Marc and Crispin,
This is a useful document for assessing boiler/furnace performance,
especially when they are on all the time. Seasonal space heating
efficiencies need to take into account standby losses.
The masonry heater folks are grappling with this. Can we cap that
chimney in the off cycle? Thats frowned upon. What is the true cost of
that chimney? My daughter needs a new way to heat her home. There are
interuptable electricty rates which together with mass thermal storage
for off peak power rates allow for chimneyless space heating. (Still
need some air change is humans are involved) Competing combustion based
systems require venting where the effects of barometric draft dampers
and their kin are often left out of the sales literature.
Alex
Gleaned by google;
Heating Efficiency
__
High efficiency heating systems have become popular because they save
fuel and, one would suppose, save money. But what do they mean when they
talk about efficiency? What is furnace efficiency? Actually, there are
two ways to measure it -steady state and seasonal.
Steady State Efficiency
Steady state efficiency refers to how much usable heat iscreated when a
furnace is running. Conventional gas and oil furnaces have steady state
efficiencies of roughly eighty percent. When the furnace is on, .twenty
.percent ofthe heat that is generated goes up the chimney while the
remaining eightypercent is transferred through a component called a heat
exchanger and stays in the house. This isthe heat that travels through
the ductwork and ends up coming out of the registers in each room. But
even in the dead of winter, furnaces are not on all of the time. They
cycle on as the thermostat calls for heat, and off when the thermostat
is satisfied. During start-up and cool down, the furnace is not
operating as efficiently.Also, when the furnace is not on, heat from
inside the house escapes up the chimney flue the same way that heat
would escape from an unused fireplace, if the damper was left open.
Seasonal Efficiency
If you add these off-cycle losses to the steady state losses,you end up
with the seasonal efficiency. Seasonal efficiencies for conventional gas
and oil furnaces are about sixty to sixty-five percent.
Therefore, if you buy a thousand dollars worth of gas or oil over the
course of a winter and you have a conventional furnace, only about $600
to $650 worth ofthe fuel will be used to heat your house and rest will
be wasted.But what about mid and high efficiency furnaces? How efficient
are they, and what makes them so efficient?
Mid-efficiency Furnaces
Mid-efficiency furnaces have a seasonal efficiency of about eighty
percent. They achieve this by cutting off-cycle losses. Mid- efficiency
gas furnaces do not have a continuously running pilot. Itis shut off
when the furnace is idle. Also, to prevent heat from escaping up the
chimney when the furnace is not on, some manufacturers install a
motorized damper in the exhaust flue to close it during idle periods.
(Other manufacturers use other approaches to accomplish the same thing.)
*_High-efficiency Furnaces_*.
High efficiency furnaces employ similar techniques to reduce on-cycle
losses, but go further to improve the steady state efficiency. Instead
of having one heatexchanger, most have two or even three heat exchangers
to extract more heat from the burning fuel. High efficiency systems can
be 95 percent efficient.Because high efficiency systems are more
complex, they cost more -typically$1,000 to $1,500 more than a
conventional furnace. If you look at fuel costs only, the house that
cost $1,000 to heat with a conventional furnace should cost only $650 or
so to heat with a high efficiency system, yielding a savings of $350 or
so a year. In a few years, the system would have paid for itself.
Some models of high efficiency furnaces, however, have had reliability
problems and virtually all high efficiency systems require more costly
maintenance which eats into the savings. Therefore, if you are
considering a high efficiency system, on a retrofit basis, or
as an option on a new home, it is best to speak to a reliable heating
contractor to discuss .the pros and cons of various models and the
estimated increase in maintenance costs.
*Source : Dwelling View Engineers 2000*
On 15/10/2012 3:04 PM, Marc Pare wrote:
Sure thing, Crispin.
Crispin help me set up an interesting efficiency calculation the last
few weeks, and the results may be of general interest to the list.
I have been trying to quantify the efficiency of the direct combustion
of rice hull for heating applications. This should have some interest
in stove design since sometimes stoves are used for such applications
e.g. space heating.
The big question was how efficient is direct combustion of rice hull
when it is powered by natural draft?
The tricky part of answering this question is that the amount of
excess air (the amount of air pulled into the system that isn't used
for combustion) has a great effect on the overall efficiency. It's too
hard to measure the outlet velocity of chimney. Plus, you would need
an accurate measurement of fuel consumption. Finally, it's not really
feasible to measure the temperature of the products of combustion.
Luckily, you don't need any of these things. By leaning on the
chemistry of the combustion reaction, you can calculate the overall
"efficiency as a heater" with just a gas concentration reading (O2 or
CO2) and an outlet temperature.
In other words, with just a combustion analyzer, you can see how much
heat energy was available for your system. This is exactly the number
you need to compare between options for energy sources.
The details of the calculation with this method are in the attached
document -- BS845, a British boiler efficiency standard.
The results from direct combustion of rice hull are really
fascinating. I attached a plot of the measured efficiency.
To get most of the energy out of the fuel in the traditional process,
the combustion is done in phases.
Phase 1 (5-10 minutes): A pile of raw hull is dumped on an inclined
grate. It roars up into flame. Reaches > 1000 C
Phase 2 (10-20 minutes): The rice husk turns to char. It's still red
hot and still burning. This proceeds slowly and the temperature
gradually decreases to < 500 C
The char is scraped off of the grate. It hasn't combusted all the way
to ash.
What's interesting about this is that the efficiency of "Phase 2"
drops considerably. This isn't the case with other fuels I have seen
reports of. I plugged in some of the data from an assessment in India
of charcoal kilns, and even with similar excess air values, the
efficiency doesn't drop as low as rice hull.
What I think is responsible is that the particular chemical structure
of rice hull (high ash, a silica skeleton) acts as a brake on the
"char reactions" (I'm not sure of the technical term for this). You
have to strike a balance between getting the energy out of the rice
hull char and running at low efficiencies in your process.
I'm going to take a deeper look into this area some time soon.
Any others have thoughts to weigh in?
Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de Compiègne
my cv, etc. | http://notwandering.com
On Sun, Oct 14, 2012 at 11:19 PM, Crispin Pemberton-Pigott
<[email protected] <mailto:[email protected]>> wrote:
Dear Marc
Would you like to describe some of your recent work on using rice
hull to fire bricks in Vietnam?
It is an interesting application and he methods you used are of
general interest.
Thanks
Crispin stuck in Beijing
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
[email protected]
<mailto:[email protected]>
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web
site:
http://www.bioenergylists.org/
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
[email protected]
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web site:
http://www.bioenergylists.org/
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
[email protected]
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web site:
http://www.bioenergylists.org/
