AgSpa: Compost Hot Tubs
~ quadruple-dip utilities and value, even earn cash ~
You live on a farm? You pay for fertilizer, plus home heating half of the year, hot water heating, a humidifier in the winter, and you were thinking of saving-up for a hot tub? Get it all-in-one! You can even rig a Stirling Engine up, for efficient power, while heating your greenhouse. Heck, if you have a spare quarter acre, your hot tub can be multi-tiered sauna pools, rented-out for locals’ event nights, if you don’t mind the company.
YouTube Geniuses
A number of YouTubers have unveiled their compost-powered heaters, especially for greenhouses, even heating a steamy hot tub. I bow to their inspiration, and I have taken note of their complaints — there seems to be a larger, unifying theme and engineering problem; unlocking this could make compost-heaters really shine: Heat Balance.
The compost naturally generates heat — you aim for it to be in the 150F to 170F range, which is hot enough to scald! The Johnson & Su Bioreactors, in particular, are sized so that they naturally generate the right amount of heat to support fungal culture, without the heat of the fungi’s digestion cooking itself! Yet, if you want to DRAW heat from your pile for other uses, then you’ll NEED to make it too-big, so that it generates an excess of heat that you can take-away. If you don’t make the pile over-sized, then drawing heat away will *silence* the fungal activity and the pile will stay cold for a while.
So, the First Rule of Ag Spas: make the pile WAY too BIG! That’ll give you a lot of heat, and you’ll need to pump that heat out regularly. We’ll want thermostats — but not in the pile itself. Instead, we’ll measure the temperature of the water that we *STORE* inside the pile. Yes, PVC pipes will be storing water inside the thick of the pile, as a reservoir — a ‘thermal battery’. By keeping multiple times our tub’s water in those pipes, you can draw-down a full bath without chilling the pile when fresh, cold water spills into the pipes. You level-out that temperature-swing, so the compost stays active, continuing to generate heat.
That heat has a LOT of potential uses. In general, though, you’ll want that same Unit of Heat to travel through multiple utilities and values before it leaves. For example: compost heat can be fed into a hot-pond at 170F, where it steams the room for an enclosed sauna; it then forms a hot waterfall down to a larger pond, 104F hot tub; it spills from there to an even larger, warm-ish pool, with a waterfall from the hot top-zone as a shower; the steam can be let-into the house or greenhouse, without bringing any compost gasses or smells with it; that heat is humid, too, while most heat-sources dry-out the air; finally, if that greenhouse condenses that water along its surfaces, you’ve naturally desalinated and purified whatever water you had in the sauna (I’d recommend mineral salts in the sauna, to prevent bacteria and for your skin!).
Another option: when the compost water-pipes register 170F and start pumping, they pump into an insulated tank with a Stirling Engine on top; that voluminous device is simple, cheap and easy to maintain, and it can grab high efficiencies from low temperature differences, pumping with mechanical power directly (for water wells or machine power) or powering a generator and battery pack. The cold-reservoir of the Stirling Engine still captures a lot of that heat, diffused; if you use that slightly-warmed water as a pre-heater for aquaponics in a greenhouse, you still add 70% of the heat that you would have gotten from burning the fuel, while capturing 30% as electricity. The fish won’t boil; output temperatures would be just a few degrees higher than intake — it means the fish won’t freeze, and neither will the plants!
So, there are a variety of ways to arrange compost-heat utilities for your needs, but the underlying theme is our Second Rule of Ag Spas: Combine & Chain Uses for Heat and Humidity.
The Pipes of Dross
Setting-up this particular sort of sauna is a bit different from the YouTube originators; they all run a loooong loop of hose through layers of the pile. That means, when they run fresh, cold water into the pile, they cold-shock it and stall-out their heat production! No good! Instead, we want vertical, wide PVC pipes, in pairs. Each pair has an elbow joint connecting them at the bottom, so that the pair forms a long, U-shaped reservoir. Use stakes to prop-up a whole forest of these pipes, with about 2+ ft spacing for aeration and compost mass. Bigger, taller piles & pipes are more economical overall, especially wherever harsh winds and dry winters normally rob your compost of its heat. That’s the Third Rule of Ag Spas: LOTS of Water-Elbows as Reservoirs.
With these U-pipes standing, and compost layered all around them, we can fill them with water, slowly. The U-shape lets us fill from one side with a hose, water falling freely, and a second *siphon* hose on the other end can pull water out — no fancy extra devices or pressurized head, while it separates floating and sedimentary debris. Put your thermostats in those pipes, nearest the center of your pile, to tell the pump when to re-circulate water between your coldest pond back into the compost pile. The U-pipe pairs need to be strung in series, with a few pairs of pipes in a row, so that the last U-pipes in the row of pipe-reservoirs spills-out its undiluted hot water.
In addition to the pipes, there’s aeration. Most of the YouTubers relied upon mesh, and stacking atop a palette, as well as just making it small enough to vent passively. Johnson & Su’s Bioreactor adds PVC pipes down through the entire mass of the pile, to be removed once the material has settled. Those are all amazing, if your goal is *just* compost — they’re simple and they work. For heat, however, we need a nice BIG pile, which means going fancy on aeration in one of two ways: 1) forced-air, which is expensive power and equipment, or 2) BIG disposable vents. I like the big vents, and there’s an easy way to make them:
The Gingerbread Apartments
Lay two big sheets of sturdy cardboard to form an A-frame shape, as if you were making a tower of cards, or the roof of a Gingerbread House. You’ll be sticking triangles upright like pine-trees underneath them, in a row along their belly, pointed up with the triangle’s tip into the peak of the rafters of your Gingerbread Roof; those triangles are the cross-beams to hold the roof rigid when you pile compost on top of it. You’ll also need to cut rows of ‘upside-down check-marks’ all along the outside faces of your roof — imagine each one is going to be an apartment window along this huge surface.
The two cuts making the upside-down check-mark should be steep angles that meet at the top, like an exaggerated 7, with that top line of the 7 cutting up steeply in the same direction as the diagonal body. Try it on paper — you’ll see it work when you fold that narrow triangle out sideways like a door, forming a ‘rabbit ear’, and then you can fold the rabbit-ear in half, pointed down in the direction of your window! The rabbit-ear now forms an awning over half of the window, so just tuck it in that spot, and you have two holes which will stay un-plugged by compost and let air rise through the thick heat. Your rows of windows are an array of vents.
You’ll want to make the Gingerbread Roof a few feet shorter than your final desired pile, so that a thick layer of compost sits between them. A key problem with the Johnson & Su Bioreactors and similar designs was that their aeration-pipe went *straight* through from TOP to BOTTOM — and so did all their watering! Instead, water must hit a mass-break; no matter which path water takes, it needs to stop inside composting material for a moment, to be wicked away and absorbed. That’s Rule Four of Ag Spas: Water Must Hit Fibers; No Direct Exit. So, your steep Gingerbread Roof may be 5 ft tall; pile the compost an extra foot and a half over the roof, and leave it flat on top, *without* any wide puncture-holes for water to escape quickly. If the compost is tall enough, and hot enough, it’ll drive its own convection. [[Additionally, by spacing the compost material with these Gingerbread Rooves in-between, along with the voluminous PVC water-reservoirs, then there is less composting material per volume, chunks of compost are never that thick by themselves, and each chunk of compost is near some vent or other surface. As a result, good ventilation shouldn’t require forced-air.]]
Yields, though?
If a quarter of your compost material was chewed-away microbially, that’s giving you the same overall heat as if you’d burned equal weight in dry firewood. A cord of firewood usually weighs 1 to 1.5 tons, and costs $100 to $500 depending on how fancy you want it and if you’re in a place that doesn’t have plenty already laying around. So, suppose you piled 6 tons (dry) straw and manure, Johnson & Su Bioreactor-style, to heat your hot tub all winter? It’ll be doing what a cord of wood would, but slow and steady, with NO ‘heat lost in the flue’ or ‘unburned wood gasses’ to lower efficiency! That’s roughly 24 Gigajoules of energy, which’d heat ice-cold water to 104F (at the rate of 168kJ/kg) for 142 cubic yards of water — or 20 weekends hot-tubbing in a 7 cubic yard pond with steamy sauna! From just six tons of rubbish!
[[Bonus Points for collecting urine or manure, and mixing it into the spray you use for keeping compost-moisture at 70%; you can boost the heat of the pile by adding nitrogen, a few days before you need the pond hot! Adding silt-clay to that spray will also slowly coat the compost fibers, so the ‘chemotroph’ microbes can go to work making soil-particles’ minerals bio-available. And, don’t let the spray for the pile’s moisture-level get into the tub-water — keep the PVCs’ lids on and the sauna tarped-off!]]
Johnson & Su’s Bioreactor is alarmingly good at getting the right *microbial* ecosystem into your soil — that’s why I keep mentioning them, and their easy, no-turn compost method! They developed it with University Ag-programs, looking for the best way to compost dairy manure, and they found it! They report that just 400kg (less than 900lbs) is enough to inoculate an acre with good fungi and mineralizing bacteria — barely a pound per fifty square feet! So, while you are sitting with a drink in your double-wide hot tub, heating and humidifying the greenhouse, you are also preparing a potent fertilizer for your entire acreage. Enjoy!
