I‘m planning to buy some black tarp, put it on my roof, lay out a meander of copper pipe on it, and cover the whole thing with clear, corrugated polycarbonate panels. Will that work?The climate where I live is a bit like that of Oregon, or the southern UK (I‘m in New Zealand). So, usually no ice, and a narrow temperature band tending toward 15 Celsius.
What do you intend to do with the warm water? Heating the house or coming out the hot water tap? How will you move the water? The longer it sits in the pipes (paint the copper black, it's too reflective all shiny) the warmer it gets, so you don't want to have it move only when you turn on the tap. What most people do is get a new water heater for traditional heating and use the old one with a little circulating pump (solar panel to power it with battery backup) to store the solar-heated water. A thermostat tells the pump how fast or often to circulate the water through the heater. A slightly alternate method is to use your solar system to preheat the water coming into your regular water heater. Way less energy is used to heat warm water than cold. You won't get much result on cloudy days, such systems require direct sunlight. I'm sure thermostats for moving water can be had which can be set to quit at night.
In the 70's, magazines such as Popular Mechanics and Mechanix Illustrated often featured articles on building thermal solar collectors just like that though they often used plate glass which would trap and concentrate the infrared heat (greenhouse effect). Polycarbonate panels wouldn't last very long as plastics break down in UV light. Usually the process is to build an insulated box in which copper pipe meanders over a copper plate to which it's welded to and the box is covered with a glass pane. It should get plenty hot enough for heating and for hot water, you could use small gas fired tankless heaters as backups near to the faucets, ie.: one per bathroom and one for the kitchen and laundry room, that will also give you instant heat instead of waiting for the hot water to come from the storage tank. It won't be hot enough to run a lithium bromide chiller for air conditioning or refrigeration, that would require an evacuated tube system but it sounds like your climate doesn't really need air conditioning.
Yes and No, your design has a flaw. A flat plate solar collector panel works by transferring heat through radiation, convection and conduction. A coil of copper pipe (or even black polyethylene) will collect heat (by radiation.) Glazing on the top in the form of your piece of clear corrugated polycarbonate will trap in radiation and create a greenhouse effect. Also the piece of black polycarbonate (or tarp) on the bottom will act as a target for the sun's heat (transfering radiation to heat in the tarp which will have to heat the air.) But in the usual flat plate solar collector design the target transfers heat to the pipes through direct conduction in addition to what the air may transfer to the pipe through conduction and convection. The black polycarbonate on the bottom will not be conductive of the heat as if you had put aluminum or copper painted black and connected to the copper pipes. As a result you will capture less heat than you would otherwise. More heat will tend to build up within the air above the tarp. If the air leaks away you will be losing much heat. You might instead consider putting black fins on the copper pipe as the main transfer of heat will be through the air and radiant heat. The typical solution also involves capturing the air within a box. In your area you may also wish to consider the benefits of double glazing for additional insulation to prevent heat loss through the front glazing. The sides and back of your box should also be insulated. You are thinking.
