Solar furnace plan

I've seen so many preppers/survival's worried about how to power the gadgets after SHTF.Some ideas are great,others are a little wasteful..more worried about charging a cel phone than anything else.Most people in that scene would prefer to stay warm and dry,rather than worry about watching a movie on the laptop.If shtf were a solar flare or emp,most of this will be lost,even a solar panel and the controls.So lets ignore all electrical stuff for now. You can cut firewood to cook with,stay warm,light,but if you could reduce the amount of wood needed,you'd cut your work in half,so let's look at that.
Look up fresnel lense on youtube.These things are super cool,but all I see them used for is fast cooking or melting steel,all useful in shtf,but they do nothing else with them.Follow along with my ingenious thought.
You've probably seen the racks used in a fireplace to hold the logs,with big gaps to let the ash fall thru.Since we're not burning anything,lets close the gaps.Use pipe or steel tubing,thick walled,welded together to make a L shape,with holes drilled to allow airflow thru it.Say 2 ft wide,deep,and tall.Take the top ends,connect to a pipe leading into your trailer,house,cabin.Set the lens up so its out of focus on the bottom section.If its in focus,you will melt it!Now,the sun will heat the bottom legs,heat rises thru the uppers and into your home!
Now you have a solar furnace!
Yes,I know,no airflow.Add a little muffin fan from a computer or such.
But what about at night?
A little more work.You could plumb water thru the bottom of the legs to heat water,pump it into a tank,like an old water heater,and run that thru old car radiators at night.Or pipe the air over a bed of gravel to act as a heat sink during the day,stay warm thru the night.
You probably won't stay 80 degrees all nite,or all day,depending where you are.But aside from the initial cost,its free,easier than hauling wood all day,and maintenence free.
Any thoughts?

It would only work when the sun is out and you'd have to keep repositioning the lens.
It wouldn't work at night, when you really need it.

reply to post by GalaxyEyes


you want it out of focus,so you won't melt it. Use the excess heat for water,or a heat sink.....read it all