COMPARISON OF CONCENTRATING SOLAR COLLECTORS

Presentation by:

Srikanth Reddy

Prakash Chandra sharma

Contents Introduction Basic characteristics

Components and structureSpecifications(size, material, HTF(heat transfer fluid))

Operation and maintenancePower generation technology(thermal cycle)CleaningDrives tracking

Performance comparison Cost comparison Globally installed capacity of each technology

List of abbreviations:

PTC: Parabolic Trough Collectors CRS: Central Receiver Systems LFC: Linier Fresnel Collectors SD: Solar Dish CR: Concentration Ratio HTF: Heat Transfer Fluid

IntroductionParabolic Trough collector(PTC)

Central Receiver Systems(CRS)

Linear Fresnel Collectors(LFC)

Solar Dish(SD)

PTC is a line focusing system that uses a moving parabolic reflector to concentrate direct solar radiation onto linier receiver.

CRS is a point-focusing system that uses individually tracked heliostat mirrors to concentrate solar radiation onto the stationary receiver located on the top of solar tower.

Fresnel Collector is a line-focusing system which uses individually tracked reflector rows to concentrate onto a stationary linier receiver.

Solar Dish is a point focusing system that uses curves solar tracking mirrors to concentrate direct solar radiation on to a receiver

Components and Structure-PTC Pylons: To support the system, these

were attached to some body or rimmed into ground.

Torque body: Which could be made of some kind of framework or a simple solid tube, is mounted onto these pylons.

Cantilever Arms: To hold the mirror facets.

Absorber: To absorb the concentrated sunlight and convert it with high efficiency to heat.

Metal bellows: These are used at either side of absorber tube to accommodate thermal expansion difference between steel and glass.

Getter: To keep and maintain vacuum.

Specifications Size: The normal range of the collector/aperture area ranges from 817 m2 to 1000 m2

these values may reach about 1700 m2 . Material: The pylons, torque body and cantilever arms are made of steel with paint or

galvanization to avoid corrosion. Stainless steel, plastic is used for smaller applications[1] and to build more rigid PTCs concrete structures are fabricated on-site[2].

Heat Transfer Fluid (HTF): Thermal oil(biphenyl-diphenyl oxide) : Up to 300oC[3]

Advantage: Low vapour pressure

Disadvantage: Low viscosity

which is critical in start-up after the plant is cooled down. Water/steam:

Advantages: Reduction in thermal loss through elimination of thermal oil.

lower pressure drop resulting in lower pump work

Disadvantages: Eventual instabilities at the two-phase flow. Molten nitrate salt: 1000oC[3] and above

Advantages: High operating temperatures

Disadvantage: The freezing points were typically too high to prevent freezing during off-sun and winter periods

Operation and maintenance Cleaning: The mirrors should be cleaned/washed at least once in 2 months. Demineralized

water must be used for wet cleaning. The reflectivity of surface washed with hard water is lower than that of the surfaces left with dirt. Usually spraying washer is used.

Maintenance of HTF quality[4]: The normally used HTF is synthetic oil with lower and upper temperature levels of 14 and 400 degrees respectively. For satisfactory operation it should be operated in that limit. Operating at higher temperatures increases the thermal degradation.

Drives: Most of the currently installed systems are using hydraulic drives which are robust and can provide strong forces with small steps(typically 1/10mm).

Tracking system: Either one or two dimensional to attribute the daily and seasonal variations, the sensor is located on parabolic trough.

Layout of PTC plant[5]

LOCE[5] and Resource Requirements[6] of PTCs

Performance and cost analysis[7]-PTC

Components and structure-CRS A CRS includes solar tower and heliostat field. Tower: Normally the tower is a monopole whose

structure is similar to that of a wind turbine or steel framework constructions. The shape can be :

1.Cylindrical

2.Rectangular Heliostat field: It is made up of individual

heliostats. Each heliostat follows the sun in two dimensions with the help of dual axis tracking system and concentrates the radiation onto receiver area.

Receiver: The receiver shape can be flat, cylindrical, pyramidal depending upon the heliostat field. To minimize thermal loss receiver is covered by cavity of quartz glass.

Tower Reflector: In this the receiver is placed on ground[8]. The radiation from the heliostats is reflected on to hyperboloidal mirror which is placed on tower and then the radiation reaches to receiver. It helps in solar chemistry where the reactants are solids.

Specifications Size: The height of tower depends on the area of

heliostat field, varies from as low as 150 ft. to order of hundreds with the tallest tower in world is 750ft. [9] high in  California.

Material: Towers are mainly constructed of steel or concrete. Factory made tower structure which are used for wind turbine were also been available, old models used the microwave relay towers.

Heat Transfer Fluid(HTF):

Water/Steam: It is used as HTF for a long time as it is used in conventional steam cycles with good thermal conductivity, absence of heat exchanger and high specific heat capacity as advantages and storage, two phase flow and corrosion as disadvantages.

Molten salt: The molten salt for both heat receiver and storages yields high capacity factors[10].It is best developed CRS technology today. It can be operated only in open cycle.

Air: Air as HTF is environmentally Benin and free. The disadvantage being low flow rate and high convective losses.

Operation and Maintenance-CRS Cleaning: Cleaning is normally carried at night times. The adjacent heliostat rows

were driven in opposite direction to facilitate cleaning. Drives: Depending upon the size of heliostats either hydraulic or electrical

drives(stepper motor) were used. Tracking: Two dimensional tracking is used in most of the cases. Separate

microwave towers are used to hold sensors which detects the reflected sun rays and tracks the sun accordingly.

Layout of CRS plant

Performance and cost Indicators[11]

Components and Structure-LFC It consists of a collector consists of a receiver with selectively coated absorber tube,

concentrator, cover plate and thermal insulation. Reflectors: The reflectors were long flat linear mirrors which were tracked individually.

The adjacent reflectors should be spaced carefully to avoid shading losses. Receiver: The receiver is based on a vacuum tube and a secondary reflector on top of it. Secondary Reflector: It has the task not to concentrate but to direct the radiation that

misses the entrance of the absorber aperture again to the absorber tube. Most common are CPCs which are designed for temperatures above 1200oC.Hexagonal shapes were often used.

Specifications: Size: The size width of the mirrors should be carefully co-ordinated with the gaps

between mirrors and height of receiver[12]. Todays power plants with Fresnel collectors have a typical size of 50MW gross electrical output. The risk is high due to limited experience and the technology has not yet proven in full-scale size.

Material: These uses flat glass mirrors and has a significant material reduction compared to parabolic trough collectors.

HTF: Water, air or oil is used as HTF. When using Direct Steam Generator(DSG) no heat exchanger is required but with thermal oil heat exchanger is imperative.

Layout of LFC plant

Components and structure-Solar Dish The major parts are solar concentrator(dish) and power

conversion unit. Dish: It is mounted on a structure that tracks the sun

continuously. The solar concentrators are supported with a truss structure in order to hold the mirrors of the concentrator.

Power conversion unit: It includes the thermal receiver and the engine/generator.

Thermal Receiver: It is the interface between dish and engine. It absorbs the radiation and transfers the heat to engine. The thermal energy cab be either transported to central generator or converted directly into electricity. The receiver transfers its heat to HTF.

Engine/Generator: It is the sub system that takes heat from thermal receiver and uses it to produce electricity[13].

Stirling Engine: It heated HTF to move pistons and create mechanical power which rotates the generator. It has higher efficiency high power density, low maintenance and long life.

Specifications: Size: They were flexible in terms of size and scale since they can be operated

individually or a in cluster. The dish/engine systems produce relatively small amounts of electricity typically 3-25 kW.

Materials: concentrators use a surface of Aluminium or silver, deposited on glass or plastic. Since the focal length is low thin glass mirrors are required to accommodate required curvatures low iron content ,silvered solar mirrors can have high reflectivity up to 94%[14].

HTF: Usually liquid hydrogen or helium is used as HTF to facilitate the heat transfer between receiver and engine[13].

Operation and maintenance Cleaning : The key difference between solar dish and

other systems is that the solar dish technology does not use water in the power conversion process(neither for steam nor for cooling),and water is used only for washing/cleaning.

Maintenance: Scheduled maintenance can be done on individual units while the others continue to generate power.

Tracking and control: The solar dish system is mostly controlled by microprocessor based control which also enables switching off from solar to fuel operation is a hybrid system is integrated.

Layout of solar dish system/plant

Performance and cost analysis[15]:

Performance and economic comparison:   Parabolic Trough Dish/Engine Power Tower

Size 30-320 MW 5-25 kW 10-200 MW

Operating Temperature (ºC/ºF)

390/734  750/1382 565/1049

Annual Capacity Factor  23-50 % 25 % 20-77 %

Peak Efficiency  20%(d)  29.4%(d) 23%(p)

Net Annual Efficiency  11(d)-16% 12-25%(p) 7(d)-20%

Commercial StatusCommercially Scale-up

Prototype Demonstration  AvailableDemonstration

Capital requirements($/kW)

3000 1690 2605

O&M costs $/kW-yr 43 11 30

Technology Development Risk

Low High Medium

Storage Available Limited Battery Yes

Hybrid Designs Yes Yes Yes

Globally installed capacity of each technology

References: [1] Anthrakidis A, Kroker J, and Rusack M, et al,(2009) technical improvement of

small parabolic trough collector. Berlin, Germany: solarPACES [2]Bader R,Haueter P,Pedretti A, and steinfeld A(2009) Optical design of novel2-

stage solar trough concentrator based on pneumatic polymeric structures. Berlin, Germany: solarPACES.

[3]Comprhensive Renewable Energy- Elsevier. [4]Gamble C and Schopf M(2009) optimization opportunities for thermal oil VP-1

heat transfer fluid in concentrating solar facilities.Proceedings solar PACES. [5] Status Report on Solar Thermal Power Plants, Pilkington Solar International:

1996. Report ISBN 3-9804901-0-6. [6] Assessment of Solar Thermal Trough Power Plant Technology and Its

Transferability to the Mediterranean Region- Final Report, Flachglas Solartechnik GMBH, for European Commission Directorate General I External Economic Relations, and Centre de Developpement des Energies Renouvelables and Grupo Endesa, Cologne,Germany: June 1994.

[8]Segal A and Epstein M(2006) practical considerations of designing large scale beam down optical systems solarPACES conference.19-23 june,Sevilla, Spain.

References(cntd.): [9]

http://www.treehugger.com/renewable-energy/worlds-tallest-solar-tower-be-erected-california-twice.html

[10]Ortega JI, Burgaleta JI, and Tellez FM (2008) central receiver system(CRS) solar power plant using molten salt as heat transfer fluid. Journal of Solar Energy Engineering.

[11] Stoddard, M.C., et. al., SOLERGY - A Computer Code for Calculating the Annual Energy from Central ReceiverPower Plants, Sandia National Laboratories, Livermore, CA: May 1987. Report SAND86-8060.

[12]Mertins M, Lerchenmuller H, and Haberle A (2004) Geometry optimization of Fresnel collectors with economic assessment, proceedings Eurosun conference, Freiburg.

[13]Kroposki B, Morgolis R, and Ton D (2009) An overview of solar technologies. IEEE power & Energy magazine.

[14]Winter C-J, Sizmann RL, and Vant-Hull LL (1991) solar power plants. Berlin, Germany: Springer.

[15] Kolb, G.J., “Evaluation of Power Production from the Solar Electric Generating Systems at Kramer Junction: 1988 to 1993,” Solar Engineering 1995, Proceedings of the ASME Solar Energy Conference, Maui, HI (1995).