After decades of slow progress, the popularity of concentrated photovoltaics is now increasing with installations poised to grow exponentially at double digit percentages every year through to 2020, according to a 2013 report by IHS.
The market research group IHS further explains that the global market for concentrated PV is about to experience explosive growth, with installations across the globe set to skyrocket by about 750% between 2013 to 2020. The report predicts concentrated photovoltaic installations will rise from 160 MW in 2013 to 1,362 MW in 2020. Currently, Concentrated Photovoltaic panels are considered the most reliable PV solar technologies yielding the highest solar cell efficiencies.
So what exactly is a Concentrated Photovoltaic?
Concentrated photovoltaics (CPV) are systems that use photovoltaic technology to generate electricity from sunlight. This technology uses lenses or curved mirrors to focus sunlight onto a dense array of small but highly efficient and multi-junction (MJ) silicon solar cells. During this process, the concentration decreases the required solar cells area while increasing the efficiency of cells.
Generally, the CPV systems, MJ-cells, and particularly the High concentrating photovoltaic (HCPV) systems are more expensive than the conventional PV used to produce the flat-plate photovoltaic systems. Since the CPV systems use more optics and possess the highest efficiency levels, their costs are higher compared to conventional PV installations, hence limiting the wide acceptance of photovoltaic solar solutions. However, with technological advancements the costs are set to reduce.
“What is happening in today’s CPV market is very similar to that of the overall PV space in 2007, beset by high costs and an uncertain outlook,” said Karl Melkonyan, photovoltaic analyst at IHS. “However, the CPV market in 2013 is on the verge of a breakthrough in growth. Costs for CPV have dropped dramatically during 2013 and are expected to continue to fall in the coming years. Furthermore, when viewed from the perspective of lifetime cost, CPV becomes more competitive with conventional PV in large ground-mount systems in some regions.”
Unlike the traditional PV systems, Concentrated Photovoltaic systems use solar thermal and energy production elements of photovoltaic cells. The system uses optics to direct and intensify inbound solar radiation onto small-sized photovoltaic panels. During this process, few high-efficiency photovoltaic materials harness sunlight from a larger area covered by inexpensive concentrating lenses, thereby reducing overall costs.
The cost analysis and future market performance of concentrated photovoltaics and thermal
As manufacturing processes of CPV move down the learning curve, improved system efficiencies and rising volumes also keep driving the price decline of concentrated photovoltaic panels. The IHS report further pointed out that the cost of CPV systems would slide further at a compound rate of 15% every year from 2012 to 2017, falling to lows of $1.59 by the end of 2017. On average, the installed price for a high-concentration PV (HCPV) system declined by 25.8% from $3.54 per watt in 2012 to $2.62 per watt in 2013.
The IHS also shows the cost analysis for PV systems in the conventional market mainly focuses on the price-per-watt and the aggregate cost-per-watt module. In the long run, when the cost per watt of any installed conventional PV is compared to the Concentrated Photovoltaic system, it will be significantly lower.
“This is mainly due to the higher panel cost of CPV, given that CPV suppliers have yet to achieve the economies of scale, as well as a better balance of system and installation cost, because of the required tracker system,” IHS says.
“To be sure, conventional PV has a lower upfront cost and appears to be a more attractive option based on upfront system costs. However, this does not take into account the overall cost of the system over its lifetime, nor does it consider the energy yield of the system.”
Future of CPVs
Concentrated photovoltaics and thermal panels have a great potential of becoming highly competitive in the near future. Albeit being less common in the PV roof top segment and more expensive compared to the conventional PV systems, ongoing research and development of concentrated photovoltaic technology is focused on enhancing competitiveness in areas of high solar insolation as well as utility-scale segments, making it a viable alternative.
The IHS report adds that it is important to consider the levelised cost of electricity (LCOE). The LCOE estimates the overall cost of producing electricity at the point of connection, and divides the total lifetime costs of a system by the total energy generated over the system’s lifetime.
“Such a calculation is also necessary in order to compare the competitiveness of PV and CPV with that of conventional power generation.”
Based on the LCOE, IHS predicts that HCPV costs will significantly decline to compete with the conventional photovoltaics for large commercial in the target areas. Target areas are the regions with dry, hot climates and high irradiation exceeding 6 kWh per square meter of uninterrupted normal irradiation daily.
In order to generate extremely high temperatures needed by concentrated photovoltaic panels, solar systems track the sun’s course throughout the day, effectively maximising efficiency and solar concentration. The efficiency of CPV technology is enhanced by installed computers and other advanced sun monitoring devices which help control every feature of the system.
For CPV to reach their maximum efficiency, they must operate in areas where direct sunlight is concentrated since diffuse light – which often occurs in overcast and cloudy conditions, can’t be concentrated. This is one reason why concentrated photovoltaics use solar trackers and a cooling system which utilize heat sinks to enhance their efficiency.
In contrast to the traditional solar systems which are usually mounted on a solid frame, concentrated photovoltaic systems have to be mounted on an axis, around which they will rotate. In the case of solar dishes and power towers, some of the system parts are mounted on a double axis frame to help focus it directly towards the sun.
How to get a CPV system
There are numerous companies that are now designing concentrated photovoltaics and thermal systems with unique features than the normal optical design. The state-of-the-art CPV which are suitable for mass production are also highly efficient and concentrated, and provide uniform cell illumination besides being insensitive to mounting and manufacturing inaccuracies.
The CPV technology is best suited for regions with high direct normal irradiance and the features below set it apart from the conventional PV system:
- No moving parts
- Scalable to a wide range of sizes
- Fast response; no thermal mass
- No intervening heat transfer surface
- Near ambient temperature operation
- Reduction in costs of cells relative to optics
- Potential for solar cell efficiencies greater than 40%
The high-efficiency and high cost of advanced cells requires use of extremely concentrated sunlight for the CPV to achieve a cost effective comparison with both the conventional PV systems and concentrator optics. Currently, most Concentrated Photovoltaic companies are developing dense arrays (multi-cell packages) to enhance the system’s overall performance and cooling.
Classification of Concentrator Technologies
Below are examples of both line and point concentrator technologies.
Parabolic mirrors use the collector (the first mirror) to reflect the incoming parallel light via a point onto a second mirror. The second mirror is usually much smaller in size and acts as a parabolic mirror. Primarily, the second parabolic mirror reflects light beams to the core of the first parabolic mirror where it directly hits the solar cells. The main strength of this configuration is that it doesn’t need any optical lenses. However, both mirrors result in some losses.
Named after a French physicist, Fresnel lens consist of various sections customized with different angles, which eventually reduce the weight and thickness of the CPV in comparison to a conventional Len. With Fresnel lens, large aperture and short focal length can be achieved without adjusting the lens’ length.
In most cases, Fresnel lenses are constructed in a circle-like shape, providing a point focus with concentration ratios of about 500, or in cylindrical shapes to offer line focus with low concentration ratios.
Fresnel point lens have high concentration ratios making it ideal for use by the multi-junction PV cell with maximum efficiency. It’s worth noting that high efficiency silicon is commonly used in a line concentrator.
Low CPV modules use mirrors to concentrate sun rays onto a solar cell. Usually, these mirrors are produced with a silicone-covered metal. This technique reduces the losses due to reflection by effectively providing another internal mirror.
The angle at which the mirrors are set depends on the latitude, inclination angle and the module design, but is normally fixed. The concentration ratios range from 1.5 – 2.5. No cooling is required for low concentration cells which are made from monocrystalline silicon.
So far, Sevilla PV is the world’s largest low-concentration photovoltaic plant with modules from three major companies: Isofoton, Artesa and Solartec.
For luminescent concentrators, luminescent films are used to refract light before channeling it towards the photovoltaic material. This technology is indeed promising as it requires no mirrors or optical lenses to operate. In addition, it uses diffuse light and hence no tracking is needed. It has a concentration factor of around 3. Several other developments are in progress. For instance, the Prism Solar uses holographic film while Covalent uses organic material for their film.
In addition, luminescent concentrators require no cooling since the film is designed in such a way that wavelengths that cannot be converted by the solar cells simply go through. The unwanted wavelengths are eventually removed.
As seen earlier, most CPV systems require cooling. This takes place in two ways;
Passive Cooling: This involves placing the cell on a cladded ceramic substrate that has high thermal conductivity. The ceramic also acts as a source of electrical isolation.
Active Cooling: In this case, the liquid metal will be used as a cooling fluid, able to provide cooling from 100°C to 1,700°C.
Photo courtesy of Sandia Labs