Jeroen van Luijtelaer: Rotating solar boiler (2005)

The proposed solar collector (ref 1) has the potential to greatly expand the use of solar energy. This will decrease the world’s dependency on fossil fuel and bring our society closer to a sustainable future.

I am convinced this is the future for solar collectors, and determined to make my design work. However the project is becoming too expensive and time consuming for a spare time project and I have no space for constructing a larger, more sophisticated collector, therefore I request assistance. Any assistance is welcome, financial assistance as well as (a sunny) space.

Solar collectors are well known devices. The sun heats up everything and so constructing a solar collector is easy. The tricky part is to produce high temperature heat with a dirt-cheap collector. So far solar heat collectors are either too expensive to compete with fossil fuel or supply low temperature and low value heat that limits its uses.

The idea is to make a solar heat collector that costs less than a flat plate solar collector yet has the efficiency of an evacuated tube collector (ref 2). Convective losses are eliminated by rotating the collector (app 1). This is a completely new method. No patents or publications were found after an extensive search. Furthermore experts in the field have not seen it before (ref 3 and 4). I have done preliminary experiments in my spare time and concluded this idea is feasible. For less than 100€ I built a 0.5kW solar collector that produces steam at 100oC. The motor consumes 8 watt to achieve sufficient rotation. The collector essentially consists of two tubes that rotate (at 75 rpm). The simplicity of this design is a huge advantage.

Steam evolves from the rotating solar collector (February 5th 2005, Delft)

The small-scale prototype

The plan is to construct the collector from two inflated plastic film tubes. The inner tube (the absorber) may be made of metal for now just like the prototype. The tubes will have diameters of about 2 m and 2.5 m and approximately 10 meter length. This will bring the cost and weight per square meter down to an absolute minimum (app 2) and yield a 40kW collector that produces steam.

The shoestring budged was not sufficient to obtain all the appropriate materials. Advanced materials can make this design much more efficient and reduce the pay-back time. For instance an advanced coating on the inner tube can improve the efficiency from 50% (for the prototype) to over 80% as well as increase the operation temperature from 100oC to 200oC

Soloar Boiler Theory

The rotating solar collector prevents convection from taking place. In this appendix it is explained how that works.

1.1 Convection theory

The design I came up with has a means of preventing convection. First I will have a look at how to prevent convection in a solar collector.

Fig 1.1: gravitation causes convection

Hot air rises due to gravity. The hot air inside near the absorber of a flat plate collector is lighter than the colder air near the glass cover and hence it tends to rise towards the glass. There it cools down and sinks to the absorber plate. This effect cools down the absorber plate, because air transports the heat from the absorber to the glass cover.

Placing the solar collector upside down prevents this cooling but obviously the sun does not shine bottom up. A mirror can be used to reflect the sun onto the absorber and operating a collector this way may seem promising.

1.2 Rotating the collector

Convection occurs due to gravity. Adding the centrifugal force to the system can have a dramatic effect. In effect the rotating collector is a centrifuge. In a centrifuge denser material is pushed outwards. This also applies to air and other gases. In a centrifuge the colder (denser) air accumulates on the outside and the hotter (lighter) air is pushed inwards as can be seen in figure 1.2.

Fig 1.2: centrifugal force prevents convection

The two tubes and the air in between rotate preventing convection in the inside air. A modest rotation speed can already achieve this desired effect. The effect prevents convection completely when the centrifugal force is greater than the gravitational force in the entire air layer.

This simple phenomenon has not been described before. This may be due to the fact that neither air nor convection can be seen. This also made the phenomenon subject to discussion on whether it occurs and to what extent. The prototype proved the effect occurs. Stopping the motor also stops the steam flow completely due to the added convective heat loss. Theory says no convection occurs at all if centrifugal force exceeds gravitational force in the whole air layer. Furthermore convection is suppressed incompletely at lower rotation speed.

The benefits of scaling up

The air layer thickness should be at least 10-20 cm thick to minimize heat conduction. If the inner tube’s diameter is 0.3 meter the outer tube should be at least 0.5 m. If the inner tube is 2 meters the outer tube should be 2.2 meter. So a bigger collector yields more black surface per total collector surface and hence a higher ground surface efficiency.

The ends of the collector should be insulated well. This is an investment that is not dependent on the length of the collector. Making the collector twice as long halves the cost per square meter for this insulation. This also applies for the motor and the water and steam connections. These investments are almost independent of the collector size.

A drawback of scaling up is that rotation drag increases. The drag of the smooth rotating tube in the outside air is very small and motor energy consumption will be a fraction of the heat gain. Problems with transport are expected to become a somewhat bigger problem.



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