There is a pressing need to accelerate the development and deployment of advanced clean energy technologies in order to address the global challenges of energy security, climate change and sustainable development.
Current trends in energy supply and use, are patently unsustainable. Without decisive action, energy-related emissions of CO2 will be more than double by 2050. Renewable energy, together with carbon capture and storage, may form technologies to reduce greenhouse gas emissions.
Solar heat may make a substantial contribution to meeting climate change and security objectives. Solar heating and cooling (SHC) is a straightforward application of renewable energy; solar domestic hot water heating is already widely used in a number of countries but on a global level contributes only 0.4% of energy demand for domestic hot water. The roadmap of the International Energy Agency envisages that by 2050, solar energy could annually produce more than 16% of total final energy use for low-temperature heat, and nearly 17% of total energy use for cooling.
Companies working on large-scale solar thermal projects are invitingly close to achieving the replacement of conventional coal-fired energy around the world. The technology has been proven in many countries, amongst them Spain, and projects are viable, but the challenge is getting major investors to gamble on something new.
The theory behind large-scale solar thermal is simple. Curved mirrors called heliostats are positioned in a field, reflecting the sun’s energy onto a tower with a receiver on top. A liquid material, such as molten salt, is pumped through the receiver where it is heated and then pumped back down to be stored in a tank. When electricity is required, the hot material is used to heat water, creating steam and turning a turbine. Early designs used the focused rays to heat water and the resulting steam to power a turbine.
In order to make the system more efficient the thermosolar industry has replaced heated water with molten salts.
These salts are usually a combination of potassium nitrate and sodium nitrate at a 40/60 ratio. The working fluids have high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. These designs also allow power to be generated when the sun is not shining.
Heliostats are key components in central receiver plants. A heliostat is composed of mirror segments supported by hot dip galvanized steel structures. A typical heliostat can vary from 1 m2 up to more than 120 m2. A solar plant capable of delivering more than 500,000 MW-hours of electricity per year consists typically of a large central tower with a receiver at the top surrounded by thousands of heliostats.