Manned Operations
	Introducing EVE Hull Design Exostructure Power Propulsion Docking Ballast Sensors & Sampling Manipulators Life Support Navigation Communication Viewing Systems Control Systems ROV

Powering EVE

Energy is by far the most important limitation that faces designers of deep sea submersibles. Most submersibles today use high-energy density batteries. These are divided into secondary batteries, primary batteries and fuel cells.

Secondary batteries: They are rechargeable and examples include the lead acid battery used in cars, nickel cadmium, silver zinc, silver iron and lithium ion.

Primary batteries: They can only be used once, and examples include lithium sulfur oxide and aluminum-seawater. These batteries may sound less attractive than secondary batteries due to the lack of the recharging option, but on the other hand, they have up to four times the energy density of their secondary counterparts, which means that the submarine can stay for a longer time in water.

Fuel cells: Fuel cells have an energy density in between, and can be recharged by refilling their reactant tanks with the fuel. Examples include aluminum-oxygen fuel cells, alkaline fuel cells and proton exchange membranes.

Heat engines like diesel engines are also an option.

The factors that determine the choice of the energy source are:
1. Energy density.
2. Cost.
3. Life cycle (number of times it can be recharged).
4. Cost.
5. Maturity and reliability (has it been tested thoroughly for submersibles or is it just a lab demo?).
6. Safety concerns (such as hydrogen generation, fuel fires, thermal runaway, toxicity of chemicals used).
7. Power density (ability to supply the power needed at the necessary rate).
8. Ability to operate over a wide range of temperatures and pressures.
9. Environmental friendliness.

After a careful comparison of 26 energy sources and the application of each of the above criteria to it, we have decided to use lithium ion cells. Before going into the details of why we chose this cell, let us consider how much energy we shall need for each trip we make. Our submersible uses power for the following things:

Propulsion

Energy for propulsion can be calculated as follows: Energy = Power x Time of operation x Efficiency. The power of a propeller is usually measured in horsepower. The effective horsepower of a propeller is given by: EHP = Total resistance (drag) x Velocity / 550. The total resistance is the sum of the bare hull resistance and the resistance due to the appendages. The calculation of the resistance is not easy at all. Actually, it can only be roughly estimated using potential-flow computer models. So, does doubling the power double the submersible's velocity? No! The relation between velocity and EHP is not linear as you may think when you first look at the above formula. Remember that the resistance is also a function of the velocity (in fact it varies as the square of the velocity. Refer to the hull design section for more information about calculating the resistance.) Thus, it is very hard for research submersibles to achieve high speeds. For example, the maximum speed the Alvin can achieve is 2 knots (3.7 km/hr.) We aim at a higher speed of about 3 knots (5.6 km/hr.) Therefore, EVE should be much more streamlined and should have propellers that can provide much more power. Going at higher speeds is almost impossible because as mentioned, the power needed is proportional to the cube of the velocity. That explains why cars can easily go up to 150 km/hr, but jumping to 200 km/hr requires very good design and a very powerful motor. Moreover, we shall have to use several thrusters. We shall use 4 thrusters for forward and backward motion, 2 for vertical motion and 1 as a rudder. No more than 5 thrusters shall be used together at the same time (this being controlled by computer systems on board.) EVE shall be equipped with 5hp electrical thrusters. Therefore, a maximum of 18.65kW shall be used for propulsion (1hp = 746W) at any time.

Lighting

We shall have four floodlights attached to EVE's exostructure which need 500W each. In addition, we shall need lighting inside the submersible. We have estimated the total power used for lighting to be around 3kW.

Other equipment

Other equipment such as computers, life support equipment and communications equipment is estimated to need a total of 3kW. Thus, the total power we need shall be about 25kW. Our main power supply shall be an lithium ion cell, which can easily meet this requirement. In fact, recent advances have made it possible for lithium ion cells to supply up to 200kW!

Advantages of the Lithium Ion Cell

This cell has many advantages. First, it can meet the high power requirements of our submersible. Also, it is rechargeable, so we do not have to replace the battery frequently. Another advantage is the high energy density of the fuel. It has a gravimetric energy density of 170 watt-hours/kg and a volumetric energy density of 440 watt-hours/l.

How much fuel do we need to carry?

We have calculated our power needs to be 25kW. For a six-hour journey during which all submarine systems are working at full power we shall need 150kW-hours of energy. We shall carry 300kW-hours for safety reasons. This means we need to carry batteries of a total mass of 1740kg and a total volume of 0.67 meters cubed. We shall also carry a small lead acid battery within the pressure hull to operate life support systems in case the lithium ion battery malfunctions.