Bob Ramlow

Water is the best heat transfer fluid from the respect of efficiency. It is also the best heat storage medium from the respect of heat able to be stored per cubic foot.
If water did not freeze at 32 degrees F., then you would see water as a heat transfer fluid in every residential and most commercial solar water heating systems. So all solar water heating systems that operate all year round in the continental US must be freeze protected. There are 2 types of systems used worldwide for residential and most commercial solar heating/cooling systems, pressurized and drainback. You can read about how those systems work in my book (it is available for purchase and also should be available at your local library). Drainback systems use either pure water or a weak antifreeze solution as the heat transfer fluid and the fluid drains from the collectors every time the system turns off. In a pressurized system the collectors are filled with a non-tozic antifreeze.water mixture at all times.
solar water and space heating and cooling systems do not require temperatures above the boiling point of water, so there is no reason to go there. Because solar energy is low-grade heat, it is difficult to make steam on a reliable basis except in very specific places on earth and with very specialized collectors. These systems are called CSP (concentrating solar power) and are all large facilities that are in high desert regions of the world and are very sophisticated systems that typically operate around 600 degrees F. They in fact often use an oil as their heat transfer fluid. They do make steam and turn a turbine to make electricity. These plants are the cheapest way to make electricity on the planet, with the added benefit of no pollution.

I recommend my book as a resource for solar water heating systems. While there are plenty of good resources on the internet about this subject, I would caution you that there is a lot of mis-information out there too.

Bob
www.arthaonline.com

Back in the 1980's some companies advocated using oil as a heat transfer fluid. The most popular oil was a brand called Bray 888. I imagine that oil is still made.
The problems with using oil are: it can not, in fact, carry a lot more heat than water. Water is actually the best fluid and all other fluids heat carrying capacity is compared to water. The seals and gaskets, including the bladder in expansion tanks are not compatible with oils. When oils are used in solar water heating systems, special seals must be used in all components. Viton seals are typically used.

I don't have any specific examples of mold polluting the air. Perhaps one reason is that all of the systems like this that I have encountered have been capped off and abandoned. All the systems like this that I am experienced were in homes. They were used for cooling during the summer and for make-up air in the winter. There is no question that mound will grow in these buried plastic pipes. I know it is well documented, but I do not have any references on hand. Perhaps the mold issue would not be as big a concern in a greenhouse as it was in a home where people are living all the time. Also, if it wasn't an issue, then why would a manufacturer go through all the trouble of making anti-microbial drain pipe.

Tube collectors do work best in warm climates and flat plate collectors work great in all climates. My experience with tube collectors has been less than perfect while I have seldom had a problem with flat plate collectors.

In a freezing climate a solar fluid is heated in the collector and is typically pumped to a heat exchanger where the solar heat is transferred to the domestic water and that water is stored in a storage tank. There is little heat loss in the solar piping loop if they are properly insulated, so distance in this loop is not critical. I have a 30+ year old solar water + space heating system here in Wisconsin that is 100 ft. from the heat exchanger and it works fantastic.

Now, the distance from the solar storage tank to the back-up water heater is much more critical - you want these close together because when you use hot water, solar preheated water flows from the solar storage tank to the back-up heater and if the distance is long and the draw is small, then the preheated water never gets to the back-up water heater.

So the distance is not critical from the collectors to the heat exchanger but is critical from the solar storage tank to the back-up water heater.

Bob

A flat plate collector is the most common solar water heating collector used in the world today. Ask google.

Bob

Xisca, with a direct solar water heater in a non freezing climate, then distance certainly is an issue. Posts below give you an idea of the volume of water in a pipe. Make it as short as possible.

Bob

Same deal with the solar oven, single glazed is fine.

I am reluctant to use plastic collectors for domestic water heating, From the plastic collector mfg. web sites they claim that these heaters will only heat the water to 15 degrees above ambient, therefore it is difficult to get the temperature of the water very hot most of the year. I prefer flat plate collectors in almost every case. they do not cost that mush, last a generation and get the water hot, but not boiling in most situations. Also, I have little experience with what you are asking for, so I must defer on this one.

Bob

The simplest seasonal solar shower is a barrel mounted on a stand. preferably at least 6' high to the bottom of the barrel. The barrel is painted black. There is a shower faucet attached to the barrel. You can even have a shower curtain attached to the stand for privacy.