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Electrification of a Sightseeing boatChristoph Wijsen, Jan Fissette, and Wouter Vanaken

Abstract—

Index Terms—brol brol

I. I NTRODUCTION

In many cities, sightseeing boat tours on the availablewaterways are organized. These small boats are typicallypowered by an internal combustion engine. Electric boats arestill rare because of a variety of technical and economicalreasons. However fully electric boats have some characteristicsthat makes their application in these boat trips very interesting,as listed below:

• No polluting exhaust gases• Quiet operation• Possibility to make use of solar energy• Well suited torque speed characteristic of electric motors• Fixed trajectory allows for adequate battery pack sizing

However, electrification also has some serious disadvantages:

• Charging time reduces flexibility• More complex system• increased investment cost

In this paper, these different aspects will be addressed andaneconomic analysis of the electrification process is performed.In section II, the boat under consideration is described. ... NOGAAN TE VULLEN

II. B OAT DESCRIPTION

The boat under consideration has a length of 8m an a widthof 3m. 80% of the surface is covered by a roof. Based ondata of an existing and commercialy available electric boat,the Duffy 22 Bay Island, such a boat’s displacement can beestimated to be around 2000 kg. Based on this figures, anestimation of the required propulsion power can be made. [1]describes how the required power of low speed vessels can beestimated based on the Yokoyama model.

IP = 0.0018v3W (0.0158v3K1

W1

3

L2+31.6K2

√

L

W)K3 (1)

• IP = Indicated Power [W]• v = Speed [km/h]• W = Displacement of the boat [kg]• L = Hull length [m]• K1 = Hull resistance coefficient(≈ 0.5)• K2 = Friction resistance coefficient(≈ 3)• K3 = Wave resistance coefficient(≈ 0.5)

The results are shown in figure 1. It is shown in [1] that theYokoyama formula slightly underestimates the required power.Therefore, the red line in figure 1 represents the ‘Yokoyamaestimation’ multiplied by a factor 1.5. It can be seen from the

0 1 2 3 4 5 6 7 80

2

4

6

8

10Estimation of required power versus shipping speed with Yokoyamas formula

Pow

er [k

W]

speed [km/h]

Yokomyama Estimation1.5*Yokomyama Estimation

Fig. 1. Estimation of the required power

45 min Trip Fuel Consumption/Trip Fuel Cost/Year (e 1.4/l)

12.5% average efficiency 3 l e 9175

20% average efficiency 1.875 l e 5749

30% average efficiency 1,25 l e 3822TABLE I

FUEL CONSUMPTION ESTIMATION

curves that at low speed, the power requirements are low. Atspeeds higher than 6 km/h, the power requirements start toincrease very fast. However for a sightseeing boat, a modestmaximum speed of only 6km/h is no problem. At this speed,the Yokoyama model gives a required power of 3.22 kW.Multiplied with a factor 1.5, this gives a value of 4.83 kW. Thismeans that a drive able to produce 5 kW of mechanical outputpower at the propeller shaft is sufficient to power the boat. It isassumed that the boat makes 6 tours of 45 min per day, everyday of the year. In between two tours, the boat docks for halfan hour. Except for the stop after the third trip, which takes1 hour. The trips are performed at a nearly constant speed,requiring an average mechanical output power of 5kW.

III. I NTERNAL COMBUSTION ENGINE DRIVE

Three theoretical combustion engine drives are consideredwith an overall average efficiency of 12.5% , 20% and 30%respectively. Based on the LHV (43.4MJ/kg or 36.1 MJ/l),the diesel consumption per trip can be estimated. The resultsare shown in table I. The power rating of internal combustionengines typically refers to the peak mechanical power output.Therefore, the power rating of the combustion engine shouldexceed the nominal output power demand. For an averagedemand of 5kW, a power rating of 10 hP (7.5 kW) isappropriate. REFERENTIES A typical outboard marine engineof this power rating has a cost ofe 2300. REFERENTIES

IV. ELECTRIC VEHICLE SYSTEM ARCHITECTURE

The combustion engine drive is compared to a fully electricsystem. The system layout is given in figure AANVULLEN

2

+ BESCHRIJVEN

A. Solar Panels

The boat has a roof surface of19m2. 18m2 is covered bysolar panels. (REFERENTIE UTRECHT) proposes a yield ofapproximately 875kWh/kWP. A PV system with a surface of18m2 has a peak power of roughly 2.25 kWp. This gives atotal annual energy yield of approximately 2 MWh or a dailyaverage yield of 5.48 kWh/day. REFERENTIE WEBSITEgives the following formula to calculate the average dailyenergy yield:

E = 100×A× r ×H × PR (2)

• E = Energy (kWh/day)• A = Total solar panel area = 18m2

• r = Solar panel yield (electrical peak power in kWpeakdivided by the area inm2) = 0.125

• H = Average solar radiation energy = 3.2 kWh/m/dayREF

• PR = Performance ratio, coefficient for losses (rangebetween 0.5 and 0.9, default value = 0.75) REF

This gives a value of 4,53kWh/day. It can be concluded fromthe previous two calculations that 5kWh/day is a reasonableestimation for the solar panel yield. This gives a yield of 1.825MWh/year. The cost per square meter of solar panels is inthe order ofe 300-500. For the economic analysis, a price ofe 450/m2 is used. This gives a total investment cost for thesolar panels ofe 8100.

B. Power Converters

C. Motor

Different types of electric motors can be deployed in boatdrives. Commercially available electric boats mostly use

• Brushed DC motors• Permanent magnet AC / Brushless DC• Induction motors

Switched reluctance machine could also be used, but are atthe moment not used in commercial electric ships. BrushedDC motors have the disadvantage of higher maintenance costsand will not be considered in this paper. Permanent magnetmotors are more expensive than squirrel cage induction motorsbut are typically a few percentage points more efficient. Forthe electrification of the ship, a 7.5 hP squirrel cage inductionmotor is selected, with an efficiency of 90% and a cost ofe 750. REFERENTIE

D. Battery Pack

1) Battery Types: Important factors in selecting a batterytype are the investment and maintenance cost, the energy andpower density and the lifespan of the battery. Lead-Acid basedbatteries are by far the cheapest (125$/kWh) but they areheavy and have a low energy density (30-40Wh/kg) and anda low power density (85W/kg). Their charging efficiencies arerather low (< 90%). The number of cycles of a Lead-Acidbattery at high depth of discharge is very limited. However

Mechanical output 22.5 kWh

Total Battery Output 29.16 kWh

Charging Energy from Grid (3h) 19.8kWh

Charging energy to Battery 16.98 kWh

Net Battery Output 12.18 kWhTABLE II

ENERGY NUMBERS PER DAY(NO SUN)

NPC capital [e] NPC fuel [e] NPC total [e]

η = 0.125 2300 140596 142869

η = 0.2 2300 88093 90393

η = 0.3 2300 58568 60868TABLE III

FUEL CONSUMPTION ESTIMATION

because of the low investment cost, so called deep cycle Lead-Acid batteries are sometimes found in marine applications.REFERENTIE Other battery types such as NiCd and NiMHbatteries do slightly better in terms of energy and powerdensities (up to 70Wh/kg) but they are more expensive andsuffer from high self discharge and faster aging at highertemperatures. Also their charging efficiency is rather low.Thebattery that seems the most suitable for the use in the electricboat is the Li-ion battery. It has a high energy density (95-140Wh/kg) and a power/energy ratio up to 35. It charges fastand at high efficiencies (> 90%) and has a long lifetime ( 3000cycles at 80% Depth of Discharge). The biggest disadvantageof Li-ion batteries is their cost, currently in the order of 560-600$/kWh. (REFERENTIE MCKINSEY)

2) Battery Pack Sizing: The boat has to be able to operatewhen there is no sun. In this case, all power has to be deliveredby the batteries. All convertors are assumed to work at anefficiency of 0.95. Charging and discharging efficiency is alsoassumed to be 0.95. As previously stated, the induction motorhas an efficiency of 0.9. Charging the batteries is assumed tohappen at a rate of 6.6 kW. REF The Li-ion battery has to beable to deliver 12.18 kWh. Complete discharging damages thebattery. A safety margin of 20% at the end of the day preventsthis damage. This results in a battery pack of 15.23 kWh. Witha Li-ion battery able of delivering 100 Wh/kg, this results ina battery pack of 152.3 kg. The cost of Li-ion batteries isin the range of $560-800 REF. This gives an investment costbetweene 6140 ande 8770 for the entire battery pack.

V. ECONOMIC ANALYSIS

A cost comparison between the ICE boat and the electricone is performed, based on the notion of the Net Present Cost.A discount rate 1.03 is used. The fuel price is fixed ate 1.4/l.For the electricity price, a constant value ofe 0.21/kWh isused. A period of 20 years is considered. Maintenance costsare not taken into account. For the ICE boat, it is assumed thatthe engine does not need to be replaced during the 20 years.The cost figures for the ICE boat, based on the numbers givenin section III are given in table III. In the case of the electricboat, first an estimation of the electric energy bought from thegrid is made. During a trip, power from the solar panels isconverted into mechanical power with an efficiency of 0.81.

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When the ship is docking, energy from the solar panels is firststored in the battery. The overall efficiency of the electrictomechanical energy conversion in this case is 0.66. A weightedaverage of these two, based on the ratio between operatingand docking time results in an efficiency of 0.75.

REFERENCES

[1] S. Minami and N. Yamachika, “A practical theory of the performance oflow velocity boat,”Journal of Asian Electric Vehicles, vol. 2, no. 1, 2004.