Quote:
Originally Posted by StuM
You are VERY confused. You can't just mix power and energy like that.
And you can't take "total lifetime efficiency" and apply it to a single part of the life cycle.
To take it to more practical terms:
Using the common rules of thumb for marine auxiliary diesel engines of 1 gal per hour per 20 HP
36 USG of diesel will provide roughly 700 HP hours. or 500 kWh of propulsion energy.
2 x 12 kW h batteries will provide 24kWh. Assuming a realistic 90% efficiency in converting that to propulsion energy, that's about 21 kWh of propulsion energy .

Yes you can mix power and energy like that more info below
Yes you can take total life time efficiency and apply it to single part of the life cycle more info below
Confusion reins supreme in the world of rules of thumb whos thumb is the measure
The
rule 1
usa gallon per hour for each 20 HP
On a need to know basis it works for average cruising boats cruising in reasonable
weather
There is no real data from this
rule of thumb on weather boat types speeds all sorts of factors that can make the rule fall apart
Many rules of thumb are iffy at best
In a
hurricane going against 50
knot head
wind and boats cruising speeds are knocked to close to zero knots often means you might need more like 100 gallons to 20HP so the rule falls apart
In a
hurricane going with a 50
knot tail wind cruising speeds are exceeded and engine has hardly any RPM means you might only need more like 1 gallon to 200 HP so again the rule fell part
All energy can be measured very very exactly given all the data speed weight time etc
If the data is not exact examples like plus or minus 50% then the results will risk to be close to useless
So most energy is measured in egg head world laboratory in terms of JOULES and then converted into other energy forms like Watts amps horsepower newton kilos etc
So it is possible to convert Diesel fuel into watts but its not normally done to turn it into volts and amps even though volts multiplied amps makes watts
Electric power from battery can be turned into watts and volts and amps and horsepower but not so easy to turn it into fluid onzes or gallons or kilo newton force equivalents.
As a example There exists ( mtoe ) metric ton
oil equivalent to replace electric power generated by coal or gas or hydroelectric power or nuclear power .
Yes we can measure watts power from any source compared to
oil or coal or
solar power
However most every type of energy can be converted to JOULES but then if I do that in this forum I could lose the people reading the thread because most sailors would not know a JOULE from a dead
fish that fell onto their head from the sky
That means when I convert battery watts to horse power I can use the simplex systems 750 watts equals one horsepower instead to convert everything into JOULES
If your not used to converting energy then comparing electric watts power to horse power is gonna be problem.
Example
One boat goes with 35% efficient diesel engine in calm waters top speed full throtle at 10 knots and the global efficiency of the fuel burn to distance covered is 5%
The same boat another day goes the same journey with 35% efficient diesel engine into strong head winds full speed full throtle but can only get 5 knots forward speed and the global efficiency of the fuel burn to distance covered is 2.5% because the same boat had to use twice the time and twice the fuel to go the same distance as the first journey .
The same boat another day goes the same journey with 35% efficient diesel engine with strong tail winds full speed full throtle but can get 20 knots forward speed and the global efficiency of the fuel burn to distance covered is 7.5% because the same boat had to use the half the time and half the fuel to go the same distance as the first journey.
We can infer a rule of thumb this diesel engine is always 35% efficient when it goes flat out top speed but we don't know it's horse power or its tourgue and its efficiency at a Lowwer RPM and worse we know nothing about the boat it is attached too
Therefore this data above is of marginal to possibly no benefit to us .
The above data allows us from three journeys to infer a rule of thumb that the certain type of boat with that certain type of engine has a average global efficiency of 5% but its suspect information with only three journey data base used
If we did 3000 journeys and the average global efficiency of fuel use global efficiency was 5% we would then be more able to infer that most results of this boat with this type of engine will be global efficiency of 5% plus or minus 50% (so global efficiency maximum is 7.5% and minimum is 2.5% )
We still don't know the Horse power or type of boat or distance traveled or energy of fuel or force of wind in knots but the important detail we do know the global efficiency if that information is needed .
We know more than the information the engine is 35% efficient
which is not a lot of useful information as all journeys including running the engine at zero speed the engine is always 35%
Many experts over time have accumulated lots of useful data on marine engines coupled to many types of boats in many sea conditions.
These experts vary in their rules of thumbs on the subject of global efficiency of boats with marine diesel engine but most would agree it is some where between 2.5 % to 10% for most small boats and higher for larger ships due to economy of scale .
Sales reps selling you a marine engine need a useful number for them to sell so they will look the highest efficiency number they can find and if it is 37% for a modern diesel engine versus typical petrol engine 25% that sells it is a no brainer diesel is so much better
A sales rep could maybe tell you the global efficiency of the petrol engine version on that boat is 4.5% costing $ 50000 versus the
Diesel engine version higher global efficiency of 5.5% costing $ 60000
Now you can figure out how many hours of engine use over so many years will it take to break even and be ahead of the
game if you buy the Diesel engine version costing $10000 extra with a minor better global efficiency rate
As sales reps prefer a easy life it is more easy to give you a near useless number diesel 37% versus petrol 25% and get you to fork out the extra cash with MumboJumbo numbers called dales
pitch numbers
Its also with life cycle for resale more easy for sales rep to show the idea the diesel engine version of the same boat retains its resale
price better but you got to pay more to get more
This resale factor is sales
pitch on cycle life. How true this data is again suspect
Batteries have various important factors cycle life, amp or Joule
storage , real use able power ,
safety, recharging times, recharge efficiency , discharge efficiency , cost per unit cist per power unit to charge and discharge, robustness to misuse and temperature to big to cover here all these facts today
All chemical
batteries types suffer one big flaw if you demand continuous high work loads versus their capacity in amps their efficiency tends to be seriously reduced
Light loads on most battery types will often return the highest efficiency.
A typical LiFe type battery will have maybe 95% return of the power in efficiency with light loads and can drop to lower than 90% with high workload demands
Cycle life of the battery will go from as high ad , 4000 cycles if the cycles are shallow like 50% DOD
depth of discharge . Continuous 100% DOD the cycle life will be greatly reduced often to less than 1000 cycles
We can calculate the cost ratio benefit
buying larger capacity battery so as to increase cycle cost ratio
A typical lead acid battery when we factor in all the above data for these types we find that often the lead acid types cycle costs per cycle are four times larger than the LiFe types even though the lead acid types costs are often four times cheaper per amp equivalent than LiFe
So yes we can measure individual cycles versus life cycles for batteries
2 times 12 kW battery will equal 24 kW of power to start with.
If the specifications of the battery shows at the normal work load discharge rate that it can deliver ,90% the power the a maxim of 90% of the battery power is available to be used much the same as we can say a 90% full gas tank has the potential to deliver that much fuel energy .
This is merely the measuring of the battery one component and not the entire drive system .
This would be a typical sales rep sales number
This electric power from battery now ,90% will go through wires and suffer losses from heat and Resistance .We would expect 5% losses
Then the power now 90% reduced by 5% will go through speed controllers and suffer losses often from MOSFET duty cycle issues . Typical losses are 5%
Then the power now 90% reduced by ,,,,5% reduced by another 5% will through electric motor and suffer losses best vibration etc. Losses of 15% tend to be the real expected losses
Then the power now 90 %. Reduced by 5% then reduced by 5% then reduced by 15% will go though shafts to
propeller and suffer losses . losses from craft with water
seals resistance make 5% losses the normal
Then the final drive of the
propeller will duffer losses often more than 40%
So when we figure the total losses we find that final global efficiency can be as low as 5% and rarely can attain more than 40%.
Lower cost simplex electric power systems will tend to return global efficiency of about 10% and higher cost versions would expect to return global efficiency number closer to ,20%
So we could say many 24 kilowatt electric power units will at ,10% global efficiency return 2.4 kilowatts of final power in the real world
The sales rep will be stuck on the upstream number 22 kilowatts