16:00
Parallel Session: Cycle efficiency
Chair: Rene Pecnik
16:00
20 mins
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TWO STEP OPTIMIZATION APPROACH FOR INCREASE OF ENGINE-ORC EFFICIENCY
Daniela Gewald, Andreas Schuster, Sotirios Karellas, Hartmut Spliethoff
Abstract: Due to their high electrical efficiency (up to 47 %), internal combustion engines (ICE) are widely used as independent power producers. In order to further increase the electrical efficiency, an Organic Rankine cycle (ORC) can be applied for waste heat recovery (WHR) of the ICE.
The recoverable engine waste heat is usually available at two different temperature levels: 300 °C to 400 °C (exhaust gas) and 90 °C (engine cooling water). The integration of both heat flows and especially the integration of the cooling water heat is a certain challenge for ORC operating with one specific working fluid which is usually optimized to just one temperature level. Therefore only few examples of the recovery of both heat flows can be found both in literature and as existing power stations.
This paper presents a two step optimization approach in order to increase the overall system efficiency of an engine-ORC combined cycle by means of the integration of the heat flows at both temperature levels.
In a first step the variable cycle parameters both in the topping and the bottoming cycle (e.g. fluid evaporation pressure and temperature of the cooling water) will be defined and evaluated for different working fluids (R245fa, pentane, MDM among other) within the boundary conditions given by the technical feasibility. Furthermore the application of a recuperator is investigated since recuperation and the integration of low temperature heat are often competing efforts. The effect of higher pressure losses throughout the recuperator has to be taken into account as well. This step is aimed at the definition of a set of working parameters, which leads to an optimal system configuration for each working fluid. The optimization parameter is the net electrical power output of the system.
Within the second step important constructive aspects of the engine-ORC are rated for the selected fluids. A fast indication of the expenditure of each power-optimized system can be generated by the evaluation of specific constructive parameters such as heat exchanger surfaces and volume flow rates. Based on these design parameters a power independent comparison can be obtained by calculating the influence of relative deviations of the constructive parameters from the optimal system on the net electrical power output.
By means of this two-step approach a fast assessment of an ORC system both under thermodynamic and cost-related considerations is possible. Both aspects are necessary for the next steps towards realization of the system.
The same approach for the evaluation of an ORC system is used in another investigation presented at the conference [1].
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16:20
20 mins
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EFFICIENCY OF ORGANIC RANKINE CYCLE: POTENTIAL AND LIMITATIONS
Hilel Legmann
Abstract: Efficiency of Organic Rankine Cycle: Potential and Limitations
Hilel Legmann
Ormat Technologies, Inc.
e-mail: hlegmann@ormat.com
ABSTRACT
Organic Rankine Cycle (ORC), sometimes referred to as “Binary power generation systems,” are typically used to exploit low- and medium-temperature geothermal and recovered energy resources.
There are numerous technical variations of such plants using different organic fluids, as well as Kalina and Trilateral cycles.
Optimizing the efficiency by matching the cycle to the heat source:
This presentation will compare theoretical cycle efficiency, field measured and net power output and performance for the different heat sources and cycle configurations.
The maximum available energy produced as work for electricity from any heat source is specified by the second law of thermodynamics. Because the rate of the sensible heat carrying fluid is not infinite, its temperature decreases as it transfers the heat to the motive fluid in the heat engine. Thus, the overall process must be envisioned as a summary of an infinite number of infinitesimally small engines. Any heat exchange increases the irreversibility i.e.: reduces the efficiency. A temperature heat transfer diagram illustrates the differences in the temperature drop between a Steam Rankine Cycle and an Organic Rankine Cycle. Because of the lower heat capacity of organic liquids and their much smaller latent heat of vaporization, these fluids let too much smaller losses of availability in the utilization of the low- or medium-temperature predominantly sensible heat streams.
The process of designing a geothermal or waste heat recovery power plant can be considered one of matching and optimization. We have a source and a sink of heat of certain characteristics and the problem is to match them with the working cycle, match the working cycle with the working fluid, and match the working fluid with the expander. What matters most is the optimization of the whole system, involving the well-known process of trading-off a loss or gain.
Examples will be given showing the impact of different factors on the net power delivered to the grid.
REFERENCES
[1] Bronicki, L.Y., 1984, “Twenty Five Years of Experience with Organic Working Fluids in Turbomachinery”, ORC-HP Technology Conference, Verein Deutscher Ingenieure, Zurich
[2] Bronicki, L.Y., 1989, “Organic Vapor Turbogenerators using Locally Available Heat Sources – 25 Years of Industrial Experience”, Congress of the World Energy Conference, Montreal
[3] Legman, H., 1999, Power Returns from Waste Heat, International Cement Review
[4] Bronicki, L.Y., 2002, “Geothermal Power Stations”, Encyclopedia of Physical Science and Technology, Third Editions, 6, Academic Press, San Diego
[5] Legmann, H., 2003, “The Bad-Blumau Geothermal Project”, Proceedings of European Geothermal Conference
[6] DiPippo, R., 2004, “Second Law Assessment of Binary Plants Generating Power from Low-Temperature Geothermal Fluids”, Geothermics, University of Massachusetts, Dartmouth, USA
[7] Bronicki, L.Y., 2008, “Ormat Rankine Cycle Configurations for Utilization of Low Temperature Heat Sources”, Proceedings of the 9th Biennial ASME Conference on Engineering Systems Design and Analysis ESDA08
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16:40
20 mins
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TO RECUPERATE OR NOT TO RECUPERATE - ORC CYCLES COMPARED TO IDEAL CYCLES
Pall Valdimarsson
Abstract: The performance of real power cycles for heat source in the temperature range from 100°C to 300°C is studied in this paper. A reference is made to four ideal power production cycles: Carnot, Reversible Heat Engine (RHE), Triangular and Lorenz.
The real cycles are assumed to have infinite heat exchanger area, and it is as well assumed that cooling fluid is produced by external means, without being a parasite of the power plant.
The binary cycles studied are ORC with a single high pressure level as well as two pressure levels, a saturated Kalina cycle and a supercritical cycle. Single and double flash geothermal power cycles are included as well. A few different working fluids are considered for the ORC cycles, and a few different ammonia concentrations for the Kalina cycle.
The produced power for these cycles from the same source is then compared and a range of superiority for each cycle presented. The effect of recuperation on the produced power as well as on the calculated efficiency is shown.
Finally the influence of finite heat exchanger area is analyzed and an estimate of the cooling fluid generation parasitic power made both for air and wet cooling tower system.
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