“So, which lowers my carbon footprint the most, solar thermal for hot water or photovoltaics?”  (A review of the two technologies is here.) A colleague has bought a house in Brooklyn and wants to make it as green as possible.  I think you’ll be hearing a lot more about this house as time goes by, but let’s look at this question now, because it’s actually possible to answer it.

But first, a little ecological scolding:  You know before you go solar you should plug leaks, insulate pipes, caulk windows and doors, insulate roofs and walls and purchase efficient appliances, right? It’s boring, it’s not glamorous, and it won’t get you into Dwell magazine, but it is what you owe your mother, the earth.  And by the way, have you called her lately?

That said, why choose? Why not do both? Well, the roof is only 20 feet by 45 feet, and the fire department needs a six-foot passage, and there’s a stairwell and skylights and…So there may be room for some of both but certainly not all you want. Which comes first?

The super-simple analysis:  Full sun provides a resource of 1,000 W per square meter on a clear day. Running for one hour at noon, a perfectly oriented, 1.0 square meter, 12% efficient PV cell will collect 120 W-hr or 0.12 kWh of energy.  In New York State, each kWh saved lowers fuel use at some fossil fired power plant by about 11,400 Btu, so the PV cell lowered fuel use by about 1400 Btu in oil or gas.  (Of course there are other non-carbon sources of electric power, but they don’t get turned down or off when the sun fires up the cells, the fossil fuel power plants do.)  If it’s gas, those 1400 Btus are about 0.16 pounds of CO₂ that won’t be released. (For #2 oil, it would be 0.23 lbs.)

That’s PV.  What about solar thermal? Well, a perfectly oriented, 1.0 square meter solar thermal collector will operate at perhaps 70% efficiency, collecting (during the same one hour) 70% of the same 1.00 kWh of available solar energy.  But this is collected as heat, so here in the USA we convert the energy to British thermal units and find 1.00 kWh x 3413 Btu/kWh x 70% = 2389 Btu. This is heat in the hot water tank for which there is no need to burn gas, and let’s assume the rest of the hot water is supplied by a condensing water heater that operates at 90% efficiency.  That means 2389/0.90 = 2655 Btu of gas will not be burned and 0.31 pounds of CO₂ not released, almost twice what the PV cell would save.

Now there are a gazillion caveats and hundreds of papers published about all this, but these corrections will only cancel enough to make the emissions comparable in very rare circumstances.  A few of the points:

• If the building burns oil to make hot water, solar thermal looks even better; if the power plant does, the ratio will be smaller, but not enough smaller to reverse the conclusion.

• This calculation really should be done for annual energy output (rather than for one hour of peak output), but the answer will be about the same, despite a few corrections like:

• PV is better at continuing to work at low light levels; solar thermal really needs direct sunlight, so on an annual basis the savings will be a little closer to each other.

• The efficiency of solar thermal depends strongly on how hot the water gets in the collector (the hotter, the lower the efficiency).  Here we’re on strong ground since we only want to take it up to 130 – 140oF.

• Of course you can spend more money and get more efficient PV cells, but it will take a LOT of money to get you anywhere near 20% efficiency, and that will only get you up to 0.27 pounds of CO₂ avoided, still less than 0.31 pounds for solar thermal.

I didn’t differentiate single crystal or polycrystalline or amorphous PV cells – the only thing that matters for this calculation is the efficiency.  Similarly, the solar thermal collector can be flat plate or evacuated tube or whatever, as long as you have one square meter of collector at the specified efficiency.

For the detail-oriented: Burning natural gas (mostly methane) produces about 117 pounds of CO₂ per million Btu of heat; for #2 oil, it’s 162 pounds.  Unfortunately, leaking gas can increase the carbon footprint of gas appliances and gas power plants substantially.  That’s not accounted for here, and will be the topic of a future blog.

For the PV cell:  0.12 kWh * 11,400 Btu/kWh = 1400 Btu

Be Sociable, Share!

•