Re: Redox batteries

Evgenij Barsukov wrote:
Jean-Marc Delaplace wrote:

You are right about the concern with the leaks. However, I still do not know how toxic vanadium is for the environment. Of course, containment measures should be taken in the design so as to avoid any spillage.

I want to answer your question about not using lead acid batteries. I hope this will raise an interesting debate.

First, let me recall that my purpose is to build a hybrid propulsion house boat. That is, a boat that will sometimes ride at top speed (15 km/h), sometimes at half speed (most canals are limited to 6 km/h) and sometimes it will stay at a mooring for days.

The objective is to be energy-efficient. The energy source will be one or more of the following:
-a diesel generator: 20 kW;
-photovoltaic cells: at most 1 kW peak;
-wind turbine: at most 500 W peak.

To achieve optimum fuel efficiency, the generator must be used near full power exclusively. Then, the role of the battery is to buffer this pulsed power source to deliver just as much power as needed when needed.

To save energy, using the generator should, as much as possible, be avoided and power should come preferably from the sun and/or the wind.

This is where the lead-acid battery has two major drawbacks:

1) A lead-acid battery should always be fully charged, i.e. it should be partially discharged for the shortest period of time possible, to avoid the degradation process to come to play. This is not compatible with waiting until sun or wind are available.


This is true. However, once your boat is moored, what prevents you from
connecting a charger to it and charging the batteries to full?
Deep cycle batteries can tolerate even full discharge, as long as
they are recharged soon to prevent crystallization of PbSO4.

2) A lead acid battery must be charged using precise current rates according to the state of charge. This is incompatible with the idea of accumulating all and as much energy is available at a time, i.e. always using the generator at nominal power.


Lead Acid battery is one of the least sensitive to charging rates (probably only cylindrical NiCd beats it in this respect, but they
are not available in right sizes). If you take the Lind book on batteries, there are over 10 different charging methods described for lead-acide, which have little in common. Which basically means, it does not matter how to charge it as long as you do not cause
1) persistent extensive gasing
2) persistent undercharged condition (unsufficient voltage)


What do you mean with being little sensitive to charging rate? Apparently you can charge a LA battery at C rate until 80% SOC, but the remaining has to be charged at about C/10 ("finishing and equalizing"). This is where the benefit of hybrid propulsion is lost: whereas I could charge 80% at full power of my generator, that is 0.8 h, to charge the remaining 20% at C/10 would require running the generator at 1/10 its power for 2 hours ! I would then spoil the efficiency advantage i was planning stating that the generator need be used at full power only for the best efficiency. If you consider that friction consumes more than 10% of the total engine torque, when you run an engine at 10% load it consumes two times more oil than required, which mean half the expected engine efficiency. In addition, you come into problems such as cylinder glazing, etc.
In short, I think the constraint of slow charging over 80% ruins the advantage of using a hybrid propulsion chain. This i why I think lead acid is just not adequate for hybrid propulsion.


Occasional gasing is not harmful, and even good for the battery (equalizes, mixes the electrolyte). So if you have short spikes
of charging, it is no problem.
More of a problem can be persistently low voltage (or current so low
that it is below self-discharge rate). But it is the matter of designing
your charger properly (e.g. having bust converter if voltage from
your generator/solar cells etc is not enough, and provide sufficient
power capability of your power generating equipment.

In fact I can turn the table here - vanadium flow battery is bound to have low power capability/capacity ratio, because its electrode surface is much smaller than any conventional battery. Low power applies both for charging and discharging - so at high charging spikes it might overvoltage and you would have to stop charging. At high load spikes it might not be able to provide sufficient power, and your equipment will shut-down.

3) Everyone knows that event with the utmost care, a lead acid battery shall not withstand more than say 1,500 equivalent full-discharge cycles.


Here vanadium flow might have advantages, but this is a big "might" as this is a completely unproven technology.

Btw if we are talking graphite-foam electrodes that vanadium batteries needs to employ to achieve high surface area electrodes and therefor acceptable power capability, there have been a recent development in lead-acid cells that also use carbon-foam electrodes to greatly increase its longevity:
http://www.powermanagementdesignline.com/news/177101291
In both cases, the problem with graphite-foam being brittle would have
to be addressed.


I think that vanadium batteries completely address these three points, which make me hope they become available - and affordable - within a few years.


My main concern (in addition to environmental) is power capability, that
I pointed out in the paragraph 2).

Regards,
Evgenij



Sorry to be lengthy but I would be very pleased to discuss these arguments with as many people as possible.

Jean-Marc

Evgenij Barsukov wrote:

Jean-Marc Delaplace wrote:

I am currently trying to define what the appropriate battery would be for a hybrid boat. I found information on Vanadium batteries, that apparently are close to commercialization. Their biggest drawback is apparently the low specific energy, but this is not very critical on a boat. I also read about Cerium that could provide a higher energy density.
Does someone know about these? What is their future? Are the claimed advantages of Vanadium batteries true?
In short, I would like to discuss the topic in all its aspects, including hazard and ecologic risk.
For those who have no documentation but are chemists - which i am not - I can supply them with information I got by writing to a company that is currently developing a vanadium battery.
Jean-Marc



From the technical point of view using vanadium battery on the boat
appear to be a good idea as specific energy is basically irrelevant.
But than why not use deep-cycle lead acid cells, as same argument
applies there.

Main issue with using vanadium flow battery in a boat is the possibility of a leak. Soluble vanadium salts are toxic and the chance of damping a ton of this solution into a picturesque bay killing all forms of life for years to come appears scary.

Regards,
Evgenij
.

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