«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»
For the sauce we fry the onions (150 Wh), add the meat and fry again (150 Wh), add fresh tomatoes, herbs, etc and bring the sauce to the boil (1 litre, so 100 Wh) and leave the sauce simmering for 20 minutes (200 W during 20 minutes), total 150 + 150 + 100 + 200 x 20 / 60 = 470 Wh.
For the desert we heat 2 litres of cold milk right from the refrigerator (300 Wh), plus 3 minutes of simmering (30 Wh), total 300 + 30 = 330 Wh.
I have also verified the above in practice and the result is that for most meals with 3 hot courses and intended for 4 persons indeed 1200 to 1400 Wh, or 50 to 60 Ah from a 24 V battery is needed.
6.7. The diving compressor I like diving. What I do not like is that after the dive I have to lift anchor, head for a harbour and lug my bottles to a diving club in order to have them refilled. Why not install a diving compressor on board?
A small diving compressor is powered by an electric motor of around 3 kW, and the start-up current is about 10 times the rated current. It will trip the shore power circuit breaker in the harbour, and a diesel generator will have to be substantially over dimensioned to start it.
The solution is to drive the compressor with a 3-phase motor and ad a variable frequency drive, with a three phase output to drive the motor and a single phase input to connect to an inverter, diesel generator or shore power. The 1 to 3-phase frequency drive (readily available up to 3 kW output power from several frequency drive manufacturers, like ABB, Hitachi or Mitsubishi) will eliminate the start up surge and allow the 3-phase motor to be supplied by a single phase supply.
6.8. How to deal with the inrush current of AC electric motors Electric motors rated in the kW range have very high inrush currents and an inverter or diesel generator has to be substantially over dimensioned to run them (examples: pumps, air conditioning, and the diving compressor discussed in the previous section). As discussed in the previous section, a solution is to use 3-phase motors and a 1 to 3-phase variable frequency drive.
The refrigerator, a continuous consumer of electricity, will if not carefully engineered, drain the battery and consume more energy than high power but short time consumers like a washing machine, dishwasher or even an electric stove.
7.1.1. A diesel engine will last longer if it has to work In order to generate a stable 50 Hz or 60 Hz output, the diesel engine powering the generator must rotate at a fixed and stable frequency. For 50 Hz output this is 3000 rpm or 1500 rpm, depending on the number of poles of the generator (3000 rpm / 60 seconds = 50 rotations per second = 50 Hz). When a diesel engine runs at relatively high rpm and with nearly no load the internal temperature will be low and service life will be reduced.
It is therefore not recommended to run a genset 24 hrs per day, with nearly no load. And the noise, fumes and odours are not to look forward to either.
7.1.2. A hybrid or battery assisted AC system A first improvement is to run the generator during periods of high power demand only, and install a battery and inverters to generate AC when the generator is off.
An even better system is obtained by operating one or more Phoenix Multi’s or MultiPlus units in parallel with the genset (see for example par. 10.6).
The advantages are:
- uninterrupted AC supply
- relatively more load on the generator, less space needed, less noise and less weight because a smaller genset can be used: the MultiPlus will absorb peak loads taking energy from the battery, and recharge whenever “surplus” power is available (see for ex. par. 10.6.5. or “Achieving the impossible” and many other examples on our website).
7.1.3. Don’t forget the problem of limited shore power A washing machine, dishwasher, electric cooker, air-conditioning: it is all feasible with a big enough generator. But in Europe power from the shore side is often limited to 16 A or even less (16 A x 230 V = 3,68 kW). Here also the MultiPlus can help to increase available power to the required level.
7.1.4. 3000 rpm or 1500 rpm (in a 60 Hz environment: 3600 rpm or 1800 rpm) A, more expensive, 1500 rpm genset is the right choice if intensive use is to be expected.
A 3000 rpm genset is in general designed for a limited number of operating hours, and is not made to operate at full load for long periods of time.
Some generator suppliers are wildly optimistic about the maximum output of their product. A way to find out is to look for gensets from different suppliers but with the same engine and then compare the rated output.
7.2. DC Generators
Next to conventional 50/60 Hz AC generators, some generator suppliers are also offering DC generators.
Outputs of up to 10 kW, which means a battery charging current of up to some 300 A at 28 V, are attainable.
DC generators are smaller and lighter, and have a higher efficiency than AC generators. Moreover, engine rpm 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.
Please keep in mind however that the battery should be sized for the huge charge current. For a charge current of 300 A for example, battery capacity should be 300 A / 5 = 1500 Ah (see par. 2.5.6.) It should be noted here that some manufacturers of AGM batteries claim much higher charge currents without appreciable reduction of service life.
As electronic power conversion technology improves, more and more household appliances, which do require an AC supply, are also being connected to the DC bus, with an inverter.
Next to conventional 50/60 Hz AC generators, some generator suppliers are offering DC generators.
DC generators are smaller and lighter, and have a higher efficiency than AC generators. Moreover, engine rpm can be harmonised with current demand, so that fuel efficiency remains high even under partial load.
8.2.3. Unlimited inverter power Sinusoidal inverters have now become generally accepted.
New is the possibility to connect inverters in parallel.
Victron Energy has developed inverters and inverter-chargers (bi-directional converters) that can be parallel connected in either single or three-phase configuration.
The parallelable inverter/ charger modules are the Multi 12/2500/120 and Multi 24/3000/70, which have a continuous output power of 2 kW at 12 V input and 2.5 kW at 24 V input respectively.
Up to 6 modules can be connected in parallel per phase. Taking as an example the 24 V model, the
output power which can be reached is as follows:
Where previously installation of an AC generator was a must, parallel inverters are now an alternative.
8.3. The AC concept can be improved with PowerControl 8.3.1. The AC concept In the AC concept one or more petrol or diesel fuel-powered generators are the hart of the system.
Whenever AC power is needed a generator is started. The generator has to be rated to meet the highest power demand that is expected.
In general the generator, together with a battery charger, is also used to charge one or more small service batteries for navigation equipment, lighting, DC pumps, etc.
Likewise, shore power has to be rated to meet the highest power demand that is expected. Shore power must also match the frequency and voltage of the on-board AC equipment. If not, a frequency converter (also called shore converter) is needed.
The AC concept is the preferred solution when a lot of power is required.
8.3.2. The AC concept with generator free period As power demand decreases, the drawbacks of the AC concept become more and more prominent.
The generator will operate without any load at all for long periods of time, or will have to be started and stopped frequently, often operating with hardly any load. This of course means noise, pollution, fuel consumption, wear and maintenance while at same time, on average, electric power consumption is low.
A way to improve on this situation is the generator free period, which requires in addition to the generator a big battery, battery chargers and inverters. When the generator is off, all consumers are supplied with energy stored in the battery. Periodically, in general when a lot of AC power is required anyway, the generator is started and then also used to recharge the battery.
Although much better than the “generator only” concept, there still is a lot of room for further improvement. This is the subject of the next sections.
A more effective solution is PowerControl.
With PowerControl the output current of the generator is continuously monitored and the power taken to recharge the battery is automatically adjusted so that the total load of the generator remains within a pre-set limit.
Similarly, with PowerContol charging would be reduced, but not stopped, while using the microwave, a hot water kettle, an AC motor powered water maker, etc.
The example above is also applicable to shore power. The current limit should then be set at the rating of the circuit breaker protecting the shore power outlet. Of course this rating should be sufficient to supply the most power hungry piece of equipment on board. In our example this would be the washing machine, so in Europe the minimum shore power rating should be 2 kW / 230 V = 9 A.
Often the rating is lower, for example 6 A or even 4 A, not enough to run the washing machine. This brings us to PowerAssist, the subject of the next section.
Continuing our example of section 8.3.3., one may want to run a 2 kW air conditioning compressor and at the same time do the washing, bringing the peak power to 4 kW. Or to heat water in a hot water kettle (2 kW), or simply make coffee (1 kW), or use an electric stove (6 kW) instead of gas.
With the MultiPlus this is all feasible. When the AC power required increases beyond a pre-set limit, the Multi will stop charging the battery and operate as an inverter in parallel with the generator or shore power. In our example the generator would be boosted from 3 kW to 3 + 2.5 = 7.5 kW. As will be shown in the next chapters, the saving in weight and fuel consumption of the electric power supply system can be substantial.
The MultiPlus solves the problem of insufficient shore or generator power by adding additional power taken from the battery.
8.4.2. Other advantages when operating Multi’s together with a generator In the previous sections we explained the advantages of PowerControl and PowerAssist: the possibility to use a smaller generator, or to reduce generator running hours, to increase the AC load, or to boost shore power.
Redundancy When several Multi’s are operating in parallel, a faulty unit (although unlikely that this would happen) can be isolated from the healthy ones. There is a second AC redundancy because of the presence of
the Multi’s and a generator. And finally there are at least 2 sources of DC power to recharge the battery:
one or more Multi’s and the alternator on the main engine.
We have seen that one way to cope with insufficient shore power is the MultiPlus: with PowerAssist shore power can be boosted to up to 4 times its nominal rating.
An alternative is to use the DC concept for shore power. In other words: use a battery charger to convert shore power to DC and convert DC back to AC with the inverters or Multi’s which are on board anyway. The house battery will supply additional energy when a lot of power is required on board, and will be recharged by the battery charger during periods of low power demand.
The daily run-time needed to produce the required energy is calculated with the following formula:
run-time (hours) = daily energy need (kWh) / output of the source(s) of electric power (kW)
The rule of thumb from practice is that in case of 2 recharges per day, battery capacity should at least be twice the daily Ah consumption.
If for example daily consumption is 128 Ah (see sect. 9.3), battery capacity should be 256 Ah. Assuming a constant discharge rate over 24 hours, our 256 Ah battery would be subjected to a discharge of 128 / 2 = 64 Ah over a period of 12 hours.
The rule of thumb derived from theory is that the usable battery capacity is 32 % of the nominal capacity. Assuming a maximum period of 12 h between recharges and a consumption of 128 / 2 = 64 Ah during that period, 32 % usable capacity would in this example mean that we need a battery of 64 Ah / 0.32 = 200 Ah.
The positive difference between practice and theory of 265 - 200 = 65 Ah can be seen as compensation for the fact that the discharge rate is not constant but will depend on which consumers are switched on, and when. Recharge periods may also vary in length.
In other words: theory leads to the same result as the rule of thumb.
We now have two simple methods to estimate the capacity needed for the house battery:
1) The capacity of the house battery should be at least three times the expected discharge during the generator free period. (100 % / 32 % = 3.1)
2) If the house battery is recharged two times per day, its capacity should be at least twice the daily Ah consumption.
Maximum amount of energy that will be taken from the battery during the generator free period: 4 kWh Minimum capacity of the battery (12 V system): 4 kWh x 3 / 12 V = 1000 Ah Minimum capacity of the battery (24 V system): 4 kWh x 3 / 24 V = 500 Ah
8.5.3. Shore power When the generator on board has been sized to supply the maximum expected power need, quite naturally, the shore power connection will also have to be rated to supply the maximum expected power consumption on board.
Let’s assume that the microwave oven, rated at 1500 W, is the most power hungry appliance. At 1500 W, the microwave will take 1500 / 230 = 6.5 A from a 230 V shore outlet. This is already more than the usual 4 A or 6 A shore outlet rating. If at the same time the electric water heater switches on (4 to 5 A) and your coffee machine (4 A) is just starting to spread the lovely smell of freshly made coffee, your power draw increases to 6.5 + 4 + 5 = 15.5 A. In other words: you are not far from tripping even a 16 A shore outlet!
Not to mention a washing machine (9 to13 A), a dishwasher (also 9 to 13 A) or an electric stove (16 to 35 A).
The result is that the generator has to be started even when moored in the marina. Not the way to make friends on the neighbouring boats.
The solution is to think differently and to implement the DC or the hybrid concept for shore power. Once more the question then is not “what is the maximum AC power to be expected?” but instead “what is the daily electric energy need?” The microwave for example takes 6.5 A, but only for 5 minutes, at most. If this current could be averaged over 50 minutes, then the 6.5 A would reduce to one tenth (0.65 A) but during a ten times longer period: 50 minutes instead of 5 minutes.
This is exactly what the DC or the hybrid concept do: using the house battery to average peaks in power consumption (“peakshving”).
© Victron Energy The example described in chapter 9, where indeed a microwave oven is the most power hungry appliance, will show that the daily energy consumption when moored is 1.6 kWh, which translates to an average power of 1600 / 24 = 66 W, or 5.6 A taken from a 12 V house battery. And 66 W is a current of only 66 / 230 = 0.3 A from the shore power outlet!