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
For this case, the CSP plant supplies electricity when needed to help covering demand peaks.
Firmness and dispatchability are the main advantages over other intermittent forms of renewable energy, such as PV or wind. These technologies require additional investments in conventional power plants to back up generation to follow demand: in addition to the extra investment required and the implications for resource security, they will not provide a CO2 free generation system.
The production of electricity in a conventional power station is based on steam generation with a heat source and the use of this steam to make the turbine and the alternator works that converts mechanical power into electrical power. Traditionally, the heat needed to generate that steam is supplied by combustion of coal, leading emissions of CO2 and other gases; another option is nuclear fission, which requires strict security and management of radioactive waste associated.
However, solar thermal power plants produce electricity through a similar process, but prevent greenhouse gas emissions or production of hazardous waste to directly use the sun as an energy source. In addition, unlike other renewable sources under the variability of meteorology, the large thermal storage capacity that have these plants enables them to be manageable, that is, produce energy when needed and in the right quantities.
On the one hand, the dispatchability reduces the amount of rapid-response plants connected to the electricity grid in order to balance the possible variations of resources such as solar radiation or wind energy. Considering that rapid-response plants combined are cycle natural gas or hydroelectric power, this further reduces emissions of greenhouse gases and optimizes the use of water resources. CSP could avoid fluctuations energy produced and its storage enables them to produce at peak hours, which in many cases are the first of the night. This decreases the amount of energy generated by other plants during those hours, leading to a reduction in fossil fuel consumption, emissions and even the price to be paid by the consumer.
In contrast to other plants based on renewables sources that use power electronic devices and only inject the maximum power available at all times, CSP plants can contribute to the stability of the system, i.e. to maintain tension and frequent in the range established by the system. This fact avoids that fossil fuel plant must be connected, not only perform this function instead of these plants, but other renewable technologies that are not able to provide these services plants. Thus, thermal solar plants allow increasing the proportion of renewable energy that can be accommodated without the need for conventional backup plants, leading the way towards a more sustainable electricity system.
To provide solar electricity after sunset with CSP, thermal energy is stored in very large quantities. There is no other commercially available solution to deliver such base load with renewables, except with hydro storage. Thermal energy storage (TES) systems are an integral part of a CSP power plant, allowing eliminating short term variations of electricity production: the thermal energy collected by the solar field is stored for conversion to electricity later in the day. Storage can adapt the profile of power produced throughout the day to demand and can increase the total power output of a plant with given maximum turbine capacity. This by storing excess energy of a larger solar field before it is used in the turbine. Eventually the plant can be operated nearly at 100% capacity factor as a base load plant in appropriate locations.
Thermal storage is used in 40% of Spanish plants since 2010. A number of 5 to 10 hours storage, depending on the DNI is an average. The IEA reports that “when thermal storage is used to increase the capacity factor, it can reduce the levelised cost of solar thermal electricity (LCOE). Thermal storage also has remarkable “return” efficiency, especially when the storage medium is also used as heat transfer fluid. It may then achieve 98% return efficiency – i.e., energy losses are limited to about 2%”.
We can distinguish 3 categories of storage media that can be used in CSP plant but their maturity degree is different:
- Advanced sensible heat storage systems
Such system is used in most of the state-of-the-art CSP plants, with a “two-tank molten salt storage” (two tanks with molten salts at different temperature levels). The development of new storage mediums with improved thermal stability, such as molten salt mixtures will allow higher temperatures to be attained. Higher temperatures enable increased energy density to be achieved within the TES and hence lower the specific investment costs for the system. Improvements to TES systems would have the potential to reduce CAPEX and also to improve efficiency.
- Cost-effective latent heat storage systems
Latent heat storage has not been implemented in commercial STE plants yet but there are several research activities going on supporting the introduction and use of phase changing materials in TES technologies. The use of latent heat storage offers new possibilities for DSG helping in achieving cost competitiveness with sensible heat technologies.
- Thermochemical storage systems
To date, there are no known commercial systems for thermochemical TES in STE plants. Research into the application this technology started 40 years ago. Development projects assume potentials in energy density up to 10 times higher than a comparable sensible heat TES.
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.
Figure: Combination of storage and hybridisation in a solar plant