Comparison of Enhanced Organic Rankine Cycles for Geothermal Power Units

Binary cycles have drawn the attention as a technical solution for the geothermal power production. This attention is mainly due to the huge potential of medium-low temperature geothermal sources, typically exploited by means of a binary cycle, and the relevance of the environmental concern, which can be conveniently dealt with by means of a closed cycle. The binary cycle has been therefore the object of an extended research activity, in order to attain higher plant performance. A crucial matter is the improvement of the heat introduction process. For a given geothermal fluid in liquid state, i.e. for a variable temperature heat source, in a conventional ORC the working fluid evaporation process is responsible for an important second law loss: removal of this loss allows greater power and possibly higher cycle efficiency to be attained. Aim of the present paper is to investigate and compare recently proposed technical solutions based on the current technology, which do not entail considerable operating risk or relevant investment; they can however lead to an improvement in plant performance and economics. The selected cycle options were dealt with in the open literature, and try to reduce the heat introduction second law loss: in the first one, the so called OFC, this loss is strongly reduced, because heat is introduced in the cycle when the working fluid is in liquid phase, but a dissipative flash process is then required. In the second one, the so called Pinch Point Smoother, this loss is reduced because the working fluid heating curve is smoothed by means of a flow split, which allows a fraction of the working fluid flow to evaporate at a pressure lower than the pressure of the main flow, but mechanical recompression is then required to inject the separated flow fraction into the turbine. The result of comparison may depend both on the temperature level of thermal sources involved and on the working fluid selected: the present paper will discuss several examples, representative of geothermal applications, and try to assess whether the adoption of these solutions can be convenient for geothermal exploitation

Thermodynamic assessment of liquid metal–steam USC binary plants to break 50% efficiency in pulverized coal plants

Nowadays the state-of-the-art technology to convert coal energy of combustion into electricity is to adopt a pulverized coal boiler coupled with an Ultra Super Critical (USC) steam cycle. The total installed capacity of this well-proven configuration is of hundreds of GW worldwide with an increasing share respect to both supercritical and subcritical cycles. Typical coal USC cycles have maximum pressures of around 270 bar and maximum temperatures of 600-620°C for the high pressure and the mid pressure steam respectively. Maximum attainable efficiency is close to 45% in favorable locations and is mainly penalized by two irreversible processes: coal combustion (about 30%) and heat introduction (about 10%) that is characterized by large temperature differences between the hot flue gases and the steam. The main strategy to reduce the second loss is focused on the development of new super alloys able to withstand higher temperatures, higher pressures and water corrosion and so bring efficiencies close to 49% in the so called Advanced USC plants (AUSC). However, the increasing of maximum cycle pressure and temperature results in a relatively small increase of cycle efficiency due to the large increase of specific heat around the critical point but, on the other hand, it involves a considerably increase of equipment’s cost. Another option to increase cycle efficiency is represented by the introduction of a high temperature and low pressure power cycle between the flue gases and the steam cycle. In this case, the topping power cycle could be (i) an external combustion gas cycle, (ii) an open gas cycle fueled by syngas produced by coal gasification or (iii) a Rankine cycle that uses a proper working fluid with a very high critical temperature. This study aims to define a number of optimized binary plant configurations with saturated Rankine potassium cycle as top cycle and a conventional USC plant as bottom cycle. Top cycle receives heat from the flue gases within the coal-fired boiler while bottom cycle recovers heat from the top cycle fluid condensation and the flue gases cooling before the Ljunström air preheater. Potassium thermodynamic properties are computed with a proper equation of state calibrated on experimental data from reference [2] and able to predict accurately both the volumetric and the thermodynamic behavior of potassium in liquid, vapor and two-phase conditions. Different liquid metal cycles have been designed and the trends of the main quantities (heat of condensation, turbine isentropic enthalpy drop and plant efficiency) have been correlated to both evaporation and condensation temperatures. This information is implemented in the USC scheme, calculated with an in-house process simulation code GS developed at the Department of Energy at Politecnico di Milano [3], which has been validated and used on hundreds of publications and projects. Analysis is completed by the evaluation of the potassium turbine design in terms of number of stages, need of cross-over and optimal rotational speed. A double condensation level configuration is also considered for the top cycle in order to further reduce the temperature difference between the top cycle condensation and evaporation process in the bottom cycle, which further increases the efficiency. The thermal input of coal to the burner is fixed for all the simulations to 1.66 GW, five plant configurations have been selected as the most promising ones and fairly compared with a conventional USC coal-fired power plant having a calculated efficiency equal to 44.72%. Limiting the maximum potassium temperature at 800°C, which corresponds to an evaporation pressure of 1.5 bar, it is possible to reach electric efficiencies close to 51% with a single condensation level top cycle and value close to 52% with a double condensation level top cycle. Power produced by the metal cycle ranges between 25 and 30% of the net system power output. As general conclusion the adoption of binary cycles with a top Rankine liquid metal cycle is demonstrated to be an attractive option from a thermodynamic point of view leading to an electric efficiency larger than in AUSC plants. However, these binary metal-steam cycles still need to face a number of technical and safety issues mainly related to the use of liquid metals. Technical issues are related to the high temperature of heat exchange surface of the boiler, to the very high vacuum at condenser, the need of limiting air leakages and the design of a turbine expanding a fluid with an increasing liquid fraction. Safety issues are due to working fluid reactivity with water that requires the need of expensive solution to limit fire hazard. [1] World Energy Council, 2016. World Energy Resources: Coal. [2] Reynolds, W.C. Thermodynamic properties in SI - graphs, tables and computational equations for 40 substances. Department of Mechanical Engineering, Stanford Univ., 1979 [3] GECOS, GS software. www.gecos.polimi.it/software/gs.ph

Adoption of CO2 blended with C6F6 as working fluid in CSP plants

The adoption of CO2-based mixtures as power block working fluid for CSP plant can turn supercritical CO2 cycles into efficient transcritical cycles even at high ambient temperature, with significant performance improvement and potential power block cost reduction. In this work, the use of CO2+C6F6 mixture as working fluid for a power cycle coupled with a solar tower is analyzed. Two different cycle maximum temperatures (550°C and 650°C) are considered and for both configurations the overall plant design is performed. The yearly energy yield is computed with hourly data and the LCOE is minimized varying storage and cycle recuperator sizes. Results show comparable results for the innovative working fluid and for the sCO2 cyclesEuropean Union’s Horizon 2020 No 81498

Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700ºC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 15-25%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55ºC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions.Unión Europea SI-1900/10/201

Potential and challenges of the utilization of CO2-mixtures in supercritical power cycles of Concentrated Solar Power plants

The potential of supercritical Carbon Dioxide power cycles to supersede subcritical steam turbine technology in Concentrated Solar Power applications is widely acknowledged. Some differential features of the former are higher efficiency at similar temperatures (in the range from 600 to 750ºC), smaller footprint, higher flexibility and lower cost. Several theoretical and experimental R&D projects are currently working on aspects such as component development (turbomachinery and heat exchangers), system integration into the solar subsystem (receiver and thermal energy storage system), operability, materials… Nevertheless, whilst progress is being made at a very high pace, there is still a great deal of uncertainty regarding how much sCO2 technology will be able to reduce the cost of solar thermal electricity with respect to contemporary CSP technology. This is mostly caused by the sensitivity of cycle performance to ambient temperature, bringing about a large efficiency drop when this temperature exceeds 35ºC. The root cause for this performance drop is the unfeasibility of compression near the critical point, where the very high density of the fluid reduces density and, therefore, compression work. The SCARABEUS project is based on the addition of certain dopants to carbon dioxide in order to yield a working mixture with higher critical pressure and temperature. As a consequence of these modified critical properties of the fluid, compression near the critical point is enabled even at ambient temperatures as high as 40-45ºC. Moreover, at these high temperatures, condensation and compression in liquid state are still possible. The characteristics of the new working fluids have been proved to enable thermal efficiencies higher than 50% for minimum cycle temperatures as high as 60ºC, hence boosting the performance of CSP plants well beyond of the capabilities of systems based on steam turbines. This implies a substantial reduction of the cost of the plant. Nevertheless, whilst the thermal and economic performances are more favourable for CO2-mixtures, new technical challenges must be faced if the technology is to be mature: thermal stability and potential hazards of the dopants, new turbomachinery and heat exchanger designs adapted to the composition of the mixture, phase separation, materials (selection, compatibility and degradation) and others. This paper introduces the main advantages and technical potential of the SCARABEUS technology along with a discussion of the main challenges faced by the consortium in order to demonstrate the technology and beyond.Unión Europea H2020-81498

High-Efficiency Small-Scale Combined Heat and Power Organic Binary Rankine Cycles

Small-CHP (Combined Heat and Power) systems are generally considered a valuable technological option to the conventional boilers, in a technology developed context. If small-CHP systems are associated with the use of renewable energies (biomass, for example) they could play an important role in distributed generation even in developing countries or, in any case, where there are no extensive electricity networks. Traditionally the considered heat engines for micro- or mini-CHP are: the gas engine, the gas turbine (with internal combustion), the steam engine, engine working according to the Stirling and to the Rankine cycles, the last with organic fluids. In principle, also fuel cells could be used. In this paper, we focus on small size Rankine cycles (10–15 k W ) with organic working fluids. The assumed heat source is hot combustion gases at high temperature (900–950 ∘ C ) and we assume to use only single stages axial turbines. The need to work at high temperatures, limits the choice of the right organic working fluids. The calculation results show the limitation in the performances of simple cycles and suggest the opportunity to resort to complex (binary) cycle configurations to achieve high net conversion efficiencies (15–16%)

Prospects of Mixtures as Working Fluids in Real-Gas Brayton Cycles

This paper discusses the thermodynamic characteristics of the closed Brayton cycles in which the compression is placed near the critical point of the working fluid. Under these conditions, the specific volumes of the fluid during the compression are a fraction of the corresponding values under ideal gas conditions, and the cycle performances improve significantly, mainly at moderate top temperatures. As the heat is discharged at about the critical temperature, the choice of the correct working fluid is strictly correlated with the environmental temperature or with the temperature of potential heat users. To resort to mixtures greatly extend the choice of the right working fluid, allowing a continuous variation of the critical temperature. These cycles have a high power density, and the use of ordinary turbomachinery is accompanied by high capacities (tens of megawatts). In the low power range, microturbines or reciprocating engines are required. One important constraint on the choice of the right working fluid is its thermochemical stability that restricts the operative temperatures. Among the organic compounds, the maximum safe temperatures are limited to about 400 ◦C and, forecasting high temperature applications, it could be interesting to explore the potentiality of the inorganic compounds as secondary fluids in binary mixtures

Valutazione della stabilità termica di fluidi di lavoro per cicli Rankine: apparato sperimentale e risultati di calibrazione

E' presentata una apparecchiatura in grado di fornire misure della pressione di saturazione di fluidi puri e nello stesso tempo impiegabile per ricerche sulla loro stabilita' termica. Il sistema è stato collaudato: rilevando la tensione di vapore di alcuni classici e ben noti fluidi di lavoro in cicli Rankine con positivi risultati (errori medi inferiori all'1%); per il solo toluene, valutando la sua massima temperatura di impiego in cicli termodinamici. L'apparecchio approntato permette una duplice metodologia per la ricerca della soglia di stabilità termica: rilevando minime decomposizioni con misure sub-atmosferiche di pressione; evidenziando le massicce alterazioni in condizioni isoterme alle alte temperature

Thermal stability of organic fluids for Organic Rankine Cycles systems

Each organic fluid is characterized by important thermal decomposition above a specific temperature called thermal stability limit. The correct evaluation of this limit is extremely important to avoid massive cracking phenomena which cause fouling of heat exchangers surfaces, material erosion and changes of fluid thermodynamic properties. After a brief discussion about the mutual correlation between the chemical structure and the thermal stability of working fluids, a review of the main experimental data available is worked out. Finally, an experimental apparatus and a useful methodology to determine limit temperatures for the working fluids is presented and discussed

Prospects for real-gas reversed Brayton cycle heat pumps

Ideal-gasreversedBraytoncycles are shown to be intrinsically inefficient owing to the high level of turbomachinery losses. An appropriate selection of the cycle operating parameters leading to the location of the expansion process in the vicinity of the critical point, where specific volumes and turbine works are small, allows the design of regenerated gascycles with efficiencies similar to those of conventional vapour compression cycles, at least in the generation of high-temperature heat. A number of working fluids are presented (both pure substances and mixtures) yielding a good conversion efficiency at various source/sink temperatures. Basic optimization rules are given for fluids of different molecular structure. Fluids with a simple molecule (Xe, CO2 etc.) tend to produce heat at very high temperatures and with an excessive temperature change: compression staging is effective in correcting this trend. Moderate pressure ratios (2 to 4) are sufficient to yield a good cycle efficiency; however, operating pressures are intrinsically high, since a minimum pressure around pcr is in general requested. The main features of the real-gasheatpumpcycle can be summarized as the large power density, the ability to operate at high temperature with a small pressure ratio, and non-isothermal heat generation. Whenever such characteristics are of particular value, as, for example, in the production of heat for a long-distance conveyance, as needed for urban heating systems or for industrial heat networks, the real-gasreversedBraytoncycle should be examined as a possible alternative to conventional heatpumpcycles