Thursday, 21 February 2013

More on Heat Exchangers & Generators

Aldi have replied to my query about their Heat Exchanger.  I wanted to know whether it contained any aluminium as I understand the metal is incompatible with copper.  They advised the heat exchanger is made from stainless steel so there is little risk of galvanic corrosion.  However we have already decided the idea of installing a heat exchanger on Waiouru isn’t a financially viable option.

I’ve now being doing further research into petrol generators.  Initial thoughts focussed on the generator output but then I realised size and weight would be critical factors.  We’re not going to be continuously moored which means the generator will have to be regularly carried on and off the boat.  Moreover I was thinking it would have to be stored in the engine compartment when not in use.  Reaching down and retrieving a heavy generator was a recipe for a strained back or shoulder.  But I’ve now realised we could possibly stow it in the starboard cratch locker.

However I’ve now had time to consider the matter further and asked myself again “Why do we need a generator?”  At the moment we have two sources of power; the engine and the solar panels.  The solar controller is rated at a maximum of 15 Amps for a 265W solar array.  Our panels have a rated output of 200W so the most we’re likely to get from the panels is 10-12 Amps.  We have a 900Amp flooded lead-acid domestic battery bank which needs to be regularly charged.  Lead-acid batteries go through three stages of charge.  The first is “Bulk” where the batteries will accept a relatively high charging rate.  The combined output of the 175A & 50A alternators on the engine can produce this high rate.  The “Bulk” stage can be quite brief.  As little as 30 minutes!  The second stage is “Absorption” which occurs when the batteries are approximately 80% charged.  During Absorption the batteries cannot accept a high rate of charge which means the engine is producing a large charge, most of which will not be accepted by the batteries.  Therefore running the engine during the Absorption stage is very inefficient.  The last stage is “Float” where the batteries accept an even lower level of charge to get them to 100%.   

It appears there are two options when considering generators.  Have one with the necessary output to complete all three re-charging stages (which avoids running the engine) whilst simultaneously provide power for any 12v and 240v load from the boat.  Alternatively, have a smaller generator capable of completing the final two stages of the re-charging.  As the size and weight of the generator is a critical factor in our situation I have eliminated the idea of a large generator from our considerations.

There are a wide range of small petrol powered generators.  As we are going to be continuously cruising the open frame “box” type generator doesn’t seem appropriate.  They are noisy and their bulk makes them difficult to stow.  This leave the “suitcase” style generator.  The options are Japanese, Korean or Chinese with the Japanese Honda considered to be the best (and by far the most expensive).  A 2KW generator weights approximately 21kg whereas a 1KW weighs 13-14kg.  The latter is a better option from a handling and storage perspective.  However the 1KW output is actually the ‘peak’ output and the generator is more likely to have an 800-900W continuous rating. 

We are rather fortunately that our Victron Multiplus inverter/charger has a current limiter function.  This means that it is possible to set a maximum power level that the Multiplus will attempt to take from the external power source (eg, generator).  If we didn’t have this functionality then the inverter/charger would attempt to take more power from the generator that it was capable of producing which would result in the generator either overheating or stalling.  So we can use a small generator.

Some rough calculations

Assume our batteries have received their “Bulk” charge.  They would be at 80% and require a further 180 Amps.  But nothing is 100% efficient so we allow an additional 50% of charge.  Therefore the batteries require 270 Amps.  A 1KW generator produces 900 watts which should provide approximately 30-40 Amps of charge.  This means the generator will need to run for 6-8 hours (270÷40) to fully charge the batteries.  I suspect I’ve been very conservative with the figures and the actual charging time will be less.  Moreover I haven’t included any input from the solar panels.  However it does appear using a small generator is a viable alternative to completing the last two stages of charging the batteries.  I should do some calculation regarding the generator purchase and running costs.

But I’ve probably bored you enough already! Smile

We purchased six more Dri-Deck 300x300mm interlocking mats from the chandlery this morning.  I’d already worked out for this small number of mats it was cheaper to purchase them from the chandlery than order them online and pay the delivery.  The mat will go on the roof of Waiouru and the folding ladder/gangplank will sit on top.  My idea of the rubber tape on the underside of the ladder didn’t work and I’ve scratched the new paintwork removing the ladder from the roof.  Now I have yet another painting task to compete this summer arrives.


Jenny and Robin said...

Hi Tom,
Regarding your thoughts on a generator. You are more likely to need it in winter when you are possibly frozen in and very little solar activity. Have you thought of using the 230v ac output into a mains voltage charger then the only consideration is the input requirements of the charger, so match the Gen to this. The charger will look after all three stages of charge and the Gen will cope. Just some thoughts. We have a Honda 2kw for this purpose. The added sizes allows for the mains microwave to be also used.

Tom and Jan said...

Yes, more use in winter, but also used in summer on the days we don't cruise. The Multiplus is a sophisticated mains charger and an inverter. I suspect 2KW is too large and heavy to both carry and store. The Multiplus will supplement the power from the generator by drawing on the batteries for any load that exceeds the capacity of the generator.

nb Achernar said...

Your post was far from boring. To someone like me at the early stages of having a boat built they are most informative. I was also interested in your previous posts about the Hurricane as we are having one of these also. So from an interested reader of your blog please don't stop with your technical posts

nb Achernar said...

Your post was far from boring. To someone like me at the early stages of having a boat built they are most informative. I was also interested in your previous posts about the Hurricane as we are having one of these also. So from an interested reader of your blog please don't stop with your technical posts

Tom and Jan said...

Congratulations Phil

You just proved Jan wrong..... Brave man! :-)

Derek Bird said...

Hi Tom

After over six years of cruising and being iced in for over six weeks one year, I have never felt the need to have a generator, I do however carry a spare alternator and fan belts, plus you should not store petrol in the engine compartment

Tom and Jan said...

Hi Derek,
Yes, I definitely would not keep petrol in the engine compartment. We have room in the starboard bow locker. I'm not convinced a generator is necessary or even desirable for our cruising requirements. I'm just exploring the idea.

Geraldine Starks said...

Thanks for sharing your experiences Tom. You never got me bored! Thanks for the information. I will share it with my dad! :)-