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Integrated Solar Combined Cycle Using Particles as Heat Transfer Fluid and Thermal Energy Storage Medium for Flexible Electricity Dispatch

• M. A. Reyes-Belmonte, M. Romero & J. González-Aguilar

§ Solar plant concept being investigated under NEXT-CSP project § Dedicated modelling of Integrated Solar Combined Cycle (ISCC) pure-solar plant that uses particles as heat transfer fluid and storage medium § Plant components

• ptimization

(solar field, receiver, Brayton topping cycle, Rankine bottoming cycle, particles-based heat exchangers network) to maximize ISCC efficiency (design-point conditions) § Multi-tower solar plant arrangement for commercial scaling-up (150 MWe) § Dispatch strategies definition to maximize electricity power output (annual performance)

Objectives

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§ Solar particles receiver was designed to reach 825 ºC § Double-reheated Brayton & reheated Rankine cycles to maximize ISCC efficiency § Dedicated design of particles-based heat exchanger network connecting solar loop & power cycle

mexh mreh mlive

Heat Recovery reheater

HP TURBINE Condenser LP pump HP pump deareator

Heat Recovery Steam Generator Particles HOT tank Particles COLD tank

Particles receiver

DPS-air HX

Compressor Turbine

DPS-air reheater DPS-air reheater

Heliostat field MP - LP TURBINE

TDPS = 825 ºC Tin = 800 ºC Tin = 585 ºC Tin = 575 ºC

Plant Layout Description

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Solar plant location Ouarzazate, Morocco 30.9°N, 6.93°W Design DNI 900 W/m2 @ noon 21st March Heliostats area 49 m2 (Stellio heliostat) Aperture incident flux 2,000 kW/m2 Thermal power onto aperture 55 MW Particles maximum temperature (at receiver

• utlet)

825 ºC Preferred commercial size (electrical power

• utput)

150 MWe

Multi-tower (& solar fields) configuration is required to achieve design- point dispatch power (150 MWe – pure solar)

Boundary conditions (design point)

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DNI (W/m2) Wnet (MW)

§ Annual DNI (15-min based) from Ourzazate § Typical electric grid demand curve from Mediterranean area

Boundary conditions (annual performance)

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Unit 4 Unit 5 Unit 6 § Multiple solar-fields & towers feeding single Combined Cycle Power Block § Particles transportation system between multi-solar fields units & common particles TES & heat exchangers network

Plant Layout Description

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Topping cycle Bottoming cycle HP inlet pressure 14.3 bar HP inlet pressure 160 bar MP inlet pressure 6.1 bar MP inlet pressure 20 bar LP inlet pressure 2.5 bar HP inlet temperature 585 ºC HP – MP – LP inlet temperature 800 ºC MP inlet temperature 575 ºC

Power cycle Solar plant & receiver

Solar Field Solar Receiver Power incident on field 75.5 MW Power onto aperture 55 MW Number of heliostats 1731 Absorbed thermal power 44 MW Heliostats area 49 m2 Thermal efficiency 79.4 % Design day noon 21st March Tubes height 7 m Design DNI 900 W/m2 Number of tubes 240 Tower optical height 110 m Particles inlet temperature 606 ºC Aperture tilt angle 30º Particles mass flow 165 kg/s Aperture incident flux 2,000 kW/m2 Receiver average flux 500 kW/m2

Design-point optimization

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C T2 T1 T3

Air (Brayton cycle) Hot particles Cold particles

from ambient

to HRSG (Rankine cycle)

Main heat addition First reheating Second reheating

HP DPS-HX1 IP DPS-HX LP DPS-HX

Cold air in Cold particles out Hot particles in Hot particles in

800 ° C 800 ° C 800 ° C 407 ° C 600 ° C 600 ° C 600 ° C To particles receiver

From particles receiver

600 ° C 600 ° C 800 ° C

HP DPS-HX2

m1 m2 m3

mtotal = m1 + m2 + m3

Cold TANK

Very regenerative configuration (double turbine reheating of the Brayton cycle) leading to reduced temperature difference across heat exchangers à high particles mass flow

Particle-based heat exchanger network

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Very regenerative configuration also leading to high temperature of “cold” particles sent back to the tanks and receiver à higher particles receiver area and storage tanks capacity Temperature (C) Specific entropy (J/kgK)

4 6 7 9 12 8

Tsteam,live + ΔT

Double reheating (higher pressure ratio) 10 13 1

407 ºC 800 ºC 600 ºC 800 ºC Mass flow distribution: § mair (HP hot DPS-HX) = mair (IP DPS-HX) = mair (LP DPS-HX) = mair (HP cold DPS-HX) § mDPS (HP hot DPS-HX) ≈ mDPS (IP DPS-HX) ≈ mDPS (LP DPS-HX) ≈ 3 · mDPS (HP cold DPS-HX)

Particles-based heat exchanger network

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Strategy A Constant power output (nominal power) from 17h – 22h 30 minutes ramp-up & ramp-down Strategy B Following grid demand during morning-peak (9h – 11h) and evening- peak (17h – 21h)

Thermal energy surplus to be stored as hot particles (tanks sizing)

Dispatch strategies (plant operation)

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Thermal power reaching the solar field Thermal power reaching particles receiver Thermal power absorbed by particles Thermal power absorbed by particles Electricity production

September 3rd Strategy A Strategy B § Energy harvesting and electricity production are decoupled. Power cycle runs taking thermal energy from storage tanks § Tanks sizing according to dispatch strategy and electricity demand § Depending on DNI, grid demand and existing TES capacity, dispatch electricity could not be enough to cover the demand Grid demand

Particles stored from previous day

Results Analysis: Daily Operation

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Electricity annual production (MWe) § Winter months: not enough thermal power to dispatch 5 hours @ full load Turbine ramp-up Turbine ramp-down Particles stored at tanks (tons) Maximum amount of particles stored during central months of the year and before power cycle

• peration (before 17 h)

Strategy A Strategy A

Results analysis: Annual performance

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Strategy B Strategy B

Results analysis: Annual performance

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• Exceeding thermal power stored as hot

particles by the end of the day (during those months when the solar resource is higher but the electricity demand is lower)

Power cycle operation hours Ø Strategy A: operation hours depending on DNI evolution Ø Strategy B: operation hours depending on grid demand curve & DNI Strategy A Strategy B

Surplus of thermal energy to be stored as hot particles (TES tanks oversized)

Results analysis: Annual performance

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§ Pure-solar Integrated Solar Combined Cycle (ISCC) optimized at design-point conditions § 2 dispatch strategies analyzed: § Constant nominal power output (17h – 22h) § Flexible dispatch power output to cover morning & evening peaks § Thermal storage sizing largely depends on dispatching scenario, so that a case- by-case analysis (dispatching, resource, demand) is necessary § Solar-to-electricity efficiency is not a good figure when similar times of full load plant operation and turbine ramp up/shut off penalizes solar-to-electricity efficiency. § More detailed analysis on operation modes based on dynamic modelling of transients and economic analysis are absolutely needed to support power plant viability

Conclusions

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Integrated Solar Combined Cycle Using Particles as Heat Transfer Fluid and Thermal Energy Storage Medium for Flexible Electricity Dispatch

THANKS FOR YOUR ATTENTION !

The research leading to these results has received funding from European Union’s Horizon 2020 research and innovation program under grant agreement No 727762, Next-CSP project. jose.gonzalez@imdea.org

Motivation

v Novel heat transfer fluid based on Dense Particles Suspension (DPS) to be used at central solar receiver and for direct Thermal Energy Storage (TES) v Excellent thermophysical properties of DPS

ü High Temperature (> 650 ºC) ü High heat transfer coefficient (> 2,000 W/m2 K) ü No freezing risk ü Cheap and abundant ü No hazardous ü High energy density CSP plant layout suitable to be coupled with several power blocks DPS HTF advantages

Plant layout proposal: A high-efficiency solar thermal power plant using a dense particle suspension as the heat transfer fluid, J. Spelling, A. Gallo, M. Romero, J. González-Aguilar. SolarPACES 2014