Questions answers

Dueling Charge Controllers

My stand-alone hydroelectric system has been in operation for over 20 years, and it wasn't until last summer's East Coast drought that I decided to supplement with PV. The hydro's charge controller is a vintage Enermaxer with a simple, one-knob adjustment for the float voltage. The solar-electric system uses a Trace C35 charge controller, which has both bulk and float voltage adjustments. I know that the simplest arrangement would be to set the Trace controller to "load diversion" and get rid of the Enermaxer, but this would mean upgrading the controller (combined output of hydro and PV is too much for a C35) and adding more resistance load. My budget won't allow for that now. What I need to know is how to set up the two controllers so that they work together. Vinnie Valentino • valentinomasonry@hotmail.com

Vinnie, Set the bulk and float regulation point of the C35 at 0.5 VDC below the regulation point of the Enermaxer so that the PVs regulate first. If the Enermaxer regulates first, it will attempt to divert the entire load (both PV and hydro), which could possibly damage the controller or the diversion load. The C35 will regulate the PV output, and the Enermaxer will continue to regulate the hydro output. The accuracy of controllers' voltage sensing can vary. Make sure to observe the first couple of regulation cycles to make sure the settings are appropriate and that the controllers are regulating properly. You may need to increase the differential between them in some cases. Joe Schwartz • joe.schwartz@homepower.com

Homebrew Heat Exchangers

I am considering some solar water heating and want to make my own system. I was wondering if you have heard of anyone using car or truck radiators in reverse to collect heat. The basic principle of heat transfer would seem to indicate that something that gives off heat well will collect/absorb it well. I was thinking of building a glass-covered box to contain used radiators from the junkyards, which should be easy to get to keep cost down. Please point out any errors in my thinking if you can so that I won't waste too much of my time and money. Thanks Patrick Mckowen, Toronto, Canada • elyse.cohen@sympatico.ca

Patrick, You are correct that heat exchangers can be used in either direction to heat or cool depending on the desired application. An auto radiator is designed to achieve a high rate of heat transfer from a hot fluid to a high volumetric flow of cooler air. Therefore it would be just as effective in reverse, transferring heat from a flow of hot air to a cooler fluid circulating within. This suggests that you may have better results by passing solar heated air from one or more air collectors through an auto radiator.

However, it may not be the most cost-effective approach. Solar air collectors are generally less efficient than liquid collectors; ducts are larger than pipes, and blowers more costly to operate than small circulating pumps. You should also consider that those auto radiators have been used to circulate ethylene glycol, which is highly toxic. I suggest you flush them well and design the collectors into a closed loop type of system with a double-walled heat exchanger. Use a 50-50 mix of propylene glycol with distilled water. This will give you a good measure of safety from contamination of domestic water. You'll also have the freeze protection you need in a cold climate.

One more thought: I suggest you look for used solar collectors in your area. If available, they will work better than the auto radiator. Either way you proceed, please keep us posted. There are plenty of folks interested in your experience with it. Ken Olson • sol@solenergy.org

Patrick, A car radiator will work, and in fact some collectors manufactured in the U.S. used a radiator-type design for the absorber plate. None I know of are manufactured like that today— too much cost to value. Make sure the fins and tubing are black; paint them with flat black engine paint if needed. The only significant problem with a radiator is the typically large pipe connection size. If you use radiator hose for connections, they will probably deteriorate quickly. Everything inside a collector should be metal or a synthetic that will take both temperature and the harmful spectrum of sunlight. And you must use tempered glass, since plate glass can't take the temperatures.

The reason more people don't do this is cost. Once you have gathered enough radiators for the surface area needed, enclosed and insulated them, and covered them with a good glass cover, your cost probably exceeds the cost of a good used collector. If you already have the radiators and glass, then this probably won't be true. Chuck Marken • chuck@aaasolar.com

Clocks on Inverters

Hey, I really like your magazine. I have been receiving it for the last four years. I'm looking for a brand of alarm clock that will work with my Trace DR2412 modified square wave inverter. I have tried the Sylvania brand; they do not keep time properly. Do you have any ideas? Thanks, Rocky Etherington

Hello Rocky, First off, there are many different types of clock circuits. Synchronous motors haven't been used in clocks for decades. Ever since the displays went digital, the circuitry in clocks has been electronic, not electromechanical.

Some AC powered clocks rely on the fact that the grid is very frequency stable and accurate (60 cycles per second). These clocks basically count cycles as the waveform crosses zero volts, divide by 120 (each AC wave crosses zero twice during one cycle), and increment the result as a second of time. The transition from full voltage (about 160 volts) to zero volts is very rapid (less than 20 microseconds) with inverters. The clock is unable to detect the zero crossing point quickly enough to get an accurate count. Many of these clocks will actually run "double time" when powered by a modified square wave inverter. Inverter waveforms—both modified square and sine—are also not anywhere near as frequency stable as the grid. It's very common to find an inverter running at 59 Hz or 61 Hz, and even this small a difference will render a clock wildly inaccurate if it is counting cycles.

Noise on the AC line is also a factor for some clock circuits. The sharp corners on the inverter's AC waveform contain odd numbered harmonics up to infinity. Some clock circuits are confused by these harmonics and do not count cycles accurately. This problem varies depending on what appliances the inverter is powering. Large inductive loads, such as motors, will make the harmonics larger in amplitude and make the clock even less accurate.

It's common today for electronic clocks to use their own crystal time base and not to count the AC power's cycles. The reason for this is two-fold—higher accuracy and reliability. You can easily recognize these clocks because they use a small battery (usually a 9 VDC battery) to keep time when the grid fails. In this type of clock, the incoming 120 VAC is converted into low voltage DC that powers a crystal time base usually running at several thousand cycles per second. The result is greater accuracy due to the time base's greater number of cycles; and reliability, since the clock's time base is backed up by battery power. Note that in almost all cases, while the time base functions during a grid outage, the display and alarm will not function in the absence of the grid.

The obvious clock solution for inverter users is the strictly battery-powered clock with no connection to 120 VAC whatsoever. These are accurate, inexpensive, and available everywhere. I do not recommend trying to power these clocks with rechargeable batteries. The self-discharge rate of rechargeable batteries (NiCd and NiMH) is far too high to make them suitable for use in clocks. A rechargeable cell will self-discharge in months while a primary cell will last for years in most clocks.

If an inverter user (either modified square or sine) must have an accurate clock powered by 120 VAC, look for two things. Both are easy to scope out in the store before you buy the clock. One, does the clock use a battery to back up the 120 VAC power? Two, does the clock come with a low voltage power supply that plugs into the 120 VAC line? If the clock has these two easily viewed items, it should run accurately on any type of inverter.

Probably more than you wanted to know about electric clocks Richard Perez • richard.perez@homepower.com

want to consider using a Vanner equalizer. With that hardware, you tap into a 12 V half of your battery. The equalizer detects when the two sides are unbalanced, and charges the lower half from the higher. Check out www.vanner.com.

The other method is to use a DC-to-DC converter, which will take 24 V and step it down to 12 V to your loads. This is most commonly used if the current draw is not too large. The loads you mentioned could easily be serviced by this method, which would be more efficient than an equalizer. Check out www.zaneinc.com.

Another option would be to install a small, one panel, one battery system just to service your DC loads. This would require an inexpensive charge controller, separate overcurrent protection, and a DC distribution system (which you would also need for the other two solutions). Peace. Michael Welch • michael.welch@homepower. com

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