HVAC circuit

In Figure 3.7, the layout of the HVAC circuit can be explored. The discussion of this circuit will be divided in three sections, in order to better clarify the developed control strategies.

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Figure 3.8 AC Compressor working map

3.3.1 Model and layout description

The refrigerant fluid for this circuit is the R1234yf and a total charge of 0.85Kg has been used to test the system. The main components are the ones of a common HVAC circuit, as described in Section 1.3. The compressor works according to a map taken from similar applications found in the GT open library (Figure 3.8), and its speed is controlled by an additional module. The two condensers are the main refrigerant-side templates coupled with the underhood condenser and the cabin condenser, respectively.

They are both thin tube heat exchangers, but the underhood one has an horizontal configuration (550x350x20mm) with 2 passes and a total of 50 tubes in the only row, the cabin one has a vertical configuration (250x300x25mm) with 3 passes and a total of 22 tubes in the only row. So, it is reasonable to say that different heat exchanges are expected.

The two condensers are used alternatively depending on the passengers AC requests. If the passengers activate cabin heating, the underhood condenser is not used, so that the heat coming from the battery and dissipated by the chiller can be re-used and not wasted in the environment. On the contrary, if the AC request is off or the conditions are such that it is more convenient to use the electric heater, the underhood condenser can receive all the flow. The refrigerant flow diversion is managed by EV1 and EV2, two simple open/closed solenoid valves. Two additional similar connections (EV5 and EV6 or EV9 and EV7) are employed also at the outlet of the heat exchangers so that the one that is not active cannot alter the fluid state in any way. These connections are not real valves, as they represent a workaround needed to avoid numerical problems. For what the evaporators are concerned, the chiller in the map is the main refrigerant-side template coupled with the coolant circuit and it has been already described in Section 3.1. The other evaporator is coupled with the cabin circuit and therefore it is used to cool down the cabin when the AC request is on. The specs of the cabin side evaporator include a thin tube exchanger once again (200x265x30mm, vertical tube flow

orientation), with 6 passes splitted into two rows configuration. This means that the exchanger has two rows of tubes along its depth, and each of them allows 3 flow passes, improving the heat exchange. This particular configuration has been selected

considering the tests that needed to be carried out and similar cabin evaporators as well, tuning the layout so that the calibrated controls could reach the target in several

operating conditions. The valves highlighted in blue in the map, EV8 and EV4, are employed as expansion valves (EXV). Although GT-SUITE does already provide a standard template in the library for TXVs, their control strategy is rather simple, acting

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Figure 3.9 Compressor control strategy

on the opening area to target the fluid superheat in a specified circuit location. In this study case, the double evaporator configuration needs a more refined control strategy for the EXV opening areas. For this reason, a custom connection has been chosen, whose control strategy has been developed from the ground up and explained in Section 3.3.3.

To conclude this section, the switch system inserted just downstream and upstream of the compressor will be discussed. The idea behind those custom connections is the same of the already explained EVA and EVB valves in the cooling circuit. EV3, EV14, EV15 and EV16 are just connections that close when a certain layout of the circuit is required.

HP-switch1, HP-switch2, HP-switch3, HP-switch4 are, on the other hand, just FlowSplit templates used to model the valve volumes, whose inlets and outlets are managed by the linked custom connections. The purpose of the linking system is to invert the direction of the refrigerant towards the heat exchangers, while keeping the same direction across the compressor at the same time. If the refrigerant mode is required, EV14 and EV15 are open, EV3 and EV16 are closed, therefore the refrigerant goes through the exchangers at the top and condenses, and then evaporates. If the heat pump mode is required, EV3 and EV16 are open, EV14 and EV15 are closed. Therefore, the refrigerant condensate in the exchangers at the bottom, releasing heat to the battery or to the cabin, and the upper condensers are used as evaporators.

The different layouts and configurations explained in this section have been used to run battery cooling tests and cabin heating tests. The results of each test with the relative layout will be discussed in Section 3.5 and Chapter 4.

3.3.2 Compressor controls

The compressor is responsible for the refrigerant flow rate in the HVAC circuit, on which the heat exchanged and the variations in temperature depend. This means that a control strategy is needed in order to generate the right amount of flow to reach the temperature targets. In Figure 3.9 the developed controls acting on the compressor speed are shown.

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The control is assigned to a PID Controller, which uses a feedback loop mechanism to evaluate the error between a variable and the target that has been set and tries to correct it by means of Proportional, Integrative and Derivative gains, hence the name. In this case, the gains have been calibrated iteratively and a target of 2 as been set. The PID has three possible input signals: chiller outlet coolant temperature, evaporator outlet air temperature in cooling and in heating. All signals are normalized to the target value of temperature, so that a single target can be set in the controller (Equations 3.1). In case of cabin heating, the input signal is the inverse of the normalized fraction, in order to make the PID behave in the same way in the three cases: an input signal bigger than 2 will make the compressor speed up, a signal lower than 2 will make the speed decrease.

If the normalized signals were all the same, in case of heating, the PID would correct the error by acting on the speed in two different ways with respect to the cooling cases.

2𝑇𝑐ℎ𝑖𝑙𝑙𝑒𝑟𝑂𝑈𝑇

𝑇𝑡𝑎𝑟 ; 2𝑇𝑒𝑣𝑎𝑝𝑂𝑈𝑇𝑐𝑜𝑜𝑙

𝑇𝑡𝑎𝑟 ; 2 𝑇𝑡𝑎𝑟

𝑇𝑒𝑣𝑎𝑝𝑂𝑈𝑇ℎ𝑒𝑎𝑡

The CompressorLogic block is in charge of passing to the PID the input signal relative to the configuration the system is into. The Table 3.2 clearly summarizes the passed signal depending on the cases.

The Table 3.2 does not include the evaporator outlet air temperature in the heating case because in the tested configurations, the cabin is warmed up by the heat extracted from the battery circuit, that can be partially dissipated in the underhood condenser or can be increased by the electric heater, depending on the cases. The cabin heating-up with a non-active chiller can be managed only in heat pump mode, not included in this project.

As Figure 3.9 shows, the RPM PID output is not directly passed to the compressor speed. As a matter of fact, while attempting to reach the temperature target, the PID could force a high value of speed that results in a pressure of the refrigerant at compressor outlet that goes beyond the saturation limit maximum pressure, causing unrealistic results. For this reason, a second PID is added to the strategy, this time targeting a maximum pressure at compressor outlet of 28bar.

Therefore, two values of compressor RPMs are calculated by the strategy, one needed to reach temperature targets, the other to reach a pressure of 28bar. The minimum signal

Table 3.2 Compressor logic

CASE PASSED SIGNAL

Cabin Heating + Battery cooling Chiller temperature AC off + Battery cooling Chiller temperature Cabin Cooling + Chiller off Evaporator temperature Cabin Cooling + Battery cooling Chiller temp + Evaporator temp

(3.1)

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Figure 3.10 Expansion valves control strategy

between the two is passed, so that as soon as the temperature targets are met with a value of speed that does result in an excessive pressure, the strategy switches to the value of speed that maintain the pressure at 28bar.

Also for the secondary PID the input is a normalized signal and it is multiplied by a factor of 2, so that the two PIDs are targeting the same value and there is no problem in switching.

3.3.3 Thermal Expansion Valves controls

In Figure 3.10 the developed strategy for the expansion valves (EV4 and EV8 in Figure 3.7) is shown. If the goal of the compressor control was to generate enough flow rate to allow the required heat exchange, in this case the objective is to reach a superheat value of 10K at compressor inlet. As a matter of fact, the normalized superheat is given as input signal to the two EXV PIDs, deactivated alternatively depending on the cabin being cooled down or heated up. The gains of the two PIDs have been calibrated to be able to meet the target robustly in different conditions, and in this purpose prospective 4 compressor speed-dependent maps have been generated, by tuning the relative gain at 3 compressor speeds (500, 3000, 7000 rpms) in each of the two cases. The derivative gain has been set to zero.

The PIDs calculates an equivalent EXV area that is needed to achieve 10K superheat.

Their outputs are given to an if block that simply let through the signal depending on which PID is active. A third PID block receives as an input signal the chiller outlet coolant temperature, the same that enters the compressor control PID and it is needed in order to meet the target temperature of the battery coolant. The EVs_Area_Control block menages the three cases the circuit can work in, deciding the areas that have to be sent to the chiller side EV (EV8) and to the evaporator side EV (EV4).

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If there is no AC request, then the equivalent EXV area is sent to EV8 and the

refrigerant evaporates in the chiller. If there is no need to cool down the battery, but the driver wants to cool down the cabin, then the equivalent EXV area is sent to EV4 and the fluid evaporates in the cabin heat exchanger. In these two cases, the targets on temperature are met thanks to the control on the compressor speed. If both chiller and cabin evaporator are active, the equivalent area represents the total area needed for the expansion and it is split up between EV8 and EV4 in order to reach the temperature targets on the specific components. That’s where the ChillerTarget_PID comes into play: once the equivalent area is found, it decides how much of that area is needed to reach the desired battery temperature; by subtraction the area for EV4 is evaluated. So, to sum up, the compressor provides the total flow needed to satisfy both battery and cabin requirements, the EV logic evaluates the area needed to meet the superheat requirement and splits it up between the chiller and evaporator. Table 3.3 schematizes the logic.

Obviously, the area evaluated to reach the target on the battery is saturated by the total EXV equivalent area: in the worst case, all the fluid evaporates in the chiller, and EV4 remains closed. Operating in this way, a priority of targets is introduced: the battery, which integrity is extremely temperature-sensitive, must be cooled down in every scenario; the cabin on the other hand has no problem to wait a little bit longer or to settle for a slightly different temperature it asked for.

Knowing the EV logic, it is also easy to understand why the compressor PID has been chosen to have as input signal the sum of the normalized chiller and evaporator temperatures in case they are both active. Even if it seems a 2 degrees of freedom equations, it comes to just one if there is another PID targeting one of the two signals.

CASE CHILLER EV EVAPORATOR EV

Cabin Evaporator OFF Equivalent TXV area 0

Cabin Evap ON + Chiller OFF 0 Equivalent TXV area

Cabin Evap ON + Chiller ON Chiller PID area Equivalent TXV area – Chiller PID area

Table 3.3 HVAC EV areas logic

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Figure 3.11 Cabin circuit