System and Method of Conveying Thermal Energy
This is a comparison of invention, U.S. Patent No. 7,475,543 B2 “system and method of conveying thermal energy”, to other technologies being used in the solar energy industry. The best systems using other technology are called “SEGS” (solar energy generating systems). I know of two plants currently in use in the U.S., 150 MW at Kramer Junction, and 160 MW at Harper Lake. Both are located in California. A new plant is currently being built for APS in south west Arizona by a Spanish company called Abengoa Solar at a cost to APS (Arizona Public Service Company) of about one billion dollars. The plant is intended to be used to power approximately 70,000 homes at a projected cost to the consumer of more than $0.11 per Kwh.
Here are some of the existing solar options and the cost of their electric power as of 2008:
SEGS (Solar energy generating systems) $0.11 per KWh
Central Receiver $0.14 per KWh
Solar Chimney $0.16 per KWh
Dish Sterling $0.60 per KWh
Solar Pond $0.14 per KWh
Photovoltaic $0.66 per KWh
As you can see the SEGS is the least expensive of these. It's lower cost for power comes from it's relatively simple configuration that enables it to collect large volume of energy at fairly high temperatures. All of it's parabolic trough mirrors follow the sun by use of a very inexpensive single axis tracking device. Every other configuration misses what is needed by not reaching a high enough temperature, or by reaching the high temperature in a small area only. The central receiver misses by being very complex. It has many flat mirrors driven by a pair of expensive 2 axis CNC positioning devices that must track the sun and calculate the angle needed to make the reflected light hit the receiving tower. These devices must compensate for the time of day, the time of year, as well as it's position relative to the collecting tower to be able to hit the tower with the light. I have read reports that indicate that the cost of maintaining these tracking devices is a major factor in this systems cost for generating electrical power.
The only item on the list that is not “Solar Thermal” is the photovoltaic. It's drawbacks are it's high production cost and relatively short service life.
The major difference between the current SEGS systems and the concept in my patent is the substance being used to convey the thermal energy. The current systems use mineral oil whereas my system uses solid steel spheres.
Here is the comparison of some of the values for the physical properties of mineral oil and steel spheres in relation to using them to convey thermal.
Factor Mineral oil Steel spheres Ratio
Usable conveyed 1 2.2 1 to2.2 thermal energy
Working 800° max 1500° estimated 1 to 1.875 temperature
Energy needed to 1.67 .49 3.41 to 1 heat conveying substance (Cp) (kj/kg K)
Thermal .15 43 1 to 286.67 conductivity
Although the heat capacity (Cp) of mineral oil (kj/kg K) is 1.67 compared to carbon steel at 0.49 indicating that only 3.41 times as much energy per degree kelvin is needed to raise it's temperature. An experiment that I have done indicates that if both mineral oil and steel of equal volumes are heated to the same temperature, and used to convey that energy to equal volumes of water, the water receiving heat from the steel has 2.2 times the temperature increase than that of the water receiving heat from the oil. This test was made with a hot temperature of 150° F & water starting at the ambient temperature of 80° F. All of the temperatures were monitored by calibrated digital temperature probes.
This gives us the working difference for the substances as far as the ratio between energy applied, to the energy delivered. The ratio is 1 for mineral oil to 7.99 for steel. But this number is correct only if both systems are running at the same temperature. The system with the steel spheres will be running at 1.88 times the temperature of the oil system, for an actual increase on the order of 15.02 times, plus whatever advantage we get from the thermal conductivity being 286.67 times as good as the oil. These two factors together indicate that the steel spheres can deliver thermal energy on the order of 4306 times as fast as the oil.
If convection is prevented the oil that they use in these systems is actually a very good thermal insulator. I have read reports that give the oil filled systems an efficiency rating of 20%.
Here are some of the disadvantages of the current design of the SEGS using mineral oil and explanation about why they will not apply to a system that uses steel spheres.
1.a. The temperature of the mineral oil in these systems, on the hot side is above it's open air flash point, so that if there is a leak in the pipes the oil will ignite spontaneously in the air. I have read reports of this happening many times on systems of this type. The fumes of burning mineral oil contain toxic gasses. This can make these systems hazardous to use, or to have near populated areas.
1.b. When using the steel spheres the system may operating at 1500° F on the hot side but if there is a break in a pipe there will be no leak of the steel spheres unless the pipes separate enough for the spheres to escape. Any opening from the outside into the system would need to be fixed immediately because the system needs to run with argon not oxygen in the tubes to keep the system free of corrosion. But there should be no danger to people from the leak unless someone gets hit by the spheres, or if there are combustible substances under the pipes (it should be flat bare ground with no weeds or chemicals).
2.a. Solar thermal energy collection systems of the SEGS type have black receiving tubes at the focal line of the parabolic mirrors troughs. These require evacuated glass tubes around the black receiving tubes as an insulator to minimize losses due to convection and wind. As these systems are being used the elements in the mineral oil slowly make their way through the glass wall of the tubes that it is flowing thru entering into the vacuum. I have read that this migration of chemicals thru the solid glass wall of the tube is possible because of the small size of the molecules that are formed as the mineral oil ages and from the heat. This makes it necessary to test and re-evacuate the insulating tubes periodically.
2.b. With the steel spheres the evacuated tubes around the receivers will get much less contamination from the black tube because it will only contain the steel spheres and argon gas. The argon gas will not migrate thru the wall of the tube as easily as the elements in the oil because it's molecules are much larger than the elements in the oil. The argon is already in it's basic form (being an element) and it is an inert gas.
3.a. The low grade of heat and, low thermal conductivity of the oil in the existing systems necessitates the use of very large heat exchangers and boilers.
3.b. The much higher working temperature, and much larger thermal energy package being delivered by the steel spheres make it possible to greatly reduce the size and cost of most of the equipment in the system between the solar field and the boiler. After the boiler both systems (oil & steel) will be the same. At that point it is steam turbine, electric generator, and power control systems, as are all of the municipal power systems, except photovoltaic, and hydroelectric. It makes no difference to the steam turbine where the steam comes from as long as it is clean, has a constant flow with sufficient volume, and the temperature stays above 400° F.
4.a. Sodium nitrate (salt) is often used in conjunction with this type of system as a medium to store thermal energy during the day. It's advantage (above oil) is that it's storage capacity for thermal energy is higher than a simple liquid would be, because it undergoes a phase change which lets it store more thermal energy than the oil would be able to store within this temperature range. In the oil systems the salts used to extend the amount of time that the system can be used to generate electrical power during a day. The systems to handle this salt when it is in it's liquid phase must be made of expensive high temp alloys or the service life would be very short.
4.b. The steel spheres themselves can be stored at high temperatures (1500° F) for use at night, and cloudy days to generate electric power, They don't have the extra advantage of saving power in a phase change as you get with the salt but the fact that they start with much more power than the mineral oi, makes the phase change of the salt no real advantage. Also this eliminates the need of having extra heat exchangers, pumps etc. to handle an additional type of substance in the system.
5. Another factor that has a large effect on the output of a solar energy collections system is the fact that light is a dynamic energy as opposed to static. In other words you can't stop light and still have its energy. For example, if one puts an empty cardboard box open in the sunshine and permits lots of light to go in. If this box is then closed, taken indoors, and re-opened, most people will not be surprised to find that there is no sunlight coming from the box. This is because light is a dynamic energy, not static energy. To save the energy of light it must be converted to some other form of energy. For example a parabolic reflector of a certain size focused on a collection tube is able to raise the temperature of the tube to a certain temperature. After the tube has reached that temperature no more photonic energy will be converted into thermal energy until the thermal energy in the tube has been conveyed away so that the tubes temperature is lowered. In essence the more efficiently you convay the thermal energy, the more thermal energy the system can collect.
One problem that often happens when trying to have thermal energy flow from one solid to another solid is that the actual point of contact may be very small, drastically reducing the effective thermal conductivity. In the solar field where the steel spheres are heated I feel that issue will be fairly negligible because the spheres will be clean and they will have contact with the collection tube at the bottom. The spheres will also be in very close proximity to the tube on a line that goes all the way around the sphere, and the argon gas in the tube should be very close to the same temperature as the tube. My main concern as far as being able to get good contact is when the spheres need to give up their thermal energy at the boiler. This can be improved by wetting the spheres and the outside of the boiler with liquid metallic tin. Tin has almost the same thermal conductivity as steel, and is a fairly safe non-reactive substance, with it's liquid phase covering most of the working temperature range of this system. There are chemicals that can be added to the tin to increase its surface tension and keep it from sticking to the steel spheres. Having the tin make contact with all of the outside of the spheres will make it possible for the spheres to supply thermal energy to the boiler fast enough to enable it to create a continuous jet of steam to drive the steam turbine, without the need of having a large tank (boiler) full of super heated water under pressure. The type of boiler we would use would be similar to a liquid fuel rocket engine, in that you add high grade thermal energy to the water in a small area, and you get a continuous jet of steam.
All of these factors working together will enable us to use solar energy to generate electrical power at a cost that is much less than the cost of fossil fired power plants.
After the cost of the equipment and the land have been paid for, the electrical power is almost free. There is no fuel cost, barren land in the desert southwest far from the cities can be had for a very low price. The only expense at that point is maintenance and a small amount of personnel to act as caretakers, and to protect the equipment.
The only problem I have been able to find with this new concept is the fact that most people are rather conservative, and reluctant to change their point of view, or to embrace a new concept.
Most people seem to think that the difference between the substances being used to convey the thermal energy would not make much difference in the overall result in this application, or that if this concept was as good as I say that it is, then someone would have noticed it a long time ago.
People have been dealing with many of the concepts of thermodynamics for more than a thousand years without coming-up with this idea, almost as if they were wearing blinders like a race horse
The subject of thermodynamics seems to be misunderstood, or not understood by most of the people that should understand it.
There are three power plants that I mentioned plus many other, un-named that could benefit tremendously by being converted from mineral oil to steel spheres. Their outputs could be increased to between four to five times their current levels without increasing the amount of parabolic collectors. Also there is the possibility of building new power plants to replace other types of energy.
All of the physical constant values in this comparison came from www.engineeringtoolbox.com except for the “Usable conveyed thermal energy” ratio of 1 to 2.2 which was determined by experimentation done by me.
Kenneth Bruce Martin