We've been trying to decide how cost-effective putting in a biodigestor might be. The first part of this calculation was how big we should make the biodigestor.
When we talked to Jorje, who is in charge of the current biodigestor, he informed us that they have much more dung available than we expected. If this is true, then a household-wide biogas system would be better than a community system. We have been told to stick with the community system because then people will be able to pay off the high initial cost.
Nevertheless, it is possible to design a household system, if the assumptions of how much manure is produced per household are true. If we also assume families aren't paying for water, aren't paying for this dung, would spend 1 hour maintaining the biodigestor every day, are spending 4 hours collecting firewood every day, and if they weren't collecting firewood would be earning minimum wage, the cost-analysis curve looks like this for biogas. (This is not selling the biogas, this is just in terms of opportunity costs).
In this scenario, the biogas pays itself off at about 600 days (2 years) if the family was only buying propane gas, and 800 days (less than 3 years) if the family was using only firewood as their source of fuel.
Another scenario is to have the family again supply its own dung and not pay for water, but instead decide to pay off the initial costs in five years by selling the dung. This ignores maintenance costs of the biodigestor and opportunity costs of earning minimum wage if not collecting firewood.
If this occurs, the family sells the gas for $.79 daily, and it's great deal for the families using propane (who are spending $1.08 daily), but doesn't do anything for the families using firewood ($.43 daily)).
Doubtless the actual scenario lies between these two scenarios, and their assumptions need to be combined.
However, this is just on a household level. We were told originally to design this for a community level.
We emailed Gunther, the engineer of the original biodigestor, this week. He was very helpful, and asked us a whole bunch of questions that basically outlined our entire project :). Two important things he brought up is that the village is drawing water from a single well, and there is probably not enough dung to expand the biodigestor beyond 8 m^3. This implies that our estimate for how much dung each household has access to is too high, and also that we can't make a bigger biodigestor because of water supply issues. Thus, we need to run all of the household simulations on the community biodigestor idea.
Additionally, Gunther pointed out that there will be lots of issues with a community biodigestor, because what if some families are contributing more than others, and how will this be organized? Beyond all of the calculations that need to take place regarding how much to sell the gas for and opportunity costs, we also need to design a plan for how this will work with the actual users. Much remains to be done here.
As for what we have accomplished, last class we spent much of the time collecting assumptions for the true cost of the biodigestor. This is why we now have estimates for how much water costs, maintenance costs, fertilizer costs (selling it is not worth it in this village), manure costs, opportunity costs, propane costs, etc. An interesting part of this project is that it takes a lot of time to collect each of these assumptions, and then the assumptions are definitely not guaranteed anyway. Plus, the actual analysis of all of these assumptions (via Excel-- or I actually think I'm going to move to Matlab for some of this) takes much more time than originally anticipated.
Next time, we hope to generate more graphs for a cumulative scenario, with different estimates of price and opportunity costs, as well as refine the scenarios we already have (there are almost always problems). It would also be nice to make progress on a community-wide system for how the biogas would work on a people-level.
Ciao!
Monica
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