D. Verdier, Alain Ferriere, Q. Falcoz, F. Siros, R. Couturier
To cite this version: D. Verdier, Alain Ferriere, Q. Falcoz, F. Siros, R. Couturier. Experimentation of a High Temperature Thermal Energy Storage Prototype Using Phase Change Materials for the Thermal Protection of a Pressurized Air Solar Receiver. Energy Procedia, 2014, 49, pp.1044 - 1053. 10.1016/j.egypro.2014.03.112. hal-03368238 Energy Procedia 49 ( 2014 ) 1044 – 1053
Available online at www.sciencedirect.com ScienceDirect 1876-6102 © 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://www.frankenthalerfoundation.org Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections. doi: 10.1016/j.egypro.2014.03.112
SolarPACES 2013 Experimentation of a high temperature thermal energy storage prototype using phase change materials for the thermal protection of a pressurized air solar receiver D. Verdier a, *, A. Ferrière b , Q. Falcoz b , F. Siros c , R. Couturier d a PhD student, CNRS-PROMES UPR 8521 Laboratory, 7 rue du Four Solaire, 66120 Font-Romeu-Odeillo-via, France bProfessor and Associate Professor, CNRS-PROMES Laboratory, France c Researcher Engineer, EDF R&D, France dResearcher, CEA-LITEN, France
The work addresses the issue of fast variations of temperature of a central solar receiver under cloud covering. A specific attention is paid to the situation of Hybrid Solar Gas Turbine (HSGT) systems using pressurized air as Heat Transfer Fluid (HTF), as it is considered in the Pegase project (France). A Thermal Energy Storage (TES) unit integrated in the receiver is proposed for smoothing the variation of temperature. The technology is based on the utilization of both Phase Change Material (PCM) and metallic fins in order to enhance charge and discharge capability of the storage unit. A test-bench is designed with copper fins and is experienced with paraffin wax and with Li 2CO 3 successively as PCMs. In the same time, the test unit is modeled and the charging and discharging modes are simulated. The results show that the full charging is achieved in about 4 hours starting from 700 °C when the receiver is maintained at 900°C, whereas the discharge from 900°C to 700°C is achieved in 2.5 hours. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG.
Keywords: CSP ; HSGT ; central receiver system ; high temperatures ; thermal energy storage ; phase change material. * Corresponding author. Tel.: +33 4 68 30 77 43 ; fax: +33 4 68 30 77 99. E-mail address: pcb@frankenthalerfoundation.org © 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://www.frankenthalerfoundation.org Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections. D. Verdier et al. / Energy Procedia 49 ( 2014 ) 1044 – 1053 1045
This paper is dedicated to the design of a test-bench for a low capacity Thermal Energy Storage (TES) system for Concentrated Solar Power (CSP) plants. This work is related to the research project aiming at developing a prototype of Hybrid Solar Gas Turbine (HSGT) power plant to be installed and experienced in the solar tower of Themis (France). In this tower technology, air is the working fluid used in the power block (gas-turbine). Air is also the HTF which is preheated in the solar receiver before feeding the combustion chamber and the expander downstream. This hybrid system does not require any TES unit to provide firm capacity and dispatchable power. The advantage of a TES unit of high capacity for such a hybrid system is presented and discussed by Grange et al. [1]. Various technologies of pressurized air solar receivers are currently offered or investigated by the developers and promoters of HSGT systems. The technology of pressurized air solar receiver developed by CNRS/PROMES and CEA/LITEN with the support of EDF features a rather low thermal inertia. It is therefore expected that cloud covering of the solar field results in strong thermo-mechanical stresses on the solar receiver. Moreover, fast variations of the air temperature at the outlet of the receiver make difficult the control and the stability of the combustion chamber and expander regime. To address this issue, we intend to integrate a low capacity TES unit to the solar receiver. This TES unit is intended to stabilize the outlet air temperature in case of cloud covering, thus protecting the solar receiver and other critical components downstream. This concept of “protection TES” is completely different from the conventional “production TES” which is generally intended to shift and/or maintain the generation of electricity in the late hours of the day according to the demand of the grid.
The solar receiver represents a big share of the total investment of the plant (20 % for Gemasolar tower plant, Spain [2]). It must combine an elevated solar absorption factor with a high heat transfer capability with the Heat Transfer Fluid (HTF). The solar receiver must also resist to high temperatures, thermal chocks, corrosion, oxidation, and keep high thermal performances during decades. According to a criterion proposed by the Solar Heating and Cooling Program of the International Energy Agency (IEA/SHC), the decrease of thermal performance of solar receivers should remain lower than 5 % over 25 years. Kunic [3] has demonstrated the limits of this criterion, arguing that it is only appropriate for selective surfaces operating below 500 K. This author also established that protection storage is strongly valuable to keep high performances of solar receivers operating at high temperature. As an example, the degradation of the solar receiver of SolarTwo CSP plant has been observed [4]. This solar receiver was operated during more than 1 500 hours before being dismantled. The stainless steel tubes, coated with black paint Pyromark 2500, have endured thermal and mechanical fatigue. Paint was removed in few areas, and cracks have propagated in the steel, caused by strong thermal stress.
The Pegase project (Production of Electricity with GAs turbine and Solar Energy) is currently carried out by Nomenclature Greek symbols L latent heat (J/kg) β kinetic constant (-) m mass (kg) O thermal conductivity (W/m.K) P power (W) ρ density (kg/m 3) q mass flow rate of air (kg/s) Q heat source term (J/m 3) Latin symbols T temperature (K) Cp heat capacity (J/kg.K) t time (s) fl fraction of liquid of PCM 1046 D. Verdier et al. / Energy Procedia 49 ( 2014 ) 1044 – 1053 CNRS/PROMES, France, with research and industrial partners (CEA/LITEN and EDF). One major work package of this project has focused on the design of a solar receiver capable to heat 8 kg/s of air in the range 6-10 bar from 350°C at the inlet to 750°C at the outlet, with a pressure drop below 300 mbar. The purpose is to demonstrate a solar share of 60% for the 2 MW HSGT plant and to achieve a technology transfer to the industry sector in the short to medium term. A pilot scale solar receiver of 450 kW th has been designed and is currently under testing to validate the main options and to assess the optical and thermal performances. The receiver is made of metallic materials. It features a flat surface modular and multistage absorber and a cavity for enhancing the efficiency. The pilot absorber is made of 16 modules, each of them having an area of 20 cm x 40 cm. At design point, the absorber temperature is estimated to reach 860°C for an outlet air temperature of 750 °C. Starting from this design point, Grange [5] established that a 15 minutes cloud covering yields a drop of outlet air temperature down to 400 °C. By integrating a TES unit to the absorber (see Fig.1), our objective is to keep the outlet air temperature above 600°C after 15 minutes of full shadowing of the solar field. Fig.1. HSGT principle with TES module integrated in the receiver For this purpose, a small storage capacity is sufficient but a high discharge capability is needed. The stored energy must be released and transferred to the air in a range of temperature 600 – 750°C. The TES unit must also have a high density because it will be integrated at the back of the receiver.
Phase change materials are known for their high capacity of thermal storage. A study of Farid [6] has listed the main PCMs for solar energy applications. In addition, Zabla [7] has given an inventory of more than 150 PCMs over the last 20 years. He has referred to hydrated salts, paraffin wax, fatty acids and eutectics of organic and non-organic compounds. According to Kenisarin [8], the basic requirements imposed upon phase change heat storage materials are: the demanded melting temperature, a high thermal capacity and conductivity, a reliable convertibility, a minimum volume change during the phase change, an insignificant undercooling, a chemical stability and resistance with other material of the TES, nontoxic, flame safety, availability, and low cost. In Zabla’s list of PCMs, we have selected inorganic substances for our study because of their resistance at very high temperatures. The lithium carbonate (Li 2CO 3), an inorganic PCM, is well suited referring to Kenisarin’s criteria . Li 2 CO 3 melting temperature is calibrated at 723°C, which is within the operation range temperature of our study. Its thermal conductivity is among the best in its class [7], and its latent enthalpy is relatively high. In experimental analysis carried out at CNRS/PROMES, we have observed that the Li 2CO 3 does not present a high difference between the solid and liquid density and that its undercooling is negligible. In addition, Li 2 CO 3 is nontoxic, flame safety, and available enough D. Verdier et al. / Energy Procedia 49 ( 2014 ) 1044 – 1053 1047 for our application. However, Li 2CO 3 has to be pure, indeed a mixture of carbonates (containing lithium, sodium and potassium carbonates) produces a significant decrease of the melting point. Inorganic substances can also be source of undercooling and corrosion. The thermal conductivity of the storage medium is a key parameter in our work since it will directly affect the heat exchange between the TES module and the receiver and thus affect the discharge efficiency of the storage. Li 2 CO 3 offers a low value of thermal conductivity over the range of temperature which is considered here, i.e. 2.6 W/m.K (see Table 1). This is the reason why we have investigated how to enhance this thermal conductivity. Among the all techniques for thermal conductivity enhancement, we have retained the coupling of a metallic matrix filled with PCM such as Hasse [9] has done with a honeycomb wallboard filled with paraffin wax at low temperature (50°C average). Indeed the use of metal, because of its very high thermal conductivity, is a good solution to provide the heat transfer into the PCM. High temperatures impose the structure of the storage to be robust. Therefore encapsulation techniques, such as micro-integration of graphite particles into the PCM, would not be appropriate. Aluminum and copper have been preliminary chosen because they are often used in such applications. But aluminum has been excluded because of its low melting temperature. Besides, copper presents a high thermal conductivity and remains solid until 1 080°C despite its higher cost and density and its lower thermal capacity, compared with aluminum. Finally lithium carbonate and copper have been selected to be used for the design of the TES module; their main properties are listed in Table 1. We have also checked the thermal compatibility of these materials in an experimental work by putting a sample of copper into a container filled with lithium carbonate. We have observed the formation of a thin layer of copper oxide which does not progress in time and seems to protect the sample from further oxidation. We concluded that there is no major physicochemical interaction between both materials at very high temperatures, in the range 100 – 1 000°C.
The objective of the design is the study of the thermal behavior of a TES cross-section at different temperature levels from 600°C to 800°C. For this purpose, we have designed a TES test-bench by calculating our needs in terms of energy for one module. First we evaluate the needed energy to maintain the outlet air temperature of one module of the receiver above 600°C using the following calculation: ܧௗௗ ݉ൌ ሶ ܥ ݐ ܶο ௦௧ ൌ ͲǤͳ כ ͳͲͷͲ כ ሺͻͷͲ െ ͺͲͲሻ כ ሺͳͷ כ ͺͲሻ ൌ ͳͶǤʹ ܬܯ (1) Second, we express this needed energy in terms of sensible and latent heat that storage materials ( i.e. copper and PCM) can provide.
