Big Solar, Small Footprint: The Role Innovative Technologies Play in our Energy Future

by Nancy Hartsoch, SolFocus Inc.

Meeting accelerating energy demands is always a challenge. Meeting those demands in the best way for the environment, and the economy is an even greater challenge.

The potential for solar energy, the most abundant renewable energy source in the world, in meeting all of these challenges is great, yet it has barely been tapped as an energy source. Why?

The Solar Landscape

First, a quick look at the solar landscape is required for perspective. Solar technology today can be broken into two primary buckets.

One is concentrating solar power (CSP) or concentrating solar thermal technology (CST). The technology, called by both names, involves using lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant.

This technology has matured and is becoming successful for power generation typically at a large scale of 100-MW plants or more. To be economic, the plants must be large, so CSP is not a good fit for large-scale distributed generation. 

Scalability of plant size is not economic, capital investment to deploy a plant is high and it easily can take four or more years for deployment including permitting, construction, etc.

The plants require acres of flat land, which create large footprints and significant environmental disruption around the plants.

Most CSP technologies consume large amounts of water in the production of electricity: 850 to 1,000 gallons per megawatt hour produced.

Since CSP plants need high solar resource and large land areas, they are targeted for areas such as deserts where water shortage is a significant issue.

The second bucket is photovoltaic (PV) technology, in which sunlight is converted directly into electricity using types of solar cells.

PV technology has been critical in development of the solar industry. New challenges facing the energy business easily indicate that PV technology will be even more critical in the future. As with any energy, the driver is cost: cents per kilowatt hour.

For the PV industry to reach its growth potential, current technologies like silicon PV and thin film must continue to evolve, squeeze more energy from the cells and drive to lower cost.

But that alone won’t take us far enough. There is a need for disruptive technologies, which leverage existing technology but are more advanced at providing benefits not achievable with current approaches.

The latest PV technology on the landscape is concentrator PV (CPV), which is being deployed commercially and is proving its ability to provide a new trajectory for efficiency improvements and cost reductions.

In addition to the cost benefit, CPV meets other challenges facing renewable energy, including distributed generation, optimized land use, environmental sustainability and a strong roadmap to future advancements. It is important to understand the technology before looking for opportunities.

CPV

A CPV system converts light energy into electrical energy the way that conventional PV technology does. The difference in the technologies lies in the addition of an optical system that focuses a large area of sunlight onto individual PV cells.

Also, the solar cells used in CPV systems are different from silicon PV cells in their capability to convert large amounts of sunlight into energy at high efficiency.

CPV systems may be thought of as telescopes trained on the sun’s position and feeding the concentrated light to the cell.

Optics in a concentrator system are significantly less expensive than the PV cell. The less cell area used per unit, the lower the overall cost of the system.

High-efficiency PV cells and precision optics are the essential building materials for all CPV technologies.

Optics. While there are multiple approaches to CPV systems, SolFocus has selected reflective, nonimaging optics (customized, precision mirrors) to collect and concentrate the sun’s energy (see Figures 1 and 2).

A primary mirror collects direct sunlight and focuses the reflected light onto a smaller, secondary mirror. The secondary mirror then redirects the reflected light into a glass prism, channeling the sunlight onto the PV chip. The result is a compact, efficient CPV system.

 

CPV is on the fastest cost-reduction path. (Levelized cost of energy encompasses all costs of ownership.) This example is in Phoenix.

Another approach chosen by some manufacturers is the use of refractive optics, which usually are plastic lenses through which the sun’s light is focused. Regardless of the approach, the concept of concentrating sunlight remains the same.

Multijunction PV cells. The photovoltaic cells used in high-concentration CPV systems differ from traditional, crystalline silicon cells that make up traditional PV systems. CPV cells, known as multijunction cells, provide energy conversion efficiencies of about 38 percent in contrast with the typical 12 to 17 percent of crystalline silicon. 

These cells are based on device technology used in space applications since the early 1990s. Solar cell designs originally built and rigorously tested for demanding space requirements have been optimized for performance under a somewhat less-demanding, terrestrial solar spectrum.

While these high-efficiency cells are too costly for use with traditional PV, they are ideal for CPV systems where a large amount of light (500 to 1,000 times) is collected and focused on a small amount of solar cell material.

Dual-axis tracking. Because a CPV system is like a telescope, each unit needs an unobstructed view of the sun and must actively track the sun in its progression across the sky on two axes.

The technology will not respond well to light scattered by clouds or reflected off other objects. CPV is most effective when deployed during clear weather and lots of sunshine hours and in combination with a precision sun-tracking mechanism.

The Path to Grid Parity

There is a lot of talk about solar at grid parity. While the industry is getting closer, there is still a way to go. Ongoing technology improvement, innovation and an industry appreciation for the support required are needed to reach that point.

CPV might be the fastest path to that promised land.

Low cost driven by high efficiency. There are two factors to understand related to cost.

First is efficiency. The biggest impact on the cost of delivering solar energy is system efficiency. CPV has the highest efficiency levels: nearly twice that of most PV.

These efficiency levels are increasing steadily upward with tremendous headroom before they begin to approach any theoretical limits for the cell technology.

The second thing to understand is manufacturing costs. From a volume manufacturing perspective, CPV is in its infancy. As volumes accelerate, manufacturing costs come down rapidly as they have done in automotive and electronics industries.

Also, CPV systems (using SolFocus CPV as the comparison) typically have only 50 percent of manufacturing costs driven by materials, whereas for PV, it is around 80 percent material costs. This is another factor contributing to the rapid cost-reduction map for CPV technology.

When you combine the increasing efficiency and decreasing manufacturing costs, CPV leads the industry in its cost-of-energy reduction potential. Also, there is an ability to ramp quickly with CPV.

Because CPV systems use little, specialized PV material and are built primarily from readily available materials–in the SolFocus design, this is glass and aluminum–supply constraints are not an issue.

CPV also has a much lower cap-ex requirement than other solar technologies. This is an important element of rapidly building capacity as the deployment of CPV systems moves from a projected 30 to 50 MW this year to gigawatts of capacity in the not-so-distant future.

High energy output. CPV will provide the highest energy output per megawatt installed of any PV technology (see Figure 2). When hot, silicon PV and thin film suffer temperature degradation resulting in a much lower energy production as temperature rises.

This chart illustrates the upward trajectory for efficiency improvements with CPV technology.  Source: Manufacturer-reported efficiencies from industry reports

On the contrary, the multijunction cells used in CPV systems do not suffer from significant temperature degradation. Energy producers get the highest energy output per megawatt installed with CPV.

Also, CPV systems include dual-axis tracking, a requirement for the technology.

This provides a consistent level of energy production throughout the day, with energy production continuing to near sunset, supporting peak demand requirements of the late afternoon and early evening.

Scalable. With the high energy yield from CPV, it is possible to generate a lot of energy from less equipment than would otherwise be possible.

Ideally suited to large-scale distributed generation in the 5- to 20-MW range, CPV is also appropriate for utility-scale deployments, as well as smaller, industrial deployments of hundreds of kilowatts.

This flexibility allows for land use to be optimized and projects to be laid out in irregular, available land areas. With the ability to scale sites, energy deployment can be staged and matched to the demand requirements of off-takers.

More to environment than just clean energy. All solar produces clean energy, but not all solar provides the same environmental impact. CPV has a better cradle-to-cradle footprint than other solar technologies.

Because less PV material is used, CPV systems provide high recyclability. For the mostly glass and aluminum SolFocus system, the systems are 97 percent recyclable with no consumption of plastics.

CPV systems also offer an energy payback that is about 25 percent of traditional PV. Being mounted on trackers instead of directly on the ground, the systems disrupt less land than many technologies, and dual use of the land is possible.

Sensitive environments are better protected because there is no permanent shadowing, minimal land disruption and wildlife corridor protection.

Flexible deployments allow projects to be cited on already disrupted land, which typically speed permitting and project deployment. CPV also uses water only for cleaning, with no water consumption in electricity generation.

CPV Delivers Big Solar, Small Footprint

CPV solutions are changing PVs in many ways. Deployment of ground-mounted, utility-scale systems is growing dramatically. The cost of solar energy in the fastest-growing solar markets is falling rapidly.

The environment benefits from clean energy created with a small carbon footprint and environmental impact. Politicians, industrialists and environmentalists talk of the new energy future. With industry support of full commercialization and deployment, CPV will be a critical element of that future.

Author

Nancy Hartsoch is the vice president of marketing at SolFocus and chairwoman and director of the CPV Consortium. She received a bachelor’s degree and MBA from San Jose State University. Reach her at nancy_hartsoch@solfocus.com.

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