It’s no secret that the earth receives virtually all its energy from the sun. The earth converts this energy into all kinds of useful forms, including the gasoline we put into our cars. Unfortunately, that process takes millions of years and we appear to be using up our supplies at an alarming rate. The ideal would be to harness the sun’s energy directly and convert it rapidly into useful energy.
This, of course, is not a new concept. Even ancient man figured out that storing the sun’s energy in a mass of stone to be used later for heat was a pretty easy trick.
By the time the sun’s energy makes it to the surface of our planet, it produces on average about 430 Btus of heat per hour per square foot. Unfortunately, that solar energy is not constant and varies considerably with location and weather patterns. Even so, the average solar energy on a surface tilted at an angle equal to the latitude in the mid-latitudes of the United States is about 2,000 Btus/ft2 per day.
Collecting Solar Energy
There are two basic ways to collect this solar energy. One is with photoelectric cells, which convert sunlight directly into electricity. They are great if you want to run an electric appliance or charge a battery, but to convert the sun’s energy to electricity and then back into heat is not very practical or cost-effective, at least at this time.The second, more efficient way is to convert the energy directly into heat and transport it to the space requiring heating, or store it in a thermal mass. For this we use thermal collectors, and the best economical and efficient transfer fluid is water.
If we could capture and store all the available energy, all we would need is eight 4-foot x 8-foot solar collectors to provide the domestic water and space heating needs for a typical 2,000-square-foot home year-round. The reality is that we can’t get anywhere near that efficiency. The average hydronic flat-plate solar collector can capture about 30 percent of the sun’s energy in a day, providing the sky is unobstructed.
Evacuated tube collectors have a slightly higher efficiency when generating higher water temperatures. There are even collectors that track the sun to squeeze out the last bit of collectable energy as it travels through its daily arc. Still, even with the limited performance of equipment, there is a lot of “free” heat to be collected from the sun and, when properly designed, can contribute a considerable amount to the space and domestic water heating needs of a residence.
A basic fact when dealing with solar collectors is that, as the required water temperatures go up, the efficiency of the collectors goes down. This becomes a challenge for solar designers. To provide warm air from solar-heated water to heat a home requires air that is 120 degress F or warmer for comfort. Any cooler and the air feels drafty and cold.
The solar-heated water is typically piped through fan coils, so the water has to be hotter than the air it produces. This requirement is not a good match for thermal solar collectors. Take a look at the typical performance chart in Figure 1. A flat-plate, glazed collector producing 150-degree F water captures only about 25 percent of the sun’s energy.
That same collector under the same conditions producing 100-degree water captures about 50 percent of the sun’s heat. That is double the efficiency.
Spreading The Warmth
Which brings us to radiant floor heating. A radiantly heated home designed to take full advantage of solar-heated water can utilize as little as 90-degree F water to provide a comfortable and energy-efficient environment. Unlike air systems that are difficult to zone, hydronic radiant floor systems are easily zoned and allow the heat to be placed exactly where it is needed. No cool air drafts, energy-hogging blowers, or ceiling fans to keep the warm air off the ceilings, just soothing, quiet, comfortable warm floors filling the space with warmth.Not only does a radiant floor dramatically stretch the capabilities of the solar collectors, it is an extremely energy-efficient method of distributing and utilizing the heat so generously supplied by the sun. The heat is put exactly where it will do the most good and is kept away from ceilings, windows and other high heat-loss areas where heat from hot air systems is generally concentrated. Couple that with the fact that people actually feel comfortable at lower thermostat settings when living on a radiant floor and it all adds up to reduced energy requirements.
To get the most out of what the sun has to offer, the energy needs to be stored for later use when the sun’s rays are weak or not available. Water is a very convenient and economical storage media. It can store 1 Btu per pound per degree F. Using our previous example of 2,000 Btus/ft2 per day of solar energy and an average of 30 percent collector efficiency, we could store 153,600 Btus of heat in under 400 gallons of water by raising the water temperature 50 degrees in one day with our eight solar collectors.
Of course, if we had radiant floor heating, we could conceivably take advantage of the higher collector efficiencies at the lower temperatures and double the amount of heat stored to 300,000 Btus. Depending on the design of the home, that amount of heat could carry it for a day or two. By using a concrete slab floor, even more heat could be stored over several days.
The same principles can be applied to ground source heat pumps as well, since the ground is, in effect, simply a big solar collector. The geothermal heat pump extracts the heat from the solar-heated ground. Like a solar collector, a heat pump’s efficiency falls off rapidly as the delivery water temperature rises. Coupling a geothermal heat pump with radiant floor heating reaps benefits at both ends, just like the solar match. The heat pump is more efficient and the heat delivery system is more efficient. That means reduced energy use and lower utility bills.
Solar energy is free, but it costs to harness it, just like air is free, but it costs something to put it in your car tires. It requires special equipment unless, like the Anasazi Indians of New Mexico, you build your home in a south-facing cliff that is nature’s solar collector.
Like so many things in life, it comes down to return on investment. With a newspaper in one hand and a crystal ball in the other, we try to determine if the money we invest today will pay dividends in the future. But if a solar radiant floor heating system can save homeowners some money, reduce their home energy bills and make them more comfortable that will get their attention.
Being part of the “green” scene just makes it that much better. Solar heating has a great potential for saving money, saving energy and improving the environment. It is an unbeatable combination if we in the industry can keep our claims realistic and deliver what we promise.
