Off Grid Geothermal Cooling and Heating

This week marks the 2 year anniversary of our Off Grid electrical system. Two years ago I turned off the generator and commissioned the solar panels and we haven't looked back since. In fact, we haven't so much as run a backup generator at the house site in those two years. (In the interest of full disclosure: I did run my welder from a tractor PTO generator, independent of the house system. But that's it - I promise!)

Currently, we have two computers, a refrigerator, lots of lights, a 220 air compressor, random power tools, a small window air conditioner, and a deep freezer running flawlessly on the system. (We have expanded the solar array from 1250 Watts to 5000 Watts as we've added appliances.) At present, our inverters can produce 7200 watts, but soon I'll be expanding the inverter system's capacity to 14,400 watts to accommodate the most ambitious off-grid appliance yet... but first some background.

When my wife and I were just in the planning stages for our new house, I was insistent that it should be off grid. I wanted to produce all of our own power and not rely on public utilities. I realized that once you convert the biggest electrical pigs in the household (furnaces, electric ranges, water heaters, and clothes dryers) to gas or wood, then the electrical requirements for most homes can be met with an array of solar panels equivalent in price to a new SUV. I conveniently left air conditioning out of the equation, but my wife started asking about air conditioning, so I thought I had better look into it.

One wild, but no so practical, idea I had was to buy a tractor-trailer refrigeration unit on ebay and run home grown soybean oil in it (in place of diesel) to cool the house. Hmmmm. I got bogged down at the "squeeze/press the soybeans and filter the oil" stage. I'm an eternal optimist, but that idea was starting to look doubtful even to me. So I continued to dodge and dismiss the air conditioning question for as long as I could. This only aroused my wife's suspicions that I had no viable solution.

I know, I know... why not just suck it up and live without air conditioning? I thought the same thing too, and I was just about there, but then my wife said in no uncertain terms that if I wanted to live without air conditioning, then I was going to be living without her. In other words, she wasn't at all against my dream of living off the grid... it just had to mesh with her dream. You see, the 900 sq foot mobile home we've been living in for 5 years has air conditioning... and it's hard to go back once you've lived the good life. ;)

The solution, I realized, was to compromise on some other aspect of the house (sorry kids, there went the pool!), increase the eventual size of the solar array to 10,000 Watts, and find the most efficient geothermal heat pump in existence. I decided to go with the two speed Envision series from Water Furnace. An on-the-grid friend of ours had two Water Furnace systems installed in his timber frame ten years ago and he loves them. (I think his are single speed models.) He introduced us to his installer (a man who has put in several hundred of these systems) and we hit it off. I don't just trust he can do the job - I like the way he thinks.

The systems have a COP (coefficient of performance) that can approach 5 under ideal conditions. In other words, for every 1 unit of electricity they consume, they can add or remove 5 units of heat. For instance, 2 Kw of electricity could yield 10 Kw of heat (equivalent to 34,000 BTU/hr). Our house will have two, 2 ton units. Under full power, these units will consume a combined total of 4 Kilowatts of electricity, which is within the capability of our solar array when the sun is shining. On cloudy days we can run at least one of the units at the low power setting so long as we have 1200 watts of available sun power. In fact, one of the units (the one dedicated to the bedrooms), can run for 8 hours at night and use only 25% of the KwHrs that our batteries can store.

I doubt I'll be able to keep the house at 68 degrees when its 105 degrees outside, but I'm confident that we'll be able to keep the house in the low to mid seventies in the summer. More importantly, these units will reduce the extreme humidity we see here in Kentucky. Make no mistake, the primary purpose of installing these geothermal heat pumps in our house is to provide air conditioning, but we can and will use them to provide heat for the house when the sun is shining and our batteries are full (like today!!). I won't count on them for heat, but when it is available, it'll be that much less wood I'll have to burn!

Fifteen years ago, our installer put these systems in exclusively with ground loops buried in trenches (that he dug with his own back hoe). He has since become convinced that drilled well systems are superior, and installs almost all of his systems in wells now (he farms out the drilling process). He convinced me that wells are superior (especially for our clay soil), so we're having ours installed in four wells, similar to this picture from Water Furnace's web site. If we can hit the ground water table, the system will work much better, so he recommended that we go to 200 feet instead of 150 feet, due to our location on a hill. Regardless of whether we hit water, the extra loop length can only improve the efficiency of the system.

I'll be updating the blog with pictures of the installation. I also plan to develop some automated way to turn the ground source heat pumps on and off, depending on the availability of sun and the temperature of the house. For instance, if the sun is shining and the batteries are charged, the system could go ahead and lower the thermostat from 74 to 70 (in air conditioning mode) in order to bank some of the solar energy. When I get that working, I'll describe it on the blog. But first, I must go add two inverters to the system, so I'll have more than enough current to start the compressors in the heat pumps when they're installed.