Efficiency - All about Energy, Power and Work
Updated: Aug 26, 2020
In today's world we all need to be careful to minimize our use of precious resources. It makes sense from a "green" perspective, reducing our carbon footprint, and from a financial perspective, reducing the cost of cruising. There are many things which go into the efficiency of narrowboat operation, and here I am going to describe the elements that affect a serial hybrid installation. Later blog posts will examine separate elements in more detail, but first let's look at some theory.
We are going to burn diesel fuel in an engine to convert chemical energy into mechanical power, then transfer this into electrical power. We translate this into a direct current voltage which we store in a battery. Later we recover the electrical energy, shape it into alternating current for the motor which converts the electrical power into mechanical power again. Some power is lost in the drive train before the propeller works to push water backwards, which creates the force that pushes the boat along. The drag of the boat in the water is what limits our speed.
In this short description I have, intentionally, included all the words efficiency, energy, work, power, force, drag, voltage, current and speed. Some people get confused about these terms, so I will explain how they relate to each other. Then we'll get back to the topic in hand.
Energy and Work
Energy is the ability to do work. Energy can be stored in many forms. For example in a hydro electric plant water is stored high up and has energy of position (potential energy). It can be converted from one form to another, so a ski-jumper turns potential energy into energy of motion (kinetic energy) as he slides down the ski ramp. In our case, we have energy stored in the fuel or batteries.
Mechanical energy is a force moving through a distance, both in the same direction. Think of the two "zero work" extremes; the foundations of your house exert a huge force to keep the walls up, but they don't move, so no energy is used. Equally an ice-skater gliding across the ice uses no energy as there is motion but no force. Only when a force moves through a distance is energy consumed, and when this motion is what we wanted, we use the term Work. The work we need on a narrowboat is the force to push the boat forward times the distance we travel.
Efficiency is how much work we achieved for the energy we used up. Unsurprisingly, efficiency will always be less than 100%, otherwise we get perpetual motion... The energy that does not end up as work is called wasted energy which is a bit rude, because it is an essential part of the process.
Power and Speed
Power is how quickly we are using energy. Since work is just the energy we wanted, it's also the rate of doing work.
But hold up, I said that mechanical energy was moving a force through a distance, without saying how fast this happened. Power is how quickly we are using energy, so putting these two ideas together we find that mechanical power is a force moving at a speed.
We can reverse things and say that energy is power applied for some time. If you drive up the road at a steady speed (i.e. constant power) , then the longer you do this for, the more fuel (chemical energy) you will use.
Force and Drag
Force is easy; it's what moves things. Drag is a type of force and acts to try to stop things moving.
Voltage and Current
I said that energy could be stored in many forms, including chemical, position, motion and of course we can store electrical energy. Voltage is the electrical equivalent to force, and current the electrical equivalent to speed, and in the same way that mechanical power is force times speed, so electrical power is voltage times current.
In the same way that mechanical energy is how long power is applied, so electrical energy can be computed from multiplying electrical power by the time that power was used.
There are different units to measure some of these things, but I will try to keep things simple where I can by using consistent units. Let's do the easy ones first !
Efficiency is a ratio, expressed in %.
Voltage and Current
Voltage is measured in Volts, and current in Amps.
Force is measured in Newtons, and a Newton is about the weight of one apple. A bit small, so watch for kN values.
Power and Speed
Power is measured in Watts (W), and a Watt is one Joule per second. Mechanically, a Watt is a force of one Newton moving at a speed of one metre per second. Electrically, a Watt is one Volt and a current of one Amp. A Watt is a bit small, so we are more likely to use kW which are just a thousand Watts.
Energy and Work
Mechanical energy and work are measured in Joules; a Joule is a force of one Newton moving through a distance of one metre.
Electrical energy is measured in kWh. That is one thousand Watts running for an hour. It's what your electricity bill is measured in; a "Unit" of electricity is a kWh.
AC or DC?
If the electricity has a constant current it is described as "Direct Current" or DC while if goes backwards and forwards it is described as "Alternating Current" or AC. The current or voltage in an AC circuit is measured using values which allow us to stick with Volts times Amps = Watts. I will go into this in more detail when I explain how manufacturers exaggerate the power of their generators.
Almost there - just one thing to add. We often talk about engines in Horsepower, and a Horsepower is about 750 Watts (there are Imperial and Metric definitions which differ, of course!) Also, horses are not all the same, and for a short period a horse can exert up to 15hp, but most can't manage 1hp for extended periods. So I will try to leave horses out of the conversation and stick to kW where I can for the units of power.
This blog is about efficiency and I seem to have got slightly diverted, but it's all in a good cause, as we now have a firm foundation to work from.
The overall efficiency of our system is found by multiplying the efficiency of each part of the system together. Let's look at each in turn:
We burn diesel fuel in an engine to convert chemical energy into mechanical power.
We convert mechanical power into AC electrical power using an alternator.
We translate AC power into DC power ready for storage.
We store the electricity in a battery, and later recover it.
We convert DC power into a different type of AC power for the motor.
We use a motor to turn AC electrical power into mechanical power.
We lose some energy between the motor and propeller.
We use a propeller to create the force that pushes the boat along.
Now, I did say that efficiency is the ratio of the useful energy (work) to the energy used up. In a narrowboat the useful energy is not only the energy used to move the boat, but also the energy used to cook dinner, watch television and keep the boat warm. So an overall view of system efficiency needs to take account of all the energy consumers, and not just propulsion.