09:00
Session 6A: Novel/advanced architectures (1)
Chair: Tryfon Roumpedakis
09:00
20 mins
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Investigation of an ORC System with Integrated Phase Change Engine Cooling
Oliver Dingel, Daniel Luederitz, Thomas Arnold
Abstract: The CO2-emissions of on-road vehicles have to be reduced significantly in the upcoming years in order to achieve overall climate goals. ORC applications are an interesting option to contribute to these goals. Due to the high fuel consumption and mileage of long haul trucks they are of special interest for ORC systems since a long time. The upcoming CO2 limits for these vehicles in the EU are an additional driver for this combination.
The main reason why ORC systems have not been introduced into the market so far is the very ambitious goal of the shipping companies to achieve a return of invest of these systems within two years. To reach this goal, future systems will have to be either cheaper in production or perform better fuel economy. IAV found an approach to achieve the latter one.
Current ORC systems for long haul truck application make use of so far wasted exhaust or exhaust and EGR heat. Another big source of wasted heat, the engine cooling was not in the focus so far, as this energy is only on much lower temperature level available, which has negative impact upon the recuperation efficiency. This drawback can be overcome by replacing the conventional liquid convection cooling by a high temperature phase change cooling system. As this system is integrated into the exhaust heat ORC circuit it promises several advantages at the same time: more heat recuperation, less wall heat losses, and higher exhaust gas temperatures. Due to the integration in the ORC circuit, it does not need additional components but replaces some of them which at the end leads to a much better performance to cost ratio.
In a detailed simulation study based on GT-power and Matlab/Simulink models IAV has investigated the potentials of such a combination at three characteristic load points, accompanied by a detailed loss split analysis. In addition also the impact of phase change cooling upon NOx-emissions has been evaluated. As a result it can be said, that the results underline the theoretical potential of this combination in an impressive way.
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09:20
20 mins
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Mapping of Performance of Pumped Thermal Energy Storage (Carnot battery) Using Waste Heat Recovery
Olivier Dumont, Rémi Dickes, Mitchel Ishmael, Vincent Lemort
Abstract: The growth of renewable energy requires flexible, low-cost and efficient electrical storage to balance the mismatch between energy supply and demand. Pumped thermal energy storage (PTES) converts electric energy to thermal energy with a heat pump when electricity production is greater than demand; when electricity demand outstrips production the PTES generates power from two thermal storage reservoirs (ORC mode). Classical PTES architectures do not achieve more than 60% roundtrip electric efficiency. However, innovative architectures, using waste heat recovery (thermally integrated PTES) are able to reach electrical power production of the power cycle larger than the electrical power consumption of the heat pump, increasing the value of the technology. In this paper, a general model is developed to draw mappings of performance depending on the two main inputs (waste heat and ambient air temperatures). Whatever the storage configurations, the best performances are reached when the waste heat temperature is high, the air temperature is low, and the lift of the heat pump is low. Finally, the thermally integrated PTES technology is compared with other technologies of energy storages and is theoretically promising due to its high roundtrip efficiency, its low specific price and no specific geographical conditions.
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09:40
20 mins
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Flexibility and Economic Dispatch of Island Power Systems with Integrated Thermal Energy Storage in Smart Grids
Panagiotis Romanos, Emmanouil Voumvoulakis, Nikolaos Hatziargyriou, Christos Markides
Abstract: The operation of combined cycle and steam-turbine power stations must meet the requirements for the high penetration of renewable energy sources under the EU framework of 2050 commitments. A modular, expandable, generalised adaptable configuration is investigated with the integration of Thermal Energy Storage (TES) systems.
The efficiency of combined-cycle and steam-turbine power plants varies with the load demand. During off-peak demand, steam is extracted from the output of the boiler for charging an array of thermal tanks which are the evaporators of organic Rankine cycle (ORC) units and contain suitable phase-change materials (PCM). The outlet steam from the tank-ORC arrangement is returned to the inlet of the boiler.
An Energy Management System (EMS) regulates the system such as to optimize the charging of the thermal tanks-evaporators during the off-peak demand. In the case that excess electrical energy from renewable energy sources is available on the grid, the ORC units also charge the thermal tanks operating inversely. Furthermore, due to the constraints for the minimum temperatures of PCMs related to the condenser minimum temperatures of the conventional power cycles, the EMS controls the ORCs units so that these constraint requirements are satisfied. At a later time when this is required and/or economically favourable, these thermal energy storage (TES) tanks can act as the heat sources of organic Rankine cycle (ORC) plants that are suitable for power generation at reduced temperatures and smaller scales. This type of solution offers greater flexibility than TES-only solutions that store thermal energy and then release this back to the original power station.
This paper investigates the integration of such a Thermal Energy Storage System in the power System of Crete, Greece. The power system of Crete is currently an autonomous power system which is going to be interconnected to the Greek mainland System in the next few years. It operates with high levels of wind power penetration which is curtailed to avoid violation of technical constraints of compatible units and to ensure dynamic security of the system. It also comprises a lot of units with high fuel cost which operate during peak hours. Thus a storage system would provide both necessary flexibility and reduce cost during peak hours.
In a case study where a 50-MW rated oil-fired power unit is considered, it is found through a thermodynamic analysis that a maximum combined power of 70 MW can be delivered during peak demand, which is 40% higher than the oil-fired plant’s full-load rating. The scenario also allows the reduction of the unit’s minimum power output from 27 MW to 16 MW, enhancing thus its flexibility.
In order to perform a cost-benefit analysis of the TES of the above case study, a realistic model of the generation system of Crete projected to the year 2020 is considered as well as annual time series of load demand and Renewable Energy Sources generation. A Unit Commitment and Economic Dispatch algorithm is implemented in order to simulate the operation of TES over the long term. The algorithm is based on fuel cost and start-up/shut down cost minimization, subject to the technical constraints imposed by the generating units and TES. A stochastic algorithm is also used to simulate the generation outages incidents.
Two cases are examined and compared to each other; the first represents the operation of the power system with the integration of TES, while the second is the system operation without TES installed. The benefit of the TES would be the difference in the annual cost of operation of the power system for the two cases. In addition, the contribution of TES to the generation adequacy of the System is estimated and the possible revenue from a capacity remuneration mechanism is taken into account in the cost benefit analysis. The above analysis is performed for various storage capacities in order to provide the optimal dimensioning of TES. The above analysis is also performed both for the case of autonomous and interconnected operation of the power system in Smart Grids.
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10:00
20 mins
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Performance Comparison of Different Rankine Cycle Technologies Applied to Low and Medium Temperature Industrial Surplus Heat Scenarios
Goran Durakovic, Monika Nikolaisen
Abstract: Energy intensive industries generate vast amounts of low-grade surplus heat, and potential for direct re-use can be limited due to insufficient local demand. Conversion of heat into electric power thus becomes an attractive option. However, heat-to-power conversion is limited by poor efficiencies for heat sources at low and medium temperatures. To optimally utilize the available energy, the choice of technology could be important.
In this work, single-stage subcritical, dual-stage subcritical and single-stage transcritical Rankine cycle technologies were compared. The investigated heat sources are an air equivalent off-gas at 150 and 200°C and a liquid heat source at 150°C. These were all evaluated with the same available duty. The Rankine cycles were compared with equal total heat exchanger areas to enable fair comparisons of different cycles, instead of comparing cycles based on equal pinch point temperature differences as is frequently reported in literature. Six working fluids were investigated for both heat source temperatures, and cycle optimization was performed to determine the best Rankine cycle for each working fluid. This means that the optimizer could choose between single-stage and dual-stage cycle during one optimization, and determine which gave the highest net power output. The results were benchmarked against single-stage subcritical Rankine cycles.
The results showed that the cycle yielding the highest net power varied depending on the allowable total heat exchanger area. When the total heat exchanger area was large, the dual-stage and transcritical cycles performed better than the benchmark, and the improvement became larger as total area increased. For the total heat exchanger areas investigated for the gaseous heat sources, the maximum performance increase was 3 %. Below a certain total area threshold, the benchmark performed the best.
For the liquid heat source, the transcritical and dual-stage cycles increased performance by 10 % and 5 % compared to the benchmark, respectively. This larger performance increase is primarily owed to an improved heat transfer in the heat recovery heat exchanger. In fact, using a liquid heat source could improve the net power production by up to 75 % for the same total heat exchanger area compared to when using a gas heat source.
Dual-stage and transcritical Rankine cycles seem to offer only a slight improvement over the benchmark for air-equivalent gas heat sources, but only for large total heat exchanger areas. Performance improvement is higher for liquid heat sources. The results indicate that the performance increase for dual-stage and transcritical Rankine cycles compared to the subcritical benchmark depends on the total heat exchanger area and heat source composition.
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10:20
20 mins
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Experimental Results of a Waste Heat Recovery System with Ethanol Using Exhaust Gases of a Light-duty Engine
Jelmer Rijpkema, Fredrik Ekström, Karin Munch, Sven Andersson
Abstract: Organic Rankine cycle (ORC) waste heat recovery (WHR) systems have the potential to improve the efficiency of modern light-duty engines, especially at high-way driving conditions. This paper presents and discusses the experimental results of an engine connected to a compact ORC-WHR system with ethanol, suitable for integration in a modern passenger car. The aim is to show the added value of this ORC-WHR system for passenger cars by presenting the experimental results with the focus on the expander power output. The experimental setup consists of a Volvo Cars VEP-4 gasoline engine, which has an evaporator integrated in the exhaust pipe. During operation, one of two different states can be selected: electrical feedback (EFB) or mechanical feedback (MFB), where the expander can be either coupled to a 48V generator (EFB) or directly to the engine (MFB). Control strategies were developed to allow for operation of the system without interference of the driver. The results show that the current setup and control strategies can be successfully employed with significant expander power outputs for both MFB and EFB. The expander power outputs, similar for both states, go up to 2.5 kW, recovering 6.5% of the available exhaust energy and giving more than 5% improvement in fuel consumption.
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