Richard, You got me thinking (which can often be dangerous).

Reading further on the ASU Isotherm spends a lot of time talking about
monitoring system voltage and increasing compressor speed when there is
extra power available i.e. battery charging. This is great for holding
plates but of no particular value in evaporator systems. The one thing I
see that the ASU has over the AEO or the SSC is that it looks at evaporator
temperature as well as run time to optimize compressor speed.

The key to maximum efficiency in an evaporator system is to keep the
compressor running as long as possible as slow as possible. In process
control systems the easiest systems to control are the ones that are stable
or change slowly. A top opening refrigerator qualifies as long as you don't
add a lot of warm items or accidentally leave the hatch open. The AEO works
fine in this situation but will require an hour to adjust to a major
variation like adding a lot of warm items and then it has to start readjust
back to normal over several cycles after the box recovers. The AEO
algorithm also reaches equilibrium at 32 minutes run time. It seems to me
that with a well insulated box that stayed under the on point for longer
than 6 or 7 minutes the speed would not get as slow as it could. The SSC
has a target of 50 minutes run time in an hour which might give a better
overall COPE.

I would like to see a system with 3 set points; off, on and exception and
maybe an adjustable duty cycle. As long as the box stays below the
exception point the control would adjust compressor speed to maintain a 90
to 95% duty cycle. If the box warmed above the exception point the
compressor would run at high speed until the box recovered but that cycle
would not be used to calculate the normal run speeds. That way the control
could react quickly to new loads and return to normal operation as soon as
the box recovered. The duty cycle could also be adjusted downward when the
boat was on shore power or charging and power was not at a premium.
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

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ups.com...

The reason we want to control compressor speed is to use the lowest
amount of daily onboard power and maintain the box temperature we want
at all times. We by a compressor that can produce the results we want
and generally more capacity output most of the time. Cutting down on
capacity by lowering compressor speed improves SCOP (System Coefficient
Of Performance). Whenever the box temperature can be maintained at the
compressors slower speeds and still have it cycle off the end result
will be fewer daily amp-hrs consumed. What all three of these speed
controllers do is compensate for day night and seasonal temperature
changes. They all will change speed but slowly when warm product is
added to the box. The SSC and ASU both can be set to manual to speed up
the cooling process. I believe only the SSC will provide actual
compressor speeds at the panel. All of these units change the milliamp
flow in the thermostat circuit to control compressor speed.