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Energy Management Part 1 - Consumption and "Use Cases"

A reliable and continuous supply of electric energy is not only necessary for convenience reasons (although it is nice to always have a cold fridge...) but it is actually a critical requirement to assure safety of boat and crew when cruising.

Navigation lights, instruments, positioning devices and communication do not work without electricity.

 

Check out discussion forums, magazines, books or simply talk to people who live on their boat. It's a common theme - many cruisers struggle with proper energy management when going offshore for a prolonged time.

 

One example is this article from Yachting World, giving an overview on energy management solutions in the ARC 2013 crowd.


We assume you are basically familiar with creating an energy balance for your boat when reading this article. If you do not know how to do this please check out the links section.

 

So, How much energy do we actually need?

It may seem a very basic question but actually it is a very important one. And there is no generic answer, you really need to calculate - and if possible - measure this for your own situation.

 

For practical reasons we were initially not able to perform measurements, so we first started with assumptions and calculations.

 

Assumption 1: We will distinguish two basic scenarios which will result in net discharge of the batteries:

  • Case 1: At anchor (not connected to shore power)
  • Case 2: Offshore, sailing

Assumption 2: We will be running at least one fridge unit all the time, regardless where we are and what we are doing.

 

Assumption 3: As we are "only" crossing the Atlantic Ocean we will not fit wind vane self-steering gear, hence we will have to factor in the auto pilot when sailing offshore.

For safety reasons we will also use the full navigation gear, including running the chart plotter, AIS and radar when cruising.

 

Assumption 4: Our longest offshore sailing leg will be max. 45 days (which should be an extremely safe estimate for the classic route Canaries - Caribbean, roughly 2700 NM).

 

In our case we measured the following energy consumption, measured over the course of 24 hours:

  • Case 1: At anchor we measured roughly 110 Ah/day
  • Case 2: Sailing offshore we measured roughly 250 Ah/day (assuming no engine operation, with navigation instruments, lights and Radar active)

Once the energy consumption has been determined, the interesting question is:

HOW CAN WE MAKE SURE THAT THE CONSUMED ENERGY IS REPLENISHED AND THE ENERGY BALANCE IS NEUTRAL OVER TIME?

And actually this is one of the most important and central questions when designing the system.

 

Factoring in total energy consumption and recharging over the total time spent offshore you will always need to stay within the acceptable "state of charge windows" of the battery system. Practically this means that the battery state of charge must never fall below the lowest acceptable state of charge of the battery system used in order to avoid damage to the battery.

For Lead Acid batteries e. g. this lowest acceptable state of charge level is around 50 % and this also means that you can effectively only use half the advertised capacity of a conventional lead acid battery bank.

Saving energy: How can we reduce overall energy consumption?

The numbers above were measured with the equipment that was available to us, in normal use, and without any optimization.

 

Luckily our boat is already equipped with a complete set of LED-only internal and navigation lighting.

There is a standard fridge on board which probably does not use the most efficient compressor and more importantly isolation, but it will be quite difficult to improve the isolation without dismantling most of the pantry - so this is basically not an option.

 

For now we'll ignore this topic, but we may revisit saving energy later when trying to optimize for longer offshore trips.

2018 Update: Real world numbers

This section was added in 2018, after we finished our cruise and had an excellent time on the boat.

Average daily energy consumption (measured/experienced during the year)

 

At anchor:

100 - 120 Ah typical. Peak usage 150 Ah when recharging the portable dinghy battery bank from the solar panels (about once a week).

 

When sailing 24/7:

250 - 350 Ah typical

 

The 250 Ah are experienced in calm to moderate conditions. The 350 Ah were only seen in prolonged times of bad weather (Bft 7 - 8, nasty waves) when the autopilot had to do some real work.

 

Summary of "use cases" from the core/main cruising time (2016-11-09 - 2017-08-14, 279 days)

 

1. Ocean passage; 34 days (La Palma - Grenada)

We were trapped in 13 days of total calm with 700 nm of zero wind around us. Using the engine would have been totally pointless. We had enough water and food and hence decided to have a vacation on the ocean instead of motoring.

We switched off navigation and drifted for a few days. This means that a few days of this passage were identical to the "at anchor" use case - with neutral energy balance.

 

Total engine hours during this passage: 52.0 h (1.52 h per day).

Propulsion/charging ratio: 100 % charging

 

2. Ocean passage; 21 days (Guadeloupe - Horta/Azores), see below for details

Total engine hours during this passage: 117.3

Propulsion/charging ratio: about 20 % (4 days of calm in the Horse Latitudes) propulsion, 80 % charging

 

3. Ocean passage; 8 days (Azores - Spain)

Total engine hours during this passage: 37.7

Propulsion/charging ratio: 15% propulsion (about one day of very low winds), 85 % charging

 

4. Cruising mode; most of the remaining time (216 days)

Total engine hours: 135.4

Typical was to stay at one place for 1 - 4 days. In rare cases up to 10 - 14 days.

0 days of shore power (exception: charge to 100 % before leaving for the long cruising passages)

Propulsion/charging ratio: 100 % propulsion, 0 % charging (to confirm: never used the engine to charge batteries at anchor)

 

SoC regime:

- SoC mostly between 30 % and 90 % during cruising. This did not require any particular attention during the "at anchor/leasure" times.

- During the passages recharging was done daily or every other day (depending on SoC). 

- Passage sailing SoC policy: SoC at dawn no less than 45 %. Typical energy usage over the night (lights, nav, autopilot, fridge): about 20 - 25 %.

- When recharging with the engine, delta charge of at least 30 % or to SoC of 70 %

- During the year SoC was at 100 % some time when motoring for prolonged times, e. g. during calms on passages. During normal cruising we avoided high SoC values by making liberal use of the electric kettle and induction plate.

Detail data for one ocean crossing

Real world scenario from our ocean crossing.
Real world scenario from our ocean crossing.

This diagram shows actual statistics from the last leg of our cruise. It is interesting because it actually shows several different usage scenarios:

  • From the beginning of the diagram to the 23rd of May we stayed in a marina but not connected to shore power. The solar panels typically allow us unlimited time without need for other charging sources at anchor, so we saw no need for shore power connection in the marina. We were careless with energy because the shore power readily available.
  • On 24th of May we left the marina for the 21 day ocean crossing from Guadeloupe to the Azores. Or course we charged the batteries before leaving. This does not show in the SoC statistics (blue) because we charged to 100 % immediately before leaving.
  • From May 24th to June 15th we crossed the Atlantic Ocean. At first we had quite some wind and wave to struggle with, the red consumption bar shows that the autopilot was consuming a decent amount of energy. Solar roughly stayed the same.
  • We  crossed the "Horse Latitude" calms between May 31rd and June 3rd and did a lot of motoring then. Hence SoC was near 100 % (which is not desirable for LiFePO4, but on a crossing we take what we get - you never know). Solar power was not accepted by the batteries due to high SoC, hence small orange bars.
  • Between June 9th and June 11th we crossed the edges of a low pressure area with heavy winds and steep waves. The autopilot did a good job but consumed a lot of energy.
  • We arrived on the Azores on June 15th

We had to run the engine for about 1.5 hours a day to compensate the consumption of the navigation electronics and autopilot, but overall the system worked extremely well. We also learned that in extreme cases we were consuming up to 350 Ah a day! Also, consumption at anchor was more like 150 Ah due to liberal usage of electrical conveniences.

Conclusions after completing the cruise

A LiFePO4 battery bank allows to store energy much more efficiently than a Lead Acid battery and is particularly interesting for long-time offshore cruising.

Particular benefits of LiFePO4:

  • Charge efficiency near 100 %
  • No diminishing charge acceptance rate like Lead Acid battery
  • Prefer to be stored and operated not fully charged

These properties allow to dimension the charging system of a cruising boat to exactly match the requirements. They also mean that charging sources can be chosen independently of bank size (which is not advisable for Lead Acid banks due to efficiency losses and long tail charging).

 

In our case our solar panel input almost exactly matches our daily consumption, covering the use case "at anchor" which is the majority of the time.

When cruising we have a "energy gap" which needs currently to be filled by running the engine for 1.5 hours a day on average. We intend to fill this gap later by adding a hydro generator which will fill exactly this gap for this use case.

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