Saturday, April 30, 2016

Photo Essay of the 2016 Pellet Stove Design Challenge

Patricia Fritz of the NY Department of Health and
Dr. Barbara Panessa-Warren, a nano particle expert
from Brookhaven.  Their panel was one of the most appreciated!
Dave Atkins, moderator of the wood stove retrofit panel,
introducing Jeff Hollowell, a retrofit builder.

Stove installation and set-up prior to the event. (Norbert Senf)
Marius Wöhler came from Germany to share experiences of BeReal, a
European round robin testing project and a multi-year European
 Union funded survey of how people actually use their stoves at home. (AGH) 
Geoffrey Johnson and the Torrefire Pellet Stove. (BNL)
SUNY Buffalo students (from left: Kevan Darmawan, Kyle Hinman,
and Steven Widdis) using the Testo 320. (BNL)
The VibraStove, invented and
 designed by Stephen Spevak. 
A drawing of what the PELLWOOD, first place winner, by Wittus,
may look like when it goes to market. (Wittus)

Geoffrey Johnson, inventor of the Torrefire stove, with Jytte
 Illerup, a Danish researcher and Ricardo Caravahlo, a
Portugese Ph.D. student from Denmark. (AGH)

Mark Knaebe, from the US Forest Service and John Crouch, from
HPBA at the automated cord wood stove panel. Ben Myren
 was presenting the VcV valve technology via telephone. (AGH) 
The Alliance for Green Heat team: Board members Dave Atkins,
Jonathan Kays, and Norbert Senf with AGH President John
Ackerly in blue and staff member Gabriella McConnel. (AGH)
René Bindig, a member of the first
place team,Wittus - Fire by Design.
Craig McKim of Testo,  discussing
  testing procedures with the SUNY Buffalo team. (AGH)
Tom Butcher and Rebecca Trojanowski, biomass testing
 experts from Brookhaven National Laboratory. (AGH)
John Ackerly and Gabriella McConnel, of the Alliance for
 Green Heat, presenting two of the commercial
demonstration stoves. (Norbert Senf)

Bill Clark of the Osprey Foundation (right) shows off the
Mimi Moto, an ultra clean pellet fired cook stove
that he distributes in Africa. (AGH)
Norbert Senf of the Masonry Heater
Association presenting findings on PM
 repeatability testing. (AGH)
Judges meeting before the Closing Ceremony.  From the left:
 Rebecca Trojanowski, Mark Knaebe, Tom Butcher, Ray
Albrecht, Ellen Burkhard and Phil Hopke. (Norbert Senf) 
Stephen Spevak, inventor of the VibraStove, explaining
 his design to the students of SUNY Buffalo.  (AGH)
Team Wittus - Fire by Design, seconds after hearing
the news that they received first place. (AGH)
Rebecca Trojanowski, Craig McKim, Geoffrey Johnson,
and Mark Knaebe admiring the testo model capable of
measuring PM. (Norbert Senf)
Scott Williamson,,
touching on the best and worst of
 innovation in the pellet stove market. (AGH)
Jock Gill, representing Jerry Whitfield,
speaking about the history of pellet stoves
and the potential of biochar. (AGH)
Dr. Joseph Mollendorf, advisor to the SUNY Buffalo team,
 speaking on the automation and controls
 his students are working on. (Norbert Senf)
Adam Baumgart-Getz of EPA with Geoffrey
Johnson and the Torrefire stove, which
 burns torrefied wood pellets. (AGH)
Second and first place teams congratulating each other
(From left: Vance Hirst Sr., Vance Hirst Jr., and
Vance Hirst III of Team Seraph and Niels Wittus and
 René Bindig of Team Wittus - Fire by Design) (AGH)
Alliance for Green Heat staff (right) congratulating Niels Wittus and
 René Bindig, who won first place at the 2016 Pellet Stove Design Challenge.

Tuesday, April 12, 2016

Wittus and Seraph win Pellet Stove Design Challenge

Rene Bindig and Niels Wittus,
designers of the Pellwood stove.
A German designed Wittus stove that is distributed by a New York company, and a stove made by Seraph Industries, the smallest U.S. pellet stove manufacture, won first and second place in the 2016 Pellet Stove Design Challenge. 

This was the third Stove Design Challenge promoting innovation in wood and pellet heating to help consumers reduce fossil heating fuels with appliances that burn far cleaner and more efficiently than average stoves.

The Wittus Pellwood stove is an extremely innovative prototype that can burn both pellets and cordwood, bringing advanced technology from basement furnaces up into the living room to achieve very low emissions of less than half a gram per hour.  The second place stove, Seraph’s Phoenix F25i, is nearly ready for certification testing.  It also achieved a very clean burn, consistently under 1 g/hr. and has innovative features to help and encourage the consumer to keep the stove operating well.

The Seraph team with AGH President
John Ackerly (right).
Other stoves featured extremely innovative designs, including the futuristic looking, radiant heat Torrefire stove with a glass burn pot. In addition, the gravity fed Vibrastove, with a burn plate instead of pot, used only one small fan and made its own electricity for off-grid use.

The Department of Energy’s Brookhaven National Lab hosted the event. The Lab conducted extensive testing of the competition stoves and will provide valuable data for the EPA, industry and other stakeholders about the strengths and weaknesses of testing protocols.  Each stove was tested three times, to see if the stove operated consistently or whether the testing protocol may lead to variable results.
The Torrefire pellet stove.
“Designing a very affordable, high performing pellet stove should not be rocket science,” said Dr. Tom Butcher, Head of the Energy Resources Division at Brookhaven National Laboratory. “But in some ways its harder than rocket science because solid fuel combustion is extremely complicated to design for and test,” he said.  “What makes this competition great is the new ideas from the competing teams and the spirit of collaboration.”

Pellet stoves are widely seen as a modern, cleaner, and a user-friendlier alternative to cord wood stoves.  More states and programs are starting to give larger rebates and incentives for pellet stoves than cord wood stoves, and are beginning to focus on the stricter emission standards that will take effect in 2020.  This Design Challenge showed that the 2020 standards for particulate matter would not be difficult for pellet stoves to attain, but that many pellet stoves have mediocre efficiencies.

Steve Spevak, designer of the
Vibrastove and Dr. Tom Butcher
(right) during testing.
The Pellet Stove Design Challenge is a partnership between various organizations and agencies that are interested in exploring the potential of technology to meet a growing demand for renewable energy.  The principal funder,  the New York State Energy Research and Development Authority (NYSERDA), runs Renewable Heat New York, a multi-layered incentive program for pellet heating equipment at the residential, commercial and industrial scale.  Other partners include the United States Forest Service, Brookhaven National Lab and state agencies from Massachusetts and Washington, along with leading experts from Clarkson University, the Masonry Heater Association and the Osprey Foundation.

The Design Challenge brought nearly a hundred students, stove builders, backyard inventors,  academics, regulators and experts together to discuss and debate the state of the pellet stove technology, indoor and outdoor air quality issues and deployment strategies.  Of particular note were three university teams that are designing stoves from engineering departments at SUNY Buffalo, SUNY Stony Brook and the University of Maryland.  The speakers included Adam Baumgart-Getz from the EPA, Marius Wohler from the European BeReal initiative, nanoparticle expert Dr. Barbara Panessa-Warren and scores of others.  Presentation abstracts are available along with most of the powerpoint presentations.
Marius Wohler, one of the European
presenters, describing the BeReal survey
and testing, leading to new testing
protocols in Europe.

 The event coordinator, the Alliance for Green Heat, is exploring a return to advanced cord wood stove technology and using the National Mall in Washington DC again as a venue in 2017.  Stakeholders are invited to contact with input about the next Design Challenge.
- - -

The Alliance for Green Heat promotes modern wood and pellet heating systems as a low-carbon, sustainable and affordable energy solution. The Alliance works to advance stove innovation through technology competitions and advises state and federal agencies on improving programs that involve wood and pellet heating. Founded in 2009, the Alliance is an independent non-profit organization based in Takoma Park Maryland.

Friday, April 1, 2016

Efficiency of Popular Pellet Stoves

This is an except of a much longer, and more technical paper by Prof. Gael Ulrich -“BioCombustion Institute Bulletin #3.” Prof. Ulrich calculated the efficiency of six popular pellet stoves, finding a wide difference.  The highest, the Italian made Piazzetta Sabrina was 76% efficient and the lowest was the Enviro M55 Insert at 51% efficient.  In between were the Ravelli RV80 (62%), Englander PDCV55 (63%), Quadrafire Mt Vernon AE (64%) and Harman Accentra 52i (71%).

He did this by using performance data produced by the Alliance for Green Heat, who tested these 6 stoves over a 30-day period.  The Alliance operated the stoves, often for 24 hours a day, testing them almost every day at various heat output settings and averaging the results. All the stoves were purchased new, without the knowledge of the manufacturers and operated with the same PFI certified pellets.  The Alliance produced an in-depth report about the findings, but we did not report the efficiency values because the instrument we used was a Testo 320, which produces a proprietary European (LHV) number, not the kind of efficiency values that are used and reported in North America. 

Gael’s full paper can be downloaded as a PDF here, which is quite technical.  We reproduced the less technical parts which are accessible to a wider audience. 

One conclusion is that many pellet stoves lack a very simple solution to increasing their efficiencies – larger heat exchangers.  Gael found that “All [the stoves], except the Enviro and Quadrafire, appear capable of adding another 5 to 10 percentage points by increasing heat exchange area to reduce the flue gas temperature.”  This solution may only add $100 - $200 to the price of a stove but would save consumers far more in fuel costs. 

One thing is clear: more expensive stoves do not necessarily provide consumers with higher efficiency. The Englander is sold by big box hardware stoves for $1,100, and is on par or better in efficiency than stoves that sell for $3,000 or $4,000.  This is significant because the big pellet stove manufactures do not release the actual efficiency of their stoves to consumers and consumers have virtually no way to tell which models are lower or higher efficiency.  The EPA contributed to a myth that pellet stoves have high efficiencies by giving them a default efficiency of 78%.  Emerging data shows the average pellet stove is likely around 70% efficiency, but many big name brands make pellet stoves that have efficiencies in 50s and 60s. This analysis begins to dismantle the lack of transparency in efficiency values that manufacturers have tried to maintain for many years.

Biomass Combustor Efficiency
BioCombustion Institute Bulletin #3

(Gael Ulrich: 16 March 2016)


Gael Ulrich was a professor of
Chemical Engineering at the
University of New Hampshire
If flue gas temperature and composition are known, one can calculate the efficiency of a biomass combustor using the so-call "stack loss" technique.  This paper explains in detail why that is possible and how to do it.  Fortuitously, during the preparation of this bulletin, the Alliance for Green Heat published data from their testing of six pellet stoves this past September.[1] Test equipment used in the AGH study delivered composition, temperature, and efficiency numbers.  Investigators declined to report the efficiency numbers for various reasons, although they do mention a range of 60 to 75%. 

Using the AGH temperature and concentration data, I made independent calculations as described in detail herein.  I find one of the six stoves operating at 51% efficiency, three in the low 60s, and the remaining two operating at 71 and 76%.  I also conclude from my analysis that some of these units use "dilution as the solution to pollution."  If we consider actual emissions in grams per hour or milligrams per MegaJoule of heat delivered instead of parts per million in flue gas, the rating is rearranged with one stove deemed second dirtiest becoming the cleanest and that ranked third cleanest becoming the dirtiest.  Factors that influence efficiency and cleanliness and how to improve these important performance properties are also discussed herein.  


As pointed out in BCI Bulletins #1 (Units) and #2 (Emissions), biomass is intrinsically a clean fuel composed primarily of carbon, hydrogen, oxygen, and ash.  If burned properly, with ash un-entrained, flue gases from biomass can be as clean as those from natural gas and perhaps even cleaner than from oil.  The problem, of course, is that biomass is neither a liquid nor a gas like these fossil fuels.  Burning a solid cleanly and efficiently is much more difficult.  Bulletin #2 dealt with cleanliness and the standards expected.  This one focuses on efficiency and how it can be measured for a biomass burner.

Instruments and software are available to deliver efficiency ratings and other data even to an ignorant user with enough money to buy them. But to use these tools intelligently, one must know how they function and should be able to calculate efficiency separately and from scratch.  This bulletin describes how to do that. 
Efficiency is a concept that everyone understands, but different people often define it differently.  Let's solve that problem first.  For simplicity, visualize a biomass combustor as a black box with fuel and air flowing in; flue gases and ash flowing out.[2] ... As defined by logic, efficiency is the ratio of useful heat released to fuel energy provided.  Fuel Energy is the Higher Heating Value,[3] a quantity that has been carefully measured over the last couple centuries by scientists for all common fuels. 

For highest efficiency,

1.  Burn with the least amount of excess air possible.
2.  Operate with the lowest feasible flue gas temperature.
3.  Use dry fuel.
Alliance for Green Heat Data
The AGH study ran for a period of 30 days.  Investigators found results that showed little drift with time.  Five of the six stoves operated with more than 200% excess air; beyond maxima considered in Figure 7.  One could derive additional curves for these high air rates just as was done for the lower percentages of excess air, but I chose to extrapolate instead, creating the dashed lines in Figure 9.

Efficiencies for the six AGH pellet stoves as read from Figure 9b are listed in Table 6. 

Table 6.  Calculated efficiencies of pellet stoves studied in the September 2015 Alliance for Green Heat test series.

Stove                O2         % X's Air           Flue Gas            Efficiency e                             
Brand                 Conc.    (Figure 8)           Temp. (oC)         (Figure 9b)
Enviro               18.7%       800%                        150                        51 %           
Ravelli              16.8%       400%                        195                        62 %           
Englander         16.0%       315%                        222                        63 %           
Quad                  17.4%       480%                        160                        64 %           
Harman             15.0%        245%                        205                        71 %
Piazzetta            13.5%       175%                        203                        76 %

One of the six operated at 51% efficiency, three in the low 60s, and the remaining two operated at 71 and 76%.  These numbers are consistent with the range mentioned in the AGH report. 

Piazzetta achieves superiority through low excess air rate. Enviro, at the other end of the spectrum, would have an even lower efficiency if its flue gas temperature were as high as the others.  All, except Enviro and Quad, appear capable of adding another 5 to 10 percentage points by increasing heat exchange area to reduce the flue gas temperature. 


What about Pollution?  The AGH data demonstrate an interesting application of using "dilution as a solution to pollution."[4]  The Harman emitted flue gases containing about 820 ppm CO while the Enviro emitted 534 ppm.  But the Harman operated with about 240% excess air; the Enviro with 800%.  And, the Harman was 22% more efficient. 

At the same pellet burning rate, the Enviro produces roughly 900/340 or 2.6 times as much flue gas as the Harmon, and its useful heat delivery rate is only 82 percent as great.  Thus, in terms of mass of CO per kJ of delivered heat, a better measure of actual pollution, the Enviro is (2.6/0.82)*(534/820) = 2.1 or about twice as bad as the Harmon.  Based on the data provided, I calculated mg of CO per MJ of useful heat delivered for the six pellet stoves.  Results are listed in Table 7. 

Table 7.  Calculated CO emissions of pellet stoves studied in the September 2015 Alliance for Green Heat test series.  

                                                                                                CO emissions    
 Stove                                                                     (mg/MJ of       (ppm)
Brand       % X's Air           Efficiency e        (ppm)    heat   normalized**
Enviro               800%                51%                  534       3000     850  (2.7)
Ravelli              400%                62%                  428       1100    365  (1.2)
Englander         320%                62%                  542       1200    387  (1.2)
Quad                  480%                64%                  318         930     318  (1.0)**
Harman              240%                71%                  821       1300     487  (1.5)
Piazzetta            170%                76%                  648         780     370  (1.2)
                  *Normalized to Quad as the reference.
                  **Normalized to Quad as the reference using Wikipedia formula. 

In terms of mass per unit of useful heat, the Enviro emits about four times as much CO as the Piazzetta (3000 versus 780 mg/MJ or roughly 130 versus 35 milligrams per hour).

What about non-steady-state?  My analysis assumes the appliance operates at steady state with feed rates and temperatures invariant with time.  This is valid for automatic-feed pellet stoves but not for wood stoves that are fed batch-wise.  There, the burn mode migrates from de-volatilization and combustion of light organics, gradually progressing to char or carbon burn-out.  Fortunately, stage changes are slow relative to combustion kinetics.  At any given time, the analysis described herein can be used to analyze the appliance at that instant.  To more accurately reflect the performance of a batch-fired wood burner, one must record data over a complete firing cycle and then integrate results to obtain an average.  This is further complicated by the fact that heat of combustion changes with time.  That for carbon, for instance (near burn-out), is about 30,000 kJ/kg.  Since the overall HHV for biomass is 20,000 kJ/kg, that for the volatiles must be lower than this.

What about moisture condensation? Mark Knaebe advocates improving efficiency by increasing heat exchange surface to the extent that water in the flue gas is condensed, adding its latent heat to the useful Q.  This requires dropping flue gas temperature below the dew point.  With low amounts of excess air, the dew point might be as high as 60 deg-C, but with 400% excess air, where many pellet stoves operate, the dew point is nearer 30 deg-C.  

As cleaner appliances develop, the prospect of taking advantage of this extra heat becomes more intriguing because the condensate will be purer and non-fouling.  The added heat transfer surface and increased capital cost, however, may not be practical.  

Ray Albrecht suggests that temperatures in the range of 1000oC or greater are needed to achieve good burnout of flue gases.  He stresses the importance of preserving flame temperature by insulating the combustion chamber to make sure reaction is complete before gases enter the heat exchanger.

Staging the air feed can promote gasification and partial combustion at low excess air conditions where temperatures are higher.  Preheat can almost deliver a one-to-one increase of flame temperature with increased feed air temperature.  Staging and preheat are common in newer biomass burners. 

Catalysts are another important way to promote oxidation at lower temperatures than those needed otherwise.  

[1]Alliance for Green Heat press release [Oct. 27, 2015]
[2]For simplicity, assume it is burning at "steady-state" where flow rates and temperatures are constant; not changing with time.  This is true of many pellet stoves and large-scale furnaces.  Small batch-fed systems do experience cycles and are more complex to analyze, but the steady-state analysis gives a useful result even for these systems.
[3]Unfortunately, fuel energy can be expressed in multiple ways, depending on how it was measured--giving different numbers for the same fuel.  As argued in BCI Bulletin #1, HHV or the Higher Heating Value is preferred and will be the only one considered here.
[4]A phrase attributed to the 20th century comic-strip character "Pogo."