Concentrating Collectors

IntroductionFor applications such as air conditioning, central power generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective. They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used. These are devices that optically reflect and focus incident solar energy onto a small receiving area. As a result of this concentration, the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several thousand degrees Celsius. The concentrators must move to track the sun if they are to perform effectively.

Concentrating collectorsConcentrating, or focusing, collectors intercept direct radiation over a large area and focus it onto a small absorber area.

These collectors can provide high temperatures more efficiently than flat-plate collectors, since the absorption surface area is much smaller. However, diffused sky radiation cannot be focused onto the absorber.

Most concentrating collectors require mechanical equipment that constantly orients the collectors toward the sun and keeps the absorber at the point of focus. Therefore, there are many types of concentrating collectors.

Types of concentrating collectorsParabolic trough system

There are basic two categories of concentrating collectors:

Line Focusing Point Focusing(The line is a collector pipe)(The point is small volume through which the heat transport fluid flow)Parabolic dishPower towerLens Concentrator

*SOLAR FURNACECENTRAL RECEIVERPARABOLIC DISHLENS CONCENTRATORSPARABOLIC TROUGH

Concentrating collectors

Parabolic Trough SystemParabolic troughs are devices that are shaped like the letter U. The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the trough. Sometimes a transparent glass tube envelops the receiver tube to reduce heat loss. Temperatures at the receiver can reach upto 400C and produce steam for generating electricity.Parabolic Trough System

Schematic of a parabolic trough collector system

Fresnel type parabolic trough collector

Parabolic troughs often use single-axis or dual-axis tracking.One Axis Tracking Parabolic Trough with Axis Oriented E-W Two Axis Tracking Concentrator

Parabolic dish systemsA parabolic dish collector is similar in appearance to a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. It uses a dual axis sun tracker.

Parabolic dish collector with a mirror-like reflectors and an absorber at the focal point Crossection of parabolic dish

Schematic of a parabolic dish collector

A parabolic dish system uses a computer to track the sun and concentrate the sun's rays onto a receiver located at the focal point in front of the dish.Parabolic dish systems can reach 1000 C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small-power capacity range.Solar dish stirling engine

Power Tower SystemA heliostat uses a field of dual axis sun trackers that direct solar energy to a large absorber located on a tower. The heliostat collector in a system called the power tower.Heliostats Power tower system [A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky.]

Schematic of central receiver system

Linear Fresnel Reflector (LFR)

A power tower has a field of large mirrors that follow the sun's path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected rays of the sun are always aimed at the receiver, where temperatures well above 1000C can be reached. [High-pressure steam is generated to produce electricity].Power tower systems with heliostats

Lens ConcentratorThe Fresnel lens reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections known as "Fresnel zones." Lens concentrator collector

Lens Concentrator

Fresnel lens collectorHow a spherical Fresnel lens focuses light

Lens Concentrator

Stationary concentrating solar collectorsStationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle.

The wide acceptance angle for these reflectors eliminates the need for a suntracker.

This class of collector includes parabolic trough flat plate collectors, flat plate collectors with parabolic boosting reflectors, andsolar cooker.

Flat plate collector with flat reflectors

Solar FurnacesA field of heliostats tracks the sun and focuses energy on to a stationary parabolic concentrator which refocuses energy to the receiver.

Solar Furnaces - Operation

Receivers vary in design depending on process: Batch or continuous process Controlled temperature and pressure

Higher Temperatures (up to 3800oC)Higher temperatures are possible in solar furnace than in conventional combustion furnace or electric arc furnace.

Cleaner Processes e.g. Electric arc furnaces use carbon electrodes which often contaminate product.

Energy Sustainability Use of renewable energy for industrial processes.

Electricity through Solar ChemistryExample: Water splitting: 2H2O 2H2 + O2

*Centre National de Recherche Scientifique (CNRS) Solar Furnace at Odeillo, FranceMirror is 10 stories high and forms one side of the laboratoryMaximum temperature is 3800oC

Working principles of concentrating collectorsUnlike solar (photovoltaic) cells, which use light to produce electricity, concentrating solar power systems generate electricity with heat. Concentrating solar collectors use mirrors and lenses to concentrate and focus sunlight onto a thermal receiver, similar to a boiler tube. The receiver absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is converted into electricity.

There are three main types of concentrating solar power systems: parabolic troughs, dish systems, and central receiver systems.These technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid connected applications of 200-350 megawatts (MW) or more. (A concentrating solar power system that produces 350 MW of electricity displaces the energy equivalent of 2.3 million barrels of oil).

Major Components of Solar Collector SystemsConcentrating mirror(s) May use primary & secondary concentrators.

ReceiverAbsorbs energy from concentrator and transfers to process being driven (engine, chemical reactor, etc.)

Heliostats Flat mirrors that track the sun and focus on receiver or concentrator.

Trough SystemsThese solar collectors use mirrored parabolic troughs to focus the sun's energy to a fluid-carrying receiver tube located at the focal point of a parabolically curved trough reflector.Parabolic trough with mirrored parabolic troughs

Characteristics of Trough SystemThe length of the reflector unit may be 3 to 5 m and width about 1.5 to 2.4 m.Ten or more units are connected end to end in a row and several rows may be connected in parallelMany troughs placed in parallel rows are called a "collector field." The troughs in the field are all aligned along a north-south axis so they can track the sun from east to west during the day, ensuring that the sun is continuously focused on the receiver pipes.Reflectors made of highly polished aluminium or silvered glass or a thin film of aluminized plastic on a firm base The energy from the sun sent to the tube heats fluid (e.g. oil) flowing through the tube, and the heat energy is then used to generate electricity in a conventional steam generator. Individual trough systems currently can generate about 80 MW of electricity.

Concentrating collectors Reflectivity for mirror surfaces

MaterialrSilver0.93-0.95Back silvered low iron glass0.88Back aluminiumised glass0.76-0.80Plated silver0.96Aluminium sheet0.82Aluminiumised PTFE (poly tetra fluoroethylene)0.77Silvered PTFE0.86

Dish SystemsDish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish center. A dish/engine system is a stand alone unit composed primarily of a collector, a receiver, and an engine. It is about 6.6 m in diameter and made from about 200 curved mirror segments forming a paraboloidal surface.It works by collecting and concentrating the sun's energy with a dish shaped surface onto a receiver that absorbs the energy and transfers it to the engine. The engine then converts that energy to heat.The absorber, located at the focus , is a cavity made of a zirconium-copper alloy with a black chrome selective coating. The heat is then converted to mechanical power, in a manner similar to conventional engines, by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a piston to produce mechanical power.An electric generator or alternator converts the mechanical power into electrical power.

Each dish produces 5 to 50 kW of electricity and can be used independently or linked together to increase generating capacity. A 250-kW plant composed of ten 25-kW dish/engine systems requires less than an acre of land. Dish/engine systems are not commercially available yet, although ongoing demonstrations indicate good potential. Individual dish/engine systems currently can generate about 25 kW of electricity. Combination of parabolic dish system

Central Receiver SystemsCentral receivers (or power towers) use thousands of individual sun-tracking mirrors called "heliostats" to reflect solar energy onto a receiver located on top of tall tower.

The receiver collects the sun's heat in a heat-transfer fluid (molten salt) that flows through the receiver. The salt's heat energy is then used to make steam to generate electricity in a conventional steam generator, located at the foot of the tower.

The molten salt storage system retains heat efficiently, so it can be stored for hours or even days before being used to generate electricity.

The process of molten salt storage

Highlights the key features of the three solar technologies

Parabolic TroughDish/EnginePower TowerSize30-320 MW5-25 kW10-200 MWOperating Temperature (C/F)390/734750/1382565/1049Annual Capacity Factor23-50 %25 %20-77 %Peak Efficiency20%29.4%23%Storage AvailableLimitedBatteryYesHybrid DesignsYesYesYes

Concentrating collectors Concentration ratioC = Concentration ratioAa = Aperture area (m2)Ar = Receiver area (m2)

Collector PerformanceHeat from a solar collector may be used to drive a heat engine operating in a cycle to produce work. A heat engine may be used for such applications as water pumping and generating electricity.The thermal output Qout of a concentrating collector operating at temperature T is given byQout = F'[gamma . Ainqin U . Arec (T - Ta)] where:Ain = The area of the incident solar radiation (m2) Arec = The area of the receiver (m2)Gamma = Optical efficiencyqin = The incident solar irradiation (W/m2)Ta = The ambient temperature (C)U = The heat loss coefficient (W/m2K)F : Collector efficiency factor

The quantity Ain/Arec is called the Concentration ratio.

High concentration ratios are obtained by making Ain the area of a system of mirrors designed to concentrate the solar radiation received onto a small receiver of area Arec. Heat losses from the receiver are reduced by the smaller size of the receiver. Consequently, high concentration ratios give high collector temperatures. The stagnation temperature Tmax is given by:Gamma . Ainqin = U . Arec (Tmax - Ta)

For example, If the optical efficiency is gamma = 0.8, the incident solar irradiation is qin = 800W/m2, the ambient temperature is Ta = 30C, and the heat loss coefficient is U = 10W/m2K, then a concentration ratio Ain/Arec = 1 (no concentration) gives Tmax = 94C, and a concentration ratio Ain/Arec = 10 gives Tmax = 670C.

Economic and Environmental Considerations The most important factor driving the solar energy system design process is whether the energy it produces is economical.

Although there are factors other than economics that enter into a decision of when to use solar energy; i.e. no pollution, no greenhouse gas generation, security of the energy resource etc., design decisions are almost exclusively dominated by the levelized energy cost.

The expected cost of the energy produced by the solar energy system, averaged over the lifetime of the system.

Very high temperatures reached. High temperatures are suitable for electricity generation using conventional methods like steam turbine or a direct high temperature chemical reaction such as liquid salt. Good efficiency. By concentrating sunlight current systems can get better efficiency than simple solar cells.

A larger area can be covered by using relatively inexpensive mirrors rather than using expensive solar cells.

Concentrated light can be redirected to a suitable location via optical fiber cable for such uses as illuminating buildings.

Heat storage for power production during cloudy and overnight conditions can be accomplished, often by underground tank storage of heated fluids. Molten salts have been used to good effect.Advantages of Focussing Collectors

Concentrating systems require sun tracking to maintain Sunlight focus at the collector.

Inability to provide power in diffused light conditions. Solar Cells are able to provide some output even if the sky becomes a little bit cloudy, but power output from concentrating systems drop drastically in cloudy conditions as diffused light cannot be concentrated passively.

Disadvantages of Focussing Collectors

Solar Thermal Power PlantsorSolar Thermal Collectors

What is STPP/STC ?An innovative technology for harnessing solar energy for thermal energy

It is different from and much more efficient than photovoltaics, which converts solar energy directly into electricity

Classification of STPP/STCSolar thermal collectors are classified by the United States Energy Information Administration as:

Low-Temperature collectorsMedium-Temperature collectorsHigh-Temperature collectors

Low-Temperature collectorsLow-Temperature collectors are flat plates generally used to heat swimming pools

They can also be used in space heating

They use air or water as the medium to transfer the heat to their destination

Medium-Temperature collectorsMedium-temperature collectors are also usually flat plates but are used for heating water or air for residential and commercial use.

It consists ofSolar DryingCookingDistillation

High-Temperature collectorsHigh-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production.

It contains system design:

Parabolic trough designsPower tower designsDish designs

Conversion rates from Solar Energy to Electrical EnergySolar dish/Stirling engine has the highest energy efficiency

A single solar dish-Stirling engineproduces as much as 25kW of electricity, with aconversion efficiencyof 31.25%

Fresnel reflectors system has an efficiency of 19%

Solarparabolic troughplants have been built with efficiencies of about 20%

ApplicationsThe main use of this technology is in residential buildings where the demand for heating has a large impact on energy bills.

Commercial applications include car washes, military laundry facilities, eating establishments, drying, distillation andpasteurization.

The technology can also be used for space heating if the building is located off-grid.

Advantages of Solar Thermal Energy

No Fuel Cost

No Pollution and Global Warming Effects

Disadvantages of Solar Thermal Energy

High Costs Water Issue Limited Locations and Size Limitations Long Gestation Time Leading to Cost Overruns Financing

***************Liberating aluminum from bauxite, for example, is a chemical process that can be run using electricity (Hall process) or alternatively by solar energy.*IMP Laboratory studies the thermal methods for hydrogen production with solar energy among the various routes: electrolysis, photo-catalysis, biosynthesis... Typical range of working temperature is 1000C - 2500C. In a long-term vision, water decomposition into hydrogen and oxygen by closed thermochemical cycles is a very promising approach (Figure 2). In a medium term vision, solar decarbonization and up-grading of hydrocarbons are hybrid solutions that permit to reduce significantly the CO2 emission, as shown in Figure 3. Hybrid methods may lead to a reduction of 2/3 of CO2 emission.***************