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Institutionen för systemteknik Department of Electrical Engineering Examensarbete Model Based Diagnosis of an Air Source Heat Pump Examensarbete utfört i Fordonssystem vid Tekniska högskolan vid Linköpings

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Institutionen för systemteknik Department of Electrical Engineering Examensarbete Model Based Diagnosis of an Air Source Heat Pump Examensarbete utfört i Fordonssystem vid Tekniska högskolan vid Linköpings universitet av Sandra Alfredsson LiTH-ISY-EX--11/4502--SE Linköping 2011 Department of Electrical Engineering Linköpings universitet SE Linköping, Sweden Linköpings tekniska högskola Linköpings universitet Linköping Model Based Diagnosis of an Air Source Heat Pump Examensarbete utfört i Fordonssystem vid Tekniska högskolan i Linköping av Sandra Alfredsson LiTH-ISY-EX--11/4502--SE Handledare: Examinator: Emil Larsson isy, Linköpings universitet Anders Lönnstam Thermia Värme AB Per Öberg isy, Linköpings universitet Linköping, 30 September, 2011 Avdelning, Institution Division, Department Division of Vehicular Systems Department of Electrical Engineering Linköpings universitet SE Linköping, Sweden Datum Date Språk Language Svenska/Swedish Engelska/English Rapporttyp Report category Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport ISBN ISRN LiTH-ISY-EX--11/4502--SE Serietitel och serienummer Title of series, numbering ISSN URL för elektronisk version Titel Title Modellbaserad Diagnos av en Luftvärmepump Model Based Diagnosis of an Air Source Heat Pump Författare Author Sandra Alfredsson Sammanfattning Abstract The purpose of a heat pump is to control the temperature of an enclosed space. This is done by using heat exchange with a heat source, for example water, air, or ground. In the air source heat pump that has been studied during this master thesis, a refrigerant exchanges heat with the outdoor air and with a water distribution system. The heat pump is controlled through the circuit containing the refrigerant and it is therefore crucial that this circuit is functional. To ensure this, a diagnosis system has been created, to be able to detect and isolate sensor errors. The diagnosis system is based on mathematical models of the refrigerant circuit with its main components: a compressor, an expansion valve, a plate heat exchanger, an air heat exchanger, and a four-way valve. Data has been collected from temperatureand pressure sensors on an air source heat pump. The data has then been divided into data for model estimation and data for model validation. The models are used to create test quantities, which in turn are used by a diagnosis algorithm to determine whether an error has occurred or not. There are nine temperature sensors and two pressure sensors on the studied air source heat pump. Four fault modes have been investigated for each sensor: Stuck, Offset, Short circuit and Open circuit. The designed diagnosis system is able to detect all of the investigated error modes and isolate 40 out of 44 single errors. However, there is room for improvement by constructing more test quantities to detect errors and decouple more fault modes. To further develop the diagnosis system, the existing models can be improved and new models can be created. Nyckelord Keywords modelling, diagnosis, vapour compression system Abstract The purpose of a heat pump is to control the temperature of an enclosed space. This is done by using heat exchange with a heat source, for example water, air, or ground. In the air source heat pump that has been studied during this master thesis, a refrigerant exchanges heat with the outdoor air and with a water distribution system. The heat pump is controlled through the circuit containing the refrigerant and it is therefore crucial that this circuit is functional. To ensure this, a diagnosis system has been created, to be able to detect and isolate sensor errors. The diagnosis system is based on mathematical models of the refrigerant circuit with its main components: a compressor, an expansion valve, a plate heat exchanger, an air heat exchanger, and a four-way valve. Data has been collected from temperatureand pressure sensors on an air source heat pump. The data has then been divided into data for model estimation and data for model validation. The models are used to create test quantities, which in turn are used by a diagnosis algorithm to determine whether an error has occurred or not. There are nine temperature sensors and two pressure sensors on the studied air source heat pump. Four fault modes have been investigated for each sensor: Stuck, Offset, Short circuit and Open circuit. The designed diagnosis system is able to detect all of the investigated error modes and isolate 40 out of 44 single errors. However, there is room for improvement by constructing more test quantities to detect errors and decouple more fault modes. To further develop the diagnosis system, the existing models can be improved and new models can be created. Sammanfattning Syftet med en värmepump är att reglera temperaturen i ett avgränsat utrymme. Detta sker genom värmeutbyte med en värmekälla, till exempel vatten, luft eller mark. I den luftvärmepump som har studerats utbyter ett köldmedium värme med utomhusluft och med vatten i ett distributionssystem. Luftvärmepumpen styrs genom kretsen som innehåller köldmediet och därför är det viktigt att denna fungerar. För att säkerställa detta har ett diagnossystem skapats för att kunna detektera och isolera sensorfel. Diagnossystemet är baserat på matematiska modeller av köldmediekretsen med dess huvudkomponenter: en kompressor, en expansionsventil, en plattvärmeväxlare, en luftvärmeväxlare och en fyrvägsventil. Data har samlats in från temperatur- och trycksensorer på en luftvärmepump. Datan har sedan delats upp i data för estimering och data för v vi validering av modeller. Modellerna används för att skapa teststorheter, som i sin tur används av en diagnosalgoritm för att avgöra om ett fel har uppstått eller ej. På den studerade luftvärmepumpen finns nio temperatursensorer och två trycksensorer. För varje sensor har fyra olika felmoder undersökts: Fastnat, Offset, Kortslutning och Avbrott. Det framtagna diagnossystemet kan detektera alla undersökta fel och isolera 40 av 44 enkelfel. Emellertid finns det utrymme för förbättring genom att skapa fler teststorheter för att detektera fel och avkoppla fler felmoder. För att utveckla diagnossystemet ytterligare kan de befintliga modellerna förbättras och nya modeller kan skapas. Acknowledgments I would like to thank the staff at Thermia R&D for all help and support, especially the staff at the department of Electronic & Electrical Design. I would also like to thank my supervisor at Thermia, Anders Lönnstam and my supervisor at Linköping university, Emil Larsson, for all support and understanding. A special thanks to Tomas Persson, who was always helpful and took time to answer questions. Furthermore I would like to thank my examiner, Per Öberg and my fellow students for rewarding discussions and inspiration. Last but not least, a huge thanks to my family, who has always supported me. Sandra Alfredsson Arvika, September 2011 vii Contents 1 Introduction Background Problem formulation Purpose and goal Expected results Related research Outline of the master thesis Theory Heat pump construction Compressor Expansion valve Heat exchangers Four-way valve Receiver Drying filter Thermodynamics of a heat pump Ideal gas Isentropic process Superheated vapour Subcooled liquid Temperature glide The vapour-compression refrigeration cycle Heat pump models Compressor Expansion valve Heat exchangers Four-way valve Sensor fault models Diagnosis Concepts Generalised minimal hitting set algorithm Test quantities ix x Contents 3 Experiment set-up Measurements Results Model validation Compressor Expansion valve Plate heat exchanger Air heat exchanger Four-way valve Test quantity validation Model based test quantities Model based test quantities with estimated inputs Test quantities based on the variance of the sensor outputs Diagnosis algorithm validation Detectability analysis Isolability analysis Conclusions and future work Contributions of the master thesis Conclusions Future work Bibliography 37 A Abbreviations and nomenclature 39 B Estimated parameters 40 Chapter 1 Introduction This chapter gives an introduction to the system for which the diagnosis system have been created. It describes the problem, the purpose, the goal and the expected results of the master thesis. A short review of related research is given and the chapter is ended with an outline of the remainder of the report. 1.1 Background The purpose of a heat pump is to control the temperature of an enclosed space. This is done by using heat exchange with a heat source, for example water, air, or ground. A heat pump basically consists of a compressor, an expansion valve and two heat exchangers. There are also a number of sensors in a heat pump to monitor and control the heat exchange. There are four different heat sources from which the energy for the different types of heat pumps is collected: ground heat (vertical loop or horizontal loop), lake heat and air heat. An air source heat pump consists of two units: an indoor unit and an outdoor unit. The indoor unit consists of the control system and the water heater, while the actual heat pump is located in the outdoor unit. In the studied air source heat pump, the outdoor air exchanges heat with a refrigerant, which also exchanges heat with a water distribution system. The heat pump can be used for both heating and cooling. In this master thesis, the focus lies on model based diagnosis of the outdoor unit of an air source heat pump during heating production. 1.2 Problem formulation The temperature sensors play an important role when it comes to the functionality of a heat pump. For example, due to air humidity, frost starts building on the air heat exchanger at low temperatures. When this happens, the air heat exchanger needs to be defrosted, but if there is an error in any of the temperature sensors, the defrost function may fail and the pump efficiency will be negatively affected. 1 2 Introduction When the heat pump is dysfunctional it would be very useful from a maintenance point of view to be able to narrow down the number of components that might be faulty, since it may be very time consuming to manually find the faulty component. Therefore it is desirable to detect and isolate errors in the sensors to minimise the risk of losing the functionality of the heat pump and to ease the maintenance of the pump. 1.3 Purpose and goal The purpose of this master thesis is to find ways to diagnose possible sensor errors in an air source heat pump. The primary sensors to be considered are the temperature sensors in the refrigerant circuit. Other faults to be considered are broken pressure sensors. Errors to be investigated for all sensors are: Open circuit Short circuit Stuck Offset The goal is to create models to be able to construct a model based diagnosis system. The components that should be modelled are: a compressor, a four-way valve, a plate heat exchanger, an expansion valve and an air heat exchanger. The components and their functionalities are described in Section 2.1. The diagnosis system should be active during heating production, but not during cooling production and defrosting. Since it is more likely that one error has occurred than several, the primary focus of the diagnosis system is to detect and isolate single faults. 1.4 Expected results The master thesis should result in a number of models, from which test quantities should be created. The test quantities will be used by a diagnosis system to be able to detect and isolate sensor errors during stationary heating production. 1.5 Related research There are two fundamental principles for constructing models: physical modelling and identification. Physical modelling means using knowledge of the system in order to create models, while identification means adapting the properties of the model to the properties of the system. For related work on this topic, see [4]. A heat pump basically consists of two heat exchangers, a compressor and an expansion valve. The properties of these components have been studied extensively and some of the relations can be found in for example [2] or [3]. Once models of a 1.6 Outline of the master thesis 3 system are created, these can be used to create a diagnosis system by comparing the output of a model with the output of a sensor [6]. Since a diagnosis system that handles several behavioural modes in the sensors is desired, it is necessary to use an algorithm that can handle more than two modes per component. An algorithm of that kind has been developed in [5]. 1.6 Outline of the master thesis The outline of the master thesis is: Chapter 1 gives an introduction to the system and why diagnosis is desired. The chapter also describes the purpose, the goals and the expected results of the master thesis. Chapter 2 gives a description of the components and the functionality of the air source heat pump. It also provides a short description of the thermodynamics involved in a heat pump. Furthermore, the chapter contains the estimated models of the system and the fault models. The chapter is ended with an introduction to diagnosis. Chapter 3 gives information regarding test conditions, how the data collection was carried out, which test points were used and how they were chosen. Chapter 4 contains model validation and test quantity validation. The chapter also contains a detectability analysis and an isolability analysis. Chapter 5 consists of the conclusions from tests and results and gives suggestions of improvements and future work. Tables of abbreviations and nomenclature can be found in Appendix A. The estimated parameters for the models are given in Appendix B. Chapter 2 Theory This chapter gives a description of how the studied heat pump is constructed and where the sensors are placed. Some thermodynamic concepts are introduced and the vapour-compression refrigeration cycle is described. Then the chosen models of the heat pump are stated and the chapter is ended with some diagnosis theory. 2.1 Heat pump construction The main purpose of a heat pump is to maintain a high temperature in a heated space. This is done by absorbing heat from a source with low temperature and supplying this heat to the object of the heating. The studied heat pump can also be used to cool down a space. When cooling is the desired functionality, the heat pump absorbs heat from the space to be cooled and rejects it to the source used for absorbing heat during heating production. There is also a defrost mode, which is used when there is ice on the air exchanger. The air source heat pump that has been studied during this master thesis consists of two units: an indoor unit and an outdoor unit. The actual heat pump is located in the outdoor unit, while the indoor unit consists of the control system and the water heater. A schematic view of the outdoor unit, also referred to as the refrigerant circuit, can be seen in Figure 2.1. The refrigerant circuit is connected to the distribution system indoors through (8) and (10). The compressor(1) is connected via the four-way valve(3) to the plate heat exchanger(6). The receiver(11) and the drying filter(13) are placed along the connection between the plate heat exchanger and the expansion valve(14). The expansion valve is connected to the air heat exchanger(16), which in turn is connected via the four-way valve to the compressor. There are nine temperature sensors, located at (2), (5), (7), (9), (12), (15), (17), (18), and (20). There are also two pressure sensors, located at (4) and (19). The heat pump components are described in the following sections. 5 6 Theory Figure 2.1. The refrigerant circuit at heating production. The picture shows the components, the sensors and the flow direction of the refrigerant and the water during heating production. A description of the components is given below. Position Description 1 Compressor 2 Temperature sensor (discharge pipe) 3 Pressure sensor (discharge pipe) 4 Four-way valve 5 Temperature sensor (ref 4) 6 Plate heat exchanger (condenser) 7 Temperature sensor (cond out) 8 Heating system (hot supply line) 9 Temperature sensor (cond in) 10 Heating system (cold return line) 11 Temperature sensor (ref 3) 12 Receiver 13 Drying filter 14 Electronic expansion valve 15 Temperature sensor (ref 2) 16 Air heat exchanger (evaporator) 17 Temperature sensor (defrost) 18 Temperature sensor (ref 1) 19 Pressure sensor (suction line) 20 Temperature sensor (suction line) 2.1 Heat pump construction 7 Figure 2.2. Illustration of scrolls in a scroll compressor Compressor A compressor is a device that is used to increase the pressure and temperature of a fluid by reducing its volume. The compressor used in the test object is a scroll compressor and requires a power input. A scroll compressor consists of two spiral-shaped blades interleaved with each other, see Figure 2.2. Usually, one of the scrolls is fixed, while the other one moves in an orbit around a centre point without rotating. This traps the fluid between the scrolls, which compresses the fluid as the scroll moves Expansion valve An expansion valve is a device that controls the mass flow of a fluid. In a heat pump, the expansion valve is used to control the mass flow of the refrigerant into the evaporator and thereby also the superheat and the subcooling (see Sections and 2.2.4) of the refrigerant flowing out of the evaporator. The expansion valve in the test object is an electronic valve with a controllable input signal Heat exchangers A heat exchanger is a construction where two mediums exchange heat. The heat exchange usually occurs through a wall that separates the mediums. The heat exchange is intended for the two fluids within the device, so the heat exchangers are usually well insulated. The fluids can be flowing in the opposite direction (counter flow), in the same direction (parallel flow), or perpendicular to each other (cross flow), see Figure 2.3. Fluid 1 8 Theory Fluid 2 Counter flow Parallel flow Cross flow Figure 2.3. Schematics of different heat exchangers Two types of heat exchangers are used in the test object: a plate heat exchanger and an air heat exchanger. These are described in the following sections. Plate heat exchanger The plate heat exchanger consists of several plates where the fluid passes through corrugated passages. The plates are usually made of metal or another material with high thermal conductivity. The plates give a large surface for the heat exchange, since the fluids are distributed over the plates. The two fluids flow through alternating passages, which means that the colder fluid always has two adjacent passages with warmer fluid. In the test object, the fluids flow in opposite directions (counter flow) during heating production. Air heat exchanger In the air heat exchanger, the refrigerant is distributed into different sections, flowing in pipes in both the perpendicular direction and in the opposite direction of the air flow. This makes the air heat exchanger a combination of a cross flow and a counter flow heat exchanger. The air is sucked through the air heat exchanger by a fan with a controllable input signal Four-way valve The four-way valve is a device that makes it possible to control which way the fluid flows. In a heat pump, the four-way valve makes it possible to switch between running modes (heating, cooling or defrost). The four-way valve is constructed in a way that makes two flows of different temperature interact thermally with each other (this is however not the purpose of the valve, but needs to be considered in the modelling). Figures 2.4 and 2.5 illustrate the different settings of the valve. The section with dashed lines can be moved to change the flow path. 2.1 Heat pump construction 9 From compressor To plate heat exchanger To compressor From air heat exchanger Figure 2.4. Schematic view of the refrig
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