Why is a heavy stove an inefficient stove? A tall combustion chamber makes a lot of draft to keep a fire roaring, how can that be a bad thing? What is TARP-V and how will it improve your stove? Dean Still has the answers for you in the latest Rocket Stove 2020 Video.
In 1976, Aprovecho was started in Guatemala after a terrible earthquake. The founders invented a stove called the Lorena as part of trying to help folks with their farms and forests. In 1981, Aprovecho came back to Oregon and bought 40 acres to learn how to maintain a forest, grow food, and make stoves. The hardware and software were OK but the wetware was not.
Sigmund Freud and Carl Jung broke up in 1913. Freud had picked Jung as his successor, but Jung made an unforgivable joke at dinner that ended their relationship. Freud was sure that the most powerful human motivation was infantile sexuality. Jung commented that eating must be at least equally important?
That was it!
Sure, stoves are interesting, but food is certainly more important. I lived on a ranch in Mexico for eight years and when the food ran out we weren’t lighting fires to cook. We were out farming, hunting, fishing, or gathering edibles from the desert.
Growing food has always been important at Aprovecho. So has maintaining the health and productivity of our forest/woodlot. Oregon is cold and too rainy in the winter, and warm and too dry in the summer, so we’ve had to learn how to adapt to this place and how to grow food productively.
We are lucky that our garden at Blue Mountain is bottom land and is easily accessible. We buy inexpensive compost made from food scraps by friends in Eugene and our neighbor, Butch, bought a rototiller for his tractor a couple of years ago that he uses, free of charge, to till our ½ acre garden. Butch also gives us unlimited manure from his cows, so friendship has made growing food a lot easier.
Being friends with the folks on our road increases our feeling of safety, and friendly neighbors can agree to growing a wide variety of foods from garden to garden. Being friends is important on lots of levels and when food is involved, breaking up, especially because of an ephemeral idea, is something to be carefully considered.
Maybe better to share a slice of that good pumpkin pie?
How can burning wood, agricultural waste or even cow dung be a carbon neutral energy source? How do you start a fire without making a lot of smoke? How can a metal skirt around a cooking pot help with fuel efficiency? Dean Still has the answers for you in this new video.
Find out more about the Jet-Flame combustion accessory used in this video at www.jet-flame.com.
David Evitt, ASAT COO, and Sam Bentson, ARC GM, have been adding capacity to the Laboratory Emissions Monitoring System (LEMS). So far, four oxygen sensors, temperature probes in the fire and under the pot, and a velocity sensor give us a clearer picture of what’s going on in a stove. Knowing PM2.5, CO, CO2, and firepower at the same time, combined with the improved testing protocols in ISO 19867, is making us more confident that iterative improvement can be accomplished relatively quickly.
Starting in early December, Dean Still and a research assistant will be doing 20 tests per week to create an optimized forced draft insert that cleans up the combustion of found biomass fuels and improves thermal efficiency in open fires, high mass, and Rocket stoves. A screen in the hood showing real-time data helps reduce the needed repetitions to achieve statistical confidence. The 90% confidence interval has to be less than 1/3 of the range of the Tier that contains the conservative bound of the confidence interval. When Tier Confidence Interval Range is equal to or less than 0.33, the number of tests is deemed sufficient to meet the Aprovecho data repeatability quality standard (seven to nine tests each for high, medium, and low power are usually sufficient).
David is in grad school with Dr. Nordica MacCarty at Oregon State University and the ARC lab is supplying them with information. We’ll keep you in the loop as we make discoveries. Part of the goal is to keep the optimized insert as close to a $10 wholesale price as possible.
Here we go! Eco-Science marching forward!
In this video, Dean Still explains why mixing air into flame is important for cleaner combustion. He uses several Rocket Stoves to demonstrate the effects of both natural draft and forced draft secondary air jets. Which style is more effective? Watch to find out!
For a simple way to add mixing to a Rocket Stove, check out the Jet-Flame.
An Important Health Intervention
When cooking stoves are tested in the field the emissions of PM2.5 and CO are often higher than lab results (Roden et al., 2009). The wood can be wetter, the fire is made with less attention, and many real life variables create higher levels of pollution. It’s hard to imagine that unvented cookstoves for indoor use can be invented that will protect health when too much wet fuel is pushed quickly into the combustion chamber. Even modern cars make a lot of smoke when trying to combust bad quality gasoline.
Clean burning stoves require clean fuel just like automobiles. The sticks of wood need to be relatively dry and the metering of the sticks into the combustion chamber cannot happen too quickly. Perhaps batch fed pellet stoves will have more similar lab and field results if the pellets are well made, dry, and clean?
It’s illegal to install most types of unvented combustion devices in the United States and Europe. Even natural gas room heaters and gas cooking stoves are vented. For realistic protection of health, ARC consultants try to attach chimneys to biomass cookstoves whenever possible. When the stove smokes at least the pollution goes outside above the roof line where it becomes diluted.
Health Supportive Alternatives
Adding a chimney is not always a possibility. In these cases, it is helpful to move cooking out of the closed kitchen, for example under a veranda in the open air. Increasing air exchange rates by cooking under a veranda has been shown to dramatically lower concentrations of harmful PM and CO. Even opening the door and window in a test kitchen lowered the particulate matter 1-hour concentrations between 93% to 98% compared to the closed kitchen, and the CO 1-hour concentrations were 83% to 95% lower (Grabow et al., 2013).
Hundreds of years ago in
Europe chimneys were developed as a first step to take smoke and gases outside
of the kitchen. In the United States millions of wood burning heating stoves
are used indoors every winter. Chimneys transport the pollution outdoors where
it is mixed with the outside air.
Roden, C. A., Bond, T. C., Conway, S., Osorto Pinel, A. B., MacCarty, N., & Still, D. (2009). Laboratory and field investigations of particulate and carbon monoxide emissions from traditional and improved cookstoves. Atmospheric Environment, 43(6), 1170–1181. https://doi.org/10.1016/j.atmosenv.2008.05.041
Grabow, K., Still, D., & Bentson, S. (2013). Test Kitchen studies of indoor air pollution from biomass cookstoves. Energy for Sustainable Development, 17(5), 458–462. https://doi.org/10.1016/j.esd.2013.05.003
Dean Still and Sam Bentson have started collaborating on a series of videos that explain the basics of how Rocket Stoves work, so that stove designers and stove users can get the best performance out of this popular stove design. In this first installment, “Time and Temperature,” Dean explains the importance of high combustion temperature in a Rocket stove where there is limited time to burn up smoke particles. He demonstrates how the Jet-Flame (www.jet-flame.com) helps to increase combustion temperature by blowing air under the fire.
Be sure and subscribe to Sam’s YouTube channel so you never miss an episode! New videos will be added every other week.
Aprovecho’s General Manager Sam Benston recently returned from a trip to Rwanda, where he helped to set up a new ISO compliant cookstove lab. Here are some photos and information from Sam about his work there:
I was installing the LEMS (Laboratory Emissions Monitoring System) and PEMS (Portable Emissions Monitoring System) and the rest of the new ISO 19867 cookstove laboratory at the Rwanda Standards Board in Kigali. The lab started as an empty room full of equipment in boxes. I trained the laboratory staff on the set-up and use of the equipment for cookstove evaluations according to ISO 19867. Shortly after I left there was a Grand Opening to celebrate on the ISO’s World Standards Day. Here is a twitter link with photos: https://twitter.com/REMA_Rwanda/. Our new PEMS with the battery powered gravimetric system is visible.
Aprovecho provides a turnkey cookstove testing laboratory which is useful for cookstove performance certification, design, and basic research. The lab is centered around the ARC manufactured LEMS. It consists of a gas and particle analyzer with a pump and filter PM2.5 sampler, an emissions collection hood, and a dilution tunnel. The LEMS is the result of 20 years of development that started due to the lack of affordable and easy to use equipment suitable for cookstove emissions monitoring.
Aprovecho develops its equipment as the need arises during research and development activities that occur in its laboratory. Aprovecho’s ability to commission the other instruments that makeup a cookstove testing laboratory is the result of a similar depth of experience.
It was remarkable to observe how the Rwandan people have protected themselves against COVID. It was a great honor to be part of their community at this time.
Here are the TLUD (Top-Lit Up Draft Stove) derived heat transfer principles that ARC designers use when designing and improving stoves. They are just as important for Rocket stoves as TLUDs:
T: The temperature of the hot gas contacting the pot or griddle should be as hot as possible.
A: Expose as much of the surface area of the pot or griddle to the hot gases as practical.
R: Increasing heat transfer by radiation is important. Move the zone of combustion as close to the surface to be heated without increasing harmful emissions.
P: Optimize the proximity of the hot gases to the pot or griddle by reducing the channel gap without reducing the velocity of the gases. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. Match the firepower to the channel gap size and to the size of the pot or griddle.
V: In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increase the velocity of the hot gas as it flows past the pot without reducing the temperature of the gases. As a rule of thumb, heat transfer efficiency can double when the velocity of the hot gases also doubles (N. MacCarty, et al, 2015).
There are three types of heat exchangers generally used to capture the heat produced in a combustion chamber.
The hot flue gases can:
- Heat mass, like heavy stone or masonry
- Heat water which then warms the house or…
- The easiest and least expensive route – make the hot stove gases efficiently heat the air inside the room
In modern houses with limited air exchange rates heating the air has become the popular option. High mass heat exchangers were created in the days of drafty houses when heating air was a losing proposition. Old houses had air exchange rates of more than 10 exchanges per hour. All the air in the house was replaced ten times or more every hour! It didn’t make sense to heat air that would quickly be outdoors.
Heat exchangers increase heat transfer to the room by making sure that the hot flue gases leaving through the chimney are as cool as possible. Even a smoldering fire turns about 90% of the wood into heat. But, heat transfer efficiency (heat delivered to the room) can be less than 20% in poorly designed systems. As the cartoon shows, a little improvement in heat transfer equals impressive increases in fuel efficiency.
Retaining Heat is Part of the Equation
We cook beans (and other long simmering foods) at Aprovecho using a “haybox.” The pot of food is boiled for ten minutes on a stove and then placed in a well-insulated, airtight box. The beans inside the pot get soft and palatable because the retained heat is sufficient to finish cooking them. We end up using a great deal less fuel because the haybox has improved the heat transfer into the pot. (It’s also a much easier cooking method!)
The reason that beans are usually simmered over a fire for a couple of hours is because the pot constantly loses heat to room air. The reduced flame underneath the pot replaces the lost heat.
In the same way, a furnace or a wood replaces the heat in our houses because the house allows the heat to constantly leak away. The house loses heat and the burning wood replaces it. If the house loses a lot of heat, we use a lot of wood per season. If the house loses less heat, we can save trees and are better stewards of this precious resource. If the house loses very little heat, the stove is frequently not even lit because energy in sunlight and interior sources of heat are now equal to the heating demand.
Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA