Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control

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SCHOOL OF DESIGN

MSc in Design and Engineering

Alternative solutions in automotive HVAC systems.

Comfort, efficiency and sustainability in car cabin

temperature control

Master of Science Thesis

Thesis supervisor: prof. Silvia FERRARIS Adjunct supervisor: prof. Emmanuele VILLANI

Author: Manol DIMOVSKI (10522815)

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 1

Abstract

The vehicles cabin temperature control is essential to car passengers

and thermal comfort, providing appropriate conditions all seasons. Taking into consideration that in a hot weather sunny day, due to the higher solar radiation, it is normal every car cabin to suffer from high rise of the indoor heat, which leads to peak in the surface and interior air temperatures. Controlling the thermal comfort with adjustment in temperature, however, is acknowledged to be a difficult task especially when the cabin is a rapidly changing environment, non-uniform with respect to parameters such as air temperature, air velocity and solar load. The current existing Heating, Ventilation and Air Conditioning (HVAC) mobile system have a main disadvantage it is a big consumer of energy. Since it needs to achieve the desired thermal comfort temperature as fast as possible, it directly reflects in fuel consumption in the combustion engine cars, but also affects electric vehicles as it could substantially reduce the . In both cases this requires more frequent charges of petrol or electric power. Additionally the excessive usage of private vehicles nowadays directly leads to an increase in over-consumption of natural resources, producing more CO2 and NOX greenhouse gas emissions. This redirects in increasing the natural imbalance and over-pollution. It could be easily seen that the problem, based on the passengers microclimate comfort adjustment, reflects directly into economic and ecological problem, which need to be managed.

This thesis paper investigates the possibilities of the existing alternative solutions for assisting or improvement of the mobile air-conditioning system performance in hot weather. The focus of the research is directed to private vehicles, which are more affected from the temperature misbalance in the summer since they are left parked up to several times and hours per day under hot solar radiation. Thus the simultaneous prevention of overheating inside the cabin, necessary in moment of entering of passengers, is much harder to be controlled and managed. In the process is researched variety of design solutions for active and passive influence over interior air temperature and heat reduction of the cabin. Based on multiple comparisons of experimental works and literature surveys, the research is exposing effectively ways of preventing the rise in cabin temperature. The installation of such products in mass production cars as a standard or optional feature will significantly help reducing the overall vehicle compartment needs of energy for acclimatization and therefore extend a combustion engine travel mileage or hybrid/ electric vehicle travel distance per charge, reducing the impact of the heat over the HVAC work. High lightening the opportunities for optimization of vehicle cabin physical characteristics and properties from the further research results will be made conclusions and further suggestions for possible directions for future design of heat-preventing solutions and product improvements in reduction of the usage of the HVAC system and increasing its efficiency/performance.

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Abstract

(Italian)

Il controllo della temperatura della cabina dei veicoli è essenziale per la salute, le prestazioni e il comfort termico dei passeggeri, condizioni appropriate per tutte le stagioni. In una giornata di sole calda, a causa della maggiore radiazione solare, è normale che ogni cabina dell'auto soffra di un aumento elevato del calore interno, che porta a un picco nella superficie e nelle temperature . Il controllo del comfort termico con la regolazione della temperature in un autoveicolo, tuttavia, è riconosciuto come un compito difficile, perché è un ambiente in continua variazione, non uniforme rispetto a parametri come la temperatura dell'aria, la velocità dell'aria e il carico solare. l sistemi mobili esistenti di riscaldamento, ventilazione e condizionamento dell'aria (RVCD) presentano un grande svantaggio sono grandi consumatori di energia. Dal momento é necessario raggiungere la temperatura di comfort termica desiderata il più velocemente possibile, questosi riflette direttamente sul consumo di carburante nelle auto dei motori a combustione, ma colpisce anche i veicoli elettrici in quanto potrebbe ridurre sostanzialmente l autonomia. In entrambi i casi ciò richiede più frequente rufornichento di benzina o energia elettrica. Inoltre, l'uso eccessivo di veicoli privati oggigiorno conduce direttamente a un aumento del consumo di risorse naturali, producendo più emissioni di gas serra CO2 e NOX. Si puó facilmente vedere come il problema, basato sulla regolazione del comfort microclimatico dei passeggeri, si riflette direttamente nel problema economico ed ecologico, che puó e deve essere gestito.

Questa tesi di laurea indaga sulle possibili soluzioni alternative esistenti per l'assistenza o il miglioramento delle prestazioni del sistema di climatizzazione mobile nella stagione calda. Il focus della ricerca è rivolto ai veicoli privati, che sono più colpiti dallo sbilanciamento della temperatura in estate poiché sono lasciati parcheggiati fino a diverse volte e per piú ore al giorno sotto la radiazione solare. Pertanto, la prevenzione del surriscaldamento all'interno della cabina, che si riscontra al momento dell'ingresso dei passeggeri, è molto più difficile da controllare e gestire. Nella ricerca sono state studiate una varietà di soluzioni progettuali per l'influenza attiva e passiva sulla temperatura dell'aria interna e sulla riduzione del calore della cabina. Sulla base di confronti multipli di lavori sperimentali e indagini sulla letteratura, la ricerca sta espone in modo efficace per prevenire l'aumento della temperatura della cabina. L'installazione di tali prodotti in auto di serie contribuirà in modo significativo a ridurre le esigenze complessive di energia per l'acclimatazione del veicolo econtribuendo a prolungare del motore a combustione o la distanza percorsa dai veicoli ibridi / elettrici, riducendo l'impatto di il calore sul lavoro RVCD. L'ottimizzazione delle caratteristiche fisiche e delle proprietà della cabina del veicolo sonoi ulteriori risultati della ricerca da cui saranno tratte conclusioni e ulteriori suggerimenti per la progettazione futura di soluzioni di prevenzione del

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List of figures

Figure 1 http://www.hybridcars.com/wp-content/uploads/2015/09/vauxhall-astra1.jpg Figure 2 http://gtspeed.us/wp-content/uploads/2015/12/danger-to-children-sun-heat-750x400.jpg Figure 3 http://www.adnradio.cl/images/3639675_n_vir3.jpg?u=161155 Figure 4 https://fthmb.tqn.com/fxoGknFuFhT-5TQjT_c0eTrzc3s=/768x0/filters:no_upscale()/ tesla_model3_roof-57044e5f3df78c7d9e800918.jpg

Figure 5 Redrawn from Atkinson and Hill (2015)

Figure 6 http://atermalnaya.ru/wp-content/uploads/2013/05/tonirovochnaya-plenka.jpg Figure 7 Redrawn from Farrington, et al.(1999a)

Figure 8 https://dantricdn.com/2017/o-to-do-duoi-nang-1498634227012.jpg Figure 9 Redrawn from Aljubury, et al. (2015)

Figure 10 Al-Kayiem, et al., (2010)

Figure 11 http://www.theseus-fe.com/ths_content/images/thematic/thermal/manikin-with-seat-and-steering-wheel_transparent.png

Figure 12 Thermal comfort factors

Figure 13 Redrawn from https://static.wixstatic.com/media/

5dfbab_441b9188859e4100b8bac948df406524~mv2_d_3004_2272_s_2.jpg Figure 14 Redrawn from Fanger (1972)

Figure 15 http://technox1.cafe24.com/wp-content/uploads/2016/11/newton-thermal-manikin1-1-1-1024x613.jpg

Figure 16 Ivanescu, et al. (2010)

Figure 17 http://www.kramautos.nl/wp-content/uploads/2017/09/ac-730x350.jpg Figure 18 http://www.linzing.de/wp-content/uploads/2012/01/rl11Klima02_01_1024.jpg Figure 19 http://www.audiocoustics.co.za/car%20air%20conditoning%20diagram.jpg Figure 20 Redrawn from Farrington & Rugh (2000)

Figure 21 Redrawn from Farrington & Rugh (2000) Figure 22 Redrawn from Barrault, et al. (2003) Figure 23 Redrawn from Farrington & Rugh (2000)

Figure 24 https://qph.ec.quoracdn.net/main-qimg-68b8b6d8a46a7f7cd46c30981e04de7a Figure 25 Redrawn from Clodic, et al. (2005)

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Figure 27 Ivanescu, et al. (2010) Figure 28 Ivanescu, et al. (2010)

Figure 29 https://www.liebherr.com/shared/media/aerospace-and-transportation/transportation/images/ products-and-solutions/air-conditioning-systems/liebherr-air-cycle-air-conditioning-system-architecture-zoom.jpg Figure 30 https://thermaxprofetherm.files.wordpress.com/2016/01/img_4273.jpg?w=2000&h=1500&crop =1 Figure 31 http://www.apexinst.com/cms/wp-content/uploads/2015/06/Stirling-Cooler1.jpg Figure 32 https://daisanalytic.com/applications/nanoair/ Figure 33 http://www.gentherm.com/sites/default/files/Zonal_Hvac_product-shot.jpg Figure 34 https://www.researchgate.net/profile/Gevork_Karapetan/publication/258436493/figure/fig1/AS: 297375907237888@1447911349632/Figure-1-The-semiconductor-thermoelectric-cooler-of-old-type-with-hot-and-cold-sides.png Figure 35 https://blogs.scientificamerican.comblogsassetsFileBeCoolSchematic.png Figure 36 https://www.cibsejournal.com/technical/the-appeal-of-magnetic-refrigeration/ Figure 37 http://snowders.com/curtains-for-cars-2/ Figure 38 http://www.theseus-fe.com/ths_content/images/thematic/applications/climatization/ applications_climatization_climate-chamber-results.png Figure 39 http://bronxmrc.com/wp-content/uploads/2015/12/techpage_applications3.jpg Figure 40 http://www.nissan-global.com/JP/TECHNOLOGY/FILES/2013/05/f51a4547ab11fd.gif Figure 41 https://www.virtualmarket.innotrans.de/en/Thermobreak-RT,p1509489 Figure 42 http://www.classicgarageblog.com/wp-content/uploads/2013/07/1968-camaro-headliner-b-1024x768.jpg Figure 43 https://www.heatshieldproducts.com/hp-stealth-shield Figure 44 Redrawn from Purusothaman et al.( 2017)

Figure 45 http://www.conservationsolutions.com/assets/images/insulation%20classes.jpg; Figure 46 https://i.pinimg.com/564x/1a/1d/ca/1a1dcab5e7aa67333d47d659121ee659.jpg Figure 47 Redrawn from Levinson, et al. (2011)

Figure 48 https://www.dispersions-pigments.basf.com/portal/streamer?fid=814255 Figure 49 Redrawn from BASF( 2017b)

Figure 50 http://www.aerogel.org/wp-content/uploads/2009/03/fenanofoamsem-lanl.jpg Figure 51 https://upload.wikimedia.org/wikipedia/commons/6/69/Aerogelflower_filtered.jpg Figure 52 https://cnet4.cbsistatic.com/img/1FFvwaDU-SleB33nAPen_W6M3eM=/770x433/2010/02/02/ ec51ee01-f4d5-11e2-8c7c-d4ae52e62bcc/Aerogel.JPG Figure 53 https://upload.wikimedia.org/wikipedia/commons/2/2c/Aerogel_hand.jpg Figure 54 http://www.johnsonwindowfilms.com/dealer/articleView.php?ARTICLE_ID=244 Figure 55 Redrawn from Atkinson and Hill (2015)

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Figure 56 http://www.thecrashdoctor.com/Images/PGW-sungate-weathermaster-glass-windshields.gif Figure 57 http://midwestglasstinters.com/wp-content/uploads/2017/08/Window-Films.jpg

Figure 58 Isa, et al. (2015)

Figure 59 https://www.raynofilm.com/wp-content/uploads/2014/09/nano-carbon2-01.png Figure 60 Redrawn from Rugh, et al.(2006)

Figure 61 https://www.6speedonline.com/forums/attachment.php?attachmentid=507903&d=1501791993 Figure 62 Redrawn from Aljubury, et al. (2015)

Figure 63 https://asklegal.my/p/what-does-malaysian-law-say-about-car tinting?fb_comment_id= 1419052528202099_1421105507996801

Figure 64 http://veteranlending.info/retractable-window-blinds/retractable-window-blinds-solar-screens-windows-motorized-back-awning/

Figure 65 http://www.audizine.com/gallery/data/500/273IMG_0245.jpg

Figure 66 https://ssli.ebayimg.com/images/g/CLMAAOSwnHZYV~Cm/s-l640.jpg Figure 67 Redrawn from Jasni & Nasir (2012)

Figure 68 https://sc02.alicdn.com/kf/HTB1ASy1JVXXXXcUXVXXq6xXFXXXC/outdoor-portable-car-covers-garage-automatic-universal.jpg

Figure 69 https://sc02.alicdn.com/kf/UT824x.XMhaXXagOFbXh.jpg Figure 70 Redrawn from Aljubury, et al. (2015)

Figure 71 http://assets.weathertech.com/assets/1/22/713x535/82556_Odyssey_2011.jpg Figure 72 Redrawn from Manning & Ewing (2009)

Figure 73 http://www.automobilesreview.com/gallery/toyota-prius-solar-pack/toyota-prius-solar-pack-03.jpg Figure 74 https://blogmedia.dealerfire.com/wp-content/uploads/sites/190/2015/07/How-Does-the-Toyota-Prius-Solar-Roof-Feature-Work-1024x400.jpg Figure 75 http://3.bp.blogspot.com/-eW4J4j8tZHo/VUDrIS9RqiI/AAAAAAAA5-4/7g3_GphPXEM/s1600/1kulcar.jpg

Figure 76 Redrawn from Rugh, et al.(2006)

Figure 77 https://www.audiworld.com/forums/attachments/q5-sq5-mki-8r-discussion-129/8778d1271910380-ventilated-seats-availability-074__scaled_600.jpg Figure 78 Theseus-Fe (2008)

Figure 79 https://marketinginsidergroup.com/wp-content/uploads/bfi_thumb/lead-scoring-criteria3-n3nbow2brrcie38ru4xvse4dj6ifog9zd59rcok3o4.jpg

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Introduction

1.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 10 PROBLEM DEFINITION

1.1

Nowadays the automobile is one of the most important transportation and spread vehicles for a lot of people compared to public transport. Since its debut in the end of 18th

century, the gasoline cars have being improved and modified, enlarging their role for the human life, becoming an asset that every modern family or individual needs to own. Meanwhile the extensive usage of these vehicles has brought an enormous change to the The high demand of the private transportation causes multiple circumstances in multiple terms of traffic organization and available spaces for parking not only in the downtowns of the big cities. This leads to alternative ways for parking, one of the most popular of which is leaving the car at opened parking space outside, in a lot of cases without any sufficient shadow nearby, which is critical in the summer time.

In hot weather, when the ambient air hits over 40°C, such exposure to the sun radiation causes multiple further consequences for the car and its passengers. The amount of heat, received from the intensive sun radiation activity leads to highly uncomfortable and disturbing warmth, which is incompatible with the sense of comfort and even dangerous for the driver and the passengers once they enter inside the vehicle. The reached heat is harmful and even fatal for any left belongings, including pets as well.

Hitting more than 72°C in a summer day, the interior of a car becomes as a real oven.

year in the United States (GTSpeed, 2015).

From another point of view, the comfort inside the cabin of vehicle is expected by default from the user. In the high-end class of vehicles, where the buyer is ready to pay more than 60-100 000 for a luxury car, is inadmissible to have such disturbing factors as Fig.1 Vehicle

exposure outside a hot and clear sunny day

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 11 the unpleasantly high temperatures. Of course, the luxurious and higher class vehicles already have multiple installed features as an option. But this does

all so effective in terms of sustainability. An example of this could be the remote accessed Heat, Ventilation and Air-Conditioning (HVAC) system of most high-class vehicle manufacturers, which start working several minutes before the passenger enters the vehicle in order to reach the desired internal air temperature without suffering from the over heat.

This significant amount of heat, which the vehicle gains from the sun activity in the hot weather, affects directly on the fuel consumption of every gasoline, hybrid or electric vehicle, because further usage of the HVAC system consumes a lot power. In general, a reliable car nowadays is expected to be driven for at least 500 km before another easy and quick refueling. In addition the vehicle is made to be capable of keeping up with the overall flow of traffic, which additionally increases the fuel consumption. Thus the tailpipe smoke and burn emissions from gasoline vehicles have lead in greatly pollution of the environment and is believed to be one of the leading source to recent dramatic change in climate.

RESEARCH OBJECTIVES 1.2

Since the first implementation into the vehicle, throughout the evolution steps which it has made up to nowadays, the development of the interior thermal comfort maintenance with different solutions is object of unstoppable evolution. The main purpose of this thesis work is to examine the potential and the opportunities of the alternative systems and solutions reliable for this thermal comfort of the occupants. The directions of the research topic are:

 giving a guideline in improving and developing the overall thermal comfort of the occupants in terms of better temperature and heat distribution inside the vehicle compartment;

 limiting or reducing the influence of factors leading to excessive usage of the HVAC system and thus minimizing the effect of its circumstances;

 exploring possibilities of reduction the heat load in a vehicle in warm weather through investigation, comparing and highlighting the most effective existing or conceptual solutions above them;

 analysis of their advantages and disadvantages as a general impact of improving their further development.

Highlighting the reasons and main related factors for extensive rising of cabin temperature, when the vehicle is closed and parked and under direct sunlight, is critical for further analysis and proposal of proper and effective solutions for ensuring safety and normal thermal conditions in vehicle compartment. The existing risk of damage and harm

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 12 children or pets left by chance in the vehicle or property and belongings, is enough powerful motivation for the manufacturers to qualify the problem as a not only thermal discomfort. Inside of the vehicle cabin, the users get in touch with the circumstances of thermal misbalance, led by the exposition of the vehicle compartment to the intensive solar energy (heat) high temperature of inside breath soak air, extensively hot surfaces as dashboard, steering wheel, consoles, burning seats (especially leather ones) and other cabin objects, lack of airflow and fresh air.

elopment, as seen from the evolution of the HVAC design, is led by the user needs in such conditions, which are related to their first instinctive reactions:

 eliminating the subtractive factors for the heat;  applying temperature reducing methods;

 avoiding immediately entering inside the car if possible.

Taking action in this proper order is a key point for a fastest, safest and less disturbing way to remove the heat from the internal compartment and to use the car without further thermal comfort disorders.

of view in two main groups:  instantaneous;  preventive

The heat reduction devices and solutions are being classified in terms of their action to the given temperature of the vehicle interior. The active solutions are designed to manage directly with decreasing of the environment temperature and act simultaneously, while the passive ones are preventive or just limiting the heat increase, so their action is indirect and generally not related to the present moment.

Appropriate design improvements and practices will be essential to help generalizing more objective and fair overall model pattern, which creates future opportunities of improving and revolutionizing of temperature management in the cabin comfort via alternative and environmentally friendly solutions for cabin HVAC systems. The final conclusions will consider the diverse priorities for future development and improvements in this area in order to give a reliable direction for new design concepts and solutions.The right oriented proposals for such of improvements are essential for revolutionizing the comfort inside the vehicle cabin and the overall work of the HVAC system, making it much more efficient. Reducing the indirect pollution made from its usage will decrease its footprint index to the environment with closer to zero values and thus helping to the efforts of mankind for a balanced nature.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 13 METHODOLOGY

1.3

In this research will be analyzed several main aspects related to the continuous development and evolution of the interior thermal comfort technology in vehicles. Starting from the main factor the solar load and its influence over air-temperature, continuing to the consequences related to them, which are directly affecting the physical and psychological perception of the human body for comfortable conditions. The definition of thermal comfort is going to be presented, and issues in terms of its application, related to the drivers and passengers of motor vehicles, will be examined. Thus will be revised already made models, testing methodologies, and applying current standards. The leading purpose in the development of this research work is the sustainable and ecological aspect of assuring comfort temperatures in vehicle cabin. After deep analysis it will be assumed that even simple design steps and improvements such as good cabin insulation, window sunlight tinting/glazing, solar reflective paint or conserving temperature devices/solutions could be essential key points in reducing the amount of internal heat, further less usage of the HVAC system and assuring the comfort traveling of the passengers, without negative affecting of the environment. This will directly cut the overall vehicle consumption coming from smaller amplitude in cabin temperature between operating/ non-operating HVAC system, which will increase and assure the better comfort in the cabin as well it is essential for the driver, passengers, pets and goods.

In the active and passive products research sections will be investigated the solutions and technologies for thermal comfort coverage. It will be examined multiple literature surveys and research papers, based on study cases, scientific publications and reports, including multiple scale model experiments, temperature distribution investigations, passengers and cabin heat experiments, existing HVAC efficiency tests and calculations as source of a deeper research on the topic for better understanding the main challenges in providing effective cooling in vehicle cabins nowadays. Experimental reports from developing concepts will be also analyzed. It will be considered the user perceptions and existing habits for solving problems related to the thermal comfort area. Thus will be examined different scenarios of overheat prevention, but also different possibilities for maintaining cooler temperatures inside of the car cabin in hot days, leading to evaluating the opportunities and to find possible alternatives and assistance to the existing HVAC solutions. The variety of researched possible solutions will be examined in terms of overall temperature/ heat reduction of the vehicle cabin, coefficient of performance (COP), feasibility and user interaction point of view and cost for implementation.

In the final section, based on the realized findings from the research process, will be made conclusions and further suggestions. There will be proposed alternative ways in solving the problem that could have the potential to change the further development of the automotive HVAC systems, thus making vehicles much more effective, eco-friendly, but also much more comfortable for their passengers.

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Analysis of the heat

2. factor

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 15 There are several factors that cause a parked vehicle interior to become that hot almost like an oven place. Based hese include its exterior and interior surface colors, their materials, the type, shape, angle and size of the windows, and also the size of the passenger compartment.

According to numerous experiments, including a detailed one of Atkinson et al. (2015), the sun radiation (Figure 3) is considered as a main factor on elevating the vehicle interior temperature even on mild temperature days (Atkinson & Hill, 2015).

Due to the necessity of reduction of the environmental poluion made of the usege of fosil fuels (directly from combustion engine vehicles or indirectly by producing energy for charging electric vehicles), the car manufacturers have been taken in consideration the aerodynamic aspect of the vehicles deeper in development of every new model for less airflow resistance and thus reduction of the overall vehicle fuel consumption.

Since the modern cars are designed to have better air-dynamic performance, but also increased perception for the surrounding environment and overall visual comfort, their windows use an extensive amount of glass. It is coincidence of the lower-mounted angle of the windshield and windows, which shape have become more oval, thus including more space, which directly leads to a bigger impact of sun energy entering the interior of the vehicle. The implementation of the panorama windshield and roofs nowadays (Figure 1, Figure 4) is a common design solution, which affects all interior surfaces that can be subjected to direct sunlight.

Of course, the mounted glass in a lot of the modern cars, specially those on the rooftops and bigger windshileds, has been tempered with particular technologies in order to reduce their impact of gained solar light and energy from the solar load. Therefore, multiple experiments confirm the influence of the window size and angle over the soak temperature in vehicles. An experiment of Atkinson (Atkinson & Hill, 2015) gives a

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 16 good idea of the effect of window properties (Figure 5) over 7 vehicles, produced in 1985. It could be seen the huge difference in the average breath temperatures in the cabin of several different model types of cars, especially the gap between Mercedes 190 and Chevrolet Camaro of more than 10°C. The key point of understanding and managing the

GREENHOUSE 2.1

The solar load is consisted of ultraviolet (UV), visible and near-infrared (NIR) radiation.

can be found in conditions where the short wavelengths of Figure 6 Transmittance of conventional cabin windshield

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 17 visible light pass through transparent environment and being absorbed after that (Figure 6). However the longer wavelengths (750-1000nm) (Figure 7) of the infrared waves, re-radiated from sun-warmed objects are unable to pass through the environment since it cannot transmit them effectively.

The trapping of the long wavelength radiation in vehicle compartment is the main factor in increasing the heat of the interior objects and air (Figure 6), causing drastic higher resultant interior temperature since the car is left completely closed as an empty taped glass jar

compartment.

The short infrared waves are slightly transmitted through the clear glass which is proved by the warmer surface of the glass when it is exposed to solar light. The long infrared radiation waves (mainly in the far infra-red band above 5µm) are being 100% blocked. Even ordinary float glass is practically opaque to radiation with a wavelength higher than 5µm. Short wavelengths of visible light are readily transmitted through the transparent

of ultraviolet light are largely blocked by glass since they have greater quantum energies which have absorption mechanisms in the glass, which could be proved even if a person is uncomfortably warm with bright sunlight streaming through, he will not be sunburned (Nave, 2015).

CAR CABIN HEAT ACCUMULATION AND THERMAL DISTRIBUTION 2.2

The temperature of car cabin can rise drastically if the vehicle is left parked outside without shade. This is due to the materials which the automobiles exterior is made from. In

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 18 order to be enough strong, rigid, cheap, easy to maintain and reliable, almost every car chase, body parts, panels are made from metal, mainly steel sheet panels. The windows are commonly produced from glass and in some rare cases from transparent thermoplastics. Since the steel is one of the best thermal- and electro conductors, this reflects directly to the temperature distribution from the warmed body panels by the solar load into the other parts of the vehicle compartment, which are in contact with the metal chase. Since the engine is not working, it cannot provide effective ventilation in order to decrease the rising amount of heat from the interior. The level of heat from the solar energy is so big that it can damage some internal parts, left property and harm children or pets left in the car, even in a short time of 10 min. As described in Section 2.1, the direct sunlight over vehicle is converted by its windows from solar radiation into long wave thermal radiation, . The already entered sunlight is absorbed and re-radiated from the warmed interior air, objects and surfaces, and thus being trapped inside with no way to escape (Figure 8).

The process of warming the cabin interior is consequence of 3 processes, attributed to conduction (air volume inside), convection (presence of metals and heat absorbing materials inside) and radiation (from the glass and body of the car by the sun). Thus the internal soak temperature amplitude of the vehicle is directly related to the amount of thermal radiation exchanged between the ody and the environment, but also to the radiation absorbed and eradiated by the cabin compartment.

Multiple experimental, numerical and simulation tests and investigations of different studies and researchers show the exact process of increase and diffusion of accumulated heat from the cabin, its further consequences to the exposed surfaces and the regularity in the temperature distribution. The optimal and hottest possible scenario could be seen in the research report of a team from Baghndad, Iraq (Aljubury, et al., 2015) as a refference.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 19 Their experiment investigates the change in the breath soak temperature of a vehicle cabin, which is left under hot for couple of hours. It gives a lot and precise information (Figure 9) about the effects of solar radiation on car cabin components (steering wheel, dashboard, seats, inside air temperature) in an intensive case, where the ambient temperature reaches 43°C. Orienting the vehicle to face south ensures tracking the sun most of the day with maximum (thermal) sun load from the front windshield the biggest glass surface in most of nowadays . Taking in consideration the average values of the actual , the proposed exploitation of energy loads and heat transfer into the cabin can be examined accurate and dependable.

The results from measurements (Figure 9) show the progress of change and maximum temperature values of front dashboard, inside air, front seats and internal air in the parked car. The temperature inside the car rises in the morning more rapidly than the ambient. The rising in the first one hour is the highest compared to the other periods, which shows the impact of - the visible radiation is transferred into thermal infrared re-radiation. It is also observed that the temperature of the measuring points cool in the afternoon faster than the ambient. Reasonably, around 12:00h the values of the temperature reach its maximums due to the maximum value of solar load of 954 W/m2

available around 13:00h. The dashboard has the maximum temperature of 99°C between components due to the largest projected area of glass facing the solar rays, the dark color and the big exposed surface.

The results of another experimental research of a university team (Al-Kayiem, et al., 2010) could be used as confirmation of the overall result from the numerical simulation they are publishing. It gives much better illustrative information of how the temperature is

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 20 distributed inside the common cabin interior. It is noticed that the highest temperature spots are close to the front and rear windows (Figure 10). The warmest air regions are near the top roof and near the windshield glass at the front and the rear, confirming almost the same temperature distribution pattern from the experiment of Aljubury et al.(2015).

It can be assumed that the experimental results indicate the dashboard as a functioning thermal sink of solar radiation and source of convection heat, which is transfered to the adjacent air particles and directly affects the cabin air temperature (Al-Kayiem, et al., 2010). Elimination of this factor will improve the overall effect of less usage of HVAC system and further fuel consumption.

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Thermal comfort

3. research

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 22 Taking in consideration that thermal comfort is that state of mind the individual expresses satisfaction in relation to thermal environment and is assessed by subjective evaluation (ASHRAE Standard 55, 1992), achieving and maintaining it inside the cabin is a fundamental ergonomic aspect for customer satisfaction in any kind of vehicle. Through the years it has become more important due to the increasing mobility of people, which results into more time spent and distances overtaken by people inside the vehicles.

The thermal comfort is subjective perception for the surrounding environment, because it is different for every individual. It is maintained when the heat generated by the human metabolism is able to distribute at a rate that maintains thermal equilibrium in the body,

produced from the body stays in balance with the heat loss. However, as consequence of changing factors, the balance is able to incline the body temperature as well. When temperature changes above the scope of a human body automatic temperature regulation, the body produces more heat than it dissipates, which leads to substantial discomfort. If it is available a big difference between gain and lost heat from the human body, there are even severe risks for human health and specially babies and children, but also pets, deliberately left or inadvertently trapped inside a closed vehicle. If the body temperature of an adult person exceeds 40°C, the chance of getting heat stroke or hyperthermia is absolute real. The situation with the children is even worse since they can reach and maintain dangerous body temperatures much faster.

Despite the human perceptions of receiving feedback from the surrounding environment are very sensitive, it is common issue that a thermally comfortable environment generally is accepted unconsciously, but an uncomfortable and disturbing one is recognized simultaneously, leading to immediate response of impatience and further necessity of taking sudden actions in order to change the conditions.

According to ASHRAE Standard 55 (1992) and Fanger (1972) the heat balance of a human body is affected by six environmental and personal key factors (Figure 12) air temperature, air motion, relative humidity, mean radiation, metabolic rate and clothing insulation. These factors are always kept in mind in developing effective and precise air-conditioning solutions. The t

to surrounding thermal environment, which makes ambience quality an important criterion inside the cabin. It influences not only the thermal comfort inside the vehicle, but

it also decreases the risk of possible , led from

damaging and disturbing factors of environment perception. Every one of the six factors of thermal comfort plays significant role in the overall perception, so analyzing them is obligatory in providing a precise thermal comfort model and building a real perception for the environment.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 23 AIR TEMPERATURE

3.1

The air temperature is an average temperature of air, which surrounds the body, with the respect of location and time. The spatial average takes into account the ankle, waist and head levels, which vary for seated or standing occupants (ASHRAE Standard 55, 1992). Since the interior has a relatively small air volume, it is much easier to be influenced by heat exchange or HVAC system airflow. It is also an inhomogeneous zone with significant differences between soak air temperature and surface temperatures due to the unequal exposure to the sun, respectively the area of the exposed surfaces of the different components. T air temperature is taken as the average of several positions measured if not specified.

Since humans create different temperatures at the different parts of the body, ASHRAE Standard 55 prescribes 3ºC for the vertical air temperature difference between head and ankle level. (ASHRAE Standard 55, 1992)

The air temperature alone is one of the leading and most sensible factors for thermal comfort. Its change is able to easily manipulate the levels of perceptions for it in humans. Thermal comfort and the related air temperature effect driver alertness. For example, in an experimental study, examined by ASHRAE Standard 55 (1992), drivers of a moving vehicle missed 50% of test signals at 27 with reaction times 22% slower than those at 21°C.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 24 MEAN RADIANT TEMPERATURE

3.2

The Mean Radiant Temperature of any environment is defined as that uniform temperature of an imaginary black enclosure which would result in the same heat loss by radiation from the person as the actual enclosure (ASHRAE Standard 55, 1992)

It is related to the amount of radiant heat which is absorbed or emitted from a surface, depending on the of soaking, but from the emissivity of the surrounding

The precise measurement of mean radiant temperature is very complex and time consuming in the case of vehicle interior, because it is consisted of the temperatures of all surrounding surfaces and additional index factor for any of them. So it is introduced the operative temperature, combining the effect of air and mean radiant temperature in a given place, which could be easily and directly measured. In the mean temperature the influence of the air velocity is neglected. It should be taken in mind that the operating HVAC produces high local air velocities, which cooling effect will be neglected, and this could result to false conclusions. (Zhou, 2013)

AIR VELOCITY 3.3

Air velocity is the average speed of the air to which the body is exposed, with respect to location and time, without regard to direction. (ASHRAE Standard 55, 1992)

Since the HVAC cools or heats the interior space with concentrated air-flow, which increases the velocity of it has an interconnection with many other factors such as heat exchange and human physical activity. Controlling the relevant air-flow and speed is essential for the perception, because the presence of large or irrelevant air motion is leading to thermal discomfort to the most sensitive parts of the body as human neck and head. According to ASHRAE Standard 55 (1992) the acceptable value for the air velocity is between 0.1m/s~0.4 m/s.Due to the existing nature of temperature distribution in the air and the nearest areas with higher temperature (bigger exposure to the sun) as roof and windows, the higher levels of the interior have bigger temperature, thus the cool air should be directed to the upper body parts. In the article published by Rugh & Bharathan (2005) they introduce the equivalent temperature to express the combined effect of air velocity, air temperature and mean radiant. Thus Madsen et al. (1986) explains that the equivalent temperature is the preferred parameter for the evaluation of thermal comfort, if there is presence of high air velocities. (Madsen, et al., 1986)

RELATIVE HUMIDITY 3.4

It is defined by ASHRAE Standard 55 (1992) that the ratio of the amount of water vapor in the air to the amount of water vapor that the air could hold at

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 25 the specific temperature and pressure Humidity inside the vehicle is influenced by the evaporation of sweat from the human body in a warm temperature. Sweating is an effective heat loss mechanism that relies on evaporation from the skin. Keeping the relative humidity in the cabin within a proper range is essential in the hot weather. However at RH>70%, the air has close to the maximum water vapor that it can hold, which limits the evaporation and therefore the heat loss. The recommended level for RH is between 30%~70% for automotive interiors. If the RH<30%, the dry air is going to lead to uncomfortable dry sensation and discomfort. (Zhou, 2013)

Relative air humidity is directly connected with interior air temperature. In the related research of Zhou (2013) the increase of temperature leads to decrease of humidity and contrary. Additionally, when the difference between the inside temperature and outside temperature increases, the maximum relative humidity decreases. There is a particular zone of comfort, which is a ratio between air temperature and humidity (Figure 13).

HUMAN ACTIVITY LEVEL (METABOLIC RATE) 3.5

As the human generates internal heat, the amount of the generated heat generated is assumed as a quantity called Human Activity Level. It is an index of the work intensity or action performed by the subject, valued as Metabolic Equivalent of Task (MET). It is a human personal parameter (1 MET=58.2W/m2

). which is equal to the energy produced per unit surface area of an average person seated at rest. According to ASHRAE Standard 55 (1992), which provides numerical rates for variety of activities, typically the value for the driver is 1.2 MET and 1.0 MET for other passengers. As a refference, the resting condition is normed between 0.7 MET for sleeping and 1.0 MET for seated position. For heavy physical work and normal sports activities, values between 8 10 MET are reached.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 26 CLOTHING INSULATION

3.6

Clothing insulation reduces the heat loss from the body and thus effecting on the heat balance, which directly means that it could either keep the body warm or lead to overheating. Normally, the thicker clothes have higher insulating ability (Innova AirTech Instrument, 1997). Of course, this depends on the type of material the clothing is made from. The insulation value of the cloth depends on the Clo unit value 1 clo = 0.155 m2

*K/W. (ASHRAE Standard 55, 1992)

The skin temperature differs in different parts of the body. Clothing insulation increases will prevent less heat loss, which means the differences between air temperature and skin temperature of each part decreases as shown. (Zhou, 2013)

ADDITIONAL FACTORS FOR THERMAL COMFORT 3.7

The establishing of thermal comfort in vehicle cabin is consisted not only by personal and environmental factors. The thermal sensation of the occupants is affected also by the physiological and psychological aspect of their perception for level of comfort. These, according to Zhou, include shades, glazing, coating, light intensity, sun load (see Section 2.1), acceleration, internal and external colors and size of vehicle. (Zhou, 2013) (These factors and their influence over the vehicle and passengers are going to be analyzed further in Section 5 as a part of product research).

Due to non-uniform conditions in interior compartment of vehicle and the variable differences in thermal perceptions of every individual occupant, it is hard to be given exact criteria of what is an optimal thermal comfort in numeric numbers. For this reason the definite for optimal environment must satisfy the majority of the occupants, which according to ASHRAE should be at least over 80% (ASHRAE Standard 55, 1992).

As Musat and Helerea (2009) have described,

ensuring temperatures of 20°C ÷ 22°C, as a result of air temperature, delimitation areas, humidity and air velocity in accordance with the activity level and clothing insulation of the occupants, (ii) by avoiding situations such as the occupants coming into contact with very cold or very hot surfaces, (iii) by avoiding air currents. These requirements must be

(Musat & Helerea, 2009). ASHRAE suggests temperature between 20°C~23°C for winter and 22°C~26°C for the summer as a standard, which matches with the Psychrometric chart (Figure 13) (ASHRAE Standard 55, 1992).

THERMAL COMFORT RESEARCH MODELS 3.8

The convective, conductive and radiative heat exchange inside a vehicle interior effects the thermal environment, characterizing it as a non-uniform environment. It is mainly due

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 27 to the action of solar load and HVAC system, which at the different part of the cabin are felt different. In extreme weather conditions like hot summer the heat balance is influenced by evaporation as well. The researchers have developed effective methods for evaluating and collecting the necessary properties of the environment. So far they are two a thermal comfort test and numerical simulation.

3.8.1 MODEL

The thermal comfort model, introduced by Fanger (1972) uses a time-dependent heat balance equation between thermal heat developed by metabolism of a human body and the heat transferred in environment (through convection, conduction, radiation and evaporation). According to Fanger, the thermal comfort is able to be predicted if the six with two coefficients: PMV (Predicted Mean Vote) and thermal discomfort, analyzed by PPD (Predicted Percentage Dissatisfied). They are based on the physiological processes that underlie human heat balance and establish the thermal comfort levels according to exposed to different thermal conditions and circumstances. (Fanger, 1972).

The PMV index has a range between -3 to +3 (-3: cold, -2: cool, -1: slightly cool, 0 neutral, 1: slightly warm, 2: warm, 3: hot), seen on Figure 14, relevant to the human sensations for cold and warm conditions. The point of thermal neutrality is pointed with PMV=0, where the temperature of a human body (or rather that on its surface) remains constant over time. The PMV model is used as a basis for most current standards prescribing methods for evaluating thermal comfort in vehicles (ASHRAE Standard 55, 1992), (Fanger, 1972).

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 28 The PPD index (Predicted Percentage of Dissatisfied) is associated with the parameter, indicating the percentage of occupants under thermal discomfort. A PPD=10% index corresponds to the interval between -0.5 a

PMV=0, about 5% of occupants are in discomfort. This means that in any case there will be dissatisfied users. (Zhou, 2013)

Unfortunately, the PMV model was developed based on data from uniform thermal environments. So it has limitations related to: (i) dynamic state, (ii) distinction between local and whole-body thermal comfort, (iii) environmental particularities of the vehicle. it represents the entire body as one object since the clothing is assumed to cover the entire body uniformly. It does not distinguish between different parts of the body, which has different sensation and local temperature and thus is unable to predict local discomfort. If one side is warm and the other cold, the PMV model will calculate a zero thermal load PMV=0, defined as a neutral condition. It is noted that the optimum value for thermal comfort (PPD is 5% and PMV is 0) can be obtained only with automatic HVAC systems, because of the influence of outside temperature. (Musat & Helerea, 2009)

3.8.2 THERMAL MANIKIN

Nowadays several tools were developed to apply on the research of thermal comfort which makes the thermal comfort test much easier and more accurate. These tools include thermal comfort manikin, physiological model and human comfort empirical model. Each model has its function and provides accurate feedback while measuring. For instance the

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 29 advanced automotive manikin (ADAM) on Figure 15 is composed of 120 individually controlled surface zones, which are connected with sensors and collect multiple type of data. The physiological model is used to simulate the human body internal physiology system and provide physiological responses as a real human. The comfort empirical model predicts local and global thermal comfort based on the collected data from manikin and physiological computes. Unfortunately, the cost of a thermal manikin and test are relatively high for researchers (Zhou, 2013).

3.8.3 COMPUTATIONAL FLUID DYNAMICS (CFD)

With this method, the passenger compartment of a 3D model car (Figure 16) is set up as figure shows, which adds the human body, a computational grid model, considering the thermal radiation and adding the solar ray tracing, calculates by using CFD software. The thermal comfort of passenger compartment is evaluated by equivalent temperature. Dividing the human body into 15 segments, it calculates the heat exchange between each segment and surrounding environment. This method provides a precise guidance for the design and development of automobile HVAC and overall interior design. (Zhou, 2013) (Theseus-Fe, 2008)

The rapid development of computer technologies and their cheaper to build virtual models has overtaken the traditional thermal comfort test method and made this method a primary for researchers. The factors in examining the thermal comfort are properly analyzed in order to be implemented into realistic simulation human (manikin) and environmental models (3D cabin), which are essential in predicting the possible change in the cabin environment and passengers, thus improving the existing solutions for thermal comfort. The aim is reduction of the negative effect of extreme environmental conditions. Examining the impact on the thermal conditions within a passenger cabin by different

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 30 manikin simulation drastically reduces the costs and time compared to physical experiments and thus providing reliable information for further product developments. (Zhou, 2013)

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Research of active

4. HVAC solutions

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 32 The main instantaneous method for climate control in the vehicle is its Heat, Ventilation and Air-Conditioning (HVAC) system. Nowadays HVAC is a basic standard equipment installed in almost every automobile (Figure 17). The demand for more comfortable and luxury vehicular thermal environment has led to a promotion in vehicles thermal control. For less than 60 years since its first implementation in vehicles (Swenson, 1995) HVAC develops from high-end extra feature for luxury vehicles into a standard system for almost every class vehicle sold nowadays. HVAC system is a technology for a precise climate control in vehicule compartment. It is designed to provide constant fresh air (through filtering system) and at the same time controlling the interior temperature by cooling or heating, in order to meet the comfort demand of the passengers. It also plays additional significant role inside interior safety considerations such as clearing the fog, mist and moisture from the windshield and windows and thus providing adequate thermal comfort in any condition.

HVAC includes three main functions heating, ventilation and air conditioning. They are interrelated and work together in order to provide and ensure the occupants a safe and comfortable temperature, good air quality in summer or in winter.

The fresh air enters into the vehicle through vents near the base of the windshield. The air is drawn into the HVAC module by a blower motor and later directted to the heater core in order to be warmed or through the air evaporator to be cooled. It continues then directed by air flow controls to the area selected by the user toward the windshield, in defroster mode, to the floor, in heater mode or through dash vents in HVAC or vent mode.

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 33 The modern HVAC systems can have several individually ventilated thermal zones, with up to 2 blowing motors, situated in the front and back compartment of the vehicle. In this

thermal zones, where the temperatures can vary between each other in order to satisfy any Figure 18).

CONVENTIONAL AUTOMOTIVE HVAC SOLUTIONS 4.1

4.1.1 MECHANICAL VAPOR-COMPRESSION HVAC SYSTEM

Briefly, the basic working principle behind the most popular type of HVAC system is exchange between conduction and convection of low/high compressed vaporized refrigerant. Absorbed heat is transferred from a low-temperature area to a higher-temperature area in the vehicle, which is caused by the pressure difference, called refrigeration. The system is based on use of five major components: Evaporator, Compressor, Condenser, Receiver/Drier, Expansion device (Figure 19). They are divided into two pressure regions: high-pressure side, including a condenser and a receiver/drier unit, and low-pressure side, with air conditioning evaporator. The separation between low and high pressure is the compressor from one side and the expansion valve from the other, situated as it is shown on Figure 19. (Gupta, et al., 2012)

4.1.2 ALTERNATING CURRENT (AC) VAPOR-COMPRESSION HVAC SYSTEM

The hybrid and electric vehicles have a little, but significant difference in the HVAC anatomy. Since an electric motor powers the car when it is driven as an electric/hybrid

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 34 vehicle, the belt-driven compressor from the HVAC system is unable to run with non-working combustion engine. Most of the hybrid and electric vehicles have a 200V AC (alternating current) electric motor implemented into the compressor assembly. Thus it takes the place and function of the belt-driven pulley found in the typical combustion engine vehicles. There is also another difference since the

heated coolant from the engine. In this case the heat for the cabin comes from an additional electric heater. All of this energy power used for the HVAC system is delivered

-voltage battery.

4.1.3 IMPACT OF HVAC SYSTEM USAGE

Since the HVAC system has become an inseparable part of ensuring thermal comfort for automobile cabins, it needs to be very efficient and reliable system. The high-efficient overall performance of the present system is capable to satisfy the occupants even with drastic amplitude difference between the ambient temperature, internal breath air temperature and user desired temperature in significantly short time (depending on the condition). Of course, the high COP of 2.5 in vapor-compression HVAC (Clodic, et al., 2005) has its coincidences, which means that there is an influence over other aspects of the system like fuel/energy consumption, environmental impact, economic situation, thermal and other factors.

4.1.3.1 Fuel/energy consumption and equivalent CO2 emissions

The vehicle HVAC system is sized to provide adequate cool down time for a high cooling load even for extreme places like Arizona, United States or Baghdad, Iraq, where the solar load can reach up to 1000 W/m2_{and ambient temperature up to 44°C. According}

to several experiments, including that of Aljubury et al (2015), such extraordinary weather conditions can lead to surface temperatures of more than 100°C and cabin air temperatures higher than 70°C. Thus the requirement for necessary energy power of the cooling mode is significant.The combustion engine vehicles use a belt-driven mechanical compressor to produce the necessary power, directly connected to the engine. In this way it puts additional load to the motor when is working, which could rise up to 5 kW (6hp).

A vehicle simulation of Farrington and Rugh (2000) has shown that although the impact of the additional load is significant for combustion engine (conventional) vehicles, it is much more critical for high fuel economy vehicles (HEV). From the experiment is visible, that moment fuel economy of a nominal 37km/L vehicle could drop to about 21km/L if the additive loads increase from 400W to 4kW (Figure 20). (Farrington & Rugh, 2000)

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 35 The big additional energy consumption due to the HVAC usage in the HEV is unacceptable. From the table is seen as well that even a minimal load of 400W on a conventional engine can decrease the fuel economy by cutting 0.4 km/L (adding 0.01L/100km to fuel economy). (Farrington & Rugh, 2000)

According to the federal US tests FUDS (an urban driving cycle) and HWFET (a highway driving cycle), the direct impact of the hybrid regime of a vehicle in motion has a big influence over the fuel consumption. Combined with a higher energy load by the HVAC unit, the overall distance range of HEV is significantly affected (Figure 21).

Though the HVAC compressor in hybrid/electric vehicle is controlled by an electronic control module (ECM) (minimizing the electrical power draw), the air-conditioning usage (Figure 22) has a significant negative effect over the fuel economy, reducing the mile range

Figure 21 HEV range simulation

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 36 of the vehicle with 9-22% at high-speed (129km/h), respectively with 16-38% in urban driving with low speed and frequent start/stops. (Farrington & Rugh, 2000).

The additional fuel consumption due to HVAC operation mainly depends on climate conditions Figure 22 shows calculated mechanical power required by the mechanical compressor in conventional gasoline vehicle. It varies between 0.9 3.5kW, depending on the ambient temperature and the engine speed (Barrault, et al., 2003). It is visible the significant increase in the energy consumption related to higher air temperatures, when the compressor works on maximum rotation speed of 3500 rpm. The biggest load is at maximum power, which happens when the occupants enter inside a left under the sun vehicle and turn on the HVAC on maximum speed for cooling down the temperature as fast as possible in order to not struggle from the excessive warm saloon.

The huge difference between real air temperature and desired comfortable temperature lead to excessive amount of power loads necessary to compensate the hot interior, mainly in the first 20-30min, when the control knob is positioned on Maximum cool airflow.

Normally, the HVAC systems are not used the whole period of the journey, but in warmer weather conditions its usage respectively increases. For instance in northern Europe, where the temperate climate is moderate and the summer temperatures are not passing over 30°C, the HVAC operate for about 24% of the vehicle running time (reference with Frankfurt, Germany). Going on the south, where the climate of some regions is subtropical, will increase the usage of the HVAC up to 60% in southern Spain and up to 70% in Phoenix, Arizona. For European diesel engines the additional consumption ranges from 21,5 L/year (Frankfurt) to about 80 L/year (Sevilla). If climatic conditions, engine type (diesel or gasoline) and user profile for thermal sensation are taken into consideration, the annual additional fuel consumption is between 2.5 and 7.5% (Clodic, et al., 2005).

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 37 Based on Fischer (1995) research, the annual fuel required to run the additional load of HVAC system is about 12.7 liters per vehicle. Given the above assumptions, the estimated total fuel used for air-conditioning is about 40 billion liters of gasoline annually, and this is only for the United States. (Fischer, 1995)

4.1.3.2 Tailpipe emissions

From a published investigation model of Farrington & Rugh (2000), (Figure 23) is visible the increase in tailpipe emissions depending on the engine modeled for a conventional vehicle in the Federal SC03 test drive cycle more than doubling the CO and NOx, when

the baseline load without HVAC is sat as an auxiliary load of 500 W. Despite the significant model variation in the test of HVAC use, where the net coefficient of performance (COP) is sat as a product of the HVAC COP and the compressor efficiency, the results confirm the fact that smaller and high economy engines suffer from bigger consumption/ tailpipe emissions when they are overloaded.

Using the Federal SC03 test, Farrington measures the effect of the air-conditioning system on fuel economy and tailpipe emissions for a variety of vehicles. The average increase (Figure 23) of CO2 with 0.42g/km and NOx with 0.053g/km in HVAC operating

mode take a big impact to the 19,300km driven annually, with HVAC operating 45% of the time. In this case it is assumed that vehicle air-conditioning usege increases CO2 by 655

ktonnes and NOx by 82 ktonnes only in United States (in case 80% of the light duty vehicles

has HVAC system installed) (Farrington & Rugh, 2000).

4.1.3.3 Greenhouse gases and footprint impact on the environment Figure 23 Predicted Increase in tailpipe emissions and fuel consumption

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POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 38 Another sufficient disadvantage, related to the HVAC usage in vehicles, is its direct effect over the atmosphere. It is already well known and proved that during the last two decades the O3 layer of the atmosphere is slowly, but effectively destroyed because of the

wide usage of refrigerant (Freon) for the refrigeration and air-conditioning purposes, which are fundamental part of working principle of these systems. The refrigerants used in the automotive HVAC are Chlorofluorocarbons (CFC), Hydrochlorofluoro-carbons (HCFC) and Hydrofluoro Carbons (HFC) (Figure 24).

According to report of Clodic et al.(2005), the properties of three of them have similar distructive and/or harmful effects to the atmosphere. The CFC refrigerant (also known as CFC-12) has the highest ozone depleting rating among the three and is a greenhouse gas as well. That leads to total ban nowadays from use or production of this gas in all countries covered by the Montreal Protocol. Unfortunately there are still a lot of old produced systems in operation with it. The HCFC (CFC-22) refrigerant has a potential to damage ozone (rating 0.05) and is also a greenhouse gas. There are still many systems utilizing these refrigerants, as well, nevertheless it is also banned for use and production since 2015. The HFC gases (also known as HFC-134a) are used extensively in every day RAC systems. Actually, there is no current ban upon these gases but it is subscribed mandatory presence of responsible usage and regular equipment inspections under the "F gas" regulations. (Clodic, et al., 2005)

The HFC refrigerants (the latest category) do not have ozone depletion potential, but unfortunately, they act as a greenhouse gases as well. Implemented for a very first time at 1990, by 1994 almost all vehicles, including cars, light commercial vehicles, and truck cabins sold and manufactured in developed countries use this refrigerant. This is a circumstance of a big environmental conserving step the implementation of the Montreal Protocol from 1989, which represents a global decision of switch from CFC-12