Primary energy

I wrote in in an earlier post (Out with the Old, In with the New) about my heating energy usage with the oil-fired boiler vs. the usage with the minisplit heat pump. The average daily energy input to the heat pump in BTU/day was 43,000 BTU, compared to somewhere around 290,000 BTU into the boiler (note that the boiler also uses electricity, a fact often ignored in comparisons between heating systems - the controls draw power all the time, and the burner uses about 300W, so maybe 1-1/2 kWh/day - that's over ten percent of the energy going into the heat pump!) So is the heat pump really over six times as efficient at heating my house?

Well, sadly, no. The electricity going into that heat pump, at least for now, comes from the utility grid. Somewhere, often far away, are generators, powered by coal, natural gas, fuel oil, hydropower dams, and nuclear fission. When electricity is generated, no matter what the process is, there is energy lost, usually as heat - recall those plumes that come off of the cooling towers at powerplants. Closer to home, maybe you have a small gasoline-powered generator - that exhaust is pretty hot when it's running, right? That's energy that isn't being used. (An aside - the most sensible thing to do with power plants is to use the waste heat energy also - that's called cogeneration or combined heat and power, and it's more common in Europe than North America, where district heating systems are fed with hot water heated by powerplant waste heat.) So energy is lost in the generation process, and that number in fuel burning generating stations is 40 percent for the best plants to 70 percent for poor ones.

Once the electricity is generated at the powerplant, it has to be transmitted down the grid to us. In this step, energy is lost to the resistance of the conductors, and also at transformers, where voltages are changed. So we take another hit!

Overall, depending on the mix of power generators in a region, the generally-accepted figure for the US power grid is that about two thirds of the energy that goes into the powerplant is lost before it gets to us at our homes. At our buildings, we call the amount of energy we use "site energy" - it's energy consumed within the site boundary. As you can see, if that energy is electricity, the "primary energy", sometimes called "source energy", used to produce that electricity is three times the site energy. So the Primary Energy (PE) Factor for electricity in the US is three.

So maybe my Fujitsu heat pump isn't as good as it seems - if I multiply that 43,000 BTU by three, it's about 130,000 BTU of primary energy consumed to deliver that electricity to my heat pump. That's still under half of the energy in the oil the boiler was using, so the heat pump is still looking like a good climate choice in addition to the great cost reduction of operation.

We should note that fossil fuels also take energy to extract and refine and transport to the end user. Some sources assign a PE factor of 1.1 to these fuels, although this is clearly a catch-all, since the extraction energy needed for an oil well in Saudi Arabia is likely quite different from that required when drilling in the Gulf of Mexico (if one were so foolish as to do such a thing.) The Germans who run the Passivhaus Institute assign a PEF of 0.7 to solar electric systems. This says that it takes 0.7 kWh of energy input to produce the system for every 1 kWh of energy it produces. This number is more debated than the electrical generation factors. Even so, there's no question that with on site generated renewable electricity, the heat pump looks a lot better than it does with grid power.

What about biomass? Advocates often say that biomass is carbon neutral, because a tree absorbs carbon as it grows, and as it decays, that carbon is released. By burning biomass, we're just hastening the release. I don't think it's quite that simple. Pellets, especially if they are made from wood chips (they used to be wood waste, less so today) need to have the wood chipped, then dried, then ground up and pressed into pellets. There is some fractional PE factor there. Firewood cut on your own place with a handsaw has a pretty low PE factor!

The climate impact of our houses is proportional to primary energy consumed, not site energy. If you use electric resistance heating (COP of one - one unit of energy in, one out; efficiency on site of 100 percent) supplied with grid electricity with a PE factor of three and another house consumes the same amount of site energy of natural gas (assume an efficiency of 90%) with a PE factor of 1.1, the climate impact is 3/(1.1/0.9) or 2.45; the climate impact is almost two and a half times worse. So don't use resistance heat, use your electricity in a highly efficient heat pump (which incidentally doesn't mean a ground source heat pump, commonly mistakenly called geothermal...) and then you can do better than natural gas, and you also can make that energy on site, which is hard to do with natural gas-fired equipment. Of course, you could bone up on your fracking...and I don't mean what Starbuck meant.

Finally - the climate is affected by all the energy used in a house, not just thermal energy. And the fastest growing portion of residential energy usage is in stuff we plug in, all using electricity with that lousy PE factor. So please turn that damn big screen plasma TV off once in a while!

 

Comments

[Dodd Stacy]

Marc,

In doing cursory research for our PV system, I came across a PEF of more like 0.1, vs the Passivhaus Institute value of 0.7. I could possibly resurrect the source. Can you cite the German source?

[Marc]

I get this value at page 166 of the Passive House Planning Package 2007 Manual, which lists the PE factors to use in the Primary Energy worksheet in PHPP. One of the four PH criteria is that total PE does not exceed 120 kWh/sm/year (note that PHI calculates floor area according to a German standard that does not include stairs, walls, etc., and therefore the area is often 75-80 percent of the gross floor area - making all three area-based criteria harder).

In no way do I endorse the 0.7 PE factor, but I'm ignorant to say what the correct value is. It's not zero and it better not be one, but I don't what it is.

[Small Water Pump]

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[Bruce Harley]

Really useful stuff. So, unless I missed it, I don't think you said how you're getting domestic hot water now. I'm very curious, because this is a non-trivial end use that was clearly included in the daily oil consumption of the Buderus. I haven't seen you account for hot water use in the present situation. If I missed the discussion, which is entirely possible, just point me to the right spot.

[Marc Rosenbaum]

I threw a 10 gallon electric water heater in as a stopgap for a few weeks. I'll get to the DHW soon!

[gas furnace]

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[provera online]

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[Tad Montgomery]

The heat pump option improves from a climate standpoint inversely proportional to how much fossil fuel is in one's utility's mix. Here in VT, with VT Yankee and Hydro Quebec being our major sources, the heat pump option looks quite good, at least from a climate standpoint and if our lower winter temperatures don't drain too much efficiency.

The peak oil folks say that the average EROEI (Energy Returned On Energy Invested) for liquid fossil fuels is down to around 7. If I convert this to your PE figure it looks to be 1.125, which is pretty close to your 1.1.