Solar heat, update.

Some points I noticed while my solar heat system was installed last spring.

The glass cracked in couple of days, most likely the window frame was too tight. It was probably loose enough for normal use but in my use it heated to over 50 degrees and I'm guessing that was too much. Next time I'll make a new frame and make sure the glass is really loose in there.

I'm measuring the small solar panel wrong, I should be measuring amps and not voltage. Needs to be fixed sometime in the future.

ESP8266 real time clock is very inaccurate. I fixed this by ignoring the time sent and using the receiving servers time.

There's not enough error handling in my code and sometimes it crashes, most likely it's in the MQTT or NTP part, network trouble probably.

Using just convection didn't give that much heat so I added a fan. Currently it starts the fan when heated temperature is 20 degrees above inside temperature, when heated temperature is less than 10 degrees above room temperature then fan speed is decreased.

I used a standard 120 mm computer case fan I had lying around. "Case" for the fan was fabricobbeled from plastic cellular board I had salvaged from trash.

Here's an example of a sunny day, blue is outside temp (north side of the house), red is bottom of the collector, yellow is heated air and green is inside of our garage. Magenta is the "speed" of the fan, it's actually 10 bit pwm value of the ESP8266, as the fan doesn't have a pin for measuring speed, but it gives some idea on how fast the fan is running.

Updated source and Fritzing scheman can be found on github. https://github.com/mika-koivusaari/solar_air_heat_controller

Solar heat

Our garage currently has no heating, sometimes when it's cold for a long time I put a electric heater there to make sure it doesn't go below freezing. For some time now I have thought about making a solar heater there and now I had some time to do it. One of the outside walls faces south and there is already an air intake, so that's a good starting point. I decided to only heat the incoming air and not make a full blown solar heat system with water circulation, it's cheaper and also I like to start small.

Collector

A house near us had it's windows replaced so I got one of the old ones for free. It was about the right size which reduced the work needed. Basically it's just a wooden box with polyurethane insulation I found in a dumpster, with the window put in front. Just behind the window is a tin foil paper with the paper side painted black and facing the window and sun. That way space where air heats and comes inside is covered with aluminium and there is no paint that might give out some fumes when heated. I ran out of paint so the collector isn't painted as black as it should, I'll probably paint it better in the summer as I have to take it out anyway so our garage wont heat too much. But I thought it would be good to get it installed and making heat, I can make it better later when I have some more data on how it works.

Controller

The controller needs foremost to control the flap on the air intake, because there are usually no people in the garage there is no need to get fresh air all the time, so the flap can be closed when there is no heat coming. Generally when air is hotter inside the collector we open the flap more, and when it's colder we close the flap some more. For operation there's only need for two temperature sensors, heated air and inside air, but I also want to monitor cold air coming to the collector so I have three sensors. That way I can monitor how much air heats and whats the temperature difference between north and south sides of our house. There is no fan, air is only moved by convection.

I also had a small solar cell from a Ikea garden lamp, which I added to get some reference to compare to when I make changes to the collector or controller. I put some resistors and diodes as a load, and scaled the voltage to 0 - 1 volts with a voltage divider.

All the values, temperatures, solar cell voltage and servo position are sent out with MQTT and then logged to a database so I can monitor how it works. And when I make changes I can see if it was a good or bad change.

On the left the protoboard version, and on the right the final veroboard version. Power supply is just some generic psu I had lying around as I use two dc-dc converters to make the needed 5 (LCD and servo) and 3.3 (ESP8266 and DS18B20 sensors) volts.

Controller in it's "final" installation. After one day I lowered the position of the servo some. It only opened the flap a few centimeters, now it opens some more.

Source and fritzing files can be found in github https://github.com/mika-koivusaari/solar_air_heat_controller