«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»
In practice, due to losses and some reserve to charge the battery, shore current will be 2 to 4 times higher, but even 1 A still is next to nothing.
The example from chapter 10 shows that with more electric equipment on board, shore power can be reduced from 8 kW (3 phase 16 A shore outlet needed) to a mere 1.3 kW (6 A 230 V shore outlet).
What the examples show is that the DC or hybrid concept reduces the shore power rating needed by a factor of 4 to 10, making it much easier to find a suitable berth in today’s overcrowded marina’s.
But reduction of shore power rating is not the only advantage of the DC and the hybrid concept,
here is the complete list:
Up to ten times less shore power needed As will be shown in more detail in the next chapters, implementing the DC or hybrid concept really results in a breathtaking reduction of the required shore outlet rating.
The average power demand is in general less than one ¼ or even, depending on the power consumption profile on board, less than /10 of the peak power demand. Therefore the battery charger needed to connect to shore power will also be quite small and represents a small investment compared to the total cost of the electric infrastructure on board.
And a low power shore socket to connect to will be much easier to find in an overcrowded marina than a 16 A or a 3 phase socket!
Built-in clean, stable and no-break AC power Whatever goes wrong with shore power, the battery + inverters or Multi’s are there to guarantee uninterrupted power.
DC concept only: built-in frequency and voltage conversion Battery chargers will operate on a 50 Hz and on a 60 Hz supply. With an autotransformer or a wide input range (90 V to 260 V AC) battery charger one can connect to shore power anywhere in the world, without the need for an expensive and cumbersome shore power converter.
It is now time to go on board and see how things work out in practice.
Of course all boats are different, depending on purpose, budget and ownership. Some boats are equipped to cross the Atlantic or to sail around the world. Others are intended to travel along rivers and canals. And still others go out fishing for a day. Some boats are sailed and maintained by the owner, others are part of a charter fleet. Then similar electric installations can be found in mobile homes for example, or off-grid houses.
I have chosen here to take sea-going yachts as the example, because that is what I know about first hand. It is not very difficult to adapt the reasoning given in this and the following chapters to other applications.
The first boat we will board is fairly simple in terms of electric installation, and electric power consumption has been kept as low as possible. It would typically be a motorboat of up to 9 metres or a sailing boat of up to 12 metres.
The boat has a 12 V electrical system and, to start with, we list all electric equipment and current consumption.
9.2. Equipment and current consumption 9.2.1. Navigation instruments (wind set, log, depth sounder, etc): less than 0.2 A
Standby consumption is low (approx. 0.1 A). Transmitting does take a good deal of current (approx. 5
A) yet is brief, so that consumption in Ah remains quite low.
These days lighting consists of halogen lamps (10 W to 20 W) and fluorescent tubes (approx. 8 W).
Incandescent bulbs are not recommended because they take up to 5 times more current for the same amount of light produced. Assuming 10 lighting points and thrifty use, consumption would be limited to approx. 10 Ah per 24-hour period.
9.2.8. Refrigerator Refrigeration has been discussed in sect. 6.2.
In this example we will assume that we have a refrigerator on board with a 50 W compressor running with a 50 % duty cycle. In my experience this is an average refrigerator in terms of energy consumption, when cruising in a temperate climate.
Our starting point is one 24-hour period under sail (when travelling under power the current consumption is not of importance, because the alternator on the main engine can easily keep up with consumption).
We will now determine the battery capacity needed for supplying all consumers during one 24-hour period.
In the table that follows the consumers have been divided into continuous (C), long duration (L), and short duration (S) consumers.
It is noticeable that the refrigerator is by far and away the biggest consumer. The refrigerator’s current consumption could be halved by using a more expensive water-cooled heat exchanger instead of an air-cooled heat exchanger and by improving insulation. The total consumption per 24-hour period would then reduce to 103 Ah. Using a gas refrigerator (only useable on motorboats in calm waters) would even reduce current consumption to 78 Ah.
9.4. At anchor or moored without 230V shore power pick-up Once again, our starting point is one 24-hour period, but this time the following applies for motorboats and sailing boats.
Even the relatively small boats that we are considering here often have (or the crew might wish to have!) some extra safety and comfort on board. A few optional extras are suggested below. For some an inverter is needed.
Because today inverter efficiencies are higher than 90%, the losses in the inverter are ignored in the energy consumption calculations.
One should always opt for a diesel burner so that current consumption stays confined to diesel pump and fans. The current consumption is then kept down to approx. 5 A.
Some very efficient water makers are now available that work on 12 V DC. Current consumption is only 10 to 20 A for 30 to 60 litres of fresh water per hour. This has made a water maker (and thus also a freshwater deck shower!) a realistic extra for small boats used for blue water cruising.
Which translates to the following minimum battery capacity and average discharge current:
- when sailing: 297 x 2 = approx. 600 Ah and 12.3 A discharge current
- at anchor : 254 x 2 = approx. 500 Ah and 10.5 A discharge current We are now going to see how to produce the required energy, for the “basic” yacht (1.0 to.1.5 kWh needed), and for the “full featured” yacht (3.0 to 3.5 kWh needed).
9.6.2. Increase battery capacity so that you can sail or lie at anchor for several days.
This is a simple and inexpensive solution that only makes sense, however, if you always expect to be travelling for longer periods under power within a few days, or will have shore power available.
9.6.5. Wind generator A wind generator with a rotor diameter of 1 metre delivers approx. 25 W (2 A in a 12 V battery) at a wind speed of 10 knots. A contribution of 40 to 80 Ah per 24-hour period can be expected.
Where current consumption on board is low, solar cells and a wind generator can make a considerable contribution and drastically reduce engine running hours needed to generate power.
Even on somewhat bigger yachts, solar cells and / or a wind generator are very suitable for charging the batteries and keeping them 100 % charged during periods that the boat is not used. However, good charge regulators are very important to prevent overcharging.
9.6.6. Water generator (shaft or towed) Under sail, extra current can be generated with a propeller shaft generator (disadvantage: significant drag, noise, and wear and tear), or with a small stand alone water generator, transom-hung or towed.
The latter will generate about 12 W, or 1 A per knot of speed through the water, i.e. 40 to 100 Ah in a 12 V battery per 24-hour period, and adequately covers the increase in current consumption while under sail compared to lying at anchor.
The additional drag of about 30 kg will, however, reduce speed by about 0.5 knot.
9.6.7. Shore power
The best way to connect to shore power is with a battery charger, in other words, to also use the DC concept (see sect. 8.2.1 and 8.5.3). As will be shown below, the rule of thumb here is that the battery charger should be able to supply at least twice the daily energy need, and 3 to 4 times the daily energy need if a discharged house battery must be recharged within a day.
For the full-featured yacht for example, the daily energy need when moored, (which means no power needed for the masthead light, navigation, SSB, radar and water maker) is 132 Ah or 1.6 kWh.
Alternatively, if AC power is required when sailing, a 1200 VA Phoenix Multi or MultiPlus could be installed: it has sufficient power for the microwave oven and doubles as a 50 A battery charger.
- One or more sources of alternative energy such as solar cells, a wind generator, or a water generator can contribute a very substantial 1 to 2.5 kWh (100 to 200 Ah in a 12 V battery) to the daily energy required.
- The practical limit to daily recharging a 12 V battery with alternators on the main engine is about 4 kWh, or 300 Ah at 14 V. This is however sufficient for all the extra luxury as listed in sect. 9.5!
At 24 V, and again with 150 A alternator output (a load of 8.4 kW on the main engine!) one could move up to 8 kW daily energy consumption.
- An inefficient refrigerator (50 W compressor running at 100 % duty cycle) will consume up to 100 Ah per day of your precious battery capacity.
- Implementing the DC or the hybrid concept also to connect to shore power will limit the required rating of the shore outlet to 4 A.
In chapter 9 we saw that up to 4 kWh of electric energy required per day, a relatively simple DC system is perfectly adequate.
Next to the basic consumers, for which about 1.5 kWh was required, 2.5 kWh was available daily to operate additional equipment, increasing safety and / or comfort on board.
The required battery capacity did range from 250 Ah / 12 V for the “basic” yacht to 600 Ah / 12 V, for a “full featured” yacht.
Even above 4 kWh per day, a system with powerful alternators on the main engine is feasible (and the main engine will be more reliable than a small 3000-rpm genset). For reasons of noise, redundancy, and efficiency it is, however, certainly worthwhile to explore alternatives.
The available alternatives are:
- An AC diesel genset to directly supply AC consumers: the AC concept.
- A (smaller) battery assisted AC genset: the battery assisted AC concept.
- A DC genset: extension of the DC concept of the previous chapter to higher power ratings.
A daily energy consumption of 14 kWh is quite substantial and could very well be the average consumption in your land based home. Check your electricity bill!
In terms of boats, we are looking at motorboats and catamaran sailing yachts of up to 15 metres (49 ft) or monohull sailing boats of up to 18 metres (59 ft).
The calculations that follow are based on a 24 V house battery. To convert to 12 V, simply double current and Ah required.
To start with, the list of standard electric equipment:
10.2. Equipment: the minimum 10.2.1. Navigation equipment
10.3. Sailing As has been done in chapter 9, we will know calculate the power consumption over a 24 hour period, when sailing.
- By investing in efficiency and insulation, the power consumption of the refrigerator and freezer is now in line with that of other consumers. All too often bad design results in the compressors running nearly full time, which would add another 60 Ah to daily power consumption!
From the extra’s mentioned in section 9.5 of the previous chapter, the electronic navigation system, SSB and radar have been included in the list of section 10.2.
In addition to the other extra’s mentioned in 9.5 there are a few more to consider, now that we are looking at bigger boats.
10.6.2. Alternative sources of energy As explained in chapter 9, solar cells can be an excellent means to recharge the battery when the boar is left in the slip for a week or more.
When sailing, solar cells (1 m ), a wind generator (1 metre diameter) and a water generator (say 60 W at 5 knots speed through the water) together deliver almost 2.4 kWh (= 100 Ah in a 24 V battery) per 24-hour period. In other words: the contribution of alternative sources of energy can reduce engine hours substantially if not much more than the basic equipment is on board.
But when the daily energy needed increases further, other means of generating electricity are needed.
The alternatives will be discussed in the next sections.
10.6.3. With an AC generator The time-honoured method to cope with the high power demand of for example a washing machine or electric stove is installation of an AC diesel generator, to be started when power demand is high. The generator would for example run every evening for 2 to 4 hours during cooking and until the dishwasher has finished. During these same 4 hours one could run the washing machine and dryer, the water maker, the battery chargers, and heat-up the boiler (either electric or with cooling water from the generator).
If needed, one could have a second generator period of 1 or 2 hours during breakfast in the morning.
Battery sizing Knowing how long the generator will run, in our example during at least 4 hours per day, we can
calculate how many Ah the battery has to supply during the generator free period:
a) If under sail, roughly 24 - 4 = 20 hours of the basic power consumption as outlined in sect.
10.3, or 160 x 20 / 24 = 133 Ah.
b) Regarding other equipment, let us assume that the microwave, kettle and two thirds of the airconditioning time will also come within the generator free period. This means 16 + 25 + 175 x 2/3 = 158 Ah.
Total: 133 + 158 = 291 Ah. For this we then need a battery of at least 600 Ah, (2 recharge periods per day, see sect 8.5.2.) and with a little reserve (airco use during the night), that becomes 800 Ah.
Inverters and battery chargers In the inverter mode, the Multi 24/3000/70 combined inverter-battery charger will in general provide sufficient power because the generator can be started whenever more than 2 kW AC power is needed.
But as a charger the Multi only delivers 4 x 70= 280 Ah during the 4 hour generator period. During that period 291 Ah have to be charged, plus yet another 4 hours basic load, i.e. 6,7 x 4 = 27 Ah. With a little margin we then arrive at 100 A charging current. So, in addition to the Multi, a 24 V / 50 A charger needs to be installed.
For the electric stove 6 kW would be needed, plus 100 A x 30 V = 3 kW for battery charging. Total:
9 kW. With some margin to run additional equipment (the airco, for ex.) the right choice would be a 12 kW genset.
Shore power connection and frequency converter In our example the shore power pick-up can be limited to approx. 8 kW (35 A at 230 V in Europe), because there is far more time available to charge the battery.
A 50/60 Hz shore converter will be needed to connect to shore power on both sides of the Atlantic.
Halfway through the calculation you probably thought that this is not practical in a 15 meter sailing boat or a 13 meter motor yacht. The system becomes too big, costly, heavy and complicated. Indeed!
Consequently, the kind of installation described above only tends to be found in rather bigger boats, for example a 20 meter sailing yacht or a 15 meter motor yacht.
What can be done to make it work on smaller boats?
1) The obvious solution would be to drop the electric stove, and use gas instead.