Then I started thinking, I find some skrap plastic pipe, paint it in black or find a black one, place it on the sun and attach it to the water container. I'm sure it won't work that way, water has to circulate, so I need water circle. Ok, if I dug another water line in the ground, or leave it on the surface but paint it white, water will heat in one pipe, cool in other, and convection wil start if pipes are near the ground, and water barrel is above that level.
Slept over and imagine what will happens with water in the pipes when it's cold outside. I figured out already that when sun heats the black pipe, convection start water circulation. Since water is not heated inside white pipe, obviously it is cooled. I'm cooling my heating system. Now I'm thinking, what if I place white pipe inside the black one?
Does picture explane what I'm thinking?
Let's say it's freezing outside, and my water container is warm. Water wil not go down, for sure, if the pipes are cold. But when sun heats the black pipe, it heats the water between 2 pipes. When this water is warmer than in the container, it goes up and replaces it. Water from container goes through thin pipe down, but it doesn't cool since it is "insulated" with hot water. Once it hits the bottom point, it is heated again and goes up Of course, black pipe is closed on the bottom. If I can guess, I would say I can build quite efficient system with pipes and a bucket or barrel? To make it more soficticated, I would probably place water container inside insulated locker and whenever I need heat, I would open the door, or something like that. But what you say about the system itself?
Brian Knight wrote:I think that making the pipe more of a loop would create more circulation and thus more thermal transfer to the tank.
I agree. I only don't know how much flow I need. Need experiment.
Brian Knight wrote:I also think your cabin shell would need to be ultra efficient to be heated by the standby losses of a hot water tank.
It's nice issue. I guess thermal loss of metal water tank would be huge, I can't imagine that when water is not heated anymore, it will stay warm for long time. I need actualy to check does a water have enough energy to cover thermal losses of my cabin. I calculated thermal losses of 50W, more or less. But to be sure let's say it's 100W, depending on temperature and many other factors. During 24 hours it is 2.4 kWh or 8.64 MJ. Water heat capacity is 4 J/gK. If my tank is 100 litre, water weight is 100.000 g. If water is heated to 30C, before it reaches 15C it will loose 6 MJ. So to keep cabin heated to 15C(60F) I need more than 100 l tank (26 gallon) heated to at least 30C (86F). Right?
thermosiphon should stop after tank is hotter than hose.
I think the bigger variable is your cabin heat loss calculation. As you mentioned, there are many variables that affect this math. There are lots of programs available to do the calculations but I suspect with your simple cabin shape, there should be a free, simplified spreadsheet out there to get close enough. Here is a great, new article that explains how to go about sizing your heating equipment; http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
I commend you for trying to go solar Milan, but are there reasons you are avoiding a woodstove or the rocket mass heaters that are so popular on this site?