Milking the Underground Thermal Aquifer

CaliforniaGeo 12-17-16

If you’re concerned with winter heating where you live, it might be helpful to know there’s a free, carbonless, renewable heat source under your feet.  That’s right. 

When paired with heat pump compressor technology (well over 50 years old) you can concentrate low-grade heat from under the earth and deliver it inside your house to heat your space and hot water.  A fluid loop called a heat exchanger dips into dirt (which usually holds 48% of the sun’s seasonal radiation striking the earth) and delivers it for a boost from your heat pump into your home.

As you can see from a U.S. Low Temperature Forecast Map, it would be mighty nice to disconnect from piped methane or truck deliveries of expensive and  climate damaging fossil fuels.  Just make a connection with the earth’s thermal battery under your feet and milk it for all it’s worth.  Don’t worry, because it will be there for you next winter and all the winters that follow.  The colder your climate, the more money you’d save because after ditching fossil fuels, all you pay for is the electricity to run the heat pump and its circulating fluid loop.

You won’t need a separate air conditioner because heat pumps can reverse the direction of thermal flow from gathering heat (in winter) to rejecting it (in summer), without any attention from you.

There’s increased focus on making ours a heat pump, compressor-driven economy instead of one powered by furnaces and air-based air conditioners.  In the mildest climates, air-sourced heat pumps (ASHPs) can work okay.  But in cold conditions, they spend hours during cold periods in defrost cycles and must use supplemental electrical resistance heat to satisfy your thermostat.  Geothermal heat pumps (GHPs) don’t need that because their fluid loops don’t freeze and because the thermal source they either (draw from or reject to) is in a much narrower temperature band.

 In the heat pump installation above, notice how the vertical magenta lines fluctuate up and down in temperature on a daily basis.  Not so the orange (3-foot depth) and the black (7-foot depth) temperature lines that only go slowly up and down seasonally in this year-long example.  The heat exchange loop in this system resides at 7-feet.  Notice how it’s warmer during winter temperatures and cooler during summer temperatures compared to outside air.  That’s a big part of why the GHP exerts less effort to do its job than the ASHP, and for far less money the more extreme your climate.  Partly because of this, they last much longer.

The deeper you place your heat exchanger, the less the temperature fluctuates as illustrated in Figure 4.  Below 20-feet, the temperature never changes for this renewable heat source.  Vertically bored holes get U-bend loops dropped into them and grouted,  sometimes to a depth of 500 feet, and this insures a rich thermal pool to draw from that’s especially welcome if you’re dealing with an upper midwest cold spell.

—Bill Martin