Don't waste solar energy on coal

Port Augusta is the ideal location in South Australia for a solar thermal power plant, due to its very good direct annual solar radiation and its proximity to a strong piece of grid infrastructure that services the old lignite burning power plants that are located there, owned by Alinta.

There has been a campaign for some time to repower Port Augusta, after the town was named as one of 12 key power generation sites in the Zero Carbon Australia stationary energy plan.

This campaign has garnered a lot of support and gained a great deal of momentum.

But now we're at a turning point where we may get a type solar thermal plant that is of little use in promoting a shift away from fossil fuels.  A plant that will not create an inspiring vision, nor support greater understanding and learning-by-doing that will shift us from a 19th century fossil fuel economy, to a 21st century renewable-powered, cleantech economy.

The plant being proposed is a cheaper option being proposed by electricity company Alinta. But buyer beware - you get what you pay for.

The marketing name sounds alright - it's a “solar booster”.  The idea is that steam is preheated in a solar thermal mirror field then fed into the steam cycle of the existing coal fired power plant.

The problems with this are many and varied and it would be far better to invest in a 100% solar thermal plant independent of coal, with molten salt energy storage**.

Lock-in of inferior technology

Firstly, if you only want to replace the daytime output of a coal plant, then it is cheaper today and achieves almost the same outcome to build a solar photovoltaic (PV) array next to the coal fired power plant.  Just ramp down the plant during the day when the sun is shining.

According to all of the cost curve reduction forecasts being proposed by proponents of both solar PV and solar thermal technologies, it will still be cheaper in 2020 and beyond to use PV for generating daytime power than to build a coal-solar thermal hybrid.

By installing steam generating solar thermal technology at the coal plants, we are actually locking that solar technology into daytime only use. And by pairing it with the coal fired power plants, effectively locking in coal as well for another few decades and leaving Port Augusta's people with local air pollution problems.

Inadequate learning through doing benefits

There is a very low return for the public in putting the government's money behind a solar-coal hybrid plant. These will use a lot of technology only suitable to that specific kind of application, and not scalable for use in a 24 hour solar thermal with storage power plant.

While solar thermal technologies may be higher cost now than alternatives, the whole point of investing in them today is that it should provide learnings that will allow us to achieve 100% zero carbon energy at lower cost in the future.  A solar coal hybrid plant is not viable in achieving zero carbon emissions, so it is only useful in so much as it helps to progress a 100% solar thermal technology.

In this respect it doesn't offer us anything in terms of energy storage (it won't have any and it employs steam as the working fluid which is a terrible storage medium). And it also won't offer us much learning in terms of receiver design (because the receiver would be different for 100% solar with molten salt working fluid).

There are some advantages to cost reductions in mirror fields but this amounts to less than half the capital expense of a plant. 

And we still lock-in to coal at that particular facility.

Why solar thermal with storage?

Solar thermal with storage is cheaper today and promises to remain cheaper over time than solar PV plus chemical electric batteries.  The whole reason Beyond Zero Emissions is campaigning for solar thermal is so we can have around the clock, 24 hour solar power.

What about the politics of it?

Well we need lots of solar thermal plants to be built, and Port Augusta is really just meant to be the start of something bigger.  It needs to be a demonstration site that shows the public how solar can provide large-scale, reliable and controllable power even when the sun isn't shining.

However a lot of people opposed to solar thermal and renewables more generally could use a coal-solar hybrid to argue a few old chestnuts such as,

"It needs an existing coal fired power plant to back it up and provide baseload.",

And, “Fossil fuels keep it running at night, it proves you can't run a modern economy on  renewables like solar and wind.” 

The story will go according to the detractors that the plant gets just a few percent of its annual electricity from solar, meaning coal is the main game and therefore solar will never be able to do it.

Do it once and do it right

Lastly, the prospects for replacing the entire coal output of the plant would be reduced. That's because retrofitting storage and/or expanding the mirror field to an existing coal plant faces costs that you avoid with a greenfield site.  Plants with storage have a different receiver, heat exchangers and offer more reliable steam (optimal operating conditions at full output are available at any time after a short ramp up).

What's really needed is a solar thermal plant with molten storage, in fact it is the only option if repowering Port Augusta is to be more than just a green-washing option.  The plant needs at least 6 hours of storage in order to accommodate the evening peak

Variable renewables in wind and solar PV are already making good progress and we need lots more of them.  Yet to complete the picture requires dispatchable power which is best delivered from solar thermal plants with molten salt storage.

Matthew Wright is the Executive Director of Beyond Zero Emissions

**A Spanish concern Torresol Energy and its parent SENER produce molten salt power towers as does American company Solar Reserve.   Both have in the past and present used the construction expertise of ACS.  ACS is the effective parent of Leighton Holdings, Australia's biggest construction company.