Energy saving ideas using solar in sunny areas of Earth

Various posts that do not fit in any other category
Santeri
Posts: 332
Joined: 2017-7-5 09:58

Unread post by Santeri » 2025-1-28 09:18

After freezing indoors a few winters, I got the following ideas how to utilise solar energy more efficiently. Houses are built to keep the heat out because cooling during very hot summers is more important. On winters that becomes an issue when temperature is often lower indoors than outdoors. Yet there is plenty of sunshine also during winters which is not really exploited.

Houses could be painted with thermochromic paints so that they would change color according to the temperature. Black is absorbing sunlight while white reflects it. For example a paint that changes the outside color to black when temperature is under 25 degrees Celcius and white when it is over, would help to warm up the house when it's cold and keep it cool on hot days. Energy saving ideas using solar in sunny areas of Earth: thermochromic paint The tanks of solar water heaters are often white or shiny. Black colored water tanks would improve the efficiency as also the tank would collect heat from sunlight. Energy saving ideas using solar in sunny areas of Earth: black solar water heater tanks Further more, the hot water from solar heater tanks could be circulated in radiators warming up houses instead of just heating the shower water. And windows could be coated with thermochromic glazing to let the sunlight in on cold days and reflect it back on hot days. These small ideas could help reducing energy costs while making life more comfortable. Sun is the only almost limitless very low pollution energy source on Earth at the moment.

Cheers,

Santeri



Thomas

Unread post by Thomas » 2025-2-5 14:10

Heat transfer happens through 3 mechanisms, each of which is sufficient to make you uncomfortable:
  • Conduction (solid to solid or liquid) -> you want centimeters or more of insulation, ideally some inertia, and an eye on your thermal bridges.
  • Convection (gas movement) -> you want to keep an eye on ventilation or air leaks.
  • Radiation (infrared, sunlight...) -> here's the domain of reflective thin film insulators, heat-absorbing paints, well-placed windows, and thoughtful usage of sun shades.
Solar tanks for hot water should be insulated (typically a stainless steel + PU foam sandwich), so whatever their color doesn't matter much to their performance. Thermal solar panels already have thermochromic coatings. Mixing the function of the tank and the panel only makes them more complex. Efficiency for thermal solar is not a big differentiator from one technology to another, so I suggest going for the cheapest/most widely available one in your particular market.

There are some solutions out there for thermal solar panels connected to radiators or radiant floor. It's what we call active solar (passive being when you just have big windows or the like, as in our straw-bale house). They generally have a lower water temperature than furnace-powered radiators (meaning larger radiators for the same heating power). The problem is when there's no sun. You can have either a back-up heating system, a well-insulated house with high inertia (outside insulation), or a multi-day hot water storage tank (a few cubic meters ; again, it will require insulation). Looking up historical weather data on the Internet for the past few years would help you assess how much storage you'd need.

Thermochromic paint is tricky, because if it's inside the insulation layer, then it's going to dissipate the heat it collects inside whether it occludes or not. I'm not aware of any solution that would work in that scenario. However, you can lookup the Trombe wall technique that was developed in the 1970es: a small green house made from a glass panel installed a few cm away from your sun-facing wall, and vents that can direct the convection flow either inside or outside the house to regulate the inside temperature. They're obsolete for new constructions, but can make sense in a low-tech renovation project, especially when the weather is not extreme. Another solution is to leverage outside sunshades in front of windows, a veranda, or even a large dark wall with no insulation (playing on the inertia here). You probably want to run the math before you commit to renovation, especially if you are richer in time than in money.

I'm not sure about photochromic windows. The devil is in the details. Electrochromic is probably a no-go, because its power consumption is high (~100W for a large window). Photochromic is going to heat your glass, so if it's on the outside pane of high-performance double-glazed windows, maybe it's OK ; but you'd want to look into the absorption spectra (light, infra-red...), and having it turned on by the UV exposure robs you of the benefit of actual regulation (sunny winter days are typically the coldest). In any case, it's likely to be more high-tech, and as such more expensive (but not necessarily more efficient) than just hanging a piece of cloth outside your window, that you can raise or lower depending on the house temperature. Just as windows-with-solar-panels-inside, it's a blingy high-tech solution that fits well some prestige architecture projects, but not necessary home renovation plan.