The IndustryThe PV market has grown rapidly over the past decade, achieving over 6.3 GWp of sales in 2009 despite the global economic slowdown. Even given this meteoric growth in the market, PV still provides less than one percent of total global energy production. The future growth of the industry will depend on the industry’s ability to deliver cost competitive performance and improved efficiency.
PV modules are rated in watts of peak power (Wp) which is the power the module would deliver under ideal conditions. Technologies are usually compared based on their cost per Watt, or $/Wp. Although sunlight is free fuel, PV total installed cost per watt is currently considerably higher than conventional fossil fuel-powered plants. It is vital to understand that what is important when comparing the cost per watt of different PV technologies is the total Installed Cost per Watt. Often, PV manufacturers will quote module production costs, or selling costs per watt. Ultimately, it is the total installed cost per watt that determines the competitiveness of a technology, since this metric includes all costs to an end-user, including systems integration and installation.
These costs are currently still high, so the growth of solar power has been assisted largely by aggressive government subsidies and policies, which take a variety of forms, such as grants, renewable portfolio standards, and tax credits. PV installations are most successful in countries which subsidize the cost of installation. And as illustrated in the below chart, future demand is heavily dependent on policies of the world’s governments. Despite impressive historical growth, the total installed price per watt remains high. Much of the market is driven by aggressive regulatory subsidies. But for solar to reach its full potential, PV manufacturers must continue to drive down costs to become cost competitive without subsidies. To date, only a few large scale power generation deployments have been realized, primarily because existing photovoltaic technologies remain too costly to effectively compete without government subsidies. Long term, if PV prices can be driven down below $3/Wp total installed price, solar energy can be cost competitive without subsidies. For utility-scale applications, the market potential has been estimated in the Terawatt range (1 TW = 1000 GW). This number dwarfs the market projection ranges in the chart below and would make solar energy a major component of total world energy supply.
Technology developmentsThe first practical PV device was invented at Bell Labs in 1954. PV cell designs have improved during the past 50 years, but most of the technology sold today remains remarkably similar to that first solar cell. The photovoltaic industry has spent billions of dollars searching for ways to reach low enough price points to capture larger markets such as utility-scale electricity generation but has had limited success. The effort to reduce costs can generally be grouped into three broad thrusts:
Cell Cost Reduction:Thin films are an example of the attempt to reduce the cost per area of solar cells. Heavy investments have been made in thin films, but their efficiency is low and cost of production is high and problematic in large volumes. In the 1980s, thin films were expected to be the breakthrough that would drive PV to lower price points, but the technology remains only approximately 10% of the total PV market today.
Cell Efficiency Increases:If efficiency of cells could be increased, the energy returned per cell area would increase. Alternative cell structures and exotic materials have been developed in the effort to achieve efficiency improvements. However, the results of these efforts have been mixed because the improved results shown on a small cell area under controlled lab conditions have not translated well to significant improvements in practical application and have generally also driven up device costs. Increased cell efficiency is not a substitute for reduced installed system cost per watt, which is the most important overall factor.
Tracking and Concentration:
Conventional flat-plate (‘one-sun’) panelsBy tracking the course of the sun throughout the day, more sunlight can be captured and converted to electricity. Tracking increases the solar array output, particularly early and late in the day (when energy demands are the highest), and critical to the development of the industry, the cost to implement tracking has fallen dramatically over the past 20 years. By concentrating the sunlight using lenses and/or mirrors, the area of cells can be held constant while the amount of sunlight directed at the cells is increased. Concentration has seen limited research and development investment primarily because of concern about increased system complexity and cost and because conventional cells degrade in efficiency as temperature and operating intensity increase.
Thermal troughsConventional flat PV panels (such as those mounted on rooftops) and thin-film panels are said to be “one-sun” panels. “One-sun” means that there is no concentration of the sun’s rays. There are many different types of solar concentrators. Low intensity (< 200 ‘suns’) Concentrator PV (CPV) systems have been developed, but the economic advantages of concentration aren’t really achieved at such low concentration levels. Thermal concentrators produce heat and use the heat to produce electricity, while PV concentrators produce electricity from solar cells. Parabolic troughs (concentration in one dimension) are one form of solar thermal system, and typically use a concentration ratio of 60 to 80 suns. A receiver pipe that runs the length of the trough reflector collects concentrated sunlight and heats oil, which is run through a heat exchanger to produce steam that is then fed to a turbine, which produces electricity.
HeliostatHeliostats are another form of solar thermal system. Also known as ‘power towers’, heliostats use many tracking mirrors aimed at a central tower. This technology eliminates the need for a receiver assembly on each mirror, but has a significant disadvantage in the form of a “cosine loss”, which results from the fact that the sunlight must be aimed at a central tower. Another problem for the central tower approach is that unless the tower is very tall (driving up the cost), the mirrors shade each other more than other concentrators do. Furthermore, heliostats must be very large to pay for the fixed costs associated with the central tower. Stirling dishes are another form of thermal concentrator. Using large mirrored concentrators to focus light onto a heat engine (also known as an external combustion engine) at the focal point, stirling dishes use solar heat to drive a piston which produces electricity. While solar thermal systems have been developed and tested for decades, the primary drawback they face is high maintenance cost, and an inherent limitation to large-scale installations only.
Technology that more naturally lends itself well to distributed generation, high intensity (200+ ‘suns’) concentrator PV technology is still in its infancy. While it represents a very small share of the overall PV market today, market share for high intensity PV concentrators can be expected to grow as the technical barriers are removed and PV concentrators demonstrate commercial market viability. Because of the technology’s potential to dramatically reduce costs, concentrator technologies hold promise for solar in utility-scale installations.
Scalability factors:Lack of scalability for many solar technologies comes from two sources: (a) a requirement for large quantities of semiconductor material, and (b) use of rare semiconductor materials. The PhotoVolt™ cell is silicon based and is therefore abundantly available in the environment. In addition, GreenField’s PhotoVolt cell requires only a small fraction of the material utilized in conventional panels, meaning that supply of raw materials will not limit our growth.
Some concentrator cell suppliers have focused on producing cells made from other materials. However, these materials are generally expensive, often toxic, and in very limited supply. In addition, many of the non-conventional cell technologies that have been introduced require complicated production processes that are expensive and difficult to keep running smoothly.
GreenField responds to world’s material constraints in a more cost effective and operationally safe way. With a factor of 1000 suns, the economics allow the use of the highest grade silicon to maximize performance because the silicon material cost is a minor factor in the finished cells and the finished system.
Differences in CPV Designs:Some high intensity CPV systems are designed with a ‘distributed array’, where each concentrator has one cell. This approach is necessary when using ‘triple-junction’ cells that cannot be packed densely due to their interconnection requirements, the cell’s low voltage/high current characteristics, and the cell’s need for a bypass diode on each device. GreenField’s StarGen™ system design uses a ‘dense array’ approach, where a multitude of cells are densely packed together and share a large optical concentrating element. The PhotoVolt™ cell facilitates this approach due to its side contacts, high voltage output, and no requirement for bypass diodes.
There are two primary advantages to using a ‘dense array’: First, thermal energy can be removed through active cooling, which allows for better thermal management (more efficient cell operation) and provides a secondary energy source in the form of the thermal energy, which may be used for heating or co-generation, a benefit valued by utilities and end-users. The second advantage is the simplification of optical and mechanical design. With a ‘distributed array’, each concentrator must be carefully aligned, and the tracking system and mechanical structure must meet very tight tolerances. The system must be factory assembled and adjusted. Conversely, the GreenField StarGen™ concentrator uses off-the-shelf flat mirrors that are deflected and held in a parabolic shape with an innovative mechanical design. There is no secondary optical element required, and the system is naturally tolerant to tracking inaccuracies and looser fabrication tolerances. The structure can be field assembled by two men in half a day without a crane or ladders.
ConclusionsWhen considering the potential of different solar technologies, one should consider the following factors:
- How aggressively can the technology drive down the cost per watt over time? High intensity concentration promises to drive down the cost per watt of PV by virtue of its dramatic reduction in required solar cell area.
- What material limitations does the technology face? The PhotoVolt&trade cell utilizes up to 1/1000th of the expensive semiconductor material utilized by one-sun PV panels, greatly reducing the cost of the silicon on a per-watt basis, and ensuring no material shortage develops.
- How scalable is the technology from a manufacturing throughput perspective, and what is the cost to achieve that throughput? The PhotoVolt™ cell can be produced in very large volumes without production bottlenecks, and with low capital equipment costs.
- How easily can the technology be installed in the field, and what are the maintenance costs? The StarGen™ concentrator can be easily assembled and installed in the field, without time consuming processes and expensive equipment, and the system can be serviced and maintained without cranes or ladders