New 6-inch Turbine for Low Horsepower Applications!

September 2004

As we mentioned last month, we are going to take a look at a new turbine design specific to low-horsepower systems.

Solar collectors are thermal transfer systems. The available horsepower of solar systems is expressed in terms of Btu's - one horsepower being equivalent to 2,542 Btu's - or roughly 2,500 Btu's per horsepower/hour.

The work derived from a solar collector is dependent on several factors:

Designing for the Load

When we consider the overall system design, it is important to size the components to the load. Most homes draw about 18 kilowatt hours per day for normal use, so we will need a solar system that provides 18-20 kilowatts during a normal sunrise-sunset period of roughly 12 hours. -- Since the sun produces usable heat for only 8-10 hours, we will design for a 10-hour energy day.

That means we will have to produce roughly 2 kilowatts for 10 hours per day, and store that energy for later use. Several storage options are:

Energy Transfer Efficiencies

Another major design factor to consider is energy transfer efficiencies. Energy losses will occur in three major areas:

1. thermal to mechanical
2. mechanical to storage
3. storage to use

Thermal to mechanical is the biggest loss and requires the most attention. Traditionally, single stage steam systems have been very low in efficiency - averaging about 8 percent for piston engines and 12 percent for turbines. 

High tech boilers developed in the auto industry have pushed that figure up to about 22% for pistons. Since most experimenters will take a more conservative approach to boiler building, we'll use the 12% figure.

That means we will have to produce about 8-10 times the number of Btu's the engine will actually see at the inlet. Then, as the efficiency of a Tesla turbine is 30%-38% across the plates, that translates into a 30X factor at the focal point of the dish. -- Add in losses from the boiler to the turbine, and we are looking at a grand total of about 30X to 35X. 

In other words, we will have to concentrate 35 times the Btu's at the focal point!

Next we throw in our 20% to 40% energy conversion and storage losses, and we are looking at a Btu demand of between 50-100 times the required daily kilowatt demand.

Project Heat Requirements

Let's say we need 18-20 kilowatts per day. With electrical conversion and storage losses factored in, we will want to produce 3 kW for ten hours. Converting back to horsepower (745 watts/hp), we will need about 4 horsepower from the turbine.

Working back to the inlet nozzle (at 30% efficiency across the rotor), the turbine will require 12 horsepower of steam. Working back to the steam generator head, we will need to see about 120 horsepower of heat at the focal point (per hour). Translated into Btu terms, that's about 300,000 Btu's per hour.

A properly designed 9-foot diameter dish will deliver 2,000 degrees at its focal point in direct sunlight -- which should be sufficient for our project.

Now on to the turbine.

Water Conservation

Since a typical Tesla turbine requires around 40 pounds of steam per horsepower-hour, our system will consume about 1,200 pounds of water per day. On our residential well system that would be equivalent to ten tanks of water per day. Rather than waste 1,200 pounds of water every day, it would be more prudent to design our turbine as a closed-loop system. Besides saving water resources, it would also give us the option of working with other types of working fluids.

Turbine Size à la Nikola Tesla

Another factor to consider is the size of the turbine. In the past we have demonstrated turbines using 10-inch rotors. In small horsepower systems, it will be more efficient to work with smaller, lighter weight rotors. Since Nikola Tesla's first model was 6 inches in diameter, this is a good place to start.

Tesla's 10-inch turbine delivered 110 horsepower using 25 disks, or roughly 4.4 hp per slot. The difference in surface area between a 10-inch disk and a 6-inch disk is about 2.77:1 -- a 6-inch slot yielding 1.5 horsepower. A (3 slot) 6-inch rotor with four disks will theoretically deliver the required 4 horsepower.

The Containment Vessel

Since we are designing a closed-loop system, the turbo-generator unit must be enclosed in a hermetically sealed containment vessel to allow recycling of the working fluid.

After building a couple of turbines using bearing blocks and industry standard rubber shaft seals, we found that the inherent friction caused by the seals robs a tremendous amount of start-up torque from the turbine -- in some cases disallowing spool up. To get around this problem, the entire turbine-generator unit must be placed in a containment vessel.

Bearing Options

Also, when using a water-steam working fluid, stainless bearings and turbine components must be used.

One way around the use of exotic bearings and components is to use another type of working fluid such as CFC. Another way is to use a vacuum pump on the exhaust outlet to recover most of the spent steam through condensation.

Figure 45a shows a computer model of our new 6-inch turbine design. 

Next month we will show yet another method of constructing turbines using laminated techniques. In the meantime, let us know how your work is progressing.

Ken Rieli