There’s a rush on to capture the sun with mirrors. Fields, deserts, and areas the size of capital cities, are being covered in big shiny heliostats, and sunlight focussed on troughs, tubes and towers, designed to bring us out of the fossil-fuel age and into a greener, more sustainable future.

Wes Stein CSIRO solar research leader / Credit: CSIRO

Australia’s concentrated solar power (CSP) plans have so far met with mixed success. Two demonstration plants are now turning heat into electricity for the grid.

RayGen commissioned a solar plant in Mildura in July, delivering 4MW of solar and 50MWh of storage.

Vast, which has a small demonstration plant in Forbes, western NSW, is hoping to be the first cab off the rank with commercial scale CSP in 2024. When the first sods are turned in Port Augusta, South Australia, the commercial scale plant intends to deliver 30 MW and 288 MWh of energy and storage respectively.

Now the CSIRO has made a breakthrough in the use of concentrated solar thermal (CST) to replace fossil fuels for high-temperature heat production. An innovation focussed on supplying heat directly into a manufacturing process — extracting iron from ore, or making cement or aluminium, for example, and could also be used to create electricity by making steam to run a turbine.

The role of heat in industry is often under-appreciated. The Australian Renewable Energy Agency (ARENA) says that, of the 1500 industrial sites using process heat, most is produced at a small number of large sites such as alumina refineries and iron and steel production facilities.

Using CST for process heat is not a new idea, but the temperature achieved by CSIRO is a major milestone.

“We are looking to decarbonise large industrial processes by replacing natural gas used for heating.”, says Dr Gregory Wilson, Research Commercialisation Lead, Solar Technologies, CSIRO.

How does it work?

Most CST uses power towers, and until now solar energy has been captured using molten salts (typically 60% sodium nitrate and 40% potassium nitrate). These salts are initially melted at about 220^oC, using gas. Pumped through pipes up to the power tower’s receiver, the fluid captures the reflected heat from concentric rings or semicircles of mirrors, called heliostats. Because each mirror is slightly concave, the energy reflected is magnified when it hits the receiver.  “Higher temperature means higher heat transfer efficiency” says Dr Jin-Soo Kim, CSIRO’s Leader for Solar Technologies.

So, you have this bright point of energy coming in through the receiver, a hole at the top of the tower, heating pipes carrying molten salt. That heat is transferred down to hot tanks and used to run turbines for electricity and for process heat.

And that’s where CSIRO comes in.

No pipes. Past that receiver hole, through a sliding hatch from a hopper above, cascades a thick, opaque, curtain of small, black spherical particles.

They are calling this iteration CST specifically to include high-temperature process-heat capability as a point of difference, says Wilson. The CSIRO team has achieved 803^oC at the tower, with the possibility of reaching 1000^oC, creating the potential to directly supply heavy industry in Australia and perhaps the world.

Those tiny (0.35mm), but mighty particles, with enough weight to flow like liquid in large volumes, are made from bauxite (aluminium ore) and currently used for hydraulic fracturing (‘fracking’). Able to withstand enormous heat and pressure, they keep those fractures open, deep in the earth, to let gas escape for collection. Manufactured through ‘ceramic sintering’, applying heat and pressure to create a solid material without melting — a bit like making a snowball — they are also inexpensive, and being dark-coloured, are heat-absorbing, perfect for CST.

Although directly exposed to light, this cascade is well protected from the elements (and heat loss) as gravity pulls it down the middle of the tower. The essential opaque-curtain form (10-20mm thick) is maintained, and heat transfer optimised at the receiver, by controlling the granules’ cascade down a wall through a series of vertical troughs, spaced 0.5m apart — slowing the fall, and mixing the particles, to ensure an even spread of heat.

Particle flow is regulated, depending on target temperature — sunnier days mean more heat and a greater flow; less sun, less flow — optimising heat gathered depending on conditions.

The cascade ends in a hot tank at the bottom of the tower and the particles are moved to insulated tanks — like large grain silos — ready for use.

The higher temperatures could mean more efficient power cycles in steam turbines supplying most of the world’s power, says Keith Lovegrove, Director of the Australian Solar Thermal Energy Association. This includes supercritical CO₂ turbines, which run at about 700^oC, and are currently in development. “Your thermal storage must be hotter than your target temperature”, he adds.

And, the critical thing is making that storage cheap, says CSIRO’s solar research leader, Wes Stein. “One of the great things about this technology is you’ve just got hot particles stored in a tank or in a silo. The storage is very cheap, a critical advantage of the technology over batteries”.

Cheap storage has its advantages

CST also doesn’t face the curtailment issues routinely plaguing solar PV and wind power because it can generate electricity and store up to 15 hours of excess output. “Of course, if the storage is fully charged and there is still no need for further power, then you will be curtailed, but a lot less than you would be with a VRES (variable renewable energy system),” says Stein.

And if there’s not enough wind and sun over a longer period than storage can supply, then you have a problem, which “highlights the difficulties with a purely 100% renewables-based system supplying power 24/7, 365 days”, says Stein. But he adds that, if paired with CST, you’d only need 3% of the gas used in a purely gas-fired system. “So having natural gas backup as a transition and maybe hydrogen down the track, makes a lot of sense”.

Dust and embedded carbon

But capturing the Sun’s energy with mirrors presents challenges.

Increased droughts and bushfires, associated with climate change, are likely to increase air-borne dust, causing dust storms and adding to the costs of keeping mirrors clean and performing well. Inland Australia provides our largest solar resource, and our highest dust concentrations. A recent review of the literature concluded that dust on CSP mirrors, particularly in desert-sand regions, could result in thermal losses of 20-50%, compared with 15-30% on photovoltaics (PV).

CST may also not have the craving for rare earth metals for which photovoltaics are often criticised, but it does take the lead in bulk materials.

Setting up thousands of mirrors means a lot of concrete and steel, and the greenhouse gas emissions involved in getting it all there. Often unreported, this embedded carbon must be ‘repaid’ for the development to be considered carbon neutral.

In 2013, embedded carbon was calculated for Solar Reserves 110MW, 10,000 mirror, molten salt CSP, built in Nevada, USA.  An estimated 86,350 cubic yards (66,019 cu m) of concrete were used —accounting for 43.1 million pounds (19,550 tonnes) of CO₂. Steel and transport emissions were not calculated. Scale that by the intended development size and add the missing elements — that’s a lot of embedded carbon to be ‘paid back’ before such a development could be considered carbon neutral.

Where to from here?

CSIRO still needs to prove this concept at the pilot scale, says Wilson, from the existing 1MW plant at their Newcastle campus, to a pilot 50 times larger, gradually scaling up to a fully commercial plant — 50,000 mirrors producing 500MW.

Discussions are now being held between CSIRO and various Australian companies to build a multi-megawatt precommercial demonstration pilot plant.

By Richard Musgrove