Solar cooking in a family is normally limited to a short period of time before the meals. If the Scheffler reflector could also prepare hot water (always needed) for the home in the remaining sunshine hours, the installation of a Scheffler reflector would be even more attractive for the user.

 

 

A water cooled shutter in front of the focus allows to regulate cooking power and to heat water at the same time

The shutter is build as a flat, hollow vessel of 20 cm diameter, through which the water circulates. The connections are made with two heat resistant, flexible silicon rubber tubes of 10 mm inner diameter and 1 mm wall thickness. The cooler water enters the shutter at the bottom, while the heated water leaves near the top.

 

Insulated hot water storage tank inside the house

The hot water storage tank is placed inside the house. It is an insulated stainless steel vessel of 38 cm diameter and 50 cm height (50l). The cooler water leaves the tank about 4 cm above the bottom, the return- connection for the heated water is in the centre of the tank. The bottom of the tank is higher than the receiver, and the water moves by thermosyphon from the tank to the receiver and back.

 

 

Circulation of the water

The flow resistance of the flexible hose is such, that 15° cold water is heated to a temperature between 60° and 70° C in one pass through the receiver. Once the whole content of cold water has passed, already hot water is entering the receiver and is heated until it boils and some steam bubbles are formed. The bubbles move upward rapidly tand pull the water in the hose along ( like in a coffee machine ). This way the water moves more rapidly and enters the tank with about 100° C.

 

No plumbing required

The 50 litre tank is not pressurised and can be used independent of the availability of piped water. Water is filled through a bucket, which is integrated into the top of the tank insulation. From the bottom of the bucket a pipe passes through the lid and ends just above the bottom of the tank into a diffuser, which slows down any turbulence. This way the different layers of hotter and cooler water are not disturbed. The displaced volume of air leaves through a venting pipe in the lid.
A transparent hose fitted to the side of the insulation serves as water-level indicator.

 

Temperature layers in the storage tank

Inside the tank, the hot water from the receiver moves through another flexible silicon rubber hose, whose end is fixed to a float. This way it is always entering near to the water surface, independent of the water level, and does not mix with cooler layers below.
The hot water for use is also drawn near the surface ( where the temperature is maximum ), with the help of another hose fixed to the same float.

 

Scaling

Whenever water is heated above 65° C, deposits are formed on the walls of the vessel and the tubes. These are easily removed from the silicon rubber tubes by gently pressing and deforming them. The tank itself has a large screw on lid for periodic cleaning.
Most scaling happens in the receiver, where the water often reaches boiling point. Here the deposits are also harder. The receiver can be easily disconnected from the two silicon rubber hoses for cleaning purposes. It can be opened and the residues removed mechanically or with acid.
Because of the frequent thermal cycling, the deposits tend to brake into small flakes and come off easily.
We have one system installed here at our home, and are presently collecting data on the long term experience with scaling.

 

 

First experiences

Now the system at our home is in operation for one month ( a very sunny month indeed, with 22 sunny days out of a total of 32 ), on sunny days it covered the total hot water requirements of three people ( taking bath, dishwashing, washing of clothes, hot water for cooking ). Even when 6 people were there on weekends, there was sufficient hot water. We take bath the comfortable traditional African or Indian way , where 10 litres of 42° C water are plenty for a splendid bathing session.

The hot water is tapped straight from the tank, so that there is no disadvantage of any piping losses. Otherwise often more than 5 litres have to pass through the piping until you get the comfort of the hot water, and this every time you open the tap!

During daytime, the water can be taken from the tank with more than 90° C. The next day, if the sun does not shine again, it still has a temperature level between 80° C to 70° C.

 

Insulation

The storage tank is insulated 10 cm thick with Styrofoam and sheep wool. The heat loss in 24 hours is about ¼¼ of the stored energy content. This quite rapid cooling is mainly caused by the relatively small volume of 50 litres.

 

 

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Instrumentation

Four temperature sensors are fitted to the tank at different levels, one at the top, one at the bottom and two in between. This way visitors can easily visualise the maximum temperature and the temperature distribution inside the storage. Also the heating-up during the day can be followed, as the hot zone slowly extends downwards.