> Compact linear Fresnel reflector (CLFR) power plant technology

1. TECHNICAL AND COST FEATURES

CLFR is a linear Fresnel reflector system with several or many linear absorbers in the system, allowing the construction of plants in the multi-megawatt range (Fig. 1). CLFR individual reflectors can have the option of directing reflected solar radiation to at least two absorbers in linear systems.

Fig. 1: Small segment of a large CLFR array showing segments of two absorber lines. The tower height is about 15 m and typical absorber lines will be 600 m long. (Raytrace by P. Le Lievre)

The classical linear Fresnel system has only one linear receiver, and therefore there is no choice about the direction of orientation of a given reflector. However, if one assumes that the size of the field will be large, as it must be in technology supplying electricity in the multi-megawatt class, it is reasonable to assume that there will be many linear receivers in the system. If they are close enough, then individual reflectors have the option of directing reflected solar radiation to at least two receivers. This additional variable in reflector orientation allows much more densely packed arrays and lower absorber tower heights, because patterns of alternating reflector orientation can be set up such that closely packed reflectors can be positioned without mutual blocking. The interleaving of mirrors between two linear receiving towers is shown in Figure 2.

Fig. 2: Schematic diagram showing interleaving of mirror rows to achieve high site coverage without shading between adjacent mirrors.

The avoidance of large reflector spacings and absorber tower heights is an important issue in determining the cost of ground preparation, array substructure and absorber tower structure costs, steam line thermal losses and steam line cost. The more flexible CLFR still delivers the traditional benefits of a Fresnel reflector system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and the ability to use non-cylindrical receiver geometry. But further, our CLFR power plant concept is a serious attempt to reduce all major cost in a solar thermal electrical system, and includes the following additional features which enhance the system cost/performance ratio:

• The array uses flat or elastically curved reflectors instead of costly sagged glass reflectors. The reflectors are mounted close to the ground, minimising structural requirements. The heat transfer loop is separated from the reflector field and is fixed in space thus avoiding the high cost of flexible high pressure lines or high pressure rotating joints as required in the trough and dish concepts.
• The heat transfer fluid is water, and passive direct boiling heat transfer can be used to avoid parasitic pumping losses and the use of expensive flow controllers.
• An inverted cavity receiver has been designed using steel boiling tubes which can be directly linked with an existing fossil fuel plant steam system. This is much cheaper than evacuated tubes used in trough plants. Direct steam generation is much easier with this absorber than with tubular absorbers in trough collectors.
• Maintenance will be low because of ease of reflector access for cleaning, and because the single ended evacuated tubes can be removed without breaking the heat transfer fluid circuit.

Basic CLFR arrangements include analogues of the east-west axis parabolic trough, the north-south axis parabolic trough, and the polar axis parabolic trough. Large arrays are horizontal and NS axis arrays collect slightly more energy than EW arrays.

2. APPLICATIONS

Basic CLFR arrangements include analogues of the east-west axis parabolic trough, the north-south axis parabolic trough, and the polar axis parabolic trough. Large arrays are horizontal and NS axis arrays collect slightly more energy than EW arrays.

Retrofit Supply of Heat to Fossil Plant

The technology was designed for saturated steam boiling and is suitable for this purpose up to about 365°C. An ideal market is provision of thermal energy between 300°C and 365°C to large coal and oil fired Rankine cycle generating plant. Approximately 100 MW(e) peak equivalent of solar energy can be supplied to such plants for reheat alone at 300°C, and the latter can be retrofitted to existing plant.

The CLFR powerplant design was a winner of a A$2 million Australian Greenhouse Office Showcase commercialisation grant and was intended be built near Rockhampton by AUSTA Energy and Stanwell Corporation, but this project fell into difficulty after the original project manager AUSTA was abolished by the Queensland Government. However, due to new legislative incentives in NSW, the project activity has now shifted to the Hunter Valley there where a 25 MW(e) project is planned for 2003/4 next to Liddell power station. Two other similar sized sites have been located for future projects nearby.

The solar array will be a direct steam generation system and will feed steam or hot water directly into the power station steam cycle. The first CLFR plant will be used for preheating feed water going into the reheating circuit, although subsequent plant will be able to be used for main boiler steam injection in to the cold reheat line. The design steam delivery conditions for the Liddell project are 265°C and 5 MPa wet steam.

The technology can, in principle, allow a higher peak output per km2 of ground area, but the current cost optimum is to be pegged at about 125 MW(e) per km2. As a comparison, an 80 MW(e) LS3 plant in California occupies about 1.35 km2, about 60 MW(e) per km2.

The design approach is to minimise costs by using existing power block equipment and to maximise greenhouse gas savings by directly offsetting coal usage. This project is particularly attractive because it offers a low risk, low cost transitional path for commercialising solar thermal energy. The cost of electricity is estimated to be about $A 0.07 per kWh, close to the price of wind generation. These costs are well below those of competing technology, and occur for a variety of reasons.

Technology

The CLFR will use an inverted cavity receiver containing a water/steam mixture which becomes drier as the mixture is pumped through the array. The steam is separated and flows through a heat exchanger where the thermal energy passes to the powerplant system. In the initial plant, the reheat cycle only requires steam at 265°C.

Initially a Chrome Black selective coating will be used but a new air stable selective coating is being developed for higher temperature operation required by stand alone plants (320-360°C). The optical efficiency of the receiver is very high and uses no auxiliary reflectors. The design is in confidence.

The reflectors are of glass slightly curved and laminated with a composite/metal backing. Each reflector row is 600 metres long, and contains three segments of 200 metres each of which are tracked by one motor/gearbox. The structure is below is lightweight coated steel. Headers are minimised, with steam down and up each receiver row. The entire system is extremely simple and requires one laptop computer for operation.

Costs are kept low by using water as a heat transfer fluid, a low cost structure with reflectors close to the ground, a low cost receiver which is composed of mild steel pipe, and exceptionally low reflector costs due to advanced laminated construction. The installed array and heat exchanger cost is about $A900 peak electrical kilowatt, about $US500. O&M is low because cleaning can be done manually at ground level. This is less costly than an automatic cleaning system.

3. STAND ALONE POWER GENERATION

In NSW there exist two regions which, although relatively small compared to the area of NSW, make up about 100,000 km2 of relatively flat country. In these regions, the sunlight hours are in excess of 9 hours a day on an annual basis, substantially higher than in coastal NSW and far higher than in Europe. Currently, it is thought that about 300 MW(e) of solar generation can be utilised from these sites without grid extension, but the resource is potentially enormous with grid extension. To give a sense to proportion, the CLFR technology, working with standard steam turbine and generator sets, could satisfy the entire electrical usage of Australia with only 1000 km2 of such land. There exist many such suitable sites around the world.

The CLFR can be used with solar/gar co-firing in the initial stages to allow lower cost. This is necessary because the power block is now about twice as expensive as the collector field, and the power block must be run a longer time to amortise costs. It is hoped the first solar/gas plant can begin construction in NSW at the 100 MW level by 2005. The unsubsidised cost is estimated at $A0.07 per kWh initially for combined gas and solar output. Reducing the gas fraction will raise the price.

A longer term solution to this problem is to provide thermal storage for the solar energy collected. This would allow the power block to be operated on a round the clock basis, although it is likely that a 12 hour operational period would be expected initially because of the pool pricing of electricity. Sensible heat storage in oil and molten salt are being considered for this line of development. It is hoped a storage plant can be installed in NSW in about 2007. The cost of a plant with 50% capacity factor and 80% solar fraction (20% gas) would be close to that of the solar/gas plant above. Eliminating the gas component would raise the cost to around 8 Australian cents per kWh. However, the extent of collector cost reduction is not clear, and large production quantities could lead to lower costs.

Such CLFR technology can be installed in Europe, China, North Africa, the United States, and in many other regions.

4. END USE THERMAL ENERGY SUPPLY

The technology can supply process steam up to 365°C instead of electricity. It is suitable, with towers and receivers in scaled down form, to provide low cost solar steam for large absorption chillers allowing double and triple effect chillers to operate and higher temperature ORC engines.

In this type of application it would be similar in output to evacuated tube systems, but may be less expensive in capital cost. It could help run a pressurised water storage system in combination with adjacent evacuated tube arrays. It would also be able to run more efficient ORC turbines at higher temperatures than evacuated tube systems, so that electricity generation would be cheaper. It also has daylighting attributes when mounted over large spaces which evacuated tube systems do not. But it is more complicated than evacuated tube systems because of tracking requirements.

The system would have a spectacular look from above, and an attractive appearance from below as an effective daylighting device. On cloudy days the array 'slats' could be opened up to allow more light in because no useful power can be obtained without solar beam.

5. UNIVERSITY WORK

The CLFR concept was first developed in the Solar Energy Group in the early 1990's. The primary IP is now held by Solsearch Pty. Ltd. The University is contributing optical modelling and absorber design experience to the project, in an ongoing cooperation with the project management company Solar Heat and Power (SHP). High temperature selective coatings are being developed for the technology by the University for later stand alone plants.