of solar energy impinging on the surface of the Earth in the other direction from the Sun (1367 W/m² at the outer surface of the atmosphere). Never the less it is sufficient to allow harvesting of geothermal energy on a commercial basis.

The diagram below shows the Earth's temperatures resulting from its internal heat generation and heat flows. The section on Solar Power describes the solar energy flows coming from external sources.

The Earth's Layers

• Inner Core - The inner core is solid with a radius of about 1,220 km and consists of about 80% Iron and 5% to 10% Nickel, with a temperature of up to about 7,200°K.
• Outer Core- The outer core, also mainly Iron and Nickel, is in a liquid state and is about 2,260 km thick. Melted rock is also called Magma
• Gutenberg Discontinuity Marks the boundary between the outer core and the inner mantle.
• Mantle is about 2900 kms thick surrounding the core and contains 83% of the volume and most of the mass of the Earth.
• Lower (Inner) Mantle (semi-rigid) - The deepest parts of the mantle, just above the core.
• Upper (Outer) Mantle is about 670 kms thick with two distinct regions, the hotter innermost part is plastic (flowing) while the cooler outermost part is rigid.
• Upper Mantle (flowing) = Asthenosphere - The innermost part of the upper mantle exhibits plastic (flowing) properties. It is located below the rigid lithosphere and is between about 100 and 250 km thick starting about 100-200 kms below the Earth's surface and possibly extending to a depth of 400 kms.
• Upper Mantle (rigid) - The rocky uppermost part of the mantle is part of the lithosphere.
• Lithosphere - The lithosphere is defined as the solid rocky region about 100-200 km thick which spans the crust and the rigid upper mantle.
• Mohorovicic (Moho) Discontinuity - is the boundary between the Earth's crust and the upper mantle.
• Crust - The Earth's crust occupies just 1% of the Earth's volume with a thickness averaging just 15 km. In scale size, this is only one fifth of the thickness of a typical egg shell. The temperature at the Earth's surface is typically 25 °C (298°K) 
• Continental Crust - the exposed thick parts of the Earth's crust, (not located under the ocean). The average continental crust thickness is 35 km. The maximum thickness is 90 km below Himalayas and the minimum is 25 km at its thinnest in various places.
• Oceanic Crust - The part of the Earth's crust located under the oceans is thinner, only about 5 to 11 km thick.
• Ocean - large bodies of water up to 3.7 km deep sitting on top of the oceanic crust. The water temperature at the surface is higher than the deep water temperature due to solar heating and thermal convection in the water which keeps it that way since the heavier cold water remains in the depths and the warmer, less dense water stays on the surface.
• Atmosphere - The thin layer of gases above the Earth extends to about 800 kilometres deep with a temperature of -273°C (absolute zero) at its outer limits. Most of the atmosphere (about 80%) is actually within 16 km of the surface of the Earth. In, scale this would be equivalent to a generous coat of varnish on a desktop globe.

 

Geothermal Gradient

Hybrid Power Generation Systems

Combined Heat and Power (CHP)

Combined Heat and Power (CHP) is the simultaneous generation of usable heat and power (usually electricity) in a single process. CHP plants enable the recovery of waste heat and / or better overall utilisation of the heat energy supplied to the system. They can thus be highly energy efficient.

CHP units may be characterised by their design priorities

• Heat Utilisation - Using surplus heat from another heating process to generate electricity.

Small scale or Micro-CHP installations are now becoming available for home use. The standard domestic heating boiler is replaced by a heating unit which also provides the heat to power a Stirling Engine which in turn drives an electrical generator.
The heating unit in the CHP plant must be dimensioned to provide sufficient surplus heat to power the Stirling engine. Using a single heating unit for both the heat and power generation simplifies the design and permits economies of scale.
Furthermore generating electricity in small local installations avoids the energy losses incurred in the transmission grid.
CHP can use a variety of fuels and heating technologies, however the majority of small CHP plants tend to be fuelled by natural gas.
More information on domestic electricity generation can be found in the section on Small Scale Systems.


• Heat Recovery - Making use of the waste heat resulting from electricity generation.

Hydroelectric Power

Hydroelectric Power

Hydro-electric power, using the potential energy of rivers, now supplies 17.5% of the world's electricity (99% in Norway, 57% in Canada, 55% in Switzerland, 40% in Sweden, 7% in USA). Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it is so readily stopped and started. It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations. Growth to 2030 is expected mostly in China and Latin America.

Hydro energy is available in many forms, potential energy from high heads of water retained in dams, kinetic energy from current flow in rivers and tidal barrages, and kinetic energy also from the movement of waves on relatively static water masses. Many ingenious ways have been developed for harnessing this energy but most involve directing the water flow through a turbine to generate electricity. Those that don't usually involve using the movement of the water to drive some other form of hydraulic or pneumatic mechanism to perform the same task.

Water Turbines