On Sun, 31 Aug 2008 04:20:51 -0700 (PDT), ignator
wrote:

On Aug 29, 12:04 am, Richard J Kinch wrote:
ignator writes:
Water alone has 144 BTU/lb.latent
heat, just how much better can you get by adding salts or glycol? It
must change phase to store and retrieve this energy.

My constant sermon.

Refrigeration only happens effectively when you have intimate thermal
contact with a phase-changing medium. Brines or other fluid mediums cannot
work as well, because their temperatures rise as they absorb heat, and heat
transfer efficiency is all about delta T.

Homebrewers with jockey boxes will never accept that drained ice works
better than a water/ice bath, though.

No, assuming the working fluid when through a phase change, it will
melt at a constant temperature. The OP indicated -40 degree
refrigeration requirement for his cold plates. These store "cold" in
the form of a latent heat phase change of the working fluid internal
to the plate. It will only increase in temperature once all the
working fluid has melted. And only then as a function of the many
materials specific heats, and weights, and heat source being added.
ignator

And just like ice banking for commercial air conditioning to save
money using 'off-peak' power to make ice overnight and the ice melt
cools the building during the day, that brine phase change cold plate
will work fairly well if you need a continuous low rate of cooling but
can only supply refrigeration in high-rate bursts because of a
restricted power supply - which you would see on a very small vessel
like a sailboat, or a small cabin off the power grid.

You'll get solar cell input during the day (but not much), and wind
energy when the wind blows fast enough to turn the turbine - and not
so fast it trips off the safety and stops. But you only get real high
horsepower refrigeration work done when you start a generator set or a
propulsion engine, or stoke a boiler and use steam generation, or...

And because of often restricted fuel supplies or the need to
manually monitor the prime mover (stoke the boiler) you can't do that
24/7, so you 'bank the cold'. Or you put in a Servel / Dometic
ammonia absorption refrigerator and let a small gas flame supply the
slow and steady energy input.

But a larger yacht or commercial vessel doing a 5-day run doesn't
have nearly the power restrictions as a 18' sailboat trying to do 30
days between ports. You have to be power efficient, but either the
main engines or an auxiliary generator set are running most of the
time. In that use, IMNSHO the brine plate is going to be FAR more
trouble than it's worth.

When you have to fix it yourself with only the supplies and
materials on hand, there is one rule: KISS. Keep It Simple, Stupid!

A properly sized 12/24V to 120VAC inverter system (example go look
up the "Heart Inverter" perfected in RV use) and a large battery bank
can provide seamless switchover between onboard and shore power
sources - and a few hours of overnight silence if needed. The
inverter can even auto-start the generator when large loads come on or
the batteries discharge, then shut it down again when the load drops
and the batteries are full.

And if the "Magical" inverter system craps out or the battery string
goes flat or open, you thought ahead and built in a bypass. Just
throw that big switch and go straight from the generator to the
breakers.

-- Bruce --