By Dr Dino R Ponnampalam
The development of a world powered by clean and renewable sources of energy is not an easy goal to achieve, but it is a movement that countries worldwide have invested in. The standard energy resources are declining in both popularity and in volume, allowing for countries to move away from these traditional sources and develop sustainable alternatives that will generate electricity to meet the demand of society but in a less destructive manner.
This complete reorganization of a country’s energy mix away from polluting fossil fuels to cleaner sources will depend on the natural energy resources available to that country. For countries located within the Sunbelt (boundaries of which are found at +/- 35 degrees of the equator), harnessing the power of sunlight to generate electricity to meet this power requirement is a sensible option.
Utilizing solar energy is not new; sunlight has been used to provide a diverse range of benefits for society, such as heating homes and drying food and clothes. From these basic uses, solar energy advanced rapidly in the latter part of the 21st century to the more complicated function of providing electricity to meet the needs of a growing population.
Approaches to Using Solar Energy For Electricity Generation
There are two main ways to generate electricity from sunlight, direct conversion and indirect conversion. Direct conversion concerns the use of photovoltaics to generate electricity: employing silicon or non-silicon-based semiconducting material, sunlight strikes the photovoltaic solar cell and causes an electron in the semiconducting material. The electron breaks free initiating a chain reaction that eventually results in the generation of electricity.
Indirect conversion uses sunlight to achieve some task or reaction that goes on to power homes by providing electricity. Concentrated Solar Power (CSP) is a prime example of this indirect conversion of sunlight to produce electricity. Another concentrated solar technology, termed Concentrated Photovoltaics (CPV), is partly considered as an indirect conversion technology but is also considered to be an example of direct conversion. In essence, CPV is an example of a hybrid direct-indirect conversion technology.
CPV differs from CSP in that sunlight is harnessed and concentrated till it reaches a significantly high temperature, and then this concentrated beam is focused on photovoltaic material to generate electricity. However, CSP concentrates sunlight via the use of large mirrors (or through the use of large lenses) to produce heat that would power a (steam) turbine, providing electricity to homes and businesses.
Composition of CSP Plants
As with any renewable energy technology, certain conditions need to be met before the installation can even be considered. For CSP, as with all solar energy technologies, a clear sunny sky is a prerequisite. In addition, and especially true for CSP projects, a lot of available land is required. If these two conditions can be met, then CSP is a very attractive option for generating electricity from sunlight.
Adopting one of four forms, CSP installations are able to make use of their tracking systems and follow the sun to maximize the amount of sunlight the installations direct which drives the heat engine. The four forms are: a parabolic trough; a parabolic dish (dish Stirling) system (as part of the Stirling engine system); a linear Fresnel reflector; and a solar tower.
This CSP family can be neatly divided according to focus type or receiver type. In terms of receiver type, the two groups are: fixed, and mobile. With regards to the focus type, the two categories are: line focus; and point focus. Figure 1 is an illustration showing the different focus and receiver types.
Figure 1: Illustration of the receiver types and focus types found in the family of CSP technologies. Credit: International Energy Agency
In fact, in terms of CSP projects, the use of parabolic trough and solar power towers are the most popular due to their ability to receive more sunlight (parabolic trough) and handle high temperatures and track sunlight on two axes (solar power tower). As such, this article will narrow its attention and provide an overview to both technology subgroups.
Overview of CSP Systems Using Parabolic Troughs
A parabolic trough system is a mature technology in the CSP family. This system consists of a row of (connected) u-shaped reflectors that are equipped with an absorber tube positioned at the focal point (which is roughly in the center) that runs the entire length of the reflector. Figure 2 is a schematic of a typical parabolic trough, and Figure 3 is a photograph of a parabolic trough as used commercially.
Figure 2: Schematic of a parabolic trough, consisting of reflector and absorber tube. Credit: National Renewable Energy Laboratory (NREL)
The absorber tube contains a heat transfer fluid, usually synthetic oil. As sunlight strikes the reflector and is directed to the absorber tube located at the focal point, the temperature of the oil heats up to around 400 degrees Celsius. The absorber tubes are part of a maze of tubes that lead to a heat exchanger.
As the oil synthetic heats up, the water in the heat exchanger is heated and this produces steam. This high-pressure steam is then used to power a steam turbine, producing electricity that can be utilized for businesses and homes. The water is then cooled down (the process of condensation via air cooling) and then returned to the CSP system.
Commercial Realities of Parabolic trough-based CSP
Abengoa Solar (Spain), NextEra Energy Resources (USA), and Solar Trust of America (USA) are three large entities in the field of CSP using parabolic troughs. Abengoa Solar, a subsidiary of Abengoa, operates the largest solar projects in Europe. The Solúcar Complexis a mix of solar thermal technology (another name for CSP) and photovoltaic technology. When the project is fully constructed and operational, the final output will be 300 megawatts (MW); currently, the project has an output of 183 MW, providing electricity for 94,000 households.
NextEra Energy Resources is the largest generator of solar energy in the United States, and a significant player in California in particular. In 2011, NextEra Energy Resources was granted permission to construct the 250 MW Genesis Solar Project which will provide electricity for 88,000 households.
Solar Trust of America is a project development company with around 2,000 MW of planned projects in the United States. One particular project, the Blythe Solar Power project, is a mixed solar power project rated at a nominal 1,000 MW of generating capacity making this complex the biggest in the world. The first phase of the project will be in operation in 2013 and once completed, the project will supply enough electricity to meet to demands of 300,000 detached family homes.
Overview of CSP Systems Using Solar Tower
The selection of parabolic troughs is popular when installing a CSP project. However, another popular configuration is for the use of solar towers. This configuration involves a tower surrounded by many tracker-equipped reflectors/mirrors called heliostats on a 360-degree basis, and the heliostats usually number in the hundreds (but sometimes, also thousands).
When sunlight strikes the heliostats, it is reflected to the focal point of the solar tower. Inside the solar tower, at the focal point, sits a tank of molten salt that produces steam when heated (it should be noted that a tank of water is also used sometimes). The use of molten salt is particularly advantageous as the molten salt retains heat for many hours after the sun goes down, offering the prospect of solar-derived electricity after sunset. Regardless of what is used, steam is produced and this steam will then go on to power a steam turbine, generating electricity in the process. Figure 4 is an illustration of a solar tower-based CSP project, and Figure 5 is a photograph showing a typical arrangement of a solar tower-based CSP project.
Figure 3: Illustration showing how sunlight is directed from the heliostats to the focal point of the solar tower. Credit: Smart Energy News.
Figure 4 Photograph of the 20 MW Planta Solar (PS20) project, the world’s largest commercial solar tower-based CSP installation, located in Andalusia (Spain). Credit: Abengoa Solar.
Commercial Use of Solar Tower-based CSP
As the use of a solar tower concentrates sunlight causing the temperature to rise to significant levels, the efficiency of the solar plant is increased. This is an attractive feature to manufacturers and project developers. As such, the technology became popular; industrial players such as Abengoa Solar (Spain), BrightSource (USA), and SolarReserve (USA) are leading the development of this interesting field.
In 2009, the world’s largest solar tower-based CSP installation in Andalusia (Spain) began operation. The 20 MW project, called Planta Solar (PS20) and owned by Abengoa Solar, incorporated 1,255 heliostats that directed sunlight to the focal point of a 165-meter high solar tower, producing steam that generates power for 12,000 households. When the additional solar parks –using a mix of solar technology such as photovoltaics and other CSP technology- are operational in 2013, PS20 will have an output of 300 MW and will provide electricity for 180,000 households.
BrightSource is a solar thermal project development company with interests in the United States of America and in Israel. BrightSource solar thermal plants use water instead of molten salt, and by fully utilizing its proprietary software the project development company is able to maximize power generation. In addition, to conserve water, BrightSource employs a closed-loop process where the steam is cooled and then recycled back into the system. As most of the solar tower CSP projects are located in desert areas or arid areas, conservation of water is of vital importance and so a closed-loop system is used.
SolarReserve is a solar thermal project development company that currently holds the honor of having the world’s tallest solar tower on a CSP project. Recently, construction was completed on the almost-165 meter tower for the company’s 110 MW Crescent Dunes project located in the state of Nevada (USA). The project will incorporate 10,000 heliostats surrounding a solar tower containing an innovative molten salt solution, allowing for the prospect of generating electricity for 10 hours after sunset.
Potential of Utilizing CSP to Generate Electricity
With plenty of sunshine and clear skies, coupled with considerable availability of land, CSP is a powerful technology for a select group of countries. The advantages of using either form of the two CSP technologies detailed in this article are numerous, but particular attention should be paid to the two factors concerning industry: manufacturing, component, and installation costs; and electrical efficiency.
All forms of CSP technology are expensive due to the material used and the volume required; the sheer number of heliostats makes up just one part of the entire solar power plant. Accordingly, the focus for many academic research groups and industrial research centers has been to reduce the component costs associated with manufacture, and to optimize the production process and installation process. Over time, through economies of scale and through the implementation of new technology, costs should be reduced.
The electrical efficiency of any renewable energy project/installation measures the rate at which electricity is generated from the natural energy resource. Conventional solar photovoltaic modules have an efficiency rating between 15% and 20%, depending on which generation of solar cell is selected. CSP installations generally have a higher efficiency rate. For example, CSP projects using parabolic troughs have an efficiency rating of 38% (due to the use of synthetic oil), and solar tower-based CSP projects have efficiency ratings of around 42%.
Future Outlook for CSP in Transforming the Energy Mix
A drawback to using sunlight to generate electricity is that this energy resource is intermittent. While this is no doubt true, creative ways do exist to store the energy generated for use when the sun goes down. Thankfully, for CSP, energy storage is a common addition to the solar power plant. This offers the tantalizing thought that solar-derived electricity could be used to power our energy-intensive lifestyles at night.
There are many drawbacks to CSP that are down to its inherent requirements: clear sunny skies; vast tracts of land; and project costs. However, for countries in the Sunbelt with lots of land available, covering certain states in America to countries in southern Europe and countries in Africa, CSP installations will offer the chance to generate electricity in a clean and reliable way.
With falling costs, improvements to design and technology to maximize sunlight/thermal-to-electricity efficiency rates, CSP technology will certainly transform the energy mix of certain countries and offer enhanced energy security to those countries. This clean energy will reduce the dependence on polluting fossil fuels without adversely affecting society, contributing in part to a whole new way of generating and using electricity; all of which from a focused beam of sunlight.