Solar Electricity and Radiant Heat
As electricity and fossil fuel costs rise, off-grid, so-called "solar" homes gain in popularity. More and more people are both wary of the electrical grid and aware of its fragile nature. Cascading power outages like the east coast blackout of 2003 affected 70 million people and cost an estimated $6 billion in financial losses. Rising natural gas prices (the fuel used by many power plants) and the resulting twofold increase in electricity in just the last two years, as well as, inevitable future supply disruptions making power scarce at any price, make energy independence a priority for many people. And nowadays, living in an alternatively powered home is barely distinguishable from a grid powered one. Except, when your neighbors are cursing their dead flashlights and enduring an involuntary candlelight dinner, the off grid home remains unaffected.
Of course, in fairness, it should be noted that owners of alternatively powered homes must deprive themselves of one important luxury, namely, rampant energy gluttony. Electrically speaking, it's shocking how fast off-grid homeowners become "watt conscious". Once a person leaves the world of grid energy and stops taking seemingly endless, cheap, electricity for granted, the concept of leaving lights burning in empty rooms and playing the television to entertain the cat disappears. An new attitude toward energy consumption emerges when you become an energy producer. Working with the rhythm of the sun becomes a pleasant and interesting lifestyle. High energy tasks like vacuuming and laundry are planned, whenever possible, for sunny days. An appreciation for the wonder of electricity and the magical powers it grants our lives takes hold.
But, alternative energy wasn't always so convenient.
Years ago, living in a solar, hydro, or wind powered home (or any combination of the three) required major lifestyle compromises. Because alternative power sources operate with low voltage (usually 12 or 24 volts DC) wiring a home in the 1970's often required TWO types of electrical outlets, installed side by side. One outlet was used for standard 110 VAC appliances, the kind your computer is plugged into right now, and the other for a full array of 12 volt blenders, TV's, etc. This second outlet often looked like a mutant variation of the 110V outlet, with odd perpendicular slots that refused to accept standard AC plugs. Or, even worse, one of those low voltage sockets formerly used to accommodate a cigarette lighter (now called a power point).
These complications resulted because inverters (the device used for converting low voltage to high voltage) were inefficient, and worse, dirty. Inefficient because ten percent of the power entering the inverter vanished in the process of converting from low to high voltage, and dirty, because the "modified sine wave" technology used to "invert" 12 volts DC to 110 volts AC never matched the much cleaner grid power. And, of course, the inverter itself consumed power 24 hours a day.
As a result, early inverters wasted energy, buzzed stereo speakers, and stuttered computer monitors with jagged lines. Now, some twenty years later, companies like Trace and Outback produce inverters that match or surpass the quality of grid power...and draw minimal power in standby mode.
From a radiant heating perspective, this means you can avoid expensive 12-volt DC circulators because the standard 110-volt AC circulators run fine in a modern off-grid system. Consider also that the most popular 12-volt DC circulator, the El Sid, costs over $200 but can only circulate fluid through a mere 300 ft of 7/8" PEX tubing. So, an average radiant heating system would require five or six very expensive 12-volt pumps.
On the other hand, for very modest systems, the El Sid is perfect. A 12-volt El Sid pump wired "asynchronously", i.e. connected directly to a photovoltaic panel, operates only when the sun shines. This simplifies the system, eliminates differential controllers or other sophisticated electronics, and in essence assumes that, if there's enough sun to power the pump, there is likely enough sun to make hot water. And because the El Sid only requires 10 watts of energy to circulate fluid from your solar thermal collector to your heat exchanger, slab, or storage tank, it works well with a small, cost effective, 20 watt photovoltaic panel.
Larger systems (an entire house, for example) should use standard 110 VAC radiant circulators. They consume about 60 watts each, but a single pump can power a zone as large as 1800 sq. ft. And because duty cycle informs every design decision in an alternatively powered home, design minimum zoning into your heating system. If you've never heard the term, duty cycle means: how long will a given appliance run during an average 24 hour period. Off grid homes size inverters and battery banks based on the total duty cycle of every appliance in the home, that is, huge consumers (well pump, dishwasher, washing machine), medium consumers (refrigerator, television) or low consumers (lights, computers, radiant circulators).
A huge consumer may hog lots of power, but if its duty cycle is short (a burst of energy to pump water from the well to the kitchen sink) it's more acceptable than a low consumer that runs 16 hours a day (all night security lights, radiant circulators on a bitter cold winter night). So, with this in mind, divide the heated square footage into one or two large areas, or zones, and minimize the number of pumps. In practical terms that means one zone per elevation.