Solar Power Tower

Power towers use a circular or semi-circular array of heliostats (large individually-tracking mirrors) to concentrate sunlight on to a central receiver mounted at the top of a tower.

A heat transfer medium in this central receiver absorbs the highly concentrated radiation reflected by the heliostats and converts it into thermal energy to be used for the subsequent generation of superheated steam for turbine operation. Heat transfer media so far demonstrated include water/steam, molten salts, liquid sodium and air.

Concentrating sunlight over 500 times, power tower technology has the potential advantage of delivering high temperature solar heat in utility scale quantities at temperatures of 500°C or more for steam cycles and greater than 1,000°C for gas turbines and combined cycle power plants.

Closed Cycle Ocean Thermal Energy Conversion (OTEC)

D’Arsonval’s original concept used a working fluid with a low boiling point, such as ammonia, which is vaporized using the heat extracted from the warm surface water. The heated working fluid is used to turn a turbine to produce electricity. Cold deep-sea water is used to condense the working fluid in a second heat exchanger prior to being re-circulated to the first heat exchanger. This is known as a closed cycle system (see Figure).

Figure: Closed Cycle OTEC (Image adapted from National Energy Laboratory of Hawaii Authority (NEHLA)).

Parabolic dish collector

Using parabolic dishes is a well-tested approach to concentrate solar radiation, and was an early experimental tool at many locations worldwide. The optical efficiency of parabolic dishes is considerably higher than that of trough, LFR or power tower systems because the mirror is always pointed directly at the sun, whereas the trough, LFR and power tower have a reduction in projected area due to a frequent low angle of incidence of the solar radiation. A schematic is shown in Figure.

A typical Parabolic dish collector system

Fermentation of Biomass

Fermentation of biomass is process of generation of producer gas then producer gas to ethanol.

Following the gasification of biomass to producer gas, the gas is converted into liquid products (e.g., ethanol) via fermentation.

Biological means fermentation can be used to produce fluid biomass fuels. For example, methane gas is produced in China for local energy needs by anaerobic microbial digestion of human and animal wastes. Ethanol for automotive fuels is currently produced from starch biomass in a two-step process: starch is enzymatically hydrolyzed into glucose; then yeast is used to convert the glucose into ethanol. About 1.5 billion gallons of ethanol are produced from starch each year in the United States.

Fermentation of lignocellulosic biomass to ethanol and dimethyl ether is an attractive route to energy feedstock that supplements the depleting stores of fossil fuels.

Biomass is a carbon-neutral source of energy, since it comes from dead plants, which means that the combustion of ethanol produced from lignocelluloses will produce no net carbon dioxide in the earth’s atmosphere.

Also, biomass is readily available, and the fermentation of lignocelluloses provides an attractive way to dispose of many industrial and agricultural waste products. Finally, lignocellulosic biomass is a renewable resource. Many of the dedicated energy crops can provide high energy biomass, which may be harvested multiple times each year.

Biomass Digestion (Using Anaerobic Digestion)

Anaerobic digestion (AD) is a natural process and is the microbiological conversion of organic matter to methane in the absence of oxygen. The decomposition is caused by natural bacterial action in various stages. It takes place in a variety of natural anaerobic environments, including water sediment, water-logged soils, natural hot springs, ocean thermal vents and the stomach of various animals (e.g. cows). The digested organic matter resulting from the anaerobic digestion process is usually called digestate.

Biomass Pyrolysis

Pyrolysis is the chemical decomposition of condensed organic substances by heating. Pyrolysis is the basis of several methods that are being developed for producing fuel from biomass, which may include either crops grown for the purpose or biological waste products from other industries. Vehicles were run on gas produced by pyrolysis of wood in times of war to replace unavailable fossil fuels. The improved electrical efficiency of the energy conversion via pyrolysis naturally means that the potential reduction in CO2 is greater than with combustion. Pyrolysis of biomass generates three different energy products in different quantities: coke, gas and oils. Pyrolysis as a first stage in a two-stage gasification plant for straw and other agricultural materials does deserve consideration.

The following technologies have been proposed for biomass pyrolysis:

• Fixed beds
• Augers
• Ablative Processes
• Fluidized Bed

Biomass Combustion

Biomass combustion simply means burning organic material. For millennia, humans have used this basic technology to create heat and, later, to generate power through steam. While wood is the most commonly used feedstock, a wide range of materials can be burned effectively.

These include residuals and byproducts such as straw, bark residuals, sawdust and shavings from sawmills, as well as so-called “energy crops” such as switch grass, poplar and willow that are grown specifically to create feedstock. Pelletized agricultural and wood residues are also an increasingly popular option because they are very easy to handle.

One recent technology advance is the introduction of pellet stoves, which use an electrically driven auger to deliver a steady supply of compressed pellets of wood or other biomass into the fire.

Future Of Nuclear Power

Still debatable as several other forms of renewable technologies are capable of meeting earth’s energy requirements for many centuries to come , if we find the right technology to harness them.

Besides, with improvement , the cost of generating electricity can be substantially reduced and also they are virtually risk free.

Though the importance of nuclear energy cannot be altogether neglected in present state, but the best possible approach will be to invest in it with giving equal attention to more sustainable forms of energy like wind and solar.

Issues with Nuclear Power

Nuclear Power’s environmental friendliness is still debatable with there being no acceptable solution for disposal of hazardous spent fuel.

The generation of energy again depends on natural reserves of radioactive fuel which are bound to be exhausted one day even if we are able to cope with all other issues.

This again raises the question why invest heavily on something which does not have a very long foreseeable future.

All other forms of renewable energy are in direct or indirect way dependent on sun which in all possibility is bound to shine for another million years.

Lastly again, the risk factor is the highest in this technology. No matter how many safety procedures we employ, we cannot deny the fact that human errors are bound to happen some day or other.

Schematic of LFTR (Liquid fluoride thorium reactor)