TruthArchive.ai - Tweets Saved By @JessePeltan
@JessePeltan - Jesse Peltan
The physics of heat transfer are way cooler than you think. If we designed homes with them in mind, they'd be a lot more resilient and energy efficient, and people would be a lot more comfortable. Insulation standards and HVAC systems are designed around maintaining the air temperature inside a building, but air temperature is only a tiny slice of the picture. In typical conditions, only ~15% of the human body's heat loss is from convection. ~60% is from radiation, and ~20% is from evaporation (breathing and sweating). Your body glows like a light bulb with infrared radiation. You are constantly emitting infrared radiation into the environment and absorbing infrared radiation from the environment. Radiative power scales with the 4th power of temperature, so even small changes in temperature result in large changes in heat flux. If your skin temp is 34°C (93.2°F), you emit 505 W/m^2. A wall at 10°C (50°F) emits 364 W/m^2 while a wall at 40°C (104°F) emits 545 W/m^2. Your body is ~100 W and ~2 m^2. You need a net cooling of ~50 W/m^2 to maintain your body temperature. If you weren't constantly bathed in infrared radiation, you'd quickly lose heat to the environment. If you're surrounded by hot surfaces, you can be baked by infrared even if the air temperature is low. The temperature of the walls, ceiling, and floor is more important than temperature of the air. Underinsulated surfaces can be cold even when the air is warm, and hot even when the air is cold. This (and humidity) is why your house at 70°F doesn't feel the same in the winter and the summer. Doubling the amount of insulation in a home cuts the energy required to maintain a given air temperature in half, but it also cuts the differential between the temperature of the walls and the air. With better insulation, not only do you not need as much energy to maintain the same air temperature, you don't even have to maintain the same air temperature. Air conditioning can reduce humidity, but most air conditioners can't intentionally control humidity, and most HVAC system have no way to increase humidity. Better control over humidity reduces the temperature differential your house has to maintain with the outside to provide the same heat transfer for the people inside. Having air that's not overly dry in the winter means it doesn't have to be as warm to be comfortable. Having air that's not overly humid in the summer means it doesn't have to be as cold. The goal is to maintain human body temperature, not air temperature. Radiation and evaporation make up most of the heat transfer. Looking only at air temperature completely misses the big picture. We underinsulate our homes and don't provide adequate control over the aspects that are most important for comfort. Insulation provides a much larger efficiency benefit than you'd expect from a simple Manual J calculation. That's compounded by the fact that the economic value of that energy savings is far larger than flat volumetric pricing indicates. (because insulation provides the most benefit in extreme conditions where capacity is constrained and prices are high) Heat and cooling are the majority of energy use in homes, and the vast majority of energy costs. Real consideration of the physics of heat transfer and the economics of energy systems in the way we design homes and HVAC systems would provide massive economic benefits.
@JessePeltan - Jesse Peltan
We really need to update building codes. >2" of continuous exterior insulation should be standard everywhere. The economic benefits are huge, and we're currently subsidizing inefficiency with distorted price signals. Beyond energy efficiency and resilience in extreme weather, exterior insulation is extremely beneficial for building longevity and indoor air quality. Cold condensing surfaces promote mold growth and rot. We can solve all of this if we just build better buildings.
@JessePeltan - Jesse Peltan
Poor home design is absolutely killing us on energy costs. https://t.co/WMdYgtP4ok
@JessePeltan - Jesse Peltan
Not only is HALF of peak load from heating and air conditioning, 1/3 of that is lost to DUCTS. The thing that pushes the grid over the edge is leaky underinsulated ductwork. It's absurd. "Typical systems with ducts in attics or crawl spaces lose from 25% to 40% of the heating or cooling energy that passes through them." https://www.nrel.gov/docs/fy05osti/30506.pdf Without even upgrading to more efficient heat pumps, we could cut heating and cooling demand by 1/3 by switching to ductless systems or placing ducts within the conditioned envelope. Ductwork has lots of surface area. It pulls in air from and leaks air into your attic. With an unconditioned attic, you're pulling in dusty, contaminated, unconditioned air. (this is also where most of your household dust is likely coming from) People build this way because they're optimizing for the wrong price signals. We hide the cost of peak demand in volumetric rates, so people can't internalize the full savings from energy efficiency measures. We need rates that reflect reality so people can build systems that work in the real world.
@JessePeltan - Jesse Peltan
Texas could switch to 100% electric heating, use less electricity, and lower peak winter demand. 61% of homes in Texas heat with electricity, but only 19% use heat pumps. 42% use electric resistance heating, 35% use gas, and 3% use propane. Cold weather heat pumps can maintain https://t.co/CEqvzMxAif
@JessePeltan - Jesse Peltan
We should use more concrete - not less. Concrete is a foundational pillar of civilization. It's also an easy answer to "hard to abate" emissions. Concrete is about 8% of global emissions, but we could make it negative 4%. (or even more if we use more concrete) Concrete emissions come from two places: 1. fuel use to heat up limestone 2. decomposition of limestone (process emissions) (emissions are roughly half and half) Over time, concrete reabsorbs the CO2 it gave off (process emissions) during production. The rate and magnitude vary by geometry and exposure. (thinner slabs carbonate more quickly) If we provide heat from carbon free sources and capture the process emissions, concrete can be a net absorber of CO2. Capturing a concentrated stream of CO2 from a concrete plant is a lot cheaper and less energy intensive than capturing dilute CO2 from the atmosphere. This exact same chemistry is actually exploited by some direct air capture (DAC) processes to capture CO2 from the air. We don't have to set up a dedicated direct air capture facility that makes and unmakes cement over and over again. That's a lot of energy and capex that only captures CO2. By comparison, if we just make more concrete and make it carbon negative, we can capture the CO2 and use concrete for things we need. We can even substitute it for other building materials. The world needs concrete to build safe shelters, infrastructure, power plants, etc. If we make current volumes of concrete carbon negative, they could offset all of global aviation emissions (~2.5%). If we make even more concrete, we could offset other "hard to abate" sectors too. Concrete is an extremely valuable material. There's a big push to use less concrete or find alternative concrete chemistries that don't emit CO2. That push misses the massive opportunity for concrete as a tool for negative emissions. We don't need less concrete. We just need to be smarter about how we make it.
@JessePeltan - Jesse Peltan
Concrete is DAC. We're already spending the energy necessary for global scale DAC. With a few tweaks, we can get the benefit of it too.
@JessePeltan - Jesse Peltan
Side note: "embodied emissions" is mostly B.S. - at least how we typically use it. A nuclear plant has negative embodied carbon if the concrete used is negative carbon. Steel has a vastly different emissions intensity if it comes from a blast furnace, DRI with methane, DRI with hydrogen, recycling, etc. When evaluating embodied emissions, it's critical to take into account the evolution of processes used to manufacture a given technology. Solar's embodied carbon today is vastly different than 20 years ago. In 20 years, it will be just as unrecognizable. This is true broadly across technologies as processes electrify, the grid becomes cleaner, and we deploy alternative production pathways.