A New Roof Embedded Solar Tri-generation System for Supplementary Comfort Cooling and Heating in Automotive Applications

Birol Kılkış1, Türker Güdü2, Alperen Aksoy2, Raşit Turan3, Haluk Aksel3, Alp Emre Öngüt3, Olgu Demircioğlu3

1 Başkent University 2 TOFAŞ

3 Middle East Technical University

INTRODUCTION• A solar sensible comfort system was designed and a prototype was manufactured.• Sensible cooling is especially essential for parked vehicles in hot summer days with

solar incidence.

DESIGN CHALLENGES:– Removal of heat from the car without opening the windows for safety reasons,– Heat sinking from the limited roof area to outdoors by natural convection and

thermal radiation is difficult,– Limited cooling surface area and limited PV area on the car roof,– Low efficiency of PV (Photo-voltaic) and TEC (Thermo-electric cooling) modules,– Limited battery capacity,– Indoor space, roof thickness, and weight limitation for all solar add-on systems.

• The above challenges were met by adapting a specially sandwiched, roof embedded set of a radiant-convective heat transfer module with minimum add-on

weight and least interference to the passenger cabin.

THE NEED

• In parked vehicles under the sun the cabin temperature may reach a critical value,

which has important discomfort and health implications.

• This condition may also decrease driving safety until the cabin cools down.

• According to ASHRAE HB-Applications, Chapter 9, after a one-hour soak under

clear sky in summer the indoor Dry-bulb air temperature at breathing level may

be 22oC to 33oC higher than the ambient.

• Several past incidences showed the urgency of resolving this problem.

• None of the available or claimed systems in the literature address this problem

adequately from environmental and health points of view.

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PV Temperature oC

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THE CONCEPT SOLUTION• The new concept is based on a pending patent, designed for sustainable

buildings for generating power, heat, and cold using solar energy, while acting also as an insulating, non-load-bearing building element for facades.

• The PV module generates electric power at its peak efficiency, because it is cooled. With a simple control, this solar power may be split between the power need elsewhere and the power need of the TEC module.

• When the TEC module is exposed directly or indirectly by a second thermally conducting medium to the indoor space, it electronically heats or cools the indoor space primarily by thermal radiation and secondarily by natural convection on its exposed surface, depending upon the polarity of the dc power.

• In the cooling mode, TEC module absorbs heat from the indoor space and transmits it to the same heat conducting medium sandwiched in between the TEC+PV and thus added to solar heat absorbed by the PV module.

THE PRINCIPLE “Three on one Surface”

PVHEAT PLATE

TEC

©2009 Birol Kilkis, Pat. Pend.

Heat Sink

Cabin Cooling

THE PRINCIPLE (Continued)

ADAPTATION FOR SOLAR VEHICLE COOLING AND HEATING

WE HAVE CHALLENGES

PV, TEC, and HEAT SINKING TRILEMMA

Dilemma 1. The efficiency of PV decreases with its temperature. In fact, a solar PV module by itself is a solar collector and absorbs much of the solar radiation in the form of heat.

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100120140160

f(x) = − 0.0250000000000001 x² + 1.225 x + 135

PV Output Drop With Temperature

PV Temperature, tpv oC

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Dilemma 2. The cooling capacity of a TEC module is proportional to power supply. Therefore, while the solar PV module warms up (if not actively cooled by some means), the cooling capacity of the TEC module decreases, and while the cabin air temperature increases too, the system cannot follow the comfort needs.

Dilemma 3. While the heat to be dissipated is almost constant, the ability of a unit fin surface area to cool the PV core decreases if colder PV temperatures are needed. This means that the fin area necessary to dissipate the almost constant rate of heat generated by the PV and TEC system must be large.25 35 45 55 65 75 85

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SANDWICHED, ROOF EMBEDDED SOLAR COOLING and HEATING SYSTEM

ROOF MOUNTING

FAN

NUMERICAL ANALYSISSAMPLE CASE

DRIVER’S SEAT

ACCEPTABLE COMFORT PERFORMANCE

Because radiation is dominant in this concept, the operative temperature (OT), approximately the average of the DB air temperature and the mean-radiant temperature (MRT), the limited solar cooling capacity is effectively used while the air temperature remains relatively high.

Another contribution of radiant dominance is the exergy comfort of the passengers that is shown on the exergy comfort diagram by Prek.

A THERMAL PINCH

• The surface area of a TEC module is not sufficient for the required cooling flux by thermal radiation and mixed convection, even when the original car fan is operating. If the fan is not operating, the convection component is purely natural. The design thermal flux is about 170 W/m2 if the entire ceiling interior surface is used. If one-tenth of the ceiling area is used, then this flux becomes 1700 W/m2, which is unfeasible.

ADDED WEIGHT IS CRITICAL

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RESULTS

Solar Battery kgCO2/kWh Carbon Solar Battery Carbon

TEC Wedge TEC+Metal Hydride Wedge

Qtec Tskin ΨR Unit CO2 (-CO2) ΨR Unit CO2 (-CO2) DATA POINT

-200 13.3 0.500218 0.024989 -0.73401 0.677441 0.022579 -0.73642 1-150 17.8 0.492367 0.025382 -0.73362 0.656664 0.024034 -0.73497 2-100 22.4 0.484341 0.025783 -0.73322 0.635425 0.02552 -0.73348 3-80 24.4 0.480851 0.025957 -0.73304 0.626191 0.026167 -0.73283 4-50 27.5 0.475443 0.026228 -0.73277 0.611878 0.027169 -0.73183 5

kgCO2/kWh

Embedded Exergy Based on Regular Gasoline

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-0.737 -0.736 -0.735 -0.734 -0.733 -0.732 -0.731 -0.73 -0.729

Carbon Wedge

With Metal HydrideSolar Battery and TEC

kgCO2/kWh-220 -200 -180 -160 -140 -120 -100 -80 -60 -40

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Solar Battery and TECWith Metal Hydride Cooler

Cooling Capacity

ΨR

REFERENCES• ASHRAE, 2007.ASHRAE Handbook-Applications, Chapter 9: Automobiles and Mass Transit,

ASHRAE: Atlanta.• Kılkış, B. 2009. Güneş Enerjisinden Aynı Anda ve Aynı Birim Alanda Elektrik, Isı ve Soğuk

Üreten Yeni Bir Güneş Paneli, TC Enerji ve Tabii Kaynaklar Bakanlığı Toplantısı, 30 Ocak 2009.

• Kılkış, B. , 2010. US Prov. Patent, 61304458.• Howard, T. H. 2009. Solar Powered Automobile Interior Climate Control System, US Patent

6,662,572 B1.• Gürbüz, H. and Kayfeci, M. 2008. Otomobillerde Metal Hidrit Esaslı Klima sistemlerinin

Kullanılabilirliğinin Araştırılması, Mühendis ve Makina, 49, 584, pp: 20-24, 2008.• Prek, M. 2006. Thermodynamical analysis of human thermal comfort, Energy 3, pp: 732–74,

Elsevier. • Kılıç, M. and Akyol, M. 2009. Otomobil Kabinlerinin Isıtılmasında Farklı Hava Yönlendiricileri

Kullanımının Isıl Konfora Etkisi, Isı Bilimi ve Tekniği Dergisi, 29, 1, pp: 25-36.• Arens, E. , Stephen, T. and Hui, Z. 2003. Moving Air for Comfort, ASHRAE J. No: 5, pp: 18-27. • TOFAŞ, 2010. A Hybrid, Sandwiched, Integrated, Embedded Vehicle Solar Tri-generation

System, Pat. Pend.