About STE/CSP

Concentrating solar power (CSP), also known as Solar Thermal Electricity (STE) in Europe, produces heat or generate electricity by using mirrors or lenses to concentrate the sun’s rays to a temperature typically between 400 and 1000ºC. There are a variety of shapes of mirrors, sun-tracking methods and ways to provide useful energy, but they all work under the same principle. Individual CSP energy plants in operation are now typically between 50 and 392MW in size, but could be larger still. CSP plants are typically much larger than photovoltaic installations, which use mirrors to concentrate the sun’s light and convert it to heat, which drives a heat engine (usually a steam turbine) to generate electricity; often to be fed directly into homes or buildings. CSP specifically can be integrated with thermal storage or in hybrid operation with fossil fuels, offering firm capacity and dispatchable power on demand. It is suitable for peak loads and base-loads and power is typically fed into the electricity grid. Unlike solar photovoltaic (PV) technology, CSP plants use steam turbines and have a capacity to store thermal energy for later conversion to electricity.

Among all renewable energies, STE stands out for distinct technical features, such as dispatchability and firmness of delivery. As a principle, firmness means that a once contracted delivery cannot be cancelled. Dispatchability is one of the characteristics that makes STE a favoured option among other renewable resources, thanks to its storage and the possibility of hybridization, STE plants can effectively follow the demand curve with high capacity factors delivering electricity reliably and according to plan. Technically speaking, STE plants are almost fully dispatchable and can perfectly follow the demand curve, with operational time (capacity factor) much higher than 50% all year long. They could even reach 100% in plant configurations functioning solely on solar energy. Such an advantage helps optimize the transmission network, particularly in countries with a high consumption growth. STE with storage provides sustainable (CO2-free) electricity on demand and makes a further increase of variable renewables in the electricity system possible.

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.

Below, we can use the dispatch of the CSP in several cases:

Source: AbengoaIn this example the power plant ensures the electricity generation in a 24/7 basis.

Peaker designed power plant

Source: Abengoa

Hybridisation possibilities

For CSP plants, hybridisation is the combination of the use of solar energy with heat coming from other sources, such as biomass or conventional fossil fuels.

 

The advantages of hybridisation are:

  • Making it possible to convert the collected solar power with higher efficiency;
  • Ensuring dispatchability to cover peak demand and deliver energy on demand;
  • Overcoming the variability of the solar radiation;
  • Reducing start-up time; and
  • Minimising the generation cost (LCOE).

 

Steam produced with solar energy can be used to boost the capacity of a conventional fossil-fuel power plant, saving fuel, reducing CO2 emissions and achieving higher solar energy conversion efficiencies.

 

All CSP plants (Parabolic Troughs, Central Receivers and Linear Fresnel reflectors), with or without storage, can be equipped with fuel-powered backup systems that help to prepare the working fluid for start-up, regulate production and guarantee capacity (Figure below). The fuel burners (which can use fossil fuel, biomass, biogas or, possibly, solar fuels) can provide energy to the HTF or the storage medium or directly to the power block. In areas where DNI is less than ideal, fuel-powered backup makes it possible to almost completely guarantee the production capacity of the plant at a lower cost than if the plant depended only on the solar field and thermal storage. Providing 100% firm capacity with only thermal storage would require significantly more investment in a larger solar field and storage capacity, which would produce relatively little energy over the year.

 

Fuel burners also boost the conversion efficiency of solar heat to electricity by raising the working temperature level; in some plants, they may be used continuously in hybrid mode. CSP can also be used in hybrid mode by adding a small solar field to fossil fuel plants such as coal plants or combined-cycle natural gas plants in so-called integrated solar combined-cycle plants (ISCC) There are operating examples in several northern African countries with solar fields of 25 MWe equivalent and, in the United States, there are examples with a larger solar field (75 MWe.) A positive aspect of solar fuel savers is their relatively low cost: with the steam cycle and turbine already in place, only components specific to CSP require additional investment.