2.1. Heat load reduction

Over the last several decades, many technologies have been developed to keep passengers comfortable while decreasing the AC load and fuel consumption. These efforts have mainly focused on finding ways to reduce the amount of vehicular heat absorption—a phenomenon known as heat soak or thermal soak—when a vehicle is exposed to the sun for an extended period of time [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The methods explored have included cooling vehicle cabin zones independently [14], [15], [16], [17], [18], [19], [20], implementing automatic climate control (ACC) algorithms [21], [22], [23], [24], [25], [26], [27], [28], [29], managing air quality [7], [28], [30], [31], [32], and improving electric vehicle (EV) HVAC systems [33], [34], [35], [36], [37]. Each of these approaches is discussed below.

During thermal soak, a vehicle’s cabin temperature increases until it reaches an equilibrium point. The amount of heat gained during the soak, and thus the equilibrium temperature, are influenced by the transmissivity the windows and windshield with respect to sunlight, the absorptive properties of the dashboard and interior components, and the temperature of the vehicle shell. In the winter or during colder months, thermal soaking is beneficial because it heats the vehicle’s interior using available solar energy. However, during seasonally hotter months, thermal soaking results in heat that must be removed by AC or ventilation systems, thus greatly increasing ancillary loads. Researchers at the US National Renewable Energy Laboratory (NREL) have reported that reducing a Cadillac STS’s thermal soak load by 30% during the summer months can reduce AC fuel use by up to 26% [4].

2.1.3. Ventilation

Properly ventilating a parked vehicle can decrease cabin temperature during thermal soak. In a test, the NREL incorporated an array of six solar-powered fans into the sunroof of a Cadillac STS. The results showed that pulling air out of the vehicle is more effective than pushing air in, and allows for a decrease in air temperature of 5–6 °C—approximately 26% of the maximum reduction possible [4].

In addition to venting the sunroof, the NREL performed tests on alternate ventilation configurations in a 2000 Jeep Grand Cherokee [7]. Some configurations included forced convection from instrument panel vents or sunroof fans, while others were passive and only required opening the sunroof and adding floor vents. The results from this study are shown in Fig. 1. Venting the floor increased natural convection significantly; however, the researchers noted that preventing exhaust products, dirt, and animals from entering the vehicle was problematic [7].

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Fig. 1. Results of a ventilation technique study. The arrows represent the decrease in temperature compared to an unventilated, baseline cabin temperature (45 °C), while the ambient temperature is 30 °C. Bars labeled are those that require power, while those without labels are passive. Reproduced from Ref. [7] with permission of the National Renewable Energy Laboratory, © 2008.

Saidur et al. [5] sought to improve the performance of a current solar-powered, parked-car ventilator [5]. The ventilator was mounted in the rear passenger windows of a metallic gray Nissan Sunny and was powered by the vehicle’s battery and a roof-mounted 50 W solar panel. A larger motor and fan system was used to increase the ventilator’s flow rate from 20 to 110.5 CFM (ft3·min−1; 1 CFM ≈ 0.028 m3·min−1), and data were collected between 11 a.m. and 4 p.m. Overall, the temperature was reduced by 11% compared with the unmodified ventilator. The solar panel provided 31.2 W to the ventilators on sunny days, with the remaining energy being used to charge the car battery. On overcast days, when the car battery was the sole power source, it was calculated that the ventilators could be operated for 7.2 h.

2.2. Zoned and individualized cooling

Zoned cooling is perhaps one of the most researched methods of reducing vehicular AC demand. It not only lets the vehicle occupants tailor the settings based on personal comfort requirements, but also allows better overall energy management of the HVAC system.

2.2.1. Zoned cooling

Cooling the greatest surface area in contact with a passenger—namely, the seat—permits better independent temperature control. Cooled seats (“seat conditioning”) methods are not new, but the technology is continually being optimized [14], [15], [16], [17], [38]. In 2007, one common method of seat climate control used fans embedded in the seat to push or pull cabin air cooled by a thermoelectric device toward or away from occupants. Rather than using unconditioned cabin air, engineers have designed augmented systems capable of passing conditioned air through ductwork linked directly to the seat ventilation system. Air can then be cooled or heated further by a Peltier element [14]. The Peltier effect can be thought of as a reverse Seebeck effect. In a Peltier device, electricity is used to create a temperature difference between two sides of the unit. An occupant can be heated or cooled by directed air from the hot or cold side of the module. Human tests with the augmented system demonstrated a reduced time-to-comfort of 4.5 min—2.5 min less than the time achieved using a thermoelectric device, and 1 min less than that using HVAC-air-cooled seats [14]. Another approach to seat conditioning is to use an asymmetric cooling scheme. Velivelli et al. [38] combined active cooling along the seat’s lumbar support with ventilation-only in the seat cushion. Optimization of power to, and placement of, the cooling apparatus was carried out based on a human comfort-based model with predicted temperatures validated to within 1 °C of experiments.

Beyond seat conditioning, thermoelectric devices have recently been tested for use in counter-flow, air-to-air [39], and air-to-liquid [40] AC systems. Such systems can reduce engine load and may offer a less expensive, nonflammable solution to new Environmental Protection Agency (EPA)-regulated refrigerants. Though no testing has yet taken place within the context of a vehicle, simulations already exist that suggest methods of optimization for these thermoelectric AC systems [39], [40].

To improve zoned cooling further, vents can be directed at different body parts of the cabin occupant, particularly the face, chest, and waist [15]. Tests were conducted to optimize the thermal comfort provided, and it was confirmed that this system cooled the cabin more effectively than a baseline whole-cabin cooling system. Participants were asked to rate their comfort level of different body parts during the baseline test (no spot cooling) while in the car cabin. The rating scale ranged from very uncomfortable (−4) to very comfortable (4) [15]. On average, the participants’ body parts experienced an uncomfortable (−1) during the baseline, but when spot cooling was introduced and the participants were allowed to control their own temperature settings, average participant comfort rose to a comfortable 1.75–2 for all body parts. Participants also agreed that they were cooled quicker under transient conditions using spot cooling. In an effort to optimize the system, it was determined that cooling should use low flow rates to avoid dry eyes and to overcome “warm forehead” discomfort. Since the neck is a more sensitive area, some participants objected to neck or cheek cooling; however, low flow rates in these areas could be used during a thermal soak cool-down to more quickly improve transient comfort, and could then be turned off under steady-state conditions [15]. Simulations by Ghosh et al. [16]have shown that nozzles aimed at these areas cool occupants most effectively when paired with seat cooling.

In 2011, Kaushik et al. [17] simulated the thermal sensation and comfort produced by another localized/zonal cooling system. Their model was based on steady-state conditions and incorporated a human physiology model (based on the 50th percentile male) as well as a human thermal comfort model. Test data were collected from college students of different ages and genders who rated their comfort and sensation at a steady-state cabin temperature. Fig. 2compares the predicted and measured results for several cooling methods [17]. In general, the results demonstrated that micro-cooling/heating strategies can provide sufficient thermal comfort at a potentially lower AC load based on the investigated steady-state temperature of 29 °C. A recent study by Ito et al. [41]characterized passenger comfort from similar secondary HVAC devices in various thermal scenarios. The research was conducted with actual vehicles in order to validate their heat balance model, which segmented a human body and considered a blood-flow-based heat transfer. The refined thermal comfort model provides a more effective means to design cabin/AC architectures and technologies for passenger comfort under various transient/non-uniform conditions.

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Fig. 2. Thermal comfort and sensation simulation compared with test results for three micro-cooling methods.

The effects of zoned cooling on passenger comfort have been studied extensively; however, recent efforts have focused on quantifying its impact on the prime mover (e.g., the combustion engine and/or batteries). Wang et al. [18]used test data to design, test, and simulate a system that could readily be installed for commercial use in an existing vehicle. The battery-pack-powered system was placed in the trunk with the waste heat exchanger, control devices, and pumps. Power savings were based on steady-state environmental-tunnel test data taken from a Buick LaCrosse operating with standard and zoned cooling configurations. From the data, the researchers estimated a 29% power savings from the AC compressor. For greater power savings, sections of the zoned cooling system can be disabled when the vehicle is not fully occupied.

2.3. Automatic climate control

ACC was first introduced in 1964 [21], and a similar derivative of the original analog linear controller is still widely used today. Wang et al. [22] improved upon the existing algorithm by separating transient and steady-state controls to make the path to steady state tunable/adjustable. An explicit accounting of passenger thermal inertia for thermal load estimations was also integrated into the model in order to reduce reliance on internal car temperature sensors. This new method allows a prototype vehicle to be calibrated within 4–6 weeks, and its graphical software package can be easily auto-coded and incorporated into an ACC microcontroller. The system was implemented in North American and Asia-Pacific vehicles over a three-year period and met or exceeded the standard system performance in terms of efficient and effective thermal control.

Fayazbakhsh and Bahrami [23] devised and simulated another type of AC model with the potential to optimize ACC. Their lumped system algorithm enables more efficient AC use by predicting and compensating for changes in thermal load when the algorithm is incorporated into a vehicle’s computer with appropriate sensors. Using a similar method, Marcos et al. [24] developed and validated a thermal model and found that they could adequately calculate heat transfer into the cabin and estimate fuel consumption due to HVAC usage.

Donovan and Manning [25] performed a proof-of-concept ACC using a fuzzy-logic-based proportional integral (FPI) controller. Fuzzy logic is a type of programming logic that allows for “degrees of truth” rather than fixed values of “true” and “false.” Its application allowed the model to be independent of a specific vehicle and limited the memory required for implementation. The system used carbon dioxide (CO2) and IR thermographic sensors and an externally variable displacement compressor (for temperature and humidity management) in a multi-zone environment. Their scheme was able to control temperature and humidity in the zoned environment effectively and efficiently while maintaining safe CO2 levels.

Another approach to ACC, in terms of classical control theory terminology, is to add a derivative term to the proportional integral (PI) controller in order to implement a proportional integral derivative (PID) controller, which is more precise than traditional ACC. Several industrial applications use the Chien–Hrones–Reswick (CHR) method to tune the PID gains [26]. Khayyam et al. [28]combined a PID controller with a neural network tuner (NNT) to reduce power consumption and increase the efficiency of a vehicle’s HVAC system. This approach was not uncommon at the time, as Zheer-Uddin and Tudoroiu also tested a similar scheme. Khayyam et al., however, created a coordinated multi-control system (CMCS) that included a PID controller to control AC function and three stepper controllers to adjust recirculation gates and set points. The system monitored air temperature, humidity, and CO2 concentration, and used these inputs to coordinate the evaporator (for temperature management), blower (for flow rate management), and ventilation gates (for CO2 concentration management). Three simulations were performed using American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) standards to set cabin comfort conditions, while Tokyo clear-day weather data and Federal Highway Driving Schedule (FHDS) data were used to model a real driving scenario. The first simulation used a manually tuned PID, the second was tuned using CHR, and the third used an NNT PID. The NNT demonstrated no overshoot, little instability, and a maximum energy reduction of approximately 14% for the given conditions [28].

A study conducted by Furse et al. [29] in 2014 gathered real-world data from a Hyundai Genesis climate-control system. Monitoring various parameters of American customer use via smart phones, they found that temperature and blower speed were controlled manually if the cabin temperature was under heat soak conditions at or above 35 °C (95 °F). However, under mild conditions, the ACC was used more often [29]. These findings suggest that ACC is improving and becoming more useful to customers.

2.4. EV air-conditioning systems

Ever-tightening emission standards have led to increased interest in EVs. An EV’s limited driving range can be improved by implementing the aforementioned technologies and methods to reduce AC use, or by optimizing HVAC characteristics specifically tailored for EVs. HVAC systems for conventional internal combustion engines (ICEs) dispense some of their waste heat into the AC’s condenser to enable an efficient air-conditioning cycle. As EV batteries do not reach the temperatures of ICEs, they cannot heat the condenser-side refrigerant adequately, and a different method must be used to heat and cool the vehicle.

The NREL has determined that maintaining cabin comfort in cooling situations reduces EV range by 35%–50% [33]. Simulations performed by Kambly and Bradley [34] using weather data for the United States throughout the year showed that even more energy is consumed to heat the cabin of a PHEV. Torregrosa-Jaime et al. [35] noted that the challenge of cabin heating also exists in fully electric vehicles (FEV). An EV conventionally relies on electric heaters and coolers. However, despite the necessity of using electricity to run a compressor, one way to move toward a more efficient system involves the incorporation of heat pumps.

Greater research focus has been given to implementing heat pumps in electric buses than in cars. Torregrosa-Jaime et al. [35] developed a dynamic modular model to calculate the energy consumed by a heat pump and auxiliaries (cabin blowers, circulation pumps, and radiator fans) in an electric bus. The tool was validated using a conventional vapor compression heat pump, as used on a Daily Electric minibus, but the tool can be adjusted to calculate energy consumption for any EV. This model would help size components and optimize control strategies in mobile AC systems. The researchers’ analysis revealed that blowers and fans consumed a significant amount of energy (29%–40% of the total) and could be optimized to lower energy consumption depending on the vehicle and operating conditions. Nielsen et al.’s [42] simulated energy consumption from the blower is in agreement with these values.

Cho et al. [36] designed an experiment to study the performance of heat pumps using waste heat from the electronics associated with an electric bus. A controllable test setup using R-134a refrigerant was constructed to mimic the systems typically used to heat an electric bus. It was found that increasing the evaporator volumetric flow rate and increasing the outdoor temperature decreased the coefficient of performance (COP). However, the COP increased with increased condenser flow rate. A COP of 3.0 and a heating capacity of 30.0 kW was achieved when the system ran at an outdoor temperature of 0 °C with 0.020 and 0.040 m3·min−1 flow rates through the condenser and evaporator sides, respectively. The chiller was set to pump water into the evaporator at 15 °C to approximate the amount of heat output from the bus electronics. Air temperature at the heater cores was measured and reached a maximum of 45 °C after 15 min. To increase thermal response and raise the temperature to the 50 °C minimum achieved by ICEs, it was suggested that a positive temperature coefficient (PTC) heater be added to the system [36].

In an effort to remove the dependence of cabin heating on battery power completely, Taylor et al. [37] designed and tested a thermal battery that heated an EV in a 10 °C ambient environment for 1 h. They designed the thermal battery module and cooling loop for the system, and tested the output without system integration into a vehicle. The thermal battery used the PCM erythritol, a sugar alcohol, to store thermal energy for eventual transfer to the working fluid (i.e., water) flowing through coiled copper tubing in an insulated PCM container. This design was portable, weighed 20 kg, and was roughly half the cost of the conventional lithium-ion battery used to power the heater. Although the thermal and lithium-ion battery capacity and specific power density were comparable, the thermal battery was physically larger and had a shorter shelf life. The researchers also noted that thermal management of the battery could reduce EV efficiency greatly [37], as discussed by Vlahinos and Pesaran [43].

3.1. Passive IGBT cooling

Heat pipes are a relatively mature technology, having been first introduced around 1960 [44]. Noted primarily for their ultra-high thermal conductivity, a heat pipe is a passive heat transfer device that consists of a container with an internal fluid and/or wicking structure [44], [45], [46]. One type of heat pipe is the oscillating (or pulsating) heat pipe (OHP or PHP). The OHP is a two-phase, wickless device with serpentine-arranged tubes filled partially with a working fluid. When a temperature difference is imposed at opposite ends of the OHP, vapor bubbles and liquid slugs form in the evaporator (heat reception) and condenser (heat rejection) regions, respectively. The continuous oscillation and constant phase change of the working fluid drives increased heat transfer through the device. Recent advancements in OHP technology have been reviewed [46] and factors such as fill ratio, heat pipe geometry, and inclination angle have been shown to affect heat transfer. Fig. 3 illustrates the general anatomy of an OHP.

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Fig. 3. OHP operation with evaporator, condenser, liquid slugs, and vapor bubbles illustrated.

The harsh vehicular environment—which includes accelerations, inclinations, rapid air movement, and so forth—can affect the performance of heat pipes, and must be considered in automotive applications. Such factors have been studied experimentally by Burban et al. [47] with the use of a closed-end (open looped) OHP for a hybrid vehicle’s IGBT. The performance of the OHP was tested using varying air temperatures (10–60 °C), air velocities (0.25–2 m·s−1), inclination angles (45°, 0°, and −45°), and working fluids (acetone, methanol, water, n-pentane, and R-134a). The testing incorporated six cartridge heaters inserted into a 120 mm × 80 mm copper plate to simulate a vehicle IGBT. The heat pipe was charged to a fill ratio of 50% and tested with input power levels ranging from 25 to 550 W. The researchers concluded that OHP thermal performance improves with increasing heat inputs. In general, thermal resistance decreased with increasing air temperature and velocity, with the exception of tests with acetone and n-pentane. Inclinations where the evaporator was above the condenser (−45°) resulted in unfavorable thermal performance, while the +45° position proved to be only slightly more favorable in general than the horizontal inclination. It was also found that R-134a did not serve well as an OHP working fluid under these conditions, as it was outperformed significantly by the other fluids tested. Acetone and n-pentane exhibited especially desirable results at lower power levels and air temperatures, while the thermal performance of water and methanol increased gradually as air temperature and input power increased.

The use of similar cooling technologies for military electronic devices was examined by Connors and Zunner [48]. Four methods of conducting heat to a liquid-cooled edge were modeled and tested: a heat pipe, a vapor chamber, a 6061-T6 aluminum plate, and a C00110 copper plate. Both the heat pipe and vapor chamber were made from copper with sintered wicks and used water as the working fluid. Three central heating blocks produced varying CPU-type power, while two outer blocks output a constant 40 W to emulate auxiliary components such as graphics processors. Two liquid-cooled rails at 74 °C were used to remove heat from the device. The researchers found that the vapor chamber, followed by the heat pipe, outperformed the copper and aluminum plates. By testing varying device orientations, they also determined that the thermal resistance of the vapor chamber and heat pipe was not dependent on gravity for the power level tested. It was also noted that the vapor chamber tested was more than twice as heavy as the heat pipe.

Tang and Park [49] examined the use of a novel capillary two-phase loop (CTPL) device that can withstand the high vibrations found in vehicular applications. With traditional loop heat pipes (LHPs), the capillary evaporator transfers heat to the working fluid, creating a meniscus at the liquid/vapor interface that moves the working fluid from the condenser to the reservoir via capillary action. Vibrations can destroy the meniscus and impede fluid flow within the LHP. To diminish the effects of these vibrations, Tang and Park considered a CTPL that uses an evaporator with a primary fine wick that creates strong capillary forces to counteract vibrations more effectively. The primary wick is surrounded by a more porous secondary wick that can quickly supply liquid from the reservoir to the evaporator. The CTPL was tested under stationary and shock-induced conditions. Results from the vibration tests were comparable to the stationary results.