COLUMNIST: Geothermal the latest in heating and cooling
"They're not wells, they're bore holes," said the instructor at one of the three-day-long ground source heat pump school classes I just attended.
Indeed they're not wells. It turns out that a well is something one gets water out of, while a bore hole is something one gets heat and cold out of.
Directional bore. Vertical ground loop. Slinkys. Desuperheaters. Ground temperature. Entering water temperature. The terms just keep coming and coming.
If you don't have it figured out yet, what I'm learning is the latest in heating and cooling: Geothermal ground-source heat pump systems. By circulating a fluid--usually water with some antifreeze in it, and since the ground maintains a pretty constant temperature, we can move some of the earth's stored energy into a home, or school, or large enterprise, remove some of that energy in the winter, and cheaply keep us warm and cozy.
You have heard, no doubt, about air-source heat pumps, which do a great job out of stealing heat from air that is more or less above 20 degrees. Unfortunately, they don't keep up their good returns below that temperature. On the other hand, ground-source, or geothermal, heat pumps circulate fluid down deep where the earth's temperature holds more or less in northwest Minnesota at 46 degrees. At that temperature, you can put one penny into electricity, and get three or more penny's worth of energy back.
How can this be? It turns out that the simple explanation is that the refrigeration cycle, just like the one in your refrigerator, has some natural quirks that allow it to happen. Anymore than that, you'll have to come and take my HVAC course at MTech in Wadena. Suffice it to say that if you hold your hand down at the front bottom of your refrigerator, that warm air is coming out of the inside of your refrigerator, part of which is at 40 degrees. Of course you know that the new highly efficient refrigerators run a year on very little electricity.
And so do heat pumps.
From here on in, geothermal becomes fairly complex. We'll use GSHP for an abbreviation for ground source heat pumps.
To correctly design a GSHP, one first has to figure out how much room one has, because the ground loop--through which water circulates--can be either horizontal or vertical. A horizontal loop, which because it consists of plastic pipe uncurled and stretched out like a slinky, is called a slinky. Have you got lots of room? (Which means enough room to bury enough pipe down around 9-10 feet that it will match the heating requirements of your building.) Is your soil type correct? (Sorry. Some dry sand locations and rocky terrain are just outright hostile to heat transfer.) Is there a water table shallow enough to get pipe into? (Like a slough, or swamp, depending upon the DNR's opinion of this.) The best option for any slinky is to get it down into wet ground, which is the best transfer medium one can have.
Each slinky will be about four feet wide and 80 feet long. (The actual pipe length is 600-800 feet long, but because it's all curled up, it takes up less space.) From that coil of buried plastic pipe, you can expect one ton of heating and cooling, or 12,000 Btus. The average house needs at the coldest time of the winter somewhere in the neighborhood of 50,000 to 75,000 Btus to maintain required comfort inside. Five slinkys, ten to fifteen feet between the trenches, ten feet deep, well, you can begin to calculate whether or not you have that much room.
If you don't have that much room, horizontal ground loops are out for you, and then we consider vertical bore holes, which are drilled in much the same fashion that common deep wells are drilled, and are in the neighborhood of 200 feet deep. Each of these wells can deliver approximately one ton, or 12,000 Btus of heating and cooling. Figure them in at fifteen feet separation, and as you can see, this requires a much smaller area, like even your back yard.
One would do a slinky because they are less costly to install. However, given the wide variety of soil composition and heat transfer problems involved, vertical bores, even though more expensive, quickly win this contest. However, if you can get your slinky into a water table, that's pretty hard to beat. Unfortunately, most of us don't have that option.
The biggest thing going for GSHPs at the moment is the federal tax rebate of 30 percent, which cuts a huge chunk out of the price of installation. That's not figuring in the fact that you're going to reduce your home and hot water heating costs by approximately 66 percent. That's pretty fun to think about, too.
What are the possible problems? First is not installing enough pipe in the ground. Bore holes are expensive, but if you don't have enough of them, or they are too shallow, you'll overload the ground's ability to accept or reject heat to that pipe, and it won't handle as much of the winter as you'd like. At this class, they advised drilling one more well than calculations seem to call for, just to be sure.
The holes are bored by someone else other than the HVAC contractor, so it's important that they are drilled according to depth specifications, and the pipe inserted correctly, and the hole is grouted (filled back in) correctly.
Leaks in the underground piping can be troublesome, but proper thermal welding of the pipe, and proper testing--that eliminates almost all of that headache.
For us up in the north country, a big problem is that we need a lot more Btus of heating than we do cooling, so the systems are misbalanced toward heating, and cooling comfort suffers. New technology that will allow these units to run at low and high speeds will help this situation a lot.
Air flow is critical. If one is retrofitting GSHP technology to old ductwork, it's almost a given that the old ductwork won't be adequate to maintain expected comfort and economy.
"We're doing 600 holes for a new school building," said one of the men at the class. Whew. Talk about waiting until the last minute to get your knowledge up to speed. But as time goes by and more and more engineers, architects, and home owners find out how great this method of heating and cooling is, it is becoming much more popular.