Solar Energy: Technology Overview

Solar Photovoltaic Technology and Resources

Photovoltaics (PV) have achieved considerable consumer acceptance over the last few years, with more than a five-fold increase in production between 1999 and 2004.  In 2004 photovoltaic cell and module production rose to nearly 1,200 MW.  It is estimated that the world wide grid connected residential/commercial sector installations of photovoltaics grew from 120 MW in 2000 to nearly 600 MW in 2004.  The majority of these installations were in Japan and Germany, where strong subsidy programs have made the economics of PV very attractive.    The cumulative installed world capacity at the end of 2004 is estimated by Black & Veatch to be over 4,000 MW. 

PV cells convert sunlight directly into electricity by the interaction of photons and electrons within the semiconductor material.  To create a PV cell, a material such as silicon is doped (i.e., mixed) with atoms from an element with one more or one less electron than occurs in its matching substrate (e.g., silicon).  By alternate doping, thin layers of “p” material and of “n” material are created to form a “pn” junction.  Photons striking the cell cause electrons to be set free in the junction, creating a current as it moves across the junction.  The current is gathered through a metallic grid.  Various currents and voltages can be supplied through series and parallel cell arrays.

The direct current produced depends on the material involved and the intensity of the solar radiation incident on the cell.  Single crystal silicon cells are most widely used today.  Single crystalline cells are manufactured by growing single crystal ingots, which are sliced into thin cell-size material.  The cost of the crystalline material is a significant part of the cell production cost.  Other methods of crystalline cell production (casting of polycrystalline material, pulling of cell-thickness ribbons) can cut material costs at some penalty to cell efficiency.

Another approach to reducing cell material cost is the development of thin film PV cells.  Commercial thin films are principally made from amorphous silicon; however, amorphous silicon cells suffer significant degradation and are not being seriously developed for large power applications.  Copper indium diselenide and cadmium telluride show promise as low-cost solar cells.  Thin film solar cells require very little material and can be manufactured on a large scale.  Furthermore, the fabricated cells can be flexibly sized and incorporated into building components.  However, to date, thin film technology has not proven to be cost effective compared to crystalline silicon.

Gallium arsenide cells are among the most efficient solar cells and have other technical advantages, but they are also more costly.  Gallium arsenide cells are typically used where high efficiency is required even at a high cost, such as space applications.

Applications

Current utility grid connected photovoltaic systems are generally below 100 kW in size. However, several larger projects ranging from 1 to 50 MW have been proposed.  A 10 MW facility is under construction in Arizona, with nearly 4 MW currently installed and operating.  This is one of the largest PV installations in the world.  Large scale commercial roof top installations are also on the rise with a 1.17 MW installation on the roof of a jail in California.  Utility grid connected systems are typically ground mounted.

Smaller PV installations are common for remote applications such as water pumping, telecommunications, and rural electrification.  In many applications, PV is the most economical choice for remote power supply due to the low O&M requirements and “free” fuel. 

Resource Availability

Solar radiation received at the earth’s surface is subject to variations in intensity caused by atmospheric interference.  The earth’s distance from the sun and the earth’s tilt also influence the amount of available solar energy.  The northern latitudes are tilted toward the sun during the summer months. This factor combined with the longer summer days increases the amount of solar energy available on summer as opposed to winter days. 

Generally, stationary PV arrays will receive the highest average insolation if they are mounted at an angle equal to the latitude at which they are located.  This configuration will give the highest year round performance.  To optimize performance for winter the array may be tilted at an angle equal to the latitude plus 15 degrees.  Conversely for maximum output during summer months the array should be tilted at an angle equal to the latitude minus 15 degrees.  Single and double axis tracking systems are also available that increase the system output but at a significantly higher capital cost and increased O&M requirements.  The optimum time frame for solar collection is between 9:00 a.m. and 3:00 p.m.  It is important to avoid array shading during this time frame as even a small amount of shade can reduce PV module output by as much as 80 percent.

Environmental Impacts

One of the strongest attributes of solar PV cells is that they are virtually non-polluting after installation.  However, manufacturing processes for producing some types of PV cells discharge heavy metals and can be harmful if not monitored and controlled.  By comparison to conventional technologies, these impacts are generally inconsequential.

Solar Thermal Technology and Resources

Solar thermal technologies convert the sun’s energy to productive use by capturing heat.  Early developments in solar thermal technology focused on heating water for domestic use.  Advances have expanded the applications of solar thermal to high temperature energy collection and power conversion on a utility scale.  Numerous solar thermal technologies have also been developed over the past three decades as potential sources of renewable power generation.  The leading technologies currently include parabolic trough, parabolic dish, power tower (central receiver), and solar chimney.  

With adequate resources, solar thermal technologies are appropriate for a wide range of intermediate and peak load applications including central station power plants and modular power stations in both remote and grid-connected areas.  Commercial solar thermal parabolic trough plants in California currently generate more than 350 MW. 

Solar thermal systems transfer the heat in solar insolation to a heat transfer fluid, typically a molten salt or heat transfer oil.  A steam generator converts the energy in the heat transfer fluid to steam, which is subsequently used to power a turbine.  A thermal storage tank can be used to store hot heat transfer fluid, providing thermal energy storage.  By using thermal storage or by combining the solar system with a fossil-fired system (a hybrid solar/fossil system), a solar thermal plant can provide dispatchable electric power.  Solar thermal technologies may be combined with co-utilization of fossil fuels or energy storage to provide a dependable dispatchable resource.  Parabolic dish systems use hydrogen as a working fluid to capture the solar heat and power a stirling cycle engine.

Solar chimneys do not generate power using a thermal heat cycle as the other three technologies do.  Instead, they generate and collect hot air in a large greenhouse.  Located in the center of the greenhouse is a tall chimney.  As the air in the greenhouse is heated by the sun, it rises and enters the chimney.  The natural draft produces a wind current, which rotates a collection of air turbines in the current.  The first commercial solar chimney is currently under development in Australia. 

Applications

The larger solar thermal technologies (parabolic trough, central receiver and solar chimney) are currently not economically competitive with other central station generation options (such as natural gas combined cycle).  On the other hand, parabolic dish engine systems are small and modular and can be placed at load sites, thereby directly offsetting retail electricity purchases.  These systems are in the early stages of commercialization and large-scale deployments are currently under development.

Of the four solar thermal technologies, parabolic trough represents the vast majority of installed capacity, primarily in the US desert southwest.  The Global Environment Facility is currently investigating several integrated solar combined cycle projects that will likely make use of parabolic troughs as incremental solar capacity.  Small parabolic dish engine systems have been developed by a few companies and are now being actively marketed.  These systems are typically about 25 kW in size.  The US government has funded two utility-scale central receiver power plants: Solar One and its successor/replacement, Solar Two.  Solar Two was a 10 MW installation near Barstow, California, but it is no longer operating due to reduced federal support and high operating costs. 

Solar chimney technologies are receiving significant interest around the world.  A project is proposed in Australia to build 200 MW solar chimney.  The estimated cost is $700 million and would include a chimney one kilometer (0.62 mi) tall with an accompanying greenhouse 5 km (3.1 mi) in diameter.

Resource Availability

In general, solar thermal potential is measured in terms of capacity for solar concentration.  Concentrators can only gather direct sunlight for energy generation.  Because of this, a lower latitude with minimum cloud cover will offer the greatest solar concentrator potential.  An advantage of solar thermal systems, and all solar technologies generally, is that peak output typically occurs on hot summer days when electrical demand is high.