Solar Hot water

Solar hot water

For rooftop hot water there are two types of collector. The flat plate and the evacuated tube.

Flat plate col​lector

The flat plate is an insulated box with a sheet of glass letting in the sunlight. Inside the box everything is black so it gets hot. A black copper tube runs along the black absorber late and heats the water.

Evacuated tube

The evacuated tube in more efficient, but more expensive. The insulation is by a vacuum. inside a copper tube is a liquid that will evaporate and carry the heat energy up to the  water running along the top. The vapour condenses and returns as a liquid.

 

Evacuated tube

There are two different arrangements. The water tank can be on the top, or down on the ground. 

Thermosiphon

The cheapest is the thermosiphon with the tank above the collector. Flat plate or evacuated tubes. The cold water enters the pipes from the bottom, then as they are heated by the sun, they expand, becoming less dense, so rise up to teh header tank on top. Cold water from the bottom of teh header tank sinks and falls back down the tube feeding the bottom of the collector.

Split system

If the roof it not strong enough to take say 300 KG of water, or if the owner is bothered by the look of a large tank, then the tank can be located on the ground. As the thermosiphon would then work in reverse, it is necessary to pump the water with a small pump.

 

Split system

Small scale rooftop fresnel mirrors

Chromasun is a company producing rooftop units for hospitals, food processors or any other organisatioin requiring heat and cold.

The linear fresnel mirrors reflect sunlight onto a water  pipe heating it up to 204 oC (400 oF). This can be used for processing or to drive a cooling system for air conditioning. The working fluid can be oil, salt etc.

They can be installed on a hospital and coupled with a double-effect absorption chiller to provide air conditioning directly from sunlight.

In future the units will be hybridised with CPV units to provide both electricity and hot water. The first ever such hybrid unit was installed in San Jose.

(CPV = Concentrating Photo Voltaic)

Solar pond

Solar ponds are salty water with the saltiest water on the bottom. As it heats up the heat is locked in the bottom layer as it is too dense to rise up with convection. It heats the water up to 80oC and supplies heat at 40-80oC 24 hours a day.

The energy costs $10-15/GJ. 
Heat in rural areas costs over $20/GJ for LPG (at 43 c/litre).
Heat from electricity (that is, direct heating rather than from heat pumps) costs over $45/GJ at peak rate, and $9/GJ off-peak.
Solar pond heating would not be competitive in areas where natural gas is available, since this is priced typically at only $4-5/GJ. 

The technology for extraction of heat and conversion to electricity is well understood. (Basically an industrial air conditioner run in reverse)
The scientific principles on which solar ponds operate is well understood and documented. Source

Desalination requires 1KW to produce 10 litres of fresh water per day (24 hrs?) Source - RMIT