As the name suggests, concentrated solar power uses mirrors to concentrate the sun's energy onto one point. Let's explore more...

Concentrated solar is fundamentally different from the solar photovoltaic (PV) power in that it uses the sun’s heat to generate electricity whereas concentrated solar often uses a solar-thermal method.

How Concentrated Solar Works: Mirrors surround a central tower, concentrating the sun's energy in one spot.

Solar PV cells rely on the sunlight being converted directly to electricity while CSP focus the sunlight using reflectors to generate heat that is used to run a steam engine and drive an electric generator [3].

This is the same way power is generated in a fossil fuel thermal plant – using the sun’s heat instead of heat from burning fossil fuels.

What Are The Different Types of Concentrating Solar Power Systems

There are three main types of CSP system;
1. Parabolic Trough Systems
2. Power Tower Systems
3. Parabolic Dish Systems

How Parabolic Trough CSP Systems Work

Parabolic troughs use reflective material to focus sunlight onto a focal point – a receiver pipe [5].

parabolic trough solar
Parabolic trough: The sun is focused on a central pipe.

The receiver pipe contains fluid which transfers the heat to boil water and turn generator turbines.

The parabolic trough design allows the sun’s heat to be focused to between 30 and 100 times its normal intensity [6].

A collector field is made up of an array of multiple parabolic troughs arranged in parallel along a north-south axis [4].




How Parabolic Trough Solar Panels Track The Sun

In order to maximize output, the troughs are setup with sun trackers to move about a north-south axis as the sun spans the sky from east to west [4].

This is necessary because the trough has been designed to direct all the sunlight onto the receiver pipe - therefore the angles at which the sunlight hits the trough need to be the same throughout the day.

A heat transfer fluid (HTF) runs through the receiver pipe. Oil is commonly used at the HTF but molten salt is also used frequently [2].

The main characteristics of these fluids are that they can retain high temperatures without turning into gas.

This hot fluid is channeled through a system of pipes where it heats water and generates steam, which is routed to run a steam powered turbine that drives an electric generator [5].

Some advanced systems have energy storage systems that allow the storage of hot fluid for use after the sun sets while other systems have fossil fuel or natural gas powered backup systems [4].

Parabolic trough systems are the oldest and thus most common CSP systems in use today with early developments of about 354 MW which were installed between 1984 and 1991 in the Mojave Desert in California [5].

How Concentrated Solar Towers Work

Concentrated solar towers work on the same principle as the parabolic trough system:

  1. Sunlight is manipulated to heat fluid
  2. Which generates steam.
  3. The steam powers an electricity generation power plant (see below).

The difference between solar towers and parabolic trough systems

The main difference between the tower and parabolic trough systems is the way the sun is reflected.

Solar tower systems use an array of mirrors, known as heliostats, which have been programmed to track the sun’s motion [4].

These heliostats focus and concentrate the sunlight on a central tower mounted receiver. Unlike the Parabolic trough which increases the sun’s intensity about 100 times, the tower system causes the sunlight to hit the receiver at about 1,500 times the normal intensity of the sun [6].

In 2009, a 20MW solar tower plant went online in Seville, Spain [7]. According to reports the tower systems will be developed to generate between 50MW and 200MW each in the near future [4].

 
 

How Parabolic Dish Solar Power Systems Work

The parabolic or solar dish systems focus and concentrate the sun’s intensity in a manner amplifies it up to 2000 times [6].

This high intensity makes it the most efficient system attaining a sun-to-grid conversion rate of 31.5% [7].

The parabolic dish system is made up of a solar concentrator, which is a dish, and a power conversion unit.

The power conversion unit is made up of a heat engine mounted on a receiver [4].

The Dish is mounted on a two-axis system that is programmed to follow the sun throughout the day because the concentrator has to get the most out of the sun in order to maximize the heat delivered at its focal point where the power conversion unit is located [4].

The receiver, which is part of the power conversion unit and lies between the engine and the dish, is made up of a series of tubes that carry a cooling fluid.

This system is conventionally smaller than the solar tower and parabolic trough systems. Nevertheless, what it lacks in size, it makes up for in power generation.

What heat transfer fluid is used in Concentrated Solar Photovoltaic Systems (CSPs)?

Most systems use either helium or hydrogen as the heat transfer fluid. The fluid absorbs the heat directed at the power conversion unit by the dish and transfers it in controlled amounts to the heat engine where it is converted to electricity [6].

Stirling or Brayson Cycle engines are the preferred choice of power conversion engines used in these systems [4].

The Future of CSP

It is natural that CSP would be compared to solar PV when assessing the future of the technology because they are both solar powered systems. There are three main factors considered when power utilities are debating on which renewable energy technology to generate electricity from. They are [8]

i. Competitive energy Cost
ii. Ancillary Services
iii. Delivery Upon Demand

Leading researchers argue that CSP is in a position to perform better than solar PV in all the three categories [9] especially since the inexpensive thermal storage offered by CSP systems allow for better delivery of energy upon demand [9].

Nevertheless, the recent drops in PV prices of between 30% and 40% have rocked the boat enough for investors to trust more in PV technology [11].

The bottom line is that the CSP technology still lags far behind PV as far as market penetration is concerned.

As a result, market growth surveys and projections continue to indicate that the technology will take a long time to catch up with solar PV.

CSP system installations are expected to reach about 10.8 GW and PV systems to reach 45.2 GW by 2014 while the two systems reported 0.29 GW and 7.0 GW in 2009 respectively [10].

This shows that the market growth prospects of CSP systems are much faster than PV since CSP systems are expected to increase 37 times while PV will increase about 6.5 times. Other factors such as availability of land for CSP systems may limit the developments, but only time can tell.

 
 
 




Article References

[1]. Solar Energy At Home – Types of Solar Energy: http://www.solar-energy-at-home.com/types-of-solar-energy.html

[2]. U.S. Department of Energy, Argonne National Laboratory – Solar Energy Systems; Primer on Solar Energy: http://web.anl.gov/solar/primer/primer4.html

[3]. How Stuff Works – How Solar Thermal Power Works: http://science.howstuffworks.com/environmental/green-tech/energy-production/solar-thermal-power.htm

[4]. Solar PACES, Solar Power And Chemical Energy Systems – CSP How It Works: http://www.solarpaces.org/CSP_Technology/csp_technology.htm

[5]. The Energy Blog – About Parabolic Trough Solar: http://thefraserdomain.typepad.com/energy/2005/09/about_parabolic.html

[6]. How Stuff Works – Solar Thermal Systems: http://science.howstuffworks.com/environmental/green-tech/energy-production/solar-thermal-power1.htm

[7]. Environmental and Energy Study Institute – Concentrated Solar Power Fact Sheet: http://www.circleofblue.org/waternews/wp-content/uploads/2010/09/csp_factsheet_083109.pdf

[8]. Renewable Energy World – CSP – PV Pricing… The Ongoing Price War: http://www.renewableenergyworld.com/rea/partner/first-conferences/news/article/2011/06/csp-pv-pricing-the-ongoing-price-war

[9]. Advantages Of Solar Energy – The Future Shape of Concentrating Solar Power (CSP): http://advantagessolarenergy.info/the-future-shape-of-concentrating-solar-power-csp/

[10]. Examiner – Market Growth for PV Solar vs. CSP: Which is Fastest: http://www.examiner.com/article/market-growth-for-pv-solar-vs-csp-which-is-fastest

[11]. Renewable Green Energy Power – Solar Energy Facts – Concentrated Solar Power (CSP) Vs Photovoltaic Panels (PV): http://www.renewablegreenenergypower.com/solar-energy-facts-concentrated-solar-power-csp-vs-photovoltaic-pv-panels/

 
 

If you see a solar panel, the chances are it's made of monocrystalline solar cells. They are by far the most widely used solar photovoltaic technology.

This article looks in detail at how monocrystalline solar panels work. If you're looking for a simple explanation of solar photovoltaics, you may wish to read the article on how solar panels work.

monocrystalline solar
Most solar panels on the market are monocrystalline.

Monocrystalline cells were first developed in 1955 [1]. They conduct and convert the sun’s energy to produce electricity.

When sunlight hits the silicon semiconductor, enough energy is absorbed from the light to knock electrons loose, allowing them to flow freely.




 

Monocrystalline vs Polycrystalline Solar Panels

Crystalline silicon solar cells derive their name from the way they are made. The difference between monocrystalline and polycrystalline solar panels is that monocrystalline cells are cut into thin wafers from a singular continuous crystal that has been grown for this purpose. Polycrystalline cells are made by melting the silicon material and pouring it into a mould [1].

The uniformity of a single crystal cell gives it an even deep blue colour throughout. It also makes it more efficient than the polycrystalline solar modules whose surface is jumbled with various shades of blue [1].

Apart from the crystal growth phase, their is little difference between the construction of mono- and polycrystalline solar cells.

The cells are usually laminated using tempered glass on the front and plastic on the back. These are joined using a clear adhesive and then the module is framed with aluminium. Single crystal modules are usually smaller in size per watt than their polycrystalline counterparts [1].

 
 

Why is silicon used in solar cells?

The atomic structure of silicon makes it one of the ideal elements for this kind of solar cell. The silicon atom has 14 electrons and its structure is such that its outermost electron shell contains only four electrons. In order to be stable, this shell needs to have eight electrons.

In its normal state or pure form, each silicon atom attaches itself to four other silicon atoms to form a stable silicon crystal [2].

In the silicon crystal’s pure state, there are very few free electrons available for carrying the electric current. In order to alter their electrical conductivity, other elements are introduced to the silicon as useful impurities in a process known as Doping [2].

 

Doping of silicon semiconductors for use in solar cells

Doping is the formation of P-Type and N-Type semiconductors by the introduction of foreign atoms into the regular crystal lattice of silicon or germanium in order to change their electrical properties [3].

As mentioned above, electricity is generated when free electrons are directed to carry a current within the cell’s electric field [2].

When the silicon atoms share their electrons, they can attain equilibrium easily because each atom needs four electrons and each can give four electrons.

The elements used in doping however, either have 5 or 3 electrons that they can share which are also known as valence electrons [3].

 

When elements with five valence electrons are introduced to the silicon crystals, the normal sharing of electrons begins, but the fifth electron remains unattached or unbound [2]. This unbound electron can easily be dislodged from the atomic shell when energy is introduced to the crystal causing it to be negatively charged.

This makes the crystal an N-Type semiconductor where the “N” stands for negative [2].

Some of the elements with 5 valence electrons include phosphorus, antimony and arsenic; phosphorus is the most commonly used element in crystalline solar cells.

On the other hand, when elements with three valence electrons such as boron, aluminium and gallium are introduced, there is a deficiency of electrons and instead, holes are formed [3].

This means that the crystal will carry a positive charge because it needs extra electrons to fill the hole left thus making it a P-Type Semiconductor where the “P” stands for positive. The holes move around seeking to be filled just as the free electrons move around ‘looking’ for holes to fill [2].

The P-Type and N-Type silicon semiconductors are combined to make the solar cell. When in contact, these two semiconductors generate the electric field which is necessary for electricity to flow in the solar cell [2].

The extra or free electrons in the N side are attracted to fill the holes in the P side. Unfortunately, at the point of contact, also called the junction of the two semiconductors, the electrons and holes mix to form a barrier preventing the electrons on the N side from crossing to the P side.

This barrier becomes an electric field separating the two sides when equilibrium is reached and acts as a diode allowing the flow of electrons in one direction, from the P-type semiconductor to the N-type [2].

 
 

Electricity generation at cell level

All that is needed for the electricity to be generated is the flow of electrons through a path provided within the electric field. However, we have seen that the flow of electrons has been localized and limited by the electric field which acts as a barrier between the cells. Nevertheless, this flow can be achieved when sunlight hits the solar cell.

The sun’s rays carry their energy in form of photons. Each photon usually has enough energy to dislodge one electron when it hits the solar cell [2].

In freeing one electron, it automatically causes a hole to be freed simultaneously. Due to the barrier’s disposition to allow the flow of electrons to the N-side only, the freed electron – if within the range of the electric field – will be sent to the N-Type semiconductor while the hole is sent to the P-Type.

Since this motion does not restore balance, the electron that was just displaced will seek to return to the P-Side so that neutrality is restored. Since the electron cannot cross back to the P Side from the N-Side via the barrier between the semiconductors, an external current path is built to allow this electron to return to the P – Side.

While the electrons flow through the external current path, we can make use of its energy to power electrical appliances [2].

 

Crystalline silicon solar cell efficiency

One of the major subjects of research into crystalline silicon solar cells is their efficiency. It's widely believed that the absolute limit is that 25% of the solar energy that hits a crystalline cell can be converted to electricity [2].

Researchers are hard at work to reach this efficiency with some companies like Sunpower and Sanyo achieving high efficiencies of up to 24% [4].

The wide spectrum of sun light wavelengths is responsible for the loss of about 70% of the energy that hits the solar cell [2].

The light from the sun has different wave lengths which we see as different colours. The different wavelengths also differ in energy content; some have more energy than the solar cell needs to produce electricity while others have less energy.

The crystalline silicon cell needs about 1.1 eV (Electron Volts) of energy to release an electron in the semiconductor; any energy that is more or less than this simply goes through the cell with no effect [2]. This energy used to release the electron is unique for each material and is known as the material’s band gap.

The band gap also determines the voltage of the cell. If the band gap is low, the voltage is also low. Therefore, although using a material that has a low band gap can increase the current of the cell, it lowers the cell’s voltage. Since Power is the product of current and voltage, the power output of the cell cannot be improved in this way. The optimal band gap for a solar cell made from one material has thus been found to be 1.4 eV so as to balance the effect of the current and voltage [2].

 
 

The Future of Monocrystalline Silicon Solar Cells

Having been in the market for more than 50 years, silicon solar cells are approaching if not passing their peak potential.

As such, extensive research has gone into improving the efficiency and lowering production costs of these systems.

Now, new technology is hitting the market. The introduction of thin film solar modules, for example, has attracted a lot of attention.

Market forces in countries like the U.S. have made it unprofitable for many companies to continue manufacturing the traditional silicon solar cells with companies like GE opting to shift its resources to manufacture thin film solar modules while it closes down its silicon crystalline solar cell plants [5].

This closure of firms has greatly been caused by the fact that manufacturing the traditional solar panels is a lot cheaper in other countries such as China, which is why some American companies are shifting production to the oriental super power [5].

The main difference between the traditional silicon cells is in the materials used to make them. The modifications that go into silicon for use in solar cells make it very expensive [1].

New construction methods and materials are much cheaper and although the efficiencies of the new technologies are much lower with thin film efficiencies ranging from 4% – 12% [6], there is still a lot of room for improvement in the new systems.

Multi-junction solar modules, which combine up to four different elements in their construction, have even surpassed the maximum efficiency of crystalline silicon modules with Spectrolab achieving 41.9% efficiency in the NREL Lab Test [4] while a commercial subsidiary of Boeing set a practical record of 39.2% [4].

 
 




Article References

[1] Wholesale Solar – Three Photovoltaic Technologies: , Polycrystalline and thin film: http://www.wholesalesolar.com/Information-SolarFolder/celltypes.html

[2] The Solar Plan – Converting Photons to Electrons: http://www.thesolarplan.com/articles/how-do-solar-panels-work.html

[3] Hyper physics – Doping of Semiconductors: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html

[4] The Green World Investor – Efficiency of Solar Cells Made From Silicon (, Multicrystalline), Thin Film (CIGS, CIS, aSi, CdTe, CZTS) and Multijunction: http://www.greenworldinvestor.com/2011/04/17/efficiency-of-solar-cells-made-of-siliconmulticrystalline-thin-film-cigscisasicdtecztsmultijunction/

[5] The Environmental News Service – Modern Solar Technologies Shading Out Silicon Solar Panels: http://www.ens-newswire.com/ens/nov2009/2009-11-10-094.html

[6] Civic Solar – Thin Film vs. Crystalline Silicon PV Modules: http://www.civicsolar.com/resource/thin-film-vs-crystalline-silicon-pv-modules

A solar tracker is a system where solar panels follow the sun throughout the day - retaining the best angle for maximum radiation exposure.

They change the tilt and direction of solar panels as the sun moves.

Solar modules are mounted on tracking frames, connected to a solar actuator - which acts like a hydrolic lift or extender.

As the sun moves across the sky, the actuator extends and moves the solar panel in order to keep the optimal angle with the sun.

Working with solar trackers is becoming increasingly popular in both domestic and utility scale systems as it boosts the overall efficiency of photovoltaic installations.

The LINAK Solar Park in Denmark found that by adding trackers to their solar modules, they could get 30-40 percent additional electricity output.

The solar park was a green initiative of the LINAK company, setup next their headquarters. It is expected to generate around 90,000 kilowatt hours per year.

Many large utility solar parks are also using solar tracker systems, such as Italy's 80,000 module Montalto di Castro.




Solar roof tiles are an ingenious alternative to bulky photovoltaic solar panels... Let's explore what they are and how they work!

Solar energy is typically harnessed through the use of photovoltaic solar panels made of crystalline silicon, which are attached to a roof or other structure.

These panels are positioned facing the sun (south in the northern hemisphere, north in the southern hemisphere) or on a solar tracking system that follows the sun.

Because silicon solar panels are bulky, they can be unsightly, and are also prone to damage in areas that experience strong winds.

Photovoltaic roof tiles are either made from regular crystalline silicone-based materials, or from thin-film solar cells, manufactured from layers of very thin semiconductor materials, such as amorphous silicon, or from other materials such as cadmium telluride, or copper indium gallium diselenide (CIGS).

The latter are thin, flexible, and durable, and ideally suited for use as a roof tile substitute that offers a protective roof cover, while drawing energy from the sun to provide your home with power. [1]




How Photovoltaic Shingles Work

Photovoltaic shingles work on the same principal as regular crystalline solar panels. Photovoltaic literally means 'light energy'.

The semiconductor photovoltaic cells absorb energy radiated from sunlight, which is then transformed from light (photo) energy into electric (voltaic) current. When energy from sunlight strikes the semiconductor material in the photovoltaic cells, a photon of light energy is absorbed, releasing an electron, which produces an electric current.

The current produced is direct current (DC), but as homes and business run on alternating current (AC), this needs to be converted by an inverter for domestic use. Once the current has been converted to alternating current, it can be connected to the main power board of a building to provide power locally, or it can even be connected to the electricity grid to provide power further afield. [2]

Installing Solar Roof Tiles

A regular solar panel typically consists of 40 photovoltaic cells that are installed in arrays of between 10-20 panels in a typical home system.

The panels can be installed onto an existing roof structure, or placed anywhere on the property to take optimal advantage of available sunlight. Photovoltaic roof tiles on the other hand, form an integral part of the roof structure, replacing regular roof tiles to serve a dual purpose of both repelling water, snow, hail, and wind, while absorbing the energy of the sun as a source of power.

While replacing existing roof tiles with photovoltaic tiles may be rather costly, when constructing a new home this may be quite cost effective in the long term, as it saves on the cost of roof tiles, and offers dramatic savings on energy costs.

Types of Photovoltaic Roof Shingles

Photovoltaic roof shingles are available in silicon or thin-film solar materials.

With energy efficiencies as high as 20.3% attained by silicon photovoltaic cells [3], silicon roof tiles, like silicon solar panels, are more energy efficient than thin-film solar tiles, but they are expensive, and take a long time to install.

Thin-film solar tiles are a recent innovation that are more affordable than silicon roof tiles, as they are cheaper to produce.

They are easy to install, cutting the installation time down by half – to around ten hours, which offers a further cost saving in terms of time and labor.

With ongoing research and development, thin-film photovoltaic roof tiles are catching up in terms of energy efficiency.

A new record of 19.9% efficiency has been attained for CIGS thin-film solar cells by researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory [3].

This improved energy efficiency, together with the affordability and ease of installation, may make thin-film photovoltaic roof tiles the photovoltaic option of choice for solar power installations in the very near future.

 

Article References

[1] National Renewable Energy Laboratory. Learning About Renewable Energy: Solar Photovoltaic Technology.

[2] ExploringGreenTechnology.com: How Solar Panels Works.

[3] National Renewable Energy Laboratory. Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency.

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