«Energy Unlimited Reinout Vader Electricity on Board (And other off-grid applications) Revision 9 June 2011 Electricity plays an increasing role on board ...»
11.3.6. Using a small auxiliary DC generator to reduce generator hours, battery capacity and fuel consumption On a big boat a small genset can be made inaudible and completely vibration free. So why not run a small genset during most of the day to reduce battery capacity?
- Battery capacity could be reduced substantially, for example to 1000 Ah which is the minimum needed to run 5 Multi’s.
- Main generator running hours can be reduced further, from 8 hours to approx. 6 hours, and even down to 1 or 2 hours when no airco is needed.
11.4.1. The 20 kW generator with generator free period (right-hand column) This alternative is heavy, and the 2000 Ah battery is expensive, with the risk of high expenses in case of a mistake regarding battery management or an accident, like a cell failure for example.
Implementing the DC concept solves the shore power problem (see sect.11.3.3.).
By adding an auxiliary genset (AC or DC), battery capacity could be reduced to 1000 Ah, reducing weight by 1000 kg.
11.4.3. Using a smaller generator with PowerAssist, the DC concept for shore power, and an auxiliary genset (left-hand column) The main difference compared to section 11.4.2 is the smaller generator (10 kW instead of 20 kW), reducing weight by another 130 kg.
12.1. Introduction In chapter 11 we saw that size, weight and complexity of the power supply system could be significantly reduced by designing a well balanced system consisting of a 10 kW generator assisted by Multi’s, and also a relatively small service battery assisted by an aux. genset.
In this chapter we will look at still higher power requirements
12.2. The major consumers An average consumption of 10 kW is applicable for boats of up to approx. 30 metres.
- The biggest consumer of electricity will in general be air conditioning, running day and night when cruising in tropical areas. The rated cooling capacity would for example be 100,000 BTU (= 30 kW). With a CoP (Coefficient of Performance, see section 6.2.) of 4 this means that 30 / 4 = 7.5 kW would be needed when the air conditioning has to work at full power.
On average over a 24-hour period the air conditioning’s consumption will be at most 5 kW, and that immediately explains half of the total power consumption.
- The other major consumers are galley appliances, washer-dryers, the water maker and lighting. Current consumption will be less at night than during the day, for example in a proportion of 5 to 15 kW.
12.3. Energy generation
- 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 harbours and nature reserves where running a generator is forbidden).
This alternative only makes sense if the peak power of 50 kW is an exceptional situation and of short duration, with power demand staying below 20 kW most of the time.
The battery If power consumption can be reduced to an average of 4.5 kW over sizeable periods of time, for example 8 hours during the night and 6 to 8 hours during the day, the maximum daily amount of energy to be supplied by the battery would be 4.5 x 16 = 72 kWh or 72 kWh / 24 V = 3000 Ah. With our rule of thumb from sect.8.5.2, a battery of 6000 Ah would be needed.
The generator, Multi’s and shore power Now we have to think differently, forget about peak power required and instead look at the daily energy needed (see sect.8.5.) Of the 240 kWh required per day, in this example 72 kWh is supplied by the battery, and the remaining 168 kWh directly by the generator The amount of energy needed to recharge the battery is 3000 Ah x 30 V = 90 kWh.
The daily energy to be supplied by the generator therefore amounts to 168 + 90 = 258 kWh.
The generator, running during at least 8 hours per day, should be rated at 258 / 8 = 32 kW. With some margin, 40 kW would be installed.
Adding 3 Multi’s per phase will increase continuous output power by 9 x 2.5 = 22.5 kW to 40 + 22.5 = 62.5 kW.
When AC power demand increases beyond a pre-set limit, for example 35 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 6000 Ah battery (24 x 6000 x 0.5 = 72 kWh) is more than sufficient to cover short time peak power demand.
When power demand drops below 35 kW, the Multi’s will use the surplus power from the generator to recharge the batteries. The maximum recharge current of 9 parallel Multi’s is 9 x 70 = 630 A, which would take 630 A x 30 = 18.9 kW from the generator. Much more than needed: the average recharge current needed is 3000 Ah / 8 h = 375 A.
One attractive feature of the Multi’s is that they will automatically balance the load of the generator:
the Multi’s will take most power from the phase(s) which otherwise would have the smallest load.
A solution to reduce shore power is again to implement the DC or the hybrid concept. The daily energy required of 240 kWh translates to (240 kWh / 24 h) / 24 V = 416 A at 24 V, which could be supplied by 6 off 100 A rectifiers. Shore power required would then be 18 kW (32 A 3-phase).
Alternatively, because the 18 kW is not so much less than the 50 kW peak power required, the Multi’s could operate in shore power support mode, which would likewise limit shore power to 18 kW, but frequency conversion would not be possible.
By adding a 6000 Ah battery, 15 Multi’s with PowerAssist and 6 off 100 A battery chargers 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 50 kW (3 phase) to 40 kW (3 phase).
- Reduce shore power required from 50 kW (3 phase 75 A) to 20 kW (3-phase 32 A)
- Achieve uninterrupted AC power on board
- Substantially increase redundancy and therefore safety.
12.3.3. Adding an 8 kW auxiliary AC generator On a big boat a small genset can be made inaudible and completely vibration free. So why not run a small genset during most of the day to reduce battery capacity?
- Battery capacity could be reduced substantially, for example to 2000 Ah which is the minimum needed to run 9 Multi’s.
Note: savings due to usable heat output of the auxiliary genset not included) What can we learn from the table?
The main lesson is that with 240 kWh of electric energy required per day, the limits of the new components and concepts presented in this book have been reached.
The battery needed to implement PowerAssist and the DC concept becomes really cumbersome and very expensive.
Only when a battery free period is a must, or when the energy needed is, for most of the time, much less than 240 kWh per day, will PowerAssist or the DC concept be attractive options.
13.1. Consumption of electric energy on board
- On small boats the refrigerator and freezer often are the most important consumers. Spending some money on good insulation and a good water-cooled refrigeration system can reduce battery capacity and recharge time needed dramatically.
- Similarly, small air conditioning systems can be incredibly inefficient.
- The impact on energy consumption of continuous and long duration consumers (mainly navigation and refrigeration equipment) is often underestimated.
- The impact of short duration consumers (microwave, electric stove, washing machine, pumps, electric winches) is often overestimated.
13.2. Energy generation
- The first step to have more energy available on board is to increase alternator output by installing a second or bigger alternator and to increase battery capacity to at the very least 3 times the alternator output (C / 3 charging rate). Otherwise the battery will overheat and not absorb the available charge current.
- When designing a small autonomous power supply system one should, in the first instance, ignore the maximum power required, but consider the total amount of electric energy needed over a 24 hour period.
- A problem that is often overlooked when installing an AC generator on board is shore power. When no additional measures are taken the shore power rating must match (or even exceed, because of electric boiler heating) the rating of the generator. How easy is it to find to find
- in Europe: a berth with more than 16 A (3.7 kVA) shore power?
- in North America: a berth with more than 50 A (5.5 kVA) shore power?
13.3. The DC concept
- In the DC concept a battery sits in between the consumers and suppliers of electrical power. The battery supplies additional energy when demand exceeds supply, and absorbs energy when supply exceeds demand.
- With the DC concept high power consumers (the electric stove) can operate together with low power suppliers (for ex. a 230 V / 4 A shore power outlet).
- The DC concept doubles as a 50 / 60 Hz shore power converter
13.4. PowerAssist: the hybrid or battery assisted AC concept
- Similarly to the DC concept, PowerAssist uses a battery to supply or absorb electric power, but now the link between suppliers and consumers is AC instead of DC. One or more Multi’s operating in parallel with a generator or shore power will provide additional AC power when demand exceeds supply and absorb AC power to recharge the batteries when supply exceeds demand.
- Similarly to the DC concept, PowerAssist allows high power consumers to operate with a lower power generator or shore supply.
- Like the DC concept, PowerAssist saves space and weight. Additionally the average load of the generator will be much higher. This will increase service life, decrease maintenance and decrease fuel consumption.
- PowerAssist is not suitable for frequency conversion. Maximum flexibility is obtained by adding battery chargers to the system, and using the DC concept when connected to shore power.
-On bigger boats, with a substantial house battery to cover a generator free period, a small auxiliary genset can be used to reduce size and weight of the battery and at the same time produce useful heat for the boilers and for space heating.