Concentrating Solar Power

The concept

We have known the principles of concentrating solar radiation to create high temperatures and convert it to electricity for more than a century but have only been exploiting it commercially since the mid1980s. The first large-scale concentrating solar power (CSP) stations were built in the California Mojave desert. In a very short time, the technology has demonstrated huge technological and economic promise. It has one major advantage – a massive renewable resource, the sun, and very few downsides. For regions with similar sun regimes to California, concentrated solar power offers the same opportunity as the large offshore wind farms in Europe. Generation of bulk solar thermal electricity (STE) from concentrating solar power (CSP) plants is one of the technologies best suited to mitigating climate change in an affordable way, as well as reducing the consumption of fossil fuels. Unlike photovoltaic technology, CSP offers significant advantages from a system perspective, thanks to its built-in thermal storage capabilities. CSP plants can operate either by storing heat or by combination with fossil fuel generation (gas or coal); providing firm and dispatchable power available at times at the request of power grid operators, especially when demand peaks in the late afternoon, in the evening or early morning, or even when the sun isn’t shining.

Environment

The main benefit of CSP systems is in replacing the power generated by fossil fuels, and therefore reducing greenhouse gas emissions which cause climate change. Each square metre of CSP concentrator surface, for example, is enough to avoid 200 to 300 kilograms (kg) of carbon dioxide, each year, depending on its configuration. Typical power plants are made up of hundreds of concentrators arranged in arrays. The life-cycle assessment of the components and together with the land surface impacts of CSP systems indicates that it takes around five months to ‘payback’ the energy that is used to manufacture and install the equipment. Considering the plants last at least 30 years and beyond with minimum performance losses, this is a good ratio. Most of the CSP solar field components are common materials and can be recycled and used again for further plants.

Requirements for CSP

Solar thermal power uses direct sunlight, called ‘beam radiation’ or Direct Normal Irradiation (DNI). This is the sunlight which is not deviated by clouds, fumes or dust in the atmosphere and that reaches the earth’s surface in parallel beams for concentration. Suitable sites need to get a lot of this direct sun – at least 2,000 kilowatt hours (kWh) of sunlight radiation per square metre annually. The best sites receive more than 2,800 kWh/m2/year.

csp

Typical regions for concentrating solar are those without lots of atmospheric humidity, dust and fumes. They include steppes, bush, savannas, semi-deserts and true deserts, ideally located within less than 40 degrees of latitude north or south. Therefore, the most promising areas of the world include the South-Western United States, Central and South America, North and Southern Africa, the Mediterranean countries of Europe, the Near and Middle East, Iran and the desert plains of India, Pakistan, the former Soviet Union, China and Australia. (Add the map of sun belt countries)

In these regions, one square kilometre of land is enough to generate as much as 100-130 gigawatt hours (GWh) of solar electricity per year using solar thermal technology. This is the same as the power produced by a 50 MW conventional coal- or gas-fired mid-load power plant. Over the total life cycle of a solar thermal power system, its output would be equivalent to the energy contained in more than 5 million barrels of oil.

Like conventional power plants, CSP plants need cooling at the so-called “cold” end of the steam turbine cycle. This can be achieved through evaporative (wet) cooling where water is available or through dry cooling (with air), both of them conventional technologies. Dry cooling requires higher investment and eventually leads to 5 – 10 % higher cost compared to wet cooling. Hybrid cooling options exist that can optimise performance for the site conditions and are under further development.

However, the huge solar power potential in these areas by far exceeds local demand. So, solar electricity can be exported to regions with a high demand for power with less solar resource. If the sun-belt countries harvest their natural energy in this way, they would be making a big contribution to protecting the global climate. Countries such as Germany are already seriously considering importing solar electricity from North Africa and Southern Europe as to make their power sector more sustainable. Of course, for any new development, local demand should be met first.

How it works – the technologies

A range of technologies can be used to concentrate and collect sunlight and to turn it into medium- to high temperature heat. This heat is then used to create electricity in a conventional way, for example, using a steam or gas turbine or a Stirling engine. Solar heat collected during the day can also be stored in liquid or solid media such as molten salts, ceramics, concrete or phase-changing salt mixtures. At night, it can be extracted from the storage medium to keep the turbine running. Solar thermal power plants with solar-only generation work well to supply the summer noon peak loads in wealthy regions with significant cooling demands, such as Spain and California. With thermal energy storage systems they operate longer and even provide base-load power. For example, in Spain the 50 MWe Andasol plants are designed with about 8 hours thermal storage, increasing annual availability by about 1,000 to 2,500 hours.

The concentrating mirror systems used in CSP plants are either line- or point focussing systems. Line systems concentrate radiation about 100 times, and achieve working temperatures of up to 550° C while point systems can concentrate far more than 1,000 times and achieve working temperatures of more than 1,000° C. There are four main types of commercial CSP technologies: parabolic troughs and linear fresnel systems which are line concentrating and central receivers and parabolic dishes which are point concentrating. Central receiver systems are also called solar towers. Part 2 provides information on the status of each type of technology and the trends in cost.

Since the last Greenpeace update on Concentrating Solar Power technologies in 2005, there has been substantial progress in three main types of use besides electricity, namely solar gas, process heat and desalination. There have also been advances in storage systems for this technology. These are discussed further in part 2. Part 4 lists the development in the Market by region. A full list of the plants operating, in construction and proposed is provided in Appendix 1.

Dispatchability

Dispatchability is the ability of a power-producing facility to provide electricity that can be dispatched at required amounts of power on demand of the grid operator. Power plants with dispatchable generation can be turned on or off, or can adjust their power output on demand.

Dispatchability is one of the characteristics that makes CSP a favoured option among other renewable resources, thanks to its storage and the possibility of hybridisation, CSP plants can effectively follow the demand curve with high capacity factors delivering electricity reliably and according to plan.

Thermal Storage system allows provide power during periods of absence of direct solar radiation, so that periods of solar gain and the power supply does not have to happen at the same time. Besides, it allows discharge into the power network requires it, regardless of the direct radiation that is affecting the feedback system.

All CSP plants can store heat energy for short periods of time and thus have a “buffering” capacity that allows them to smooth electricity production considerably and eliminates the short-time variations that non-dispatchable technologies exhibit during cloudy days.