THERMOSYS 4.3. Getting Started Guide

Transcription

1 THERMOSYS 4.3 Getting Started Guide February 2016

2 Table of Contents 1 Installation and File Organization THERMOSYS by Example THERMOSYS Component Descriptions Compressor Condenser Thermostatic Expansion Valve Evaporator Box Model Simulink Blocks: On-Off Controller and Simulation Settings Additional THERMOSYS Models Not Applied in this Example Running Simulations Future Updates and Advanced Use

3 Overview: THERMOSYS is a toolbox designed for the Simulink environment within MATLAB that models and simulates vapor compression systems. Component models including a compressor, evaporator, condenser, multipurpose tank, valves, and more allow the simulation of these systems. Coupled with the power of the existing Simulink library, control schemes can be applied in order to aid the design and effective control of thermal systems. NOTE: This Getting Started Guide is for versions of MATLAB prior to 2012b. A second guide for Matlab 2012b and newer will be issued soon with updated screenshots for the new User Interface. 1 Installation and File Organization THERMOSYS includes separate instructions for installation and licensing which should be followed before using the program. Before each use of THERMOSYS, ensure that the entire install folder and its subfolders are part of the MATLAB path. The Simulink Library Browser will display the THERMOSYS toolbox if this is done properly. The toolbox should display a Components and a Fluid Properties icon. The Components section is where the user can drag blocks into Simulink models. With this in mind, we can close Simulink and take a moment to browse through the THERMOSYS install directory. Within the install folder, several other subfolders and files are available and discussed below: Component S-Functions This contains the code that each Simulink block calls. These are compiled code and are unavailable for viewing or editing. Fluid Tables This directory contains fluid property tables for the different refrigerants and gases used within THERMOSYS. THERMOSYS includes properties for Air, Ammonia, CO 2, Glycol(75%)/Water(25%), R21, R134a, R245fa, R404a, R407c, R410a, R507a, and Water. In a pending update, software will be provided that allows for the generation of these tables for additional refrigerants using the NIST software REFPROP. Heat Transfer Coefficients This contains heat transfer coefficients necessary for the models. These are compiled code and are unavailable for viewing or editing. HelpFiles This contains the various help files about the various components which will be accessible in the MATLAB Simulink help browser. Icons This directory contains the images used in each of the THERMOSYS blocks. It is not recommended that these be altered. Licensing This directory contains initialization and validation files for the THERMOSYS licensure system. The thermosys.license.dat file you will receive after sending in your license request should be placed in this folder. Sample Systems This directory contains sample refrigeration systems. These are useful for reference and show many different block connections. They also demonstrate how other Simulink blocks can be used to implement control on the systems. 3

4 Single Component Models This directory contains simple examples with individual components for a basic understanding of their inputs and outputs. Support Functions Some of these functions are called within the various component models, while others are user-oriented utilities. Within this folder, Fluid Table Generation has compiled code which generates fluid property tables from REFPROP. However, the MATLAB interface provided with the purchase of REFPROP is only compatible with MATLAB versions prior to For this reason, a separate package for fluid property tables is in development that is compatible with current versions. With the current release, these functions are not packaged for standalone use, and are only called by the THERMOSYS blocks when needed. slblocks.m This file automatically adds the THERMOSYS toolbox to the Simulink Library Browser. It must be part of the MATLAB path before opening Simulink, but no other user interactions are needed. thermosys.mdl This file contains the actual THERMOSYS blocks. Within this file, the masks (which contain the user interface and default parameters) can be viewed and edited. Editing the block masks is not recommended. The user may simulate their own system by dragging and dropping blocks from this file or the Library Browser into a new Simulink model, as shown in Section 2 of this guide. 2 THERMOSYS by Example The following is a demonstration example using Sample System #3 to get users started with THERMOSYS. Open Sample_3_TXV_Box_On_Off_Cycling.mdl in the Sample Systems. This can be accomplished by either double clicking the file within your file browser (which should then open MATLAB and the sample), or by opening the file within MATLAB or Simulink. Sample 3 is used for this demonstration as it includes a wide variety of components along with a usercreated control system (using non-thermosys Simulink blocks). These include a refrigeration loop consisting of a compressor, condenser, expansion valve, and evaporator. This loop (in conjunction with a simple On-Off controller) regulates the temperature within a Box Model (essentially a room affected by its environment). This example, as provided, regulates the temperature of the box to a value between 19 and 20 degrees C. With the sample open, run the model by clicking the play button shown at the left below in Figure 1. One can then double-click the eyeglasses-shaped icon within the model as indicated in the right of the figure below to open a scope view of the box temperature changing in time. Figure 1. Running and Viewing Sample 3. 4

5 After running the simulation for a few seconds, it is advised to click the Autoscale button within the scope view or to select Autoscale from the right-click drop-down menu. NOTE: repeatedly autoscaling the embedded scopes in the examples can crash the simulation. This is not the case for the discrete scope blocks like. If the simulation crashes when autoscaling the scope accept the error message and restart the simulation. When run successfully, the scope should display a drop in temperature to 19 degrees, and at this point the refrigeration system shuts down, allowing the environment to heat the box. With more simulation time, the refrigeration system enables again when the temperature has risen to 20 degrees. A sample of system pressures and box temperature output with the TXV cycling on and off is shown below in Figure 2. Figure 2. Sample 3 Successful Code Output (3000 sec.). 2.1 THERMOSYS Component Descriptions All components applied in Sample 3, their connections, and their initial conditions are now described in detail. Special considerations for the components will also be mentioned. Following the THERMOSYS components used in Sample 3, the remaining Simulink components will be overviewed in the Section 2.3. In this example, all inputs and outputs to most blocks are linked to Simulink standard goto and from tags for neatness. Figure 3 shows the full simulation model. 5

6 Figure 3. Full Simulation Loop for Sample 3 (block displays in Text mode) Compressor Figure 4. Compressor Block. The Compressor Block shown in Figure 4 has four inputs and two outputs. The [rpm] from tag feeds into the compressor s input. The evaporator exit pressure [Pe_out] is fed into the compressor s input pressure ( ) and the desired compressor output pressure [Pc_out] is assigned to the compressor block s output pressure input ( ). Finally 6

7 the compressor s input enthalpy ( ) is set to the evaporator s exit enthalpy [He_ro]. On the output side, the compressor block produces the mass flow rate ( which this example assigns into the [m_comp] tag. Aside from the block connections, there are additional parameters that must be set by double clicking the block. The Compressor Parameters tab contains the input for the internal compressor volume and outlet enthalpy time constant, which are constant during the simulation, and the compressor map file name CompProp. The Refrigerant Properties tab has an input for the refrigerant data filename (ex. R134a ); additional fluid files can be generated and must be placed in the Thermosys/Fluid Tables folder, and the selected refrigerant should be consistently selected across all blocks connected to each flow loop (note: in Thermosys 4.3, the selected fluid name of each block is displayed at the top of the block). Inlet and outlet pressures and temperatures are specified as initial conditions and should correspond with the surrounding components. The Initial Conditions tab allows setting initial inlet and outlet pressures and temperatures, as well as initial mass flow and compressor speed. The temperature and pressure for these locations are related and should be consistent. For example, if a saturated vapor exits the compressor, temperature and pressure are direct functions of one another. Compliance with the refrigerants properties will help ensure a stable system. Mass flow and compressor speed define their respective initial conditions and are straightforward. The Display tab allows the user to choose between Symbol and Text display modes Condenser Figure 5. Condenser Block. The Condenser Block shown in Figure 5 takes in an air flow (with temperature) as well as a refrigerant input mass flow rate, output mass flow rate, and refrigerant input enthalpy, and it returns the refrigerant output pressure, enthalpy and temperature along with some additional diagnostics. This totals five inputs and seven outputs. As with the compressor, most of the inputs and outputs are either pulled from from tags, or pushed into goto tags, to keep the inter-block connections clean. The incoming air mass flow rate ( ) is assigned from the [mc_air_in] tag. The input air temperature ( ) pulls from the [Tc_air_in] tag. The refrigerant mass flow from the compressor [m_comp] is supplied to the refrigerant mass flow 7

8 input ( ), and the desired condenser output mass flow rate (determined by the TXV valve) [m_valve] is passed into the appropriate condenser block input (.. The enthalpy of the refrigerant (coming from the compressor) [Hk_ro] is fed into the appropriate input ( ). The important outputs for the condenser are refrigerant output pressure ( )) which feeds into the [Pc_out] tag (and therefore into the TXV valve), and refrigerant output enthalpy ( ) which goes into the [Hc_ro] tag (and from there the TXV valve).). In addition to these two outputs, another five are provided, but for this example, they are not used to feed into any other blocks, and are simply attached to scopes so their values can be observed. These additional five outputs are: the refrigerant output temperature, the air output temperature, the wall temperature during the 3 operating phases (Superheated, Two-phase, Sub-cooled), the lengths for each phase zone, and the current mass in the condenser. When double clicking the block, its parameters are shown. The Condenser Block has six tabs with inputs to be set by the user: Condenser Exchange Geometry, Wall Properties, Parameter Adjustment Factors, Refrigerant Properties, Air Properties and Display. The user should refer to the THERMOSYS help files for further information Thermostatic Expansion Valve Figure 6. Thermal Expansion Valve Block (TXV). The Thermostatic Expansion Valve (TXV) Block shown in Figure 6 is a metering device, placed between the condenser and evaporator, which has some means of control based on the evaporator exit conditions. This block takes in the desired input and output pressure and input fluid enthalpy, and returns the corresponding mass flow rate and exit fluid enthalpy, as well as some valve parameters. As in this example, the pressure exit on each side of the valve is determined by other components, so [Pc_out] and [Pe_out] are inputs feeding in from the 8

9 compressor and the evaporator into the corresponding block inputs and, respectively. Note that [Pe_out] is used twice as an input. The second instance corresponds to the label P eqlzr in the library blocks, and in this case is set to match the downstream pressure of the evaporator [Pe_out]. This allows simulation of an internally equalized TXV. Attach a different pressure source to the P eqlzr to simulate an externally equalized TXV. The valve has a switch for enabling and disabling, which is supplied a constant value of 1 (indicating enable) in this Sample. When the switch has input equal to 0, the valve is closed, mass flow rate will report zero. Outputs for the valve are mass flow through the valve ( which is tied to the [m_valve] tag, and output enthalpy sample. which is not used by any other blocks in this Double clicking on the TVX block brings up its parameters which are organized on five tabs: Valve, Cv Data, Bulb, Display, and Refrigerant Properties, which all contain inputs to be set up by the user. More information on how to set up these tabs is available in the THERMOSYS help files Evaporator Figure 7. Evaporator Block. Much like the condenser, the evaporator takes in an air mass flow rate and temperature, as well as a refrigerant input and output mass flow rate and enthalpy, and returns the corresponding output refrigerant pressure, temperature and enthalpy, as well as the output air temperature. It further returns some additional evaporator diagnostics and values. In this example, the evaporator air input mass flow rate ( ) takes in the [me_air_in] tag, and the air temperature (coming from the [Te_air_in]) is fed into the evaporator air 9

10 temperature slot ( ). As the evaporator is placed between the TXV valve and the compressor, its input refrigerant mass flow ( ) comes from the [m_valve] tag, which takes it values from the TXV block, and the desired output mass flow rate ( ) takes its value from the compressor mass flow rate [m_comp]. Finally, the refrigerant s input enthalpy ( ) takes its value in this example from the refrigerant coming out of the condenser, [Hc_ro]. The output refrigerant pressure ( ) is fed into the [Pe_out] tag for use in the TXV and Compressor, as is the output refrigerant enthalpy ( ) to the [He_ro] tag, also needed for the Compressor s input side. The refrigerant output temperature is moved into the [Te_ref_out] tag for use with the TXV, while the air temperature is moved into the [Te_air_out] tag for use as a Box Model Block input. The refrigerant s effective cooling power ( ) is passed into the [Capacity] flag for use with the box model. The remaining the parameters are not needed for this example, and are attached to the scope for observation. For a full description of these as well as the Evaporator mask parameters inputs, see the THERMOSYS help files Box Model Figure 8. Box Model Block. The Box Model block is used for simulating a refrigerated trailer or similar enclosure exposed to outside elements with the effect of altering the interior environment. The inputs are: Q ref. [W] which is fed from [Capacity] from the Evaporator V dot [m 3 /hr] which is determined from [me_air_in] and [Te_air_out] from the Evaporator T amb. [C] which is assumed to be the same as the Condenser air input [Tc_air_in] 10

11 Various input conditions [Wind], [Solar], [Door], and [Product] defined as Box Model Inputs (see Figure 9 below) Outputs include the resultant internal air temperature (which is subsequently pushed to the [Te_air_in] flag, and used as input air for the evaporator), the internal and external Wall temperatures, the product temperature, and a load vector containing various heat loads in Watts. The Box Model parameters are organized on five tabs: Box listing box dimensions, UA value, and initial temperatures Product listing product dimensions and thermal characteristics Door listing door dimensions, and protection rating Solar listing emissivity of the box and the solar view factor Display which allows the user to select between the Text and Symbol display modes Figure 9. Box Model Inputs. 2.2 Simulink Blocks: On-Off Controller and Simulation Settings 11

12 Figure 10. Simulink Blocks. This example uses Simulink blocks to define constant environmental inputs for the Box Model. In addition, a simple controller is made in which the air temperature leaving the box is compared to a set point. If the temperature reaches the set point, an off signal is sent and the compressor is spun down (rpm = 0). In addition, the air cooling for the evaporator and condenser is disabled. When the temperature reaches an upper bound these are re-enabled. More complicated controls can be implemented (for instance, see Sample 1: TXV PI Control for an example of a proportional-integral controller), and parameters held constant in this example could be allowed to vary, such as RPM. PID controls are also possible, varying parameters such as compressor rpm or external air temperature. This illustrates the power of THERMOSYS within Simulink : any control scheme imaginable can be applied to these thermodynamic systems. 2.3 Additional THERMOSYS Models Not Applied in this Example Electronic Expansion Valves (EEV): These have very similar inputs/outputs/parameters to the TXV. There are both map and poly versions. The former has its parameters empirically mapped, while the latter is instead fitted with a polynomial. The Electric Expansion Valve - Poly is included in Sample 4. Multipurpose Tank Block: This block can be used to model a variety of refrigerant tanks applied in a system design, for example a receiver placed between the evaporator and compressor, or at the exit of the condenser to avoid undesirable fluid phases in the compressor and expansion valves respectively. 12

13 Hydraulic Resistance Block: This calculates a mass flow rate from a pressure difference in a tube and returns the new enthalpy. There are three versions of this corresponding to three different models: (1) The Static Valve model which requires only specification of a flow coefficient and a max flow; (2) the Full model which accounts for the pipe dimensions and uses an adaptive iterative model dependent on the flow being vapor to vapor, two-phase to two-phase, or two-phase to vapor transition; and (3) the Swamee- Jain model (non-iterative). Plate Heat Exchanger Blocks: Two fluid models for plate heat exchangers for both condenser and evaporator roles have been added to THERMOSYS. These operate on similar principles to the finned tube heat exchanger blocks. The details are described further in the help files. 3 Running Simulations Once a model has been built, linked, and has all its initial conditions defined, it is ready to be simulated. In the example, press Ctrl + e to enter the configuration parameters, or choose Configuration Parameters from the Simulation drop down menu. Start and stop times are editable problem variables within this window. It is strongly recommended to begin any new THERMOSYS models by using the parameters defined here and in the other Samples as they are relatively stable for similar configurations. If stability or accuracy is problematic, adjusting step sizes and error tolerances are options, but will increase computation time. See Figure 11 for the recommended settings. Figure 11. Configuration Parameters. 13

14 Results can be stored with Simulink scope blocks and exported at will, or passed back into MATLAB, using standard Simulink blocks. Users can create their own blocks to streamline this process in the event large amounts of data collection are necessary. 4 Future Updates and Advanced Use As mentioned above, while several fluid property tables are currently included in THERMOSYS 4.3, a software package for creating fluid property tables is in development. The default maps for the compressor and TXV performance are calibrated to specific hardware. The help files contain instructions on how to create user-defined maps for valves and compressors. Additionally, if THERMOSYS 4.3 is used with Matlab 8 (R2012b) or newer, the fluid tables will automatically populate a drop-down list, instead of the user needing to type in filenames. 14