System cost and efficiency optimization by heat exchanger performance simulations

Transcription

1 Available online at ScienceDirect Energy Procedia 00 (2017) IV International Seminar on ORC Power Systems, ORC September 2017, Milano, Italy System cost and efficiency optimization by heat exchanger performance simulations Andreas Möller a*, Dr. Vijaya Sekhar Gullapalli b a Senior Expert Application Developer, SWEP International AB ( ), Box 105, Landskrona, Sweden, a.a.moller@me.com b Manager SSP Core Devlopment, SWEP International AB, Box 105, Landskrona, Sweden, vijaya.sekhargullapalli@swep.net Abstract Heat exchangers of any types are fully necessary for sourcing heat energy to, as well as disposing the low temperature waste energy from, the ORC system. Depending on the temperatures and the number of heat sources; the working fluids, choice of thermodynamic cycle and internal cycle, operating conditions can be varied to determine the optimum fit of the system design for the available heat source and sink. Brazed Plate Heat Exchangers (BPHEs) are used for a wide variety of applications and are very suitable for ORC applications due to their compactness and high efficiency. In this article, we will present a tool which is useful for early phase decision making, such as system optimization based on brainstorming of the concept and choice of thermodynamic cycle, during the design phase involving system design and heat exchanger selections, and for the late post-launch phase where an existing system design might be adopted for use with different heat sources. The principle strength of the developed freeware tool is that it combines system efficiency calculations for various thermodynamic cycles and comes with a large fluid database as well as a heat exchanger price indicator. The tool communicates with a commercial heat exchanger selection software (SWEP SSPG7), in which detailed calculations at component level can be performed followed by possibility of configuration of heat exchangers, check stock availability, compare price and generate 3D drawings. Cases studies within low and medium temperature applications, based on different installations of the Viking CraftEngine, are presented in a separate performance verification section. Several variations of the same basic concepts are obtained by varying temperatures, internal recuperation, working fluid and secondary fluids, which all in all shows some of the possibilities which can be explored virtually with good reliability The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems.

2 2 A. Möller, Dr. V. S. Gullapalli / Energy Procedia 00 (2017) Keywords: ORC; Brazed Plate Heat Exchangers; Simulation; Design 1. Introduction ORC systems are not particularly more complex than for example refrigerant cycle chillers, or heat pumps, in terms of number of components, or system design, but differ in a few aspects making system design dramatically more challenging. First of all the operation window is very large in terms of temperature range which the system may have to accept, secondly the impact on the system internal process from the surroundings, is normally huge. Use of intermediate circuits for thermal energy input and output provide some smoothening of the environmental impact as well as some decrease of the required dynamic range, but nevertheless ORC system design remains complex. The consequence is plentiful where the match against a suitable working fluid is the most dominant, which in turn impacts and limits primarily the choices of the expander. In combination with capacity range, the temperature range and the physical properties of the working fluid strictly limits the applicability of a specific set of system components. Heat exchangers are normally more versatile - and depending on technology also more modular - than the expander. However, there are clear limitations for different heat exchanger technologies, both in terms of practically offering a good performance fit for an arbitrary operating point and also in terms of pressure and temperature limits. For the remaining part of the paper, the applications referred to will dominantly be such where brazed plate heat exchangers (BPHE) are, or may be, used, but with examples also showing where they evidently should not should be used. In other words, we look on how the BPHE technology is applied within low temperature ORC, how it is and could be adapted to cope with medium temperature ORC, all in the capacity range where each module is rated below 200 kw e. For a certain technology, the pressure rating commonly has an inverse relation to its capacity. As an example, the heat exchangers used in the examples in this paper are of BPHE type, of SWEP brand. To illustrate the pressure class vs. capacity relation, the size ranges Large ( 3 in. connection diameter), XL ( 4 in.) and XXL ( 6 in.) are plotted in Fig. 1. Similar relations could be established for any BPHE brand. Fig. 1. SWEP BPHE models on x-axis (port size) with pink curve, left vertical axis, visualizing maximum water maximum 5,5 m/s port velocity and green curve, right vertical axis, indicating maximum pressure rating for each model

3 Author name / Energy Procedia 00 (2017) In addition to the pressure rating s relation to certain capacities, there is a degradation as a function of operating temperature. This relation differs considerably from different pressure vessel directives and takes material type into account, where the de-rating at 200 C, from 20 C, is up to 25 %. 2. Functionality requirements on the simulation software At least two heat exchangers are needed to make an ORC system; the evaporator and the condenser. For high temperature rise in the liquid pre-heating phase, it might be desirable to add a pre-heater to the evaporator and with an analogue argument, it might be desirable to also add a super-heater after the evaporator. The condenser may benefit from a sub-cooler companion for certain operation points and on a system level an internal heat exchanger, a so called recuperator, increases the efficiency as well as reduces the condenser load and capacity requirement. Fig. 2. System schematics of most fundamental ORC system covered by the software Nevertheless, all these heat exchangers do not necessarily create an increased value at the bottom line for the end user, when CAPEX and OPEX sum up. Neither is it clear that each heat exchanger position contributes to increased efficiency for all refrigerants in dynamic as well as static operation, to just list a few concerns. The software must provide guidance to the user in order to sort out which additional heat exchanger positions that are beneficial operation wise and add efficiency without increasing return of investment. Particularities of BPHE s; such as internal pinch with mixed fluid phases, maldistribution of flow within the heat exchanger at part load as well as with two-phase flow and heat transfer characteristic s dependence with flow, all points in the same direction: Black box model treatment of the heat exchangers in an ORC system comes with an increased risk of time consuming manual iterations after empirical tests, or separate heat transfer calculations. Therefore a key feature for the software developed must be to model the heat exchangers behavior realistically. When all the requirements on functionality and user value were summarized, it ended up in the following

4 4 A. Möller, Dr. V. S. Gullapalli / Energy Procedia 00 (2017) definition: Fig. 3. Definition of required software functionality and user value 3. Implementation of concept in SSP Organic Rankine Cycle Simulation The approach chosen by SWEP was to develop a freeware tool where the full system performance could be simulated, with state of the art heat exchanger (BPHE) and fluid models. On the top level, SWEP s SSP Organic Rankine Cycle Simulation software intends to offer two features; design of heat exchangers for a certain design case, respectively conduct performance calculations for a given system design at any operating conditions. Based on which mode the simulation is conducted in, different inputs will be required; for design mode maximum allowed pressure drop for each unit has to be specified as well as dew point for the evaporator and the condenser. If the evaporator operates supercritical, dew point is replaced by outlet pressure and super-heat is replaced by outlet temperature. A certain number of information blocks was defined by the functionality requirements from a market need perspective. In principle the functionality and the logical order can be visualized as follows in figure 4: Fig. 4. Software functionality map Details of numerical schemes, heat transfer equations and data format is described in the paper Modeling of brazed plate heat exchangers for ORC systems, by Dr. Vijaya Sekhar Gullapalli Fluid data base sub-critical operation A large number of working fluids are available within SWEP SSP G7, but only those of any practical interest for ORC has been implemented in SSP Organic Rankine Cycle Simulation. A demand for high critical temperature for the system efficiency has already streamlined the ORC industry around R245fa and a few other working fluids. Additionally density is a critical parameter for plate heat exchangers due to the fact that volume flow indirectly is a cost driver since the connection diameter has a strong impact on the fix cost of the heat exchanger. All in all about 20 working fluids, both natural and synthetic, are available for sub-critical system simulations at the moment Fluid data base trans-critical operation To maximize the utilization of heat sources with large temperature differences, trans-critical cycles can be used. The obvious reasons are less pinch sensitivity and higher thermodynamic efficiency, by higher operating pressure. However, even fewer working fluids fulfil the basic requirements when operated in a trans-critical cycle. Studying previous research in the field [1][2], suggested candidates such as R134a, R152a and R245fa. To select some hydrocarbon working fluids as well we used the following criteria: Highest possible critical temperature Operational sweet spot in the range 1,2-1,5 times critical pressure [2] preferably not exceeding 45 bar Expansion ratio, with 20 C condensing temperature, in the range 2-35

5 Density at 20 C dew temperature not below 5 kg/m 3 Author name / Energy Procedia 00 (2017) The only candidates that came out of this selection process was the three Pentane isomers, resulting in six transcritical work fluids in total for use within the SSP Organic Rankine Cycle Simulation software Simplifications of the physical model To limit the complexity of the interface as well as for the code and increase the stability of the simulations, a few simplifications has been made to the code: Parametric expander performance o % isentropic efficiency o The software is prepared to take performance maps Parametric recuperator performance o % of available gas cooling range - expander outlet to dew point o Automatically prohibits partial condensation in the recuperator No heat loss or pressure drop outside specified components, i.e. heat exchangers The practical implications of the parametric representation is some manual steps depending on how the program is used. For the expander you need to know a good estimate of its efficiency in advance to perform a design simulation with good accuracy. For a performance simulation, the test data can be used together with tabular data of the working fluid, such as REFPROP for example, to calculate the isentropic efficiency for each operation point. The recuperator is either selected separately in SSP G7 before or after a design simulation, or its efficiency is calculated from test data. The pressure drop over the recuperator is relatively stable for operation points at similar capacity and pressure, which in combination with low impact on the overall efficiency, can kept at a certain value for several operation points. 4. Evaluation As a primary source of operational data for evaluation of the SSP Organic Rankine Cycle Simulation tool in this paper, test data from Viking Heat Engines has been used where the Craft Engine CE10, rated from 2 to 10 kw electric output, has been applied to both a low temperature heat source (<120 C), as well as a medium temperature heat source (<200 C). So said, the heat source temperatures generally exceed the critical temperatures of the working fluid used, R134a, respectively R245fa with operation up to 73 %, respectively 82 % of critical pressure. The system layout includes a recuperator as well as a sub-cooler operational during start up conditions, utilizing the same heat sink as the condenser, connected in parallel. In the software, this system is treated as shown in figure 5. a b

6 6 A. Möller, Dr. V. S. Gullapalli / Energy Procedia 00 (2017) Fig. 5. (a) Layout of Craft Engine CE10 used for evaluation of the software (b) Cycle nodes As an example for evaluation of the simulation accuracy, this system layout is very good. It highlights some of the challenges you are facing when test data are to be fitted to system simulations; both recuperator and sub-cooler adds challenging complexity and are designed to provide tight temperature approaches. Table 1. Measured data and deviation in mechanical output from the simulation Work fluid Expander speed p evap [bar] T in [ C] Output [kw] Deviation R134a R245fa 2/3 23, ,7-3 % 2/3 23,5 99 6,7-4 % 2/3 23, ,04 0 % Full 29, ,17 3 % Full 29, ,99 6 % Full 29, ,85 6 % Full 23, ,09 4 % Full 29, ,5 9 % Full 26, ,3 5 % 2/3 21, ,7 14 % * *The desired super heat could not be used (-8,5 K) due to pinch, which leads to increased evaporation temperature and efficiency 4. Analysis All in all the software deviates in system efficiency/output compared to the test data in the range of -4/+6 % for the low temperature simulations (R134a), while the error grows to % for the medium temperature simulations (R245fa). With correct super-heat in the last R245fa simulation, the deviation is expected to be in the range of 10 %. Given the physical conditions for the R245fa cases, especially for the evaporator; such as pre-heat, superheat and total temperature difference over the heat exchanger, they are all further out in the periphery of the calibration range of the heat exchanger models, so that the error is expected to increase. R245fa is also, despite its popularity within the ORC industry, not as thoroughly investigated as the well-established R134a. On a detail level, the evaporator performs well over the calculated performance in temperature approach, while the pressure drop is substantially higher. Pipe and valve pressure drop may be part of the reason for the discrepancy between calculated and measured pressure drop. Concerning the thermal performance of the evaporator, pinch in the super-heating region is identified as potentially the main reason behind lower evaporating pressure in the a b simulations. Fig. 6. (a) R134a temperature profile (b) R245fa temperature profile Heat carrier, wall temperature and working fluid temperature

7 Author name / Energy Procedia 00 (2017) At its extreme, the medium temperature cases uses only 10 % of the evaporator surface for evaporation and 80 % for super-heat, while the low temperature case uses up to 60 % for evaporation and equally 20 % for pre- and superheat. 5. Conclusion SSP Organic Rankine Cycle Simulation provides the accuracy needed to replicate real operation performance of the studied ORC system, which can be used to evaluate different concepts as long as the performance differences are larger than 5 %. Such a tolerance is lower than the tolerance for most components. If operation in essentially different temperature ranges and with different working fluids should be compared, then calibration against tests will be needed to reach accuracies within 10 %. With reference to figure 5 b, correct expander efficiency (3-4) as well as recuperator thermal and hydraulic performance (4-5) are critical to not push the condenser ( ) and the sub-cooler (8-9) out of range. A system design calculation can be performed with less effort, since the software self-adjusts the heat exchanger performance to fit the available temperature ranges, while a performance calculation more easily can end out of range, such as wet expansion, pinch and sub-cooling below heat sink temperature. With high heat exchanger thermal performance, such as in the studied system, small errors in the temperature measurements makes it complicated to fit experimental data to the simulation for calibration of the model. Finally, with further calibration of the physical models throughout applicable temperature ranges, combined with improved system analysis in case of non-converging parameter setup, the program can provide the same quick (~10 s per simulation), powerful and reliable system development support to a wide range of ORC system setups. Acknowledgements Data for verification shared by Viking Heat Engines, Norway References [1] Van Long Le, Thermodynamic Laboratory, University of Liège, ASME ORC 2015 Proceedings, Presentation 189, p. 7 [2] Tian Ran, Department of Thermal Engineering, Tsinghua University, ASME ORC 2015 Proceedings, Presentation 053