Post date: Nov 20, 2014 10:55:16 PM
Today I met with architect and a heating engineer, who convinced me that my concept of how to heat my house with solar energy was outdated. So, what is outdated? Certainly not the sun. But houses now are more heavily insulated and, more important, have fewer air leaks.
This means not only that less sunlight is needed to heat them (because they retain the heat much better), but that they will overheat more easily if the sun is streaming in the windows. They will also not cool as much during the night, nor do they need to be quickly brought back to comfortable temperature - just before you jump out of bed.
Thus the idea of a hot-air furnace that can quickly heat the house is outdated and unnecessary, and the need for air filtration can be met by the heat recovery ventilator, which is required in tight, new houses.
So what does this engineer recommend?
He recommends an indirect hot water system, where the sun heats water, that heat is transferred to other water which circulates in the floor. As a side benefit, that heat can heat domestic hot water for bathing and washing.
Because the nights are cold in Taos, the solar water collectors must either be filled with anti-freeze solution or be drained. And there are two ways of draining: drain down and drain back, drain back being preferable because it works if the power fails (the pump simply stops pushing water into the collector so it drains back into the heat exchanger reservoir. Asking gravity to drain the system does require that all the collectors and pipes be properly sloped and the pipes not sag over time.
But here is the rub. Where do you put the collectors so that they look good on or near the house? Having the collector panels at ground level means that they must drain into an underground reservoir, which is out of sight, but contains the pump and heat exchanger - perhaps not a good idea. Having the panels on the roof means a somewhat more expensive roof - we need supports for the panels and the piping - but they are at least partially out of sight behind the parapet. And the roof will get more foot traffic, so must be more robust or better protected.
Each of us left the meeting with an assignment:
My work is to study and become familiar with the subject.
The architect needs to hide the panels.
The heating engineer needs to provide a rough estimate of the cost. [note: $15,000]
If you are curious, I already found one good article on Drainback Solar Hot Water Systems. It lacks illustrations, but searching Images for that phrase will get you plenty. Here is another source with diagrams, though not quite so thorough: Active and Drainback Closed Loop Solar Hot Water Systems.
These systems require about $200/year in electricity to pump the water around. Hopefully at least some of that electricity will be recovered as hot water.
What if the sun is not sufficient? Say the power goes out for three days or we have a week-long freeze with no sun. Then we need a backup source of heat. The easiest is to use electric heat, just as in an electric water heater. Next (for me) would be a tankless gas-fired water heater. The fancy way is to install an air-to-water heat pump. which is more efficient in operation, but costs more to buy and install. On the other side, it could be used to cool the floor during hot summer days.
Decisions, more decisions.