Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions: Thermodynamic analysis
Introduction
Sunlight is by far the most abundant energy resource available on earth. However, solar irradiation is relatively dilute and intermittent. Measures have been taken to overcome these obstacles in regions with abundant solar resources by concentrating solar irradiation and using the resulting heat to produce electricity [i.e., concentrated solar power (CSP)]. Solar concentrating facilities equipped with power towers with heliostat fields and parabolic dishes are capable of achieving solar concentrations in excess of 1000 suns (1 sun = 1 kW/m2). Major challenges to current CSP systems are effectively storing intermittent sunlight and operating at high temperatures, directly translating to higher efficiencies. Increasing CSP heat storage capabilities offers a renewable pathway forward toward significantly extending solar electricity production to periods when sunlight is unavailable. On-demand production greatly improves the value of CSP, and the economic competitiveness of CSP facilities is inherently tied to the capacity for energy storage (Pitz-Paal et al., 2012).
Three methods are currently employed to store solar heat: (1) sensible energy storage (SES), (2) latent energy storage (LES), and (3) thermochemical energy storage (TCES). SES systems store sensible energy using heat transfer fluids (e.g., oil, synthetic oil, molten salts, etc.). SES is commonly used for commercial CSP systems, but it affords numerous challenges due to high pressure components, cost of different heat transfer media, and temperature limitations that impede facility operations (Medrano et al., 2010). This is especially prevalent with molten salts (e.g., NaNO3 and KNO3 mixtures) that solidify below 200 °C and often degrade at temperatures greater than 600 °C (Medrano et al., 2010, Ho et al., 2014). Maintaining these temperatures is challenging, with the maximum temperature limiting the facility output work potential. An alternative SES system is a falling particle receiver, which utilizes a curtain of ceramic particles or proppants as a thermal storage medium. The particles are predicted to heat rapidly to temperatures greater than 600 °C and can be temporarily stored to retain sensible heat while remaining stable. The high temperature particles can be accessed on demand to supply process heat to the working fluid of a power cycle (Siegel and Kolb, 2009, Ho et al., 2014).
CSP facilities with LES capability store heat in a phase change material (PCM) with a high heat of fusion. Heat is stored when a thermal gradient exists between the heat transfer fluid (HTF) and the PCM. A heat exchanger then transfers heat to the PCM, storing heat by driving the PCM phase change. Heat extraction occurs when solar conditions fluctuate and the HTF becomes cooler than the PCM, removing the heat from the material. Challenges to LES include the selection of a PCM and the large thermal resistance between the PCM and HTF. A favorable PCM must have high thermal conductivity and a constant charge/discharge temperature during phase change that is tailored to the working conditions of the CSP facility (Sharma et al., 2009, Liu et al., 2012). The low thermal conductivities of most PCMs require specialized storage/heat exchange units with a series of highly conductive fins to promote heat transfer (Steinmann et al., 2009). One of the most extensively studied PCMs includes NaNO3. Previous works have assessed the eutectic mixture of NaNO3/KNO3 in a 100 kWth latent heat storage system. The system successfully stored and extracted heat from the PCM, providing process heat to a Steam–Rankine cycle despite low HTF temperatures (Bayón et al., 2010).
TCES systems utilize concentrated solar irradiation as process heat to drive reversible chemical reactions, enabling the storage of sunlight in a chemical form. Stored heat is extracted by running the reverse chemical reaction, releasing heat to drive power cycles. Previous work has resulted in an ammonia-based TCES able to deliver process heat at 475 °C to a power cycle (Lovegrove et al., 1999). Alternative TCES and solar fuels studies include two-step solar thermochemical cycles based on metal oxide redox reactions (Bilgen et al., 1977, Fletcher, 2000, Kodama, 2003, Steinfeld, 2005, Meier and Sattler, 2009, Perkins and Weimer, 2009, Loutzenhiser et al., 2010a, Neises et al., 2012), with a range of redox pairs having been evaluated for both energy storage and extraction potential (Abanades et al., 2006, Wong, 2011). Evaluations have encompassed both thermogravimetry and economic analyses for different metal oxides with the most promising redox pairs, including BaO2/BaO, Mn2O3/Mn3O4, and Co3O4/CoO (Wong, 2011). Co3O4/CoO redox reactions have been the focus of further TCES studies aimed at characterizing reaction parameters for complete and cyclic reduction and oxidation with particulate and monolithic structures (Agrafiotis et al., 2014, Pagkoura et al., 2014). Complete reduction and oxidation was demonstrated between 800 and 1000 °C. The cycle was additionally demonstrated in a rotary kiln with directly irradiated Co3O4 particles followed by off- sun re-oxidation of the reduced particles in air (Neises et al., 2012). Partnering the high energy density with encouraging experimental results, Co3O4/CoO is an excellent redox pair for TCES.
In the present work, an Air Brayton cycle is thermodynamically examined with an integrated two-step solar thermochemical cycle for heat storage based on Co3O4/CoO redox reactions. The first step is the thermolysis of Co3O4 using concentrated solar irradiation as process heat, represented as:
The particles can then be stored in an inert environment and re-oxidized in the reversible second step that does not require solar energy and releases heat, represented as:
Co3O4 is then recycled to the first step, completing the cycle. The steps of the cycle may be decoupled, enabling the long-term chemical storage of sunlight to provide the necessary heat inputs to the power cycles to produce on-demand electricity. This work builds on previous analyses that examined thermodynamic constraints for CSP facility operation and two-step solar thermochemical cycles based on metal oxide redox reactions for fuel production (Lovegrove et al., 1999b, Sturzenegger and Nüesch, 1999, Loutzenhiser and Steinfeld, 2011, Xu et al., 2011, Mahfuz et al., 2014).
Ranges for reactor temperature and O2 partial pressure were determined thermodynamically to identify temperatures above which the Co3O4 → 3CoO + ½O2 reaction is spontaneous (i.e., ΔG ⩽ 0). ΔG is given as a function of temperature and O2 partial pressure in Fig. 1. Three different O2 partial pressures were plotted to show the impact of O2 on the reaction. ΔG is positive at 800 K for = 0.001 bar, indicative of a favorable back reaction, and decreases monotonically with increasing temperature until reaching T = 1036.6 K, where the reaction becomes spontaneous. The same trend for ΔG is shown for both = 1 bar and 6 bar where the reaction is shifted further to the right according to Le Chatelier’s principle, resulting in equilibrium temperatures of 1252.8 K and 1351.8 K respectively, where the back reaction is favored.
Section snippets
Thermodynamic analysis
The Air Brayton cycle integrated with the two-step solar thermochemical heat storage cycle is schematically depicted in Fig. 2 with relevant energy flows. Concentrated solar irradiation enters the solar thermochemical reactor through a quartz window and impinges directly upon Co3O4/CoO particles entering from a cold storage bin. Particles are heated rapidly to Treactor and undergo thermolyses to O2 and CoO. The solar thermochemical reactor is maintained at a reduced pressure by a vacuum pump to
Results and discussion
Parameters for the thermodynamic and exergy analyses are provided in Table 1.
Trecover and Tturbine as functions of were determined using the energy balance across the re-oxidizer employing the thermodynamic constraints previously described and are given in Fig. 4 at Treactor = 1050 K and pcomp = 30 bar. At < 13.7 mol/s, Tturbine ∼ Tturbine,max and is greater than Treactor as calculated from the thermodynamic limit where ΔG = 0. Tturbine increases slightly with as O2 concentration increases
Verification
Thermodynamic analysis can be verified by performing an energy balance with a defined control volume that encompasses all components. This was done to compute the incoming concentrated solar irradiation, given as:
For operation at Treactor = 1050 K, pcomp = 30 bar, and = 16 mol/s both and are equal to 306 kW.
The
Summary and conclusions
A thermodynamic analysis was performed for an Air-Brayton cycle coupled to a two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions through direct integration of direct counter-flow heat exchange between the high-temperature particles and the working fluid. Cycle performance was quantified and major sources of irreversibilities (i.e., exergy destruction) were identified using exergy destruction results. The analysis used a windowed cavity receiver for a solar
Acknowledgements
This work has been financially supported by the U.S. Department of Energy, SunSHOT Initiative for the High Performance Reduction/Oxidation Metal Oxides for Thermochemical Energy Storage (PROMOTES) project: DE-FOA- 0000805-1541. Contributions by A. Muroyama were supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1148903.
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