Solar. Solar heating. For some folks those are political fightin’ words. For others it’s hope for the future. Those of us who were around in the late-1970s and into the 1980s can remember solar as an energy laughingstock. Now it’s once again becoming an exciting idea.
Solar heating may be what in-floor radiant was 20 years ago. It’s an old idea come back to a new life; an idea that has “failed” in the past because of misapplication, but if used with technical smarts has great potential. When I first came west in 1987, only a few folks, and only in Santa Fe, N.M., were talking about in-floor radiant heating. Half of those were full of horror stories about how it had failed back East - leaks inside concrete, not enough heat, too much heat. The other half couldn’t say enough about the great new technology. And look at it now.
A Bit Of History: By accounts from those who were “there” during the heyday of solar in the 1980s, the solar heating craze wasn’t really about technology or energy savings. It was all about tax credits. Bruce Faul, now a rep for Shamrock Sales in Denver, was a contractor back then. He says you could pay your $7,000 in taxes to Uncle Sam or you could pay it to a contractor for a solar heating system.
Unfortunately, some smooth “gold chain guys” spoiled the industry. They were fast talkers and didn’t know much about technology. But they knew how to sell tax credits. The tax credits were about energy savings. But the energy savings were hard to document, and probably weren’t about solar energy.
Energy savings were most likely generated by the installation of a night set-back thermostat to reduce nighttime temperatures, and an intermittent pilot retrofit kit to do away with a gas-consuming standing pilot in the supplemental heat appliance. Many say that the energy savings could have been accomplished with just these controls, without the solar collectors, piping and tanks.
The solar equipment itself and the installations often were substandard and didn’t last long. End-users didn’t understand that there was maintenance involved. Those who bought a house with solar already installed were clueless about what the apparatus even was, let alone how to keep it running.
What About The Future? This time around, the industry can be more based on professionalism and technology. Today, solar is not sold on only reducing energy costs. The appeal now is based on concern for the environment and the future inherited by children and grandchildren. Solar equipment is expensive and expensive to install. It has a long return on investment.
This is one reason why the United States is not a leader in solar. Experts say that solar is used much more worldwide than in North America. Guess the largest user of solar. Ready? It’s China. Second is Europe.
Solar heating is more similar than different from what we already do with hydronics. Just imagine a solar panel rather than a boiler as the first stage of heating water. That’s the difference. What’s the same is that you have a heat source, hot water storage tank or tanks, a piping distribution system, pumps, and controls that determine how hot the water is and where the pump puts it.
Although some products are unique to solar because of their temperature range or capacity, their function is the same as with a conventional boiler system. You heat a fluid, and when it’s the right temperature you pipe it to where it gives off its heat.
Solar Controls: Picture the controls on a conventional boiler: expansion tank, air eliminator, system fill valve, sensors, freeze protection, relief valve, a controller or two. Exactly the same functions are used on a solar system. Some are the same as with a boiler; some are specific to solar.
The expansion tank needs to be a lot bigger than for a boiler. That’s because the temperature changes with solar are much larger. With a boiler system, you’re dealing with relatively small temperature changes. With solar the changes might be as extreme as hundreds of degrees in an hour.
The air eliminator would be the same as on a boiler, usually 1 inch or 1 1/4 inches.
The system fill valve is one time, not constant. Oops, it shouldn’t be constant with a boiler system, either, should it?
Sensors are located on the roof to sense collector temperature, and in the storage tank or tanks.
The controller is similar to what could be used on a boiler. Remember that the simple definition of a controller is a device that measures temperature and causes an action based upon temperature change. For example, falling temperature in the common controller called a thermostat causes a switch to close, which allows electricity to flow to bring on a burner.
Another common boiler and water heater controller is an aquastat. It has two settings. One is the setpoint, which is the ideal temperature of the water in the tank. The other setting is the differential. That’s the number of degrees the temperature in the tank will be allowed to fall before the burner comes on to reheat the water. Typical differential is between 10 and 15 degrees F.
A solar controller also works on a differential. The controller measures the difference between the fluid in the solar collector and in the collection tank. When the difference is greater than the differential setting, the controller closes a switch to bring on a pump. The pump then moves the hotter water in the collector to the storage tank.
Like with a boiler system, we often call the fluid “water.” We really mean any heat transfer fluid used to absorb and give off heat. It could be glycol. But solar equipment manufacturers specify the fluid to be used in their equipment so that we don’t put in a fluid that will harm the equipment or piping, for example something that is too acidic or just plain dirty.
Also like boiler controls, modern solar controls are more complex than just an aquastat. For either a boiler or solar, the modern controller is an electronic box that does several things at once. For example, the solar controller can have a number of sensor inputs so that it can control water going to several different collection tanks of different temperatures.
That makes it possible to provide water to domestic hot water, a pool and even some living space heating. The controller also makes it possible to integrate the use of solar with a boiler, so that the end-user doesn’t come up short on hot water or heat.
Too Much Heat: One of the surprising challenges of solar heating is too much heat - in the summer. Solar collectors collect year-round, whether we have a use for the energy or not. In the winter, when less solar energy is available, we have a higher demand for heat, and can use every bit of solar energy that is collected.
In July and August, however, unless the collector is covered, it absorbs much more energy than there’s a use for. The problem the system designer faces is to not size the collectors bigger than the energy that can be dissipated in the summer. Besides the seasonally constant demand of 15 to 25 gallons per person for hot water, the summer heat can be used for a pool. An innovative use is also replacing the heat in the ground water when it has been depleted over the winter by a ground-source heat pump.
Because we’re in the comfort business, and because comfort means having the heat we want whenever we want it, a solar system doesn’t do away with the need for a boiler. Solar means that the boiler can be used much less, as a back-up rather than as a primary heat source. Solar does not reduce the cost of a heating system. It mostly increases it.
But for energy-conscious customers, using less fossil fuel can be much more important than saving money. You and the solar business can help them do it. For checklists for a solar project, check out the Web site www.retscreen.net. To find out the status of federal and state tax incentives today, try www.energytaxincentives.org or www.dsireusa.org.
However, if you’re curious about solar, give yourself a head start and remember that the controls do the same thing as on a boiler system - just sometimes bigger and at a larger temperature range.