Ecopower Thermal Dispatch
How much does the micro-CHP contribute to satisfying the thermal load of our house, and what would be needed to put the entire thermal load onto a co-generator? That is the question that this post tries to answer.
The above plot shows the amount of electric power generated over a 24-hour average versus the average daily temperature. The temperature reading is from the Hartford, CT airport, but has been adjusted upwards by 2 degrees Fahrenheit, for that appears to be the average temperature difference between the thermometer at our house and at the airport. It is important to note that the y-axis is the electric power and not the thermal output. To get the thermal output in KWH, multiply the y-axis value by 2.6 (the long-term average of the Ecopower's thermal/electric ratio). To get to BTUs/hr, multiply the thermal output by 3412.
Let's go through all the different things on the plot. The blue points are ones from the past year where the Ecopower was run as the primary heating source for the house. The purple dots are from the period where the Ecopower was used as hot water and second-stage heat. Recent analyses suggest to me that this 2nd stage usage is almost entirely hot water and not space heating, and maybe with time I can get a post out on that.
The black dots are from an empirical linear fit to the non-space heating data. What is interesting about it is that the points colder than 50 F are not needed to get the slope. In other words, there is a slight temperature dependence to the amount of fuel needed to heat the hot water, and extending that dependence to lower temperatures lines up with the actual data at lower temperatures. Generally, people speak of base load and heating load where base load is found by taking the fuel consumption in the absence of the space heating. The small base load temperature dependence is usually ignored (well, it is small). I found the temperature dependence somewhat surprising, but of course, now it is obvious that there should be such a dependence at some level. Don't take the exact match at the temperatures below 40 degrees to mean that hot water load is totally responsible, but it almost certainly is most of it.
The red dots are from a (loose) fit to and extrapolation of the space heating data, and they have the same range in temperature as the black dots. They span the entire daily data set for Hartford from 1948 up to the present. The light blue dots show the Ecopower's maximum output stuck at 4.7 KW. Indeed, you can see that the actual data (the blue dots) do turn over just under the power cap. The blue "sagging" points are from days where either we were away or where we used the geothermal for part or all of the space heat.
The blue vertical dashed line at zero degrees F represents the so-called "Manual-J" esitmate as filed with the local building department when our original heating oil boiler was installed in early 2006. The heating unit is supposed to be sized to keep the house at 70 degrees Fahrenheit at the minimum expected temperature for our geographic location, which for our location, just happens to be 0F. The expected heating load is 99,111 BTU/hour, or, about 29 KW which corresponds to just more than 11 KW electric output from a co-generator with the Ecopower efficiency and thermal to electric output ratio.
Recall that the red line is the extrapolation of the actual load experience through the expected temperature range. The light dashed green line is just the line from the Manual-J level to 65F, the place where traditionally the heating load starts. You can certainly quibble that the red-linear fit isn't exactly right, and I won't argue too much, but it seems pretty clear that our heating load is lower than that estimated by the person who installed the oil burner--a burner that was rated at 125,000 BTU/hr, a full 34% higher than the Manual-J calculation.
The real irony here is that the person who installed the geothermal called me up after I had sent him that previously filed load calculation and told me that that calculation had under-estimated the house's true load, and he estimated the load to be about 25% higher, and he sized the geothermal system based upon his calculation. With that, I decided at the time to try to do the load calculation myself, and from some web surfing, I found www.hvaccomputer.com, where for $49, you can get a 2-month use of software that does the Manual-J calculation. It takes some time to input all the data, but once I did, I got somewhere around 80,000 BTU/hr as the proper heating load, a value that would be equivalent to about 9KW electric output for an Ecopower type system. After arguing in vain with the geothermal guy that we didn't need such a big system, I finally caved in and got the geothermal--primarily because I really really wanted to get off of oil as much as I could. This was in late 2006.
Anyone who has made a Manual-J calculation will tell you that there are more than a few places to vary the inputs to get some different outputs. It appears that the installers tend to be a little loosey-goosey. There are two good reasons. The first is that they probably have an arrangement with the equipment provider to get a cut of the sale, and that would bias folks to a larger system. I want to stress here that I haven't any proof that that happened in this case and am not accusing anyone of any thing. The second reason I think may be more plausible, and that is that with a slightly larger size, the HVAC installer knows there won't be a problem with heating or cooling due to insufficient capacity, and so they would naturally want to oversize by a little to provide for some room for growth and to insure they don't have to come back any time soon. At any rate, it looks to me that my calculation was probably the most accurate, but frankly, why not just take people's actual data and use that?
At any rate, to get back to my question asked at the beginning, a 10KW system would probably be sufficient to meet the entire heating load for our house as we use it. The Manual would say we'd want 12KW at least, and the local installer, if he had a choice, would probably recommend 15KW. There are all sorts of questions that this answer raises, namely what about the economics, etc? Presently, to recover the capital costs involved with the systems, you really need to be running the devices at 80-100% of capacity, and we are at about half of that. It is interesting to note that most of the present systems seem to match a clear load, namely water heating.
My question next is to go in the opposite direction, namely, are there things I could do to have the current system be sufficient for total heating load? And a related question really is whether or not one would want only an engine heating the house. That second question is a trade-off of economics versus security as I see it, and today I don't have a satisfactory answer as to how to make the appropriate size, and it isn't clear to me that anybody else has the answer to that either, but I could be wrong there.
One last thing on the plot is the blue dashed line on the right-hand side. This doesn't really belong on this plot at all, and it is misleading to put it on the plot, but I did anyway. The value there corresponds to the cooling load of 58,447 BTU/hr as filed with the Building Dept here. The geothermal guy was of course higher, and our geothermal system is rated at 6.5 tons or 78,000 BTU/hr (an 8.8KW machine), roughly, it seems, to match our heating load and not our cooling load! I think the hot water/2nd stage heating only data support that notion.