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Two months ago I made a post asking about Compost Heat Recovery System designs that are compact enough for household usage.

Though I got no answers from that post, I managed to develop a prototype reactor which I think is showing promising results. Its volume is only 0.24 cubic meters (50cm * 40 cm * 120 cm). While the ambient temperature is around 26~29 degrees Celsius, the compost heap in the reactor stayed above 50 degrees Celsius for 14 days.

My goal is to integrate it with heating system of a house to replace natural gas and reduce the demand of electricity produced from fossil fuels.

The general setting is illustrated below: enter image description here

At least one of the reactors is at thermophilic stage to provide continuous heat source. The heat is pumped out from the reactor to the thermal reservoir by Stirling heat pumps. This reservoir is just a regular hot water tank with fortified thermal insulation.

This system will have far lesser supply chain issues since it completely runs on local resources(grass clippings, food scrap, etc). The compost, as a byproduct, can also reduce the need for chemical nitrogen fertilizer.

I'm now building Stirling heat pumps to test how much and how intense the prototype can heat up. According to this review article: A kilogram(dry weight) of compost can produce as high as 7084kJ in total.

So, do you think this system can reduce the amount of oil and/or gas used in heating?

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  • How do you define "damage from current energy crisis"? Damage to who or what?
    – THelper
    Jun 14 at 10:20

3 Answers 3

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Reduce yes. Reduce markedly, no, unless you live in a passive house.

A normal human eats about 12 megajoules per day. If waste food is at such a high level as 3 megajoules per day (most people on this site wanting to be sustainable probably produce less waste), that's about 0.8 kWh per day or about 300 kWh per year.

The house where I live requires 15 000 kWh per year to heat (southern Finland). So if I produced massive amounts of biodegradable waste (I don't), I might be able to cover perhaps 2% of the heating needs at most. Ok, I live alone in a large house but I don't think more than 4 persons could live in a house of this size, and if every one of those would produce 3 megajoules of waste per day, that still would be only 8%.

I don't think the idea is worth implementing. The heat gets lost in the noise.

A passive house would be different, on the other hand. A passive house uses less than 15 kWh / square meter (the standards are bit looser than 15 kWh/m2 in Finland but let's use 15 here) so a house of the size where I live would use 1500 kWh / year of heating energy at the most. If four persons live in a house of this size and everyone produces 3 MJ / day of waste, then the waste would cover 80% of heating energy needs.

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  • Thank you for reading and answering my question! You offered practical data on first hand. And I do think this system will be difficult to maintain by only one person. What I planned is to run at community scale, which includes major organic waste producers (Supermarkets, coffee shop, local parks, etc). The waste will be collected and delivered to nearby reactors on regular basis. This will also decrease the fuel consumption of garbage trucks.
    – gitpharm01
    Jun 17 at 4:08
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My answer is yes if this system is run at the size of a town/city and it could run as a rental service.

A medium-sized facility will be the center of this operation. First it collect organic wastes from major producer of this town.(Supermarkets, coffee shops, food factories, parks) enter image description here And these waste will be made into fresh compost piles in correct C:N ratio. Most of compost is put into reactors and sent to "subscribers" who needs heating: enter image description here This will reduce the space needed for storing the compost in the facility.

A compost pile can be "revived" or "boosted" by adding fresh material while turning the pile. This means the thermal output of a compost pile can be maintained at high level for a long time. I did a experiment on my prototype reactor and proved this. Here's the record I collected so far: enter image description here

A stands for steam temperature in upper area of the reactor.

B is the core temperature of the heap.

Average ambient temperature = 30 degrees Celsius

This experiment started at 2022.06.12(Day 0) and on Day 1 the core temperature already reached 64 degrees Celsius. I boosted it on day 5 and day 17, and you can see the core temperature remained above 50 degrees most of the time.

Another important result is that it is an efficient way to shrink the volume organic waste. Here's the record of volume change of the pile: enter image description here

Total volume of the reactor is considered 100%. From day 1 to day 5, it lost 25 percent. From day 6 to day 17, it lost 30 percent. From day 18 to day 23, it lost 20 percent.

It's 75 percent in total. This is quite normal since the raw material is only grass, which has high water content. The compost pile shrinks quickly due to dehydration. If food waste, which usually contains even more water, is added into the mix, the shrinking will be even greater.

If we process organic waste in a town in this way, a large amount of diesel/gas will be saved since the waste will not be carried by garbage trucks and travel to distant incineration plant. Instead, it stays locally and produces useful heat and fertilizer.

Moreover, the compost can be swiftly turned into a possible fuel source: black soldier fly oil. This insect grows fast( 14 days to get mature)

There are 12–34% percent of fat by weight in this insect's larvae( see reference 1).

They have no problem thriving on my finished grass-based(manure-free) compost pile as long as it's wet and shaded. After all, manure from herbivores is just a pile of well fermented grass.

BSF oil, as a heating fuel, can be sent to extremely cold areas where hot composting might fail and have limited electricity supply. It can also be converted into biodiesel for diesel engines.

This operation creates a series of valuable products which are illustrated in the following picture:

enter image description here

Advantages:

1.Reduce gasoline/diesel consumption of waste transportation and fuel for incineration.

2.Reduce natural gas usage in heating.

3.Reduce natural gas usage in chemical fertilizer synthesis.

4.Carbon fixing effect from compost usage and no dig farming.(reference 4,5)

5.Create a fast shortcut from waste to protein and fertilizer

6.From 1 to 5 combined. This system can help securing national food and energy safety by reducing imported resources from other countries)

7.Create new jobs


Potential income sources:

1.Sell compost / vermicompost

2.Provide rental service of heating

3.Charge for organic waste disposal

4.Sell BSF derived products(animal feed, fuel for heating)


Challenges:

1.No one ever tried this approach and it's unclear if it can generate enough income to sustain its operation. An article(reference 2) which proposes a medium-scale BSF based facility which looks quite promising.

2.Its operation is complicated and requires a lot of workforce to handle it:

a.Mini truck drivers to transport organic waste and compost

b.Professionals who can sort organic waste, turn them into good compost piles, and determine a compost pile is ripen or not.

c.BSF farmers

d.Engineers who can maintain equipment in the facility

e.Managers who take care of financial issues and supervise operations of the facility.

--------References---------

ref 1. Review of Black Soldier Fly (Hermetia illucens) as Animal Feed and Human Food Foods 2017, 6(10), 91; https://doi.org/10.3390/foods6100091

ref 2.Opportunities and constraints for medium-scale organic waste treatment with fly larvae composting. October 2015 Conference: 15th International Waste Management and Landfill Symposium, Sardinia,At: S. Margherita di Pula, Italy.

ref 3.Comparison of the Effect of Vermicompost and Inorganic Fertilizers on Vegetative Growth and Fruit Production of Tomato ( Solanum lycopersicum L.) January 2015Open Journal of Soil Science 05(02):53-58 DOI:10.4236/ojss.2015.52006

ref 4.No-dig gardening https://en.wikipedia.org/wiki/No-dig_gardening

ref 5.Koplowicz, Sarah R., "Utilizing Compost for Carbon Sequestration: A Strategy for Climate Goals and Land Use Management" (2019). Master's Projects and Capstones. 945.

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A few thoughts:

  • if you take too much heat from the compost, the lower temperature impacts the rotting processes. In the end, the same amount of energy will be released, but the thermal power will be much lower.

  • "Baseline" energy expenditure calcuation would be helpful: do the saving outweigh the energy needed to move the compost from your collection facility to the households? Compost is a low energy high mass (moisture) substance...

    Found some numbers: 

    • a 26 t waste truck uses, say, 75 l diesel/100 km
      (side note: this could likely be reduced a lot by having little "herds" of big rubbish bins instead of the bin-per-house setup we use in Germany which means that the truck has to stop and go every few meters. Also, these trucks, like buses, would likely be the cars where electric drive has huge advantage over combustion. The Berlin newspaper article from 2018 reports 60 electric trucks vs. 1000 diesel ones...)

    • Website linked below says, energy produced via their anaerobic process (biogas + heat) from one 120 l bin is equivalent to 3 - 4 l oil.

    • Which gives us some energetic leeway after the collection (and financially even more since at least over here people are used to pay per kg of organic waste that is removed by the communal waste collection), but your proposed distribution of compost would likely need to stay quite local.

  • Here in rural Germany, a traditional use of the compost heap heat is to have e.g. zucchini or pumpkins grow there. This boosts the plant productivity because the compost supplies both nutrients and a position that is a bit warmer than usual over here, which both agrees well with those plants.
    The energy savings from having home-grown vegetables may be higher than any direct use of the heat.

  • Have a look at this combined biogas and compost facility (link to German language site - I only found general information on the English version).
    Rather than distributing the half-done compost, they use the gas to cogenerate electricity and heat. Interestingly, they say part of the heat is needed to keep up the process temperature in the composting/fermenting facility. Iow, if your compost produces excess heat, that energy could have gone into gas by a different (anaerobic) process conduct.

    They say that 120 l organic waste (one household waste bin) will produce the equivalent of 3 - 4 l of oil. The plant works on a scale of 20000 t/a.

  • Fly production: black soldier flies can be used for oil production, yes. But they're also discussed as food & feed protein source (over here in the EU, AFAIK you can use them as feed but they are not approved for food [yet]). Protein is a higher value product than oil/gas, but I'm not sure you can grow them in the after-fermentation composter of the plant linked above. However, it may make sense to have them as first step and put the remainder they didn't use into the anaerobic fermenter.

Digging a bit further, biogas miniplants are available in various scales, e.g.

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  • Thank you for answering! You shared many valuable data and researches. I'm glad that bio gas technology is still going and improving in Germany. There are some pig farms here in Taiwan also trying to do it.
    – gitpharm01
    Aug 4 at 3:57
  • @gitpharm01: slightly nitpicking: the linked biogas facility is in (a German-speaking region of) Switzerland. Aug 4 at 13:38
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    It's always good to be accurate. But Germany does have advance technologies in bio gas. Many bio gas projects in Taiwan are actually using systems from German companies.
    – gitpharm01
    Aug 5 at 4:16

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