Our Sun: All the energy we need

You can heat and cool your home, heat your bathing water and make electricity by the sun.
Now you can also make clean drinking water.

Solar collectors – heat or electricity ?

We use thermal solar collectors to harvest the energy for the distillation process. This (old) technology is simple and much more efficient than photovoltaic panels for heating / evaporation of water. There are mainly 2 types of solar collectors: Flat plate and vacuum tubes. For high temperatures, the vacuum tubes are preferred since the heat loss to the surroundings practically is eliminated.

Vacuum tubes are either “heat pipes” or “U”. They look exactly the same but are different on the inside. The U-type is a continuous pipe that runs in/out of all glass tubes – whereas the evacuated heat pipe tubes (EHPTs) are composed of multiple evacuated glass tubes each containing an absorber plate fused to a heat pipe. The heat is transferred to the transfer fluid of the WaterStillar system in a heat exchanger called a “manifold”. The manifold is wrapped in insulation and covered by a protective sheet metal case.

The vacuum inside of the evacuated tube collectors have been proven to last more than 25 years, the reflective coating for the design is encapsulated in the vacuum inside of the tube, which will not degrade until the vacuum is lost. The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss, therefore achieving greater efficiency than conventional flat-plate collectors, especially in colder conditions.

WaterStillar uses mainly the heat pipe type (although the U-type is a bit more efficient), which has a dry connection between solar vacuum tube and the manifold allowing very easy replacement of tubes. Replacing a tube is done while the system is running and there is no risk of pressure loss or hot water pouring out. Transportation of a whole collector is easier as well.

Efficiency and type of solar collector

Thermal vacuum tube solar collectors has an efficiency of approximately 60-70% at high temperatures. Comparing this to photovoltaic panel’s efficiency of app 10-18%, the thermal collectors are the clear choice. Both these numbers has assumptions to them and the industry has developed standard tests to make collectors comparable – but in short thermal solar collectors are 3-4 times as effective compared to solar PV’s

During daytime the solar collector(s) will provide sufficient energy to run the system. During night time the WaterStillar system will either come to a complete hold or be run with a heat controllet and electrical heater. For Works this is mandatory; for Family it is an option. 

Start up time for any WaterStillar system is crucial. The time before the solar collector become hot enough to reach production temperatures may take a long time and in the meantime the production window gets smaller. Therefore all systems are designed to start quickly and use the energy quickly. The downside is that once the sun is down, the production of water stops as well – if not an electrical heater kicks in.

Solar distillation – old news!

Earliest recorded solar stills are from mid 18 century – miners in Las Salinas, Chile got their drinking water from solar stills in 1870. Today solar stills still are a niche product, since filters and deep ground water wells provide fresh water in most places. There are many different solar stills, most of them are not available on the market.

Distillation of water is quite simple. A black box with a sloping glass cover and a collection gutter – that is a traditional basin solar still, just as the picture fra Las Salinas shows

The power of the sun allows a certain volume of water to be evaporated and the efficiency of solar stills are often measured by how much water per m2 per day the still will give. The solar power of a certain day, on a certain location is easily measured. The amount of energy needed to evaporate water at a certain pressure is well known. These 2 factors will give a certain efficiency of the solar still – an oversimplified example: The daily irridiance in hot and sunny place is 6 kWh per m2 and is takes app 650 watt to evaporate 1 liter of water  – then a daily output of (6/0,65) 9 liters from 1 m2 would mean 100% efficiency.

The performance from simple solar stills are always less than 100%. Typical production is in the range of 1-4 liters/m2/day. When the glass gets milky, the efficiency simply goes down.

Improving this small amount can be done in many ways. Reflectors, sloping design, fans, flow through wicks, thermal solar collectors, insulation, active cooling, multieffect design, heat storage and the list goes on. (REF: P. Vishwanath Kumas et al. 2015). Adding this to the basin still improved performance – but also cost and complexity is added to the equation.

Most energy efficient are the multi effect solar stills. WaterStillar Works is a multieffect still. WaterStillar Family is a improved basin still. Both using high performance thermal solar collectors. At WaterStillar we believe that simple technologies are the longest lasting – and we try to design our systems to be as simple as possible. Improving production rates by adding more and more technology to our design is possible – but we also know the consequences. 

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