A new thermal energy storage technology for power system ser- vices

The decarbonization of the electrical energy sector is in progress for contrasting the climate changes, with a relevant increase of the Renewable Energy Sources (RES) power plants, mostly in Dispersed Generation (DG). The adequacy and the security of power systems, with a huge penetration of RES in DG is possible with a suitable integration of energy storage. In fact, energy storages are able to provide many different services for long-term adequacy and real time security. In this framework the present paper deals with a Thermal Energy Storage (TES) proposed for power system services. The technology presented is made up of modules containing a bed of fluidizable solid particles, which can store thermal energy from waste heat, process heat and/or from electricity. Stored thermal energy can be released, e.g. as superheated steam, for thermal uses or converted into electricity, by means of steam turbines. Some possible applications are then reported explaining advantages and limits.


Introduction
To contrast climate changes, new energy policies are in progress worldwide, pursuing the goals set with the COP21 agreement [1-3]. The energy sector plays a decisive role in reducing CO2 emissions and limiting the increase of the Earth surface temperature. Targets of energy policies include the decarbonization of the energy sector, as a primary source, with its replacing with an ever-increasing penetration of Renewable Energy Sources (RES). The strong development of RES implies benefits in environmental impact of electricity generation, but, at the same time, it requires a deep change in the power systems: Large, concentrated power stations will give way to a plurality of smaller, distributed power plants, named Dispersed Generation (DG), characterized by non-programmable generation profiles.
Concerning the management and the operation of power systems, the development of RES DG will lead to new criticalities to be addressed, e.g., the reduction of the adequacy margin on the grids, a reduced power system inertia, and the lack of power plants ready for frequency and voltage control services. RES uncertainty also implies the need for greater reserve margins, with the related economic consequences for the entire system.
In this new scenario of the energy system, an important role is expected by storage systems. Various types of storage, with their respective characteristics and technical performances, can allow coping with the problems illustrated above [4,5]. Based on different technologies and features, storage can be implemented in different scales and at different levels of the power systems, ranging from large, concentrated storages to small, distributed storages.
In the present paper, a novel technology of Thermal Energy Storage (TES) is presented. The paper is organized in other 6 sections. Section 2 is dedicated to a literature review on different technologies of TES existing and under development. Section 3 provides a focus on storage services and technologies for power systems. Section 4 deals with the proposed TES technology, pointing out its main figures, its advantages and disadvantages and its positioning as energy storage technology. Section 5 includes some possible applications of the proposed TES. Section 6 contains the conclusions.
Applications of TES can be the coupling to concentrating solar power (CSP) [6,10] or combined heat and power (CHP) [11] plants, as a buffer to provide balancing services to the grid. Other applications concern off-grid buildings [7] or the use of buildings in demand response (DR) [12,13].
CSP is among the most attractive technologies, due to efficiency, environmental compatibility and scale-up potential, but lacks continuity of power generation due to the strong dependence on available solar radiation: A common solution is to incorporate a buffer TES system [10]. CHP is also interesting due to efficiency and opportunity to use a programmable RES (biomass) [14,15].
Ma et al. [6] present a modular, inexpensive TES system based on particles within a concrete silo, for concentrating solar power or grid energy storage. Meroueh et al. [9] present an electrically charged thermal energy storage system, releasing radiant heat to a supercritical Rankine cycle, to support the power grid against demand variations, using existing power units in thermal power plants to reduce capital cost. Bachmaier et al. [11] investigates the spatial distribution of TES systems in district heating networks, to increase flexibility of central CHP units and of households.DeValeria et al. [7] present a chilled water system for daily storage supporting seasonal chemical (hydrogen) storage, in a cluster of grid-independent buildings. Kohlhepp et al. [12] review large-scale grid integration of residential thermal energy storage for demand response (DR), via thermal storage attached to building heating, cooling, and air conditioning, or to domestic powerto-heat. The latter shows large and predictable DR capacities compared to smart appliances or vehicle-to-grid. Finck et al. [8] investigate an optimization strategy for scheduling various thermal energy storage technologies in an office building (building thermal mass, PCM, HW, TCM).
Mitsubishi Heavy Industries (MHI) also recently produced an article [16], referring to the use of storage systems for the flexibility of thermoelectric power plant. Attention is drawn to the way the Thermal Energy Storage can enhance electric power resilience when renewable energy supply is significant, estimating of energy-saving effects and economic efficiency.

Services
The integration of storage in power systems is generally a measure that increases the flexibility of grids or of users connected to the grids (generators and passive customers), able to give fast responses, intended as implementation times, without major changes on the operation and interventions on the equipment.
A classification of storage technologies can be made according to the different services for the power system and its usefulness for the specific operators. The mentioned services can be provided to different entities within the power system, as it is illustrated in Table 1. Characteristics and features of each technology determine the suitability to perform certain services or to provide specific applications for the different power system entities. A classification exists between power-intensive and energy-intensive performance: powerintensive technologies are able to supply high power and low energy values, being suitable for fast, short applications; on the other hand, energy-intensive technologies can supply high energy, low power values, being suitable for slow, long uses. The mentioned services are characterized by different response speed and discharge duration. The services listed by DOE/EPRI [17] can be classified as per Table 2.  Table 3. Possible applications of different energy storage technologies, elaborated from [17] 1

Role Application PHES CAES Flywheel Li-ion Sodium sulfur Lead-acid Vanadium redox flow Hydrogen Supercaps
Energy arbitrage Magaldi STEM-RES [18] belongs to TES technologies. It consists of an insulated tank 5 containing a bed of fluidizable solid particles capable of storing high temperature (over 6 600 °C) thermal energy (Figure 1). The solid particles can be heated via steam coils or 7 electrical resistors, in case it is desired to store thermal energy from heat or electricity, 8 respectively. Stored energy is returned in the form of superheated steam, conveyed via 9 the mentioned steam coils or via dedicated heat exchangers. Then, superheated steam can 10 be expanded in a steam turbine, as in traditional steam power plants. STEM-RES is made 11 up of modules with typical mass inventory of 500-1000 tons that, combining thermal and 12 electrical charges simultaneously or not, allow to reach thermal capacity in one module of 13 approximately 20 to 150 MWht, according to the desired temperature of produced steam. 14 Several modules can be coupled in parallel or serial configuration with limited land 15 requirement, to increase thermal capacity and provide steam quality to match customer 16 different needs. STEM-RES presently classifies Technology Readiness Level (TRL) 5-6, 17 "Technology demonstrated in relevant environment", within the Horizon 2020 definitions. 18 The time required to bring the technology to full maturity (e.g., TRL-8 or TRL-9) is rela-19 tively short and not determined by the technology itself but from the creation of a first-20 mover commercial application, where favorable market conditions are present; since the 21 technology is modular and uses common industrial materials and components, it can be 22 easily scaled. The bed of fluidizable solid particles allows to store thermal energy more 23 easily ensuring response time in the order of minutes. When fluidized, the solid particles 24 will behave as a fluid and the immersed elements (e.g. the steam coils and electrical heat-25 ers) can freely expand at high temperature in any direction without permanent defor-26 mations. Thanks to the solid particles properties, STEM-RES technology is open to reach 27 very high temperature of storage media, allowing to couple with high performance cycles, 28 like sCO2, with efficiencies in the order of 50%. The plant can be installed anywhere without any geographic or orographic impact 43 compared to other high-capacity energy storage systems, e.g., PHES, or CAES in 44 caves; 45 • The plant has minimal territorial environmental impact, simplified permitting and 46 licensing, short overall construction time and high social acceptability. 47 The main drawback of TES systems in power applications is the necessity to use a 48 Rankine cycle to convert thermal energy to electricity, with relevant efficiency, in the 49 range of 30% to 40%, approximately half of that of PHES and one third of that of Li-ion 50 BES.  64 The decarbonization of the power system and the diffusion of RES plants push for 65 more flexible transmission and distribution grids, to face the variability of injected power 66 and the reduction of reserves available by conventional generation for transient control 67 and quality of service. This process will be slow and over span of years (horizon 2030). In 68 a presumable first phase, RES share will be less than half of total energy generation. In 69 this phase, the following entities will contribute to the flexibility of the power system, 70 based on measures taken by grid operators worldwide: reserve, based on their modulation capability [21]. 75 Batteries, particularly Li-ion, are regarded as the most suitable technology. BESs 76 should be sufficient to provide bulk energy services, as long as the excess of renewable 77 energy occurs for a few hours per day, seasonally and in relatively modest quantities. 78 Projects are in progress around the world on the integration of BES into the power system 79 for different services, see e.g. [22,23]. 80 In a presumable second phase, RES production will exceed 50% and the mentioned 81 measures could not be sufficient. The integration of energy storage in the power system 82 could be mandatory. The size of energy storage and the variety of services towards grid 83 operators could increase based on the characteristics of the power system itself. BES could 84 not be sufficient and suitable for a large-scale diffusion, technically, economically, and 85 environmentally, considering the huge quantities of materials potentially necessary to 86 manufacture batteries and then to be decommissioned. Different technologies will be nec-87 essary, including systems with larger storage capacity, possibly combined with batteries 88 for power-intensive services. The most suitable technologies for high and very high ca-89 pacity could be PHES, power-to-fuel/gas (including hydrogen), LAES, adiabatic-CAES (in 90 caves or seabed balloons), TES (including STEM-RES). 91 Possible applications of STEM-RES could already be existing in the next decade, par-92 ticularly in hybrid systems with Li-ion batteries, considering that: 93 •

Integration in transmission and distribution grids as bulk energy storage
In developed and densely populated areas like Europe, the construction of new PHES 94 plants can take up to ten years from decision to operation, including the authoriza-95 tion process and environmental impact assessment; 96 • Bulk energy storage technologies other than PHES still have a low TRL and at least a 97 decade is required for a large-scale diffusion;

98
• PHES systems can outperform STEM-RES in power and energy capacity, based on 99 orography and hydrogeology, but energy densities of PHES is much lower than 100 STEM-RES (in the order of 1kJ/kg for PHES with 100 m head vs 320 kJ/kg for STEM 101 with 300°C temperature drop), lack modularity and suffer strong territorial con-102 straints, leading to authorization difficulties and long development and construction 103 times;

104
• STEM-RES has a maturity level of at least TRL5-6; design and construction of com-105 mercial STEM-RES systems are ordinary industrial activities; the implementation of 106 a first-mover requires a short time, lower than that of other bulk energy storage tech-107 nologies. 108 The last remark concerns inertia. In power systems with large penetration of RES and 109 BES, there is a need for inertia, presently mainly provided by rotating masses of turbo 110 generators. Research in converters is focusing on synthetic inertia, but the prediction of 111 the behavior of power systems with numerous distributed converters and artificial 112 inertia is difficult. Without any doubt, the presence of rotating machines remains a ben-113 efit for the regulation and safety of power systems. STEM-RES, making use of turbo 114 generators, is useful in this regard. 115 These issues are stressed in small, weakly interconnected, or stand-alone power sys-116 tems. In such systems, energy storage is mandatory in presence of RES. One possible application is in disused thermal power plants (whose number is des-120 tined to increase following the decarbonization of power systems) to recover power blocks 121 and create a storage hub. Turbo generators groups can be recovered (with significant eco-122 nomic savings) and connected to STEM-RES modules, which can be installed e.g. in the 123 areas dedicated to fuel depots. Storage hubs can be created with large capacity, already 124 connected to grid nodes, with no further land use, and recovering existing facilities and 125 equipment ( Figure 3). Storage hubs can be hybrid, integrating STEM-RES and BES, 126 providing both power-intensive and energy-intensive services. Another possible application is in existing thermal power plants, to increase flexibil-131 ity, in particular, to reduce the minimum acceptable load [16]. In fact, in certain market 132 conditions, steam power plants are pushed to operate at minimum load, to avoid shut-133 downs and startups. This occurrence is increasingly frequent in advanced power systems, 134 because of the increasing penetration of RES, with dispatching priority, and the conse-135 quent reduced dispatching of thermal power plants. Reducing the minimum acceptable 136 load, thermal power plants can face this market condition more advantageously, in two 137 ways: 138 1. Reducing the amount of electricity sold at minimum load point, thus reducing mar- 139 ket losses in the day-ahead market; The economic benefit depends on numerous factors, e.g., plant marginal production 158 cost at minimum load compared to electricity market price in the time frame, revenues on 159 flexibility in services markets, investment, and operating costs, and returns of the energy 160 storage system. 161 . 162 163 Figure 4. Schematic of implementation for improvement of existing steam power plants 164 165 The versatility of STEM-RES such as electric and/or thermal charge, the possibility of 166 sizing the storage system according to needs and the wide range of operating tempera-167 tures, makes it suitable for the integration into existing Combined Heat and Power (CHP) 168 plants serving industrial and civil users or district heating networks. 169 The benefits of this type of application can be summarized in the following points:

Integration in CHP systems
Partially or fully decarbonize the fossil-fueled CHP segment through renewable elec-171 tricity;

172
• Balancing of variable renewable energy in electrical systems with a high penetration 173 of wind and photovoltaic systems, thus avoiding curtailment; • Increase the flexibility of existing CHP plants, by decoupling in time electricity and 175 heat generation. 176 STEM-RES can be charged by the excess of electric energy, generated by variable re-177 newable source plant during time when overgeneration occurs (and therefore when the 178 electricity price is low/zero/negative), and by the excess of thermal energy generated by 179 the CHP plant itself when the heat demand of the user decrease ( Figure 5). 180 During discharge phase, STEM-RES can replace or integrate the thermal energy gen-181 eration of the CHP plant. In a first phase, the STEM-RES system can be integrated into existing CHP plants to 185 make the operational process more flexible, as mentioned above. In a scenario in which 186 the penetration of variable renewable energies within the electric system is strong, it is 187 possible to hypothesize to use the STEM-RES system in a stand-alone configuration for 188 the combined production of thermal and electrical energy. In this case, coupling with a 189 powerblock is necessary for the conversion of thermal energy into electricity (same as-190 sumption made in paragraph 5.2.1 are feasible for disused power plant recovery). • Versatility, being able to take advantage of both electricity and thermal energy, both 207 for charging the thermal storage system and for discharging both electricity through 208 a turbo alternator unit, as well as thermal energy in the form of superheated steam. 209 The range of STEM-RES potential applications is wide both in the power systems and 210 in all industrial processes in which there is a need for high temperature thermal energy 211 storage.