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Micro-CHP

Ronald Ottney | Posted in Energy Efficiency and Durability on

A company called Enginuity Power Systems recently won the best of show at the 2020 International Builders Show in Las Vegas.

https://enginuitypowersystems.com/

They build small natural gas (and other fuel) fired piston engines that generate electricity.  The exhaust from the engine is used to heat water.  Their website says to “think of it as a conventional water heater (that just happens to power your home)”.

I’m a builder (Northern Michigan) and requested further information on their 6.2 kw unit, specifically; how many cubic feet of propane required to generate the equivalent of 2.6 kilowatt-hours of electricity?  I used 2.6 kwh because it’s my homes average monthly use.  Below is the response I received:

Our testing shows that the Systems use 10% to 15% more Ng or propane than a standard hot water uses just to heat a homes hot water. The EPS system for the average home will create all the hot water a home needs in a 24hr period while also making 36kw over a 24hr period. Running 6 hours a day.” 

Anybody here aware of this company?  Thoughts?

Ron

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Replies

  1. Expert Member
    Peter Engle | | #1

    I haven't heard of this company, but here's another one:

    https://www.marathonengine.com/cogeneration.html

    The idea is not new, but it is interesting. Marathon sells their unit as a power producer with the heat as an extra bonus. Great if you need process heat 24/7. It's interesting that Enginuity is taking the opposite approach: a water heater that also generates power.

    I haven't found many instances where the heat and power needs of a building are similar enough that these make economic sense. They are waay more expensive than a water heater or boiler. If you're only using it to produce DHW, then it's not going to run a very high duty cycle, and it's not going to produce much electricity, especially in summer. If you try to amortize the excess costs through the electricity, the ROI is terrible. If you run it 24/7 to produce electricity, it has a pretty decent ROI (depending on local electric rates and tarriffs, of course), but then you've got a ton of excess heat to dump somewhere.

    I almost had one client who was interested in a system like this: large house with hydronic heat for wintertime loads, and they had a swimming pool that they liked to overheat with extended spring/fall seasons. That's a big sink to dump excess heat when the house doesn't need it. Matching the loads to the equipment still didn't work out and his contractors were very scared of the complexity.

    Bottom line for me so far is that CHP systems can be awesome in industrial settings where the loads are nearly constant and very predictable. They are very hard to justify for residential use.

  2. Expert Member
    Dana Dorsett | | #2

    >"The EPS system for the average home will create all the hot water a home needs in a 24hr period while also making 36kw over a 24hr period. Running 6 hours a day."

    It's pretty pathetic for a cogen company to use kw where they meant kwh. That's the sort of thing clueless TV reporters do. (Marketing people...)

    >" I used 2.6 kwh because it’s my homes average monthly use."

    A 6.2 kw generator makes 2.6 kwh in about 2.6/6.2= 0.42 hours, or (x 60 min/hr=) 25 minutes.

    I'm pretty sure you use more than 2.6 kwh per month, and probably more than 2.6 kwh per day. It's also unlikely that your average load is 2.6 kw too. Please clarify!

    One of my biz-partners has heated part of his home & pretty much all of his hot water with a 1-kw Honda cogen married to a condensing hydronic boiler. The 1-kilowatt Honda puts out about 12000 BTU/hr of thermal output and the combined efficiency is alleged to be about 85%. One kwh is 3412 BTU, so the electricity to total output ratio is 3412/15412 = 22% of the total, and if the net total efficiniency is 85%, it's efficiency as a generator is 0.85 x 22%= ~19%

    So if it were propane fueled (his is natural gas) 19% of the 91,600 BTU source fuel in a gallon of LP would be delivered as electricity, which is 17,404 BTU (/3412=) ~5.1 kwh/gallon. Alternatively, every kwh of electric output takes (1/5.1=) 0.196 gallons of LP.

    The efficiencies of micro-cogens using reciprocating engines are all pretty similar, so it's in that range.

    Since his runs pretty much 24/7 when it's under 40F outside it delivers about 24 kwh, per day, which is only slightly less than that home's average power use. Most reasonably efficient homes don't use as much 36 kwh per day (the 6kw x 6 hours cited) unless all space heating/cooling is done with electricity.

    1. Ronald Ottney | | #6

      Hello Dana.

      Thank you for your reply. It will take me some time to digest your post. In the mean time, I've attached my electric CO-OP's monthly breakdown of 2019. I looked at that a long time and apparently could not figure it out. Any help would be very helpful.

      Ron

      1. Expert Member
        Zephyr7 | | #7

        What are you trying to determine from that report? It’s giving you your monthly energy consumption numbers in kilowatt-hours (kWh), which is the standard unit of electrical energy used for metering purposes.

        BTW, I recommend you take that pic down, and repost it with your account number redacted (blacker out). You shouldn’t post your account numbers, even utility account numbers, due to identity theft risks.

        Bill

      2. Trevor Lambert | | #8

        You're off by three orders of magnitude. 2600kWh, not 2.6kWh.

  3. Expert Member
    Peter Engle | | #3

    Dana,

    Your example is exactly the issue I've got with these. He's got the Honda married to a condensing boiler. So it's an "extra" piece of equipment. Extra space, extra maintenance, extra plumbing, just extra stuff. Yes, he gets electricity from it, and cheaper than from the local utility. The heat is delivered for "free" if the electric is saving money, but it's being delivered at a lower efficiency than the condensing boiler. From GHG perspective, the 85% (claimed) total efficiency is about even with a modern, noncondensing boiler. His electric is probably a little bit greener because he doesn't have the transmission losses, but he's also burning propane which is not the greenest of fuels with lots of leakage in the pipeline.

    So again, it's an interesting technology, but it's hard to justify.

    1. Expert Member
      Dana Dorsett | | #10

      >"The heat is delivered for "free" if the electric is saving money, but it's being delivered at a lower efficiency than the condensing boiler."

      The more legitimate comparison is to the efficiency of a gas or LP water heater, not a mod-con. The fact that a fraction of the output is a premium quality energy (electricity) as opposed to low-grade thermal (hot water) makes it somewhat more compelling.

      >"You're off by three orders of magnitude. 2600kWh, not 2.6kWh."

      What's an order of magnitude or three- it's just adding zeros! :-)

      If the house is burning through 2600 kwh/month it means a 6 kw cogen would have to run (2600/6=) 433 hours to cover the whole thing. At 5.1 kwh/gallon of LP that would take (2600/5.1=) 510 gallons of LP.

      If it's only being used as a water heater you'd need to have truly HUGE water use numbers to cover much of the power use. This is why the Climate Energy folks married the Honda cogen to a heating system.

      At 2600 kwh/month the average load is about 3600 watts, which is quite a lot. (Typical US homes not heated with electricity run about 1000 watts, give or take.) Where is all that juice going?

  4. Expert Member
    Zephyr7 | | #4

    I usually quote 18% as typical efficiency for these types of generators, and I design these plants (much larger ones for commercial sites, I’ve never done one of the little residential type units) as part of my work. I also ABSOLUTLY agree with Dana about a vendor, or anyone else claiming to be knowledgeable about these things, confusing kilowatts (a rate) with kilowatt HOURS (a unit of energy). It’s equivalent to confusing gallons with gallons per minute. An example would be buying a gallon jug of milk and taking it home compared with buying one gallon PER MINUTE which would be like buying 1,440 gallon jugs of milk EVERY DAY, forever. This is a newbie mistake, and you see it all the time with people who don’t know what they’re talking about.

    The large cogen plants don’t usually heat domestic hot water, at least not as the primary use for the thermal energy. Normally the thermal output goes into heating or cooling (using an absorption chiller) a building or campus. The last system design I did was for 500kwe (electric output from the generator), and it provided most of the heat needed for a small (maybe 25 acre campus with several mid-size buildings) college and a big chunk of their electric power.

    Here is the big issue with a residential plant: the electric efficiency number varies with load, and is not constant. These plants want to run at full capacity continuously for maximum energy efficiency. This means you have to use ALL of BOTH the electrical AND thermal output of the unit ALL the time. Cycling the plant kills your efficiency.

    The other big one, and usually the biggest operating cost issue to be factored in, is that reciprocating engines have a pretty limited life in terms of number of operating hours. That 500kw system I mentioned had a BIG engine overhaul planned for every 2 years as part of the project cost projections, and lower levels of maintenance every 8,000 hours. Expected plant life was 5-7 years before the engine would need a full rebuild or replacement if I remember correctly. The smaller engines usually have shorter lives than the large ones, so be sure to factor this into your total costs.

    The big cogen plants all use gas turbines, or boilers running steam turbines. Turbines have MASSIVELY longer lifespans in this type of operation. Turbines run much, much more smoothly than reciprocating engines and usually last for decades. They also tend to be much larger (usually multimegawatt range), although there are some micro turbines out there. I’m only familiar with the larger plants.

    If you want to see how a large cogeneration plant works, Michigan state university (first or second largest university in the country by number of students) heats their entire campus and provides all (or at least almost all) of their electricity with such a plant, and they have a pretty good website about it here:
    https://www.chems.msu.edu/classes/321/powerplant/

    Bill

    1. Craig | | #9

      Here's a fun one from my neighborhood:
      New power plant was installed with 3 turbines. 2 produce energy, 3rd picks up unused heat from the steam generation. Gets better. Next they send that mildly hot medium to a heat exchanger and heat the cities water supply. After that it creates a larger dT for the turbines which increases efficiency by a few points.

      Yes, they heat the cities water supply. By 7 degrees.
      What does this mean really? On a whole city utility scale, every residence consuming hot water has a 3-5% reduction in usage (electric or gas) because the incoming water is warmer. From the utility's rate, this translated into level consumption instead of increases from the new power plant.

      Now do note, were talking cold water at 48-52F, warming to 55-60F. There's a roughly 4F dT drop from near the plant to the far side of the city as well due to our cold ground temps. But it's still impressive.

  5. Expert Member
    Akos | | #5

    When the Honda micro CHP units came out I crunched some numbers. With our cheap natural gas and expensive electricty, there was a long ROI on part cost (I think it was around 20 years). The issue I had was the unknown amount of maintenance cost with something that will need to run 24/7 to get the return.

    About the only spot that a micro cogen can be justified is off grid where you sometimes need to run a propane generator in the winter time. Here, along with producing the power that you must have, you also get some "free" heat from the cogen unit. With a properly sized setup, this should happen rarely, so the ROI for that the extra bit of heat is probably never.

    The problem is that these units are so expensive, that you are much better off to get more solar+battery.

    Cogens are also a maintenance item. Keeping a backup genny going is annoying, something that runs many hours a day will not be cheap.

    If you need power, solar+batt will always be cheaper even with heat capture taken into account. If you need heat, burn the fossil fuel in a modcon as it is more efficient than cogen (combined electric+heat output).

  6. Ronald Ottney | | #11

    Thanks to everyone responding to my questions! Very helpful. I'm not as excited about this product as I once was. But I'm glad not having to learn the hard way.

    In regards to my energy use. I'm all electric in 3,000 sq. ft. timber frame home. Ductless mini splits for heating/cooling main level and in floor heat in walk out basement. SIP panel insulation, 6" walls and 8" roof. Well with 1.5 HP motor. Wife and daughter with hair dryers and long hot showers.

    Does this usage seem high?

    Thanks again.

    Ron

    1. Expert Member
      Zephyr7 | | #12

      Your usage would be measured in kilowatt-hours, usually per day or per month. Without that information, it’s hard to say if your usage is “high” or not.

      Bill

      1. Ronald Ottney | | #13

        2,600 kwh/month

        1. Expert Member
          Dana Dorsett | | #14

          >"2,600 kwh/month"

          x 30 days = 87 kwh /day

          /24 hours= 3600 watts (~12,300 BTU/hr) average load...

          ...which is unusually high for a normal sized house/household if the house that isn't being heated with electricity (but it is.)

          >"I'm all electric in 3,000 sq. ft. timber frame home. Ductless mini splits for heating/cooling main level and in floor heat in walk out basement. SIP panel insulation, 6" walls and 8" roof. "

          It's still somewhat high for a 3000' all electric house built with SIPS in northern MI heated with mini-splits. The in-floor heat is probably a significant factor driving up the electricity use numbers.

  7. Expert Member
    Zephyr7 | | #15

    If we assume 2,000 square feet to be an average size home, and 1-1.5 kw the average load for a home (which is what the utility industry uses), then an average home of your size should be somewhere in the 1,080 to 1,620 kWh/month range. As Dana mentioned though, an average house won’t be all electric, so take that into consideration when making comparisons.

    Your water well will be a very small fraction of that total electric use. My guess is your well pump runs less than a total an hour over an entire month, which is probably somewhere around 2 kWh total over the entire month.

    Hair dryers are usually 1500 watts, so 1.5 kWh for every hour one of them is running. Even with lots of hair drying, you’re probably under 10 kWh over the entire month.

    Bill

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