Concentrated solar power plants belong to the category of clean sources of renewable energy. The paper discusses the possibilities for the use of molten salts as storage in modern CSP plants. Besides increasing efficiency, it may also shift their area of application: thanks to increased controllability, they may now be used not only to cover baseload but also as more agile, dispatchable generators. Both technological and economic aspects are presented, with focus on the European energy sector and EU legislation. General characteristics for CSP plants, especially with molten salt storage, are discussed. Perspectives for their development, first of all in economic aspects, are considered.
The European energy sector is going through a transformation caused by the commitments to reduce CO2 emission, the increasing penetration of renewable sources (RES) [
Recent years have seen an increase in the percentage of energy from renewable energy sources (RES), particularly from biomass combustion and wind farms [
Electrical energy stimulates the economic and civilisational development of the world. The level and dynamics of electricity used in individual countries or regions depend mainly on the population, the economic and civilisational development, and the structure and efficiency of energy use [
Management and control of energy transmission and distribution at an industrial scale has its own specificity. On the one hand, power demand is significantly variable. On the other hand, currently, possibilities of electrical energy storage are strongly limited; thus, means of storing large volumes of energy in other forms have been studied intensively. Such solutions enable us to control the energy supply and optimise the economic aspects of power plant operations and energy market parameters.
This paper analyses molten salt power plants as energy reservoirs that enable us to achieve the specified goals regarding flexible energy control and storage. The topic is crucial because, at the present stage of power industry development, molten salt power plants are pioneering solutions promoted mainly in Spain and the US. Molten salt reservoirs have high storage efficiency (above 90%), but the efficiency of the energy transformation from heat to electricity is much lower at about 50%, which is a significant disadvantage. The presented studies have been conducted in the framework of the PreFlexMS European Project.
The EU environmental and climate objectives require the European countries to introduce changes concerning power generation. Instruments supporting the renewable power industry in the form of subsidies and privileged access to energy infrastructure change the energy mix of the European Union [
The situation in the energy market makes the entities which aim to cover the deficiencies in wind and photovoltaic generation or cater for peak consumption ineffective. Increased green energy supply causes the conventional energy sources with high generation costs to lose profitability. However, even unprofitable generating entities are crucial to energy security because 90% of wind or solar energy require full backup. Energy deficits in times of slowdowns in renewable generation can be observed in many EU countries. These problems are intensified by unplanned transborder flow of energy from the RES of neighbouring countries. The ongoing changes bring hazards for the safety and stability of power systems [
The biggest challenge hampering the economic optimisation of power generators of any type is the lack of ability to react to sudden changes in demand. Depending on the technology, the following factors contribute to this state:
Unpredictability and little controllability of factors affecting the generation (such as the weather) Lack of headroom with regard to power generator capabilities (as it is unfeasible to deploy generators with much power reserve) Lack of agility with regard to controlling the power generation process (usually reflected by long startup and shutdown times) [
The approaches used to remedy these problems may include the following:
Deployment of energy storage devices to bridge the temporal gap between generation and consumption of energy Implementation of modern technologies to improve the agility by reducing startup and shutdown times Utilization of prediction and simulation methods both to assess the requirements for power generation and storage capacity during the design phase and to optimise the dispatch plan during operation
A concentrated solar power plant (see Figure
Functional schema of a concentrated solar power (CSP) plant.
Various technologies are in use regarding each of the steps of light-electricity conversion. A solar field is composed of reflectors concentrating light onto a receiver. They are usually equipped with trackers which follow the sun position to maximise the amount of harvested energy. The receiver can be integrated with the reflectors (which is the case with parabolic trough, enclosed trough, and Fresnel plants), or it can stand alone (e.g., in solar towers). The latter approach seems to be the most promising [
Molten salt CSPs seem to be the most promising regarding both economic and technical factors. In such a plant, molten salt is used as the HTF, hence the name. The technology was developed in the 90s by the Sandia National Laboratories [
Currently, molten salt CSP plants are designed as baseload plants, e.g., plants which generate electricity to constantly satisfy minimum demand. Therefore, they replicate the characteristics of nuclear or coal plants. This is due to using energy storage in the form of tanks with heated molten salt. Discharging stored energy, however, does not have to follow the baseload strategy. It can be more dynamic to satisfy a particular demand that is unaccounted for. However, it needs to be stressed that such actions have to be supported by a proven predictive model of the plant itself to identify if such additional dynamic discharge is economically feasible. This is especially important, as only a limited amount of energy can be gathered and successfully stored, depending on insolation. Thus, weather forecasting, indicating the amount of energy that can be harvested, plays a major role in such a case.
While evolving, state-of-the-art CSP technologies have to comply with
dynamic tariffs and the tariff bidding process introduced by regulators being a flexibility to adjust plant operations as needed, to react to changing market conditions
Contemporary CSP is mainly limited by two factors:
Plant hardware is designed for baseload operations, having slow ramp-up time Simplistic dispatch strategies, accumulating energy during the day and managing storage to ensure delivery at night
The first one makes the plant incapable to respond to demand fluctuations. The second one prevents energy dispatch management, reducing its flexibility. Thus, these factors lead to corresponding challenges that need to be addressed, which are:
to improve the flexibility of a CSP with molten salt energy storage to improve the predictability and provide not only baseload operations but also on-demand supply driven by market economics
One of the hardware culprits hampering flexibility is the steam generator. It usually consists of four heat exchangers: a preheater, an evaporator with a steam drum, a superheater, and finally a reheater. The preheater heats up feedwater just below the boiling point. The evaporator and the steam drum boil water and separate moisture from steam. The superheater heats up dry steam to superheated conditions, and the reheater reheats the steam exiting the turbine. Currently, there are several kinds of steam generators, including kettle or drum types. They share a common limitation which is a slow ramp-up rate of 2-3 centigrade per minute. It is one of the most limiting factors, preventing using CSP in a flexible manner. Replacing this technology with the once-through approach allows achieving supercritical steam conditions which, coupled with supercritical turbines, is more efficient and increases flexibility. That makes it ideal for CSP applications requiring rapid load changes [
Providing on-demand supply, challenge #2, requires applying coupled storage capacity prediction and dispatch optimisation. The storage capacity prediction is solely based on weather forecast. However, depending on the demand, how much thermal energy is stored compared with immediate electricity generation has to be balanced. On the one hand, there is the weather forecast and on the other, the dispatch strategy and optimisations based on current demand and the electricity market. In order to make it work, there is a need for an accurate plant model covering the solar field, receiver, storage, steam generator, and electric generator. It enables decision-making providing the balance, thus making the plant more predictable and capable of reacting to energy market fluctuations. There are several papers that tackle the technological and economic benefits [
On 27 September 2001, the European Parliament and the Council of the European Union established a directive on the promotion of electricity produced from renewable energy sources [
The European Union aims to have 20% of consumed electricity coming from renewable sources by 2020. A renewable source is defined as a nonfossil one, which includes wind, solar, aerothermal, geothermal, hydrothermal and ocean energy, hydropower, biomass, landfill gas, sewage treatment plant gas, and biogases. The directive sets mandatory targets for the member states which range from 10% (Malta) to 49% (Sweden). Furthermore, it also pushes 20% energy savings to be achieved by 2020. It also provides basis for administrative procedures enabling integration of renewable energy sources in buildings, access to an electricity grid, and market cooperation mechanisms.
According to Article 3 of 2009/28/EC (see The European Parliament and the Council [
It needs to be pointed out that the targets regard gross final consumption. Such a definition makes involvement of all energy market and distribution participants necessary. It includes consumers, prosumers, distribution system operators, transmission system operators, producers, and sellers.
Furthermore, according to Article 4, member states need to formulate national renewable energy action plans which regard the forecast of estimated excess production of energy from renewable sources and the estimated demand for such energy. It allows transferring such energy to other member states to respond to their demand if it cannot be satisfied by domestic production. Similarly, cooperation among member states or member states and other countries on all types of joint projects regarding the production of electricity from renewable energy sources, which may involve private operators, is encouraged.
Member states shall also take appropriate actions, according to Article 16, to develop the transmission and distribution grid infrastructure. This includes intelligent networks and storage facilities. It is to support energy generation from renewable sources as well as its distribution and transmission to obtain interconnection among member states and between member states and other countries. Thus, it mandates focusing on interoperability and the exchange of data, and not only energy, among all involved parties and systems.
While dispatching electricity generation installations, priority must be given to installations using renewable energy sources.
According to the most recent report on renewable energy sources by REN21 [
An overview of the current trends in installed renewable power is provided in Table
Total installed renewable power (GW), broken down by technology [
Technology | Start 2004 | 2013 | 2014 | 2017 |
---|---|---|---|---|
Hydropower | 715 | 1,018 | 1,055 | 1,114 |
Biopower | <36 | 88 | 93 | 122 |
Geothermal power | 8.9 | 12.1 | 12.8 | 12.8 |
Solar PV | 2.6 | 138 | 177 | 402 |
CSP | ||||
Wind power | 48 | 319 | 370 | 539 |
Total | 800 | 1,578 | 1,712 | 2,195 |
In spite of the growing trends, CSP is in tight competition with PV technologies which, as of 2015, constitute 40 times more installed power.
Fresnel CSP plants range from 10 to 100 MW, while trough and tower installations can provide as much as 250 MW of electrical power. Energy costs of tower CSP plants amount to 0.11-0.145 EUR/kWh, and they grow to 17-38 for trough and Fresnel installations [
A breakdown of the cost of electricity production (LCOE) for CSP is presented in [
It needs to be pointed out that it looks optimistic in the light of the data from the same source which states that the CSP global capacity reached only 11 GW in 2017. On the other hand, increased transmission capacity (deployment of long-distance networks) should further increase the competitiveness of CSP.
An important advantage of CSP, compared to other renewable technologies such as solar photovoltaic (PV) plants, is its flexibility. CSP plants feature short-term heat storage, which allows them to provide more constant output even during periods of cloudy weather or after sunset. Even though this feature is mentioned in the literature, CSP plants are usually analysed with regard to their capacity of competing in baseload operations. As mentioned in Section 3.1, the baseload focus is reflected in the design of current CSP plants.
Application of state-of-the-art operational data analytics and planning algorithms may further increase the flexibility and controllability of CSP plants. CSP technology may prove more competitive if considered not only as an alternative to baseload production by fossil plants but also as a dispatchable source which may provide power on demand. However, this may require a shift in the design methodology, with more stress placed on the energy storage technologies and application of intelligent algorithms both to assess the parameters of plants and to control their operation.
Electric energy is a unique commodity. The instability of power systems and the complexity of energy storage require constant monitoring of the demand-supply balance [
Analysing the volume of output of the primary energy in the EU in the period 2004-2016, we observe a negative trend for the majority of energy sources, except for renewables (Table
Total production of primary energy in EU-28 and in Poland, 2004-2014.
Year | EU-28 | Poland |
---|---|---|
2004 | 930.1 | 78.5 |
2005 | 900.2 | 78.2 |
2006 | 881.6 | 77.2 |
2007 | 856.5 | 72.0 |
2008 | 850.7 | 70.8 |
2009 | 816.0 | 67.1 |
2010 | 831.6 | 66.9 |
2011 | 802.9 | 67.9 |
2012 | 794.3 | 71.1 |
2013 | 789.7 | 70.5 |
2014 | 771.6 | 66.8 |
Source: Eurostat (online) (cit.2016-02-17), available: data codes: ten00076. The energy balances (also called energy balance sheets) are expressed in thousands of tonnes of oil equivalent (ktoe). The tonne of oil equivalent is a standardised energy unit defined as a net calorific value of 107 kilocalories (41,868 MJ), which is roughly the net energy equivalent of a tonne of crude oil. Mtoe: million tonnes of oil equivalent.
Primary production of energy in the EU-28 in 2016 dropped by 1.5% compared to 2015 (Table
Primary energy production and share of each fuel to total production in EU-28 and in Poland, 2010-2014 (Mtoe).
Region | Total production (Mtoe) | Solid fuels | Oil (total) | Natural gas | Nuclear energy | Renewable energy | Wastes (none ren.) |
---|---|---|---|---|---|---|---|
2010 | |||||||
EU-28 | 831.6 | 164.0 | 97.1 | 159.8 | 236.6 | 163.0 | 11.1 |
Poland | 66.9 | 55.1 | 0.7 | 3.6 | 0.0 | 6.9 | 0.6 |
2011 | |||||||
EU-28 | 802.9 | 166.6 | 84.8 | 141.7 | 234.0 | 162.2 | 13.6 |
Poland | 67.9 | 55.3 | 0.7 | 3.9 | 0.0 | 7.4 | 0.6 |
2012 | |||||||
EU-28 | 794.6 | 166.1 | 76.7 | 133.2 | 227.7 | 177.4 | 13.6 |
Poland | 71.1 | 57.5 | 0.7 | 3.8 | 0.0 | 8.5 | 0.6 |
2013 | |||||||
EU-28 | 790.3 | 155.8 | 71.6 | 131.8 | 226.3 | 192.8 | 12.0 |
Poland | 70.5 | 56.8 | 0.9 | 3.8 | 0.0 | 8.5 | 0.5 |
2014 | |||||||
EU-28 | 771.7 | 149.3 | 70.0 | 118.0 | 226.1 | 195.9 | 12.4 |
Poland | 66.8 | 53.6 | 0.9 | 3.7 | 0.0 | 8.1 | 0.5 |
2015 | |||||||
EU-28 | 766.6 | 144.9 | 75.1 | 107.3 | 221.5 | 204.7 | 13.0 |
Poland | 67.3 | 53.6 | 0.9 | 3.7 | 0.0 | 8.6 | 0.5 |
2016 | |||||||
EU-28 | 755.4 | 132.2 | 74.0 | 107.3 | 216.8 | 210.8 | 14.3 |
Poland | 66.4 | 52.1 | 1 | 3.5 | 0 | 9.0 | 0.8 |
Source: Eurostat (online) (cit.2016-02-17), available: data codes: ten00076, ten00080, and ten00081.
Share of each fuel to total production renewable energy in EU-28, 2013-2014.
Year | Total production (Mtoe) | Biomass & waste | Hydropower | Wind | Solar energy | Geothermal energy |
---|---|---|---|---|---|---|
2013 | 192.0 | 64.2 | 16.6 | 10.5 | 5.5 | 3.1 |
2014 | 195.8 | 63.1 | 16.5 | 11.1 | 6.1 | 3.2 |
2015 | 204.7 | 63.4 | 14.3 | 12.6 | 6.4 | 3.2 |
2016 | 210.8 | 63.8 | 14.2 | 12.3 | 6.3 | 3.2 |
Figures do not sum to 100% due to existence of other fuels. Source: Eurostat (online data codes: nrg_100a and nrg_107a).
The key elements of electric energy supply strategy are price and supply reliability. Legal regulations on the level of the EU as well as individual states are among the factors determining the price of electric energy. They influence the market’s daily valuation and create long-term pricing conditions which are mainly connected with the basic premises of the EU climate policy. The policy regulates the models of state energy markets, the security and solidarity mechanisms, and the instruments for the unification of the EU market. Legal solutions of individual states influence the energy pricing as well. All kinds of administrative interventions on the energy market can complicate the operation of energy entities and increase the unpredictability of prices. Also, different sorts of tariffs can make it hard to properly valuate energy and determine market risk. Another factor is the RES and cogeneration support systems, which are promoted for two reasons: they are efficient and help reduce the emission of carbon dioxide and other harmful substances. Lastly, the capacity mechanisms which ensure the right and effective capacity level in entities are independent of atmospheric conditions [
The energy market is a mechanism which helps ensure the capacity needed to cover the demand for energy and maintain reserves necessary for the security of supplies in advance. The market can take two forms: it is either
As an example, the Polish energy market is a single-good market with two reserves: strategic and operational. The strategic reserve is maintained by the system operator in order to ensure long-term energy security in case of a malfunction in one or more basic generating units. As the starting of the reserve’s blocks takes about 8 to 10 hours, the decision to use it must be made in advance. Contracts for 830 MW for the period 2016-2020 have been concluded. The maximum annual cost of the strategic reserve is 40 million EUR. It still uses old blocks which have not been abandoned yet because PSA (the electricity supply operator) decided they would ensure better security for the expected capacity shortage. The operational reserve is to ensure the system’s ability to continuously balance capacity and rapidly recreate regulatory capability. The operational reserve mechanism was introduced in 2014 with a value of 102 million EUR. In 2015, it was reduced to 92 million EUR and then increased to 114 million EUR in 2016. The mechanism allows transferring of surplus capacity from system sources to the reserve. This way, the generating entities are remunerated for their readiness to supply additional power on weekdays between 7 AM and 10 PM. In order to maximise profitability, they can also choose the destination of their surplus, which can be directed to the energy market, the balancing market, or the operational reserve. The introduction of the reserve caused the wholesale prices on the energy market to increase. Other resources available in the Polish energy system are the resources of the demand side of the market and import through interconnections with the power systems of neighbouring countries of 8 GW, including 6.5 GW for the EU. However, so far, they have not been used extensively due to delays in market coupling. In 2014, only 2% of the total annual energy supply originated from imports.
While determining the costs of energy for businesses, the electric energy price is an element of international competitiveness. It varies much among the EU member countries in comparison with the prices of fossil fuels on global markets. The prices of energy in the European Union are regulated by the demand and supply in individual countries, level of excise duty and taxes, geopolitical situation, industry costs, import volume, and environmental circumstances with a particular consideration on weather conditions. The price determinants in a given country can be divided into international and domestic. The former are prices of primary fuels, prices of emission certificates (considering the carbon dioxide emission costs), currency exchange rates, development of transborder connections, and volume of energy import and export. The domestic factors include the economic growth tempo, domestic energy mix, prices and consumption of fuels, labour costs, energy sector investments, and participation in power exchanges. The result of varying local conditions is a considerable energy price discrepancy in the EU, although the change directions are convergent. Table
Comparison of prices for a kilowatt-hour of energy in Euros in the first halves of 2014 and 2015 in EU-28 and in Poland. Half-yearly electricity prices 2014-2018 (semester 1; EUR/kWh).
Region | Electricity prices (EUR/kWh) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
Households | Industry | |||||||||
2014s.1 | 2015s.1 | 2016s.1 | 2017s.1 | 2018s.1 | 2014s.1 | 2015s.1 | 2016s.1 | 2017s.1 | 2018s.1 | |
EU-28 | 0.203 | 0.208 | 0.205 | 0.204 | 0.205 | 0.123 | 0.121 | 0.116 | 0.114 | 0.114 |
Poland | 0.142 | 0.144 | 0.133 | 0.146 | 0.141 | 0.083 | 0.088 | 0.081 | 0.088 | 0.088 |
Source: Eurostat (nrg_pc_204 and nrg_pc_205).
By its nature, renewable generation is not reliable in terms of energy stability. Thus, prediction and planning plays a major role in such a case [
It increases the reliability and flexibility of such generation. While storage deployment decreases generation risks, it simultaneously increases maintenance risks, which contribute in more complicated installations.
The main energy storage technologies are pumped hydro, thermal, battery, and flywheel [
Storage technology rated power worldwide.
Technology type | Number of installations | Rated power (MW) | % | Without PH (%) |
---|---|---|---|---|
Pumped hydro (PH) | 350 | 180627 | 95.35 | |
Thermal (TH) | 202 | 3615 | 1.91 | 41.06 |
Electromechanical (EM) | 69 | 2611 | 1.38 | 29.66 |
Electrochemical (EC) | 902 | 2572 | 1.36 | 29.21 |
Hydrogen (HY) | 9 | 6 | <0.01 | 0.07 |
Among the other technologies with widespread use, thermal storage takes the first place. It offers high energy density and, depending on HTF, economic viability while offering high ramp rates. Moreover, it is not limited geographically and it does not influence the environment. Its deployment is on the rise, starting with 0.4 GW in 2007 and reaching 3.6 GW in 2016. Advancing this type of storage, focusing on HTF and CSP, is one of the U.S. Department of Energy (DOE) long-term goals [
Besides the economic barriers—mostly high capital costs—there are also other issues related to regulatory policies, market, utility business model, and technology regarding storage deployment [
Since molten salt-based power plants were designed from the ground up for base load generation, they address these storage problems; in particular, certain characteristics of molten salt-based thermal energy storage need to be pointed out [
Cost analysis for a plant with gross power output at 165 MW level is presented by Pacheco et al. [
Current energy markets are characterized by dynamically changing power demand. Furthermore, they have to comply with strong legal regulations. As a result, market players struggle to provide agile response while optimising technological and economic aspects of energy generation, distribution, and transmission. Particular plants have to plan ahead to maximise profits and minimise risk. Smart grid technology is a key enabler for the required dynamic control and data exchange.
Concentrated solar power plants belong to the category of absolutely clean sources of renewable energy. Therefore, their development matches the EU policy that consists in increase of renewable energy penetration and, on the other hand, tightening of environmental protection regulations. This type of renewable energy, however, cannot be stored directly. Therefore, energy reservoirs for such a generation must be used. Molten salt storage facilities are reliable solutions for this problem. Applying energy reservoirs in the power industry allows us to store energy at the industrial level which results in increasing control abilities at a single power plant level, the grid level, and the system level spanning across multiple countries. Increased control abilities increase stability of the power system and, as a consequence, strongly influences economic aspects. It regards both system management and energy market stability. It should be also mentioned that molten salt reservoirs are conjugate to concentrated solar power harvesting due to the lack of additional energy conversion. Such a solution allows us broader exploitation of solar energy which is one of the few absolutely clean energy sources. This is crucial in the context of protection of the environment.
Moreover, current research in the field of molten salt-based generation aims at shifting its application from the baseload to a more flexible, agile one. This need arises from increasing dynamic characteristics of energy market demand.
Given the extra flexibility provided by using molten salt energy storage and intelligent control, such plants can also be used as supplementing installations for other types of renewable generators, for instance, wind turbine farms.
It should be stressed that the considered topic is connected with the stream of studies that is concerned with the operation possibilities of systems in general. The more autonomous a system is, the more operation possibilities it has. The theory of autonomous systems, strongly referred to as multiagent system theory and game theory, has been developed intensively since the 1960s [
The authors declare no conflict of interest.
This research was supported by the AGH University of Science and Technology grant number 11.11.120.859 and the Horizon 2020 PreFlexMS Project (#654984), AGH University of Science and Technology grant number 27.27.120.70.54.