It all started with a comment on a blog were I was skeptical of the idea that the oil industry would start to vanish by 2030, given that cars have a half life of 11 years in the USA, we'd have to see some really dramatic action on the ground now for that to happen in the next 15 years. I was equally skeptical that my 60 year old oil burning furnace (and those of my neighbors here in the northeast) would be even half out of commission in a mere 15 years.
That brash statement of mine elicited this response from another commenter, "and when the cost of buying enough solar capacity to heat your home is the same as refueling the oil tank in your home...... what will you do?" This seemed to imply that a solar PV system could A) be used to heat my home, and B) cost less than filling my 275 gallon tank with $4/gal oil (or $1100). While I was debating the dubious nature of that claim, I was interrupted but someone by the name of Rick Hannah saying, not only can it be done, but he had already done it.
Is that really possible, I thought? Can a home be heated in the winter with solar PV?
I asked the Rick for more information which he kindly sent along.
Rick built himself (or had built) a beautiful home 5100 Sq Ft home at 3802 N Cottonwood Rd, Urbana, IL, but he clearly had energy efficiency on his mind as he planned out his home.
The first thing he did was plan on heating the house from day one (late in 2010) with a 4 ton ground source heat pump. This is a very efficient form of electricity where you can get much more heat out of the system than you put into it with electricity. Some of the better systems will give you 8 units of heat output for each 1 unit of electricity output. This is far more efficient than electric resistance heaters which are 100% efficient, but that is nowhere as good as the 800% efficiency that is possible with ground source heat pumps. (Note that no laws of thermodynamics are broken because heat is being moved from underground into the house and the electricity is just a means of moving the heat).
The second thing he did in April 2012 was to install a 11,280 Watt peak grid-tied ground based solar panel array which can be seen in the lower right of the picture above. The system was estimated to have a yearly output of 16,500 KWH, which was more than enough to cover his yearly energy consumption, which is pretty amazing.
The graph above shows how much energy was consumed per month in 2011 (black line) prior to solar array being installed. 15,127 KWH of electricity were consumed from the gird during that year. This is far more than the 10,837 KWH/year that the EIA says a typical American home uses. But there are two things to keep in mind. Firstly, the house is effectively being heated by electricity where as other homes, like mine, use oil or gas to heat the home. So it would be expected that this house would use more electricity. Secondly, this is a relatively large home at 5100 sq.ft which is almost twice as big as the average home built in 2014 which was 2,600 square feet. So it's amazing that even more energy isn't being used.
When thinking about the electricity consumed by this home, let's do a little comparison. Last year my 1500 sq ft home used 700 gallons of diesel fuel for heat and hot water. Each gallon of diesel fuel contains the equivalent of 40.7 KWH of energy. That means I used the equivalent of 28,490 KWH of electricity to heat my much smaller home. And that number shouldn't be compared to the 15,127 KWH of electricity used by the Hannah house, it should only be compared to the portion used to heat that house. So the value and impact of the ground based geothermal heat should be clear. It is far more efficient than burning diesel fuel, and a lot cleaner too.
As one would expect, the Hannah home uses more electricity in the winter months of December, January, February, and March than in the other months of the year.
The red line in the chart shows the amount of solar production in the year 2013. As one would expect, solar production is much lower in the winter months (down to about 800 kwh) than in the summer months (as high as 1700 kwh). This is the result of the shorter winter days and more bad weather. Still, there is a significant amount of electricity being produced even in the winter.
So, can you heat this home with solar PV panels?
The answer depends on how you look at the problem.
1) No, in the middle of the winter the heat pump and home electricity needs requires more than twice as much as electricity as is produced by the solar array.
2) Yes, in a net zero sense over the course of a year, the solar array produces more electricity than is consumed by the Hannah house. The excess production in the summer more than compensates for the shortage of KWH in the winter.
3) If only... if only the array was twice as large, then even in the winter, there would be enough electricity produced to cover the needs in the home. Unfortunately, there would be a huge excess of production in the summer that the net metering rules of the state probably don't allow the Hannah house to get full value for.
But judged by any measure, this house is are remarkable accomplishment. Leaving aside the costs of the geothermal and solar array, which are probably a small part of the overall home construction costs, the ability to power a home this large and heat it as well, is most impressive. It is not the first time I've seen this done however. James Warden of Solectria Renewables has had a similar setup at his home for more than eight years. His ground based solar array covers is home electricity needs including geothermal heating, and two electric cars (all in a net zero sense over the course of a year).
But returning to the original question of heating a house with solar rather than oil, the answer, as always, is, it depends. For my home, the energy contained in the 700 gallons of fuel used last year was 28,490 KWH. This is nearly twice as much as Hannah's 11KW array can produce in a year. So using solar PV in electric resistance heating is really a non-starter from a financial standpoint. The reality is that the really success story has more to do with incredible efficiency geothermal heating than with the abilities of solar PV arrays.
Even if one accepts that the premise of "solar heating" implies Solar PV PLUS geothermal, it is still impractical because the PV array does not produce enough power KWH in the winter.
But if one accepts that the premise of "solar heating" implies Solar PV PLUS geothermal PLUS net-metering, then it is clearly possible to do has Rick Hannah and others have shown.
As for the equating a tankful of oil (roughly $1100 at $4/gallon) to price of an 11KW solar array and geothermal heat pump (likely north of $50K, particularly if a retrofit rather than new construction), well that is not likely at all.
FOOTNOTE 1: If this setup was installed by a large percentage of the population, it would likely have to be banned as it would create a huge grid imbalance. There would be a lot of excess production in the middle of a summer day that would have nowhere to go.
FOOTNOTE 2: The full price of electricity in Illinois is about 10¢/KWH meaning that without solar, the cost of a year's worth of electricity at the Hannah house would be $1,500, which is not too bad. Placing that same house where I live in Massachusetts with electricity prices of 24¢/KWH would mean a much more significant expense of $3,600/year. So while geothermal is great, you have to watch the electricity prices carefully.
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