Solar radiation arriving at the Earth’s surface is a fairly dispersed energy source. The photons comprising the solar radiation can be converted directly to electricity in photovoltaic devices, or, in concentrated solar energy, the solar radiation heats up a fluid that is used to drive a thermodynamic cycle.
In the latter case, concentration of sunlight using mirrors or optical lenses is necessary to create a sufficiently high energy density and temperature level. Various strategies have been adopted for concentrating and capturing the solar energy in Concentrated Solar Power technologies, giving concentrations of 25–3000 times the intensity of sunlight.
Concentrating systems (which are sometimes also used in photovoltaic devices) can only make use of direct radiation, and are therefore applicable in areas where there are few clouds. In cloudy or dusty areas, photovoltaic technologies (without concentration) are likely to be preferred. A Concentrating Solar Power plant comprises four main sub-systems: concentrating system, solar receiver, storage and/or supplementary firing (back-up system) and power block. They are linked together by radiation transfer or fluid transport.
The solar receiver absorbs the concentrated solar energy and transfers it to the heat transfer fluid. Then the heat transfer fluid is used to deliver high-temperature heat to the power block and/or to store solar heat in a hot storage tank. The heat transfer fluid in the solar field and the power block working fluid may be the same, as in a CSP plant using direct steam generation.
The four Concentrating Solar Power technology families
There are four main Concentrating Solar Power technology families that can be classified according to the way they focus the sun’s rays and the receiver technology.
In systems with a line focus (Parabolic Trough and Linear Fresnel) the mirrors track the sun along one axis. In those with a point focus
(Tower and Parabolic Dish), the mirrors track the sun along two axes. The receiver may be fi xed, as in Linear Fresnel and Tower systems, or mobile as in Parabolic Trough and Dish Stirling systems.
The Concentrated Solar Power technology families differ in how they concentrate the solar radiation, which strongly affects their overall efficiency. The best annual optical efficiency (about 90%) is obtained for the parabolic dish because the concentrator axis is always parallel to the sun’s rays. The worst (about 50%) is observed for linear Fresnel systems because of poor performance (‘cosine effect’) in the morning and in the evening.
Intermediate values (65–75%) are obtained for parabolic trough and tower systems. For each family the actual effi ciency varies with the location, the time of day and the season of the year.
In each family, various options exist for the heat transfer fluid, the storage technology, and the thermodynamic cycle. Synthetic oil and saturated steam are currently used as heat transfer fluids in commercial plants, while molten salt and superheated steam are coming
to the market. Use of air (at ambient pressure or pressurised) and other pressurised gases (for example, CO2 and N2) are under development, while helium or hydrogen is used in the Stirling engines used in parabolic dish systems.
Liquid molten salt is the only commercial option today for storage for long (some hours) periods of time, allowing electricity production
to better match demand. Steam is also used for short time (less than 1 hour) storage. Thermodynamic cycles are currently steam Rankine cycles, and Stirling cycles for parabolic dish concentrators.
Brayton cycles are under development in which a gas turbine is driven by pressurised gas heated by the solar collector. The combination of Brayton cycle that supplies its waste heat to a bottoming Rankine cycle (often referred to as combined cycle) promises the best effi ciency and thus the highest electrical output per square meter of collector field.
Whereas trough plants are in routine commercial application, tower plants are currently making the transition to commercial application, and linear Fresnel and parabolic dishes are at the demonstration stage, and have not yet reached large-scale commercial application. In all cases, new technological options are at varying stages of development as discussed below.
Water consumption for cooling has the potential to be somewhat lower (around 2 m3/MWh) for tower technologies owing to their greater potential for efficiency increases than parabolic troughs and linear Fresnel systems. Conversely, the lower efficiencies of linear Fresnel systems tend to result in water consumption at the higher end of the range.
Dry cooling substantially reduces water consumption with a limited impact on plant efficiency and generating costs. For a 100 MW trough plant, adoption of dry cooling instead of wet cooling reduces water consumption by about 93%. The generating efficiency penalty is 1–3% (with respect to nominal power).
Annual production of electricity is reduced by 2–4% because of a 9–25% increase in the parasitic power requirements associated with the additional equipment for dry cooling (the ranges are due to differences in site characteristics).
As a result, generating costs increase by 3–7.5% compared with water cooling. The technical options for each CSP technology family are not currently at the same level of development.
Five development levels can be considered:
• field R&D;
• industrial/commercial application.
For parabolic troughs, an emergent additional option is the use of compressed gas as the heat transfer fluid and molten salt for storage. However, this option is at a very early stage of development and effi ciency data are not yet available.
It is noted that, although efficiency improvement is generally a strong driver of generating cost reduction for CSP, alternative strategies may be used to reduce costs, for example by reducing the cost of components of the concentrating system and solar receiver as in linear Fresnel systems.