Power production using a paraboloidal solar concentrator
by the CADDET Australian National Team

The Australian National University (ANU) has developed a demonstration paraboloidal solar concentrator with 400 m2 of mirrors. Superheated steam is produced in the dish’s solar boiler and expanded in a 50 kWe steam engine/generator set. The system has produced some 60 MWh of solar power for the local grid over the past four years.

The paraboloidal dish concentrator witht the solar boiler at the focus

Background

ANU solar energy researchers have focused their research and development (R&D) activities on the paraboloidal dish solar collector system on the basis of its potential cost-effectiveness and versatility for large-scale use. Following a period of successful technological R&D, and 20 years of demonstration experience, the economies of size and scale of production now make large-scale, cost-effective concentrating solar collectors a real prospect.

The technology

This third-generation paraboloidal solar concentrator, “SG3”, features a hexagonal aperture mounted on a network of spherical nodes on a tubular steel frame. The paraboloidal dish-frame is hinged to a support base-frame which itself rotates on a reinforced-concrete track. Fifty-four triangular reflector elements – composites of steel, polyurethane foam and silver-backed glass – are attached to the dish-frame to concentrate the sun’s rays.

Tracking of the entire dish around its horizontal and vertical axes is accomplished using hydraulic rams. These are controlled by a computer, which calculates the precise position of the sun, and gives a high degree of pointing accuracy.

An insulated, cylindrical top-hat cavity at the focal point of the concentrator houses a solar boiler rated at 400 kWth. The boiler produces a peak output of about 120 g/s of steam, superheated to about 460¡C and 4.2 MPa. The superheated steam is piped to a four-cylinder uniflow steam engine for expansion. The engine drives a 65 kVA asynchronous generator.

The dish weighs about 16.8 tonnes, a specific weight per aperture of about 42 kg/m2.

Table 1: Main technical specifications

Reflector

Apeture area (net)
Mean collector diamete
Focal Length
Mean half-rim angle
Number of mirror panels
Mirror reflectivity (new)

400m2
22.74m
13.1m
46.6o
54
86%

Tracking

Actuation
Elevation envelope
Azimuth envelope
Pointing error
Cut-off wind speed
Sun position programme

hydraulic
0o to +90o
+ or - 270o
<= + or - 2 mrad
80km/h
open-loop

Weight

Weight/aperture ratio

42 kg/m2

Performance

The 50 kWe solar thermal demonstration power plant has not operated on a continuous basis. However, the overall technology involved in the system has accumulated almost 20,000 operating hours over a period of 10 years, and this, combined with the 2,000 cumulative operating hours of the demonstration system over four years, provides confidence in the overall performance of the technology.

A broad range of operating scenarios have been examined, including at steady-state, under various solar transient conditions and at fault situations. The performance results of the concentrator dish at nearly steady-state conditions at noon (given a direct-beam solar irradiation of 881 W/m2) are summarised in Table 2.

Table 2: Performance of the SG3

Solar powr intercepted by the receiver

254 kWrad

Solar power absorbed by the feed-water in the absorbed/boiler (450oC/4.5 MPa)

231 kWth

Equivalent heat rate fed to the steam engine

224kWth

Gross electricity generated

44kWe

Net power fed to the grid

40kWe

Net solar-to-electricity conversion efficiency

11.4%

The conversion figure would have been 14.7% if new thin, “low-iron” glass mirrors with a reflectivity of more than 94% had been used instead of conventional 2 mm thick “green” glass mirrors. Even greater efficiency would be achieved in very sunny areas (1,000 W/m2 insolation or more). More significantly, a net solar-to-electricity conversion of over 30% could be achieved for multiple-dish/central-power generation systems employing conventional megawatt-scale steam turbines.

Manufacturing

Manufacturing and erection costs are $0.6 million (where $ is the Australian dollar) – $0.326 million for the collector system and $0.27 million for the power conversion system – but this does not reflect the cost potential of a commercially-produced mature technology.

The technology has recently been evaluated by an industrial manufacturing licensee who estimated the costs to be about $1,750/MWe if installed on a scale of 20 MWe. Amortised over a period of 20 years, this would lead to specific power generation costs in the order of $0.08–0.12/kWh, depending on the choice of financing and of site location.

The future

Several exciting developments have spun off from this technology.

A simplified “PowerDish” collector, for solar thermal power system applications, has been developed for small production runs. In a $6.5 million project, 18 PowerDishes will feed steam into a 660 MWe steam turbine at an existing coal-fired power station, due to be operational by June 2000. This will provide the equivalent of 2.6 MWe of solar thermal power to the grid.

A fixed-receiver solar collector system is being developed which could facilitate many thermochemical applications, particularly those involving solar gasification of carbonaceous materials, such as coal and biomass, and solar-driven direct-reduction of ores, such as zinc and iron oxides.

The ANU is also exploring the use of large-scale, paraboloidal dish solar collector systems for continuous 24-hour operation using ammonia-based closed-loop thermochemical energy storage.

Environment

The use of ANU’s large-scale solar collector systems could have a significant impact in reducing greenhouse gas emissions. In the case of the 2.6 MWe solar thermal power project (see above), some 4,500 tonnes/year of CO2 emissions will be avoided from the coal-fired power station. Greater savings would result from larger solar collector fields.

Conclusions

Although the 50 kWe prototype has performed well, there is still scope for further development, primarily to improve its economic viability. In addition to power generation, waste heat from the SG3 can be used for various forms of co-generation in general and for economically viable desalination in particular.

The SG3 family of solar collectors developed at the ANU is an emerging solar power technology that can deliver cost-effective greenhouse gas reduction associated with utility-scale power generation.

For more information contact Prof Stephen Kaneff, Energy Research Centre/ANUTECH, Australian National University, Canberra ACT 0200, Australia. Tel: +61 2 6249 0027; Fax: +61 2 6249 0506; e-mail: stephen.kaneff@galaxy.anutech.com.au; Web site: www.anutech.com.au/physci/opps/bigdish.html

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