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Solar Roasting and Charcoal

First published May-2010
last update 5-April-2016

As mentioned in the previous chapter, in order to develop solar concentration it is good to identify applications particularly compatible with solar energy and economically feasible.

So, we are looking for activities:
- not dependent on the time of day
- can be performed in rural areas where space isn’t a problem
- where manual tracking would not be a significant inconvenience
- are technically simple to construct
- create new economic activity, that did not exist before, which can provide the motivation and resources to build and adopt solar concentration, and
- is not culturally complicated for people to adopt.

Two activities of prominent note that fulfil these criteria are roasting cocoa, peanuts, coffee, etc., and (bio)-charcoal production. Solar roasted cocoa beans have been developed in Oaxaca since 1995 and a solar peanut roaster has been commercialized in India, 2009.

Both roasting and charcoal are the transformation of a local material, produce such as beans for roasting and any organic matter for charcoal, which are not at all sensitive to the time of day; i.e. it’s not a problem to wait for a sunny day to roast and make charcoal. Both are of course intrinsically rural activities so there is plenty of space. Since the roaster must be watched over, beans stirred and swapped out for instance, adjusting the concentrator every 10-20 minutes is not a problem; though charcoal takes longer to produce than more roasts it can be done in large batches. Both require ovens at temperatures of around 300 C, so don’t require any special insulation or materials to construct. Roasting beans, such as cacao and coffee, and making charcoal are both economic activities that could be created almost all over the world, and where villages don’t currently have access to roasters are charcoal could represent a significant increase in income and quality of life (as the concentrators can also be used for other things, such as boiling water). Finally, local roasting and charcoal making doesn’t present any cultural factors.

To put this in perspective, an examples of an activities that do not are cooking and electricity. Cooking is very dependant on the time of day and presents large cultural barriers for people change habits, and may not create any new activity that didn’t exist before. Electricity production is not technically simple and it’s very inconvenient to have to adjust the concentrator, so automatic tracking is very desirable in these cases. Though these activities present no fundamental challenge, they will be easier to deploy on a large scale after more basic applications are in wide circulation.


Many produce must be roasted. When fuel must be paid for, any marginal gain in fuel efficiency often pays for itself, so it becomes economical to centralize all roasting in large machines. When fuel is essentially free, such as with solar, the benefit of radically reduced transportation and complexity costs of decentralization far outweigh lower fuel efficiencies.

Roasting produce is a typical example that effects most of the worlds rural areas. Bringing roasting into the community level allows the on location transformation of produce, and so the farmer can sell at a higher price. Thus the benefit of the technology is obvious and economical to adopt.

Since there is no boiling or evaporation of water, high efficiencies are not required and so horizontal reflection roasts extremely well.

Another important factor is that there is no strict time table during the day when roasting needs to be accomplished, and so the farmer can easily wait for the sun to roast. Furthermore, one’s presence is required to monitor and stir the beans so there is not a high inconvenience if needing to track the solar concentrator manually.

An added benefit is that there is no combustion in solar heating and so no pollutants, the roast is thus extremely pure. This added purity and sustainable growing could form the basis for Solar Roasted fair trade labels, which would further add value to solar roasted beans and help spread the technology globally.

Solar Charcoal

Making charcoal, from biomass, is perhaps the most important starting application for solar concentration at a global scale.

To make charcoal simply requires heating wood, or any other organic matter, in an air tight container; the moisture in the wood will evaporate as well as the esters (other organic compounds), leaving nearly pure carbon. Without moisture this carbon will burn far more efficiently and also be able to reach much higher temperatures than a wood fire. Burning charcoal inefficiently produces carbon monoxide, but various strategies exist to avoid this.

Burning wood in a normal fire produces plenty of toxic volatile bi-products, but also plenty of tar balls (which are semi burned particles of wood, giving smoke its grey colour, and causing a significant amount of health problems in the world); these tar balls are not only toxic for humans and nature, but can travel long distances and land on glaciers significantly contributing to their melting.

The esters that evaporate can be burned directly, but more interestingly can be condensed into organic tar. This tar can be used directly things from waterproofing roofs and boats to medicine, but can also be put back into the soil to promote humus growth.

The carbon a tree requires is derived from the air, so burning the carbon now in the charcoal does not disrupt the nutrient cycle in any fundamental way. We could even be tempted to call the process carbon neutral, but this term should be avoided since biomass energy is only carbon neutral in a diverse healthy ecosystem where the rate of burning is extremely small.

Charcoal replace wood

Since charcoal can burn at higher temperatures than the wood, this is the common way to heat ovens to high enough temperatures, in uses such as ceramics and pottery, which require ovens at around or above 1000 degrees Celsius. High temperature oven’s could produce ceramics, and of course could be also heated by solar (charcoal only being burned to boost the temperature or stabilize the temperature on a cloudy day.)

By powering an oven to produce charcoal with direct solar energy, all the wood previously burned as a fuel is immediately saved. In most circumstances half of the wood is burned to turn the rest into charcoal, so solar char will either double the production of charcoal or halve the consumption of wood.

A horizontal concentrator creates a focal point close to ground level. Any insulated oven can be used to make charcoal, or a mass oven which will also serve stock energy from one batch to the next.

Other uses of charcoal

There is another interesting use of charcoal, terra pretta, invented by ancient Amazonian tribes, that could increase the health of our ecosystems and arboculture.

Bio-charcoal and bio-tar can in fact form a basis for organic chemistry, only not toxic like their fossil fuels counterparts.

Despite the myriad uses, the catch in making charcoal traditionally is the cost of the energy required to transform the wood into charcoal. Though the charcoal remaining burns more efficiently, there is a net loss of energy due to the wood burned to make the charcoal. With solar energy this equation changes dramatically.


Charcoal Limit

A entirely charcoal based society is however not a global solution, as, despite even the use of solar to produce the charcoal, the worlds forest would quickly be destroyed. However, the development of this technique accomplishes three fundamental things.

First, it saves energy in itself and allows the decentralization of the charcoal making process.

Second, it is an easy vector to introduce solar concentrators into society. The goal is not that every village will start producing massive amounts of charcoal, but rather realize even less wood must be collected if the concentrators were used for other things, such as boiling water.

And third provide a basis for the production of non-petroleum based organic compounds, for instance resins (again the goal is not to recreate a ecologically sounding consumer society but develop the techniques used at a relatively small, decentralized scale for what is actually useful).

Written by Eerik Wissenz.

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