Renewable Energy (RE) and Storage
Please scroll down to see these topics
1. Energy Demand + Supply
2. Integrated Renewable Energy Sources + RE Matrix
3. Energy Storage
4. 2000 watt Society
1. Energy Demand + Supply
The key to success in energy systems lies on the Demand side. There is a higher return on an investment in energy management on the demand side, than in looking for and developing new sources. Although it is much easier when designing a new building, there is ample opportunity in retrofitting existing buildings.
The largest user of energy in a household is heating and cooling.
- Total Residential Energy Consumption
We are addicted to energy demand, and the supply of energy causes a lot of misery. …so why do we continue doing this?
We want 24/7/365 instant access to huge demands for energy. Every pound of coal, gallon of oil requires either deep mining, or exploitation of locals to gain access and extract.
2. Integrated Renewables
Integrated Renewable Energy is the key to Community Microgrids (CMGs). There are over ten renewable resources available within 5 km of most communities. Each community’s energy demand and supply profile are unique, and unlike the massive main grid, each community’s energy solution can be tailored to fit their specific needs.
In order of relative priority this matrix shows some of these sources along with common evaluation criteria. This RE matrix is an example of one community’s selection of RE’s to explore in designing its energy infrastructure:
There are many energy storage options for RE systems. Before looking into storage, one must consider base vs variable demand and supply profiles along with carbon footprints, environmental issues, and the overall purpose for storage. In many cases, design changes can be made to minimize storage capacity, thereby reducing footprints + costs.
While solar and wind energies are intermittent; biomass/fuels, geothermal and gravity systems can provide continuous 24/7 power
3. Types of Energy Storage:
1. Batteries 2. Fuel Cells 3. Compressed Air (CAES) 4. Pumped water 5. Gravity
…under construction… please pardon our dust…
Integrated Renewable Sources
The key to using renewable resources is to have multiple local sources of this energy. Each location in the world, has access to ample renewable resources. Integrating them together into one energy system is the key to having a reliable and resilient local community microgrid.
Here are the ten top renewable resources available to most communities:
1. Bioenergy (Biomass, BioFuels, BioGas)
2. Solar Thermal (lo-temp)
3. Small Scale Wind
4.Geothermal (small scale)
5. under construction
There are at least eight proven renewable energy technologies. Some are appropriate for CMGs (community microgrids), some not.
- Biomass gasification
- Geothermal (shallow + deep)
- Solar thermal (lo-temp)
- Small + Micro Hydro
- BioGas + (some) BioFuels
- Solar PV
- Tidal + wave power
- Pumped storage
- Other (hydrogen, compressed air, etc)
Each of these renewable energy sources are available in different forms and times at most every location in the world. Some communities are blessed with an abundant supply of some and a lack of others. It is up to individual communities to observe and discover (Permaculture principle #1) each of these renewable energy sources.
Proximity – The boundaries within which each of these renewable energy sources are located is key to the long term strategic planning and holistic health of the local biosphere around each community. Having a large waterfall 50 miles away from the community is not considered ‘local’ by most people. Having rolling topography inside or adjacent to a community presents a readymade opportunity to use gravity (small hydro) and technolgies like pumped storage.
Let’s get started with one of the most efficient, easy to do, lowest cost renewable energies: Solar Hot Water (lo-temp)
- Solar Hot Water Heater
There is no other renewable energy technology that is more appropriate than solar (lo-temp) thermal, commonly called solar hot water (SHW) systems. Residential households use about 2000KWH a year or $350/year in a typical utility bill. This energy bill can easily be eliminated with a solar hot water heater. Unless the location receives no sunlight for substantial periods of time, it’s feasible everywhere. One of the BIG advantages of solar hot water is that only partial sunlight is required to make hot water, and one of the typical issues with SHW is that you get TOO much hot water – a very nice problem to have!
SHW systems are typically between 60-80% efficient in converting solar radiation (insolation) to useful (BTU) heat. The highest efficiency in large commercial power plants is 34% from a solar hi-temp thermal Stirling engine type.
There are two major types of solar HW systems: Direct or Indirect. Passive or Active. So you can have an indirect active system (most common) or a direct passive system such as a thermosiphon.
I recommend Direct semi-active systems = Drainback with a PV powered variable DC pump. Here’s a simple illustration:
In a drainback system, the water drains out of the collector when there’s no hot water – thereby preventing freezing.
I can highly recommend this book by Tom Lane. You can obtain it from the Builders Booksource here.
LCE: LoCost Energy
This is not your typical lo-cost energy: like turning off lights, changing thermostats or taking cold showers 🙂 LCE is about reducing your energy footprint DRAMATICALLY – from 11,500 watts/day to 2,000 watts.day (see the section below on the ‘2,000 watt Society’). It requires a large change in our human behavior and our expectations of having everything (30+ widgets) instant-on, on-demand, 24/7/365 over-the-top energy consumption.
LCE is mostly free (or very lo-cost) to do. But it’s painful to change habits. Here’s a way to get started:
- Observe, measure, inform your daily energy use behavior + patterns
- Learn about energy demand, load management, usage.
- Invest and use LED lights, thermal curtains, lo-cost HVAC.
- Install solar hot water WAY before solar PV
- Learn to live by the sun (current solar income).
This is the place is to start with CMGs (community microgrids). The key to energy independence is not just using local, renewables (RE), but HOW the energy is used in the home and office. Energy is all about supply + demand. This is the DEMAND side.
It is much easier to invest in appropriate technologies to reduce the demand for energy, than to scurry around and develop additional energy supplies.
Lo-Cost Energy (LCE) reduces energy costs by:
- using available FREE energy (such as gravity)
- re-discovering and adapting proven ancient principles,
- embracing leading-edge, appropriate high tech solutions.
on the demand side (not the supply side). It is much easier to invest in appropriate technologies to reduce the demand for energy, than to scurry around and develop additional energy supplies.
Example of savings potential with LCE:
If there were 20 people consuming 11,500 w/p/d the total annual energy supplies needed would be a total of 84 MW hours per year [11500 X 20 X 365]. If these same 20 people instead consumed 2,000 w/p/d it would be 15 MW hours per year [2000 X 20 X 365].
That’s a difference of ~70 MW hours. At 12cents per KWH this would be a difference of $8.3 million for 20 people!
Another way to look at this $8.3 million is that this is the equivalent of ‘avoided generation’ (supply of energy) needed to power up these 20 people. Or, that this is the amount of cash available for a community to invest in helping reduce its energy demand profile.
Getting started with LCE
So how to get started in reducing our average consumption by a whopping 82%? [11,500-2,000/11,500]? It won’t be easy! We need to retrain ourselves. We need to take incremental steps. Perhaps in increments of 20, 40, 60% reduction steps.
One of the most appropriate hi-technology devices to grace our lifestyles are LEDs (light emitting diodes). From the laboratory to the household was quite a long process (1950’s to now), but usable LEDs have finally arrived to the mass market. They are evolving quickly and the prices will drop over the coming years.
They use less than one tenth of the wattage of comparable CFL’s and last up to 10 times longer. LEDs are a no-brainer. It’s only a matter of pricing that prevents them from overtaking the entire lighting industry. Color is not an issue (contrary to popular belief) since they emulate ANY color desired. The first ‘white’ LEDs were really tri-colored LEDs made to look like white light. Now we have real white LEDs and the color issue is no longer. As with anything, change is painful. Some people still insist that LEDs give off a ‘blue’ type light – probably from their perception of LED car headlights. But truth be known, there is an infinite number of possibilities available with LED lights – from color to fixture shape to socket type to … whatever a designer or customer can dream up.
The reason why LEDs are such a perfect example of LCE at work is their ubiqutousness within the environmental area (no toxic gases or chemicals within the light source); super long life and super low energy.
My super-eco home uses 10 Ikea style ‘Dioder’ thin-film type LEDs for general lighting which draw less than 20 watts TOTAL. That’s less than 2watts per bulb!! It doesn’t get more lower than that (for now).
As you can see there are many different choices.
Sampling of LCE Database
- 1. ABS solar pre-heater
- 2. Greywater + Rainwater
- 3. Slow Sand Filter
- 4. LED lighting
- 5. DIY Solar PV + SHW
- 6. Solar Space Heater
- 7. Solar Dehydrator
- 8. Solar Cooling
- 9. Pot-in-Pot Cooler
- 10. Root Cellars
- 11. Rocket Stoves
- 12. Ridin’ the gradient
- 13. Thermal curtains
- 14. Thermal Chimney
- 15. Thermal mass
- 16. Earth Tubes
… and many more. Please email us (see ‘Stuff’ section)
There are workshops for:
- A. LCE Database overview
- B. Ridin’ the gradient
- C. Solar cooking
- D. Drainback Solar HW system
- E. Lo-Cost solar PV
- F. No-Cost Operation home
- G. Greywater + Rainwater
- H. LCE Electrical Systems
4. The 2,000 Watt Society
Could you live on 2,000 watts per day? …for all your energy needs? That’s what Switzerland is aiming for. In the USA we typically consume 11,500 watts per person per day. So that would be like cutting your energy calories by two-thirds!
The following is TOTALLY from a Swiss web site (unfortunately this link no longer works) It’s the best explanation around on how to live on 2,000 watts per day.
Worldwide, each person consumes an average of 2,000 watts of constant energy to support his or her lifestyle. The average Swiss consumed precisely that amount of energy in 1960. However, energy consumption has tripled since then and is now 6,300 watts (55,000 kWh per year). It is far lower in poorer developing countries. The vision of the “2,000-Watt Society” aspires to attain globally fair and sustainable development. To accomplish this, per capita consumption and CO2 emissions must be drastically reduced in industrialized nations. The citizens of Zurich enshrined the 2,000-Watt Society as a guiding concept in their municipal constitution. This ambitious goal calls for measures in all areas of life: technological advances and increases in efficiency, but also a rethinking of every individual’s consumption behavior.
One Ton of CO2
Another goal of the 2,000-Watt Society is the reduction of greenhouse-gas emissions because they accelerate climate change.Crude oil and gas must be replaced by renewable energy sources. Annual CO2 emissions per person should be lowered from 8tons to 1 ton.
Today the industrialized nations consume so much fossil energy that too little remains for the development of future generations and poorer countries.One of the 2,000-Watt Society’s objectives is that all regions of the world have the same sustainable development perspectives and can secure people›s basic needs: food,health, work, mobility and accommodations.
Energy-efficient houses can use up to 80% less energy in comparison with older buildings. Residents can also control consumption through their behavior with regard to heating and ventilation, as well as by preventing unnecessary standby power consumption. The size of the homes we choose is decisive. In the past 30 years, that has increased significantly. On average, the Swiss now use 470 sq. ft. of living space each, while in the U.S. the average is 720 sq. ft.
The combustion of fossil fuels in motors accounts for 35%. Americans drive 12,000miles by car each year. The Swiss, on the other hand, drive about 5,600 miles. In cities,cars become less important because of denser construction and available public transportation.
Worldwide, most electricity is generated in thermal power plants (e.g., coal-fired, gas fired or nuclear power plants). Only a quarter of the primary energy fed in becomes electricity; 75% is lost. Powering an electric car or heat pump with thermally generated electricity instead of electricity from renewable sources does nothing to counteract climate change.
Gray energy and scarce resources are hidden in everyday goods. As a rule of thumb,spending 1 U.S. dollar entails energy consumption of 1 kilowatt hour. But does quality of life always have to mean consumption?What we consume matters. Meat, especially beef, requires a great deal of land, water and energy. One pound of beef accounts for ten times more gray energy than a pound of pasta. Because of intermediate storage,processing and transportation costs, seasonal and regional foodstuffs are more sustainable than meat, fast food or convenience foods.
Every consumer good, every service and even every energy source carries an invisible“energy backpack”: the gray energy required for production, processing and transportation. What matters for the environment is not only the immediately used energy, but rather the entire input from source to final usage.
How Much CO2 Do We Produce?
0.3t –One year of train travel (daily commuting distance 15 miles)
1.4t –One year of car travel (5,600 miles/year, 15 miles/day)
3.6t –1 flight New York – Zurich – New York (7,500 miles)
1.0t –Electricity consumption in one year (7,000 kWh) with Switzerland’s consumption mix (incl. imported electricity)
0.15t –7,000 kWh of electricity from renewable sources
Rule of Thumb: Spending $1 entails energy consumption of 1 kilowatt hour.
Energy Up in Smoke – A diesel-powered vehicle utilizes only one-fifth of the energy originally obtained from the crude oil.
|What Can Your City Do?|
The vision of the 2,000-Watt Society has been a powerful tool for the city of Zurich. It simultaneously offers symbolic value and entails tangible measures. Committing to a clear policy concept is the key to enabling people and communities to act for their future. Zurich is moving toward sustainable living and every city can do the same!
Buy Electric Power from Renewable Sources
“Wind Farms in California” – High potential for renewable energy: wind farms in California. – Photo: Darrell Clarke
Coal- and oil-fueled power plants are strongly affecting our climate. Therefore voters in Zurich decided to gradually phase out fossil energy and shift toward renewable energy sources. Communities must change their consumption habits by choosing electricity from hydroelectric, wind, photovoltaic or biomass sources.
A holistic approach to urban planning allows cities to establish the core values of sustainable living. Dense developments and high-rise apartment buildings help keep daily distances short and buildings compact. Density encourages a reduced use of automobiles, especially when there is a good public transportation network and green spaces are incorporated into the city.
Strengthen Public Transportation
“Light Rail in Downtown Portland, Oregon” – Public transportation contributes to liveable cities. Photo: Darrell Clarke, light-rail.blogspot.com
Zurich is proud of its powerful network of urban railways and streetcar lines, reaching out into the suburbs and neighboring states. The United States is experiencing similar progress as the public transportation infrastructure in dense urban areas continues to become more efficient. Excellent public transportation offers a viable alternative to cars, thereby alleviating traffic and cutting down on greenhouse-gas emissions. This enhances the quality of life in the city.
|Guidelines for Sustainable Construction|
To confront the challenge of reducing energy consumption, Zurich has embraced the vision of the 2,000 Watt Society. Similar movements exist in the United States, such as the 2030 Challenge (architecture 2030… added by fred k), which aims to have all new buildings and major renovations be carbon neutral by 2030. Both visions share the same core message: To change the status quo in construction, it is important to introduce and enforce standards.
4. 2,000 Watt Society
- All new urban buildings at least comply with the Minergie standard
- 25 percent of conversions comply with the Minergie standard
- Lighting complies with the Minergie standard. Household and office appliances fit into the highest efficiency class.
- Renewable energy sources are tested in every construction project and pilot projects are supported systematically
- The city uses ecologically friendly construction materials which pose no health risks, thus providing for a healthy indoor environment in its buildings
- Sustainability is a criterion in architectural competitions and study contracts
- Buildings are managed according to ecological perspectives
- All new buildings and major renovations shall meet a fossil fuel consumption standard of 60% below the regional average
- At a minimum, an equal amount of existing building area shall be renovated annually to meet a fossil fuel consumption standard of 60% below the regional average
- The fossil fuel reduction standard for all new buildings and major renovations shall be increased to:
- 70% in 2015
- 80% in 2020
- 90% in 2025
- Carbon-neutral in 2030 (using no fossil fuel greenhouse gas- emitting energy to operate)
These targets may be accomplished by implementing innovative sustainable design strategies, generating on-site renewable power and/or purchasing renewable energy.
There is more information at http://www.theoildrum.com/node/5316/
Including this Sankey diagram of how the Swiss manage their energy flows:
……..under construction… please pardon our dust…
5. Biomass energy
There are some that believe the long-term energy source will be one large supply chain of bio-something. Bio-gas, bio-fuels, or bio-mass. Of these, Biomass is the big one for the Western USA. The sheer size of the forests along with the threat of mega-fires will result in much thinning and control of the forests. Instead of spewing wood smoke into the air, let’s gasify it, make energy and some money with it. Biomass gasification along with various biofuels + biogas are set to make a large impact on the renewable energy landscape.
Biomass gasification is the process that converts solids (such as wood chips + ag waste) to a synthetic, or producer gas. Here’s the wiki definition: http://en.wikipedia.org/wiki/Gasification
Some people believe that in the near future, bio (mass,fuel,gas) will be THE preferred renewable energy supply chain. The USA has millions of acres of forestland that are in dire need of thinning and harvesting to prevent forest fires. This is spurring a new industry to provide 2nd and 3rd generation biomass to energy plants in these forestland areas. This will also avoid the open slash pile burning and smoke, help reduce fire risks, and provide local jobs – all while creating renewable energy with minimal emissions and net carbon benefits.
stay tuned… more content to appear soon…
CHP (Combined Heat + Power) through Biomass
“Through biomass gasification, we can convert nearly any dry organic matter into a clean burning, carbon neutral fuel that can replace fossil fuel in most use cases. “
In many areas of the USA we have forest thinnings, agriculture waste and household garbage that, when properly processed can be readily used in biomass gasification, for local energy production AND LOCAL JOBS.
Biomass is already being used for many commercial activities in the USA. The most common is a simple furnace and boiler system which produces steam for tobacco curing, electricity generation, beer brewing, etc. Biomass is also used to provide direct heat for brick burning, lime burning and cement kilns. The advantage of using biomass is it’s local sourcing which avoids shortages due to poor fuel supply networks and fluctuating costs of the main grid.
CHP/Co-gen + Gasification
Co-generation is a mature industry to implies using the heat produced by an electrical generator as an energy by-product. CHP (combined heat + power) implies is identical to co-gen, except in its design intent: it is purposely used to generate heat and electricity (not as byproducts).
Most biomass operations are straight through boilers which burn biomass directly. The new biomass technologies promise a very different renewable energy source: gasification (through pyrolysis). These are fast becoming a potentially huge future renewable energy industry with minimal emissions and much higher efficiencies.
Most of electric producing biomass plants in the USA are large megawatt commercial/industrial operations by Big Ag and Big Lumber corporations. This section covers small and medium scaled biomass.
One of the best sources for information and products on small-scale biomass gasification is GEK = Gasifiers Experimenters Kit. Here’s some cut+paste from their web site…. http://www.gekgasifier.com/gasification-basics/
Gasification is the use of heat to transform solid biomass or other carbonaceous solids into a synthetic “natural gas like” flammable fuel. Through gasification, we can convert nearly any dry organic matter into a clean burning, carbon neutral fuel that can replace fossil fuel in most use cases. Whether starting with wood chips or walnut shells, construction debris or agricultural waste, gasification will transform common “waste” into a flexible gaseous fuel you can use to run your internal combustion engine, cooking stove, furnace or flamethrower.
Did you know that over one million vehicles in Europe ran onboard gasifiers during WWII to make fuel from wood and charcoal, as gasoline and diesel were rationed or otherwise unavailable? Long before there was biodiesel and ethanol, we actually succeeded in a large-scale, alternative fuels redeployment– and one which curiously used only cellulosic biomass, not the oil and sugar based biofuel sources which famously compete with food.
This redeployment was made possible by the gasification of waste biomass, using simple gasifiers about as complex as a traditional wood stove. These small-scale gasifiers are easily reproduced (and improved) today by DIY enthusiasts using simple hammer and wrench technology. The goal of this GEK is to show you how to do it, while upgrading the engineering and deployment solutions to something befitting the digital age.
What is biomass? (from http://www.appropedia.org/Biomass )
Biomass is the term used to describe all the organic matter, produced by photosynthesis that exists on the earth’s surface. The source of all energy in biomass is the sun, the biomass acting as a kind of chemical energy store. Biomass is constantly undergoing a complex series of physical and chemical transformations and being regenerated while giving off energy in the form of heat to the atmosphere. To make use of biomass for our own energy needs we can simply tap into this energy source, in its simplest form we know, this is a basic open fire used to provide heat for cooking, warming water or warming the air in our home. More sophisticated technologies exist for extracting this energy and converting it into useful heat or power in an efficient way.
The exploitation of energy from biomass has played a key role in the evolution of mankind. Until relatively recently it was the only form of energy which was usefully exploited by humans and is still the main source of energy for more than half the world’s population for domestic energy needs.
Traditionally the extraction of energy from biomass is split into 3 distinct categories:
• Solid biomass – the use of trees, crop residues, animal and human waste (although not strictly a solid biomass source, it is often included in this category for the sake of convenience), household or industrial residues for direct combustion to provide heat. Often the solid biomass will undergo physical processing such as cutting, chipping, briquetting, etc. but retains its solid form.
• Biogas – biogas is obtained by anaerobically (in an air free environment) digesting organic material to produce a combustible gas known as methane. Animal waste and municipal waste are two common feedstocks for anaerobic digestion.
• Liquid Biofuels – are obtained by subjecting organic materials to one of various chemical or physical processes to produce a usable, combustible, liquid fuel. Biofuels such as vegetable oils or ethanol are often processed from industrial or commercial residues such as bagasse (sugarcane residue remaining after the sugar is extracted) or from energy crops grown specifically for this purpose. Biofuels are often used in place of petroleum derived liquid fuels.
Biomass energy and the environment
Concern for the environment was one of the major inspirations for early research and development work on improved stoves. One of the greatest paradoxes of this work is that, the more that is learnt about people, fuel and cooking, the more it is realized how little was understood about the environment and the implications concerning domestic energy use.
Initially, one environmental concern dominated the improved stoves work – saving trees. Today, this issue is considerably downplayed as time has brought a clearer understanding of the true causes of deforestation. At the same time, other environmental issues have become dominant – the household environment with its smoke, heat, lighting requirements, etc. has been given greater consideration. These micro environmental needs are often as complex as the broader environmental concerns and this is reflected in the fact that no one improved stove design can meet the needs of a wide and diverse range of peoples.
Large-scale combustion of biomass is only environmentally feasible if carried out on a sustainable basis. For obvious reasons continual large-scale exploitation of biomass resources without care for its replacement and regeneration will cause environmental damage and also jeopordize the fuel source itself.
As mentioned earlier, natural biomass resources vary in type and content, depending on geographical location. For convenience sake, we can split the world’s biomass producing areas into three distinct geographical regions:
• Temperate regions – produce wood, crop residues, such as straw and vegetable leaves, and human and animal wastes. In Europe short rotation coppicing (SRC) has become popular as a means for supplying woodfuel for energy production on a sustainable basis. Fast growing wood species, such as willow are cut every two to three years and the wood chipped to provide a boiler fuel. In the UK there is a functioning 12.6 Megawatt power plant which burns poultry litter (which has relatively low moisture content) as fuel and others which burn wheat straw. There are also many non-woody crops which can be grown for production of biofuels and biogas, and investigation of energy crops for direct combustion is underway. In western countries, where large quantities of municipal waste are generated, this is often processed to provide useful energy either from incineration or through recovery of methane gas from landfill sites.
• Arid and semi-arid regions – produce very little excess vegetation for fuel. People living in these areas are often the most affected by desertification and often have difficulty finding sufficient woodfuel.
• Humid Tropical regions – produce abundant wood supplies, crop residues, animal and human waste, commercial, industrial and agro- and food-processing residues. Rice husks, cotton husks and groundnut shells are all widely used to provide process heat for power generation, particularly. Sugarcane bagasse is processed to provide ethanol as well as being burned directly; and many plants, such as sunflower and oil-palm are processed to provide oil for combustion. Many of the world’s poorer countries are found in these regions and hence there is a high incidence of domestic biomass use. Tropical areas are currently the most seriously affected by deforestation, logging and land clearance for agriculture.
Biomass is available in varying quantities throughout the developing world – from densely forested areas in the temperate and tropical regions of the world, to sparsely vegetated arid regions where collecting wood fuel for household needs is a time consuming and arduous task.
In recent decades, with the threat of global deforestation, much focus has been given to the efficient use of biomass (as well as introducing alternative fuels) in areas where woodfuel is in particular shortage. Although domestic fuelwood users suffer greatly from the effects of deforestation, the main cause of deforestation is clearing of land for agricultural use and for commercial timber or fuel-wood use.
For solid biomass to be converted into useful heat energy it has to undergo combustion. Although there are many different combustion technologies available, the principle of biomass combustion is essentially the same for each. There are three main stages to the combustion process:
Drying – all biomass contains moisture, and this moisture has to be driven off before combustion proper can take place. The heat for drying is supplied by radiation from flames and from the stored heat in the body of the stove or furnace.
Pyrolysis – the dry biomass is heated and when the temperature reaches between 200o and 350oC the volatile gases are released. These gases mix with oxygen and burn producing a yellow flame. This process is self-sustaining as the heat from the burning gases is used to dry the fresh fuel and release further volatile gases. Oxygen has to be provided to sustain this part of the combustion process. When all the volatiles have been burnt off, charcoal remains.
Oxidation – at about 800oC the charcoal oxidises or burns. Again oxygen is required, both at the fire bed for the oxidation of the carbon and, secondly, above the fire bed where it mixes with carbon monoxide to form carbon dioxide which is given off to the atmosphere.
It is worth bearing in mind that all the above stages can be occurring within a fire at the same time, although at low temperatures the first stage only will be underway and later, when all the volatiles have been burned off and no fresh fuel added, only the final stage will be taking place. Combustion efficiency varies depending on many factors; fuel, moisture content and calorific value of fuel, etc.
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