Andreas Nöhrig was sitting in the sunny garden of an eco-hotel on the Canary Islands and felt disturbed by the constant humming of the diesel generator, and asked the manager why he does not use the abundant solar energy. „Too expensive“, was the answer, although a kilowatt hour of electricity from the generator is also everything but cheap. „That was the moment I decided to develop a solar technology which costs only a fraction of today’s PV.”

The technical business consultant made a large matrix over all theoretical ways to use the sun. It is always easier to use concentrated energy, and mirrors are a smart means to concentrate the sunlight. So he created the SunOyster: A parabolic mirror that is bi-axially tracking the sun and concentrating the sunlight in an elongate receiver. The bi-axial sun-tracking – which is essential for concentrated PV – means that 30 per cent more sun is falling on the mirror compared with a fixed collector, and still 10 to 20 per cent more compared with a mirror rotating around one axis. If there is storm or just no sunlight, the mirror closes down through a patented linear movement along its basis. It protects the mechanics and the control system like the shell protects an oyster. This sleep mode allows building the SunOyster in a particularly light and cheap way, no matter whether it is a 5 m² plastic structure or a 500 m² steel construction. A 10 square meter SunOyster for a family home for example weighs only 100 kg.

Another salient point of the SunOyster is the hybrid Receiver. Slightly modified high-performance PV cells for one sun whose surface corresponds to just one per cent of the mirror aperture produce PV electricity, are operating more efficiently at 100 suns. The downside is normally, that monocrystalline silicon looses its efficiency with rising temperatures. So the SunOysters cools  it with water which absorbs a large part of the heat. Where conventional PV generates in average less than 15 % electricity from the sunlight and wastes the rest of the sun energy, the SunOyster comes up to five times of this value in relation to the direct sun light: The system efficiency of totally 75% is made up of 20 % per cent electricity and 55 % heat which can be used for warm water supply or heating support. So it pays the user a quintuple energy dividend. A SunOyster 10 has a peak power of 2 kW electrical and 5.5 kW thermal. That altogether equals the power of 10 horses.

However, roughly half of the energy demand in Europe is needed for heating or cooling. Therefore, the SunOyster can be operated with a purely thermal receiver which is capable to generate water steam of 150 °C. The thermal losses are minimised because the SunOyster receiver is embedded by a strong vacuum. This is so far not yet different from conventional vacuum tube collectors which already eliminate losses from the circulation of air and from heat conduction. The only loss remaining is radiation which is minimised by selective coatings. However, where the state of the art needs dozens of vacuum pipes for 10 m² of surface, the SunOyster only needs two. This reduces not only the loss surface, but also the cost.

With a temperature level of 150 degrees, existing double effect absorption chillers – which are normally powered by burning oil or gas – can twice as efficiently transform the heat into cold. In contrast to this, the lower temperatures of conventional solar thermal collectors can only power a single effect conversion. The big economic advantage of solar cooling is that there is an ideal coincidence between the offer of solar energy and the demand for cold. Combined with a gas or biomass burner, the solar cooling system becomes reliable also for cloudy days (Graphics left).

However, the high temperatures of the concentrated solar power also facilitate storing the energy with a high density. Phase change materials such as paraffin and salt hydrates or in the future thermo-chemical materials can absorb high amounts of heat, reducing the space demand for its storage. This will enable the seasonal storage of heat: The SunOyster fills in summer the seasonal storage tank which is no bigger than a former oil tank with so much energy that it can heat a well-insulated family home over the whole winter.

Moreover, large SunOysters can power solar thermal power stations. They normally heat up thermo oil or other heat transfer fluids to 300 or 400 °C, a heat exchanger generates steam which powers a steam turbine. A heat storage of molten salt helps to perpetuate the power generation for the time when the sun has set. If the sun does not shine, the power station can also fire fossil fuels or better biomass (e.g. wood waste or biogas from slurry) to generate the steam or to overheat it. Power generation based mainly on the sun is then not depending on the sunshine any more, but becomes a reliable base load power.

So far, there are only demonstration plants of the SunOyster in operation. But as soon as it is serially produced from 2010 on, the cost of a kWh PV electricity should in sunny countries fall well below grid parity, with the co-generated heat being so to say for free. And focusing on heat or cold, the SunOyster can produce the energy content of one barrel oil for below 25 US-dollars while the oil price is ranging somewhere between 50 and 150 dollars, and the costs of tanks and burner have to be added.

“The sun sends us 10,000 times the present world energy demand“ knows Andreas. And the major part of the world population lives in sunny countries. So has the vision that the technology can within two decades cover a third of the world energy demand.

Andreas Nöhrig 2009