Prime Movers vs. System design

[Crumpite]

Folks,

All of this discussion has got me thinking about all of the possible design goals vs. CHP design.
Maybe we need to look at what folks are designing for and see what engines match up with which designs.

Climate seems to make a big difference in design, the second being if electricity or heat is the main object.

Cold areas need continuous heat in the winter, occasional A/C in the summer.
This leads to 24/7 operation in winter, no excess heat and excess electricity.
If fuel were cheap you'd have occasional operation in summer with lots of excess heat.
You could get hot water in the summer by occasional operation, but would need to dump heat if you were using the electric to run an A/C

Hot areas need continuous A/C in the summer 24/7, and will need to dump almost all the heat except for hot water.
Winter operation would be intermittent if heat was wanted or continuous with heat dump if the electrical was the primary concern.

The next question would be if you were tied to the grid or totally independent.
 
A grid tie gives you big surge capacity, allowing a smaller prime mover and generator.
A battery bank is somewhat similar, except you need big bucks to match the 200 amp capacity of most standard electrical service ties.

Heat storage is pretty cheap on the other hand, letting you vary the electrical load as you see fit.

By my calculations, I'll be getting an excess of Kwh during the winter.
In Michigan, there is a grid tie arrangement you can get if you are generating with renewable fuel (WVO.)
This would allow me to generate excess Kwh credits during the winter and use them to pay the summertime electrical costs.
Any excess power over a years time is just canceled, but in the meantime, with proper management, you get to use the grid as a giant battery bank.

This would allow you to use a larger engine in the CHP unit to supply all of your heating while letting the unit sit idle in the summer instead of running at a partial load.
It sure would simplify things !

I wonder if other states have similar rules ?
This could impact the whole nature of CHP design.

Comments ?
Crumpite







 

[mobile_bob]

i think we are going to start seeing a move for the states to dictate that power generated by cogen is net metered
and it won't make a difference if it is dino or veggie based fuel, provided the system exceeds a minimum overall efficiency standard.

i read somewhere that new york state now has a by back on cogen power, and it doesn't matter what the fuel is.

once the overall efficiency gets up near 80% it just makes more sense than having the power company deliver kwatts of electricity
"and" a heating oil company delivering oil for a furnace or boiler.

cogen, and particularly micro cogen's time has finally come, the only barrier now is first cost of a system that is reliable and as simple
as a furnace or heat pump, that is still a ways off for the general population, but no problem for a good diy'er.

unlike the average homeowner a diy'er can build and service his own cogen, and because he understands his system he doesn't need
15 layers of redundancy, 5 layers of safety, and does not have to pay liability insurance or product liability insurance, so first costs
and ongoing maintenance will be much lower than a commercial offering.

as states start to adopt and embrace cogen and microcogen, the possibilities and chances of success greatly improve, along with a reduction
in cost.

having the grid to use as a big battery is a definite advantage, 

the next thing we need is a safe/simple/diy'er friendly absorption chiller to use the waste heat in summer to make cold for space cooling
but i would suspect that there will be a large segment that have a fairly balanced load where they can use all the heat in the winter and
get kwatt credits for summer airconditioning.

that would be cool!  no pun intended

bob g

[Westcliffe01]

Crumpite, once you put in a water reservoir to store heat, it becomes fairly easy to use up all your available electrical kwh for heating that water.  You could for instance add 2 resistive heating elements and have them powered by relays which drop out when the full load reaches 2 different load levels.  

For instance assume 5kw available electrical power and the home base electrical load is 2kW.  So you add 2kW and 1kW water heating elements.  When the "protected" load exceeds 2kW, the 1kW heater drops out and when the protected load exceeds 3Kw the 2kW heater also drops out, making full generator power available for the "mission critical" loads.  When the base load drops below 3kW, the 2Kw kicks back in and when below 2kW the 1kW is back on again.  Under all of these conditions the generator will see a virtually constant 5kW load and should be operating at the most efficient operating point.

Load sensing on the "mission critical" loads are needed, and one would probably need to "condition" the load sense signals (apply damping) to avoid oscillation (turning the heaters rapidly on and off).  At utility level, if properly designed, very accurate frequency measurement devices are used to manage load shedding and in a city context load shedding would start with suburbs and progress further and further into the city core with industrial areas usually the last to go due to the expense that results from shutting down foundries and the like.  From the disastrous blackouts in recent years, it appears that any vestige of sensible load shedding has been lost, maybe because of the way energy has been privatized... ?

[Crumpite]

Westcliffe01,

Beat ya to it ! 

Oddly enough, I have a 50 gal electric hot water heater set up as my primary heat store tank.
And I've got a couple of current transformers sitting on the bench right by it. 
A couple of contactors or solid state relays and I'll be set.

I just need to monitor the amps drawn from the house and load switch with a small controller.

As you say, it will take a little intelligence in the load controller to avoid oscillation.
That shouldn't be too hard to do, even with a little microcontroller.

I've got another 50 gal. tank, but gas heated, that I'm going to pipe the exhaust gasses through to act as a heat exchanger.
Preliminary numbers say that it ought to work at least 80% as well as a custom designed exchanger, with the bonus of extra heat storage and ease of cleaning.

With 100 gal. of water, and let's say, 100 degrees of water temp to play with, that gives about 10,000 BTU's of storage - perhaps a hour or so of heat in average winter weather around here.
Or 1/4 hour when it hits -30 Deg. F 
Better than nothing though !

The electrical grid is certainly in bad shape, and the powers-that-be are quite aware of the problem.
I know that work is being done on upgrading the system, and intelligent metering is being installed everywhere now on new installations.
I believe that I saw somewhere that work was done to help keep that happening again.
We shall see...

The last big eastern seaboard outage just kissed the edge of where we live.
We're out in the country though, and used to frequent outages.
It only takes a half hour at most to get our generator on line.
With candles, kerosene lamps, LED flashlights and hand-crank radios we are set.

With the new listeroid system it'll be a lot quicker and more convenient !

Thanks for the thought though, I learn more new stuff on this forum all the time.
Crumpite

[mobile_bob]

you might wanna check your Btu computations

one btu is the amount of heat needed to raise one lb of water one degree F

not the amount to raise one gallon of water, so

instead of 10k btu capacity in a hundred gallon tank you have approx 8 times that available

that is iirc

otherwise, the use of added heating elements that are controlled by the load management system is a good one
one i too plan on using as a last line of loading, the use of a refrigeration compressor will be my primary load dump
to provide for refrigeration and/or air conditioning at times when the mechanical capacity is high enough to support
its use.

bob g

[Crumpite]

Bob g

I *knew* that BTU figure sounded low...

Thanks for the correction, jeez what was I thinking.   

That's a fair bit of thermal storage after all.

Crumpite

[Westcliffe01]

Crumpite:

I too am living in south eastern MI and I heat primarily with wood.  I too use the typical 4.5 cord of wood per season, although some like last winter were milder than ever...  Doing the math, 1 cord of red oak has 24MBTU so the season total is close to 108MBTU.   The heating season is about 5.5 months long, so that equals about 165 days.   Dividing 108MBTU by 165 to get the average yields 654.5k btu/day (average).  The coldest days could be double or triple the average with mild days at both ends of the season.  Dividing 654.5k by 24 yields about 28kbtu/hr 24/7 (average).   I know this is about right, since I heated through a winter with a corn stove right before the price became ridiculous and that stove ran 24/7 putting out around 20-30k btu/hr.

Looking over my utility bills, I typically don't use over 600kwhr/month, unless my wife runs an electric heater in our basement.  Our present home does have forced air heating and in the basement the ducts are in the ceiling and with the upper floor wood heated, the central heating never turns on unless we are away for days at a time.  So the result in that the basement is cold in winter.   So my electrical needs if using a genset with 5kW output to provide direct power as well as charge batteries would need to run less than 4 hour/day.   Typically, engine thermal efficiency is about 1/3, so producing the 20kwh of electricity also creates an additional 40kwhr of heat or 40*3412.3 = 136492btu of heat.  It is unlikely that more than 85% of this heat is recoverable, even with a condensing heat exchanger on the exhaust, as well as coolant and oil heat exchangers.  But that would still yield 116kBtu per day or 18% of the average amount of heat needed for the season.  OOps, looks like that misses the requirement by a mile....  For each additional hour run, using electrical heaters, one gets an additional 50% more heat on top of the 116k so doing a bit of math, it looks like I would have to run a total of 7 hour/day, have solar heat collectors, burn more wood or a combination of all of them.

In operating cost, looking at the specs of the Yanmar 2TNV70-PGA engine, they list specific fuel consumption as 0.51lb/bhphr.  That is only 27% thermal efficiency and may be due to parasitic losses due to the radiator fan, water pump and other accessories.  Not using the majority of the accessories in a CHP application may improve the efficiency substantially, particularly at higher RPM where the fan and alternator can be energy hogs.  Thus for 7hp shaft power (I'm guessing here) and 7 hour run time one gets 7*7*0.51 = 25lb of fuel.  Since diesel is 7.09lb/gal that translates to 3.52 gal of fuel and about $10.57 in 2010 diesel fuel cost/day or about $317.22/month.   I currently pay my utility about $25/month for the privilege of access to electricity and NG before I have consumed anything.  I presently spend about $600/season for wood and my actual utility NG and electricity bills usually drops to $80/month with the exception noted.  So total utilities and heat (in winter) run around $200/month in town.  The only catch is the $400/month property taxes which won't go away no matter whether I lose the grid connection or my job (which has been a popular trend the last 10 years in MI).   So taking taxes into account one would be ahead about $300/month despite the higher energy cost of running off grid ($130/year tax is typical in Custer county CO, less if you are zoned agricultural, like leasing your land to a rancher for dry grazing of cattle).

So, if I was designing for this climate (which I'm not, I am designing the system for Southern Colorado), I would size my system for the "average" consumption and supplement with wood heat/direct solar gain and Solar collectors as needed on the coldest days.  So the co-gen source would have to be capable of generating just shy of 654.5k btu/day or about 27.27kbtu per hour (averaged over the day).   I prefer hydronic heating to forced air, due to the reduction in dust, noise and drafts, not to mention avoiding the huge ducts all over the place.  It is also possible to individually adjust each zone to as warm or as cool as needed.  For radiant heating the water temperature needs to be 90-125F, whereas for baseboard or radiator style heat exchangers it is typically 160-200F.   The higher the temperature, the less efficiently one can recover waste heat from the engine coolant, oil or exhaust, since heat exchanger efficiency drops to zero as the warmed storage fluid approaches the temperature of the source fluid.   Given that 90F is the lower limit for getting radiant heat exchange to work, the upper limit depends primarily on the desired level of heat recovery, as well as the space available for heat storage and the desired heat losses from the heat storage tanks (heat loss is higher at higher temperatures).  Generally speaking, most mixer valves have an adjustment range of 100F but in the interest of efficiency it makes more sense to stay well below this differential.  My plan is to keep the differential closer to 50F, thus 90-140F.  This requires more fluid, thus more space than living with 90-190F, but one would have a harder time insulating the tanks for 190F when they are likely to be buried deep under the concrete floor slab.  It also turns out that the 90-140 range matches solar collector characteristics very well (they typically max out at around 130F).

So, assuming the lowest limit of 90F for radiant heating, the volume of heat storage water would have to be able to absorb 654.5kBtu without raising the temperature of the fluid more than 50F is calculated as follows:  654.5kbtu / 50 = 13klb of water or about 1600gal.     So my plan is to use 400gal tanks (rectangular in section, they look like this: ).   Using 4 separated tanks gives one more options regarding how to run the system.   One can isolate them all and start the season with just 1 tank, until the depletion rate gets on the high side and then open the valve to the second tank, which will require a longer run of your heat source to "re-charge" and continue in this fashion through the season.   A small PLC could handle the logic and math to determine how many tanks are needed, based on the average temperature drop in the heating loops.  The bigger the drop, the colder it is outside...  If one has solar collectors, they start the day heating a different tank to the one used by the co-gen system and if the temperature differential of the collector drops below a determined point, the PLC switches over to the next tank.  The rectangular tanks are easier to insulate using flat rigid foam insulation, whereas cylindrical tanks need to be spray foamed.

I have probably left out a few factors in my concept, but it certainly seems feasible to live off grid based on this kind of system, without huge expense.  Depending on location, if wood is plentiful and cheap and sunlight abundant, one may want to reduce dependence on the generator and install a PV array and use solar collectors to best effect.  Provided one does a DIY instalation, the cost is modest when compared to other costs incurred in building a new home (kitchen, bathrooms, roof).

[Jens]

This is an exceptionally informative post! Thanks for taking the time to write it out.

I would only like to add one thing - verify the temperature operating range of the tanks that you intend to install. You are pushing the limit (of the standard plastic tanks) and you might need some structural reinforcement to prevent distortion and creep.

[mobile_bob]

i too have pretty much come to the same conclusions, in that the cogen will likely be a supplement to a primary heating source
or better yet one of a group of sources.

because i can design the house to take full advantage, my thinking is to insulate the floor into a huge thermal mass where
every harvested btu can be absorbed and put to use.

although on the surface it would seem hard to believe that a fuel fired cogen would be cost effective, it also seems credible that
it should be possible to compete with a fuel fired boiler.

i would also agree that the upper limit is probably about 85% recovery of waste heat, and even that is going to take some careful
design and well engineered heat exchangers matched to the available source and thermal load.

its going to be very interesting to see some solid test results, where a well designed system is matched to a home that is engineered
to take full advantage of such a system as part of a larger scheme. other inputs such as solar heat gain, PV, wind, and in some cases
hydro, all tied together with a very well thought out control system ought to return some very good numbers.

success to me would probably be measured in how well it all works together without a massive change in lifestyle, or comfort.

of course it has to be reliable and only require reasonable maintenance, this idea of having to do serious amounts of maintenance every 100,200, 500 hours just doesn't really excite me.

what would though would be something on the order of 500hr oil changes, 1000 hr valve adjustments, belt changes perhaps, fuel and air filters
and maybe 2000hr minor overhauls, for fresh rings, big end brg, freshen the valves/seats, and a minimum of 5000hrs before major overhaul.

heat exchanger service? maybe once a year do a flush and clean.

to be honest, with my plan for limited run time, of about 750-800hours per year, an annual complete service without any intermediate maintenance would be  more than adequate for my needs.

so many things to consider, and so many ways to get the job done

bob g