Cutting the cost of solar energy

Friday, 15/04/2011

Staff Writer: Colin Dunstan

Author of: Cyclic Heat to Work Conversion Systems: Elegant Simplicity and Beauty in Mathematics (Read more...)

The Kogan Creek Solar Boost facility is to be an auxiliary component of a coal-fired power station.

(Solar base-load power stations on the way - Huge solar power project approved for southern Queensland)

The investment is planned to produce 44,000 megawatt-hours per year to supply power to 5,000 homes. That is 8,500 kWh per home each year.

To do this with solar photovoltaic ("PV") panels would require a 5,000 kW solar PV system for each home, plus storage for night and cloudy days. A 5,000 kW solar PV system might produce 8,500 kWh in a year - but only on sunny days and will produce no energy at night.

The Kogan Creek Solar Boost facility shares the steam turbine generator of the coal-fired power station and as a result does not require any costly storage mechanism for 24-hour 7-day per week uninterrupted energy supply.

This is a very good example of an innovation aimed at reducing the cost per kWh of solar energy that is converted into a usable energy resource.

Solar Thermal Installations

Abengoa Solar PS10

Large fields of flat mirrors ("heliostats") that track the sun and reflect sunlight onto a single receiver for power generation have been constructed. The Abengoa Solar PS10 solar field that is composed of 624 heliostats is one example.

(Abengoa Solar PS10)

The large flat mirrors need to be spaced so that they do not obstruct reflected sunlight beams from mirrors further away from the receiver. To get enough mirrors close enough to deliver a reasonable total amount of solar energy, the receiver is mounted on a tall, expensive "solar tower".

The Abengoa Solar PS10 field illustrates a number of limitations that contribute to the cost of this approach. Compare it to the planned Kogan Creek Solar Boost facility;

• The Abengoa Solar project includes the capital cost of a steam turbine generator - and this expensive capital equipment is idle every night and on cloudy days.
• Flat mirrors need to be larger than the area of solar energy they can reflect onto a "solar tower". Each 1 metre x 1 metre square beam of sunlight needs a mirror about 1 metre x 1.4 metres if it is deflected by 90 degrees.

Ivanpah Solar Power

In the same week that the Prime Minister Julia Gillard announced in Australia the Kogan Creek Solar Boost facility, Google announced it will invest $168 million in the 370-megawatt (MW) project which relies on solar thermal technology that's sometimes informally called the “power tower”

(The Ivanpah solar power plant)

Flat mirrors also are limited because the sunlight they reflect spreads out the further it travels to the "solar tower". As a result there is a practical limit on how much solar thermal energy can be supplied, no matter how much capital investment has been made in the "solar tower" and turbine generator. The cost per kWh produced cannot easily be reduced because of these restrictions that are an inherent part of the design.

Newcastle Solar Thermal Field

In 2010 construction began on a new solar thermal field, tower and research facility at CSIRO’s National Solar Energy Centre in Newcastle, New South Wales, Australia.

Funded by a Commonwealth Government initiative - the Australian Solar Institute (ASI), this project is part of a A$5 million collaboration between the CSIRO Energy Transformed Flagship and the Australian National University (ANU).

(CSIRO's National Solar Energy Centre)

Concentrated Solar Power to Gasify Biomass

An idea aimed at further cost reductions is to use solar thermal energy from a Solar Boost facility to gasify biomass. If this was configured as part of a coal-fired power station like the Kogan Creek Solar Boost facility there would be 2 further cost benefits:

• The solar energy being used to generate power in the power station would be obtained from both the solar reflectors and the solar energy embodied in the biomass. The result is that a greater percentage of the power station's energy production would be renewable solar energy.
• Biomass often contains a surplus of water. When solar thermal energy is used to gasify biomass some is wasted converting this surplus moisture content into nothing but water vapour. The addition of small amounts of coal to the biomass could avoid this energy loss; The excess water in the biomass plus the added coal could be converted to syngas by the solar thermal energy.

Pursuing Further Cost Reductions

Another idea that may help to further reduce the cost per kWh of solar energy that is converted into a usable energy resource is to replace the flat mirrors that are used in solar thermal fields with concentrating parabolic reflectors to create compact solar energy beams that are directed at a central receiver.

One design objective is to minimize the number of components needed.

With parabolic concentrators, small beams directed towards a single target do not need a tall "solar tower".

Each parabolic concentrator points directly at the sun, so a 1 metre x 1 metre square beam of sunlight needs a mirror only 1 metre x 1 metre area of mirror. The result is a more cost-effective mirror.

A small concentrated beam will still spread out the further it travels but will fit within the same size target window of a large flat mirror from a considerably greater distance.

This short animation of the design shows that over a 12 hour period each concentrating parabolic reflector produces a vertical collimated beam along the vertical axis of rotation of the large parabolic dish.

The angle of vertical deflection is constrained between +/- 45 degrees with some careful thought on the choice of direction of the small parabolic dish.

The foci of the large and small parabolic dishes have to coincide but their axes do not need to be parallel to one another.