7) Geothermal- Ground or Air?
"No fossil fuels ever!" To paraphrase Joan Crawford. When it came to deciding on a heat source we concluded pretty quickly that a geothermal heat pump was the way to go. Unless you live in Iceland or Yellowstone on top of a geyser, there are few ways to avoid using oil, natural gas or propane to heat your home. OK, there's nuclear, but they just shut down Vermont's only nuclear power plant! Anyhow, armed with our photovoltaic tracker, and thus free electricity from the sun, what we plan on doing is pumping the free heat that lives deep down in Mother Earth. It turns out that there are two types of heat pumps: air-source and ground-source.
The former draws the latent heat from the air, and brings it inside, exactly like an air-conditioner, except in reverse.
People cool their houses, or cars, or refrigerators, by pumping heat out of the place they want cold, and blowing it elsewhere. With heat pumps we're talking about the opposite: taking the heat in the air ( or ground) and moving it inside where it's needed. Air-sourced heat pumps look like this:
You have an external unit that extracts the heat in the air, and one or more units inside the house that blows it into the living space. Either hot air or through liquid, say using embedded radiant heat tubes or radiators. The added benefit of air-sourced heat pumps is that they can act as air-conditioners, pumping excess summer heat outside when it's sweltering inside.
With ground-based heat pumps we're talking geo-thermal, i.e. 'earth heat'. The pump takes warmth from the ground itself. At 12 feet under the surface, the temperature is practically constant all year 'round. It varies by latitude, obviously, the ground is warmer under the Sahara than it is at the same depth in the Antarctic. In Vermont that deep down constant temperature is about 48°. If you have a well, you can figure what the constant ground temperature is where you live. Just run the tap for awhile and measure how cold it is. Since wellwater comes from more than two fathoms down, it's constant. But it depends where you live. Wellwater is warmer in Tampa than it is in Anchorage! The way I like to think about it is to speed up time. Imagine that a year was 7 days long. On Monday the sun is blazing and warming the ground surface to 80° throughout the day. By conduction the heat sinks lower and lower warming the earth. But long comes Thursday (winter) and it's freezing out, heat is leaking upwards, the ground cools. But a couple days later, it's a hot summer Monday again and the earth is warming up once more. It's pretty easy to see that somewhere down deep it averages out. Temperature changes don't come fast enough to heat or cool things down there. This graph sums it up very well, I think:
You can see in the sine waves that the ground surface temp swings drastically from 82 ° in August, to 40 something in February, but the farther down you go, the less it varies. At 12 feet it barely changes. What's really interesting is the delay. At 12 feet down, the highest temperature is in November, it doesn't hit the low point until late-April. What's key here is that when we want to start sucking heat out of the ground, in November, there's plenty of heat down there. And by the time it gets chilly down below, in late March, we hardly need any of that free geothermal heat. So based on such things, we decided to go for ground-based geothermal heat, though it had pros and cons. On the negative, it's more expensive, secondly you can't use it for cooling. (Which we already felt was unimportant at our locale.) One advantage is that there are no noisy, ugly fan boxes outside the house. It's not susceptible to damage by falling ice or freeze ups, or getting backed into by the snowblower. But by far the most important, it's significantly more efficient.
But we're not done yet. It turns out there are two types of ground-based geothermal: coils and wells.
Coils involve several hundred feet of piping buried 7 feet down in a field. Liquid (usually glycol to prevent freeze-ups) is circulated through the coil field to absorb the heat.
Something like this:
With wells, you don't go horizontal, you go vertical. Wells are drilled 200 feet down and heat is absorbed from the bedrock and groundwater.
We decided to go with the coils, with the idea that we would bury them deeper than normal to take full advantage of the more abundant heat and the time delay. Amazingly we couldn't find a company in Vermont to handle our installation, we had to reach out to our neighbors in New Hampshire. We fixed on Ultra-Geothermal out of Concord, New Hampshire. Click their link for more info on their systems work.
Next week, I want to talk about a strange Frenchman and free wood chips and what that could mean to ensuring that the SunCottage is the most efficient geothermal home on the planet.
