7.1 Solar Thermal Technology

7.1.1 Non-concentrating

There are two types of solar collectors that fall within this technology option—flat-plate, commonly used for residential applications and evacuated-tube, commonly used in commercial buildings.

Flat plate collectors may be unglazed, liquid flat plate solar collectors (FPC) and solar air heating collectors. The unglazed collectors (for 35–40°C) are used for low heating application viz. swimming pool heating etc. A typical cross sectional view of this type of collector is shown in Figure 7.1.

Figure 7.1 Schematic Diagram of Flat Plate Solar Collector

Source: http://www.solarfeeds.com/solar-cooling-and-heating-market-trends

An FPC is a widely used solar energy collection device for applications that require heat at temperatures below 80°C. A typical liquid FPC consists of a selectively black-coated absorber plate of high thermal conductivity (such as copper or aluminum), one or more transparent covers, thermal insulation, heat removal system, and outer casing. The transparent cover reduces the convective and radiative heat losses from the absorber plate to the surrounding. To achieve operating temperatures higher than 80°C, two glass covers may also be used. The heat collected by the absorber plate is extracted by circulating a working fluid through the riser tubes attached to the absorber plate, which are further connected to a larger pipe called header at both ends as shown in Figure 7.1.

The working fluid, usually water or an anti-freeze mixture flows through these tubes to carry away the heat. An outer casing houses all the components. This is finally placed on a stand so that the collector properly inclined to receive maximum solar radiation.

An evacuated glass solar tube (EGT) is the key component of EGT solar water heaters and solar water heating systems. The EGTs allow sunlight to pass through freely but reduce the convective heat loss from the absorber due to the evacuated space around it. An EGT is configured by two concentric borosilicate glass tubes; the outside surface of the inner tube is coated with selective coatings to increase the absorptivity in the solar spectral region and low emissivity in the infra-red region. The jacket between cover glass tube and the inner glass tube is evacuated and sealed. In order to maintain vacuum in the jacket between the two tubular glass tubes, a barium getter (gas absorbent) is used inside the bottom of the cover tube. During manufacturing this getter is exposed shortly to high frequency magnetic fields, which create high temperatures in one second and consequently the bottom of the evacuated tube gets coated with a pure layer of barium. Figure 7.2 presents a cross sectional view along with the glass based vacuum tubes, while Figure 7.3 presents various components of the vacuum tube system.

Figure 7.2 Cross-sectional View of EGT Collectors

Source: http://www.solardirect.co.nz/solar_heating_how_does_it_work_xidc25507.html

7.1.2 Various Components of EGT Collectors-based Water Heating System

Concentrating solar thermal power plants produce electricity by converting solar radiation into high-temperature heat using various mirrors/reflectors and receiver configurations. In CST plants solar radiation is converted to high-temperature heat source which, in turn, is used to produce steam or hot gas for power production through turbine generator combination where as in concentrated solar photovoltaic (CPV) technologies electricity is produced directly through semiconductor based converter/solar cell. The overall generic arrangement in a typical CST power plant is shown in Figure 7.4. The main subsystems of a CST power plant are described in the subsequent sections

Solar field

Solar field is essentially an array of the mirrors/reflectors that collects the solar radiation and focuses it on the solar receiver. The focusing mechanism depends on the type of solar collector and the tracking mechanism (one-axis or two-axis). The solar field is also known as solar concentrator or heliostat field. It is the major part of any CST power plant.

Figure 7.3 Various Components of EGT Collectors Based Water Heating System

Source: http://greenterrafirma.com/evacuated_tube_collector.html. Last accessed on 24 November 2012.

Figure 7.4 Generic Arrangement of a CSP Plant

Source: http://www.sciencedirect.com/science/article/pii/S0038092X11002441 (Research paper titled: 'Performance model for parabolic trough solar thermal power plants with thermal storage: Comparison to operating plant data' Isabel Llorente García, José Luis Álvarez, and Daniel Blanco)

Receivers/absorbers

Receivers/absorbers are part of the system that transforms solar radiation to heat. Sometimes the receiver is an internal part of the solar collector field. A heat transfer medium, usually water or oil is used in the solar receiver to transport the heat to the thermal energy storage and/or conversion system.

Energy conversion system

The energy conversion system converts heat in to usable forms of energy which could either be electricity or thermal heat for process heating. This is done in two stages:

  • heat energy is converted to mechanical power using steam/gas turbine or
  • electricity is generated through generator/alternator.

The output of the energy conversion system can be thermal heat for process heating/cooling applications or electricity. Typically in CST power plants waste heat can be used for other applications.

Thermal energy storage

Thermal storage can be for few minutes to few hours. Size of storage system influences the size of the solar field, area required for solar field and eventually the capital cost of CST power plant.

Fossil fuel back-up system

Fossil fuel back-up system is provided to improve the capacity utilization factor and / or smooth out variation due to sudden drop in solar radiation levels.

7.1.3 Classification of Concentrating Solar Thermal Technologies

Concentrating Solar Thermal technologies are categorized based on the types of solar collectors used or the method of concentration. Following four major CST technologies have reached commercialization stage or are near it:

  • Parabolic trough collectors (PTC)
  • Power towers or central receivers
  • Parabolic dishes (dish-engine system)
  • Compound linear fresnel reflectors (CLFR)

The brief discussion of all CST technologies is covered in next section. Figure 7.5 presents a schematic diagram of these CST technologies.

7.1.4 Parabolic Trough Collector System

Parabolic trough technology is currently the most proven CST technology and therefore the most developed and standardized system. Parabolic trough-shaped mirror reflectors are used to concentrate electromagnetic radiation onto

thermally efficient receiver-tubes placed in the trough's focal line. The PTCs are usually designed to track the sun along one axis, predominantly north–south. A thermal transfer fluid, such as synthetic thermal oil, is circulated in these tubes. The fluid is heated to approximately 400°C by the sun's concentrated rays and then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle. This is the first CST technology to be commercialized. Figure 7.5 presents schematic diagram of a PTC system.

Figure 7.5 Schematic Diagram of CST Power Technologies

Source: Compiled by authors.

Figure 7.6 Schematic Diagram of Parabolic Trough Collector System

Source: http://www.yale.edu/ynhti/curriculum/units/2010/4/10.04.09.x.html. Last accessed on 24 November 2012.

PTC projects worldwide

The parabolic trough projects currently in operation are between 14 and 80 MWe in size, and existing plants are producing well over 500 MW of electrical capacity. In southern California, nine plants named solar electricity generating systems (SEGS) of about 2 million sq m of mirror area were developed from 1984–1989 by Luz Company. Further development of new plants stopped in 1990s in USA due to unfavorable tax policies. However, technology development and innovations continued. Recent changes in policies and incentives have seen resumption of commercial CST plants in the US. In US, commercial construction of PTC plants has resumed with the 64 MW project called Nevada One, which will produce 130 GWh of electricity annually.

In Spain, the Andasol I, II and III and Solnova projects which are under construction will together provide 250 MW capacity (five projects of 50 MW capacity each), and more than 14 more projects based on PTC technology are being developed worldwide. The largest single parabolic trough installation yet proposed is called Solana, and is planned for a site in Nevada, US. Parabolic trough systems are also being studied to integrate them with conventional coal fired power plants in a hybrid operation called Integrated Solar Combined Cycle (ISCC), where the steam generated by solar is fed into a thermal plant that also uses fossil-fuel generated steam, generally natural gas.

Major sub-components of the PTC system

Collector field. The collector field comprises reflectors (mirrors), structure and control/tracking of the collector. Usually, low iron single piece glass mirrors are used which are parabolic in shape. Presently for PTC technology the operating reflector of 4 mm low iron glass mirrors are in use (SEGS plants in Mojave Desert in California).

Tracking system. Tracking is particularly important in solar energy collection systems that operate under concentrated solar radiation. As the earth–sun angles changes continuously hence to align the collector with the position of sun, tracking is essential with concentrating collectors. PTC comprises single-axis tracking; the frequency of tracking depends on various factors including tracking axis orientation and accuracy of tracking required which in turn depend on concentration ratio.

Thermal storage. The thermal energy storage (TES) systems are incorporated into CST installations and allow them to convert the heat collected during times of high insolation to power during times of low insolation. The kind of storage system used for solar energy storage depends upon the CST technology. Molten salt, concrete storage, phase change materials, saturated steam or pressurized air are some of the storage mediums used in CST power plants. Currently, storages for 30 minute to 7.5 hour operations are being tested and studied.

Suppliers of PTC technology. Parabolic trough collector is the most proven and mature CST technology. The major suppliers of the PTC technologies are Acciona Energia, Solel Solar System, Solar Millennium AG, Abengoa Solar: Solucar Energia, S.S., SENER Ingenieria, Sky Fuel, etc. These industries have announced more than 5400 MW capacity CST projects for the next 10 years.

Investment cost of CSP power plant based on PTC technology. Parabolic trough power plants consist of large fields of PTC collectors, a heat transfer fluid/steam generator, a Rankine steam turbine/generator cycle, and optional thermal storage and /or fossil-fired backup systems.

For investment cost analysis, recently commissioned Andasol-I CST power plant, 50 MW, PTC technology has been taken as reference. Table 7.1 presents the cost break up of CST power plant based on PTC technology along with 7.5 hour heat storage.

Table 7.1 Cost Break up of CST Power Plant based on PTC Technology

Note: 1US$ = Rs 55.7, as on 2 September 2012

Source: IRENA working paper on: Renewable Energy Technologies: Cost Analysis Series, Volume 1: Power Sector Issue 2/5 Concentrating Solar Power.

Available at http://www.irena.org/DocumentDownloads/Publications/RE_Technologies_Cost_Analysis-CSP.pdf, last accessed on 2 September 2012.

The cost break up reference has been taken from the European roadmap prepared by ECOSTAR in which the capital cost is segmented broadly into five categories.1 The investment breakdown of reference PTC power plant is presented in Figure 7.7.

It can be seen that solar field which consist of solar collectors, balance of system, and tracking comprise 51% of the cost followed by power block at 22% which comprises turbine, generator, heat exchangers etc.

7.1.5 Central Receiver System (Power Tower)

Central receiver systems use a field of distributed, circular array of mirrors that is, heliostats which individually track the sun and focus the sunlight on the top of a tower. By concentrating the sunlight 600–1000 times, they achieve temperatures from 800°C to well over 1000°C. Solar energy is absorbed by a working fluid and then used to generate steam to power a conventional turbine. The high temperatures available in solar towers can be usednot only to drive steam cycles, but also for gas turbines and combined cycle systems. Such systems can achieve up to 35% peak and 25% annual solar electric efficiency when coupled to a combined cycle power plant. Figure 7.8 presents a schematic diagram of a central receiver CST power plant.

Figure 7.7 Cost Break up of Parabolic Trough Based CST Power Plant

Source: European Concentrated Solar Thermal Road-Map (ECOSTAR SES6-CT-2003-502578), DLR

Figure 7.8 Schematic Diagram of Central Receiver CST System with Molten Salt Storage

Source: www.solarpaces.org.

There are some landmark operational projects running in Spain, notably the Sanlúcar Solar Park, the PS-10 solar tower of 11 MW and the PS-20 that has a 20 MW capacity. In India ACME Solar has invested in the start-up company e-Solar, USA which has developed saturated steam-based 5 MW capacity tower systems. The company has announced projects in Rajasthan and Gujarat.

Major sub-components of central receiver system

Heliostat field. The heliostat field comprises a large heliostat, structure, and control/tracking. The heliostat typically utilizes a mirror, which can be oriented throughout the day to redirect sunlight along a fixed axis toward a stationary target or receiver.

Storage. Central tower based systems typically use Molton salt, hot concrete storage, phase change materials, saturated steam or pressurized air as storage media.

Receivers/absorber and power block. This includes the receivers, absorbers including heat collection elements, and Power Block.

Technology suppliers

Central receiver technology is the second CST technology which is well-proven and mature. Manufacturers/ suppliers of this technology include Abengoa Solar, Bright Source Energy, Solar Reserve, e-Solar, etc. Solar Reserve had installed a power plant based on this technology named Solar Two in 1980 in California which is now decommissioned. The Brightsource Energy has announced CST power project of the capacity of 400 MW in USA (California). More than 300 MW capacity CST power projects have been announced by various companies which will be commissioned in the next 10 years.

Investment cost of CSP power plant based on central tower technology

Recently commissioned PS10 power plant of 11 MW capacity in Andalusia, Spain, based on central receiver technology has been taken as reference case. Table 7.2 presents the cost break-up of CST power plant based on central receiver technology.

Table 7.2 Cost Break-up for Central Receiver Based Solar Thermal Power Plant

Note: €1 = Rs 68.325, as on 10 July 2012.Source: Abengoa Solar.

The cost break-up reference has been taken from the European roadmap prepared by ECOSTAR in which the capital cost is segmented broadly into seven categories.2 The cost breakdown of reference central tower power plant is presented in Figure. 7.9.

The solar field which consists of solar collectors, balance of system, and tracking constitutes 36% of the cost followed by the power block at 24% which comprises the turbine, generator, heat exchangers etc. The receiver is also a major component of this technology comprising 15% of the cost.

7.1.6 Parabolic Dish

A parabolic dish-shaped reflector concentrates sunlight on to a receiver located at the focal point of the dish. The concentrated beam radiation is absorbed into a receiver to heat a fluid or gas (air) to approximately 750°C. This fluid or gas is then used to generate electricity in a small piston or Stirling engine or a micro turbine, attached to the receiver. The dishes are usually designed to track the sun along two axes to reflect the sun beam on to point of focus. Figure 7.10 presents the schematic diagram of parabolic dish-Stirling system.

Figure 7.9 Cost Break-up of CST Power Plant Based on Central Tower Technology

Source: European Concentrated Solar Thermal Road-Map (ECOSTAR SES6-CT-2003-502578), DLR

Figure 7.10 Schematic Diagram of Parabolic Dish-Stirling System

Source: Sourced from internal TERI research by authors.

Several dish/engine prototypes have successfully operated over the last 10 years, ranging from 10 kW (Schlaich, Bergermann and Partner design), 25 kW (SAIC) to over 100 kW (the 'Big Dish' of the Australian National University). Because of their size, they are particularly well-suited for decentralized power supply and remote, stand-alone power systems. The technology promoted by Stirling Energy Systems (SES), called 'Solarcatcher', is a 25 kW system. In 2008, Stirling Energy Systems claimed a new solar-to grid system conversion efficiency record by achieving a 31.25% net efficiency rate in New Mexico.

The Australian Big Dish technology is being brought to market by Wizard Power and has a surface area of 500 sq m. In Australia, Wizard Technology, which has commercialized the 'Big Dish', is proposing a project near Whyalla, Australia with applications in steel processing, of 40 MW in size was proposed to be started in 2009. The $230 million project had to use Wizard Power technology to generate 66GWh of solar electricity each year; enough electricity to power 9,500 average Australian homes and reduce greenhouse gasses by 60,000 tons per annum, equivalent to taking 17,000 cars of the road each year. The project was delayed due to the financial problems but this got some go- ahead node during March 2012, after the Solar Oasis consortium signed a funding deed with the Australian government for a $A60 million grant. The deed was crucial for the project to move forward and finalize funding arrangements with equity partners and financiers. Initial grant was awarded nearly two years ago under the Australian government's Renewable Energy Demonstration Program.3 In India Infinia Solar Systems, USA has proposed solar power plant with 3kW dish stirling system through its Indian partners, though no project with this system has come in ground in India till now.

Major sub-components of central receiver CST system

Stirling engine. The Stirling or hot gas engine is worked with a pressurized gas in a closed thermodynamic cycle. Because the Stirling engine is independent from the heat source, it is possible to use gas or oil burners as well as solar energy. Mounted on a parabolic dish, the Stirling engine transforms the concentrated solar radiation to electric energy. Presently Infinia Solar Systems is manufacturing typical Stirling engines.

Parabolic dish. Parabolic dish uses a modular mirror system that approximates a parabola and incorporates two-axes tracking to focus the sunlight onto the receiver located at the focal point of each dish. The mirror system is typically made from a number of mirror facets, either glass or polymer mirror. It could also consist of a single stretched membrane using a polymer mirror. The concentrated sunlight may be used directly by a Stirling, Rankine, or Brayton cycle heat engine at the focal point of the receiver or to heat a working fluid that is piped to a central engine. The primary applications include remote electrification, water pumping, and grid-connected generation.

Tracking mechanism. The parabolic dish-Stirling system comprises a two-axes tracking mechanism as it works on direct normal incidence.

Technology suppliers

The parabolic dish-Stirling CST technology is modular and best suited for decentralized energy supply. World-wide demonstration projects have been successfully implemented by the manufacturers but the technology has not yet commercialized to the extent of a parabolic trough and central tower system. Presently there are three major manufacturers of parabolic dish–Stirling system namely Stirling Energy Systems, Infinia Solar System, and Brayton Energy. The Stirling Energy Systems has announced the projects of 1750 MW capacity in the US.

Cost of parabolic dish–Stirling CSP technology

Figure 7.11 shows the detailed cost decomposition of the CST power plant based on the parabolic dish–Stirling technology. It has been observed that the solar field comprises 38% of the cost followed by power block at 37%.

As the power block is inbuilt with the parabolic dish system, storage is not possible with this technology. However, the efficiency of this technology is higher than other CST technologies.

7.1.7 Linear Fresnel Reflector

This system, similar to the parabolic trough collector system, consists of an array of nearly-flat reflectors

concentrating solar radiation onto an elevated inverted linear receiver. Water flows through the receiver and is converted into the steam. This system is line-concentrating, similar to a parabolic trough, with the advantages of low costs structural support and reflectors, fixed fluid joints, a receiver separated from the reflector system, and long focal lengths that allow the use of flat mirrors. The technology is seen as a potentially low-cost alternative to trough technology for the production of solar process heat and electricity. Figure 7.12 presents the photograph of a working LFR technology-based solar system.

Figure 7.11 Cost Break-up of Parabolic Dish based CST Power Plant

Source: TERI estimates

Figure 7.12 LFR Technology based CST Power Plant

Source: http://www.csp-world.com/news/20120411/0094/areva-build-250mw-csp-plant-rajasthan-india

LFR collectors are being mainly developed by the Australian company Ausra (now acquired by AREVA) in the USA. It built a test plant of 1 MW in the east of Australia in 2003, which feeds steam directly into an existing coal-fired power station. The capacity of this plant is currently being doubled. The company has one 5 MW plant operating and one 177 MW planned in the US. The Fresnel design uses less expensive reflector materials and absorber components. It has lower optical performance and thermal output but this is offset by lower investment and operation and maintenance costs. The 1.4 MW PE1 Fresnel plant from Novatec has recently started grid connected operation in Calasparra, Murcia, Spain.

7.1.8 Requirements of Concentrating Solar Thermal Power Plants

Solar direct normal irradiance

Concentrating solar power (CSP) technologies require direct solar radiation, which can be focused along a line or point through concentrating collectors. Global solar irradiance consists of direct and diffuse irradiance. Using tracking mechanism (single or two-axes) the cumulative direct solar radiation over any surface can be increased. Direct normal irradiance (DNI) is the optimal value of direct solar radiation at any location. Locations receiving less than 1700kWh per sq m annual DNI are not recommended for CSP projects.

Essentially the locations in between the latitudes 15–35oN are suited for CSP projects as the annual variation of the sun–earth angles is not extreme.

7.1.8.2 Land requirements

Concentrating solar power plants require a significant stretches of land that typically cannot be put to other uses simultaneously. The CSP project also requires the land to be graded level, except for solar dishes. The optimal area requirement for any CSP technology is estimated through shade analysis taking into account the realistic coordinates of geography and technology.

Figure 7.13 Footprint of Various CSP Technologies

Source: Global Concentrated Solar Power Industry Report 2010-11 by CSP Today (http://www.csptoday.com/globalreport/index.shtml)

From the study of commissioned CSP plants it has been observed that the land requirement for parabolic trough plants of 50 MW is about 200 ha (2 sq km). Figure 7.13 gives an idea about the foot print of various CST technologies.

Water requirement

Water is also an essential requirement of CSP plants. The highest demand of water in a CSP plant is mainly on account of evaporative losses in cooling towers. In addition water is required for cleaning of the reflectors. A summary of minimum water consumption for different CSP technologies is presented in Figure 7.14.

Figure 7.14 Water Requirements of Various CSP Technologies

Source: Global Concentrated Solar Power Industry Report 2010-11 by CSP Today ( http://www.csptoday.com/globalreport/index.shtml)

Inter-comparability of CST technologies

Based on application, advantages, and limitations a performance matrix for CST technologies has been developed and presented in Appendix V whereas Appendix VI summarizes the installed capacity and the maturity levels of various CST technologies.

Appendix VII shows the capacity of CST plants based on technology, which are (a) operational by the end of 2009 and (b) under installation and/or planned. It can be seen that the parabolic trough technology is currently the most proven CST technology and therefore the most developed and standardized. The parabolic trough projects currently in operation are between 14 and 80 MWe in size, and existing plants have a cumulative capacity of over 500 MW.

7.1.9 Solar PV Technology

Solar PV technologies can be classified broadly as conventional PV and concentrating solar PV. Figure 7.15 provides detailed classification of solar cell technologies.

Conventional solar PV

Solar cells represent the fundamental power conversion unit of a photovoltaic system, which has much in common with other solid-state electronic devices, such as diodes, transistors and integrated circuits. Solar cells are usually assembled into modules. Its operation is based on the ability of semiconductors to convert sunlight directly into electricity by exploiting the photovoltaic effect. In the conversion process, the incident energy of light creates mobile charged particles in the semiconductor, which are then separated by the device structure resulting in electricity generation.

Single/mono-crystalline silicon (C-Si)

This is the most established and efficient solar cell technology till date, which has a module efficiency of 15-18%. The cell and module fabrication technology is well developed and reliable. These cells are manufactured from single silicon crystal. During manufacturing, Si crystals are cut from cylindrical ingots (Figure 7.16).

Figure 7.15 Classification of Solar Cell Technologies

Source: http://sovoxglobal.com/cell_classification.html

Figure 7.16 Mono-crystalline Silicon Solar Cell and Module

Source: http://sovoxglobal.com/cell_classification.html

Polycrystalline silicon solar (poly-Si or mc-Si)

The production of polycrystalline cells is more cost-efficient. These are manufactured by cooling a graphite mould filled with molten silicon. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed. These cells have module efficiency of around 12-14% (Figure 7.17).

Thin film solar cell technology

In this approach thin layers of semiconductor material are deposited onto a supporting substrate, such as a large sheet of glass. Typically, less than a micron thickness of semiconductor material is required, a hundredth or thousandth part of the thickness of a silicon wafer (Figure 7.18). Some of the thin film solar cells in use are as follows:

  • a – Si
  • CdTe
  • CIS, CIGS (copper indium gallium di-selenide) Figure 7.18: Thin film solar cell and module
  • Thin film crystalline silicon.

Figure 7.17 Polycrystalline Silicon Solar Cell and Module

Source: http://sovoxglobal.com/cell_classification.html

Figure 7.18 Thin Film Solar Cell and Module

Source: http://sovoxglobal.com/cell_classification.html

A comparison of CSP and SPV technology is presented in Appendix IX.

7.1.10 Concentrating Solar Photovoltaic

Concentrating solar photovoltaic systems employ solar radiation concentrated onto photovoltaic cells for electricity production. Due to concentrated radiation on a small surface, solar cell size reduces drastically. Concentrators of all varieties (such as parabolic trough or parabolic dish) with tracking mechanism may be used. Additionally, increasing the concentration ratio improves the performance of photovoltaic materials. There are four types of CPV technologies:

  • Dish CPV
  • Lens CPV
  • Low concentration CPV and
  • Non-Tracking CPV

Dish CPV

The parabolic dishes are now being coupled with photovoltaic in dish CPV systems. Dish CPV systems are available in a range of sizes and configurations from large systems that resemble dish-engines with the engines replaced with a CPV receiver to several small dishes combined together in a tracking panel (Figure 7.19).

Figure 7.19 Large Parabolic Dish CPV

Source: www.solarsystem.com

Lens CPV

Lens CPV technology is gaining popularity as it promises lower costs than standard PV. The technology comprises full tracking panels of lens-CPV assemblies, and arrays of individually tracking facets (Figure 7.20).

Figure 7.20 Lens CPV

Source: www.solareis.anl.gov.

Lens CPV modules with individual tracking facets have a lower profile than Lens CPV tracking panels as the panel is mounted on a flat plane and only the small assemblies are used to track. This results in many moving parts and high potential for mechanical failure and consequently ballooning O&M costs. The tight packing of the facets within one panel can also lead to shading between the facets as they track the sun.

Low concentration PV

Low concentration PV is the most accessible CPV technology presently which applies simple flat reflective surfaces to concentrate light onto conventional solar panels. These systems require single-axis tracking with much lower accuracy as compared to higher concentration CPV/CST technologies. Different configurations of LCPV systems have been developed and deployed, but all are based on the same basic principle of combining low-cost and low-precision reflectors and trackers with a PV panel to improve performance (Figure 7.21).

Figure 7.21 Low Concentration PV

Source: www.news.cnet.com.

Non-tracking CPV

This technology is an approach to develop a product at a lower cost than conventional PV. This type of CPV panel looks and acts almost exactly like a conventional PV panel, but contains a third to half of the PV used in a conventional PV panel. Non-tracking CPV technologies use a variety of internal optical devices that can accept light at a range of angles and direct it towards a small amount of PV. Non-tracking CPV offer promising market application as they can be installed and operated like conventional PV panels, with low O&M costs, with a fraction of the PV material used to create PV panels. Furthermore, because these technologies accept light from a range of angles, they are not limited to DNI, but can take advantage of a greater range of the solar radiation.