Introduction
Concentrated Photovoltaic (CPV) technology refers to a class of solar power systems that use optics to focus sunlight onto high-efficiency photovoltaic cells. Unlike conventional photovoltaic (PV) panels that absorb light over a large area, CPV systems employ lenses or mirrors to concentrate the solar flux, allowing the use of small, expensive, but highly efficient cells. The resulting power density is significantly higher, reducing the amount of semiconductor material required per watt of output. CPV technology has attracted attention for its potential to provide cost-effective electricity in regions with high direct normal irradiance and for its suitability in large‑scale, utility‑grade installations.
History and Development
Early Concepts
The notion of concentrating solar energy dates back to the 19th century, when physicists explored the use of mirrors to focus light. The first practical attempts at using concentrated light to generate electricity were made in the 1950s and 1960s, primarily as part of experimental research into thin‑film solar cells. However, the limitations of early cell materials and the lack of reliable tracking mechanisms made commercial deployment infeasible at that time.
Commercialization
In the late 1990s, advances in gallium arsenide (GaAs) and copper indium gallium selenide (CIGS) thin‑film technologies provided the breakthrough efficiencies needed to make CPV commercially viable. Companies such as SunPower and SolFocus began to develop pilot plants, and by the early 2000s the first utility‑scale CPV plants were operational in the United Arab Emirates, the United States, and Australia. The technology entered a period of rapid growth, driven by falling costs of optical components and the increasing demand for renewable energy in arid regions.
Technical Overview
Principles of Concentrated Photovoltaic
CPV systems operate on the principle of optical concentration, where an array of mirrors or lenses collects sunlight over a large aperture and focuses it onto a small, high‑efficiency PV cell. The concentration ratio, defined as the ratio of the illuminated area to the cell area, typically ranges from 10× to 500×, depending on the design. The focused light increases the photon flux incident on the cell, thereby raising the electrical output without proportionally increasing the amount of semiconductor material.
Optical Concentration Systems
Two primary optical technologies are employed in CPV: Fresnel lenses and parabolic mirrors. Fresnel lenses provide a lightweight, flat‑panel solution that can concentrate light up to 200×. Parabolic mirrors, often used in heliostats, allow higher concentration ratios, up to 500×, but require precise fabrication and alignment. The choice of optical component depends on the desired balance between concentration ratio, cost, and field of view.
Photovoltaic Cells Used in CPV
High‑efficiency GaAs multi‑junction cells are the most common choice for CPV applications, achieving efficiencies above 30% under concentrated light. These cells employ multiple semiconductor junctions stacked to absorb different portions of the solar spectrum, maximizing conversion efficiency. CIGS and other thin‑film cells have also been adapted for CPV, although their efficiencies are generally lower than GaAs counterparts.
Tracking Systems
Because CPV systems concentrate light onto a small area, they require precise solar tracking to maintain optimal alignment. Dual‑axis trackers that adjust both azimuth and elevation are typically used, providing a tracking accuracy of a few arcseconds. The mechanical complexity of trackers adds to the overall system cost but is essential for maintaining high energy yield.
Performance Metrics
Efficiency
System efficiency for CPV is often expressed as the ratio of electrical energy output to the incident solar irradiance over the optical aperture. Under optimal conditions, CPV systems can achieve system efficiencies exceeding 20%, significantly higher than conventional flat‑panel PV systems. However, the actual energy output depends on several factors, including cell temperature, spectral distribution, and tracker performance.
Conversion Efficiency
Cell conversion efficiency - the ratio of electrical power generated to the incident photon flux - typically ranges from 25% to 35% for GaAs multi‑junction cells. The use of concentrators allows these efficiencies to be maintained across a wide range of sun angles, provided that the tracking system compensates for changes in incidence.
Temperature Effects
Higher operating temperatures can reduce cell efficiency due to increased carrier recombination. CPV systems mitigate this by using active cooling methods, such as liquid coolant loops or forced air, which help keep cell temperatures near ambient levels. The efficiency degradation with temperature for GaAs cells is approximately –0.3% per degree Celsius.
Applications
Utility‑Scale Solar Farms
Large‑scale CPV farms typically cover several acres and produce electricity on the grid. The high power density of CPV allows the deployment of more power per unit area, reducing land use compared to conventional PV. Notable examples include the Abu Dhabi Solar Park and the Nellis Solar Power Plant in Nevada.
Hybrid Systems
CPV can be combined with photovoltaic modules that capture diffuse light, creating hybrid arrays that operate efficiently across a range of irradiance conditions. This hybridization improves system reliability during cloudy periods, though it introduces additional complexity in system design.
Research and Development Facilities
Academic and industrial research centers use CPV modules to study advanced cell materials and optical designs. These facilities often host experimental plants that test new cell architectures, such as intermediate band cells, which aim to surpass the Shockley–Queisser limit under concentrated illumination.
Market and Economics
Cost Trends
Capital costs for CPV systems have fallen steadily since the early 2000s, primarily due to economies of scale in optical component manufacturing and the mass production of GaAs cells. Current Levelized Cost of Energy (LCOE) estimates for CPV range from $0.04 to $0.07 per kilowatt‑hour, depending on location, system size, and financing structure.
Competitive Landscape
Major CPV manufacturers include SolFocus, Heliatek, and SunPower, each offering proprietary optical and cell technologies. The market is highly concentrated, with a few key players dominating worldwide capacity. New entrants often focus on specialized niches, such as concentrated photovoltaics for niche industrial processes.
Environmental and Societal Impact
Resource Use
GaAs cells require gallium and arsenic, which are less abundant than silicon. The supply chain for these materials involves complex extraction and refinement processes, raising concerns about resource sustainability. Recycling programs for CPV modules are under development to recover valuable materials at end‑of‑life.
Land Use
Due to their high power density, CPV installations typically require less land per megawatt of capacity compared to conventional PV. However, the need for dual‑axis trackers may increase infrastructure footprints, including support structures and access roads.
Future Outlook and Challenges
Technological Improvements
Research efforts focus on increasing cell efficiency beyond 40% using multi‑junction and intermediate‑band designs. Advances in optical materials, such as low‑loss, high‑transmission coatings, will further reduce losses. Innovations in passive cooling and tracker automation are expected to lower operating costs.
Regulatory and Policy Issues
Government incentives, such as feed‑in tariffs and tax credits, have historically supported CPV deployment. However, policy shifts towards more diversified renewable portfolios may affect the attractiveness of CPV in certain markets. Streamlined permitting processes and grid interconnection standards remain critical for future expansion.
Key Manufacturers and Projects
- SolFocus – Known for its high‑concentration Fresnel lens systems and a portfolio of commercial plants across the Middle East.
- Heliatek – Specializes in organic CPV modules, integrating novel perovskite cells.
- SunPower – Offers integrated CPV solutions with dual‑axis trackers for utility‑scale deployments.
- Abu Dhabi Solar Park – One of the largest CPV farms, providing renewable electricity to the national grid.
- Weymouth Solar Farm (UK) – Demonstrates CPV viability in temperate climates using hybrid tracking systems.
Related Technologies
Concentrated Solar Power
Unlike CPV, concentrated solar power (CSP) uses thermal energy storage and heat engines to generate electricity. Both CSP and CPV employ optical concentration, but CPV directly converts photons to electrons, while CSP harnesses light to heat a fluid.
Diffuse Solar PV
Diffuse PV modules are designed to capture light scattered by the atmosphere, typically used in regions with frequent cloud cover. Hybrid systems that integrate CPV and diffuse PV aim to maximize energy capture across variable irradiance conditions.
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