Solar air over water

[gadget]

I've been studying solar heating for a few years and have collected a dozen or so sliding doors to build a very large DIY array on the south side of my garage.

I have been trying to come up with a design that is reliable, cheap to build and durable. Something that can handle freezing and high temps. I never really came up with anything that could do all this.

Then one day it occurred to me, the problem is trying to warm the water inside the panel. I thought, why not warm air then use it through a heat exchanger(car radiator) to heat the water?

I would build metal screen style solar boxes and pipe that air into my garage so it can be ran through the radiator(s). The air could also be diverted for directly heating with air.

I found one company online that sells a hybrid setup that does exactly this but that is all I have found. Had anyone seen any other info on this? I would think it has been done DIY before.

Looking forward to any thoughts.

-Gadget

[MN Renovator]

Without heating water, I think you've removed the freezing and high temp issues, I'm not sure why you want to complicate something and reduce efficiency by trying to heat air and turn it around to heat water and then try to heat air again.

I like this design and the same author has tried other less successful designs. Read up on this one and see if you can use the glass you've collected in a similar way.
https://www.builditsolar.com/Experim...r/Building.htm

[gadget]

Originally Posted by MN Renovator

Without heating water, I think you've removed the freezing and high temp issues, I'm not sure why you want to complicate something and reduce efficiency by trying to heat air and turn it around to heat water and then try to heat air again.

I like this design and the same author has tried other less successful designs. Read up on this one and see if you can use the glass you've collected in a similar way.
https://www.builditsolar.com/Experim...r/Building.htm

Thanks, I will check out that build. I love the window screen solar heaters. So simple and good performance.

If I can get 150F and above air temps, efficiency should be good as far as transfer through the radiators. The biggest loss I can see would be in air pumping losses from the pressure drop across the radiator(s). My hydronic temps are around 100-120F

You would not have to necessarily reheat air again, just change the air path around the radiators when you don't need any hot water. I like to keep my garage cool in the winter for long term food storage but a little bit of extra heat might be nice.

I use to focus allot on heating with wood but my wisdom has brought me to focus more on Solar as a first source of heat. I'm hoping to build a system this summer.

[MN Renovator]

You should have more airflow and less temperature rise for best efficiency. 150f into 70f is an 80 degree temp rise. 2-4 cfm per square foot of solar footprint is the rule of thumb I've been told.

[gadget]

Originally Posted by MN Renovator

You should have more airflow and less temperature rise for best efficiency. 150f into 70f is an 80 degree temp rise. 2-4 cfm per square foot of solar footprint is the rule of thumb I've been told.

So you are talking about within the solar panels or the HX? Makes sense to keep the temps lower inside the panels for better heat transfer and less loss through the panels.

I have heard to tilt the screen so the top is to the glazing and the bottom is towards the back. This is so the incoming air hits the glazing and not the air warmed by the screen, that way there is less heat touching the glazing and leaving through the glass

[MN Renovator]

In a solar air panel/module(whatever you want to call it), the exiting air temperature should be lower by increasing airflow so less of the heat is lost through the glazing, sides, and back of the panel as well as increasing the heat transferred through the solar capture media(such as metal window screen).

Regarding the position or angle of the screen, you want the air to pass through the screen and not around it. I imagine the airflow path is a bit turbulent in any case and difficult to control when it comes in through a small round duct into a large thin rectangular cavity. Ideally if you separated the air to prevent it from getting in contact with the glazing and the glazing was completely clear, you would probably be better off because most of your heat loss inside the panel will be through the glazing. I wonder how a double glazed panel would perform. The reality is that the faster the airflow, the lower the temperature of all of the surfaces that would transfer heat out of your solar module and the higher the transfer to the air. This is where the 2-4 cfm rule of thumb comes from. I don't remember the figures from the builditsolar site were but he tested different airflows there. Also keep in mind that you will lose heat from any ductwork between the module and the building, if it's not directly piped into the building effort to insulate the ductwork is a good idea.

[philb]

I built the window screen solar box from build it solar last fall a year ago. It ran like a 3600 watt electric heater all winter. I have made several air to air heat exchangers in the past. This is the most efficient one so far. I used a snap switch mounted in the center of the panel to turn on a 50 cfm fan when the temp reached ~120 deg F.
I took temperatures inside the box with a HF laser temp gun. The idea was to harvest some hot water also but the temp on the bottom of the box was not warm enough to get anything worthwhile out of it. I had to change the snap switch to a lower value because 3 screens were taking most of the heat out of the box!

So for my money, I'll have panels of water or air but not both.

[gadget]

Originally Posted by philb

I built the window screen solar box from build it solar last fall a year ago. It ran like a 3600 watt electric heater all winter. I have made several air to air heat exchangers in the past. This is the most efficient one so far. I used a snap switch mounted in the center of the panel to turn on a 50 cfm fan when the temp reached ~120 deg F.
I took temperatures inside the box with a HF laser temp gun. The idea was to harvest some hot water also but the temp on the bottom of the box was not warm enough to get anything worthwhile out of it. I had to change the snap switch to a lower value because 3 screens were taking most of the heat out of the box!

So for my money, I'll have panels of water or air but not both.

Thanks everyone.

I think I read through every build on that site. Screen absorber is definitely the way to go for heating air. Very simple and affective.

I was hoping to find someone that has tried heating water with solar heated air via a radiator style exchange. I checked online again to night and looks like I get to be the guinea pig. Any takers want to try it before I get to??

[tantal]

You can use antifreeze fluid to prevent it from freezing.
Or an open tank system for water. It drains water in the tank when temperatures in the solar cell are below zero c.

[gadget]

Originally Posted by tantal

You can use antifreeze fluid to prevent it from freezing.
Or an open tank system for water. It drains water in the tank when temperatures in the solar cell are below zero c.

My original plan was for a drain back system. I already had a below grade insulated tank. My concern is making a panel that can take the high heat. In theory, a cheaper lower temp panel can be made and kept cool with water pumping. But what happens the day the pump dies? Does the cheaper plastic tubing in the panel also die from over heating?