About Solar Energy

Solar energy is an all-encompassing term for energy obtained by harnessing photons of light emitted from the sun and converting their energy into other forms, such as electricity and heat, either directly or indirectly.

Solar energy is the most abundant form of energy available. The upper atmosphere of the Earth receives approximately 174 petawatts (174x1015W) of solar radiation. Of this, about 36% is reflected back into space, 17% is absorbed by the atmosphere and 47% is absorbed by the land and oceans.

To put this figure into perspective, each hour the Earth absorbs more energy than the total amount of energy in all forms currently used worldwide in a year. The amount of energy absorbed in one year is approximately twice that of all of the Earth’s non-renewable resources combined. This includes all oil, gas, coal and uranium reserves.

With the exception of nuclear, tidal and geothermal energy, most forms of energy with which we are familiar are forms of solar energy which have been transformed by some mechanism or another.

 Fossil-based energy is simply solar energy which has been transformed to chemical energy by plants over vast periods of time. Biomass and biofuels are essentially the same thing on shorter timescales.

Wind and wave energy are the result of differential solar heating between the equator and the poles and between the land and the sea. These temperature differences drive all of the atmospheric processes which are responsible for all aspects of our weather and climate, including the wind which in turn creates waves at the ocean surface.

Types of Solar Energy Conversion

Solar energy falls into two broad classes; passive and active.

In passive applications, the Sun’s energy is used either directly as light, or absorbed by some (typically dark coloured) material and used as heat. These techniques have been used throughout history, but in recent years have found their way into modern architecture (with considerable success) in order to reduce the energy consumption of buildings.

Active solar energy refers to applications whereby the energy contained in the light is converted to another form via some intermediate technology. The energy can then be transported, either in time (batteries, thermal storage media, hydrogen) or space (electricity grid, hydrogen). While there are limitless ways of achieving this, in practical terms, the two most important examples of active solar energy conversion are photovoltaic and solar thermal technologies.

Photovoltaic (PV) technologies are a direct method of solar energy conversion, whereby light is converted directly into electricity within a solid-state device. These devices take advantage of the properties of a class of materials known as semiconductors similar to those found in modern electronics.

Solar thermal technologies are an indirect method, whereby light is first converted into heat, and then the heat used to produce electricity either directly via thermoelectric materials, or more commonly indirectly by heating steam to run conventional steam turbines. Solar thermal energy is further distinguished from passive solar energy because the light must typically be concentrated in order to achieve energy densities sufficient to drive the various processes. For this reason it is commonly referred to as Concentrating Solar Power (CSP).

Photovoltaics

The birth of photovoltaics came with the discovery of the photoelectric effect by the French scientist Edmond Becquerel in 1839. While there are reports of photovoltaic effects from many materials in the ensuing years, the first rudimentary solar cell was based on silicon and was invented by Russell Ohl at Bell Labs in 1941. The first efficient cells, also from silicon, were produced by Chapin, Fuller and Pearson, again at Bell Labs, in 1954.

The advent of the microelectronics revolution, with its focus on silicon as the raw material of choice, resulted in spectacular gains in solar cell efficiency in the 1970s and 1980s. Silicon remains the dominant base material for commercial solar cells, however over recent years, many other materials have been the focus of intensive research and are now finding their way into commercial production.

How does a photovoltaic cell work?

The traditional photovoltaic cell is based on the p-n junction. A p-n junction is formed when an intrinsic semiconductor such as silicon is doped with small amounts of impurity materials to produce p-type and n-type semiconductors. P-type semiconductors have an excess of positive charge (called holes), and n-type material have an excess of negative charge (electrons). When bought together, the materials form a p-n junction.

At the junction, free holes from the p-type semiconductor migrate into the n-type material and free electrons from the n-type semiconductor migrate into the p-type region. The charges combine to form a depletion layer (i.e. a layer depleted of free charge) at the junction. The result is a net negative charge in the p-type material and a net positive charge in the n-type material. The resulting separation of charge creates an electric field across the depletion layer.

As the layer forms, it becomes increasingly difficult for free charge carriers to move across the junction and hence, at some point, an equilibrium is reached, and the layer ceases to increase in width. Typically, the depletion layer is of the order of one micron in an unbiased p-n junction, however this is largely dependent on the degree to which the intrinsic semiconductors are doped.

When a photon is absorbed in the depletion layer, a bound electron-hole pair is created, and as a result of the inbuilt electric field the exciton dissociates . The electron is then attracted to the n-type material and the hole attracted to the p-type material. These separated charge carriers can then be collected and recombined via an external circuit where they can be made to do work.

Concentrating Solar Power

Concentrating solar power has been used since ancient Chinese times. The first solar steam engine was produced in 1866 by Auguste Mouchout. The first commercial-scale CSP plant was built near Genoa in Italy in 1968.

In the intervening many variants of the technology have been built, culminating in the 354MW SEGS plant built in California in 1984 which remains the largest solar power plant in the world. A current resurgence of interest in CSP however will assure that this does not remain the case for very much longer.

How does a CSP plant work?

CSP plants use optical components (mirrors, lenses) to concentrate sunlight on a receiver which contains some working fluid. The working fluid is then used to run some type of heat engine, which in turn drives conventional electric generators.

CSP plants can also incorporate energy (heat) storage on a utility scale, which adds significantly to their commercial potential. 

Concentrator Types

As with PV, there are a large number of variants on CSP concentrators. They fall into three basic classes; trough, parabolic and power tower.

Trough plants typically use (parabolic) trough-shaped mirrors to concentrate sunlight on an elongated receiver and use single-axis tracking to follow the sun. A variant on this scheme is a linear Fresnel arrangement using flat mirrors arranged to concentrate light on a linear receiver.

The linear Fresnel arrangement has a lower efficiency than the trough arrangement but has lower costs (due to flat mirrors) and allows for closer packing of the units, thereby offsetting the efficiency deficit.

Parabolic concentrators use a parabolic dish to concentrate light on a receiver located at the focal point of the dish. They are considerably more complex than trough systems as they require two-axis tracking. However, much higher concentration ratios than those possible with trough systems are achievable. This has an advantage in terms of thermodynamic efficiency.

Power towers use a field of flat mirrors (heliostats), each of which is tracked individually to concentrate light on a single, central, stationary receiver. Again, these systems are much more complex than trough systems. Unlike parabolic dishes, which all track in an identical fashion, each heliostat must track uniquely in two axes. Again however, very high concentration ratios are achievable under this scheme.

Heat Engine Types

All heat engines are subject to a fundamental efficiency limitation, known as the Carnot limit. This limit is related to the temperature difference between the hot side and the cold side of the engine.  The two most common types of heat engines used in CSP are conventional steam turbines and Sterling engines. Each uses a different working fluid and has inherent strengths and weaknesses.

While steam turbines are not the most efficient way to convert energy, they have several advantages which make them attractive for CSP. The first of these is that steam turbine technology is mature, conventional and readily available. CSP plants employing steam can be constructed largely from off-the-shelf components, taking advantage of the inherent economies of scale implicit in this.

In theory, Sterling engines have the highest efficiency of any heat engine, their theoretical maximum efficiency being that of the Carnot limit. Although practical Sterling engines fall below this mark, they remain the most efficient of all of the heat engines. Despite this, they so suffer from significant engineering limitations which to date has limited their penetration in CSP market.

Sterling engines designed for CSP applications are typically of the order of 25kW in size. This implies that 40 individual units would be required for a 1MW CSP plant, or 4000 for a 100MW plant. This also means that a large number of small independent generators are required. Compare this with steam turbines, which can be scaled-up almost arbitrarily to many hundreds and even thousands of megawatts.

Storage

Energy storage is seen as the key to the large-scale adoption of renewable energy. Storage can be used to “smooth out” intermittent sources of energy to provide reliable baseload energy to the electricity grid.

One of the most attractive features of CSP is its potential to incorporate storage. Rather than energy (heat) going immediately to the working fluid, it can be diverted into some thermal storage media and transferred to the working fluid at a later time, when the renewable source is no longer available.

There are two basic mechanisms for heat storage: The first mechanism is referred to as sensible heat, the second as latent heat.

When energy is imparted to a material, the temperature of that material rises in proportion to its heat capacity. This energy associated with this temperature rise is referred to as sensible heat.

If the temperature continues to rise, the material will change from the solid to the liquid phase, or from the liquid to the gaseous phase. During these phase changes, the material absorbs a large amount of energy for a very small change in temperature. The energy associated with this change is called latent heat.

Both of these mechanisms can be employed as a means of storing energy, as the energy absorbed as the temperature rises can be extracted once again as the temperature falls. High purity graphite is an example of a sensible heat storage material, whilst various salts are used to store latent heat.