Would you believe that an environmental advocate hired a drill rig to help reduce global warming pollution? No, I wasn’t drilling for oil or gas, or burying my carbon dioxide (hmmm). I was installing a geothermal system for my home.
The geothermal system design we chose actually provides us with three services: cooling, heating and an improvement in the efficiency of our existing solar hot water system. It will play a key role in getting our home closer to net zero energy. The goal was to replace our gas furnace and our older air conditioning (AC) unit with the most efficient heat and air system on the market. We chose the WaterFurnace Envision unit from WaterFurnace.
For those that have been following our home project on the Southern Alliance for Clean Energy blog, you will remember that I have been concerned that the amount of natural gas needed to heat our home would exceed the ability of the 7.1 KW solar system to reach net zero. This is due to the fact that each therm (natural gas units are called therms) is equal to approximately 29 kWh of electricity. Plus our gas furnace was only about 90% efficient. So if I add in our energy use for heating, its pretty large; the last three years averaged close to 400 therms per year. We are continuing to do efficiency retrofits and finished windows this year, but its clear we need to go to a much higher efficiency heating system to reach net zero.
My AC unit was not high efficiency either. It’s was a 10 SEER AC unit and the minimum now is 13 SEER, but many AC units available in the market are much better including some as high 23 SEER.
So why go Geo?
As I did more research, it became clear that geothermal heat pumps are significantly above other choices forenergy efficiency. Here are a few things I learned on the process:
First, the term “geothermal heat pump” is a little confusing because it’s not a direct thermal power source coming from the heat of the earth’s core. More accurately, it is an exchange using the consistent temperatures of the upper earth’s core as a heat sink for cooling or heat source for heating. “Ground source heat pump,” “Earth-coupled heat pump,” or “GeoExchange” may all be better terms, but geothermal sounds fun.
The ability of the systems to produce significantly more energy output for energy input is amazing. The Water Furnace Envision unit will produce 5 units of energy for every 1 unit input. The best gas can do is in the mid 90’s for efficiency. So a 95% efficient gas unit would get 0.95 units of energy out for every 1.0 unit input. This Envision unit will get 5.0 units of energy out for every 1 unit input. That is over 400 times more efficient! This is sometimes referred to as the “coefficient of performance” or COP. (Sorry for all the Wikipedia references but I’m not a mechanical engineer.) The bottom line is that nothing I found comes close to this level of efficiency. This unit also produces hot water for home use. In our case we will tie the geothermal “desuperheater” to our solar hot water system.
One thing you must decide if you go geo is, horizontal or vertical. Basically, you are going to have to layhundreds of feet of underground pipe in your yard. We looked at laying horizontal piping, but it would have destroyed our yard and likely damaged several trees’ roots. We chose the vertical piping, which required drilling but also disrupted less of our yard. With our 3 ton system, the company drilled two, 250 foot boreholes to make two vertical closed loops. As you can see from the pics, it still required some trenches to connect the loops to the house. The mud and rock from the drilling, along with some water, can be a bit of a mess.
The unit was commissioned the first week of November. I have been amazed at how quiet it is whenoperating — it sounds like a quiet refrigerator, which is basically just a one way heat pump. We have also been happy with the air because it is not as dry so more comfortable. It maybe physiological, but I feel a greater connection to my little plot of earth out front knowing that its linked to the energy flow in the house.
Lower Carbon Footprint
I never thought I would go all electric in my home, but as clean as natural gas is, it’s not renewable.
So how does this add up on the carbon footprint. This may be hard to say for sure until we have some run-time history, but here is my thinking:
My power comes from TVA through Knoxville Utility Board (KUB) and in 2008, TVA claimed to produce 672 tons of CO2 for every gigawatt hour. So we must get to pounds of CO2 per kWh — a number we can understand.
2008 TVA CO2 Rate = 672 tons/ GWHs (1000 MWH)
672tons * 2000 lbs/ton = 1,344,000 lbs
1344000 lbs/GWH/1000 = 1344 lbs/MWH
= 1344 lbs/MWH/1000
= 1.34 lbs CO2/kWh
For heating, given that we have replaced a natural gas furnace that was about 90% efficient with the geo unit, we will look at the fuel switch from therms (gas) to kWh (electricity). There are 29.3 kWh per therm, so replacing one therm (100,000 BTUs) = (11.7 lbs CO2) with 29.3 kWh of TVA electricity (100,000 BTUs)= (39.26 lbs CO2) does not on the surface look like a good deal for the environment.
But remember that this Envision geo unit uses over 5 times less energy, so we should divide the number by 5 and this would give you 7.85 lbs of CO2 per 100,000 BTUs. We would also anticipate that part of the time, the solar PV panels will run the system, although how much solar we generate in the winter months is yet to be determined. It is safe to assume that the 7.85 lbs of CO2 per 100,000 BTUs will be lower from the solar running the system in the daylight.
The cooling is going to be a large jump because we retired the 10 SEER electric unit. While it’s a little like comparing apple and oranges, this unit has a 30 EER rating, which would be about a 34 SEER. Again this will be significantly more efficient. The bottom line: I believe this unit will cut our carbon footprint for both heating and cooling, save us money in the long run, and is more comfortable for living (humidity, sound).
The 3 ton unit with the vertical drilling was approximately $23,000. These units are subject to the 30% Federal tax credit. We received a loan from our local power company to allow payment of up to $12,500 on our bill. I’m unclear on the “payback” timeframe until we see the performance in our home and usage, but my guess at right now: about 10 years given the amount of power we use, it would be less for people with heavier use patterns.
So why go geo? We are decreasing our carbon and other air pollution, contributing less to mountain top removal mining, and coal ash from burning coal, or nuclear waste piling up at reactor sites, and hot water which gets dumped into our rivers from running nuclear power plants. Its an investment in our home and a better world, so drill baby drill…..
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