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Published: 4 May 2011

Rising CO2 plants and biodiversity

Carol Booth and Tim Low

Will increased carbon dioxide emissions usher in a new era of more abundant vegetation, enhancing plant production as well as food and shelter for wildlife? While it’s true that CO2 pumped into an artificial greenhouse is a potent fertiliser, planet Earth’s biosphere is not so simple. Carol Booth and Tim Low look at some scientific evidence that highlights the issue’s complexity.

Microscopic stomata on the underside of leaves regulate the uptake of CO2 and release of oxygen in plants.
Credit: istockphoto

The prospect of increased atmospheric CO2 has long interested biologists, not only because of the potential impacts on plants due to CO2-driven climate change, but because CO2 also stimulates plant growth. CO2 is a vital ingredient of photosynthesis – the biochemical reaction through which most plants metabolise CO2 and water into plant sugars – and higher CO2 levels increase the rate of photosynthesis.

However, Australia’s water scarcity and nitrogen-poor soils mean that increased atmospheric CO2 is likely to have limited plant growth benefits. It could also reduce our plant and animal biodiversity. This is not just because increased atmospheric CO2 will increase temperatures and reduce rainfall in some areas, but because plants that benefit from higher CO2 may have adverse impacts on native animals and other plants.

Atmospheric CO2 is expected to double from pre-industrial levels by about 2050. Because current CO2 levels limit the rate of photosynthesis, plants should – in theory – grow faster as temperatures rise and use water and nitrogen more efficiently. When plants open the stomata (pores) in their leaves to take up CO2 , they lose water from their leaves. Under higher CO2 levels, stomata can be fewer and open less, thereby conserving water.

Some scientists predict that in the absence of other influences, vegetation will become more water-efficient and store more carbon under higher CO2 levels. However, this picture may be too optimistic when climate change, Australia’s infertile soils and irregular rainfall are taken into account.

As CO2 researcher Associate Professor Mark Hovenden from the University of Tasmania says, ‘things will definitely be different, but just how different we don’t know’. He expects CO2 impacts to be subtle in the short term but cumulatively important: ‘that’s the trouble – it means they are likely to be overlooked at first’.

Free-Air CO2 Enrichment studies

The results of traditional CO2 experiments – in which plants are grown in greenhouses or closed chambers with adequate water

and nutrients – show that plants usually thrive under higher CO2. More recently, scientists like Assoc Prof Hovenden and Dr Chris Stokes from CSIRO have undertaken Free-Air CO2 Enrichment (FACE) experiments, which involve manipulating ambient CO2 levels around plants growing in the open, instead of greenhouses.

Because they are costly, few FACE experiments have been conducted in Australia. The Australian savanna (OzFACE) experiment was the world’s first CO2 field experiment in the tropics. TasFACE, led by Assoc Prof Hovenden in Tasmania’s grasslands, began in 2002 and
is still underway.

FACE experiments also have limitations. They are usually restricted to small plants grown in small areas; they impose sudden, rather than gradual, increases in CO2; they run for years, rather than decades; and they are often not coupled with climate change impacts.

The OzFACE facility in north Queensland consisted of six rings like this one shown, each 15 m in diameter. The enclosed vegetation was exposed to different levels of CO2.
Credit: Chris Stokes

Water and nutrient limitations

One of the aims of OzFACE was to demonstrate the water-efficiency response of tropical grasses in a microclimate of elevated CO2. Researchers found that grass growth tended to increase in slightly drier years but not in very dry or wet years.

Experiment leader, Dr Stokes, says greater water-use efficiency is likely to have ‘a pretty substantial benefit in offsetting the negative impacts of climate change’ for the grazing industry. He explains that modelling of grass production in tropical rangelands in northern Australia suggests that doubling pre-industrial CO2 levels could compensate for 10 per cent less rainfall and 1–2°C higher temperatures.1

Researcher Dr Chris Stokes examines a seedling growing under higher CO2 at the OzFACE facility.
Credit: Mike Whiting

However, TasFACE leader Assoc Prof Hovenden expects compensation in most ecosystems to be slight. ‘It might extend the growing season by only two weeks. And if rainfall declines under climate change are severe, as seems likely, rising CO2 won’t help then.’ With Australia dominated by old weathered soils with scarce nitrogen and phosphorus, ‘in a lot of places we might not see any changes due to higher CO2,’ he adds.

The story gets more complicated however. By increasing photosynthetic efficiency, higher CO2 can increase nitrogen-use efficiency in plants and stimulate nitrogen fixation by legumes, which include native wattles. Some plants may access more nutrients by growing longer roots. Too little is known about the soil’s bacterial and fungal communities to be sure about the outcomes. If plant litter decomposes more slowly under higher CO2, as some studies suggest, plants will have fewer nutrients to draw on.

Both OzFACE and TasFACE have measured less nitrogen available in soils under higher CO2, except in the TasFACE experiment, when temperatures were raised at the same time as CO2. How carbon and nutrient cycles will interact under global warming is clearly a key research question for the future.

Competition between plants

Assoc Prof Hovenden has concluded that plant community composition will change under higher CO2 in most of Australia’s vegetation types, ‘simply because some species will respond differently to others and the competitive balance will change’.

For example, two widespread grasses had similar responses to CO2 in a growth-cabinet experiment but in TasFACE, under higher CO2 and temperatures, the growth of one species declined, allowing the other to dominate.

Change is likely to be most marked where there is intense competition, especially after disturbances such as fire. Under higher CO2, some choking weeds will out-compete native plants and crops for water, nutrients, space or light. Trees and shrubs should benefit more than grasses and herbs, which are less responsive to higher CO2. The boundaries may shift between grasslands, shrublands, woodlands and forests, and the understorey in some woodlands and rainforests could thicken.

Bushfires could be far more damaging in future, from the combination of higher temperatures, longer droughts and thicker vegetation. However, declining rainfall could prevent vegetation thickening in many areas. And fire, grazing pressure and drought are more likely to shape the balance between woody plants and grass than elevated CO2.

In rangelands, Dr Stokes points out that, ‘changes in woody vegetation to date are more strongly influenced by human management, and this will likely continue’ – highlighting the different challenges facing those managing agricultural systems and those managing natural ecosystems.

Impacts on herbivores

Higher CO2 levels generally mean more carbon for plant construction and defence, and might mean less leaf nitrogen for plant-eating animals to convert into protein.

For herbivores, eucalypt leaves are nutritionally marginal due to their low nitrogen content. If their nitrogen levels were to drop further, consumption of other plants less responsive to CO2 is likely to increase.
Credit: Willem van Aken, Scienceimage

Australian animals living off evergreen trees and shrubs on infertile soils survive on leaves that are tougher, less nutritious and less palatable than in most places, partly due to low levels of soil nitrogen. Nitrogen, in the form of plant protein, is an essential nutrient for leaf-eating animals.

Under higher CO2, herbivore nutrition may suffer as extra carbon is converted into lignin and defensive compounds, and nitrogen is diverted to plant defence.2 This could mean fewer koalas and other leaf-eating mammals, and fewer insects – to the detriment of insect-eating bats and birds. In one experiment, when exposed to high CO2, two food plants of rainforest possums and tree kangaroos produced thicker leaves with less nitrogen, and one also had more secondary metabolites.In another experiment, eucalypts grown in infertile soil produced leaves too tough for beetle larvae to eat.

With typically less than 2 per cent nitrogen (dry weight), eucalypt leaves are already nutritionally marginal, and become inedible for insects below about 1 per cent nitrogen.3 If leaves of many species became unpalatable, consumption of other plants less responsive to CO2 is likely to increase.

However, preliminary greenhouse studies on two eucalypt species by Assoc Prof Hovenden’s team found no changes in leaf chemistry under elevated CO2. He says more research on this important issue is a high conservation priority.

Under higher CO2, herbivore nutrition may suffer if leaf-nitrogen content drops, impacting populations of koalas, other leaf-eating mammals, and insects.
Credit: Robert Kerton, Scienceimage

Impacts of pathogens

The impacts of pathogens (disease-causing organisms) on plants under a higher CO2 regime are difficult to predict. The interactions between pathogens and their plant hosts are very specific. One the one hand, higher CO2 can increase plant resistance by promoting a thicker outer layer, more protective wax, reduced stomatal opening and defensive chemicals. However, pathogens may also reproduce more rapidly once they penetrate plant tissue.

The limited global research on crops and forests under increased CO2 shows some diseases increase in severity and others decrease. For example, CSIRO researcher Dr Sukumar Chakraborty has observed greater fertility of the rust Maravalia cryptostegiae, a biological control agent for rubber vine, one of northern Australia’s worst weeds.

Dr Chakraborty says the biggest concern for crop plants is ‘more infection cycles leading to more rapid evolution of new pathogen races’.4 This could outpace breeding of resistant varieties, which takes about 10–15 years. For Australian ecosystems, any change, negative or positive, in serious diseases of native plants such as Phytophthora dieback or eucalyptus rust has substantial implications.

So, while FACE and other experiments have shown how increased CO2 stimulates plant growth – allowing ecosystems to cycle and store more carbon per unit of nitrogen and water available, and altering both the productivity and biochemical composition of vegetation – scientists are faced with a new set of questions.

How will CO2 effects combine with other climate change factors? How will interactions between different types of plants, and between plants and herbivores change? How much carbon can ecosystems sequester before becoming carbon-saturated? And how will changing water use by vegetation alter hydrology at the catchment scale?

More information

Hovenden MJ and Williams AL (2010). The impacts of rising CO2 concentrations on Australian terrestrial species and ecosystems. Austral Ecology 35, 665–684.

1 McKeon GM et al. (2009) Climate change impacts on northern Australian rangeland livestock carrying capacity: a review of issues. Rangeland Journal 31, 1–29.
2 Kanowski J, Hopkins MS, Marsh H and Winter JW (2001) Ecological correlates of folivore abundance in north Queensland rainforests. Wildlife Research 28, 1–8.
3 Lawler IR, Foley WJ, Woodrow IE and Cork SJ (1997) The effects of elevated CO2 atmospheres on the nutritional quality of Eucalyptus foliage and its interaction with soil nutrient and light availability. Oecologia 109, 59–68.
4 Chakraborty S and Datta S (2003) How will plant pathogens adapt to host plant resistance at elevated CO2 under a changing climate? New Phytologist 159, 733–42.

Published: 4 May 2011

Towards baseload solar thermal power

James Porteous

Spain’s latest large-scale commercial concentrating solar thermal plants have advanced the generation of solar energy around the clock by using thermal storage. Meanwhile, research continues on the role that concentrating solar thermal plants can play as baseload power support technology under different scenarios.

The new solar tower at CSIRO’s National Solar Energy Centre, Newcastle.
The new solar tower at CSIRO’s National Solar Energy Centre, Newcastle.
Credit: CSIRO

Traditionally, solar energy generation has only occurred when the sun shines, so it is often said that solar plants can’t ‘do baseload’ – that is, they can’t reliably produce 24-h electricity required to power society. But there is growing global interest and investment in new concentrating solar thermal (known as CST but sometimes called CSP for Concentrating Solar Power) power plants that can store energy and generate mains power, even when the sun is not shining.

CST technology

Broadly speaking, the various industrial CST energy generators in operation are made up of different arrangements of mirrors and heat ‘receivers’: troughs, power towers, linear Fresnel and dishes (the different designs are described below). All commercial large-scale solar thermal plants currently heat oil, molten salt or water to generate steam. The steam powers a turbine, which in turn spins an electric generator to create AC (alternative current) power. From the point at which the steam is generated, a CST plant is identical to a coal, gas or nuclear plant in its operating principal. CST solar plants are distinguished by how that steam is generated in the first place.

The latest generation CST plants use molten salt energy storage and can maintain full turbine output for between 7.5 and 15 h straight, without any sunlight at all. While the use of molten salt storage as a ‘battery’ is not an entirely new concept, it is the demonstration at scale that is of interest. It overcomes variable electricity supply, a key barrier facing other renewable energy technologies.

The big advance with heat storage

In November 2008 Spain’s 50 megawatt (MW) Andasol 1 CST plant started feeding power to the grid near Granada in the country’s south, supplying power around the clock using an advanced system of heat storage in tanks of molten salt. Andasol is a trough plant; when its mirrors are collecting sunlight, it heats a synthetic oil that is passed through a heat exchanger to re-heat the ‘cold’ salt before being pumped back into the hot tank. When electrical generation is required, the liquid salt is pumped through a heat exchanger: the heat is transferred to water, which becomes steam that drives a conventional rankine-cycle steam turbine (Siemens SST-700 turbines), and then returned to the ‘cold’ tank.

The adjacent Andasol 2 plant was completed soon after, and as of March 2011 Spain had 11 CST plants, for a total of 765 Mwe of solar thermal in operation, more than half of which has molten salt storage. Planned solar thermal capacity is expected to push the total to 2500 MW by 2013, with the Spanish sector standing to exceed 10 000 MW by 2020.1 These CST plants are rated with 7.5 h of thermal storage each, however, operators can produce electricity over longer periods by running at lower output, giving the plant round-the-clock generating potential.

The completed Gemasolar Solar Tres plant with adjoining salt storage tanks <i>in situ</i> beside the tower.
The completed Gemasolar Solar Tres plant with adjoining salt storage tanks in situ beside the tower.
Credit: Torresol Energy

‘Power towers’ and storage

Attention is now focused on the recently completed Gemasolar (pronounced ‘hemasolar’) Solar Tres project near Seville, also in the south. Solar Tres is the first commercial CST plant of ‘power tower’ design to have salt storage – some 15 h worth. The tower configuration allows the plant to achieve higher operating heat and efficiencies. The salt will be heated to ~565°C, meaning greater energy storage and greater thermal efficiency than for a trough plant. At these temperatures, each megawatt hour (MWh) of energy generated requires ~25 tonnes of salt. Plants operating at lower temperatures require proportionally more salt per unit of energy stored. Trough plants have an upper temperature of 400°C and store energy at the ratio of 1 MWh per 75 tonnes of salt.

Changing the relative sizing of the mirrors, storage and turbine in plants allows for a different balance between maximum power and energy storage. In the case of Solar Tres, the sizing choices mean 15 h of storage at full power, giving true baseload capacity. These levels of average utilisation (~75 per cent) compare favourably with Australian base-load coal-fired power plants (on average NSW coal plants operate at an average 63 per cent of rated capacity).

Solar thermal in the US

While Spain leads with the first commercial CST-towers-with-storage plants, the US Bureau of Land Management (BLM) has received 148 applications for solar developments (97 000 MW potential) on public land. Many of these applications are for superior tower-type systems using molten salt for 24-h supply. Fred Morse, head of the Solar Energy Industries Association, and Thomas Mancini, head of Sandia National Laboratories, expect up to a dozen large-scale commercial solar thermal plants to break ground this year.

Costs of the technology

The US, first-of-kind medium-scale (75–150 MW) solar plants with thermal storage can generate power at about AU$0.25/kWh. New technologies always follow a cost reduction curve once they mature and economies of scale are realised. Research by Melbourne’s Energy Research Institute2 suggests that CST could hit parity with new coal and gas plants once global installations are in the order of 10 000 MW, which even under business-as-usual growth could occur by 2020 or earlier with the combined growth in existing markets of southern Europe, North Africa and the United States. Companies Torresol Energy from Spain, and Solar Reserve from the USA, now provide commercially available solar power systems (with storage) with operating characteristics comparable to a conventional coal, nuclear or gas combined cycle plant.

In its recent report ‘Zero Carbon Australia – Stationary Energy Plan’3, Beyond Zero Emissions (BZE) posits that Australia’s entire energy needs could be met with a 60:40 mix of Spanish-style solar thermal, and wind. Technologies like geothermal and wave power show promise, but solar thermal and wind can be deployed at scale today and are sufficient to entirely power the country. But, while BZE support the potential of CST with thermal storage to contribute to energy supply in Australia, they believe that policy and financial frameworks are required for commercial maturity and wider application.

Australia and CST

Australia’s interest in the potential of CST has been led by CSIRO for more than a decade. Research is focused on finding pathways to least-cost but highest thermal-efficiency solar collectors and ‘receivers.’ There is now also concerted exploration underway into next-generation high temperature thermal and battery storage, which at 1400°C could achieve 50 per cent more electricity from the same collection area.

Dr Jim Smitham, Deputy Chief at CSIRO’s Energy Technology Centre in Newcastle, says while the Spanish and US CST thermal storage plants are important demonstrations of the technology’s potential, large-scale, in Australia, CST hasn’t proven itself enough commercially as standalone technology – compared to, say, wind power. It still needs to secure financiers’ confidence that here it can be a profitable, responsive supply to meet the fluctuating daily demand pricing (serving shoulder price and peak price periods is particularly important). ‘That’s why CSIRO is looking at a transition pathways role for CST technology that assists already commercially endorsed fossil fuel generation to shift to lower carbon operation.

‘As well as investigating stand-alone CST, we are investigating hybrid solar/fossil fuel and solar/geothermal plant combinations, including solar-assisted gas turbine or high-temperature steam support for fossil fuel power stations to achieve greater thermal efficiency at lower cost.

Dr Smitham also says ‘Molten salt is not the last word in CST power storage. We still need very high temperature, low-cost storage to be available at different scales. Over the last 18 months, CSIRO has started looking at the leading edge in least-cost, higher temperature storage beyond molten salt.’

More information

Previous Ecos articles: and
CSIRO’s solar webpages:
Information about Solar Tres:

More on the different CST technologies

Trough plants

Trough technology is the most proven CST design. The largest solar generation facility in the world, called Solar Energy Generating Systems (SEGS) in California, uses troughs. SEGS is a set of nine plants near Kramer Junction in the Mojave Desert. Jointly they have a capacity of 354MW.

In a trough configuration, long lines of horizontal, parabolic mirrors focus solar radiation on a pipe through which a fluid is pumped and heated to a maximum of around 400°C. The fluid is usually a high-grade synthetic oil which does not boil or degrade at high temperatures.

In a trough plant, the mirrors rotate around their long (North–south) axis to follow the Sun during the day. Because they remain horizontal, and so don't track the Sun's elevation, trough mirrors are most effective close to the Equator where they don’t suffer reduced solar efficiency – called the projection effect’ – from this inability to follow the sun’s elevation. At the latitudes of southern Australia, trough mirrors are only about half as effective as a mirror that can more closely track the sun.

Trough mirrors from a Spanish solar power plant.
Trough mirrors from a Spanish solar power plant.
Credit: BZE

Linear Fresnel plants

The curved mirror structures of a trough plant are very expensive. A less-expensive variant on the trough mirror configuration is a Linear Fresnel (pronounced 'frenell'). which uses long, near-flat mirrors close to the ground to make an optical approximation of a parabolic trough, without the structural complexity.

These systems, such as those from Biotec Novasol (owned by Australian company Transfield) and Areva Solar (formerly Australian company Ausra), have relatively low operating temperatures of around 290°C. Therefore, no commercially viable energy storage is available because not enough heat is generated to liquify storage salt. However, Linear Fresnel companies are moving to higher temperatures and pressures, such as Mann Ferrestel / Solar Power Group who are offering a 450°C operating temperature, meaning more viable efficiency for thermal storage.

A Linear Fresnel assembly.
A Linear Fresnel assembly.
Credit: Areva Solar

Dish plants

Mirrors in a dish configuration are effective at concentrating the solar rays and track the sun in two axes. They can achieve temperatures as high as 2000°C (but are typically run at between 500°C and 650°C for electricity generation). Historically they've been expensive and not often used in solar energy plants. Australia's first solar thermal power plant was a 25kW dish-based facility developed by The ANU at White Cliffs in NSW which operated from 1981 to 1996 for an off-grid community.

The ANU has since developed the world's biggest mass production solar dish system - the ANU SG4, fourth-generation dish - which is now ready for mass production. To save costs, it is built in the field on a very accurate jig, instead of adjusting the dish after it has been manufactured.

An SG4 dish mirror from ANU in Canberra.
An SG4 dish mirror from ANU in Canberra.
Credit: BZE

Tower plants

Tower-based systems use a large field of near-flat, independently controlled mirrors called heliostats to focus light on a central receiver at the top of a tower. Heliostats are spaced to ensure they don't overshadow each other. Tower configurations can scale up to involve many hundreds, or thousands, of mirrors. This gives towers the greatest capacity to concentrate the sun's rays, leading to higher operating temperatures.

A modern tower-based solar plant would typically pass a fluid through the ‘receiver’ to be heated up to ~570°C (and in future up to ~650°C). At this temperature, electrical generation can be more efficient and cheaper than that from a trough configuration which heats to ~400°C.

The turbines required in conjunction with a tower are the same as those used in coal-fired plants, whereas the turbine technology required for lower-temperature operation is considerably more expensive because of the much lower economies of scale.

Spain’s PS10 power tower near Seville.
Spain’s PS10 power tower near Seville.
Credit: BZE

Capturing the sun efficiently – the projection effect

Heliostat mirrors track the sun in two axes which makes them more efficient than horizontal trough mirrors, especially in winter and when sited further from the Equator. Compared with a dish, which gives the best sun tracking, a trough mirror captures ~75 per cent less energy in winter at temperate latitudes because of the low angle of the Sun. This reduction of collection capacity is called the projection effect.

Solar engineers use the term insolation to describe the measurement of received solar energy, and Direct Normal Incidence (DNI) to describe the solar energy available to collectors which track the sun, i.e. no projection effect. For horizontally configured mirrors, the insolation is less (due to the projection effect) and measured as Global Horizontal Irradiance (GHI).

The projection effect comparing vertical sun's rays with rays at 30 degrees.
Credit: BZE

The solar resource

It is often highlighted that Australia’s solar resource greatly exceeds our energy needs. At ground level, the power of the Sun on a one meter square surface, at right angles to the Sun's rays, is ~1 kW (kW).

Excluding cloud effects, this gives an average of ~6 kW-hours (kWh) per day for every square meter collecting sunlight. Across a sunny country as large as Australia, this represents a phenomenally large resource.

Solar energy equivalent to Australia's total current electrical peak generation capacity (~49GW4) falls as sunlight on a square area ~8km by 8km (at noon at southern Australian latitudes), or ~0.001 per cent of Australia’s landmass.

When you take into account typical sunlight patterns, and typical plant efficiency and layout, you still need less than 0.05 per cent of Australia's area to generate equivalent power. To put the required land area in perspective, it would fit six times in Anna Creek, Australia's largest cattle station.

Text explaining the different CST technologies provided by Beyond Zero Emissions (BZE).

2 Hearps, P and McConnell, D (2011), Renewable Energy Technology Cost Review, University of Melbourne Energy Research Institute, Melbourne,
3 Full report:
4 According to ABARE 2010

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