[amsat-bb] Re: Solar Power (I was wrong)

Marc Vermeersch amvm at skynet.be
Fri Oct 2 08:08:56 PDT 2009

Hi Greg and all,

> -----Original Message-----
> From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
> Behalf Of Greg D.
> Sent: vrijdag 2 oktober 2009 6:13
> To: ptrowe at yahoo.com; bruninga at usna.edu; amsat-bb at amsat.org;
> wb3jfs at cox.net
> Subject: [amsat-bb] Re: Solar Power (I was wrong)
> I've always wondered why the solar panel makers don't put water tubing
> on the back side of the panels.  Boost the efficiency, and get hot
> water for the house or pool at the same time.  Seems like a no-brainer.
> {shrug}

I guess this is too complex for the additional gain in efficiency? According
to the documentation, a raise in temperature of 25K decreases the efficiency
by about 8-10% for the Suntech panels I have installed. Being 50degrees
North, temperature is rarely a problem for me anyway. :-)

> Unfortunately for me, I have large trees shading the house (big enough
> that I'm hoping I'm coming out ahead with lower air conditioning
> bills), and a roof angle exposure that is not solar-friendly even if
> they weren't there.  So I was thinking, if I had space for just a few
> panels, perhaps I could go low-tech with a system built from a scrap
> UPS.  After all, I've got an APRS station - PC, TNC, radio, and
> internet connection - using power constantly, and I often find
> perfectly good UPSs being tossed out (i.e. free) because the battery is
> shot and not worth replacing.
> Put the solar cells in place of the UPS' battery, and modify the power
> failure detect circuitry to work backwards - give preference to the
> battery/solar side, only switching to the line side if there is
> insufficient "battery" power.  This will give me free 120 vac whenever
> the sun shines.  At night, it would power-fail back over to the line,
> and only then would the power company's meter start to spin.  I'd
> probably put a regular UPS down stream from the hacked-up one, in case
> the failover/back from line to solar wasn't real clean.

That is almost exactly what a grid-connected system does. 

In addition it also synchronizes (voltage and frequency) with the grid AND
it doesn't disconnect from the grid. This allows for sharing the load
between the panels and the grid. And for having the meter spin backwards
when it puts your "overproduction" in the grid. 

The more clever systems also search the MPP (Maximum Power Point) of the
panels and optimize production of AC Power that way. If you run disconnected
from the grid you will only produce what you use, overcapacity is wasted.

> The biggest problem I see is that most of the solar panels available
> these days are in the 50-60 volt range, which means that you'd need to
> do a conversion down to battery voltage first.  (Maybe put two panels
> in series and run them into a regular 12v DC power supply?)

A step down switching regulator can easily handle that at >95% efficiency.
For instance the MAX5035 can handle voltages up to 75V.
> Now, I just need to find a source of free-to-cheap solar panels.  I
> almost bought a set at a local Ham swap ($10 ea) that had cracked
> surface glass but were otherwise functional, but I didn't think they'd
> survive the trip home in my car (no place big enough to lay them flat).
> (I'd need to weatherproof them too....)
> Anyway, just a thought...
> Greg  KO6TH


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