Friday, January 22, 2010

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, 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.

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Saturday, January 16, 2010

Our Latest Power Bill

Due to the recent cold spell, we sold back more KWH than we purchased over the last month. We have banked 202 KWH to work against future consumption. Bill is $16 which is the monthly service fee. Click on the figure for a better view.

Tuesday, December 22, 2009

A Temperature Measurement Issue

One of the major issues in the climate change discussions is the reliability of the historical temperature records. The sceptics pounce on this one, and frankly, I have to give them some credit, because I've spent a good deal of time looking into the temperature data, and understanding some of the nuances isn't necessarily easy. In fact, I'm not convinced that the scientific community at large understands this as well as they need to. This is not to say that I think the carbon issue is a non-issue, but I would not be surprised if a significant part of the measured warmth is not due to greenhouse gases but to other things with the measurement problem being one of a myriad of outstanding analysis wrinkles.

The graph above is a rather extreme case in point. This is a plot of the outside temperature data as measured at our house and nearby. The light blue line is the temperature measurement from the thermometer associated with my generator, and a reading is taken every 15 minutes. The outside temperature is measured mainly to make sure the fuel balance is correct. I'm using the data to help understand the expected heating load.

The data are from 31-Mar-2009. Last spring I was outside and remember seeing that the sun was shining directly on the box that is attached to the back of my house and which contains the temperature reader. Most of the year this side gets shaded by three trees, but in the spring, prior to the leaves coming, the sun will peek through to the back side of the house in the late afternoon. When that happens, the process of radiative heating warms up the box beyond the ambient air temperature. I remember at the time thinking I needed to check the data for this, and well, I guess the effect is pretty big.

The other data are from the Weather Underground site. There are data from literally thousands of personal weather stations available there now, and I picked the closest stations to my house that had data on the appropriate date. There are two green-colored datasets, one of which is missing most of the data (and is the straight line across the plot). The second one is closer to the beach, and that is the primary reason it is lower in temperature. The others are further from the shore and more representative of our experience--except for the late afternoon when the sun hits our thermometer.

Alas, another thing that needs to be checked and corrected.

Just for the record, no I don't think all the NOAA (and other scientifically oriented) thermometers drift by as much as 20 degrees F, but I would not completely rule out issues at 1/20th that size, and this comment is based on not this particular graph but on my own experience dealing with NOAA data as well as other International data. Yes, there is a whole sub-industry involved in this academically, and yes they do good work, but I don't think the issue is as closed as many believe it is.


Thursday, December 10, 2009

Power Source--Relative Contributions

As an addendum to the last post, the figure above shows the relative fraction of the different power sources. Solar varies between 10 and 15 percent of our power that we use. The big shift is when the Ecopower was installed. We now co-generate locally between 25 and 85 percent of our own power depending upon the heating needs with the largest local generation naturally being in the winter. Combined with the solar, during the swing heating months we generate virtually all of our power locally.

For completeness, the two vertical lines show important operation changes. The first is when the solar panel upgrade occurred (can you tell the difference?! It sure is hard!). The second is when we switched the Ecopower from 2nd stage heating to first stage heating. The plan is to keep it that way at least through this coming winter with no other changes.

Friday, December 04, 2009

Household Energy Use

With the digitization of the propane data as described in the last post, the total household energy use can now be analyzed. This kind of analysis seems to be growing in popularity. Witness the rise of Google Power and Microsoft Hohm. The latest one I've been introduced to is I've looked at the last one and may migrate some material there, but for now, I'll use my own clunky analysis program as there are only so many hours in a day.

The following figure shows the fuel use for our house since late 2000. The dark blue is heating oil, the light blue is propane, and the big purple vertical rectangle is the period where we gutted the house and expanded it from 2600 to 4200 square feet. The size addition, I think, now files inside the "what were we thinking" folder, but we do love our home. Below the fuel figure is a figure detailing the electricity use and generation.

In the household fuel use figure, the different rectangular boxes represent one delivery by the distributor(s). The use clearly spikes in the winter and deliveries are more frequent then, so the rectangles are narrower. The base load is easy to estimate at 1.9 gal/day for heating oil--independent of time amazingly--and the propane use is 3.9 gal/day. The difference between those two numbers is discussed in the post prior to this. (That post is unfortunately a little technical and its significance is probably only appreciated by the makers of the Ecopower and maybe a few other pointy-headed geeks such as myself.)

The electricity figure is a little busy but full of information. The upper black line is the total amount of electricity consumed with the blue infill being the amount pulled from the grid. The yellow infill is solar generation, and the green infill is the Ecopower co-generation.

The number of different operation modes is beginning to become a problem in analyzing the data. Here are some of the details. Prior to the renovation, the situation was the standard old New England drafty house, poorly insulated, heat came from heating oil with supplemental electric heaters, and power all came from the grid. Just after the renovation, the solar panels were installed (2.5 KW) and they were upgraded in Sep '08 to 3KW. The Ecopower co-generator went live in Apr '08. In the fall of '06, after the renovation and after the hot summer, the geothermal (ground source heat pumps) were installed. Needless to say, this is a very non-standard situation here. One further complication is that prior to 2009, the system was run with the geothermal as first stage heat and the Ecopower as second stage heat when the geothermal couldn't quite keep up with the heating load. Since Jan, 2009, the situation is reversed with the Ecopower as first stage and the geothermal brought on only when the Ecopower can't keep up with the heating load. Very non-standard indeed.

The above figure shows the data from 2006 and is an attempt to better see the detail. The co-generation contribution is encouraging, but it comes at a cost of more fuel consumption. It seems like we are in a game of whack-a-mole here. We put in geothermal in an effort to reduce heating oil dependence only to see the power consumption go up. We put in solar, and while it helps, it sure seems less than stellar (I guess that pun was intended). Co-gen works at reducing electricity from the grid, but now we burn more fuel.

The question now in my mind is whether or not the move to propane and local generation was what I wanted, for it means more fuel consumption, or at least it seems that way. A primary aim is to reduce foreign energy dependence and increase our energy security, and given that as the goal, is burning propane, which has some foreign sourcing, better than burning natural gas at a power station and running geothermal off the power which is almost all domestic in origin? I think I've resolved how to think about that issue beyond (1) the efficiency argument that the grid structure is enormously inefficient, and (2) local power capability represents an improvement in energy security (even if it has yet to be enabled for backup power mode). These issues and more will be the subject of a later post.

When analyzing the last figure, there are two things worth mentioning. The first was alluded to earlier, and that is prior to 2009, the Ecopower ran in second-stage heat mode with geothermal as first stage. The drop-off in grid power since 2009 is substantial and installations would reflect a drop, but remember that the drop is in part from a reduction in heating with geothermal. The second item regards the small spike in heating oil use in Mar, Apr, 2008. Seeing that brought back some memories.

Throughout the winter of '07-'08, the exact Ecopower installation time was somewhat uncertain. As a result, I was ordering heating oil in 100 gallon units from a local distributor, the minimum amount they would deliver, for I didn't want to be stuck with extra oil. Timing wasn't always good, and we ran out a couple of times. With geothermal available, we had house heat but not water heat, sometimes for as long as 24 hours, much to the consternation of an increasingly impatient family. The experience was valuable for it taught us that having hot water is an enormous contributor to one's self-esteem and without it, life can appear less than dignified.

In early '08, as the Ecopower launch was getting closer, the heat was switched to heating oil as first stage heat to insure we wouldn't get stuck with any extra oil, and the spike in heating oil use is with the house just on heating oil. That period of 2 fills provides us with a powerful comparison between the old house and the new one, and the new house on heating oil still used less than the old house on heating oil, degree-for-degree, even though the house is now considerably larger.

At the tail end of the heating oil period, with the heating season over, we only needed hot water heat, and it made no sense then to buy 100 gallons from the distributor. It was funny interacting with them. Prices were soaring during that period, and I think they thought I wasn't able to afford too much oil, which was the case for many people, because we were only buying to 100 gallon increments instead of filling our tank. They knew I was in the process of getting off heating oil, and when heating oil went above $4/gal, the drivers started asking me about how the geothermal was working. It was an interesting turn of events that the people whose income depended upon a system of fossil fuels was beginning to question the value of it.

The last little heating oil fill is actually me filling it 5 gallons at a time with diesel from the gas station. We still have about 5 gallons of heating oil (diesel really) in our tanks. Some day we'll remove all vestiges of the heating oil system. Whenever we get around to it, that will be a good day.

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Wednesday, November 25, 2009

Propane Use and the Power Conversion Number

In the prior co-generation posts an important number I've used to calculate the propane to power factor via the Ecopower. That number is 5.82 KWH/gallon of propane (plus about twice that amount in heat produced which is also used). That number comes from the Ecopower spec-sheet that has the rate of 4.7KW with a fuel rate of 3.42 lbs/hr, and using 4.24 lbs/gal, the weight of propane, you get the 5.82 number above.

I've been putting off the analysis of trying to check the power conversion number, mainly because it meant aggregating my propane bills and importing the numbers into a spreadsheet. Well I did that finally and this post goes through the estimation.

Our propane tank has a 500 gallon capacity, and about once a month the propane truck shows up and fills the tank to some level near the top. Exactly where it gets filled to, I don't know, but I know it isn't to full capacity. They always leave a little room for vapor expansion and such, so there is potentially some jitter there on a month-to-month basis. I will make the assumption that the tank is filled to the same level each time, and the amount of power generated from fill-to-fill is related to the amount put into the tank.

Another complicating factor is that we also use propane for cooking, and the amount of cooking isn't necessarily a constant, so if we just take the amount of propane used and divide that into the amount of KWH generated, the result will be an under-estimate of the conversion efficiency, and there will be a fill-to-fill variation depending upon how much stove top usage there is. The only way to get a more accurate number would be to put a flow meter on the co-generator fuel line, and I may request that, but the soonest that would be done is January, 2010.

The useful data is comprised of 16 periods starting with a fill on July 22, 2008 to the latest fill on November 9, 2009. The following figure shows the daily propane usage for the periods where each box represents one fill.

The red line in the figure is an estimate of the baseline propane use, and at just under 4 gallons per day. I'm going to call it 3.9 gallons/day baseline which is where the line is drawn. That sure is a lot of fuel just to heat hot water. When we were using heating oil, we used about 1.9 gallons per day for hot water use which is still quite a lot, and it continues to bother me.

At any rate, how do the numbers compare? For starters, we have to compare with similar heat content. A gallon of heating oil, if burned, releases 139,000 BTUs of heat while a gallon of propane releases 91,333 BTUs, so heating oil has 1.52 times more energy than propane, gallon for gallon, which means that if magically we could run the Ecopower on heating oil (or diesel for that matter), we would be using 2.56 gallons of heating oil--still above the 1.9 gal/day old use.

Well, we are close to our heating oil use, but we have one more adjustment before a proper comparison. Because we are co-generating electricity, we using 27% more than we would normally use in heat production, and if we divide 2.56 by 1.27, we get 2.02 gallons per day of heating oil boiler equivalent--close to our old baseline rate, so that's good on a consistency basis. It is a little bit high, though, and that could in part be explained by the cooking use. Let's keep that in mind as we continue on our goal, the propane-to-power conversion number.

The other great thing about the Ecopower is that it keeps track of how much power has been generated, and I've been logging that information too. The next figure shows the KWH generated per gallon of propane for the 16 periods. The blue points are the raw numbers without any adjustment. The x-axis has the number of days between fills, partly as a way to separate the estimates, and partly to look for a bias that would show the cooking use. The red line is the advertised value from Marathon Engine, 5.82 KWH/gal.

Looking at the data scatter, one can argue that there is an average daily use, and it works out to about 0.28-0.29 Gal/day cooking use. This number is determined iteratively. Frankly, I haven't thought much about cooking use, so I don't know how to interpret that value, but that value would give an agreement with the old baseline heating oil use of about 1.9 gal/day (in fact, it overcorrects!). The blue line shows the least-squares fit to the data, and it intercepts out at 5.62 KWH/gal which is the number that should be compared to the advertised value. The error estimate on that is 0.1 KWH/gal (from a bootstrap technique for any geeks reading this).

One has to be careful over-interpreting these data. The two points out near 80 days between fills were early on. After those points were taken, a leak in the system was found. Was the leak present during those points, and later use has less of an effect? Also, the laggards at 20 days and near or below 5 KWH/gal can be selected out, because my in-laws were in town, and my mother-in-law loves to cook. Could she have been using 4 times as much propane per day we normally use in cooking? Seems easy to believe, and it can't be ruled out at the moment. If you discard those two laggards, you get to the 5.82 value, and it would raise the daily cooking use to about 0.4 gal/day. All sorts of data adjustments are possible. I'll leave the values as I found them for now which is the appropriate thing to do.

This post has tried to test the Ecopower propane-to-power conversion number advertised by Marathon Engine. The value I get using the number of gallons reported to me by the propane delivery company and the number of KWH reported by the Ecopower is 5.62 KWH/gal plus or minus 0.1 KWH/gal, and I estimate we are using 0.29 gal/day of propane in stove top cooking (our oven is electric). The measured propane to KWH number is lower than advertised by about 3%, but it is within the statistical error range and probably within the possible systematic errors, the bulk of which would bias our estimate to be too low.

Note: The daily cooking rate number was originally calculated incorrectly, and I have corrected that from an earlier version of this post and adjusted the text appropriately.


Saturday, November 21, 2009

Co-generation Operation Cost vs. Fuel Cost

In an extension of previous posts, I've looked at the Ecopower operating costs versus fuel costs for propane and natural gas. The above figure shows the cost of power production for these two fuels. There are three different lines drawn. The blue lines show the cost of production assuming that no "waste" heat is used, and clearly the costs are rather high.

The red lines (and next lines down from the tops) show the generation cost in co-generation mode. The main assumption is that the extra fuel requirements are 30% over a traditional boiler or furnace.

The lower purple lines in both graphs in the figure is the equivalent market generation cost in the case of a $0.05/KWH delivery cost. Currently here in CT, the delivery cost is $0.0545/KWH. The purple lines show what the market competition is (coal!). What is notable is that with a propane price of less than $1.5/gal or a natural gas price of less than $15/mcf (retail price), it makes sense always to generate the power locally in co-generation mode if there is a net metering arrangement in place, because it costs more to deliver the power than it does to co-generate the power locally.

Current costs this year in CT is about $2/gal for propane and $20/mcf for natural gas. This is the retail price, and for natural gas it is rather high as wholesale natural gas for delivery next month is currently under $5/mcf. There is always a retail mark-up due to the extra delivery costs and, in the case of natural gas which is regulated, there is probably an additional cost from prior hedging with gas procured for delivery earlier in the year or last year when prices were significantly higher. The retail power cost including generation and delivery is in the range of $0.15/KWH to $0.18/KWH depending upon the retail provider (we have choices here in CT).

The conclusion is that co-generation wins hands down, and the primary reasons are (1) close to 70% of the fuel energy is lost as waste heat up the smoke stack and in transmission, and (2) there is a delivery cost for distributing the power, and these extra costs are larger than the extra retail fuel cost over wholesale fuel cost.

I would love someone to check these numbers. The methodology is described in prior posts on the co-generation costs.

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