Heat/Power load balancing on small cogen system

[k_jab]

Comment moved to new thread and edited:

Bob, list

Load balancing is important with home scale CHP, as it is with any larger system, and I thus found as many ways possible to utilise the "spare" electricity. Running the high wattage appliances sequentially was one way to ensure the generator was kept loaded.

My house in the UK needs a constant 6kW of heat to keep it warm during the winter months, and as much as 8kW during the coldest weather. But my electricity usage is really quite low, at just 7.5kWh typically on a normal day.  Getting the right balance between heat and power is a little tricky, and using excess electricity for resistance heating , I consider a necessary step to achieve a reasonable balance.

As I normally work from home, they is a high daytime occupancy - so my work room needs to be kept at a comfortable tempertaure.

I describe my system as "semi off-grid", as I still have grid power and gas heating for times that it was not practical to run the Lister. I have subsequently installed a woodfired stove with boiler, for additional heating in the winter evenings, which has offset a considerable quantity of natural gas.

The system started with a large permanent magnet dc generator, charging a battery bank directly at 125V dc.  I have a 5kW sine inverter from which I installed four separate circuits to feed the high wattage appliances.  Unfortunately the PM generator failed through overheating, and I had to rethink and subsequently use a 3kW ST alternator and large rectifier to charge the battery bank.

I found that with my Lister set up, with typically 2.5kWe from the alternator, that I would get about 6kW of useful recoverable waste heat, which I circulate around the house central heating radiators. My house needs 6kW of heat to keep it at a comfortable core temperature so the CHP was run for its heat output.  But as I need just 7.5kWh of electricity on average for my daily activities, I had the problem of excess electricity. So I installed a 3kW electric immersion water heater, and a 2kW electric storage heater in order to trade electrical power for additional heat. 

On a typical day I would start the Lister at around 8am, just before boiling the kettle (2.2kW) for the first cup of tea.

I would then run either the dishwasher, or washing machine - again about 2.5kW max load.  By lunchtime, I would have surplus power, which I would then direct to a 2kW electric storage heater, water heater or the dc battery charger, so as to keep a reasonable load on for the afternoon session, which I would continue to around 6pm.

Thus by the end of the afternoon, I had a warm work room, with stored heat and a tank of hot water for the evening, plus the battery bank topped up.

On a typical 2.5kW mean load, the Lister would use about 15 litres of veg oil during the 10 hour run.  With the move to wood gasification, I intend to keep a similar 10 hour batch run time from a 55 gallon drum of woodchips.

The system is continually evolving, as I find new bits of salvaged equipment, and as time permits.  I intend a 700 litre (185 US gal) thermal store (holds about 60kWh thermal), made from 3 salvaged water tanks - one of which is a gas fired water heater and 200 litre tank combined.  This will be converted to run on woodgas, or as a final condensing exhaust gas heat exchanger for the Lister.







Ken

[mobile_bob]

Kjab/Ken:

my theory of operation for a cogen is somewhat different to what might be seen as typical

my goal is through programmable controlled scheduling the cogen's need to run is targeted to be approx 1 hour duration
an hour before waking, and another 1 hour duration an hour before supper time.
the idea being the cogens prime mover will be kept at full rated load operation where it is most efficient, and is most cleanest
in operation.

otherwise during the day it will only operate when the controller has a demand for power that cannot be schedule and is such
that providing for that power is best done with the cogen directly rather than through an inverter/battery system.

which explains the balance of the system, that being a hybrid cogen/ inverter battery system.

the cogen otherwise will not run unless all criteria are met, that being there is an electrical load and a thermal need that needs filled
so as to keep the unit in it most efficient and cleanest operating mode.

there may be days of the year that operation outside that of the two "one" hour runs are appropriate and make best use of the cogen
such as days when other sources of heat are not sufficient to maintain the home thermally, but primarily other sources will be used for both
heat and power primarily which will allow for a reduced run time schedule.

the basic theory of operation is there will never be a time that the cogen runs unless there is no other source of less expensive and cleaner power
available from my PV/wind/wood heat. Then when it does run it only does so under programmable control so that the unit runs at very near its full rated 9kw mechanical load which for this particular unit is the most efficient mode of operation in BSFC in gr/kw/hr.

that 9kwatt mechanical can be 120/240 AC from the ST head, 7.5 kwe from the project X alternator, or approx 3tons refrigeration from the sanden compressor individually  or some combination of any two or all three. This keeps the unit running as efficiently as any unit on the market today, and allows for the highest available heat with the most efficient recovery, and maintains sufficient heat to light it catalytic converter along
with continuous regeneration of the particulate filter on the prime movers exhaust.

two primary conditions must be met for the unit to have a demand to start, there must be enough mechanical load that needs met "and"
there must be sufficient need for the recovered heat either for domestic hot water or heating of a thermal mass such as a loop in the floor
of the home for cold weather support.

at two hours run time, split between early morning and late afternoon, there will be more need than what the unit can provide typically
so the run time can be adjusted as needed to optimize operation of the cogen. certain times of the year the thermal needs of the house
will be much lower and a smaller unit will be switched in for those months that it is most appropriate for.

i will measure the unit as a success and know i am optimized when the units overall efficiency is greater than 80%, the ability to tweak programming in order to do the optimization is key to the success of the units operation. otherwise left to operator control, my sense is
run times would be much longer, and overall efficiency would suffer.

bob g

[elnav]

Bob  correct me where I go wrong  but it is my impression that energy storage is key to successful operation.  I say energy storage  because that opens up the possibility  of storing in other forms the energy produced from a co gen power plant. this could then be use dduring periods when the  prime mover  in a co gen plant is not running.  Although  load sharing and shedding has  a place it is almost impossible to fully control loading  completely.  This is at the heart of any power utility strategy.
Of late  pumped storage  has gained prominence  But I do not know if it is scalable enough to size it down to suit a single home dwelling. 
The cost  of lead acid batteries  is rising and  transportation  costs are becoming  prohibitive. One of my customers  has one bad cell in his 1000 amp hour AGM bank and now discovers he cannot get a replacement  shipped to him due to international transportation regulations. At least the factory is unable to produce the requisite paper work  demanded  by the shipping companies. 
Few options  are available for storing electrical energy.  There is revived interest in Nickel Iron (NiFe) cells and one company is currently gearing up for domestic manufacturing. At the moment they are  busy refurbishing  old Edison cells.  Another company is located in Australia  but I do not know from where they are sourcing their raw materials. 
I have located one company in Montreal that has a wind generator that drives an air compressor   instead of  an electrical generator  but compressed air has limitations on stored volume .  The most sucessful energy storage I know of is near Niagara  Falls where they use  the electrical generators  to drive pumps to refill a rock gorge with water and this water is released during daytime peak loading periods. 
A water tower or resevoir tank on hilly terrain may be feasible  but dams  involve too much bureaucratic paper work to make it practical on a small scale.
So at the moment  storage  is still  problematic.  One other possibility  that comes to mind is kinetic storage  in a huge fly wheel.  Some of the big warships  being decommissioned  have 4 ton gyro compasses.  I wonder if it might be feasible to extract  energy from them by coupling a small  alternator  to the shaft?       

[mobile_bob]

i would agree that energy storage is a necessary evil to any offgrid system that has even a remote chance of being successful
let alone a chance at efficiency, however

under certain conditions it is preferable and more efficient to provide power directly to the load, for instance...

if i load a washing machine, and if i have more than a few sets of clothing, then my need for having the washer start now is unnecessary
i would have the following options

1. let the thing start and run using battery/inverter power and accept the inefficiency of both battery charge/discharge and the inefficiency of the inverter, or

2. wait to start it until the normal scheduled run time of either early AM or later in the PM, or

3. by a control scheme that is programmable, i might set the washer to start, but the receptical it is plugged into is under the control scheme
and cannot start until a myriad of conditions are met.

    a. program to start wash early AM or later PM, or

    b. based on programming a decision could be made that based on efficiency calculations and other requirements for thermal needs, the
       washer might get power provided at other times of the day outside the normal early AM or later PM mode.

from what i can tell a battery averages about 90% efficient, the inverter averages about 90% so the aggregate total is .90 x .90 =.81%
so it make little sense to power the washer from the battery inverter system unless there is some emergency need to do so, where the cogen
cannot be made to run or similar situation.

other loads can be supplied in a similar manner, some of which consume a significant amount of either kw/hrs or are heavy amp/hr loads
any of these heavy and hard to supply loads are far better served by the primary power source directly if possible which would allow for a significant reduction in installed battery capacity and possibly allow for the use of other battery technologies that can supply heavier amps at higher
efficiency but for shorter periods.

this leaves those loads that are very light relatively speaking but longer in duration, such as lighting or computers, tv and such. where a smaller inverter that is pure sine wave, higher efficiency and coupled to a high efficiency battery technology could be shown to be much more manageable and less expensive perhaps in both first cost and amortization.

some of this might change if there are other energy sources coupled to the system such as solar, hydro or wind power.

i too am considering the possibility of hydro using two ponds, because i have the elevation and the area to do so.
as far as i can tell a good pond based hydro might be 50% to as much as 60% efficient overall, which on the surface
does not seem like such a wonderful option until you consider that the system is much longer lived than a battery
and one cannot overcharge a pond or ruin it by running it down too low and leaving it there.

also if i were to power the hydro system via wind or solar, where i am not burning fuel to elevate water, maybe it makes sense?

in any event i think it might be interesting, even if it only provided enough power to cover lighting and maybe a puter or TV in the evening
hours.

even then it would probably be most efficient as a dump load, for the excess charging capacity of a wind gen or a solar array where either would be more efficient charging a battery bank.

so many options, so little time 

bob g

[mobile_bob]

another point that i have written about a few times in the past

the prime mover is anywhere from 2 to 3 times better at making heat than it is in make electricity, so

perhaps we need to start viewing the unit as primarily a heat generator, with electricity as a waste product we
are trying to harvest and use effectively.

thankfully the heat generated does not require very expensive, heavy to transport, finicky to charge, and in some cases
dangerous batteries to store.  also the means by which we store it will dictate the efficiency of the storage thermal battery.

so i think we make every effort to harness/harvest the heat generated, store and use it efficiently and the waste electricity is
the used as a byproduct when it is available, and we can store a reasonable amount of it if necessary fairly efficiently.

i think where we get into trouble with cogen is looking at the unit as primarily an electrical generator, and then think about it
almost as an afterthought as a source of heat.

it makes about as much sense as a furnace that might be 25% efficient at making heat, that also produces 3 times that as electricity
as a waste byproduct, with us not taking that resource seriously.  we would be so fixated on increasing the furnace heating efficiency
from 25% to 26 or 27% all the while neglecting what it is really good at.

i cannot see any argument against the fact that an engine driven genset is much more effective as a furnace or boiler than it is at making electrical power.

in my thinking it makes far more sense to optimize what a machine is good at (making heat), and then worry about how to harvest and use its waste product (electricity)in an efficient and effective manner.

when one realigns his thinking in this direction the ability to have an overall efficiency in the mid to upper 80's in efficiency can be a reality, its very hard to get even half that efficiency using the production of electricity as the focus, save for large scale power plant in the megawatt class.

my opinion only, ymmv

bob g

[elnav]

Bob  I see  where you are going with this but what about southern locations where heat is undesirable especially during a summer heat wave.   

[mobile_bob]

Elnav:

i of all folks don't have all the answers! 

i suspect that there ought to be ways of using the excess heat capacity to drive absorption chillers for air conditioning loads,
we know the prime mover can certainly drive a compressor, so maintaining load on the prime mover and by extension maintaining
heat output and efficiency would be easy enough to do.

this leaves someone wide open to explore a batch fired absorption system, which i believe would be not only the simplest but most
efficient.

basically the same type of storage system, with the same thermal concerns could be used, that is used by the northern counterparts
instead of banking thermal energy as heat, one could bank thermal energy as cold.

in either event, we might not look at the system as being the sole provider of btu's and power, either heat or cold, but rather what it does produce is produced using the fuel in the most efficient manner possible (within and acceptable standard) and that which is consistent with
clean emissions.

there are many fairly simple ways of doing the thermal cooling thing on a micro/residential size system, cooling towers can be very effective
in drier climates such as the desert southwest, rock salt makes an excellent absorption system used with ammonia in a batch fire system, where
we have adequate heat to drive off the ammonia and where the exhaust heat does not rise high enough to melt the salt.

in such an application, perhaps a standard electrical powered airconditioning system could be used to do the bulk cooling in a purpose built house
that is highly insulated and high efficiency, to get the temps knocked down, then the absorption system could then take over to maintain the
temp level over several hours, until the next programmed start cycle repeats.

i am convinced it could be done with very high overall efficiency, provided sizing is correct, the house is highly efficient and super insulated
and the occupants make a few minor alterations in lifestyle by allowing the control system to make the decisions on when and how the loads
are supplied.

certainly it might take a bit of getting used to, but if the payoff is sufficient i suspect most folks would adapt very quickly.

i know i would!

bob g

[quinnf]

Bob,

I think we're all with you re: the concept of using waste heat to drive an absorption chiller.  Problem is, those systems seem to be found in two sizes:  

1.)  Norcold  propane refrigerators for RVs (i.e. tiny) and,  

2.)  Humongous (a technical term).  

Everything in between seems to run off a rotary compressor running some version of highly-halogenated hydrocarbon.

Like you, I'm intrigued with the possibilities, and though I've worked on several refrigeration systems, and built a few from parts, absorption technology is something I haven't played with.  Then, too, the thought of dealing with high pressure gaseous ammonia rates a Pucker Factor of about a 9.2 out of a possible 10.0  Not that that would necessarily stop me.

Hope maybe someone here has more to say about the subject.

Quinn

[elnav]

Come to think about it I do know of one installation that uses  heat directly as a power source.  Ther is a radio tower  not too far away that burns propane  to power the radio equipment. The burner flame and exhaust heat passes  through a thermo couple bank that generates less than  one kilowatt  of electrical ower. Two huge propane cylinders  are filled up in fall before the winter snows make the logging road imppassible.  The propane burner provides  heat and power and I guess sone light inside the cinderblock building with no windows.
I can't recall if it was Kenworth or Freightliner that announced a thermo couple  power pack of 1 kW capacity  that replaced a portion of the exhaust stack.  At the time I was wondering why bother with  thermocouples when the engine is turning and producing power as well as waste heat. 
Stop the engine and you also lose the power from the thermo couple  oack.   The radio tower application makes more sense.

[mobile_bob]

i am with you Quinn, i hope someone tackles to the project before i jump on it down the road.

the system is not one i would want anywhere around a living space, but then again i plan on having
my cogen system in a remote purpose built outbuilding/bunker anyway.

in my opinion the likely reason we don't see much in the way of an appropriate sized system in more widespread use
is the fact that motor driven compressor units have been power historically by very cheap electricity.

aside from that i do know of several home size absorption chiller systems that are powered by natural gas, and are
a more or less continuous duty unit.

the principle is sound, and it is scalable, and would probably be more common if there was a bunch of suitably cheap
heat to drive such as system with.

having said all that, i don't see me getting involved with an experimental unit, it would have to wait until such time as
i move, get the place built and have everything else in place,  and at which time i find that i have sufficient excess heat
to warrant the possibility of reasonable success.

lots of conditions that must be met  

bob g