«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»
Then a 6 kW generator running during 4 to 8 hours a day, depending on the amount of air conditioning, would do.
(Often the generator is downsized even more, to a 1 cylinder 3,5 kW model. Using any equipment with AC electric motors, like AC powered air conditioning, a water maker or a diving compressor then becomes problematic because the generator will not be able to supply the start- up current needed).
2) Secondly, air conditioning could be limited to generator running hours.
3) And thirdly, by implementing the DC concept when on shore power (see sect. 8.2. and 8.5.3.), shore power can be reduced from 8 kW to approximately 1.2 kW (in Europe this would mean a single phase 230 V 6 A shore connection). 50 / 60 Hz frequency conversion then comes as a built-in feature of the system.
To implement the DC concept, shore power should be connected to a 40 A or 50 A battery charger, and all AC consumers on board will be supplied by 2 or 3 parallel inverters of 2.5 kW each (or preferably Multi’s, see next section).
Using Multi’s and PowerControl together with a generator has the following advantages (see also
- Uninterrupted AC supply. When the generator is off, the Multi’s will supply AC on board. After the generator has been switched on, the AC load will automatically be transferred to the generator and the Multi’s will switch to battery charger mode. The reverse will happen when the generator is stopped.
- The PowerControl feature will eliminate any risk of overload on the generator. The battery charge current will automatically be reduced if, together with other consumers, power demand by the Multi’s (which with 2 Multi’s could be as high as 2 x 70 A x 30 V = 4.2 kW) would otherwise result in an overload. Thanks to PowerControl the generator discussed in section 10.6.4. can be downsized from 12 kW to 8 kW (installation with electric stove) or from 6 kW to 3 kW (without electric stove).
PowerAssist: the MultiPlus as generator booster This is the option on the Phoenix MultiPlus to allow parallel operation with a generator or with shore power (see also chapter 8).
Let us first look at parallel operation with a generator, for example the 6 kW generator on the yacht under sail from the previous section.
Operating 2 Multi’s in parallel with the generator would increase continuous AC output from 6 kW to 11 kW, and increase peak output to more than 15 kW. This brings the electric stove back on board.
Whenever power decreases to less than a pre-set limit (which in our example would be 5 kW for the 6 kW generator, in order not to run the generator continuously at full load), the Multi’s would take the surplus power from the generator to recharge the batteries, at up to 2 x 70 = 140 A.
Similarly, when operating in parallel with shore power, which would be rated for example at 16 A (In Europe this would amount to 16 x 230= 3680 W, or 3,7 kW) the 2 Multi’s would increase available AC power on board to some 8 kW.
No need anymore to start the generator in the marina!
10.6.5. The AC generator on a relatively small boat: conclusion Sizing a generator to the peak power that may be required results in a big and heavy machine, and the shore power connection needed will be well above the rating that is generally available. If, in addition, a 50/60 Hz shore converter is needed the system becomes extremely expensive and cumbersome.
Instead of compromising with regard to comfort on board, new technology can be used to reduce cost, size and weight of the power supply system.
By adding a 24 V 800 Ah battery, 3 Multi’s with PowerAssist and a 50 A battery charger to the system
we have been able to:
10.6.6. The DC generator Next to conventional 50/60 Hz AC generators, some generator suppliers are also offering DC generators. Outputs of up to 10 kW, that means a battery charging current of up to some 300 A at 28 V, are available. DC generators are smaller and lighter, and have a higher efficiency than AC generators.
Moreover, engine speed can be harmonised with power demand, so that efficiency remains high even under partial load.
The idea is to use the DC generator to charge the batteries, and use inverters to supply the AC load.
Sizing of the DC generator is a question of acceptable running hours per day. With 14 kWh of electrical energy required per day, a 6 kW DC generator, for example, would run for 2-3 hours per day.
10.6.8. The energy supply on a motor yacht of 9 to 15 metres or a yacht at anchor.
Even with the complete wish list of section 10.5 installed, the alternators on the main engines of a motor yacht can easily supply the average consumption of 24 A at 24 V DC when cruising. In other words, if one expects to motor for several hours nearly every day or to have shore power available every night, 14 kWh of daily energy consumption can be taken care of without installing a separate generator.
At anchor consumption will reduce to approx. 22 A, because navigation equipment is switched off.
When at anchor for longer periods a generator will be needed, as discussed in sect. 10.6.3 to 10.6.6.
What can we learn from the table?
10.7.1. Let us first have a look at the conventional solution: the 12 kW generator:
This solution is heavy and takes a lot of valuable space.
With PowerControl, whereby charge current is automatically reduced whenever otherwise an overload would occur, a smaller generator, for example 9 kW, would be sufficient.
Implementing PowerAssist with a 6 kW AC generator would in our example of the fully featured yacht require 2 Multi’s. Using the DC concept when on shore power would require 3 Multi’s.
With PowerAssist most of the AC power would be supplied directly by the AC generator and the average DC charge current needed would not exceed 75 A for a 4 hour generator period.
If most of the time the daily requirement is much less than 14 kWh, one could opt for a 1 cylinder
3.5 kW generator.
Especially low power AC gensets fitted with a synchronous generator tend to overheat when used at full load: derating of up to 30 % is often necessary to prevent catastrophic failure!
Nearly all small gensets are fitted with a synchronous generator. The notable exception is Fischer Panda, who uses asynchronous generators.
11.1. Introduction In chapter 10 we looked at boats requiring up to 14 kWh of electric energy per day. We concluded that 14 kWh per day is sufficient for 1 household of 4 to 6 persons whether they live on a boat or in a house, with all usual electric household equipment at their disposal, as long as air conditioning is either not needed or if limited use is made of it.
We also saw that a daily electric energy need of 4 to 14 kWh is typical for motorboats or catamaran sailing yachts of 9 to 15 meters or mono hull sailing yachts of 12 to 18 meters.
On a yacht of just a few metres longer, electricity consumption tends to increase disproportionately. Such yachts, whether chartered or not, often carry a professional crew and instead of 4 to 6 persons the yacht carries 8 to 12. Cruising is mostly in subtropical or tropical waters and airco is on for 12 hours or even 24 hours a day.
The power supply problem is usually solved as follows:
- Running an AC generator for 24 hours per day, or
- Installing a big battery to achieve a generator free period of 8 to 20 hours, and again use AC generators to supply high power electric equipment such as the electric stove, ovens, washing machines and battery chargers.
The required shore power is also significant (and severely limits the options when seeking a berth in a marina) because the battery is in general not used as a peak shaver (PowerAssist functionality). An expensive and heavy shore power converter will be needed to convert 60 Hz shore power to 50 Hz or the other way round.
Let us first have a look on board a yacht with an average daily energy requirement of 48 kWh, which amounts to an average power consumption of 2 kW.
11.2. The major consumers
The most important continuous and long duration consumers:
- Refrigerators and freezers: 300 W average
- Air conditioning: 12 kW (41.000 BTU) running on one or more compressors together rated at 3 kW
The most important short duration high power consumers:
- A 6 hob electric stove + ovens: 12 kW peak power
- A 300 l per hour high pressure pump type water maker: 3 kW (15 kW start-up)
- Washing machine(s) and dishwasher(s): peak power between 6 and 12 kW
- Possibly a diving compressor Other consumers are of less importance for dimensioning the system. We just assume an average consumption of 2 kW.
11.3. Energy generation 11.3.1. With an AC generator running 24 hours a day Assuming that other major short duration consumers are off during cooking, and that in practice the hobs and ovens will never run at full power simultaneously, a 15 kW generator would be the minimum. In practice a 20 kW (3 phase, to match shore power) generator would be installed. Often this generator is also fitted with a hydraulic power take off for the bow thruster.
If the choice is to run a generator permanently, one could opt to add a second, smaller generator of, say 5 kW, to cover the periods that much less power is required.
Batteries and battery chargers would remain very small in this case.
Although looking simple and low cost at first sight, this solution has some serious drawbacks:
- In order to avoid a dead ship every time AC power has to be transferred from one generator to the other or from generator to shore supply, complicated and expensive synchronisation systems will have to be added.
- A 20 kW (32 A 3-phase) shore power connection will be required.
- A 20 kW generator running 24 hours a day would have an average load of only 2 / 20 = 10 %! Not good for the generator and not good for fuel consumption. Adding a second, smaller, generator would increase this figure to some 20 %. Better, but still bad.
- And then of course the noise, vibration, smell and pollution 24 hours a day…(and do keep in mind that there are more and more marina’s and nature reserves where running a generator is forbidden).
11.3.2. Adding a battery for a generator free period This alternative brings us back to 10.6.3, but with more power required.
Battery sizing Battery capacity will depend on the required generator free period, and especially on whether at all, or how much, air conditioning is required during the generator free period. Let us assume here that the generator will be running at least twice a day, whenever the electric stove & ovens are in use, when the water maker is on and during washing and / or dishwashing. In other words: during some 8 hours per day.
Furthermore, we assume an average battery load during the generator free periods of 1.5 kW (= 63 A), which results in
1.5 x (24 – 8) = 24 kWh or 24 kWh / 24 V = 1000 Ah taken from the battery per day. Applying the rule of thumb from sect. 8.5.2, a battery of 2000 Ah will be needed.
Of the 48 kWh required per day, in this example 24 kWh is supplied by the battery, and the remaining 24 kWh directly by the generator.
The generator The generator will have to recharge 1000 Ah within 8 hours. We then need a recharge current slightly exceeding 1000 / 8 = 125 A, for example 175 A. For the generator this means a load of 175 x 30 = 5.25 kW. This can be done with the 20 kW generator mentioned earlier, provided the battery chargers are switched off when peak power is required for cooking plus some other electric appliances being used at the same time.
The energy to be supplied by the generator will be 1000 Ah x 30 V = 30 kWh for the battery, plus the 24 kWh directly to the AC appliances, total 30 + 24 = 54 kWh, battery charge-discharge losses included.
But we still need a 15 kW shore power connection and a shore converter.
11.3.3. Using parallel Multi’s with PowerControl, and the DC concept for shore power:
- for automatic generator load dependent battery charging
- to reduce required shore power to 3.5 kW
- and have frequency conversion nearly for free
Installing 5 Multi’s in between the 20 kW generator and the battery will result in the following:
- Instead of a three phase generator, a single phase model could be used: shore power will also be single phase (see below) and phase balancing problems will be eliminated.
- The Powercontrol feature will eliminate any risk of overload on the generator. The battery recharge current will automatically be reduced if power demand by the Multi’s (which could be as high as 5 x 70 A x 30 V = 10.5 kW if a fast recharge is needed, but would in general be limited to 5.25 kW) together with other consumers would otherwise result in an overload.
- Uninterrupted AC supply. When the generator is off or power is needed for the bow thruster, the Multi’s will supply AC on board. After the generator has been switched on the AC load will automatically be transferred to the generator and the Multi’s will switch to battery charger mode.
- By implementing the DC concept, shore power can be reduced from 15 kW to 3.5 kW (in Europe this would mean a single phase 230 V 16 A shore connection instead of a three phase connection) and frequency conversion is a built-in feature of the system. To implement the DC concept, shore power should be connected to a 100 A battery charger (or, for redundancy, 3 off 50 A chargers) which charge(s) the battery, and all AC consumers on board will be supplied by the 5 parallel Multi’s. The 5 parallel Multi’s are rated at 10 kW continuous output power and 15 kW short term.
At first sight 100 A might seem a bit tight: the 48 kWh required per day translates to (48 kWh / 24 h) / 24 V = 83 A. But on the other hand being able to run the ship on a 16 A shore outlet is
11.3.4. Going 1 step further: using the MultiPlus and PowerAssist to reduce generator size by 50 % With 54 kWh of electric energy needed and running the generator for at least 8 hours per day, the generator should be rated at 54 / 8 = 6.75 kW, that with some margin brings us to 10 kW (see sect. 8.3.
Operating 5x MultiPlus’s in parallel with the generator would increase available AC power to 10 + 5 x 2.5 = 22.5 kW.
When AC power demand increases beyond a pre-set limit, for example 8 kW in order not to run the generator at full load, the Multi’s will start supplying additional AC power. The available energy from the 2000 Ah battery (24 x 2000 x 0.5 = 24 kWh) is more than sufficient to cover short time power demand iwhen the Multi’s have to kick in.
When power demand drops below 8 kW, the Multi’s will use the surplus power from the generator to recharge the batteries. The maximum recharge current of 5 parallel Multi’s is 5 x 70 = 350 A, which would take 350 A x 30 = 10.5 kW from the generator. Much more than needed and even more than the generator can supply.
The AC generator: conclusion By adding a 2000 Ah battery, 5 Multi’s with PowerAssist and a 100 A battery charger to the system we
have been able to:
- Introduce 2 generator free periods per day of in total 16 hours.
- Reduce the rating of the generator from 20 kW (3 phase) to 10 kW (single phase).
- Reduce shore power required from 15 kW (3 phase 25 A) to a mere 3.5 kW (16 A 230 V shore outlet)
- Eliminate the need for a 15 kW shore converter
- Achieve uninterrupted AC power on board
- Substantially increase redundancy and therefore safety.
11.3.5. The DC generator An alternative for the 10 kW AC generator would be a 10 kW DC generator. Please refer to section 10.6.7.