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Carbon neutrality with PVs


Photo credit: Dow Chemical

Yesterday, I explored the carbon captured by a straw bale building as a factor in getting to a carbon neutral/negative household. Today, I want to look at how far building-integrated photovoltaics can get us.  Let's start with a breakdown of typical residential energy demand.  I found figures from the EIA Residential Energy Consumption Survey (Table 14, 2005) as follows, based on region.


To this, I would like to add vehicle usage and personal air travel.  Slightly out of date figures from the 2001 National Household Travel Survey show household gasoline usage at 1053 gallons/household/year, and at 115,000 btu/gallon, that's 121 million btu/hh/yr.

I couldn't quickly find good household airline usage statistics, but overall, airlines used 8.5% of transportation energy versus 76% being used on highways.  So we can guess that household airline usage is about one tenth of household gasoline usage, overall.  That gives this:


These numbers are a bit rough and ready, but will get us in the ballpark.  So, the biggest chunks, in order, are household driving/gasoline usage, then heating (except in the South), then hot water, and then it gets into the smaller pieces.  Over time, climate change is going to tend to drive down the heating piece, and increase the air conditioning piece.

Before we think about the conservation options here, let's just briefly calculate how much PV is required to generate this much energy.  For that, we need a map of the solar resource, like this one:


For my purposes, I'm going to take the NorthEast as having 4.5 kWh/m2/day, the Midwest as 5.0, the South as 5.25, and the West as 5.75 (with the understanding that these are rough regional averages - there are places in Arizona where you can get up to 7.0).  We have to multiply this by a solar panel efficiency.  For my purposes, I'm going to use the Dow integrated roof shingle pictured at the start of the piece, which uses a CIGS cell with a 13% efficiency.  Doing the math, to get from kWh to btu, days, to years, etc, we get the blue bars in the following graph for the PV requirement:


(Note that there are a variety of issues with this - in particular, you can't power a gasoline car with PV in any sensible way - so this is just a rough indication).

The y-axis is in square meters.  You can get to square feet by multiplying by 10 (pretty close, anyway).  Clearly, these totals are a bit of a stretch to fit onto the roof of a residence.  For example, our house from yesterday had a 48' x 32' exterior footprint.  Positing a 45 degree south-facing shed roof (not the most practical or appealing roof design), with a 2' overhang all round and covered in PV, we get 52 x 36 x 1.414 = 2600 square feet - not quite enough in the north-east.

The red bars show what happens if you make the following assumptions:

  • Super-insulation saves you a factor 4 on the heating and cooling energy usage
  • Use of ground-source heat pumps saves you another factor 4 on the heating usage.
  • Preheating of water via the ground source, or solar hot water, saves you a factor two on the hot water energy usage.
  • Use of electric cars saves you 3X on the driving energy usage (roughly the ratio of electric motor efficiency to internal combustion engine efficiency)
  • No other changes.

With those assumptions, I think you can get in the right ballpark.  Now you need 1000 - 1300 square feet  of rooftop PV, depending on region, which seems like something one could potentially do (at a price in $$, of course).

Finally, this is an operating energy calculation.  Embodied energy will have to wait for another time.

What do you think? Leave a comment below.

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