What are solar thermal technologies?
Solar thermal technologies harness the sun’s energy in the form of thermal energy. Solar hot water collectors, such as those on the roof of a house, are about the simplest form of solar thermal technology. The heat from the sun is directly transferred to the water so you can have a hot shower and do the dishes. However, to heat the fluid to much higher temperatures, which are required to drive turbines or for complex thermal processes, usually solar radiation must be concentrated using lenses or mirrors – ASTRI is particularly interested in concentrating solar thermal technologies.
How do we concentrate solar energy?
Concentrating solar thermal power plants use mirrors to concentrate sunlight between 50 and 1000 times its normal strength. The concentrated solar radiation is then captured as thermal energy in a working (or heat transfer) fluid, usually a gas or liquid, heated to a high temperature. Concentrating solar thermal (CST) technologies are either:
- Linear-focusing – using parabolic troughs or linear Fresnels (a type of formation of focusing mirror strips) to direct reflected sunlight along a pipe filled with a fluid that heats up
- Point-focusing – using parabolic dishes or fields of sun-focusing mirrors called heliostats to direct reflected sunlight to a specific point where the heat is transferred to a fluid or particles.
Once this the fluid’s enthalpy is increased, the thermal energy may be used straight away or stored for later use.
Why concentrate solar energy?
The high-temperature thermal energy collected by solar concentrators can be used for a much wider range of applications than low-temperature systems. Some of the most important are:
- Electrical power generation,
- Supply of heat to industrial processes (opportunities have been identified in compression stations for natural gas, in pre-treatment processes of alumina and nickel, in timber, textile and paper processing and in food production in locations with high DNI) (Beath 2012),
- Solar chemistry, where the heat is used to promote high-temperature chemical reactions. These reactions could be used to make useful products such as synthetic fuels.
How long has concentrating solar thermal power been around?
Legend has it that in the 3rd century BC Archimedes used mirrors to focus sunlight and burn an enemy ship, which shows that the idea of concentrating solar thermal power has been around for a very long time! Modern industrial applications, however, began in the 1880s when it was demonstrated that sunshine concentrated by reflective dishes or troughs could be used to run printing presses and motors. In the early 20th century the technology was used commercially to power an irrigation system in Egypt, however the availability of cheap fossil fuels meant this technology was not taken up more widely.
When the price of energy began to increase in the 1970s, a number of countries became interested again in concentrating solar thermal power research and development, and during the 1980s research facilities were constructed in Europe, USA, Japan, Israel and Ukraine. The first commercial power station run from concentrated solar thermal energy was built in California in 1984, and was quickly followed by several others. In the last decade there has been massive growth in the concentrating solar thermal industry, mostly in Spain and USA, and this growth is expected to continue.
What are the costs involved in a concentrating solar thermal power station?
Although the sun is free, the collection systems are not and these are the main cost of the power station. The collection systems are made of reflectors, which reflect the sunlight, and receivers, which absorb the concentrated light at the focal point of the reflectors. With refinement of the design and technology involved, it is expected that the cost of the collection systems will drop drastically.
An advantage of concentrating solar thermal power is that once the thermal energy has been collected and transferred to a ‘working fluid’, most of the rest of the components (such as the turbines that turn the thermal energy into electricity) are similar or identical to those already used in other power stations. This means that solar power can benefit from the decades of technical improvements made in the fossil fuel power industry.
How do the mirrors manage to reflect the sun to the right point?
Do you remember at school learning about parabolas? Perhaps you didn’t realise how cool they were at the time, but the shape that the function f(x)=x2 makes (and all its variations), is the shape required for parallel beams of light, falling from above to reflect towards a particular point. In reverse, parabolic shapes can be used to shine a source of light into one direction, like on headlights. This parabolic shape can guide the development of the reflectors and control systems to ensure the sun is being concentrated to the right point.
What’s the difference between concentrating solar thermal power and photovoltaic panels?
Photovoltaic panels, or solar cells, use the light from the sun to excite electrons to create electricity instantly – this is great when the sun is shining at the times we need electricity. With concentrating solar thermal power, the energy of the sun is used to heat a fluid or particles, and isn’t instantly transformed into electricity. It therefore has the advantage of having more flexibility in how and when it is used. Once the thermal energy of the sun is captured, it can be used in a number of ways:
- The thermal energy can directly be used in a turbine to produce electricity
- The thermal energy can be stored, usually by heating a substance that is stored in an insulated tank for use later. The storage of thermal energy is much more efficient than the storage of electricity and so there can be flexibility as to when it is converted into electricity and dispatched to your home.
- The thermal energy can be used in combination with other energy sources, such as bioenergy, to create a consistent energy output where required.
Is solar thermal an intermittent form of power generation?
Solar thermal power stations rely on the sun, so if there was no sunlight at all, it wouldn’t work. However, the advantage of solar thermal power is that with inbuilt storage capacity, it becomes less reliant on the time of day or the weather.
While a Spanish solar thermal power station has shown it is possible to operate for 24 hours a day using storage, the aim of most solar power plants would be to store just enough solar energy – say 4-6 hours worth – to compensate for cloudy periods as well as to match power output to when electricity usage is highest.
It has been shown that the value of CST increases in a network where there are intermittent forms of power generation (NREL 2014) (see link http://www.nrel.gov/docs/fy14osti/61685.pdf) – so you can forget the idea that CST is adding to the problem of intermittency – it’s actually helping to solve it!
In what ways can solar thermal contribute to Australia’s energy supply?
Concentrating solar thermal power has been forecast by independent academics and by government commissioned reports to play an important role in Australia’s future energy supply (Elliston et al. 2012, AEMO 2013). Solar thermal with storage can play an important role in providing electricity for the evening peak, because the thermal storage will provide the flexibility to produce electricity when the sun has set. Concentrating solar thermal stations can also help solve other energy problems in Australia by:
- Providing cost-effective power for remote locations
- Preventing costly network upgrades
- Providing a source of auxiliary power for times when other parts of our energy supply fails
- Providing a secure supply which is not vulnerable to carbon pricing and can contribute to reducing the carbon intensity of our electricity supply and our energy intensive industry
What happens on a cloudy day?
Direct Normal Irradiance (DNI) is the portion of the sun’s light capable of casting a shadow. When a cloud comes over, the DNI might fall to zero, meaning a concentrating solar thermal power station will lose its supply of ‘fuel’ temporarily, but continue to run from its thermal inertia or storage. When the sun returns, the receiver will soon start absorbing heat again. However, if it is cloudy all day; the CST power station must use stored heat in order to produce electricity.
What happens on cold days?
Cold weather can actually improve the efficiency of the CST power station! This is because in converting heat to electricity there is a greater difference in temperature between the temperature of the heated fluid and the ambient temperature (Carnot’s Law). The concentrator can still collect radiation on colder days because the sun is still reaching the earth (as you know when you bask in the sun on a crisp sunny winters day). Often in winter the DNI is actually higher than in summer due to there being less aerosols, water vapour and/or dust particles, in the air.
Does the wind affect concentrating solar thermal power stations?
Consideration of wind is an important factor in designing the solar thermal station and in choosing a location. Wind can affect CST stations in a number of ways including:
- Convective heat loss at the receiver –however evacuated tube or cavity receivers provide partial sheltering from the wind that reduces the heat losses
- Dynamic and static wind effects on the reflectors (particularly if they are heliostats) require appropriate engineering and control solutions to ensure that the reflectors are stable and continue to be accurate in their position to concentrate the sun’s radiation towards the receiver
- Wind can carry dust and other particles towards the surfaces of the reflectors which will then require clever cleaning solutions to ensure they stay highly reflective.
Why have I heard so little about concentrating solar thermal power?
Until now, there has been a lack of research and development in CST compared to other renewable energy solutions such as wind or photovoltaic panels, particularly in Australia. One reason for this difference is that the scale and cost of testing CST ideas is much greater than a single wind turbine or small (1 cm2) photovoltaic cell. The growth in global solar thermal development is presently exponential and you can expect to hear much more about it in coming years. To ensure this growth takes places in Australia, it is important that there is significant support both for research and the development of CST industries. ASTRI is a big part of this research and capacity building effort.
How long does a concentrating solar thermal station last for?
Different components of solar thermal stations will have different lifetimes. Durability is an important requirement for stations being researched as part of ASTRI, with lifetimes of 30-40 years expected. This is similar to the lifespan of conventional coal and gas power stations.
Can I put a concentrating solar thermal power station on my roof or in my backyard?
Power stations are cumbersome creatures which require high temperatures, and therefore large solar fields, to make them efficient – so you would need a very big backyard! However, regular (non-concentrating) solar thermal technology is being used to power many hot water systems throughout Australia, with collectors put on people’s roofs. Concentrating solar thermal stations will have applications in larger-scale facilities, or in outback environments where they can help power remote settlements, farms and industries.
How do you keep all those mirrors clean?
The field of reflectors required for a solar thermal power station can be immense and the reflectors need to stay shiny and clean in order to reflect the sun’s radiation. High-pressure water spray can clean off dust build-up, but surface film growth, caused by materials becoming chemically attached, will be harder to clean. Methods for cleaning a reflector include:
- Let the rain wash the mirrors naturally. This can be sufficient for normal operation but may not be practical for power stations located in more arid regions!
- Spray washing delivered by a truck at night – is fast and not invasive (and therefore less likely to cause damage)
- Contact scrubbing at night – is more thorough and less water intensive than spray washing but is slower and more invasive (which could lead to damage)
Cleaning reflectors has similar issues to washing your car – the more often you do it, the easier it is. Work in ASTRI is helping to identify the most effective methods of cleaning including the development of self-cleaning surfaces.
How hot are we talking about?
The more the working fluid is heated, the larger the heat difference between the hot fluid that runs the turbine and the ambient temperature. This heat different allows a more efficient conversion of heat energy to electrical energy. However, avoiding heat losses during the transfer of heat from the sun to the fluid is much harder at higher temperatures. Balancing out the pros and cons of higher temperatures in CST power plants, theoretical optimal temperatures are shown in the graph below with the pink dots. Note that the optimal temperature is dependent on the type of technology used to concentrate the sun.
There are other advantages to using higher temperatures, which is that more efficient electricity production technologies become available. For example, a trough collector with a concentration factor of about 50 can run a normal steam turbine, which has an efficiency of up to 45%. However, a central receiver system, which can concentrate sunlight up to about 1000 times, it can be possible to run a combined cycle system (gas and steam turbines in combination) with an efficiency of as high as 55-60%. Higher temperatures are also required to run certain industrial or chemical processes. For example, the production of syngas – an important chemical feedstock – requires temperatures higher that 800⁰C.
Beath AC, Industrial energy usage in Australia and the potential for implementation of solar thermal heat and power, Energy (2012), doi:10.1016/j.energy.2012.04.031
Elliston, B., et al., Simulations of scenarios with 100% renewable electricity in the Australian National Electricity Market. Energy Policy (2012), doi:10.1016/j.enpol.2012.03.011
AEMO (Australian Energy Market Operator) 100 percent renewable study – modelling outcomes. AEMO (2013). http://www.climatechange.gov.au/sites/climatechange/files/documents/08_2013/100-percent-renewables-study-modelling-outcomes-report.pdf
J. Jorgenson, P. Denholm, and M. Mehos. Estimating the Value of Utility – Scale Solar Technologies in California Under a 40% Renewable Portfolio Standard. NREL (2014). Technical Report NREL/TP-6A20-61685 http://www.nrel.gov/docs/fy14osti/61685.pdf