Reducing Your Energy Requirements – Ground Heat Exchanger
If the grid should fail for whatever reason, you might be able to use alternate sources of power to heat and/or cool your home and/or safe haven. One such system is a Ground Heat Exchanger.
If you dig down thirty feet or more, the ground temperature stabilizes at roughly the same temperature as ground water – this is called mean earth temperature and is depicted on the figure below (I pulled the figure from http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm but they got it originally from Virginia Tech webpage that is no longer functional).
But you really only have to do down 12 feet to get within plus or minus 10F of the mean earth temperature (depends of various factors like type of soil, moisture content, and vegetative cover). At this depth you can leverage the temperature differential between cold air above ground in the winter and warmer ground temperature and conversely cooler ground temperature in the summer compared to the warmer or just plain hot temperature in the air. [Fun fact: the ground temperature goes up approximately 25C (or 45F) for every 1000m of depth]. This lag dampers out at roughly 30 feet as depicted on the figure to the right.
Additionally, the soil temperature lags changes in the temperature in the air so while August might be your hottest month, the soil 12 feet below the surface continues to heat up for several months before it peaks and then starts to cool back down. This works to your advantage as you have a higher temperature heat sink to use in cooler weather. And while February might be your coldest month, the soil 12 feet down continues to cool for several months after this before bottoming out and beginning to warm up again.
So my home is near Memphis TN and I should see a mean earth temperature of 62-67F. Pulling air from a buried ground chamber would really allow me to air condition my home in the summer with only the energy required to run a blower. And in the winter, instead of pulling air from outside where the temperature could be in the 40s, I could pull air from this below ground reservoir and would only have to heat up air a few degrees to be comfortable.
In the typical American home, two-thirds of the energy use is heating space (42%), heating water (18%) and air conditioning (8%) (Source: US Energy Information Administration http://www.eia.gov/energyexplained/index.cfm/data/index.cfm?page=us_energy_homes This will vary depending on your location but if you look at your energy bill may be able to see where you are using most of your energy (my largest electric bills are in the summer when I use the air conditioning since I use propane for heat). My point is that if the grid goes out and I have to rely on a solar system, I can size that system for 1/3 of my current power usage if I employ a Ground Heat Exchanger.
Now when I referred to a subterranean air chamber, that was a simplification. You really want to build a pipe / tube manifold that maximizes the contact of the air with the sides of the pipes/tubes and facilitates absorbing or dissipating heat to the soil. The pipes/tubes can be configured in a horizontal orientation or vertically encased in a bore hole or in a coil or slinky configuration in ground or maybe even at the bottom of a pond if you have one on your property.
There are systems that use water or a refrigerant in the pipes and circulates the liquid. My opinion is that water is harder to move than air so I am going to recommend air systems unless you have special requirements that prohibit it. Narrow tubes take more energy to push the air through the tube so there is a trade off in size of the pipes. And you have to plan for condensation in the pipes/tubes so you will need a way to remove moisture that may accumulate. My recommendation is that you talk to a reliable HVAC contractor who can size the system properly for your specific requirements (and based on space available on your lot, where the water table is, etc).
Payback periods vary but I have seen some sources that quote payback periods of only one year. Some of that depends on your location and your mean earth temperature. Some of it depends on your contractor’s price. Cooling systems may not economical if the mean earth temperature is too high (such as in Florida, southern Texas and parts of southern California and Arizona). But they will always assist in reducing your heat requirements (since even in the northern states, the soil temperature will be much higher than pulling from ambient air).
As you develop your prep, you may consider installation or construction of a Ground Heat Exchanger either in your home or at your safe haven to reduce your energy requirements if and when the grid goes down. If you are installing other upgrades in your home at the same time (maybe a rain water run off retention tank, a root cellar, a bomb shelter, or a septic drain field), you may be able to get all your sub surface requirements done at once at a more economic cost.
De Oppresso Liber