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Compost heating for this winter? --- a compact reactor prototype and experiment results

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Hi, I want share a compost reactor design which created some interesting results.

It's only 0.28 cubic meters large but the compost managed to get above 60 degrees Celsius.
This means you can use far lesser materials to build a hot compost pile.
I'll describe its design and share some experiment results.

1.The design of this reactor:

It's actually quite simple. Just a rectangular box with two openings.


The building material is just some discarded Polystyrene boxes.
I cut these boxes and glued them together like this.

I also drilled 2mm holes on the bottom to allow fresh air to come in.
The holes and slits between Polystyrene boards are sealed with spray foam.

The raw materials are dumped into this reactor through upper opening.
The lower cap is usually sealed. It's opened only when the compost inside is going to be harvested.

In fact, this is not a new design.
Here's a more sophisticated and elegant version done by an Irish guy.
And there is a commercially available product called "Hotbin" which is sold in UK.

2.A compost heat generation test
Due to some reasons, I want to test its capability as a "heat cell" and see how long I can keep it steaming.
The material for composting is quite simple: Fresh grass and coffee grounds in 4:1 ratio.
The grass was cut into 10cm-long segments.
Every time the compost pile gets near 45 degrees, I added and mixed new materials(the same as the initial pile).
The experiment persisted for 33 days and I got it above 45 degrees for most of this period.

A stands for steam temperature
B stands for core temperature
Ambient temperature was around 28 to 32 degrees.

Fresh grass shrank rigorously due to dehydration.
Here's the volume change( Assume full volume of the reactor is 100%)

And after 33 days of fermentation, the compost looks like this:
Most of the soft parts are decomposed and only woody twigs remain visible.

It showed great potential for becoming a heat cell.
Due to its compact size, it occupies much smaller area than a traditional compost mound.
The square shape and lesser weight make it easier to be transported by a common pickup truck.
The compost pile inside requires less turning due to larger surface area per unit of volume.
Since it can preserve most of the heat and steam, every part of the compost is under the same speed of fermentation.

This design may be able to allow reaction to continue in a harsh winter.

3.Making myself a cup of coffee:

In order to test intensity of its heat output, I did another test during day 26.
I used a stainless pot as the "heat reservoir".
It contains a tiny water pump(5 Watt) and two water tubes.
I filled the pot with 5 liters of water and placed it inside the reactor.

The concept behind this is to create a buffer zone between the compost heap and the heat pump device.
When there's demand for heat, the heat pump device will drain heat from the reservoir first.
This will prevent over cooling of the compost core which may slow down reactions inside the compost heap.
Besides, the reactor is not air-tight and still loses heat to the environment.
The heat reservoir can trap such heat energy with materials which has high specific heat capacity like water.

After the water temperature inside got stable (36~38 C), I started the water pump.
The hot water will flow through an aluminum heat exchange block and then back to the pot.
A 70 Watt Peltier plate is attached to the block with thermal paste and pumped heat from the water.

Its hot side reached 80 degrees Celsius instantly.
After 40 minutes, it remained at the same level.

The target of heating is 500 ml of water inside a steel tray.
Initial water temperature is 29.6 degrees Celsius.
The ultimate goal is to make myself a cup of coffee from this tray of water.

Though the heat on the hot side looks amazing, things turned out that it's not efficient at all.
After 40 minutes, the water in tray only reached around 39.1 degrees Celsius.
The heat energy pumped into the target tray of water is:
500 g * 4.2 (specific heat capacity) * (39.1 - 29.6) = 19950 Joule
And to convert it into Watts:
19950 Joules / ( 60 * 40 )Seconds = 8.3125 Watt

Such power output is miserable considering the electric power consumption was 70(Peltier plate) + 5 (water pump) = 75 Watt.
Apparently, this setting is not efficient enough to be a cooking heat source.
I made a short video review on YouTube:

However, the water temperature inside the heat reservoir remained at the same temperature after the experiment.
It might still be able to become a stable household heat source.

4.1 This compost reactor is effective in making hot compost with much lesser materials.
4.2 It's possible to generate heat for a month by periodically adding new materials into it.
4.3 The heat output is not strong enough for cooking.
4.4 In order to turn it into a viable heat source for household, a efficient way of tapping the heat must be established.

5.The reasons I want to create a modularized heat cell

The current situation of this world is dire and may get even worse.
There are food shortage, heat waves, and energy crisis(Mainly oil price and natural gas supply) going on.
And composting has a potential to mitigate all of them:
a. Synthesized nitrogen fertilizer requires natural gas and causes heavy carbon emission.
  On the contrary, compost usage and no-dig farming have carbon fixing effect.
b. Household heating consumes natural gas.
  If compost heating is applied at large scale, it can lower the dependence on natural gas.
c. It can turn organic waste into compost locally at each community. This will reduce fuel consumption by garbage trucks.

In order to achieve these goals, a modularized compost reactor/heat cell and a system which can utilize locally generated organic wastes are necessary.

------The following is a wild plan I made. It might have many holes since I'm just an amateur. You are welcome to comment on any part of it ---------------------

6.A localized compost heater rental service.

6.1 The system
a.Modularized and standardized compost reactor
 The goal is to maximize its endurance to cold while maintain decent heat output.
 Odor control and protection from insect are also good features that are worthy of developing.
 Most importantly, there must be unified sizes and design for compatibility among reactor units that are made by different manufacturer.
 Every single reactor is considered a "module" and its heat output can be parallelly connected to the house's heat exchange system.
b.The facility
 The center of this service is a bio-recycling facility which can collect organic wastes and process them into fresh compost piles.
 It runs locally and serve for nearby cities/communities.
 It may require full-time workforce for routine operations.
 There are two major structural compartments that will occupy most of the area: ducking site for organic waste
                                                                                                                                            compost pile storage for final curing/
 Some equipment are needed: mixer and other tools to handle the compost
                                                      small trucks for transportation
c.Collecting raw materials
 The facility will collect organic waste on regular basis.
 After screening out impurity that can not be composted, these raw materials will be blended and turned in to compost piles with proper constituents.


d.Compost heater renting service to household
 The fresh compost will be filled into reactors and transported to subscriber's location.
 They are charged on monthly basis.
 The facility will periodically mix fresh materials to the reactors to boost heat production.
 When the compost inside can no longer provide enough heat output, the reactor will be emptied and refilled with new compost pile.

e.Compost heater renting service to farms/greenhouses
 Compost heating for greenhouses is not a new thing.
 But it requires a lot of work and would become less efficient if the practitioner has no experiences in it or does it in the wrong way.
 Instead of doing it all by oneself, subscribing services from a nearby service provider is more viable.
 In this case, empty reactors are sent to these farms/greenhouses and filled with organic wastes which are generated by the farm itself.
 Ventilation/heating systems will be installed to transfer heat to greenhouses and animal sheds.

f.Selling compost
 The compost which has passed thermophilic stage will be transported back to the facility for final curing process.
 When it's ready, it can be sold to nearby farms, parks, and gardens.

6.2 Advantages
a.Modularized and standardized compost heater design
This will lower cost of manufacturing and maintenance and increase compatibility between different maker's product.
These reactors can be linked and transfer heat to the target.
Instead of making different models for different building sizes, using different number of reactors would be easier.

b.Subscribers pay money and give space for composting
This means the facility requires much smaller area for storage.

c.Integrated resource
In small scaled CHR projects which is done by a few person, it's extremely restricted in terms of quantity and variety of raw materials.
They may have to build reactors, collect materials, set up heating systems and maintain them all by themselves.
By subscribing to a specialized service provider, things will be much easier.
The rich resource generated from nearby communities will also become available.

6.3 Benefits
a.Reduce gasoline/diesel consumption of waste transportation and fuel for incineration.
b.Reduce natural gas usage in heating.
c.Reduce natural gas usage in chemical fertilizer synthesis.
d.Carbon fixing effect from compost usage and no dig farming.
e.From a to d combined. This system can help securing national food and energy safety by reducing dependence on resources from other countries.
f.Create new jobs

6.4 Challenges
a.Though it's possible to keep a compost pile going during cold weather, it might still fail if it's too cold.
b.It may face low renting income during summer.
c.Foul smell and insect infestation, which are the stereotype images of compost piles, may dissuade people from subscribing this service.
d.No similar business model has been done. Its profitability and self-sufficiency are unknown.

I'll try to make another batch of compost with this prototype reactor and do a heat recovery test with different setting.

There is one last thing I want to say.
The reason I do this is not about money, it's about sending a message:
Everything goes on even without fossil fuels.

The mass majority are educated to do almost everything on fossil fuels and has become ignorant about the abundant resources around them.
This must be changed.
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