Let's Talk Electric Propulsion Part 3 - Power Management, Recharging, and Regeneration
29 12 2011 Posted by Daniel
This is part 3 in the Electric Propulsion mini-series here at Oddasea.com. So far we’ve covered the ‘why’s, and to a limited extend, the ‘how’s for my particular case. Now let’s dive into a more detailed discussion of what helps keep this whole system working, puts energy back into it, and how I go about managing the whole process.
First, a bit of background. Combustion engines require fuel to produce heat, the energy from that heat is captured via the resultant pressure change as force and, hence, power. When the fuel is consumed, the engine is useless until the fuel is replaced. While this can be accomplished fairly quickly when the vessel is physically at a fuel dock, on the order of 10-20 minutes or so, if the vessel is not so located at the time the fuel is needed it can be quite an ordeal to refuel.
Electric motors run off of any available electrical supply. If the supply is not sufficient to meet the current power demands of the motor, the reserve capacity of the batteries is utilized. Once the batteries are depleted, a similar situation remains for the electric motor as for the combustion engines: the energy needs to be replaced. While hydrocarbon fuel is a physical material and can be relatively rapidly transported from one location to another, it is not yet practical to swap out entire packs of batteries aboard a ship, thus the much slower process of replacing the energy within the batteries - energy which is not in a direct material form and therefore much less able to be rapidly transported. So while the combustion engine, when in the vicinity of a fuel dock, can be replenished in a handful of minutes, the electrical motor’s battery bank requires several hours to properly recharge. This can be seen as a clear advantage for a diesel setup, for instance, but where the electrical system falls short, it makes up for that shortcoming in some very significant advantages elsewhere: namely:
Thus it can be argued that the electric motor enjoys a host of advantages over a diesel engine in its ability to gain additional energy for propulsion from nearly any location and at nearly any time via a diverse and redundant array of means without forfeiting the ability to utilize rapid replenishment via fossil fuel means if so desired as a fallback strategy.
Let’s cut the academic talk a bit and see what I’m actually talking about.
On my boat, I charge my batteries in three primary ways, with one fallback method. These are:
1. Wind Power. At the conclusion of my current rigging project, a 48V Air Breeze Marine wind generator (nominally rated at 160W average output power) will directly put power into the primary propulsion battery bank whenever it has enough wind to be useful. This is mounted on the mizzen mast so it is largely clear of all wind disturbances and from similar installations on my friend’s electric boats, I expect it to be a primary power source for keeping the batteries topped off under normal use. Those of you who are saying it won’t be enough without having tried it: its being done right now by many folks and it works for them just fine.
2. Solar Energy. My boat has been equipped with a pair of Kyocera 135W panels on a custom aluminum arch just underneath the mizzen for nearly a year now. These supply 95-100% of my house loads on a daily basis depending on how bad the weather is and for how long. I’ve been quite happy with this installation and only find a need to top off the batteries once every couple weeks if we’ve had a significant spell of bad weather. These will, for the time being, continue to be routed primarily to house loads, but I am adding a switch to a 48V boost charger which can directly use them as a charging source for the 48V bank as well. This lets them be a solid alternate power source for motor charging when the house bank is just fine and doesn’t need a top off.
3. Prop Regen. The least useful but most powerful method for regeneration is a feature uniquely provided by the electric motor itself: the ability to turn into a generator when the boat speed is high enough to spin the prop under sail. Since my boat has a big honking 3-bladed 17x9 prop which does not feather, it is capable of generating a significant amount of power - as long as the boat is going fast enough. This typically means speeds in excess of 5 knots, with the power just about doubling each extra knot. If the boat is really doing well, close to hull speed, this will be the single biggest recharging component onboard, bar none. When I’m just noodling along, well, its pretty darn useless. So, it becomes the third option for charging the batteries, but data from other boaters suggests that its one of the more useful options on longer range trips as that is when it has the best opportunity to be useful. Its a nice fit for my use case.
4. The fallback is the good old gas generator. In my case, the cruiser’s favorite: a 2kW gasoline unit outputting 120V into the 48V shore power charger. This can recharge my 48V bank in anywhere from 4 to 8 hours depending on how empty it was and whether I want to charge it all the way to float or not. It can also provide enough continuous output power to run the vessel at a smidge over 3 kts for as long as I can keep it full of gasoline. Since I was going to carry this anyway based on the general majority opinion of fellow cruisers, it made sense to set it up so that it could be used as a fallback propulsion assist.
I should note that if I am ever to operate in the “genset running, directly powering the electric motor while underway” mode, my ship would basically be a series-hybrid propelled vessel, much like many hybrid cars are today. This dual-mode operation is, remarkably, still more efficient than a direct combustion->transmission->prop drivetrain due to the ability of the genset to run at its most efficient RPM.
Managing Power
I manage the battery state of charge via two main information sources. While operating the electric motor, the motor controller’s display calculates current power draw, estimated remaining charge, and remaining runtime at current draw, as well as engine rpm and environmental data such as winding temperature, etc. This information, much like the “efficiency” and “regen” charts in a Prius, helps me to balance power consumption versus boat speed, conserving my power and knowing how much of it I have left. Its like a hyper-accurate fuel gauge.
When I am not in operation mode, a Bogart Trimetric 2025-RV 48V battery monitor gives me current battery state of charge, amps/volts, and historical data. I use it to keep track of my charging systems, know when I need to recharge or top off the batteries, and generally ensure that I am staying on top of the system as a whole. Its a bit more accurate than the motor controller for things like “how full are my batteries”, “did I give them a complete charge yesterday”, and “how much power am I using right now” (when the motor is not what is drawing the power).
Strategy and Resource Utilization
The unique requirements and capabilities of the electric motor setup for a long range cruiser require much more careful power management versus a diesel - much as the diesel requires much more careful maintenance. If I don’t want to get caught with my proverbial pants down, from a power perspective, I need to keep an eye on my battery charge state, whether or not my charging systems are doing their job, and whether or not I am likely to need power in the near future if I am choosing to use a lot of it right now. While I expect much of this will come from experience, a little forethought and learning from others is entirely appropriate right now. As a result, there are two major mindsets I am combining to develop my current strategy on power use: that of the cruisers without engines (the Pardey’s, Slocum, et. al) and that of my fellow electric boat owners - most of whom do shorter coastal passages or day trips. By planning my routes and sailing my passages with the mentality that I don’t have an engine, I’ll minimize situations where it becomes needed. And conversely, by strategically using the engine when I have an excess of natural power to regenerate, I’ll cut my power use and ensure a useful reserve. But its still there to help me fight a tide rip through a cut, or to catch that mooring when the wind dies, or to squeeze an extra knot or two out of a light headwind between islands.
A different kind of sailing? Indeed. And one I’m quite happy to look forward to.
Stay tuned for the next part: dealing with range anxiety.
______ /) _______
Categories : Ship EquipmentFirst, a bit of background. Combustion engines require fuel to produce heat, the energy from that heat is captured via the resultant pressure change as force and, hence, power. When the fuel is consumed, the engine is useless until the fuel is replaced. While this can be accomplished fairly quickly when the vessel is physically at a fuel dock, on the order of 10-20 minutes or so, if the vessel is not so located at the time the fuel is needed it can be quite an ordeal to refuel.
Electric motors run off of any available electrical supply. If the supply is not sufficient to meet the current power demands of the motor, the reserve capacity of the batteries is utilized. Once the batteries are depleted, a similar situation remains for the electric motor as for the combustion engines: the energy needs to be replaced. While hydrocarbon fuel is a physical material and can be relatively rapidly transported from one location to another, it is not yet practical to swap out entire packs of batteries aboard a ship, thus the much slower process of replacing the energy within the batteries - energy which is not in a direct material form and therefore much less able to be rapidly transported. So while the combustion engine, when in the vicinity of a fuel dock, can be replenished in a handful of minutes, the electrical motor’s battery bank requires several hours to properly recharge. This can be seen as a clear advantage for a diesel setup, for instance, but where the electrical system falls short, it makes up for that shortcoming in some very significant advantages elsewhere: namely:
- That it can be charged at nearly any time using a variety of sources which harvest the free and abundant energy directly provided by nature.
- That such charging sources can and often are even utilized AS the engine is running, to lessen or occasionally even eliminate the drain on the reserves, reducing the amount needed for replenishment later, and
- That it does not remove the ability to use fossil fuels as a strategic power source should they be the most expedient for a given situation.
Thus it can be argued that the electric motor enjoys a host of advantages over a diesel engine in its ability to gain additional energy for propulsion from nearly any location and at nearly any time via a diverse and redundant array of means without forfeiting the ability to utilize rapid replenishment via fossil fuel means if so desired as a fallback strategy.
Let’s cut the academic talk a bit and see what I’m actually talking about.
On my boat, I charge my batteries in three primary ways, with one fallback method. These are:
1. Wind Power. At the conclusion of my current rigging project, a 48V Air Breeze Marine wind generator (nominally rated at 160W average output power) will directly put power into the primary propulsion battery bank whenever it has enough wind to be useful. This is mounted on the mizzen mast so it is largely clear of all wind disturbances and from similar installations on my friend’s electric boats, I expect it to be a primary power source for keeping the batteries topped off under normal use. Those of you who are saying it won’t be enough without having tried it: its being done right now by many folks and it works for them just fine.
2. Solar Energy. My boat has been equipped with a pair of Kyocera 135W panels on a custom aluminum arch just underneath the mizzen for nearly a year now. These supply 95-100% of my house loads on a daily basis depending on how bad the weather is and for how long. I’ve been quite happy with this installation and only find a need to top off the batteries once every couple weeks if we’ve had a significant spell of bad weather. These will, for the time being, continue to be routed primarily to house loads, but I am adding a switch to a 48V boost charger which can directly use them as a charging source for the 48V bank as well. This lets them be a solid alternate power source for motor charging when the house bank is just fine and doesn’t need a top off.
3. Prop Regen. The least useful but most powerful method for regeneration is a feature uniquely provided by the electric motor itself: the ability to turn into a generator when the boat speed is high enough to spin the prop under sail. Since my boat has a big honking 3-bladed 17x9 prop which does not feather, it is capable of generating a significant amount of power - as long as the boat is going fast enough. This typically means speeds in excess of 5 knots, with the power just about doubling each extra knot. If the boat is really doing well, close to hull speed, this will be the single biggest recharging component onboard, bar none. When I’m just noodling along, well, its pretty darn useless. So, it becomes the third option for charging the batteries, but data from other boaters suggests that its one of the more useful options on longer range trips as that is when it has the best opportunity to be useful. Its a nice fit for my use case.
4. The fallback is the good old gas generator. In my case, the cruiser’s favorite: a 2kW gasoline unit outputting 120V into the 48V shore power charger. This can recharge my 48V bank in anywhere from 4 to 8 hours depending on how empty it was and whether I want to charge it all the way to float or not. It can also provide enough continuous output power to run the vessel at a smidge over 3 kts for as long as I can keep it full of gasoline. Since I was going to carry this anyway based on the general majority opinion of fellow cruisers, it made sense to set it up so that it could be used as a fallback propulsion assist.
I should note that if I am ever to operate in the “genset running, directly powering the electric motor while underway” mode, my ship would basically be a series-hybrid propelled vessel, much like many hybrid cars are today. This dual-mode operation is, remarkably, still more efficient than a direct combustion->transmission->prop drivetrain due to the ability of the genset to run at its most efficient RPM.
Managing Power
I manage the battery state of charge via two main information sources. While operating the electric motor, the motor controller’s display calculates current power draw, estimated remaining charge, and remaining runtime at current draw, as well as engine rpm and environmental data such as winding temperature, etc. This information, much like the “efficiency” and “regen” charts in a Prius, helps me to balance power consumption versus boat speed, conserving my power and knowing how much of it I have left. Its like a hyper-accurate fuel gauge.
When I am not in operation mode, a Bogart Trimetric 2025-RV 48V battery monitor gives me current battery state of charge, amps/volts, and historical data. I use it to keep track of my charging systems, know when I need to recharge or top off the batteries, and generally ensure that I am staying on top of the system as a whole. Its a bit more accurate than the motor controller for things like “how full are my batteries”, “did I give them a complete charge yesterday”, and “how much power am I using right now” (when the motor is not what is drawing the power).
Strategy and Resource Utilization
The unique requirements and capabilities of the electric motor setup for a long range cruiser require much more careful power management versus a diesel - much as the diesel requires much more careful maintenance. If I don’t want to get caught with my proverbial pants down, from a power perspective, I need to keep an eye on my battery charge state, whether or not my charging systems are doing their job, and whether or not I am likely to need power in the near future if I am choosing to use a lot of it right now. While I expect much of this will come from experience, a little forethought and learning from others is entirely appropriate right now. As a result, there are two major mindsets I am combining to develop my current strategy on power use: that of the cruisers without engines (the Pardey’s, Slocum, et. al) and that of my fellow electric boat owners - most of whom do shorter coastal passages or day trips. By planning my routes and sailing my passages with the mentality that I don’t have an engine, I’ll minimize situations where it becomes needed. And conversely, by strategically using the engine when I have an excess of natural power to regenerate, I’ll cut my power use and ensure a useful reserve. But its still there to help me fight a tide rip through a cut, or to catch that mooring when the wind dies, or to squeeze an extra knot or two out of a light headwind between islands.
A different kind of sailing? Indeed. And one I’m quite happy to look forward to.
Stay tuned for the next part: dealing with range anxiety.
______ /) _______
