Portager's Generator

I’ve invested a great deal of effort into reducing the weight and cost of the electrical generation system for Portager. The basic problem is the AC generator is sized to the peak loads and the AC motor driving the air compressor has a hefty startup load. The smallest air compressor for the SCUBA tanks that I have found so far is the Bauer Junior Compressor with a 3 HP electric motor. According to Bauer it takes 17 amps of 110 AC steady state and 90 amps for startup which is a ratio of 5.2 which is typical for induction motors. They also claim it will operate on an 8 KW generator although I calculate that it should have a 10KW generator.

My idea is to run the air compressor off the auxiliary engine/hydraulic pump that Michael Kasten is recommending for the get-home-drive. So I did a comparison.

An example of a high quality marine 8 KW generator is the 8 KW Northern Lights M753Ka which costs $8,021 at http://www.clis.com/mmarine/NorthernLights.htm and weighs 529 LBS. A lower cost option is the 8 kW Spartan Series Marine Generators with an Isuzu Diesel Engine which costs $4,299 @ (http://www.americasgenerators.com/products/product_view.php?ProductID=873 ) and weighs 446 lbs. This engine is rated at 22.4 BHP Continuous @ 3000 RPM (http://www.frontierequip.com/isuzu/isuzu.htm). Now if you are wondering why it takes a 22.4 HP engine to drive an 8 KW generator i.e. 10.7 HP output, there are three reasons. First the generator runs at 1800 RPM to produce 60 Hz power and the power rating at 1800 RPM is 15.4 HP continuous (http://www.tuban.net/Model8_5Isuzu60HzDataSheet.html ). Second the typical AC generator efficiency is ~85%. Third most generators operate at ~10 to 20% power margin to assure they can maintain the required speed (this is where many manufacturers cheat to get cost down). I calculate the Spartan power margin at 10.7 /(15.4*.85) = .817 or 18.3%.

Using a hydraulic pump and motor with a 70% power transmission efficiency (typical for a well designed hydraulic pump and motor system) would require a 4.3 HP diesel engine minimum. One example of a liquid cooled marine diesel is the Kubota Z482E which is rated at 10.4 HP continuous and weighs 117 lbs with an end plate and 179 with SAE flywheel housing (http://www.frontierequip.com/kubota/kubota.htm ). The Z482E would need to run at at least 1800 RPM to produce 4.2 BHP. The Z482 cost US $2,617 (http://www.rs-refrigeration.co.uk/Kubo.asp ). Without taking the time to engineer the hydraulics system, I anticipate the cost of the hydraulics will be far less than the cost difference between the Northern Lights 8KW generator and the Kubota and a little higher than the cost differential relative to the 8 kW Spartan generator. However, if the hydraulic pump and accumulator are provided as part of the get-home-drive, then the hydraulic option is much cheaper. In addition, operation and maintenance costs for the smaller Kubota should be significantly lower.

Even though I am a Mechanical Engineer and I could design this system myself, I would contract the design and assembly of the complete system to professionals such at Frontier Equipment who provide custom auxiliary design assembly services (http://www.frontierequip.com/powerunit/marineaux.htm ). I would have it built-up, tested and delivered as a palletized subassembly. Take a look at some of the examples they provide, some of them are close to what I am proposing.

Next I decided to integrate the total electrical requirements of the boat and compare DC power generation (alternators) running off the auxiliary versus AC generators. I also compared belt driven alternators and hydraulic pump driven alternators. I've started a generator load estimating spread sheet and uploaded it to my web site www.portager.info the direct link is http://www.portager.info/generator_loads.htm but this requires Windows XP, so I also provided links to down load it in Excel 2002 or Excel 5/95 versions. I started from a list of loads originally intended for home use, so there are still some items in the list that don't belong. These loads are only preliminary estimates intended for comparison purposes. I sized the DC refrigerator/freezer load at 65 watts/hr so I just entered it as a 65 watt DC load with a 100% duty cycle.

Load case #1 shows everything I think I might want to be able to run at the same time with an electric air compressor. This is a worst case day where both A/C systems are running with a 50% duty cycle, i.e. the hottest day the boat should ever see. You will notice that I have included a combo washer/dryer, dishwasher, convection oven, ... I figure if I'm going to have my air compressor how can a deny the Admirable her quality of life items? In the startup load calculations, I assumed that the startup loads are short in duration and therefore they never occur simultaneously (not always a good assumption, but…). In this case the steady state power load is 12.3 KW and the peak load is 20.4 KW so I rounded down a little bit to 20 KW. Load 2 is the same except the air compressor is hydraulic. In this case the steady state load is 10.4 KW and the peak load is 13.2 KW. So using the hydraulically powered air compressor allows the generator to be reduced from 20 KW to 12 KW (I rounded down to 12 KW since the next step up was 16 KW). Using Northern Lights generators the cost savings is $4,616 and the weight savings is 262 lbs.

Load case #3 uses the 18,000 BTU Flagship Marine heat pumps instead of the HFL 16,000 BTU air conditioners. I used the cooling steady state power requirement instead of the greater of the two (heating), because the duty cycle is worse in the cooling mode. Despite the slightly higher steady state power requirement, due to the lower startup load, the required generator size decreases 1 KW, however this benefit just reduces the negative margin. The primary feature of the Flagship Marine heat pump is that it does double duty by providing both heating and cooling.

Column "K" provides an estimated duty cycle and column "L" provides the average power per day, which is used to determine the DC generator run time. Cell L158 provides the required DC power input to the invertors to run the AC loads and cell O159 provides the total DC load. Note that in load case #1, the AC generator required a 20 KW capacity while the DC load is only 2.5 KW after accounting for the inverter efficiency. Adding in the DC load brings the total to 3.2 KW. This is due to the AC system being sized to the peak loads while the DC system is sized to the average load and the peak requirements are provided by the battery bank. (I know this seams like I’m an incredible power hog, but it is the hottest day of the year and it is only an example.) Load case #2 requires 13.2 KW AC versus 2.4 KW DC and load case #3 requires 12.3 KW AC versus 2.8 KW DC.

In calculating the alternator output, I deducted 10% for hot operating conditions. To determine the alternator output of the belt driven alternators I set the drive ratio so that the alternator speed is = the rated speed when the engine is running at it’s maximum speed i.e. 3,600 rpm. I then determined the output with the engine operating at 1,800 rpm. For the hydraulic alternator I assumed the alternator would operate at 5,000 rpm. Note that since the Balmar 98-24-220-BL has a maximum speed of 6,000 rpm, the hydraulic alternator has a 22% higher output when the engine is running at 1800 rpm than the same alternator with belt drive. If the engine speed drops to 1200 rpm the hydraulic alternator produces 52% more power than the belt drive alternator. At 900 rpm the difference is 78% (assuming the engine produces enough power at 900 rpm to drive the alternator). Since the 97-24-140-BL alternator has a maximum speed of 10,000 lbs the maximum drive ratio is 2.78:1, so with the engine operating at 1800 rpm it produces its full rated 140 amps but 39% loss than the hydraulic 98-24-220-BL. At 1200 rpm the 97-24-140-BL produces 44% less power and at 900 rpm 50% less. The real benefit of the hydraulic alternator is when the engine is oversized for the alternator. In other words if the auxiliary is sized at 40 HP for the get home drive and the maximum alternator load is 12.3 HP, when only the alternators are on the engine will be able to slow down and still provide maximum power output. With one 98-24-220-BL alternators the output is 5.5 KW of 24 VDC and the run time is 17 hours on the hottest day of the year. With two 98-24-220-BL alternators the output is 11 KW and the engine run time is 8.5 hours and the time to recharge the battery bank is <1 hour.

Finally, I estimated the weight and cost of the all AC and all DC alternatives. The DC system turned out $1,062 or 6.2% higher than the AC system, however the DC system will have significantly lower operating cost and shorter run time. In addition, the DC system is 358 lbs or 34% lighter than the AC system. Finally, cell E47 shows that if hydraulic system is already onboard, say for a get-home-drive, then the DC system is $3,463 or 20.3% cheaper than an AC generator.

I’ve concluded that using DC alternators instead of an AC generator will allow a smaller engine to meet the electrical requirements and provide shorter engine run times. If the hydraulic pump and accumulator are required for the get-home-drive, then the hydraulic driven DC alternators are more cost effective that an AC generator. Finally, the smaller engine will be easier to accommodate and easier to soundproof.

Please let me know if you see a flaw in my approach. I would prefer to be embarrassed now than surprised once Portager is complete. In addition the further I go down this road the more difficult it will be to change the design of Portager to accommodate a more conventional AC generator approach.

Regards;
Mike Schooley