Solar Water Taxi

Optimum speed & Area of wingThank you Andy for his summarization. So right after the last mid-interim report, We started to focus on a lot of calculation of how fast our bot should actually travel, and how large should be the wing panels.

As we all know, the initially objective of the project, was to create a solar plug-in boat, that go with 10 knots and able to finish 10 round trip a day.However, we discovered, that, traveling with 10 knots, would spend us 2-2.5 hr for one single tripSo it really impossible to finish the 10 round-trips in a daywe had to cut the number of trips if we go with 10 knots

At the same time, we knew that drag is proportionally , to velocity squareSo reducing the traveling speed, would hugely help for lowering the power against drag.

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Higher traveling speed ----- require larger area of solar wingLower traveling speed --- reduce the number of successful tripsOptimum speed & Area of wingEnergy requiredEnergy harvestedVSSpeaking in another way.A higher traveling speed , produce larger drag, and require larger area of solar wing for harvesting more sunlightWhile a lower traveling speed, would spend more time to finish a single trip, this reduce the number of successful trips.

So what have done, was to build a model, analysis the energy required and energy harvested, to find out the optimum speed & area of wing

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SpeedTime10 mins1 min1 minDockingConstant SpeedAccelerationDecelerationEnergy required & harvestedIn our model, We simply made some assumption first.

Here shows the speed time curve for the boat traveling from one station to another station.We assume, the boat goes with 10 mins at docking, 1 min for acceleration from 0 speed , to desired speed.Then travel at constant speed,Till the end, deceleration constantly for another1 min4

Excel model

Energy RequiredEnergy HarvestedPossible number of tripsTravelingSpeedWing areaService hourWith this assumption, we move on to build the excel model.

In this model, We got 3 inputting variable, the traveling speed, wing area and service hr.We input different values for these 3 parameters, Then our model, would analyze between the energy input and output, Telling us the possible number of trips,

Our aim was to maximize the possible number of trip in a day5

Control parameter

Variable input PanelsFor the control parameters, We got a control panels here, Allow us inputting value of speed , wing area and service starting hr.The rest are the boat and solar panels specification, the weight, the efficiency etc.

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Energy RequiredNegligible Air Resistance Acceleration parameterPower for acceleration, & Against the dragand behind the data analysis step, The model , would calculate the power required for acceleration and against the drag, itself, This area is the water drag, simply role v square, time drag coefficient and area, And we tried to calculate the effect of air resistance, But seems negligible at no wind condition.We didnt really count it in.

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Hottest day 24 hr insolation data Throughout year Insolation data Energy HarvestedWhile to calculate how much energy we could harvested at diff speed and diff area of wing,Behind the data analysis stage,

We looked for the hottest day 24 hr insolation data, it shows how much energy we could get at every hr in hong kong at the hottest day.And we looked for the insolation data throughout the year. It shows the total energy we could harvest at different day of the year.

We combine to 2 data8

Energy HarvestedAverage 24 hr insolation data Scale to Scaled down to something we need.

What we need, is the average 24 hr insolation data.It tells us, at 8 oclock, the power of the sun reaching the panels is 2 kW, in average through the year9

Possible number of tripsOur model, goes with the data analysis step, would tell us the actual number of trip

So with 10 knot speed, no solar wing, and we start our service at 7 oclockIt was impossible for us to finish a single trips, 10

Possible number of tripsUnless we go with a later service hr, Say we start our service at 12 oclock, But still it could allow us to finish 1 single trip only.

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Possible number of trips7107So after different iteration steps, We find out the optimum speed, should be something around 7 knots, and 10 meter square for the solar wing, And we start our service at 7 oclock

Then we are able to travel with 5 successful trips.

And at the end of the 5 trips, we still got 5.64 kWh in the battery, And this give extra pre-stored energy in the battery for another day use.

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Optimum speed: 7.5 - 6.5 knotsArea of wing: 1o m^2Number of possible trips: 5 trips per day

Service starting hr: 7 amService closing hr: 23pm 01 amTime for single trip: 3.24 hr 3.63 hrOptimum speed & Area of wing

A summary of the data.we find out the optimum speed should be go with something around 7 knots, instead of 10 knotsThe area of solar wing we need to put on the boat, would be 10 meter square.And this allow us to averagely having 5 trips in a day, instead of 10 trips.

Our service start at 7 amClose at 23 pm 01 amAnd time for single trip, would around 3 and a half hr.

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Solar BatteryAfter these, we started to look for the suitable solar battery for our boat.14

Solar Battery

Flooded Lead Acid(require adding water for function)

Gelled Electrolyte Sealed Lead Acid

Sealed Absorbed Glass Mat (AGM)Deep Cycle - Lead AcidBattery3 conventionally usedThere are 3 conventionally used solar battery in the marine market.the flooded lead acid, which require keep adding water for functioning the gelled electrolyte sealed lead acidAnd the sealed absorbed glass mat (AGM)

And these 3 are very commonly used, And they are deep-cycle lead acid battery.15

Solar BatteryAGM

- No Gas leakage problem- Higher Power density- Faster Charging and discharging rate- Slower Self discharge rate- Resistance to vibrationAmong these 3, agm probably is the best,

It got no gas leakage problem , Having higher power density, Faster charging and discharging rateSlower self discharge rateAnd resistance to vibration.

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Solar Battery

Weight: 1/3 of lead acidSpace: volume of lead acid

Impedance: of lead acid save power from heat lossLifetime: lithium ion 10 years lead acid 5 yearsSelf-discharge rate: 1/5 of lead acid much less stored energy loss at nightLithium ionBut if we look for the more latest technology, Lithium ion battery is getting more advance

It weight is one-third of the lead acidVolume half of the lead acid, Internal impedance, one-fourth of the lead acid,, it saves lots of power from heat lossIt having longer life time than lead acid,

And 1/5 self-discharge rate than lead acid, Self-discharging meaning when the battery is not in used, it would discharge itself, A lower self-discharging rate meaning much less stored energy loss at night.

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Solar Battery

Discharging behavior:

Lithium ion

Flatter discharge curveSteeper discharge curveAnother significant advantage of lithium ion is the discharging behavior, Lithium ion got a flatter discharging curveLead acid having stepper

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Solar Battery

Discharging behavior:

Lithium ion

- Voltage not constant- Require voltage regulator- Not too good for high power applicationWhat does this imply?It implies that a steep discharge curve, would lead to an not constant voltage output, And it would need to install voltage regulator on the boat, Which it not an advantage for high power applicatio/.19

Solar Battery

Cost: Lithium ion much expensive?

Purchasing cost: lithium higherLifetime price per kWh: Lead acid much higher

Lithium ion

the biggest concern about lithium ion battery should be the cost.Is lithium ion actually much expensive?

The answer is yes, for purchasing cost ,But no , when we look for the lifetime price.

So basically, we could benefit a lot from lithium ion battery in term of its function and money consideration.20

Solar BatteryBatteryTorqueedo 26-104(Marine intended)Tesla Model S(2015)Nissan LEAF(2013)Marine Lead AcidTypeLi-onLi-ionLi-ionLead Acid (AGM)Cost/kWh$970