When the sun is overhead, each square metre of ground on the Earth receives 1kW of solar energy. This is relatively easy to collect and, once solar collectors have been built, they can produce free energy without CO2 or other emissions. The problem up until now has been harnessing the energy at low enough cost and then getting it to where it is needed.
Photovoltaic systems generally use silicon cells to generate electricity directly. They can be excellent for small-scale power generation, for example for powering remote houses or communities instead of connecting them up to the grid. The relatively low efficiencies and high costs of silicon wafers make it difficult for them to compete at the largest scale.
Solar thermal systems, where the sun heats water, often driving steam turbines to generate electricity, can have much higher collector efficiencies and faster repayment times.
They also scale up well against larger powers. With energy storage, they can even continue to generate appreciable power when there is no direct sunlight.
HISTORY OF SOLAR POWER
The first major solar thermal power plants were built in California's Mojave Desert in the 1980s. They have been producing over 350MW of reliable (99% field availability) solar power for more than 20 years. While these 1980s plants produced power above market rates, new solar thermal technologies are coming to market with prices below 10 cents per kWh – better than new fossil and nuclear plants.
Although there is still less than 1GW of solar thermal power around the world today, that looks set to change, with a flurry of projects in Spain including Abengoa's PS-10 Solar Tower near Seville. Like other towers, this uses flat mirrors to focus solar heat and so produce steam to drive a turbine.
Another innovative system, from Australian-US startup Ausra, is now moving from prototype to commercial deployment. The technique is already being used in pilot plants to feed solar generated steam into the Liddell coal-fired plant in New South Wales, Australia.
In September 2007 Ausra announced it had secured more than $40m in funding from Silicon Valley venture capital firms Khosla Ventures and Kleiner, Perkins, Caufield & Byers (KPCB).
FPL recently announced a planned 300MW project with Ausra to be built in Florida, and Ausra recently jointly announced with Pacific Gas & Electric (PG&E) a 177MW power plant in California planned for 2010.
Ausra's system replaces parabolic mirrors with Compact Linear Fresnel Reflectors (CLFRs), long flat-glass rotating reflectors which concentrate energy on water pipes to produce steam for a turbine. The simplicity of the system – even with solar storage – will help it compete with oil, gas and even coal-fired power stations. That could spur the large solar plants needed to make solar thermal the 'renewable technology of choice'.
BENEFITS OF AUSRA'S SOLAR SYSTEM
Ausra's first advantage lies in the CLFRs. Their many reflective surfaces allow a flat plate to concentrate light onto a focal line. These flat-plate reflectors are robust, easy to control and maintain, and have fewer foundations and positioning motors per square metre of mirror than parabolic mirrors. Standard flat glass replaces the trough systems and all mirrors are close to the ground to lower wind loads and steel usage. At night and during stormy weather, the steel-backed reflector units invert to improve resistance to ice, hail and high winds.
The flat receivers are computer controlled, and mounted on giant hoops to follow the path of the sun. They concentrate the light by a factor of 30 onto fixed water-filled pipes hung several metres in the air, running north to south. Several mirrors share a single tube and need only single-axis rotation – tracking the sun from east to west.
Most parabolic trough systems supply heat at between 320°C and 400°C to a main boiler and superheater of a conventional turbine. These relatively high temperatures allow more efficient high temperature turbines, but add to costs. Ausra has instead opted for the lower-cost lower-temperature approach.
The sun boils water in the pipes, generating steam at 285°C and about 70 atmospheres. The steam is passed to thermal storage and then drives conventional turbine generators, which use a simple Rankine cycle system to generate power. The cooled steam is recondensed to water and recirculated.
The system has been designed for mass manufacture. Mirrors can be made in less than ten minutes on the production line and are lightweight. Heat is transferred to the turbine directly by water, rather than inserting another material like (corrosive) molten salt in the way.
Ausra is now developing low-cost storage systems (possibly using underground 'cavern' storage) which will store enough heat to run the power plant for up to 20 hours throughout the day. The company is using pressurised water and low-cost materials to give 20 hours of operation daily while adding less than 10% to estimated costs.
The plant will therefore continue to produce electricity during cloudy periods and provide peak, shoulder and base electricity loads. That means they provide reliable night-and-day power, with availability that closely matches human energy demand. Unlike photovoltaic and wind power systems, which cannot easily store energy, the plants are therefore suitable for load-following and base load operation.
Ausra estimates it can generate 60MW from a square-kilometre solar thermal power station. Plants will break the magic 10 cents per kWh for 200 to 500MW plants, which is the rough cost of building and running fossil-fueled power stations. Cost per kWh drops with larger plants, and should be as low as 8 cents/kWh for plants above 500MW. That is as cheap as coal without CO2 sequestration and cheaper than the 10 cents/kWh of natural gas. These figures also ignore future fossil fuel price increases and pollution taxes.
WHY CHOOSE SOLAR POWER?
Solar thermal plants need large mirrors and a lot of piping. However, much of their attraction lies in what they don't need. They don't need the advanced combustion control and equipment required by fossil fuelled systems to limit NOx and SO2emissions. They don't need the triple redundant safety systems and massive containment vessels needed by nuclear stations. Nor do they need constant input of natural fossil resources.
The trees laid down over millions of years to form fossil fuels concentrate incredible amounts of energy, which is why fossil fuelled plants have been so difficult to replace. The continuing sunlight that formed them also concentrates huge amounts of energy, if we would only use it.
The best locations are those with highest daily sunshine, such as southwest US, southern Europe, North Africa, India and China – in fact a broad band across the equatorial belt. Of course, the best places for generating solar power are not necessarily the areas that consume it. Some of the world's fastest-growing cities are now in sun belts, however, making solar an excellent choice for new builds.
Ausra also claims that CLFR plants in states like California or Texas could supply over 90% of the annual national US grid electricity load. Power plants occupying a total area of less than 150km could power the whole US grid day and night. Ausra remarks that this amount of land is readily available without significant impact; it corresponds to less than 10% of the Federal land in the state of Nevada.
Less than that area could power Europe. DLR's TRANS-CSP (Concentrating Solar Power) study has shown the potential for large solar thermal power stations in North Africa for powering all of Europe, using less than 3% of the land area of Morocco alone for the solar plants.
All this would, however, need connecting with existing grids with ultra-high-voltage transmission lines. These are expensive, costing $1m to $1.5m per mile. China is, again, taking the lead by building ultra-high-voltage transmission lines to move energy derived from coal and hydroelectric power over 2,000km away to where it is needed.
The US and European 'grids' have limited interconnections, but states and countries are increasingly linking their own grids with neighbouring ones to transfer power when needed. Solar power could be injected into national/international grids in a distributed way for extra supply security.
Ausra's energy storage will meet head-on a major criticism of solar systems: that they cannot supply base-load demand. Solar output actually matches human demand more naturally than either fossil fuelled or nuclear plants – we need more energy during our waking hours when the sun is up – and so solar power does not need additional expensive gas-fired peak-demand plants. Choosing solar sites in geographic locations according to time zones will allow fine tuning of load matching hour by hour.
With a solar-supplied base load, infrequent long-lasting cloud formations could admittedly close down a solar thermal plant. Distributed sites connected by high-voltage transmission lines would again alleviate the impact of such regional difficulties, with the possibility of using fossil fuels (or biogas or biodiesel) to provide emergency steam for the solar thermal generators themselves.
But with proper planning and investment solar thermal power plants could provide a viable 'zero carbon' grid within a few decades, according to experts in the field.
Better still, solar plants have been found to create about twice the construction and permanent jobs of fossil-fired power stations, and retain more than four times the earnings within the local area. So with new technologies like Ausra's energy storage coming on the market, it will be interesting to see just where solar takes us next.