There are several Design Choices that have been left open in the design process. It is difficult to test these on a small scale so it is important that we make the right choice before full scale testing. If the wrong design choice is made, we might have to rebuild the vehicle at great cost.
We have listed some of the more difficult design choices below. Any advice to help make the right choices will be greatly appreciated.
1 Heat or electrical storage- In very early designs we wanted to use solar salt to store heat. A well insulated tank filled with a particular salt that would be heated by hot oil in order to melt the salt and store the heat. It would be difficult to run the system at 300C in order to melt the salt. The oil would have to be pressurized and more heat would be lost at this high heat. Heat storage is also heavier than most batteries types. A large 30hp steam turbine would need to be used to convert the heat to propel the car. All these complications resulted in electrical battery storage being our preferred design.
Heat storage has great potential for stationary systems where weight is not a concern (see heat storage). A very inexpensive system can be made using rocks and oil to store large amounts of heat energy for home electrical or heat use. Heat storage tank insulation in larger tanks can last days without losing much heat.
2 Car or Trailer driven- To simplify the system I would like to drive the wheels of the trailer instead of driving the rear wheels of the car with solar power. In this way the trailer system would be independent from the car. Only the trailer hitch and a small electrical cable with data and throttle control would connect the trailer and could be easily disconnected to separate the trailer so it could act as a stationary power unit. The trailer wheels would be driven using the electrical drive motor and a differential. My biggest concern with this design would be the trailer getting unstable when it pushes the car. Also during heavy steering the trailer would push the car sideways.
The other option is installing the large electric drive motor in the trunk of the car and driving its back wheels using solar power. The problems here have to do with space and installation. The car will have to be heavily modified in order to install the large 100lb drive motor. The car will also have to be modified in order to allow the drive motor to drive the rear wheels. It may also be possible to install some batteries in the car so it would just charge from the trailer and could then be separated to drive on solar power.
3 Receiver tube, stale air or Vacuum insulation- A test was done up to 199c with stale air insulation (see stale air test). The efficiency was very low at 199c with stale air. Stale air maybe satisfactory below 150c but vacuum insulation would be preferred to increase the efficiency of the system. My concern is the cost and complexity of creating a vacuum around the 24 food long and 2 inch diameter pipe. Then this whole system will be driven on bumpy roads with excessive vibration to the pipe. This could cause the vacuum tube to leak or implode. Stale air testing was done using a glass pipe around the metal pipe. It is difficult to create a vacuum seal between the glass pipe and metal pipe. The metal pipe expands and contracts as it is heated. A vacuum pump maybe needed to measure the vacuum and remove more air as the vacuum is lost.
I am also looking into double walled vacuum tubes that could go around the 2 inch diameter metal tube but cost is an issue. This system would then have stale air between the metal pipe and glass pipe and then a vacuum between the glass pipe walls. This should insulated the receiver pipe very well and contribute greatly to a more efficient overall system.
4 System operating temperature. Higher and lower temperatures each have their advantages and disadvantages. More testing will be done to find the best operating temperature. Temperatures under 100C are obviously not possible if steam engines are used.
For high temperatures (over 250C) the turbine efficiency is much better. Heat loss at higher temperatures however is also greatly increased. A greater concentration factor (mirror to pipe size) may be required to reach higher temperatures which required greater mirror structure and mirror film accuracy. Also parts like plumbing, pumps, wood supports and the oil can be damaged by these high temperatures. A large problem at higher temperature is the oil boiling (see oil). Fire danger is greatly increased.
Testing must be done to compare the extra turbine efficiency to all the disadvantages to find the best operating temperature.
5 Steam engines vs other heat engines. I have been looking for alternatives to steam engines for converting heat into electricity. My biggest concerns with the steam engines their is low efficiency, cost of small steam engines and complexity of the water loop system. I have done some research into Thermo Electric Generators (TEG) and Stirling motors. It seems difficult to get either of these to produce 3000 watts of power without getting very expensive and/or very heavy. TEG are also not possible to make at home. My current preferred heat engine is the Tesla steam turbine. I am still looking into other steam engines but this seems to be the best fit for this design so far. It is possible to build this at home and they can produce large amounts of power while being very small and light weight. I have considered build maybe 10 smaller Stirling engines and mounting them all to the return pipe in the oil loop. I figured once I built one the others would be easier and could be improved. Each Stirling engine would have a small electric generator and they would all charge the battery bank together. In this way each Stirling motor would only need to produce 300 watts. I fear that the complexity and weight of this system would be a large problem. I will be testing Telsa turbines and Stirling engines on my Test unit to find which will work better. I also see the possibility to use both. The Telsa turbine could produce most of the power but then smaller Stirling motors and/or TEGs could be used to condense the steam and produce power instead of discarding this heat in a radiator. This could significantly increase the efficiency of the system.