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VEHICLE AUXILIARY POWER APPLICATIONS FOR SOLAR CELLS

I.F. Garner

Solems S.A., France

INTRODUCTION

For almost two decades solar cells have been developed as s leading renewable energy source. Reductions io cost, helped by the creation of new markets have contributed to a host of new applications. Solar cell technology is today at the point where automotive applications can be considered. Providing an auxiliary power supply for battery trickle charging, ventilation and air circulation fans and direct power for in-car security syatems are typical uses for solar cells. In some casea after market accessory manufacturers are already supplying products for such applications. Several European, Japanese and US OEM's are examining the latest solar cell developments and making preparations for the technology changes likely in the next 2 - 3 years.

This paper reviews solar cells (or photovoltaics) as a power supply for battery charging, ventilation and auxiliary power for lighting, vehicle security and other applications. Preliminary results from testing by various authors, of solar car ventilation systems concepts are discussed. Comparisons are made of the leading solar cell technologies and design criteria for use in vehicles. Some conclusions are drawn, namely that solar vehicle propulsion is largely impractical for production cars. However, temporary solar charging can, by reducing alternator use, improve fuel economy. For vehicle security, solar cells can provide power to offset the substantial key-off loads and allow an integrated approach to vehicle electrical supply management.

The publicity attracted by solar car races and the vast budgets available for producing winning entries, shows that solar vehicle propulsion is technically feasible. However its application in large volume production cars seems likely to follow another path, at least until car buyers demand lightweight, small electric vehicles in significant numbers. Even in countries such as Switzerland where electric vehicles are sold and solar PV power systems are accepted for government funding, their use for vehicle propulsion is restricted by area considekations.

Several manufacturers have shown hybrid vehicles combining an IC engine with electric motor and solar battery charging. the most recent example being the "Duo" from Audi-VAG. Large banks of PV cells are being demonstrated in Zurich as solar filling- stations for a small number of electric cars which, once used for a short commuting journey, have several hours to recharge before the homeward trip.

Even with stored battery power, solar cannot perform more than an auxiliary charging role for such cars. This role is however, likely to prove most practical in allowing a better optimisation of electrical and other equipment in production vehicles.

SOLAR AUXILIARY POWER FUNCTIONS

An increasing range of electrical equipment is being fitted to vehicles, especially those appliances and systems designed to enhance comfort, provide entertainment and to add safety and communications features. These additional features significantly increase the load on the vehicle power supply and have an adverse effect on the starting battery, particularly in Winter.

One way of maintaining battery charge is to have an auxiliary charging system provided by an array of PV cells. Using the latest techniques to make a-Si deposited as very thin films onto glass it is possible to make an visually pleasing surface which can be used to generate power from areas of the car roof, bodywork or even windows. If the electrical appliances need to operate when direct solar power is not available, an auxiliary battery can also be installed. This supplies high current demands for short periods and can be recharged over longer periods by the solar cells ( 1 ) .

Thus the auxiliary battery and solar power supply can be used at night or in periods of several days poor sunshine. This concept can be applied to several types of electrical units of particular interest for passenger cars. buses. coaches and commercial vehicles.

- ventilator blowers supplementing AIC - alarm and security systems - reading lights/parking lights - in-car entertainment systems - in-car communications systems - main battery trickle charging

From this list of possible applications, there are several which could be treated as discrete packages for supply as optional equipment or accessories for the after-sales market. The addition of solar cells and rechargeable battery pack, independent from the main vehicle battery promotes this approach, and on highly specified vehicles can influence electrical systems significantly.

SOLAR CAR-ROOF GENERATOR TECHNOLOGY

Three main types of solar cells are available at present each distinguished by costs and light-to-power conversion efficiency.

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Crystalline silicon (c-Si) is probably the most expensive, with up to 16% efficiency, polycrystal silicon (p-Si) cells are about 14% efficient and amorphous silicon (a-Si) are the lowest cost with up to 6% efficiency. In practical terms this mean: 100 W/m' with crystalline cells and 60 W/m with amorphous, the difference from base efficiency is explained by the necessity for distinct spaces between c-Si solar cells, whereas a-Si cells are contiguous.

Figure 1 shows typical a-Si solar car roofs. The type ESG version is a thick laminate, suitable for use in the accessory market, producing typically 1OW peak output. Type VSG is a 5mm laminate suitable for OEM use in sliding roofs to minimise headroom intrusions, producing 15W peak, but destined to improve towards 2OW over the next 2-3 years. A factory manufacturing a-Si solar cells at a rate of 30 000 m'lyear, will be opened near Munich in October 1991 (2).

Several after-market solar car products are now available using both c-Si and a-Si types of solar cells. While the former is most expensive, its wide availability since the early 1970's has allowed several tests to be performed, the results of which are discussed below. In all cases there seems to be consensus that the lower costs and better aesthetics of a-Si materials will be necessary for adoption of the solar power concept by vehicle manufacturers.

Sensitivity of a-Si cells is an order of magnitude greater than other cell types in the range of visible light (400-750 nm), and a-Si requires a correspondingly smaller bulk of material for the absorption to take place. Thus a-Si solar cells can be of the thin film variety, typically several hundred Angstroms thick compared with 0.50 mm for crystal silicon cells. This has a major impact o n production economics, materials and energy direct costs and final selling price. Physically this type of thin film cell also has benefits, due to its more uniform appearance, which are of some importance io style considerations.

A further advantage of a-Si solar modules is the technique used for interconnection of individual solar cells to produce a useful voltage and current. The p-i-n structure cella are fabricated by deposition onto an electrically conductive, tin-oxide coated glass sheet which is subsequently coated with a metallic back contact. The final module voltage is achieved by a set of stepped laser-scribed divisions which allow r e a r to front series connections between adjacent cells. Ivonolithic interconnection in this manner allows a wide variety of configurations without the need for the laborious soldering required with conventional discrete solar cells.

As investment into a-Si production facilities grows, suppliers will also be able to offer semi-transparent solar cells, where the rear contact is patterned to allow light transmission. This does compromise efficiency in direct proportion, but the effect can be mitigated and this product could have universal appeal for the car roof market.

SOLAR DRIVEN VENTILATION SYSTEM

This is probably the most obvious application and seems likely to become the first offered, albeit in small volumes, by major car OEM's. Mazda have been reported a s having cars for launch in 1991 including the solar sun-roof option ( 3 ) . Several research reports have appeared with experimental results from tests on various configurations of solar ventilating fans. The most common is to use the existing aperture for the glass sunroof and replace this with a similar laminated sheet containing solar cells. With a typical area of 0.2 m*. 12W can be generated with present generation a-Si solar cells. The power generated is then used to drive standard computer-type high efficiency axial blowers.

Svstem Concept

The blowers are built into the roof recess to avoid loss of headroom. When the car is parked in sunshine, the rear rim of the roof

(see Figure 2) is tilted and the blowers swing into their correct operating position. Thus as the solar cells deliver more power with increased sunlight, s o the blowers convey more air into the car interior.

This concept uses standard commercially available parts of tilt/slide glass hatches and could easily be adapted to retrofit. The blower installation itself provides good security against theft with the hatch raised. Further options could include main battery charging and automatic closing with a rain detection system.

Svstem Test Results

Figure 3 shows the air temperatures measured in a test vehicle (VW Golf) fitted with a solar roof and blower set. lnterior overheating is reduced by 6 0 % and the whole area has a more uniform temperature. These results were achieved with a blower output of less than 200 m3/h, consuming 101; directly connected to the solar cells. It was found that blowing inwards achieved a greater cooling effect than air extraction, since the blowers must not have an "air shortage". Cy directing the air current towards the front seats and existing air vents, an almost resistance free flow can be created inside the car. A benefit noted luring trials was the reduction of plastic odours from materials used inside the car, implying increased longevity and better preservation of the upholstery and fittinss.

Other Tests and Design Concepts

A significant potential has been identified in the u s e of a solar-driven ventilator fan to enable an overall reduction in size of vehicle A / C systems. Simulations and trials show that "cool-down" time can be reduced markedly. The potential to reduce the apparent oversizing of A / C systems is large.

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As described by Ingersoll (4). in the USA market, A/C system cooling time is a major selling benefit to the buyer; resulting in typical systems rated at almost lOkW capacity, of which perhaps 3kW is the normal "cruising" cooling load. This situation has an obvious effect on fuel usage, increases vehicle weight and congestion in the engine bay reducing natural air circulation, and requires the use of more CFC's in the refrigerant with environmental consequences. In this study, at temperature reductions of 25-3OoC, the fanlblower capacity was rated at 35W to reduce A/C capacity by 30X.

A series of tests funded jointly by Volkswagen and the European Commission's DGXII has looked at retrofit accessory and OEM solar car roofs (5). In a climatic chamber a Passat saloon fitted with 0.2m' solar roof showed an 18'K temperature reduction, similar in effect to having the standard car fan running at low speed continuously. On a Jetta saloon tests with an A/C system showed that with the solar roof, differences between headroom temperature and ambient were reduced by over 50% and that cool-down time was shortened by some minutes.

More recently researchers at FIAT (6) have published results of tests on solar roofs where by careful optimisation of the built in cooling fan, motor and cooling ducts to reduce losses, temperature reductions of up to 60% have been recorded in a Croma saloon. This was achieved using a 25W solar panel, which is ultimately about the largest output possible from a glass hatch, using either a- Si or c-Si solar cells.

SOLAR POWER ACCESSORY PACKS

Assuming an a-Si roof of 0.2m' produces 12W peak under 1000WIm' and 25OC conditions, available energy can be calculated as follows:

Solar Input x Installed Power Energy = _--_------__-________________

Max Solar Input (1000W/m')

Using the annual average horizontal radiation or solar input for Berlin as typical of Euro- pean conditions (2805 Whlm'lday) the total available energy is as follows:

2805 Whlm' x 12W Energy = ---------------- = 33.6 Wh/day

1000 Wlm'

Then the amount o f electricity can also be calculated:

Energy 33.6 Wh Electricity = --_--_ = _ _ _ _ _ - _ E 2.0 Ah/day

1 zv 12v

Corresponding calculations can be performed to derive the minimum levels of power generation for the "worst" and "best" months:

Berlin Annual Average 2.8 Ah/day

Average for June 5.6 Ah/day

Average for December Ahlday

0.45 Ahlday

Using the above data, several sub-systems are examined as potential solar power units.

Solar Powered Alarm System

An alarm system comprising passive components (type a) such as a voltage comparator or infra-red sensor, draws a standby current of approximately lOmA o r l2OmW at 12V. When active components (type b) such as ultrasonic sensors are used, the standby current drawn increases to around 300mA (3.6W). A typical siren used in an alarm system is rated at 30W and when triggered, will sound f o r around 30 seconds. Assuming the alarm sounds on,ce daily, the energy requirements are:

a) 0.12W x 24h + 30W x .0083h = 0.26Ah

b) 3.6W x 24h + 30W x .0083h = 7.22Ah

The daily energy requirements of a type b) alarm indicate that systems with active components should not be supplied exclusively by solar generated power. However with passive components only there is ample power.

Solar Parkine and Readine Liehts

Miniature halogen lights rated at 5W are highly suitable for providing additional illumination in the passenger compartment. The daily energy requirements based o n a total use of 3 hours daily are:

5W x 3h = 15 Wh/day = 1.25Ah/day

The same approach using a solar cell array could also be used to power one or more parking lights fitted around the vehicle. One light could, for example, be fitted to the parcel shelf and another to the door pillar. The calculation below assumes that the lamp used is rated at 2W. which could be reduced by a flashing light, is used for an average of 36 hours per week:

2W x 36h = 72 Uh/week = 0.85AhIday

Solar Rechargeable Auxiliary Battery

The concept of using a separate battery for electrical items supplied from a solar cell array has been introduced above. Of the available range o n the market only Lead (Pb) and Nickel Cadmium (NiCd) batteries are suitable f o r use in motor vehicles, largely due to cost and efficiency considerations, The criteria used to select the most appropriate type would include an ability to be mounted in any position, temperature stability over a range from -15OC to +60°C and a tolerance of over- and under-charging from the solar cells, without any need for control electronics.

The battery type best meeting these criteria is the NiCd sintered plate high temperature design. New nickel metal hydride batteries will begin introduction to the market in 1 9 9 1 / 9 2 without the problems for disposal of Cadmium and almost double the capacity for equivalent size. In each case the NiCd sintered plate cell is most suitable for

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solar applications due to a low intrinsic self-discharge (less than O.l%/day) and an ability to accept charge rates up to 20% of capacity. For a solar cell array of 12W as described above the ideal battery capacity would be around 2Ah. This is small enough to be charged from empty in one average day and with sufficient current to attain high temperature oxidation voltages.

With this battery size the system autonomy ( o r reserve time assuming no further solar input is available) would be s s follows:

Alarm Reading Parking System Light Light

Daily Load in Ah 0.26 1.25 0.85

No. of Days Autonomy 7.7 1.6 2.35

With the 2Ah battery size, and 12W solar cell array. average recovery time from 100% battery discharge would be 0.8 days or during the "worst" month, 5 days.

CONCLUSIONS

With a solar roof hatch or larger area covered by solar cells, there seem to be possibilities of meeting vehicle electrical power requirements by solar main battery charging. The concept of fuel saving can then be introduced by cutting out the car generator, if the needs can be met by an auxiliary solar-charged battery.

According to Koenig (5) car headlight use would automatically switch the generator o n for safety reasons. However allowing for this, in road trials with a demonstration van it was possible to save about 42 on fuel consumption, if the vehicle were driven at a constant speed for 80 minutes and then allowed to stand and charge for the remainder of the day.

This points to a most significant and potentially larger saving of up to 6 k on diesel engines where specific fuel use is lower and s o generator loads are higher.

This indicates that there may be many suitable niches for the use of solar cell arrays within the commercial vehicle market.

Solar cells have a contribution to make as an auxiliary means of battery charging or as part of an independent power pack for various functions in'vehicles. The benefits can in several cases be quantified readily, but of equal o r greater importance are the unquantifiable benefits of better comfort and convenience for vehicle users. With the advent of lower coat large area a-Si solar cells there is now a viable possibility of incorporating this technology into vehicle design.

REFERENCES

1.

2 .

3.

4.

5.

6.

Hofmann, A., ASI-Glas for solar generated auxiliary power in cars. Internal report no. SR 00191, Phototronics Solartechnik GmbH, Hermann- Oberth-Strasse 9, D-8011 Putzbrunn, Germany. Fax: +49 89 607 30332.

Hofmann, A., Solar-driven ventilation system based o n the Phototronics concept. Internal report n o . SR00590. Ibid.

News item - Automotive News April 1991 Ingersoll, J.G. Integration of solar cells in automobiles as a means to reduce the air conditioner capacity and improve comfort. Proceedings of the 9th E.C. Photovoltaic Solar Energy Conference, Freiburg. Germany, Sepr isissi. (ISBPJ 079230497-7)

Koenig, A., Grundmann, E., Potential PV Applications f o r Series Passenger Cars. Ibid.

Messana, C., et.al. Design and develop- ment of a car ventilation system powered by a photovoltaic generator. Proceedings of the 10th E.C. Photovoltaic Solar Energy Conference. Lisbon, Portugal, April 1991. (in preparation for print by Kluwer Academic Publishers).

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r'igure 1 Typical a-Si solar car roof glass concepts

SC.I-tr...P.rC.t a 4 1 so1.r Cell. C u r v e d o n e piacc ..fety glass

TYPE ESG 1011 SOLAR ROOF

Obscured glass on edges

Cvrred outer safety glass leninate

TYPE VSG 12-2511 SOLAR ROOF

Figure 2 Concept for solar driven ventilation system for parked vehicles

Figure 3 Effect of solar roof and blower set on vehic

km kw/cmZ -US ... . . ..-. . .d.r noon

interior temperature :le -

1

1 MESZ 9.00 10.00 11.00 12.00 1 3 0 0 14:W 15.00 16.00 17.00 air volume 280 m3m (nominal ratel solarroof 6 W (atnoon)

august 1990 vw ~ Golf