Thermodynamic analysis of a combined-cycle solar thermal power plant with manganese oxide-based thermochemical energy storage

We explore the thermodynamic efficiency of a solar-driven combined cycle power system with manganese oxide-based thermochemical energy storage system. Manganese oxide particles are reduced during the day in an oxygen-lean atmosphere obtained with a fluidized-bed reactor at temperatures in the range of 750–1600°C using concentrated solar energy. Reduced hot particles are stored and re-oxidized during night-time to achieve continuous power plant operation. The steady-state mass and energy conservation equations are solved for all system components to calculate the thermodynamic properties and mass flow rates at all state points in the system, taking into account component irreversibilities. The net power block and overall solar-to-electric energy conversion efficiencies, and the required storage volumes for solids and gases in the storage system are predicted. Preliminary results for a system with 100 MW nominal solar power input at a solar concentration ratio of 3000, designed for constant round-the-clock operation with 8 hours of onsun and 16 hours of off-sun operation and with manganese oxide particles cycled between 750 and 1600°C yield a net power block efficiency of 60.0% and an overall energy conversion efficiency of 41.3%. Required storage tank sizes for the solids are estimated to be approx. 5–6 times smaller than those of state-of-the-art molten salt systems.


Introduction
Concentrating solar power (CSP) technologies offer the major benefit of enabling inexpensive thermal energy storage, which allows CSP plants to operate round-the-clock as well as on-demand.Hence, CSP systems are expected to play a central role in future sustainable power systems, particularly in the earth's sunbelt [1].In this study, we explore the theoretical efficiencies achievable with a solar-driven combined cycle power plant, including a topping Brayton and a bottoming Rankine power cycle, with high-temperature thermochemical energy storage.In combined Brayton-Rankine power cycles, the benefit of high turbine entrance temperatures of gas turbines (> 1000°C [2]) is combined with the high thermal efficiencies of steam Rankine cycles at intermediate operating temperatures, to maximize the operating temperature window and hence the thermal efficiency of the power system.Consequently, combined Brayton-Rankine power cycles are the most efficient commercial power conversion technology to date, with thermal efficiencies approaching 65% [3,4].
MnOx is considered as a potential thermochemical energy storage material, due to its suitable reaction temperature range, high storage density, and low cost [5].In practice, doping of MnOx with other metals may be required to improve potential issues with pure MnOx, such as sintering at high temperatures and limited reaction kinetics.However, MnOx is chosen here as a model material to develop a framework and reference for the assessment of future advanced material compositions.The reversible reduction and oxidation of MnOx between Mn2O3 and MnO occurs via the steps: The temperatures at which these transitions occur depend on the oxygen partial pressure in the gaseous environment.The Gibbs free energy minimization method has been applied, using the FactSage software [6], to calculate the transition temperatures listed in Table 1.The results show that the required temperatures for the forward (reduction) reaction steps decrease with decreasing oxygen partial pressure in the gas phase, while for the backward (oxidation) reaction steps the opposite applies.Reduction in N2 gives access to the Mn3O4/MnO transition below the slagging temperature.This transition involves a high enthalpy of reaction, and hence strongly increases the mass-specific storage capacity of the solids.The storage system can be operated with any two reversible MnOx redox pairs in Eq. (1).For example, with the redox pair Mn2O3/MnO, operated over the temperature range 750-1600°C, the thermochemical storage process is described by: Reduction:

Model system
A schematic of the model system is shown in Fig. 1.The system components are grouped into a storage block, a power block, and a heat exchanger block.The main components of the storage block are the reduction and oxidation reactors and the storage tanks for the reduced and oxidized manganese oxide particles.During on-sun operation of the system, oxidized particles are heated and thermally reduced in the reduction reactor.The energy source for the endothermic reduction reaction is concentrated solar radiation, provided by a heliostat field and a secondary concentrator (not modeled).Oxidized particles are fed to the reduction reactor in a gas flow.A cyclone separator (separator 1) is used to separate the particles from the gas flow after the reduction reactor.The hot reduced particles are stored in the reduced particle storage tank, while the hot gases provide the heat to run the power cycle.During off-sun operation, reduced particles are cooled and re-oxidized to generate hot gases that provide the heat to run the power cycle.Reduced particles are pre-cooled in heat exchanger 3 before they are fed to the oxidation reactor in a gas flow.A cyclone separator (separator 3) is used to separate the particles from the gas flow after the oxidation reactor.
The reduction reaction (Eq.2) is conducted in inert gas, modeled as nitrogen, while the oxidation reaction (Eq.3) is conducted in pure oxygen.Each reactor is operated with a closed gas loop.In the inert gas loop over the reduction reactor, oxygen is accumulated, which needs to be continuously removed from the gas loop with a gas-gas separator (separator 2) [7,8].The separated oxygen is pressurized with a gas compressor (compressor 2) and stored in a storage tank.During off-sun operation, the stored oxygen is released and expanded via a throttle, and used in the gas loop over the oxidation reactor to compensate for the oxygen consumed by the oxidation reaction.
The power block comprises a topping solar-driven indirectly-heated air Brayton cycle and a bottoming steam Rankine cycle heated by the exhaust heat from the Brayton cycle.The Rankine cycle includes a single-stage reheating process.
The heat exchanger block couples the storage block with the power block.In heat exchanger 1, the hot gas exiting the reduction reactor heats the compressed air from the Brayton cycle during on-sun operation.In heat exchanger 3, the hot particles exiting the reduced particle storage heat the air from the Brayton cycle.The oxidation reactor serves simultaneously as a reactor and heat exchanger, exchanging heat between the oxidizing particle/gas flow and the air from the Brayton cycle.
The power system is designed to operate round-the-clock at a constant power, with 8 hours of on-sun operation and 16 hours of off-sun operation.The particle and gas flow rates entering the reduction reactor (state points N2 and M5 in Fig. 1) are adjusted such that the system generates a constant power output.3 Thermodynamic model

Model assumptions
The thermodynamic model of the system shown in Fig. 1 is formulated using the following modeling assumptions.The system is assumed to be at steady state and the air standard assumptions are used.Kinetic and potential energy changes are neglected.The heat loss from the receiver is modeled as black body radiation emission.Heat losses from other system components and connections are neglected.Pressure drop in tubes or heat exchangers are neglected.The thermodynamic effect of oxygen is neglected.System design parameters are listed in Table 2.

Model equations
For the reduction and oxidation reactors, the overall steady-state energy balance reduces to: where Q is the heat rate supplied to the reactor, n denotes the molar flow rates into and where  is the Stefan-Boltzmann constant, RR T is the reduction reactor temperature, and I is the direct normal irradiation (I = 1 kWm -2 ).
For the compressors, the feedwater pump, the gas turbine and the two steam turbine stages, and separator 1, the overall energy balance is: For heat exchangers, the energy balance simplifies to: The total amount of a medium stored in a storage tank is calculated by integrating the mole balance for each single storage medium: where nnet is the number of moles of storage medium in the storage.
Separators 1 and 3 are modeled as cyclone gas-solid separators, based on the design by Wang et al., 2006 [9].Separator 1 operates at 1600°C and 1 atm.The gas-solid mixture enters the separator from the side.The gas exits the separator at the top, while the solids exit at the bottom.The pressure drop over the separator is calculated according to the model by Shepheard and Lapple 1939 [11]: where  is the density of the gas, g is the gravitational acceleration, vin is the gas velocity at the separator inlet, Hv is the inlet gas velocity head, and K is a cyclone configuration and operating condition parameter.Parameters d2, d7 and d8 are dimensions of the separator.The inlet and outlet enthalpies of the gas flow can then be obtained from their temperature and pressure.The electrical pumping power required to operate separator 1 is calculated from Eq. ( 6).Separator 3 has negligible influence on the system performance due to the relatively small oxygen gas flow rate.Separator 2 separates O2 and N2 in the temperature range of 330-370°C and at a pressure of 1 atm.Ezbiri et al., 2015 [8] showed the feasibility to separate O2 from N2 at temperatures of 327 to 627°C.The minimum required energy to separate two gas components in a mixture corresponds to the Gibbs free energy change involved when the two components are mixed.On a molar basis of the component with the smaller molar fraction, x, in the mixture, this yields [12]: where R and T are the ideal gas constant and the absolute temperature of the mixture, respectively.The actual electrical power required to separate the gas mixture is obtained from: where sp2  is the efficiency of separator 2.
The required size of each storage tank is estimated based on the molar flow rate of the material stored in the tank: E3S Web of Conferences 22, 00102 (2017) DOI: 10.1051/e3sconf/20172200102 ASEE17 where  is the particle volume fraction [13],  and M are the mass density and the molar mass of the stored material, respectively, n is the charging molar flow rate, and ton is the charging time.
The off-sun operation molar flow rate off n is calculated via conservation of mass: on on off off n t n t  (14) The combined power cycle operates under the same conditions during on-sun and offsun operation.The net thermal efficiency of the power cycle is calculated as: where gt W , st,H W , st,L W , cp1 W , pm W are the power of gas turbine, high-pressure steam turbine, low-pressure steam turbine, compressor 1 and pump respectively, while he1 Q is the heat transfer rate in heat exchanger 1.
The overall solar-to-electric energy conversion efficiency of the system is: where sp1 W , sp2 W , cp2 W are the power input into separator 1, separator 2, and compressor 2, respectively, while sun Q is the solar power input.Separator 3 only separates a small amount of carrier gas which requires negligible energy input.

Solution methods
The model equations are solved with Matlab [14].The thermophysical properties of water and air are obtained from RefProp Mini [15].Thermochemical data for O2, N2, Mn2O3 and MnO are obtained from Binnewies and Milke [16].

Power cycle analysis
The following results are calculated using the design parameters listed in Table 2.The off-sun operation duration can be set to 16 hours by changing the ratio of N2 and Mn2O3 molar flow rates (n) at the entrance of the reduction reactor.More N2 means more solar energy is used directly for on-sun operation, while more Mn2O3 means more energy is stored for off-sun operation.Numerical modelling shows that with n = 2.96, the off-sun operation duration is 16 hours.The T-s diagram for the combined power cycle is shown in Fig. 3b.During off-sun operation, the Brayton cycle is heated in two steps, which is the only difference from on-sun operation.The flow rates of the combined cycle working fluids are the same for on-sun and off-sun, while the flow rates of the thermal storage materials are different.The required volume of the O2 storage is 1.55 × 10 4 m 3 at a storage pressure of 20 atm, while the volume of reduced particle storage (MnO) and oxidised particle storage (Mn2O3) are 2.83×10 2 m 3 and 3.81×10 2 m 3 , respectively.The MnO and Mn2O3 storage volumes are significantly smaller than those of a 120 MW molten salt power plant designed for roundthe-clock operation, which are around 1.8 × 10 3 m 3 each [17].On the other hand, the required size of the O2 storage tank is one order of magnitude larger and may prove to be uneconomically large.

Summary and conclusions
In this paper, a thermodynamic model has been presented for a solar thermal power plant with a combined power cycle and high-temperature thermochemical energy storage system based on manganese oxide redox cycling.The gas-solid molar flow ratio at the reduction reactor inlet was determined for steady round-the-clock operation with 8 hours of on-sun and 16 hours of off-sun operation.The Rankine cycle reheating pressure was optimized for maximum power cycle efficiency.The overall solar-to-electric energy conversion efficiency and the net power block efficiency were determined to be 41.3% and 60.0%, respectively.Main energy losses in the system are due to blackbody emission by the solar reactor, and power demands by the pumps, compressors, and separators in the system, in addition to the heat rejected by the power cycle.For a system with 100 MW solar energy input, the volumes of reduced and oxidized particle storage tanks are estimated to be approx.5-6 times smaller than those of a comparable molten salt-based plant [17].On the other hand, the required O2 storage volume of 1.51×10 4 m 3 is comparatively large.
This paper establishes the methodology to determine the efficiency of solar combined cycle power plants with thermochemical energy storage and provides an estimation of the theoretical system efficiency for an exemplary plant design.To obtain more accurate predictions of the annual plant efficiency, year-round simulations for a particular location need to be conducted and the component models need to be refined to take into account additional irreversibilities, e.g.due to flow friction, additional heat losses, incomplete chemical reactions, part-load efficiencies, etc.It is also recommendable to study different system parameters, such as the solar concentration ratio and the reduction temperature, and to consider alternative system configurations, including the possibility of operating both chemical reactors with atmospheric air, which would eliminate the potential challenges associated with the storage of gaseous oxygen and gas-gas separation.
Financial support by the Australian Renewable Energy Agency, grant no.2014/RND005, is gratefully acknowledged.

Fig. 1 .
Fig. 1.Schematic of the model system (black solid lines: on-and off-sun operation; red dashed lines: on-sun operation; blue dotted lines: off-sun operation).

out of the reactor, o f h( 5 )
denotes the enthalpy of formation of a substance at reference temperature ref 298T  K and standard pressure p o = 1.013 bar, and a substance between temperatures T and ref T .The reduction reactor is assumed to be a cavity-type radiation receiver that behaves like a black body.The radiative heat loss by the reactor and hence its absorption efficiency depend on the solar concentration ratio, C. The absorption efficiency is given by: E3S Web of Conferences 22, 00102 (2017) DOI: 10.1051/e3sconf/20172200102 ASEE17

Fig. 3 .
Fig. 3. (a) The effect of pRC5 on Rankine cycle efficiency; (b) T-s diagram of the combined cycle for the power cycles (BC2* is only for off-sun operation).The Rankine cycle has a single-stage reheating process.The pressure at the lowpressure steam turbine outlet, PRC5, can be optimized numerically for maximum cycle efficiency (Fig3a).The resulting maximum Rankine cycle efficiency of 38.68% is obtained when pRC5 = 1.38 MPa.The T-s diagram for the combined power cycle is shown in Fig.3b.During off-sun operation, the Brayton cycle is heated in two steps, which is the only difference from on-sun operation.The flow rates of the combined cycle working fluids are the same for on-sun and off-sun, while the flow rates of the thermal storage materials are different.

Fig. 4 Fig. 4 .
Fig. 4 shows energy balance and work allocation of the system over a 24-hour period.The total daily solar energy input to the system is 800 MWh.The largest energy loss of 186.08 MWh/day is due to blackbody radiation losses from the solar receiver, which operates with an absorption efficiency, b  , of 76.74%.Additional main energy losses are due to the power input to compressors and separators.Compressors 1 and 2 and separator 2 require the largest amounts of work, amounting to 151.92 MWh/day, 19.92 MWh, and 18.88 MWh, respectively.The daily net work output is 330.32 MWh.The work requirement of the feedwater pump of the Rankine cycle is negligible.The predicted thermodynamic efficiency of the combined cycle th  , is 60.04%.The predicted overall solar-to-electric energy conversion efficiency, overall 

Table 1 .
Approximate transition temperatures for the redox steps shown in Eq. (1) and associated enthalpy changes.

Table 2 .
System design parameters.State points are shown in Fig. 1.