The solar resource: maximizing energy yield with concentrator photovoltaics

Nancy Hartsoch, SolFocus, USA

Concentrator solar photovoltaic technology, which combines high efficiency with low cost, appears poised to enjoy a bright future in large-scale power generation. Nancy Hartsoch, VP of Corporate Marketing of SolFocus, explains why the latest solar science will earn its place in the sun.

Current installed power generation capacity throughout the world is approximately 2 TW. With the predicted growth in energy consumption, replacement of aging power plants and potential for the transport industry to partially switch to electricity, as much as 6 TW of new capacity will be required by 2030.

An expansion at this pace promises to strain fuel resources and could lead to significant power shortfalls if the right approach is not employed. To relieve the pressure, many new technologies, especially renewable energy, are being developed and brought to market.

By tracking the sun throughout the day, tracker systems boost the daily energy production of solar systems relative to their peak power rating
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In just one hour, more solar energy is delivered to the earth’s surface than it takes to power the entire globe for a whole year. That being said, efficiently capturing, converting and delivering this energy is a complex challenge, requiring many new technologies, policies and business models.

An area of innovation commanding much attention is solar photovoltaic (PV) technology. Companies ranging from hi-tech startups to multi-billion dollar international conglomerates are improving existing PV technologies, as well as developing completely new approaches.

Currently, the industry standard is crystalline silicon-based technology, which uses the semiconductor materials of the microchip industry to convert sunlight into electricity. Though proven and reliable, this approach also faces serious silicon supply constraints, which have hindered cost reduction and manufacturing expansion.

As a result, solar technologies that reduce the amount of expensive PV material are quickly coming to market. One approach uses thin films, made from materials like amorphous silicon, cadmium telluride or copper gallium indium diselenide, to capture light energy. One of the latest and potentially most disruptive approaches is concentrator PV (CPV) technology, which uses optical concentrators to boost the output of small, highly efficient PV cells.

Each approach has inherent advantages and disadvantages in terms of cost, performance and scalability. Of all of these technologies, CPV holds the greatest promise of achieving very high conversion efficiencies. In turn this offers the greatest promise to deliver low-cost solar energy with technology that is reliable and scalable.

CPV and utility-scale solar

While PV technologies currently serve mostly rooftop and on-site applications, utility-scale solar arrays are growing in size and number. Large PV arrays, also known as solar farms, harbour a unique set of opportunities and challenges compared with the rooftop setting. Solar farms require land leases and act as a primary power source, so there is a growing market for PV technologies specifically suited for utility-scale applications.

CPV is the newest technology to enter the commercial solar market, and presents exciting possibilities for utility-scale applications as its production, deployment and innovation accelerate.

Focusing on CPV

In understanding the capability of CPV systems, it is important to understand how the technology functions. The PV cells in CPV systems convert light energy into electrical energy in the same way that conventional PV technology does. The difference is that CPV systems add an optical component that focuses a large area of sunlight onto each cell.

In many ways, a CPV system is similar to a telescope, trained on the sun’s position and focusing light hundreds of times onto a solar cell. Though the idea of optical concentration on solar cells seems simple and has been examined in scientific settings since the 1970s, advances in solar cells and optics technology have only recently made the designs commercially viable.

There are two main types of commercially-used concentrating optical systems: refractive and reflective. Refractive types use Fresnel lenses, while reflective systems use one or more mirrors to focus light.

High concentration CPV systems typically focus light from 100 to 1000 times, depending on the design of the system. These high levels of concentration are important for taking full advantage of the advanced solar cells at the heart of the system. Today’s CPV systems use multi-junction solar cells with an average conversion efficiency of around 35 per cent.

Figure 1: Diagram of the reflective concentrating optical design of SolFocus’ SF-1000 concentrator
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Figure1 shows an example of a reflective concentrating optical design. SolFocus’ SF-1000 concentrator design features a primary mirror, secondary mirror and tertiary optical rod, to concentrate the sun’s energy 500 times onto a high-efficiency, multi-junction solar cell. Use of all glass components is a key element in long field life and reliability of the SolFocus design.

Sixteen SF-1000 concentrators are packed into each panel, which measures 1.3 m by 1.1 m on the surface, and is covered with glass to enclose and protect the systemà‚–the glass is also where the secondary mirrors are mounted.

At the system level, 30 panels are integrated into an array. Consisting of ten panels connected in series and three strings connected in parallel, the complete array has an aperture of 46m2 and is designed for a maximum flexure of 0.3 degrees under wind load.

Being concentrators, the power units must be kept in alignment with the sun for the cells to produce power. For this reason, CPV arrays are mounted on dual-axis tracker systems, which are equipped with self-calibrating control units that automatically follow the sun. CPV systems track the sun within +/- 1 degree of accuracy, and sometimes more accurately depending on the needs of the concentrator system. The two photographs show the SolFocus tracker system, with arrays of 30 CPV panels in place as they track the sun.

By tracking the sun throughout the day, tracker systems boost the daily energy production of solar systems relative to their peak power rating. Whereas solar technologies mounted in fixed-tilt positions have a daily energy production profile resembling a normal curve, dual-axis trackers enable CPV systems to produce peak power levels starting in the early morning and continuing until dusk.

The benefits of this daily power profile actually extend beyond simply producing more energy. Because CPV produces energy at a steady rate throughout the day and the power production remains at high levels during the afternoon’s peak demand hours, tracking makes solar systems more suitable to serve a utility function.

When deployed in large volume, tracked CPV systems operate similarly to intermediate natural gas fired power plantsà‚–starting early in the day and maintaining power production until evening hours, when demand drops off. Tracking also helps CPV systems generate maximum power through high-temperature, peak demand times, which typically occur after the solar peak at noon.

Degradation due to temperature is actually an important performance issue for PV technologies. Silicon PV and thin-film PV operating in the sunny regions of the world suffer significant temperature degradation. A silicon PV panel rated at 205 W will lose over 20 per cent of its performance when operating at temperatures typical of key solar regions, such as deserts. That means that a 205 W panel will only produce 160 W. Over a complete power field, this per panel loss is a significant loss of power production, and gets worse at peak times when power demand is highest.

The solar cells at the heart of CPV systems were originally designed for the extreme conditions of space. Thus, these cells do not degrade under the intense heat generated by the power of 500 or more suns. As a result, a panel rated at 205 W will produce 205 W, even at temperatures of 30 à‚ºC or more. The first deployment of CPV at a multi-megawatt scale has been an important verifier of these performance characteristics.

CPV at the megawatt scale

Under the Spanish Ministry of Education and Science programme, Instituto de Systemas Fotovoltaico de Concentration (ISFOC), a 3 MW CPV installation is being developed. When complete, the ISFOC project will include seven CPV technologies.

In addition to producing large amounts of power, the project will provide crucial performance and reliability testing for these new technologies. These installations are not only power plants, they have also become the proving ground for this innovative new technology.

SolFocus’ CPV systems were one of the first chosen and first installed in the ISFOC project. There are two installations using SolFocus CPV systems, a 200 kW plant in Puertollano and a 300 kW plant in Almoguera. In total, 87 SolFocus arrays were installed. After installation, the plants’ total power production exceeded the contract requirement of 500 kW. The SF-1000 arrays operate at 6.2 kW, above their specifications of 6.15 kW, and all 87 systems operated within a two per cent standard deviation. The energy production from these plants is estimated to exceed 1 GWh annually.

SolFocus’ 200 kW CPV units are part of the ISFOC project located in Puertollano, Spain
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Because CPV systems require direct sunlight, shading and clouds can impact the system performance more than non-concentrating PV systems. At the ISFOC site, energy production was maximized by inverting individual and pairs of arrays, rather than using a central inverter system. With a central inverter, one CPV system’s shading would affect the operation of the entire plant.

In regions like Spain, where there is a high amount of direct sunlight, CPV is able to outperform other PV technologies. In total, the CPV power plants at the ISFOC site generate 15 per cent more kWh versus comparable fixed-tilt PV technologies, and ten per cent more than comparable tracked systems.

Taking CPV en route to market

Specifically designed to have greater energy density and unsurpassed efficiencies, the true value of CPV systems is best represented by the levelized cost of electricity. For utility-scale applications, where the cost of land, operation and maintenance is included in the price of electricity, CPV is poised to be an extremely profitable solution.

There are two fundamental reasons to concentrate. The first reason is cost. Area for area, optical concentrators are less expensive than PV cells. If the amount of expensive cell area can be efficiently reduced, then the overall cost of the system drops. In the case of high concentration, this means that the cell, per unit of energy, costs one five-hundredth as much.

The second reason couples ease of manufacturing and reliability. Concentrating systems are mechanical assemblies, and can make use of inexpensive, field-proven materials and manufacturing techniques. When deployed at large-scale, concentrator systems are less susceptible to supply constraints of specialized materials such as PV cells because they consist primarily of common materials such as glass and aluminum. As a consequence, the technology is more scalable to large volumes of production and deployment.

While CPV technology is new in terms of commercial deployment, the reliability of its optical systems, seals, coatings and many other concentrator components have been established for years in industry. These established techniques are helping CPV become a reliable source of power in a short period of time.

Even as recently as a decade ago, PV technologies were almost solely utilized in rooftop and off-grid applications. As costs come down, however, PV technology is becoming an attractive option for primary power production. With the installation of SolFocus’ CPV plants in Spain, a reference point has been validated on the CPV cost reduction road map and route to market.

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