10:30
Session 1A: CO2 Power System
Chair: Richard Dennis
10:30
20 mins
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Experimental study of supercritical CO2 heat transfer in a thermo-electric energy storage based on rankine and heat-pump cycles
Nicolas Tauveron, Edoardo Macchi, Denis Nguyen, Thomas Tartière
Abstract: Multi-megawatt thermo-electric energy storage based on thermodynamic cycles is a promising alternative to PSH (Pumped-Storage Hydroelectricity) and CAES (Compressed Air Energy Storage) systems. The size and cost of the heat storage are the main drawbacks of this technology but using crystalline superficial bedrock as a heat reservoir could be a readily available and cheap solution. SELECO2 research project1 considers a thermal doublet consisting in a “hot storage” in a crystalline superficial bedrock (e.g. granite) and a cold storage in an ice pool. The complete system includes a heat pump transcritical CO2 cycle as the charging process and a transcritical CO2 Rankine cycle of 1 – 10 MWe as the discharging process.
Various technical studies are undertaken to assess the performance of such system. Steady-state thermodynamic models have been realized to optimize system efficiency. In addition, unsteady models of geothermal heat exchanger network were developed for the ground heat storage.
An experimental device has been designed and built to test the heat-exchange performance and dynamics. The conditions are intended to reproduce real process dynamics at a laboratory scale. The heat exchanger is at 1/10e scale with a 1.6 m height and 40 mm inner diameter. Temperature (~130°C) and pressure conditions (~12MPa) follow the operating conditions of the real process coupled with a granitic bedrock. First results show that energetic and exergetic performances are better if a specific strategy of short charge and discharge cycles is employed rather than longer charge and discharge phases. Moreover experimental results will be used to improve the above-mentioned numerical simulations and to validate more complex CFD models developed within the project.
1 Project SeleCO2, grant ANR-13-SEED-0004, partners BRGM, CEA, Enertime, ENGIE, IMFT. Project website http://seleco2.free.fr/
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10:50
20 mins
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The design of CO2-based working fluids for high-temperature heat source power cycles
Silvia Lasala, Davide Bonalumi, Ennio Macchi, Romain Privat, Jean-Noël Jaubert
Abstract: The application of CO2 power cycles is advantageous to exploit high-temperature sources (500-800°C) in the case of available low-temperature heat sinks (15-25°C). However, their efficiency is strongly reduced for higher heat sink temperatures. At these temperatures, due to the low-critical temperature of CO2 (about 31°C), CO2 is in fact compressed in the supercritical vapour phase rather than in the liquid phase, thus increasing energetic demand for compression. One of the solutions envisaged to overcome this problem is to increase the critical temperature of CO2 in order to preserve the working fluid compression in its liquid phase, even in the case of heat sinks with temperatures greater than 25°C.
This research shows that the increase of CO2 critical temperature up to 45°C, by adding to CO2 a low amount of a properly selected component, enables relevant improvements of cycle efficiency, with respect to pure-CO2 power cycles. In particular, it summarizes the most relevant criteria to be accounted for when selecting CO2-additives. Moreover, the paper warns of the thermodynamic effects deriving from adding to CO2 a second component characterized by a much more high critical temperature, such as the occurrence of infinite-pressure critical points and multiple-phase liquid-liquid and vapour-liquid critical points.
Finally, the paper analyses the thermodynamic properties of a high-critical temperature CO2-based mixture, suitable for these applications, that presents multiple phase critical points. In this regard, it is specified that the paper also aims at filling a knowledge gap in the study of thermodynamic properties of mixtures presenting how do enthalpy and specific volume change in response to pressure variations in the event of liquid-liquid and vapour-liquid critical points. Finally, we present the comparison between performances of power cycles which use, as working fluid, either pure CO2 or the novel designed higher temperature CO2-based mixture.
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11:10
20 mins
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Experimental investigations on a CO2-BASED transcritical power cycle (CTPC) for waste heat recovery of diesel engine
Lingfeng Shi, Gequn Shu, Hua Tian, Liwen Chang, Guangdai Huang, Tianyu Chen
Abstract: CO2-based transcritical Power Cycle (CTPC) could be used for engine waste heat recovery as the safety and environment-friendly characteristic of fluid, which also matches high temperature of engine exhaust gas and satisfies miniaturization demand of recovery systems. In this study, a simplified version of a CTPC system was constructed as the bottoming system and experimentally investigated to recover waste heat from exhaust gas of a heavy-duty diesel engine. The CTPC hardware was unrecuperated and the turbine was replaced with an expansion valve. By monitoring key parameters of the CTPC system and DE system, good system stability and satisfying thermal states of working fluids were observed. Investigation was based on constant operating condition of engine at speed of 1300rpm (1300ES) and 1100rpm (1100ES), constant pump condition at speed of 70rpm (70PS) and 80rpm (80PS). The CTPC system performance as a function of pressure ratio was one of the main focus points. Results indicated that the change of heat absorption and efficiency of gas heater have a clear decreasing trend with an increasing pressure ratio, mainly due the decreased mass flow rate. Compared with 1100ES, 1300ES means more heat input and more net power output, and also higher thermal efficiency at high pressure ratio range (>1.4). The advantage is feeble at the low pressure ratio range (<1.4). Up to 2.05 kW net output power was expected to be obtained at 1300ES and 80ES, and 0.043 thermal efficiency was expected at 1300ES and 70PS.
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11:30
20 mins
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Fundamental thermo-economic approach to selecting sCO2 power cycles for CSP applications
Francesco Crespi, David Sánchez, José María Rodríguez, Giacomo Gavagnin
Abstract: The interest in sCO2 power cycle has grown exponentially in the last decade, thanks to distinctive features like the possibility to achieve high thermal efficiencies at intermediate temperature levels, small footprint and adaptability to a wide variety of thermal sources. In the present work, the potential of this technology is studied for Concentrated Solar Power applications, in particular Solar Tower systems with Thermal Energy Storage (TES).
A thorough sensitivity analysis based on Turbine Inlet Temperature (TIT) and Pressure Ratio (PR) is done for twelve sCO2 cycles, considering their effects on thermal efficiency (ηth) and specific work (Ws), along with the Solar Share (SS) and the temperature rise across the solar receiver (ΔTsolar). The most important conclusions of this section are that: a) maximum values of ηth, Ws and ΔTsolar are obtained for different PRs; b) ΔTsolar and Ws, unlike ΔTsolar and ηth, are almost directly proportional within technological limitations; c) for a given TIT, an increase in the PR always produces a strong growth of ΔTsolar, but the effect on ηth is uncertain as this can either increase or decrease, depending on the cycle considered.
A deeper analysis of ηth and ΔTsolarl is therefore mandatory, given that these parameters strongly affect the capital cost of CSP power plants. On one hand, a higher ηth implies a smaller solar field, the largest contributor to the plant capital cost; on the other, the temperature rise across the receiver ΔTsol is directly proportional to the size of the thermal energy storage systems, as it is also the case for state of the art steam turbine based CSP plants. An economic analysis is developed using an in-house code and the open-source software System Advisor Model (SAM) to evaluate the trade-offs between these two effects. The results obtained for the two most representative sCO2 cycles, i.e. the Transcritical Simple Recuperated and the Supercritical Partial Cooling cycles, are provided in the present work.
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11:50
20 mins
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Selection maps for ORC and CO2 systems for low-medium temperature heat sources
Marco Astolfi, Silvia Lasala, Ennio Macchi
Abstract: Nowadays, ORC is the most reliable option available on the market for the exploitation of low-medium temperature heat sources in a large range of power outputs: ORC field spans from renewable energy sources like geothermal, biomass and solar applications to waste heat recovery form industrial processes or engines flue gases. Such energy sources are characterized by either a nearly constant or a variable temperature profile. The first case includes direct solar fields and direct biomass boilers where the working fluid flows in the solar collector or in the boiler without the use of an intermediate heat transfer fluid. The maximum temperature of these cycles is defined by the thermal stability of the fluid and the heat-resistance of the material used for the hot sections of the plant. In the second case, the ORC working fluid is heated by a variable-temperature hot stream that is typically cooled down to a certain temperature limit. This is the case of, for example, geothermal brines, flue gases discharged by either a gas turbine or an internal combustion engine or waste heat recovery from a generic plant. This class also includes applications based on a HTF loop.
In recent years, the use of supercritical CO2 for power production has gained a large interest from both the Industry and the Scientific Community. SCO2 cycles are typically envisaged for large and high-temperature power plants coupled with solar tower technology or fossil fuel combustion. In these fields of application, in fact, sCO2 plants can compete with conventional steam cycles thanks to their smaller investment cost, more compact turbomachines, simpler plant arrangement, higher flexibility. Besides the already attested application of high-temperature sCO2 power cycles, this technology may be also considered as a viable solution for the exploitation of low-medium temperature heat sources, competing with ORC.
This work aims at presenting performance maps to enable the straightforward thermodynamic comparison and easier selection between ORC and sCO2 cycles, in a wide range of applications where they may compete. The analysis is thus carried out considering both constant and variable-temperature heat sources, with a maximum heat source temperature ranging from 200 to 500°C. Each point of the map provides the optimal efficiency of both ORC and sCO2 power cycles, considering their most suitable configuration currently available on the market. As regards ORC, they are modelled as saturated Rankine cycles with a limited vacuum in the condenser to limit air leakage. Several different fluids are investigated and cycles are optimized by varying evaporation and condensation temperatures and the use of the recuperator. For sCO2 cycles, different plant layouts are investigated (regenerative, recompressed, intercooled, re-heated). In all cases, the expander efficiency is evaluated with a correlation which accounts for the effect of volume ratio and last stage size parameter. The analysis is performed considering both high and low-temperature heat sinks, representative of ambient air and water. In the first case, heat is rejected to the ambient with an air-cooled condenser, limiting the sCO2 cycle to a non-condensing Brayton configuration while, in the second case, the availability of water enables the reliable condensation of CO2 thus allowing the less-power-consuming compression of highly-dense cool CO2.
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12:10
20 mins
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Comparison of micro gas turbine heat recovery systems using ORC and transcritical CO2 cycle focusing on off-design performance
Suk Young Yoon, Min Jae Kim, In Seop Kim, Tong Seop Kim
Abstract: Micro gas turbines (MGT) are widely used for distributed generation due to the advantage of being able to produce electricity with high specific output. The exhaust temperature of MGT is still high almost 300oC in recently developed MGTs even though it is equipped with a recuperator at the turbine exit. One way to reduce the energy loss due to the high temperature gas exhaust is recovering the heat of the exhaust gas of the MGT and produce additional power using a bottoming cycle. In this study, we applied two heat recovery systems to an MGT and compared the performance of them. They are an organic Rankine cycle (ORC) and a transcritical CO2 cycle (tCO2). Toluene which is generally used for the high-temperature heat source is used as the working fluid of the ORC. For the tCO2 cycle, heat recuperation was adopted to maximize the bottoming cycle efficiency. System simulation was performed using HYSYS [1]. Since MGTs usually operate to follow the electric power demand, an off-design performance is also important. Therefore, we also analyzed the off-design performance of the two bottoming cycles with the load variation of the MGT. The performance curve, the effectiveness-NTU method, and the Stodola’s equation [2] were used to model the off-design operations of all the bottoming cycle components such as a pump, a turbine and heat exchangers. According to the calculation result, the power output of ORC was higher than that of tCO2 cycle when MGT operates at a full load. However, since the power output variation with MGT load change is larger in the ORC, the power output of tCO2 becomes larger when MGT operates at part load below 80%. Accordingly, we can conclude that the tCO2 is suitable for the heat recovery system utilizing MGT exhaust gas in places where the MGT needs to operate at low loads during a lot of its operating hours. On the other hand, in applications where the MGT can operate at near full load conditions, the ORC is more suitable.
REFERENCE
[1] Aspen Technology, AspenOne HYSYS, Ver. 7.2.
[2] In Seop Kim, Tong Seop Kim, Jong Jun Lee, 2016, “Off-design performance analysis of organic Rankine cycle using real operation data from a heat source plant” Energy Conversion and Management, Vol. 133, Issue 1, pp. 284-291.
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