High Temperature Solar Thermal

History of Solar Thermal Energy Systems and Solar Engines

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. Although no proof of such a feat exists, the Greek navy recreated the experiment in 1973 and successfully set fire to a wooden boat at a distance of 50m.

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 8 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 Solar Thermal 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 Towers

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, as shown in Figure 1. Ordinary spherical mirrors are subject to spherical aberration, where all light is not reflected into a single point.

Figure 1 The parabola has an unusual shape, such that all light is focused in 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, like the demonstration system in Canberra (see Figure 3). 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 2 Schematic of a parabolic trough concentrator
Image adapted from Energy Efficiency Renewable Energy Network.

Figure 3 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.

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. The Australian National University and Wizard Information Systems have negotiated the terms of a licence to commercialise the Big Dish solar concentrator technology and is working towards construction of a demonstration plant in Whyalla, South Australia (see Figure 5).

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

Figure 5 The Big Dish at the Australian National University
(Image courtesy of Wizard Power).

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 550^oC. The heated fluid can then be used to generate steam for electricity generation.

Figure 6 Schematic of a power tower
Image adapted from Energy Efficiency Renewable Energy Network.

Figure 7 Solar Two, power tower
Image courtesy of NREL's Photographic Information Exchange.

High Temperature Solar Thermal 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 8). 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 (ECOS, 2005).

Figure 8 An artists impression of 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 9). 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 9 Conceptualisation of the solar power generation plant.
(Image courtesy of Wizard Power).

Australian Compact Linear Fresnel Reflector technology developed by Solar Heat and Power Pty Ltd (See Figure 10). 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 10 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 11), 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.

The CSIRO’s Energy Technology division has successfully demonstrated a process of reforming natural gas to hydrogen using a 107m³ solar dish concentrator at its Lucas Heights facility (see Figure 12). 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 12 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

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

Solar Paces

ANU – Solar Thermal Energy Research

Wikipedia – Solar Energy

Renewable Energy Commercialisation in Australia – Solar Thermal

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References

CSIRO’s Process. 2004 “New Energy Source from Sunlight and Natural Gas” (Online)
http://www.csiro.au/files/mediaRelease/mr2000/Prfutureenergy.htm (Accessed 23 February 2007).

CSIRO’s ECOS. 2005, “Sun-Power Technology Centre Underway” (Online) http://www.publish.csiro.au/ecos/index.cfm?paper=EC124p27&sid=11 (Accessed 23 February 2007).

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