Concentrated Solar

History of Concentrated Solar Energy Systems | Components of a Concentrated Solar System | Parabolic Trough Concentrators | Parabolic Dish Concentrators | PV Concentrators | Power Towers | Solar Concentrator Systems in Australia | Further Information | References |

History of Concentrated Solar Energy Systems

Legend has it that as early as 212 BC, the Greek scientist, Archimedes, used the reflective properties of bronze shields to focus sunlight and to set fire to wooden ships from the Roman Empire which were besieging Syracuse.

In the1860s, French mathematician, August Mouchet proposed an idea for solar powered steam engines. In the following two decades, he and his assistant, Abel Pifre, constructed the first solar powered engines and used them for a variety of applications. These engines became the predecessors of modern parabolic dish collectors. At the conclusion of World War Two, scientist, novelist and futurist, Arthur C. Clark proposed the idea of satellites using solar powered steam engines.

In 1969, the Odeillo solar furnace was constructed. This featured an eight storey parabolic mirror. However, it wasn't until the development of Power Towers in the 1980s that the first large-scale solar thermal electric generators were built.

 

Components of a Concentrated Solar System

In addition to the collector and receiver common to all solar thermal systems, high temperature systems require a concentrator to focus incident solar radiation onto the receiver. Three common forms of concentrating systems exist:

  • Parabolic Trough
  • Parabolic Dish
  • Power Tower

Other optical systems, such as Fresnel Lenses can also be used as concentrators.

Why Parabolas?
The parabola is a special curve, which can be described by equations of the type  y = Ax2 + Bx + C, and has a single focal point at to which all light is reflected. Ordinary spherical mirrors are subject to spherical aberration, where all light is not reflected into a single point. The use of parabolic reflecting systems maximises the concentrating ratio of the system by ensuring that all reflected light focuses on the receiver that is positioned at the focal point.

Parabolic Trough Concentrators

The simplest parabolic concentrating system is the parabolic trough concentrator. These systems are parabolic only in one dimension and form a long parabolic shaped trough as shown in Figure 2. Although the trough arrangement is mechanically simpler than two-dimensional systems, which require more complex tracking systems, the concentrating factor is lower. Tracking systems are required in these systems to ensure that the maximum amount of sunlight enters the concentrating system.

Trough systems can either be orientated horizontally, in long rows, like the Luz system in California, or vertically. Horizontally orientated systems are usually positioned in an east-west direction to reduce the amount of tracking required, and hence the cost. Alternatively, vertically mounted systems follow the motion of the sun throughout the day, by rotating the direction of the trough.


Figure 1 Schematic of a parabolic trough concentrator
Image adapted from Energy Efficiency Renewable Energy Network.

 


Figure 2 A solar parabolic trough concentrator

Figure 3 shows a 400 acre solar trough plant located around 30 km outside of Las Vegas in the Eldorado Valley south of Boulder City Nevada. The 64 MW solar thermal power plant will generate electricity to meet a minimum percentage of energy generated from renewable sources set by the state. (In Business Las Vegas, 2008).



Figure 3 The 64 MW solar thermal plant in Nevada. Courtesy of The Green Energy Centre.

 

Parabolic Dish Concentrators

Parabolic dish concentrating systems use parabolic dish shaped mirrors to focus incoming solar radiation onto a receiver that is positioned at the focal point of the dish (see Figure 4). Fluid in the receiver is heated to high temperatures, around 750oC. This fluid is then used to generate electricity in a small Stirling or Brayton cycle engine, which is attached to the receiver. Parabolic dish systems are the most efficient of all solar technologies, at approximately 25% efficient, compared to around 20% for other solar thermal technologies.


Figure 4 Schematic of a parabolic dish concentrator
Image adapted from Energy Efficiency Renewable Energy Network.


Figure 5 Sandia National Laboratories world record holding technology for solar-to-grid conversion efficiency
(Photo by Randy Montoya).

 

Stirling Energy Systems (SES) now holds the solar-to-grid efficiency record of 31.25% net efficiency. The net conversion efficiency is a ratio of the net energy delivered to the grid and the solar energy hitting the concentrator mirrors. This concentrator generates electricity using a sealed Stirling engine filled with hydrogen that drives a generator. During the record, the ambient temperature was around 0 degrees C, the engine operated at about 23 degrees C, and the test ran for two and a half hours, with a 60-minute running average was calculated. The testing phase of the system generated a net output of 26.75 kW (Sandia National Laboratories, 2008).

 

PV Concentrators

Concentrator systems use large mirrors or lenses to concentrate and focus the sunlight onto a string of cells, thereby increasing the illumination and power output. The saving comes from the reduction in the number of cells required for a given power output by using the concept that "more light = greater power output" for solar cells. The maximum concentration achievable is limited in practice to the equivalent of about 50 suns. Such facilities could be attractive for large, central power stations. An Australian company, Solar Systems successfully converted 14 solar thermal concentrators at White Cliffs, South Australia, installed in the 1980s, to photovoltaic power generation in 1996 (see Figure 6). The 40 kW grid connected power station ran for around 6 years, generating valuable data on the performance and efficiency of this technology. The project ceased operation in December 2004 and the installation was dismantled (Solar Systems, 2008).


Figure 6 The 14 solar concentrating dishes using PV technology at White Cliffs in South Australia
(Copyright © 2008 Solar Systems).

 

Power Towers

Power Towers use a number of heliostats (large sun tracking flat plane mirrors) to focus sunlight onto a central receiver situated on a tower, hence the name. In these systems, a working fluid (generally a high temperature synthetic oil or molten salt) is pumped through the receiver where it is heated to around 550oC. The heated fluid can then be used to generate steam for electricity generation.


Figure 7 Schematic of a power tower
Image adapted from Energy Efficiency Renewable Energy Network.


Figure 8 Solar Two, power tower
Image courtesy of NREL's Photographic Information Exchange.

Europe's first commercially operating concentrating solar thermal power plant outside Seville in southern Spain has 624 mirrors reflecting the sun to the 40 storey tower, with some night-time storage capacity. The system is known as the PS10 solar power tower and Solucar the operator states the output is 11MW (Figure 9).


Figure 9 The PS10 solar power tower in Seville, Spain.

 

Concentrated Solar Systems in Australia

The CSIRO’s $1.5 million National Solar Energy Centre (NSEC) in NSW exhibits the latest solar thermal technologies. The NSEC is the only multi-collector facility of its type in Australia and home to the largest high concentration solar array in the Southern Hemisphere (see Figure 10). At peak operation it will generate enough electricity to power more than 100 homes. The Centre comprises three main elements: a high concentration tower solar array that uses 200 mirrors to generate more than 500 kW of energy and peak temperatures of over 1000°C; a linear concentrator solar array that heats a fluid to temperatures of around 250°C to power a small turbine generator; and a control room that houses communications and control systems, and serves as an elevated viewing platform. CSIRO project engineers are working with Australian technology company Solar Heat and Power Ltd (CSIRO, 2008).

 


Figure 10 An artists impression of the NSEC. The NSEC is the only multi-collector facility of its type in Australia and the largest high concentration solar array in the Southern Hemisphere. (courtesy of ECOS and CSIRO Energy Technology).

Researchers at the Centre for Sustainable Energy Systems at the Australian National University are researching both photovoltaic trough concentrators and parabolic dish concentrators. The Australian National University and Wizard Information Systems have negotiated the terms of a licence to commercialise the Big Dish solar concentrator technology and are working towards construction of a demonstration plant in Whyalla, South Australia (See Figure 11). The project will consist of up to twenty 400m2 solar dishes producing steam that will be carried via insulated steam lines to a central grid connected generation plant.


Figure 11 Conceptualisation of the solar power generation plant.
(Image courtesy of Wizard Power).

 


Figure 12 A trough concentrator system at the Australian National University, which is designed to incorporate photovoltaic power generation or water heating and steam production. (Image courtesy of the Centre for Sustainable Energy Systems, Australian National University.

Solar Systems have constructed four new concentrator dish power stations at Hermannsburg (192 kW), Yuendumu (240 kW), Lajamanu (288 kW) and Umuwa (220 kW) . Together they generate 940 kW and save 560,000 litres of diesel and 2060 tonnes of greenhouse emissions each year. The solar power stations at these remote indigenous communities in Australia’s Northern Territory are constructed using Solar Systems’ CS500 concentrator dish systems (Solar Systems, 2008).



Figure 13 The 10 CS500 solar concentrating PV dishes at Yuendumu in the Northern Territory
(Copyright © 2008 Solar Systems).

Solar Systems are also developing a $420 million large-scale solar power plant in north-west Victoria. The 154 MW solar power station will be connected to the national grid and generate clean electricity directly from the sun to meet the annual needs of over 45,000 homes with zero greenhouse gas emissions. The power station will be comprised of around 250 heliostats (sun tracking mirrors) in multiple arrays and towers that will concentrate the sun's energy by around 500 times into the high performance photovoltaic cells in the fixed 40 m high towers. Solar Systems has attracted a $75 million grant to the project under the Federal Government’s Low Emissions Technology Demonstration Fund (LETDF). The station will be constructed and commissioned over a six-and-half-year period to 2013, incorporating technology optimisation and commercial rollout stages (Solar Systems, 2008).

a

Figure 14 The world record solar PV receiver in Solar Systems’ factory at Hawthorn is the size of a small window.
(Copyright © 2008 Solar Systems).

Australian Compact Linear Fresnel Reflector technology developed by Solar Heat and Power Pty Ltd (See Figure 15). This system is installed at the Liddell power station, NSW, Australia and preheats water for the coal fired power plant. New South Wales State Government loan funding was offered for testing of a commercial 20 000 m² compact linear fresnel reflector array which will supply thermal energy at 285°C/70 bar to a conventional coal-fired steam-turbine cycle preheater, equivalent to 38 MWe, operating by 2007. The solar array is under construction at the Liddell Power Station, which is a 2 600 MW coal-fired facility in the Hunter Valley in coastal New South Wales. Stage 1 (the first 1 MWt) is now completed with a short length of array separate from the coal plant.


Figure 15 The Concentrating Line Focus Receiver (CLFR) plant near the Liddell power station in NSW.
(Image courtesy of Solar Paces).

The solar concentrators at White Cliffs, South Australia (See Figure 16), were successfully installed in the 1980s, where they produced 25 kW of solar thermal electricity before being converted to solar photovoltaic power generation in 1996.


Figure 16 The current 14 solar concentrating dishes now using PV technology at White Cliffs in South Australia
(Copyright © 2000 Solar Systems).

The CSIRO’s Energy Technology division has successfully demonstrated a process of reforming natural gas to hydrogen using a 107 m3 solar dish concentrator at its Lucas Heights facility (see Figure 17). Combining sunlight and natural gas in a novel process will produce large-scale energy for Australia's future industrial and domestic needs. The technology provides the energy resource industry with a path to greater sustainability with significantly reduced greenhouse emissions per unit of energy generated. Concentrated solar thermal energy provides the high temperatures necessary to reform natural gas to syngas - a mixture of hydrogen and carbon monoxide. The syngas can be used wherever conventional syngas is used now, or could be further processed to hydrogen (CSIRO, 2005).


Figure 17 Dr Regg Benito adjusts the Solar Thermal Dish at CSIRO Energy Technology’s Lucas Heights facility
(Image courtesy of CSIRO’s Process Magazine and North Sullivan Photography).

 

Further Information

Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

Solar Paces

ANU – Solar Thermal Energy Research

Wikipedia – Solar Energy

Solarsphere

Sandia National Laboratories.

 

References

CSIRO’s Process. 2005 “New Energy Source from Sunlight and Natural Gas” (Online)
http://www.csiro.au/files/mediaRelease/mr2000/Prfutureenergy.htm (Accessed 11 November 2008).

CSIRO, 2008. “New solar energy research facility in the limelight” (Online) http://www.csiro.au/news/ps1i8.html (Accessed 10 November 2008).

In Business Las Vegas, 2008 “Solar plant almost set to flip power switch” http://www.inbusinesslasvegas.com/2007/04/06/feature2.html (Accessed 11 November 2008 - no longer available).

Solar Systems, 2008. “Homepage” (Online) http://www.solarsystems.com.au/index.html (Accessed 11 November 2008).