Thermodynamic systems in housing

Much of the business of a house comes down to controlling the flow of a few crucial commodities:

1. Heat;
2. Light;
3. Air;
4. Water, whether as humidity or liquid;
5. Noise.

(This is also true, incidentally, of gardening, of which more later.)

This is a Fullerian view of a house as a dwelling-machine, rather than a structural engineering effort to resist its own weight. It’s also a sort of instrumentalist worldview: the house is designed for CONTROL, so that its owners can use that control to experience comfort.

In its crudest form, a house is just an enclosure to restrict the flow of all four of these commodities. The roof and walls stop the rain from soaking the space beneath, the sunlight from heating it, the wind from casting it into disarray, the cold night from chilling it, and the inhabitants from scaring the animals with their gasps and ululations. In nature, all five of these elements generally arrive together and depart together, and to exclude one is to exclude another.

But too much restriction is usually bad, even fatal. If heat cannot escape, an inhabited dwelling will eventually cook its inhabitants until they cease to heat it with their body heat; if light cannot enter, the inhabitants are blinded; if air cannot enter, they suffocate; if water cannot exit, everything becomes waterlogged with their breath; and if noise cannot enter, well, perhaps nothing bad happens.

More elaborate contrivances allow us to separate these five elements to some extent, reducing the necessity to compromise our needs between them:

• Solid windows, whether of mica, of glass, or of polyethylene film, damp noise but permit light to pass without bringing along air and water, or letting quite so much heat pass out the other way when it’s cold out.
• Shutters allow us to further restrict the air, light, and noise that pass through windows.
• Curtains, too, are mostly used to stop the flow of light through windows; they can also be used for privacy, to stop the flow of heat through walls, and to absorb noise.
• Thermal mass, like adobe walls, damps thermal fluctuations like the big capacitors in your power supply. That same mass often serves to absorb noise and damp fluctuations in humidity.
• Furnaces, heaters, stoves, and microwaves produce heat with little light, but the flame-powered kinds require access to vent their exhaust.
• Luminaires provide a small amount of light upon demand, even at night, along with correspondingly little heat.
• Air conditioners and other kinds of heat pumps move heat in the direction of our choosing without bringing air and light along for the ride, though often at the cost of noise.
• Plumbing provides water on demand, often leavened with a precise degree of heat for luxury; like heat pumps, it, too, can be used to control the flow of heat, which will probably be its primary function within a few decades, as machines’ energy usage explodes.
• Rugs, historically, served largely to insulate us from chilly floors and absorb sound. (Since the 1930s, they have a secondary use of reducing the cost of housing by hiding plywood flooring which would otherwise have to be covered with more expensive hardwood.)
• Construction materials are often chosen to be porous to prevent condensation inside walls: if humidity can get into the wall from the warm side, the thermal gradient through the wall must be matched by a humidity gradient, or when it’s cold outside, the thermal gradient will cross the dewpoint.
• Insulation in walls stops heat transfer without affecting airflow significantly.
• Weatherstripping around doors and windows stops heat transfer, noise, and airflow.

But there are a variety of other techniques, less widely used, some even entirely speculative, which promise to offer better tradeoffs. Some of these are proven but not widely known.

• Solar overhangs over windows permit sunlight to enter the windows during the winter but not the summer, passively reducing the temperature variation throughout the year.
• Electrically controlled shutters or mirrors could provide the same service, but controlled by a PID or better negative feedback system, rather than crudely by the season.
• Countercurrent air heat exchangers allow air to enter and leave a house without taking heat and humidity with it. These are necessary for Passivhaus structures, which are designed to limit heat flow to a much lower level than traditional, in the interest of reducing marketed energy consumption.
• Speculatively, regenerative heat exchangers are another way to achieve the same goal in less space, and possibly with less expensive materials. While a countercurrent heat exchanger runs currents in both directions at once, separated by a high-thermal-conductivity barrier (for example, warm air exiting the house in winter might be used to thus warm up cool air being brought into the house at the same time, by passing the air currents through parallel pipes separated by an aluminum wall with lots of fins), a regenerator instead alternates between the two directions. So a regenerator could consist of, for example, an insulated tube full of sand, gravel, or a microencapsulated phase-change material (see below). This, however, requires the air volume inside the house to vary, which is potentially difficult to achieve. With a little water, it’s possible that the regenerator could simultaneously function as a filter, although of course water evaporation would also humidify and cool the air.
• Geomembranes could permit subterranean construction with much less compromise. One great difficulty of subterranean construction is that all traditional construction materials are porous, so below the water table, a constant trickle of water enters in unpredictable places. It must be pumped out to avoid flooding, but it still compromises humidity control; I have known subterranean offices with deadly Stachybotrys infestations. Geomembranes are sheets of tough plastic welded together, developed for keeping landfills from leaking; they could conceivably solve this problem, although I’m not familiar with examples of their use in building construction.
• Speculatively, halite (sodium chloride, table salt) is a mineral that flows under pressure that is relatively light in geological terms. Perhaps halite could serve as a sort of self-healing geomembrane to seal out water, although over long periods of time it will eventually diffuse away in the groundwater, and it would rule out the use of steel-reinforced concrete construction.
• Light pipes (or “light tubes”) are waveguides for transporting light long distances, up to tens of meters, through small apertures into a building. In essence, they are fat fiber optics. They can exclude the infrared wavelengths that carry a substantial fraction of sunlight’s heat, and they can relieve the murky dimness that often plagues subterranean construction.
• Subterranean construction is superb for noise control, thermal mass, insulation, and security; military buildings have often been constructed underground for centuries for these reasons.
• Alkali scrubbers, like those used in submarines and space stations, remove carbon dioxide from air, reducing the amount of airflow needed to keep it breathable. (When you suffocate, the level of carbon dioxide reaches fatal levels long before the level of oxygen falls dangerously.) In their simplest form, these are just curtains impregnated with lithium hydroxide or a similar alkali, which convert to carbonates upon absorbing carbon dioxide. Lithium hydroxide, though preferred in submarines and space applications for its light weight, requires heating the carbonate to 1300° to regenerate the hydroxide; the carbonates of sodium, calcium, and magnesium are more manageable, at 851°, 550° to 825°, and 350°, respectively, and they are much less caustic as well.
• Phase-change materials like ice, Glauber’s salt, or paraffin, provide effective thermal masses that are many times larger than their physical mass. A single ton of ice, with its enthalpy of fusion of 333 kJ/kg, can absorb the same amount of heat in its melting as 44 tons of feldspar rock, with its specific heat of 0.75 kJ/kg/K, over the range 20° to 30°; ice has an inconveniently low melting point, but you can use ice, rather than air, as a reservoir for a heat pump, getting a major efficiency boost, and other phase-change materials are capable of honorable performance at higher temperatures. Paraffins can be fractionated to have precisely calibrated melting points at any desired temperature, and Glauber’s salt melts at 32°, just slightly too warm for comfort, absorbing 252 kJ/kg; at 1.46 g/cc, that’s 368 kJ/ℓ. Wikipedia says, “For cooling applications, a mixture with common sodium chloride salt (NaCl) lowers the melting point to 18°C (64°F). The heat of fusion of NaCl·Na2SO4·10H2O, is actually increased slightly to 286 kJ/kg.” As a useful ballpark, a square meter of sunlight inside your house during 12 hours deposits about 43 MJ of heat, enough to melt about 130 kg of ice or 151 kg (103 liters) of the Glauber’s salt mix; a person during 24 hours burns perhaps 2500 kcal, enough to melt 31 kg of ice or 37 kg of the Glauber’s salt mix (25 liters). (Other low-melting-point materials exist, but most are not affordable at the kilogram scale; eutectic sodium/potassium nitrate is, but melts at 260°; hydrated sodium silicate melts at 72°. A variety of other phase-change materials are commercially available.)
• Seasonal thermal energy storage are large thermal masses (whether phase-change or otherwise) intended to store enough heat to keep you warm all winter or cool all summer. Ballparking, a kilowatt per person over six months is 15.8 GJ per person, which is 47 tons of ice or 55 tons of the Glauber’s salt per person, about 38 cubic meters (38000 liters); or 2100 tonnes of feldspar (820 m³ at 2.56 g/cc) with a 10° temperature swing. This may sound like an impractically large amount of material, especially the feldspar, but even in that case it fits into a 12-meter-diameter sphere; it’s house-sized, not city-sized. You don’t have to move all that material, but you do have to somehow control fluid flow through or near it, with a typical technique being to perforate a field with boreholes 3–8 meters apart.
• Double- and triple-paned windows are hermetically sealed, typically with argon in between the panes, which dramatically drops heat loss and noise transmission through the window without noticeably affecting light transmission.
• Vacuum-insulated glazing is a more advanced version of double-paned windows with vacuum between the panes, reaching R-values as high as 12.5 K m²/W.
• Evaporative coolers come in many forms, such as spray mist nozzles, box swamp coolers with coarse vegetable fiber such as wood wool, and open water pools; it trades an increase in humidity (and a loss of water) for a decrease in temperature, and in some cases the downdraft of the cooler air can also be harnessed. If you can arrange for the water to evaporate outside your house rather than inside, you avoid the humidity increase, but you also lose most of the cool.
• Flat-plate solar thermal collectors are very-low-cost ways of harvesting the sun’s heat; they can be a simple aluminum sheet painted black with a thin Styrofoam backing with water pipes welded to it, or even black-painted plastic with channels running through it. (In cold climates, you need some antifreeze in the water, too.) Typical efficiencies are in the 40% – 60% range, several times higher than photovoltaic. With transparent plastic or glass over the top, the water can heat up efficiently to over 50° — not enough to drive a heat engine efficiently, but plenty for climate control or a hot tub, at a very low equipment cost per watt. The Drake Landing Solar Community, at the chilly latitudes outside Calgary, gets 97% of the energy it uses for climate control with this technique, storing it in a borehole field as described above.
• More elaborate kinds of solar thermal collectors, capable of higher temperatures and higher efficiencies, include evacuated-tube types, concentrating types, and types with wavelength-selective paints. Higher-temperature collectors may have to be made of more expensive materials (copper rather than plastic) and use more exotic fluids (oils or molten salts rather than water).
• Thermosiphons are an arrangement of the elements of a solar thermal collecting system such that no extra pump is needed: the heat sink or heat store is placed at a higher elevation than the solar thermal collectors, so that the warmer water from the collectors will rise into the sink or store, replaced by denser cool water. Often a backflow prevention check valve prevents backward flow at night.
• Thermal radiators are simply solar thermal collectors used at night to radiate unwanted heat as infrared light into space. Wavelength-selective paint is counterproductive for this use. In some systems, the radiator is simply an open pan or floor flooded with water, which is free to shed heat both by radiating and by evaporating. Without the evaporation, the cooling rate is about 75 W/m² at normal temperatures.
• Short-term thermal storage tanks can store heat or cool (thermal absorption capacity) in thermal mass (probably of water) or a phase-change substance. In extreme cases, this could be done in a stainless steel Dewar flask, but less extreme temperatures permit the use of inexpensive plastics with no insulation or inexpensive insulation.
• Solar air heaters are solar thermal collectors that heat air directly rather than water. This reduces the need for waterproof materials, but because of air’s much lower specific heat, requires larger systems and more flow. The Trombe wall is one well-known version of this system.
• Venturis and other fluidic pumps permit the use of one fluid flow to produce another, without any moving parts. This could be useful to drive ventilation air currents from convection currents produced by a solar air heater.
• Heat pipes are a lighter, faster way of moving heat than water-filled plumbing, and they also don’t break if they freeze. They’re sealed and filled with low-pressure water vapor, which rapidly flows to cool parts of the pipe and condenses, then flows along the inner walls until they reach a warm part, where it evaporates again.
• Non-imaging optics permit concentrating sunlight to a very high brightness in a compact space without having to track the sun (much); this is useful for feeding light to light pipes and also for heating fluids.
• Desiccant dehumidifiers are an alternative to refrigerative (or “compressor”) dehumidification when humidity is too high. Instead of chilling the air to condense the humidity, the air is run through a desiccant (silica gel or synthetic zeolite) at ambient temperature, and the desiccant, usually mounted in a rotor, is then heated to release the humidity and recycle it for further dehumidification. These have several advantages over refrigerative dehumidifiers: they operate from readily available heat rather than expensive mechanical energy; they can typically dehumidify down to lower temperatures and lower humidities; the extracted humidity is in a more manageable form of vapor rather than liquid; the machinery is much simpler. They also cool the dehumidified air, so they are an alternative to absorption chillers (see below) for solar air conditioning.
• Hypocausts, or underfloor heating, are a great seven-thousand- year-old luxury, universal in Korea, in which the house is heated through its floor rather than by directly heating its air; https://www.youtube.com/watch?v=P73REgj-3UE shows the construction of one from stone and mud. This is more comfortable than the air-heating approach, because people are more comfortable when the radiant-heat temperature is higher than the air temperature. If energy is abundant, it may actually be worthwhile to use it to cool the air while heating the floor. The heat can be carried through the floor in channels for air or water or provided by electric heating elements; the standard approach currently is to use cross-linked polyethylene (PEX) pipe (good up to 85°) with water.
• Heated counters and heated toilet seats are other similar luxuries that can be provided in like manner. Defrosting plates made of thick aluminum could be useful for many purposes.
• In snowy climates, such a heat reservoir could be used to melt snow covering driveways or roads. Experiments have been done of collecting heat using cross-linked polyethylene (PEX) pipes embedded in asphalt.
• Chilled slabs are the same system, but used to provide radiant cooling rather than radiant heating. I haven’t had the opportunity to experience these, but they are in use in places.
• Absorption chillers are refrigeration systems which can be operated entirely from heat, even including low-temperature heat such as that provided by flat-plate solar collectors. They are in common use in camping refrigerators powered by propane flames, and in large-scale commercial use, but they could be used as heat pumps in a variety of climate-control uses, pumping massive amounts of heat entirely by solar thermal power. “Solair” was a 2009 EU research project aimed at commercializing small-scale air conditioning by this technique, achieving a coefficient of performance of 0.7 powered by a 65° heat source. The application to camping refrigerators requires ammonia, which is dangerous (corrosive, volatile, and inflammable, producing toxic fumes), but much safer aqueous lithium bromide is adequate at air-conditioning temperatures. This refrigerant appears to require some moving parts, even though the ammonia version does not.
• Reflective-wall booths, like those used for indoor marijuana growing, and directional LED task lighting can improve light usage efficiency. This should allow people to achieve greater well-being with relatively small amounts of light. Full daylight is on the order of 50 kilolux, which is to say 50 kilolumens per square meter. If you are sitting in a reflective-wall booth where your skin and some reading materials absorb the majority of the light, you might have 4 m² of absorbing area, thus needing 200 kilolumens. A normal GE Polylux 70-watt T8 fluorescent tube produces 6.3 kilolumens, so you would need 32 of them, a total of 2200 watts. Even at an eighth of that, though — 4 tubes, 6.3 kilolux, and 200 watts — you’re still twelve times brighter than a normal office. The GE high-efficiency electronic ballast for this setup costs US$21 at retail.
• High-efficiency lighting systems such as sodium vapor high-intensity discharge lighting (favored by marijuana growers) have even higher luminous efficiency than fluorescent lights. While a candle is has a luminous efficiency of 0.04%, quartz halogen bulbs are around 3%, and the fluorescent tube is 12–15%, high-pressure sodium lamps can reach 22%.
• Speculatively, instant hot-water heating can be carried out by running the water through a heat exchanger with a much hotter substance; for example, a small amount of molten salts could be kept around 300°, or a larger amount of oil at above 100°, and water circulated through a heat exchanger with it would flash into steam. This is primarily useful for direct steam applications like foaming milk for cappuccinos or melting cheese, since simply keeping the heat in a larger quantity of water at a lower temperature would avoid any explosion dangers and would have lower heat losses.
• Speculatively, you could heat air for cooking a convection oven in the same way. Perhaps a gravel bed or large rock riddled with air channels would be a better heat reservoir for this than liquids.
• Dehydration of food, damp laundry, and similar things can be carried out with solar-heated air, either inside or outside the house.
• Speculatively, you could use a wet scrubber to remove particulate matter from the house air. This could be helpful if you live in an area with significant particulate pollution, both for health and for keeping the house clean. Dust particles larger than smoke, which are less of a health problem but necessitate cleaning, are probably better removed with a cyclonic separator, electrostatic separator, or paper or fabric filter. A house of 100m² with 4 m ceilings contains 400m³ of air; removing 90 to 99% of the particulates from that air with a venturi scrubber or indoor spray tower, at 0.5–3 ℓ/m³, would require some 200 to 1200 ℓ of water, which is a thing you would might want to do for all the air your bring in from outside. At 1500 kPa of water pressure in the nozzles of a spray tower, cleaning the whole house’s air with 800 ℓ of water would use 1200 kJ (⅓ kWh).

Ideally, many or all of these systems would be available to the house-dweller to deploy as they saw fit, rather than hooked up in a fixed topology at build time; heat and cold reservoirs at various temperatures would be continuously replenished when possible, thermostats and humidistats would be programmed to provide a healthy diurnal variation in the living space, and when excess energy was available, it could be spent on greater illumination or radiant heating.

While such control of the indoor climate is pleasant for humans, we can after all put on a coat or take a cool bath if we’re uncomfortable. For gardening, however, the differences in productivity from even small changes in temperature can be immense. More factors begin to matter — you care not only about the soil’s temperature and humidity, but also its pH and its contents of nitrogen, phosphorus, potassium, and sulfur; and the carbon dioxide content of the air has a significant effect on plant growth. (You could obtain carbon dioxide by calcining calcium or magnesium carbonate, which you then deploy again as air scrubbers.)

In most climates, this level of climate control ought to enable immense gains in agricultural productivity: you should be able to grow sugarcane, bamboo, corn, squash, or rice even at periarctic latitudes, and with CO₂ supplementation, they should grow even faster than in their naturally optimal environments.

Plumbing with crossbars

Passing air through pipes or ducts is an efficient way to move two or three of our five crucial commodities: heat, air, and water in the form of humidity. Even if we want to heat or cool liquid water, for example for washing dishes or laundry or for a shower, it’s probably a good idea to use air to transfer the heat or cool from the relevant reservoir to a heat exchanger for the water.

If we have the desire to