Friday, November 16, 2007

So What Exactly What Are We Burning?

This post comes from a question on the geothermal analysis. We are burning less heating oil now, but we are using more electricity. What, then is used to generate the electricity that runs the heat pumps? The answer to that question depends upon where you live and what the marginal fuel is in that area. Here I look at Connecticut, because that is where I live.

The data used in the post can be found at the ISO-New England website.( In particular, I used the hourly demand data and the capacity data from the 2007 CELT report. CELT stands for a Forecast Report on "Capacity Energy Loads and Transmission", and the report is published annually.

The figure below shows both the electric generation capacity in Connecticut and the demand. To see this plot and some of the details described, you may need to open the figure in a new window (right click and select "Open in New Window").

The capacity is stacked upon each other in a general manner of cheapest to most expensive. It is the nameplate Winter Capacity which is a little higher than the Summer Capacity. There are a few caveats to the generation capacity data. For instance, on any given day, the available capacity is usually less than the total capacity depending upon which units are down for maintenance or for other reasons. There is also the issue of fuel cost. The stacking assumes all of the natural gas plants can run cheaper than all of the petro-based plants. That isn't necessarily true and it depends upon the plant efficiency as well as the current fuel cost. Also, I've put the waste and bio-fuel plants just above the natural gas. It isn't clear if they should go above or below natural gas. What is not apparent, but is present in the plot, is a small sliver of wind and baseline hydro capacity operating below the nuclear block.

Superimposed on top of the generation stack is the historical hourly demand. The data show the minimum and peak demand over a 7-day window. This time period was chosen only to make the graph easier to see. I've also added the bulk of the hydro at the top of the demand data, because most hydro is peak-capacity power. Solar capacity is also included in the peak hydro part (you can't see it on this scale because solar power capacity is presently minuscule).

From the graph, one can see that the peak power demand is in summer, usually in early August in the late afternoon. When this happens, the marginal power is usually petroleum based. Minimum demand occurs in the early morning during the spring and fall--times when there is less heating, cooling, and lighting needs. Only during this period is there any encroachment into the coal capacity. Otherwise, coal is run pretty much continuously. The vast majority of the time, the marginal fuel is natural gas.

Yet another caveat is that CT is connected to other areas, namely all of New England, and the New England electric system has connections to New York and to Canada. The interconnections can mean that New England can bring in up to 2000 MW of power from other areas, the equivalent of CT's nuclear base. Not all of this is used by CT but is shared by all of New England, and some even passes through to New York, but when we are importing from Canada, we do get a larger hydro component than is shown here. That potentially reduces the coal use in spring/fall, but in the winter and summer, the marginal fuel is still natural gas.

The answer seems clear. For the winter heating, we are using more natural gas in place of the heating oil through the increased power use. In the summer, compared to a traditional air conditioning unit, we are using less petroleum yet again, due to the better efficiency (and cooler ground temperature) with the heat pumps.

Net-net, for heating efficiency purposes, we are more efficient, because on average, the natural gas plants, which are very efficient electricity producers, have efficiencies in the range of 30-50%, while the heat pump performance is about 3.5 to 1. In other words, for every unit of electric energy used, we get 3.5 times that in thermal heating. That amount is lost, however, as a result of the natural gas (or other fossil fuel plant) efficiency. There are also transmission losses from the plant to our house. Let's assume we have an efficient natural gas plant at 50%, our heat pumps have a coefficient of performance of 3.5, the transmission losses are 8%, and the efficiency of the boiler that is no longer running is 85%. Net-net, our gain is 3.5*0.5*.92/.85 = 1.9 (or 90% increase in efficiency). Even assuming a power plant running at 30% efficiency, the net gain is 1.14 (or 14% increase in efficiency). It isn't clear if these numbers will sink in (or even if they are correct!). Suffice it to say that the heat pumps mean we are now burning natural gas, and in addition, we are using the fossil fuels more efficiently. That is something I can live with, but can we do better? I think that we can...


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