Evaluation of Waste Heat Recovery in a Close-to-Reality Environment

Transcription

1 DEVELOPMENT ENERGY EFFICIENCY Evaluation of Waste Heat Recovery in a Close-to-Reality Environment AUTHORS Timo Engel, M. Sc. is Development Engineer Dipl.-Ing. Mirko Fritsch is Development Engineer Exhaust heat recovery systems are ideally suited to heavy long-haul trucks, with the potential to cut fuel consumption by up to 4 %. At its site in Holzkirchen near Munich in Germany, Bosch Engineering operates a dynamic hot-gas test bench to support the development and testing of Waste Heat Recovery (WHR) systems. With the test bench, both individual components and complete systems can be validated and evaluated. Bosch Engineering 56 Dipl.-Ing. Josef Kurz is Development Engineer Dipl.-Ing. Heinz-Georg Schmitz is Department Head IMPROVEMENT OF ENERGY UTILISATION IS IMPORTANT Efficient trucks contribute to protecting the environment and help freight forwarders save money. To make commercial vehicles even more economical and resource-saving, Bosch supports vehicle manufacturers in developing powertrain systems that make optimum use of every drop of fuel. This reflects the decisive role that total cost of ownership (TCO) plays for freight forwarders. This encompasses a vehicle s purchase cost, fuel costs, and operating and maintenance costs over its entire service life. When added together, they can be compared with the TCOs of alternative vehicles. With competition becoming increasingly fierce, finding ways to reduce costs over a vehicle s lifetime has a direct impact on a company s economic efficiency. One of the most important tasks for freight forwarders is to reduce fuel costs, since these account for up to 70 % of the cost of operating heavy commercial vehicles [1]. Therefore especially for long-haul trucks the improvement of energy utilisation is extremely important. Experts believe the greatest potential lies in optimising the transmission and using a waste heat recovery (WHR) system for exhaust gas heat recovery [2]. The figures confirm that it makes sense to keep improving these technologies in the future: pure diesel drives are expected to continue being the main powertrain technology in heavy commercial vehicles over the next few years, with hybrid powertrains being used in up to 15 % of vehicles [1]. Both types of powertrain can be equipped with WHR systems to cut fuel consumption and costs. Bosch Engineering has been developing technical solutions for implementing WHR systems since 2009 [3]. Key consid-

2 erations in its development and customer projects include choosing the right components and a suitable heat transfer fluid as well as making sure these are compatible with each other. In 2016, Bosch Engineering took a dynamic hot-gas test bench into operation to provide an efficient and reproducible means of examining all the WHR issues involved. ORGANIC RANKINE CYCLE SYSTEMS For long-haul trucks, researchers are developing exhaust heat recovery systems based on the Organic Rankine Cycle (ORC). This thermodynamic process is well established in the field of power plant technology. To make efficient use of this technology in commercial vehicles several adaptations have to be considered. The challenge is to find the optimum balance of efficiency and costs during the development process. Other key considerations include weight, module size, packaging, and reliability. Unlike stationary applications, mobile ORC systems are subject to very dynamic changes in temperature and mass flow rates, FIGURE 1. The system must also be capable of functioning at a range of ambient temperatures. That is why the properties of the working fluid, such as its freezing point and decomposition temperature, are so important. The fluid determines many aspects of the system, including the materials used in the system components, the design pressures and temperatures, and the mass and volume flow rates. The working fluid also has an impact on the safety, the ability to gain approval and acceptance of ORC systems. Many working fluids that would theoretically lead to a good degree of overall efficiency are flammable hydrocarbons, such as ethanol. Alternative fluid with a much lower potential of danger are present in the group of refrigerants. As they provide excellent thermodynamic properties as well, Bosch focusses in the development of its components on such fluids. To develop its ORC systems, Bosch employs powerful, highly detailed simulation models that take transient driving and operating conditions into consideration, and has applied these simulation models to a European reference route. The company s results show that ORC systems can save money on fuel even if commercial vehicle engines continue to become more efficient in the future: ORC systems can reduce fuel consumption and CO 2 emissions by 3 to 4 %, even in situations where only the exhaust gas is used as a heat source. Combined with the progressive electrification of the powertrain for example with beltless engines plus automated and connected driving functions, it is pos- ATZ worldwide

3 DEVELOPMENT ENERGY EFFICIENCY FIGURE 1 Exhaust-gas dynamics in WHTC [3] ( Bosch Engineering) sible to reduce the fuel consumption of heavy commercial vehicles by over 4 %. Incorporating additional heat sources (Exhaust Gas Recirculation, EGR) can boost this savings potential even further. DYNAMIC HOT-GAS TEST BENCH Bosch Engineering GmbH relies on a combination of computer-assisted model simulation and realistic test bench trials to develop ORC systems. The company operates its own dynamic hot-gas test bench for this purpose, FIGURE 2. This enables it to test complete ORC systems, modules, and individual components in a safe and reproducible manner under both steady-state and transient conditions. The test bench simulates realistic exhaust gas temperature and mass flow profiles. It is designed to test the following individual components in whatever combinations customers require: expander: mechanical or electrical power output condenser: the temperature of the cooling water can be regulated up to 90 C, and a specific cooling performance of 8 kw/k pump: the voltage level can be selected as required (DC/AC) evaporator: exhaust gas mass-flow rate and temperature can be set dynamically in the following ranges: 100 to 1800 kg/h and 100 to 1200 C. The components can be flexibly integrated into the cycle of the ORC system, allowing researchers to make model-based predictions of the efficiency, thermodynamic states, and system behaviour, and validate these predictions. This method can also be used to measure and test all the other system components (for example pump, heat exchanger, and working fluids regardless of their hazard rating) as well as to validate the system as a whole and develop operating strategies, control functions and systems. SYSTEM EVALUATION AND VALIDATION The system evaluation is based on measurements taken of individual components or the complete system on the test bench. The measurements will be used in correlation with a customised simula- FIGURE 2 Dynamic hot-gas test bench s facility in Holzkirchen near Munich (Germany) ( Bosch Engineering) 58

4 FIGURE 3 Chosen European Steady State Cycle (ESC) operating points ( Bosch Engineering) tion model. With the aid of the simulation, engineers can quickly evaluate many different operating points and/or entire cycles. They can also optimise the system control more efficiently than it would be possible on the test bench. In an exemplified commissioning process of an ORC system, the engineers selected three steady-state operating points from the European Steady State Cycle (ESC), which cover both high and low exhaust gas temperatures and high and low exhaust gas mass-flow rates across the entire speed range of a commercial vehicle, FIGURE 3. The input parameters of the simulation model have been adjusted to evaluate the steady-state and transient behaviour of the ORC system. Using signal generators made available as a template through Gamma Technologies Integrated Simulation Environment (ISE) [4] it is possible to impose both steady-state as well as transient conditions on the simulation model. In parallel to this simulation, a complete ORC system was set up on the test bench using a prototype evaporator, a prototype condenser, an industrial supply pump, and a functional sample of a turbine to acquire component data based on a grid measurement plan. At the current stage of development it is common practice to use prototypes and functional samples, since the corresponding mass-produced components are not yet available. As it stands now, geometric data is often the only available resource, but it is not sufficient to adequately model the components. The turbine used on Bosch Engineering s test bench is characterised for ethanol; however, the chosen fluid was water, so the engineers re-characterised the thermodynamic properties of the turbine working model to reflect this new medium. For the purpose of commissioning using water as a working fluid means there is no risk of fire. It also eliminates any possibility that a leak could cause environmental damage and avoids the risk of decomposition at excessive exhaust gas temperatures. Nevertheless, this fluid allows to perform all the tests on the test bench to obtain the sought validation results. In addition, water s thermodynamic properties are very well documented. This working fluid is not, however, suitable for use in series systems on highway. The high degrees of efficiency known from power plant technology occur only at temperatures higher than those that would be expected in a vehicle. MODELLING The individual components are modelled in the computational simulation as per the customer s requirements. By way of example, this section describes the modelling of the turbo expander, a model which consists of various different components. The pipework modelling is based on physical values. The parameters for the mechanical and isentropic efficiency of the turbo expander were calculated based on measurements and mapped onto the model. In terms of mechanical efficiency, the friction in the transmission and bearings has a crucial impact on the overall mechanical efficiency. This data was taken from measurements performed by Robert Bosch GmbH, since, as a first approximation, it is not dependent on the fluid used. It is also important to take the heat into account that is dissipated via the turbine housing. The heat losses are calculated by comparing the measurements of the turbine s real power output with the thermodynamic data of an adiabatic reversible system with identical temperature, pressure, and mass flow ratios. Hence, the engineers made precise measurements of the temperature upstream and downstream from the turbo expander in order to calculate these thermal losses. FIGURE 4 illustrates how the actual isentropic efficiency can be calculated using this method. The components were characterised using steady-state grid measurements. This was achieved by adjusting the rotational speed of the turbo expander on the dynamic hot-gas test bench and varying the pressure ratios across the expander by controlling the fluid mass flow rate. When it comes to validating a model consisting of multiple components, a sensible approach is to validate the individual components separately. FIGURE 5 provides an example with the validation results for the turbo expander. Validation was performed by comparing the simulation results with the test bench measurements obtained using the test setup. The results of this validation process indicate a good to very good match (judgement in accordance with [6]). The percentage deviation between the turbo expander simulation measurements and the real measured values is 5 % or less. The deviation is larger for the power ATZ worldwide

5 DEVELOPMENT ENERGY EFFICIENCY FIGURE 4 Determination of heat losses and isentropic efficiency (thermodynamic basic data from Refprop [5]) ( Bosch Engineering) measurements than for pressure or temperature. One reason for this is the relatively low measured torque values of around 5 Nm measurement errors in this case have a correspondingly significant effect when calculating the power output through multiplication with very high rotational speeds. OUTLOOK The use of waste heat recovery in commercial vehicles offers significant potential to achieve further reductions in fuel consumption and CO 2 emissions, especially since the truck industry is constantly on the lookout for smart solutions that save money. However, there are still numerous questions to answer in regard to optimising, safeguarding, and developing an operating strategy before ORC systems will finally be production-ready. Tests on a dynamic hot-gas test bench offer the right approach to answer these questions efficiently under reproducible conditions and in a safe environment. REFERENCES [1] Beidl, C.; Schmitt, M.; Kluin, M.; et al.: Potentials of Hybridized Commercial Vehicles. In: MTZworldwide 72 (2011), No. 6, pp. 4 9 [2] State of California, Air Resources Board: Draft Technology Assessment: Engine / power plant and drivetrain optimization and vehicle efficiency. 2015, online: 9 August 2016 [3] Schmitz, H.-G.; Fritsch, M.; Huber, A.; Schmidbaur, C.: The design of Rankine systems for automotive applications [Auslegung von Rankine-Systemen für automobile Anwendungen]. presentation, 13 th International Stuttgart Symposium 2013 [4] N. N.: Gamma Technologies. Online: September 2016 [5] National Institute of Standards and Technology (Nist): Reference Fluid Thermodynamic and Transport Properties Database (Refprop). Online: September 2016 [6] Zirn, O.; Weikert, S.: Modellbildung und Simulation hochdynamischer Fertigungssysteme: Eine praxisnahe Einführung. Berlin, Heidelberg: Springer Fachmedien, 2006 THANKS FIGURE 5 Validation results for the turbo expander: pressure (top), temperature (center), power (bottom) ( Bosch Engineering) 60 The authors would like to extend their thanks to their colleagues at Robert Bosch GmbH and Bosch Engineering GmbH. Particular thanks go to Michael Bitter, Nadja Eisenmenger, Tobias Korte, Matthias Aschenbrenner, Thomas Brock, Stefan Lösch, Annett Fischer, Andreas Huber, Christian Schmidbaur, Martin Seibold, Tobias Weller and Ralph Wentker.

6 The Powertrain of Tomorrow Hybrid and Electric Powertrain Concepts 11th International MTZ Conference on Future Powertrains Frankfurt/Main ain Germany REQUIREMENTS Legislation and customer expectations DRIVE SYSTEMS Components and systems CONTROL SYSTEMS Energy and thermal management /// KEYNOTE LECTURES Prof. Dr. Peter Gutzmer, Schaeffler Prof. Dr. Ferit Küçükay, TU Braunschweig Dr. Christian Landerl, BMW Dr. Oliver Maiwald, Continental /// KINDLY SUPPORTED BY PROGRAM AND REGISTRATION Phone Abraham-Lincoln-Straße 46 Fax Wiesbaden Germany ATZ worldwide