The Suncone Solar Power Generator

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2. PROPOSED INNOVATIONS

 The Configuration of Suncone

 In Suncone, the cones consist of thin aluminized Mylar, Nylon or other film. Instead of having air pressure inside the cone to maintain its shape, air pressure is applied inside a cylindrical enclosure. The conical shape of the interior cones is maintained by tension on the film, since air pressure is pushing upward on the end of the unit. Air pressure maintains the cylindrical shape of the enclosure, which also consists of a strong plastic. The transparent films that cover the ends of the cones are made of clear plastic, such as Tefzel, which has a transparency of 96%, a tensile strength of over 30,000 psi, is UV resistant, and can tolerate weather for decades. Since Tefzel is rather expensive, other suitable films may be used.

Figure 2 shows a cross-sectional schematic of one embodiment of Suncone. The inside surfaces of the cones are aluminized for high reflectivity. The outside surfaces of the cones are coated by flat black, which radiates heat well. Computer simulations show that the cone material remains cool, since the inside reflective layer allows little solar energy to enter the plastic, but the outside black layer radiates the heat away. The enclosure should be clear so that it allows the radiant energy to pass through or should be black plastic or coated with flat black so that it absorbs the radiant heat from the cones and radiates the heat away on the outside.

The question has been asked about the convection of hot air from the target rod to the plastic. The target rod should be surrounded by an evacuated glass tube, similar to that used in trough collectors to prevent heat loss. A perfect vacuum is a perfect insulator of convective and conductive heat losses.  Radiation from the target rod can travel out through the glass, but that radiation is reflected away by the cone.  The glass transmits most of the incoming radiation, reflects some of it, and absorbs a tiny amount, so that it can begin to heat up.  It can radiate some of this heat.  The rest of the heat from the glass is carried away by air convection.  When the calculation is made, it is seen that the heat absorbed by the glass is quite small compared to the heat that can be radiated away by the plastic cone's black outer coating at temperatures well below the maximum working temperatures of the plastic.

The sun's rays are concentrated on the target rod, which has channels inside for the flow of water or other working fluid. A metal reflector surrounds the target rod. The plastic cone is attached to the metal reflector with an insulating connector. The metal reflector and the target rod are attached to the base, which is shown as a solid circular cylinder, but it may be any suitable assembly of metal beams. The structure does not have to be as heavy at that of a parabolic dish, since it does not have to be as rigid and since it does not have to support a long metal boom that holds a heavy target at the end.

Figure 3 shows a top-view schematic of Suncone with seven cones. Note the spaces between the cones that appear to be wasted area for solar energy collection. The cones could be extended so that the total area is used, but the upper end would not be circular, which would not hurt the performance but would make construction more expensive.

Figures 2 and 3 show schematics of assemblies that have only a few cones. If the cones are 2 meters (6.56 feet) long with a radius of 1.5 meters at the upper end, it would require 7 cones to provide a total of 50 m 2 of solar collection. This would be similar to the arrangement of Figure 3 with a central cone surrounded by 6 other cones. Of course, more cones can be added.

For photovoltaic applications, the rods could be larger in diameter and coated with photovoltaic films. The metal reflector might also be covered with photovoltaic films and would be conical in shape. The concentration of light would provide higher energy collection per unit area of photovoltaic material.

It should be noted that the target rods are completely shielded from ground observers, so that eye damage to passersby is impossible. If Suncone is accidentally pointed toward the ground, it will not be pointed toward the sun, so that it cannot start a grass fire. A parabolic reflector, on the other hand, can intercept sunlight even when it is not pointed directly toward the sun, and the reflected light can ignite fires on the ground. Suncone units could be mounted in parking lots above cars to generate electricity for nearby buildings without concern for the safety of people or property below them. They could also be mounted on tops of buildings. Engineers would be reluctant to place parabolic reflectors in these locations.

2.2 Structural Calculations

A small blower or pump provides the slight air pressure that maintains the shape of the plastic films. In order to show that the structure will be rigid and the plastic can tolerate the stresses, we can calculate what stresses are applied to the plastic films of the assembly. Consider a Suncone with a total solar collection area of 50 m 2 in which there are 7 cones that are 2 meters long and 1.5-meter radius at the top. If the internal air pressure is 0.2 psi, the total force on the upper end would be 19,700 lbs. Total radial force on the enclosure film would be 17,500 lbs. If we add diagonal cords or wires internally running from the base to the outside top rim and spiral cords running around the enclosure, the structure would be quite rigid. For additional rigidity, guy wires can be attached to the top rim and connected to extensions from the base. If the enclosure film is 20 mils thick, the stress on it would be 1,770 psi (neglecting spiral wrap-around cords).

The upward force on each of the transparent windows would be 2,200 lbs. Using 10 mil thick clear plastic film, the stress on the plastic would be only 590 psi, which is small compared to its tensile strength. This force is transferred to the cones, which transmit the stress to the metal reflector. The highest stress on the cone is at its narrow end. If the metal reflector is one foot in radius, the stress on the 5-mil thick plastic film at the connection point will be 5,800 psi. Metallized Dura-Lar film has a tensile strength of 30,000 psi. Of course, most of the stress on the cone can be relieved by having wires run from the base to the top where they could connect to rings that are attached to the top of the cones and to the transparent cover.

These calculations are presented here to show that it is feasible to construct the rigid structure with lightweight plastic films. If there is concern about the effects of wind on such a light structure, the calculations show that there are almost 30 pounds of force exerted outward on each square foot of surface area. By having interior and spiral circumferential cords or wires that counter the surface forces, we see that the structure will be quite rigid.

2.3 Solid Scientific Foundation

It would be difficult to determine which reflecting surface geometries would be efficient solar collectors and would be insensitive to sun-tracking accuracy just by examining a drawing. To put the determination on a scientific basis, a ray-tracing program called SUNCONE.F was written to simulate the performance of solar concentrators. Several thousand rays per second (of simulated time) are traced from random locations on the sun to random locations at the mouth of the cone. From there, each ray is traced to an intersection with the cone or rod. At each intersection, part of the ray is reflected, and the rest of the energy is absorbed into the surface. The amount of energy that is reflected and absorbed depends on the reflection coefficient. The ray continues on through multiple reflections until it exits the cone. This method is extremely accurate in determining the performance of reflectors and absorbers of various geometries, if the emissivities and reflectivities are properly defined.

After all the sunrays are traced for a one-second duration, radiation from the cone and rod are simulated. The cone, metal reflector, and rod are divided into numerical cells. Since each cell receives energy during the sunray simulation, some of that energy is used to heat the cell, and the rest is radiated according to the equation,

P = e A s T 4

where P is the radiation power, e is the emissivity, A is the area, s is 5.6699X10 -8 in SI units, and T is the absolute temperature in degrees K. The solution to the problem of how much of the energy is used for heating the material and how much is radiated is determined by iteration. The radiated energy for the one-second time interval is emitted from each cell by random rays, which are then followed through multiple reflections. These rays also impart energy to the cone, metal reflector and rod cells. After rays have been emitted from all the cells, we note that the cells have received more radiant energy, due to the radiation from all the cells. That is, the cells lose energy by emitting radiation but gain energy by radiation from other cells. Thus the process must be repeated in order to heat the material further and to radiate the extra energy. Without the iteration on the radiation, the results of the calculation would be in disagreement with experiment.

Figure 4 shows a ray-trace of only about 100 rays. Only the sunrays are shown (in red). The program has the capability to also show the rays (in blue color) that are emitted from the cone, metal reflector, and rod, but they are quite numerous and make for a confusing picture. When several thousand rays are plotted, the interior of the cone is solid red. For clarity, the rays in the plot are shown in the plane of the paper. In a simulation for real performance, the geometry is three-dimensional, and the rays move in all directions in the cone. The configurations of Suncone can be varied to those such as Figure 5, which has two plastic cone frustums, an exponential generatrix shape for the metal reflector, and the target rod is located in a hohlraum chamber. The ray-trace program can quickly show the advantages of such a configuration.

 Computer simulations with SUNCONE.F were compared to the experimental runs of Dr. Reed Jensen (3) on the Solarec Concentrator in which a parabolic mirror dish focused light on the mouth of a reflecting cone. The light intensity at the mouth of the cone was 1,000 suns. The cone concentrated the sunlight further to heat a 10-cm long zirconia rod, 0.6 cm in diameter. The cone mouth was 4.44 cm in radius, and the cone was 15 cm long.

Measured temperatures were around 2,600 ° C without CO 2 flowing. SUNCONE.F simulation showed that temperatures along the rod varied from 2510 to 2774 ° C with an average of 2670 ° C. Thus the calculation was within 3% of experimental value.

When the CO 2 was flowing, energy was used to heat the gas and dissociate it. This lowered the experimental temperature to 2350 ° C. The heating and dissociation required 1251 watts. When SUNCONE.F simulated this by extracting 1250 watts from the rod and cone, the resulting temperature turned out to be 2310 ° C, within 2% of the experimental value. The computer simulation showed that of the 6,193 watts entering the cone, 4450 watts (72% of the input power) exited from the front of the cone, and 493 watts (8%) exited the back of the cone as reflected and radiated light.

•  The Kinetic Pump

 The patent-pending Kinetic Pump provides an efficient method of pumping high-pressure water using steam generated by solar energy. It can be used to pump high-pressure seawater or brackish water into a reverse osmosis unit to desalinate water. Governor Richardson's Water Innovation Fund in the State of New Mexico has granted $458,000 for the development of the Kinetic Pump at HYTEC and for the development of a demonstration desalination plant by Sustainable Resources, Inc.

The Kinetic Pump can also be used to efficiently pump high-pressure water to drive a high-efficiency Pelton water turbine for the generation of electricity. Where there nearby hills, the Kinetic Pump can efficiently pump water up to a pumped storage reservoir during the daytime, and the water can flow back down at night to drive the Pelton turbine so that the system can generate electricity 24 hours per day using solar energy. See Section 7.

The reason the Kinetic Pump is so efficient is that it uses both the initial positive displacement by the steam and then uses the adiabatic expansion of the steam to pump the water. Without the Kinetic Pump, steam from a solar collector would have to flow through a turbine, which would drive a gearbox connected to a multi-stage centrifugal pump to produce high-pressure water, and the efficiency would be low. The combination of Suncone and the Kinetic Pump can provide an economical method of producing electric power or desalinating seawater.

Introduction | Part 1 | Part 2 | Part 3 | Part 4 | References