Energy from the Sun, Solar Energy Guide

Solar Power Types of Technologies

Many technologies have been developed to make use of solar radiation. Some of these technologies make direct use of the solar energy (e.g. to provide light, heat, etc.), while others produce electricity.
 
 

Concentrating collector with steam engine

Solar energy converted to heat in a concentrating collector can be used to boil water into steam (as is done in nuclear and coal power plants) to drive a steam engine or steam turbine. The concentrating collector can be a trough collector, parabolic collector, or power tower.

Concentrating collector with Stirling engine

Solar energy converted to heat in a concentrating (dish or trough parabolic) collector can be used to drive a Stirling engine, a type of heat engine which uses a sealed working gas (i.e. a closed cycle) and does not require a water supply. Until recently the solar Stirling system held the record for converting solar energy into electricity (30% at 1,000 watts per square meter). Such concentrating systems produce little or no power in overcast conditions and incorporate a solar tracker to point the device directly at the sun. That record has been broken by a so-called concentrator solar cell produced by Boeing-Spectrolab which claims a conversion efficiency of 40.7 percent

Solar updraft tower

A solar updraft tower is a relatively low-tech solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 8 km in diameter), is heated by the sun and channelled upwards towards a convection tower. It then rises naturally and is used to drive turbines, which generate electricity.

Energy tower

An energy tower is an alternative proposal to the solar updraft tower. It is driven by spraying water at the top of the tower, evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving wind turbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used) but does not require the large glass house of the solar updraft tower.

Solar pond

A solar pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration. It is a relatively low-tech, low-cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:

1) A top layer with a low salt content.
2) An intermediate insulating layer with a salt gradient, which sets up a density gradient that prevents heat exchange by natural convection in the water.
3) A bottom layer with a high salt content which reaches a temperature approaching 90 degrees Celsius.

The layers have different densities due to their different salt content, and this prevents the development of convection currents which would otherwise transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for heating of buildings, industrial processes, generating electricity or other purposes. One such system is in use at Bhuj, Gujarat, India and another at the University of Texas El Paso

Solar chemical

Solar chemical is any process that harnesses solar energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis in plants but without using living organisms. No practical process has yet emerged.
A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.

 While metals, such as zinc, have been shown to drive photoelectrolysis of water, more research has focused on semiconductors. Further research has examined transition metal compounds, in particular titanium, niobium and tantalum oxides.

Unfortunately, these materials exhibit very low efficiencies, because they require ultraviolet light to drive the photoelectrolysis of water. Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage.

Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light. It is also possible to use solar energy to drive industrial chemical processes without a requirement for fossil fuel.

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