About solar thermal energy
If you’re not familiar with solar thermal energy, this section will give you a quick overview of what it is – and what it is not. Scroll down to start from the beginning, or click one of the links below to skip to a section directly.
1. Introduction to solar thermal energy
2. Concentrating and non-concentrating solar thermal systems
3. Types of solar concentrator: troughs, dishes, linear Fresnel systems, and power towers
4. Using concentrated solar thermal energy
5. Storing solar thermal energy
6. Solar thermal power plants worldwide
Solar thermal energy systems are different to solar photovoltaic cells, which are the ‘solar panels’ you might be most familiar with. Solar thermal and photovoltaic systems are two separate methods of putting the energy from sunlight to good use.
With solar thermal energy, the only part of the sunlight that’s used is its heat. When you dry your washing on the line, or heat water with a solar hot water system, you’re using solar thermal energy. But it’s also possible to generate higher temperatures by using a lens or mirror to focus the sunlight onto a smaller area. With the right type of focusing system, the sunlight can be turned from something capable of merely drying clothes, to something hot enough to boil water to run a large steam turbine that powers a town – or do many other useful things besides.
Photovoltaic (PV) cells, on the other hand, make use of the sun’s visible and ultraviolet light instead of its heat. They are made of a semiconductor material that has a special property: when sunlight hits it, electrons in the material are caused to flow in one direction across the cell. If a lot of cells are connected together in a panel, a useful amount of electricity is produced. You can read more about different types of photovoltaic cell on the about photovoltaics page.
Solar thermal and solar photovoltaic systems fill different niches in the energy mix. While solar thermal systems are more economical in larger-scale utility systems (supplying power to the central grid for towns or cities), PV systems have until recently been deployed in smaller-scale systems, like houses. As PV systems become cheaper and larger, however, solar thermal energy systems will continue to play to their particular strengths: the ability to store energy relatively cheaply, and the flexibility that comes with gathering energy in the form of heat rather than electricity. Both of these will be discussed more later on.
Solar thermal energy systems can be classified as either concentrating or non-concentrating technologies, depending on whether they focus the sunlight to a higher intensity.
Non-concentrating systems, such as rooftop solar hot water units, use the sunshine at its normal intensity. They use a few tricks to trap as much heat as possible, like black materials to absorb the heat, or double-layered glass tubes with a vacuum between the layers that prevent heat escaping once it’s been trapped. Amazingly, a fluid passing through such a tube can be heated to well over 100°C just by a normal sunbeam.
Concentrating systems, on the other hand, collect sunlight from a large area and focus it onto a smaller target. In theory this can be done using a either a lens (like a large magnifying glass) or a curved mirror. In practice, however, the concentrators are almost always mirrors due to the fact they cost less, weigh less, and absorb less of the sunlight.
Every concentrated solar thermal power system in the world can be categorised as either a linear- or a point-focus system.
A linear-focus system looks like a mirrored trough that focuses sunlight onto a tube. A liquid with a high boiling point such as oil can be pumped through the tube, where it will be heated to temperatures of 200 to 400°C — the practical operating limit for such systems is probably around 550°C. The trough reflectors are usually aligned either north-south or east-west, and they rotate on one axis to follow the sun as it moves during the day.
A point-focus system such as a dish can intensify sunlight to several thousand times its normal level and achieve temperatures of well over 1000°C. A dish needs to have two axes of motion (e.g. up/down and side to side) to be able to track the sun and keep its rays focused at the target point.
Imagine, though, that we want to build a really powerful system that would require a dish as big as, say, a football field. Building such a dish would be impractical and expensive – it’d need a lot of steel to support its own weight, as well as more costly motors to move the huge structure. This is where the idea of the Fresnel system comes in. A Fresnel concentrator is one where the large curved surface has been divided into smaller, lighter parts that are each mounted and rotated separately. The diagram below shows Fresnel versions of both linear and point-focus concentrators. The Fresnel point-focus system is usually referred to as a ‘power tower’ system, and the tracking mirrors are called heliostats.
The high temperatures obtained from focused sunlight can be used in many different ways. The first and perhaps the most obvious is the production of electricity.
In Australia most of our electricity comes from coal-fired power stations. These plants operate by burning coal to create heat, which is used to boil water, creating steam that turns a turbine. A solar thermal power plant can use essentially the same turbine technology as a coal fired plant, but instead of getting its heat from coal it’ll use concentrated sunlight to boil the water. Solar thermal power plants like this have been operating in California since 1985 and in Spain since 2007.
There are many other ways in which solar thermal energy can be used. It can supply heat to industrial processes that operate at high temperatures – processes that would otherwise generate these high temperatures by burning fossil fuels. One such example is the process where natural gas is converted to hydrogen fuel.
Solar thermal energy can also be used for heating and cooling of buildings. The energy can be used directly to heat water, or even to operate an air-conditioning system. CSIRO is developing improved technologies for using heat from the sun to cool things down.
One of the challenges with renewable energies like solar and wind is what to do when it’s not sunny or windy.
Firstly, it’s worth pointing out that clouds and night time aren’t as great a setback for solar energy as it may initially seem. In Australia our household energy use tends to be greater during the day and evening (when we’re awake), and during sunny weather (when we all turn our air conditioners on). If solar energy is one part of our energy supply, it can make a difference by reducing our fossil fuel use during these peak periods.
However, there are still significant benefits to be had from being able to store solar energy for even one or two hours. The main benefit is that any solar energy collected during ‘quiet’ or off-peak times of the day can be stored until peak usage time — even if this is after dark. This not only means solar energy is more reliable, it makes it more economical as well.
Storage is where solar thermal energy potentially has a great advantage over wind power and photovoltaics. It’s cheaper and more efficient to store energy as heat than it is to store it chemically in batteries. Whereas wind turbines and solar panels would need banks of expensive batteries for energy storage, a solar thermal energy system is more like an industrial-scale version of the flask you use to keep your tea and coffee hot.
In this case, however, the fluid that’s used is not boiling water, but (in many systems) a molten salt. These salts can be heated by solar concentrators to temperatures as high as 550°C. If the energy’s going to be used straight away, the heat in the liquid salt is used to boil steam for electricity (or used in other processes). However, part of the energy can be stored for later by simply sending some of the hot salt to a storage tank. These large tanks are so well insulated that they can store hot salt for a long time and lose only about one hundredth of the stored thermal energy per day. When the energy is needed again—during cloudy weather, at night-time, or during periods of peak demand—the salt is pumped out of the tank and used as needed. This technology is already in use at the Andasol solar power plants in Spain.
CSIRO is one of several organisations around the world that are investigating new, improved methods and materials for the storage of solar thermal energy.
Solar thermal power plants aren’t just a concept—they’ve been producing electricity to power towns since the mid 1980s and are still going strong today. The first series of commercial power plants were the trough-type Solar Electric Generating Stations (SEGS) in California, which together produce 354 MW of electricity—enough to power around a quarter of a million houses.
It’s in Spain, though, that solar thermal energy is really taking off. As of mid-2010, 380 MW of electricity was being delivered to the grid by solar thermal power plants, with a staggering 2,400 MW more under construction or in advanced stages of development.
A great reference for information on operational solar thermal power plants is the SolarPACES Database of Concentrating Solar Power Projects. SolarPACES is the solar thermal program of the International Energy Agency, and their database of commercial facilities is searchable by country, project name, technology type and operational status.