Concentrator photovoltaics, or CPV, are solar energy machines that use lenses or mirrors to focus sunlight onto very small solar cells. Instead of covering a large panel with semiconductor material, a CPV module uses optics to make intense spots of light on tiny high-efficiency cells. This matters because the best solar cells are expensive, so concentrating light can reduce the amount of cell material needed.
CPV works best in very sunny locations with clear skies and strong direct sunlight.
A typical CPV module has a front lens array, a small cell array, heat spreaders, and a tracking system that points the module at the Sun. Fresnel lenses are often used because they are thin, lightweight lenses with stepped grooves that bend incoming parallel rays toward a focus. The concentrated sunlight increases the electrical power from each cell, but it also creates heat that must be removed to keep efficiency high.
Because CPV needs accurate alignment with the Sun, two-axis tracking is usually required for maximum output.
Understanding Renewable Energy Machines: Concentrator Photovoltaics
The solar cells in a CPV system are often multi-junction cells. They contain several thin semiconductor layers stacked together. Each layer absorbs a different band of sunlight.
A top layer can use higher-energy light, while lower layers use light that passes through the layers above. This reduces the energy wasted when one material receives photons it cannot use well.
These cells can achieve very high efficiency under concentrated light, but they cost much more per square centimetre than ordinary silicon cells. CPV makes sense only when the optical system, tracker, cooling parts, and maintenance costs do not cancel that material saving.
The lens must form a bright spot in exactly the right place. Sunlight arrives from the distant Sun in nearly parallel rays. A Fresnel lens bends those rays toward the cell, but small errors matter.
If the cell is shifted, tilted, or too far from the focal position, some light misses it. Dust, scratches, yellowing plastic, and moisture can reduce the light reaching the cell. The Sun appears to move across the sky because Earth rotates.
A tracker uses motors and sensors or programmed Sun positions to keep the module aligned throughout the day. This is why CPV has moving parts, unlike many flat solar panels.
Heat is one of the hardest engineering limits. Most incoming solar energy does not become electricity. Some becomes heat in the cell, metal contacts, lens, and supporting structure.
A heat spreader carries this heat away from the tiny cell to a larger area. Fins, airflow, or liquid cooling may then remove it. If the cell gets too hot, its voltage falls and its output drops.
Repeated heating and cooling can stress solder joints and electrical connections. Engineers must balance strong concentration against reliable cooling. More focused light can give more current, yet it does not guarantee more useful power.
CPV is most useful in places with frequent clear sunshine, such as dry deserts and high plateau regions. Its output can fall sharply when thin cloud, haze, smoke, or humidity scatters sunlight. Ordinary flat panels can still collect some scattered light from many directions.
A CPV lens cannot focus that scattered light into its intended spot. Students can connect this idea to a magnifying glass. A sharp bright spot forms only when the light direction is well defined and the lens is held at the right distance.
When learning CPV, pay attention to energy flow from incoming sunlight to optical losses, cell electricity, and unwanted heat. Track the difference between high cell efficiency and high system efficiency, since motors, cooling, wiring, and imperfect optics all affect the final result.
Key Facts
- Concentration ratio: C = lens collection area / solar cell area.
- Electrical power output: P = IV, where I is current and V is voltage.
- Solar energy input to a lens: P_in = G A, where G is solar irradiance and A is lens area.
- Module efficiency: η = P_out / P_in.
- Higher concentration increases cell current, but overheating can reduce voltage and efficiency.
- CPV mainly uses direct normal irradiance, so clouds and haze strongly reduce performance.
Vocabulary
- Concentrator photovoltaic
- A solar power device that uses lenses or mirrors to focus sunlight onto small photovoltaic cells.
- Fresnel lens
- A thin lens made of concentric grooves that bends light like a much thicker curved lens.
- Concentration ratio
- The factor by which optics increase sunlight intensity on a solar cell compared with normal sunlight.
- Direct normal irradiance
- The solar power per square meter arriving in a straight line from the Sun onto a surface perpendicular to the rays.
- Heat sink
- A component that carries heat away from a device so its temperature stays within a safe range.
Common Mistakes to Avoid
- Assuming CPV works well with all sunlight is wrong because CPV mainly uses direct sunlight and cannot concentrate diffuse light from cloudy skies effectively.
- Ignoring the tracking system is wrong because even a small pointing error can move the focused spot away from the tiny solar cell.
- Thinking higher concentration always means higher efficiency is wrong because intense light also raises temperature, and hot solar cells usually produce less voltage.
- Using total panel area as the solar cell area is wrong because CPV has a large optical collection area but much smaller active cell area.
Practice Questions
- 1 A CPV lens has an area of 0.040 m2 and focuses sunlight onto a cell with an area of 0.00020 m2. What is the concentration ratio?
- 2 Direct normal irradiance is 900 W/m2 on a CPV module with total lens area 1.5 m2. If the electrical output is 420 W, what is the module efficiency?
- 3 Explain why a CPV module usually needs two-axis tracking, while a standard flat solar panel can still produce useful power without precise tracking.