Solar Steam Turbine, Examining the Curnutt Solar Furnace

February 25, 2004

Introduction

Last month we set the agenda for this year's work -- solar steam to turbine-electric power. I'd like to remind everyone that the purpose of this club is to inspire innovation in key areas of growth in the 21st century. It is absolutely necessary for all of us to develop real alternatives to overpriced sources of energy so we can regain control over our lives. As long as unregulated utility companies are allowed to gouge the general public, we'll never be free. 

As founders and directors of the Global Motive Power Revolution, my small staff can set the tone & direction of the revolution from centralized utilities; it is up to all of our members to experiment and share what they discover.

Overview of the Curnutt Solar Furnace

This month we are going to take a look at the first of two solar reflector designs -- the Curnutt solar furnace. Basically this design uses an array of (100) 12" x 12" flat glass mirrors all tilted to focus solar energy to a point approximately 12 feet from east to west, and azimuth adjusted to account for seasonal changes in the sun's (north-south) path.

Charles Curnutt first developed this system in the late 1970's in 29 Palms, California for the same reasons we initiated this club effort -- to break the stranglehold of the government-big biz energy cartell, through decentralizing energy production. Charles' solar furnace design was first published in Mother Earth News magazine in their July/August 1978 issue, with further developments printed in January/February 1979, and finally in their Autumn 1980 "Guide to Home Energy".

After seeing Charles Curnutt's solar furnace in operation and producing electric power from a homemade piston steam engine-alternator arrangement, Mother Earth News staffers designed and built their own unit. Their results were quite impressive.

From a 10 ft. x 10 ft. array of mirrors, they reached temperatures of 1600 degrees (F) at the focal point, and produced about 6,000 watts of power.

In one test session they reached 300 psi of steam pressure! Normally, operating pressures are in the neighborhood of 80 to 100 psi, depending on the water inlet pressure. Pressures as low as 60-70 psi are usable with a disk turbine -- however, the higher the pressure and fluid flow, the greater the power.

Mount Construction

The first thing to keep in mind with a solar tracking system is that the reflector must rotate from east to west on a substantial mount with a clear, unobstructed view southward.

The ground portion of the mount we have used in the past for our dish systems consists of either a heavy duty I-beam or 6-inch steel tube cemented into the ground with a couple yards of concrete. It's important to make sure that the concrete mass is sufficient for counteracting wind loading. A typical wind load on an 8-10 foot reflector at 60 mph is around 3,000 pounds.

The I-beam or steel tube may extend some 5-6 feet above ground, or may be capped with a 0.5-inch plate just above ground -- ready for bolt-on extension. Make sure the steel section is perfectly level, before the cement begins to set up.

On top of the steel post is the mount which holds the reflector array and allows it to rotate east to west. The mount must be lined up with the North Star, so make sure it is built with rotational adjustment in mind.

Reflector Frame

The reflector frame is essentially a 10 ft. x 10 ft. square steel frame with eight vertical or horizontal legs positioned just slightly over 12 inches apart. One hundred (100) mirror holders are welded to the ten vertical/horizontal legs. The mirror holders must allow movement in two planes so they can be aligned to focus on the steam generator head.

The steam generator head is a steel heat exchanger, about 18" x 18" -- similar to a car/truck heater core. Pipe supports for the steam head can either run up the center or from the corners, both to support the steam head and to feed water/steam to and from the head.

Finally, a tracking actuator is attached to both the rigid post/mount and the swinging frame.

That about wraps up the basics of this Curnutt solar furnace. As we mentioned earlier, we are simply setting the course and direction of the project -- inspiring ideas for club members to act on. As with all of our projects, there are countless details in fabricating these systems. So let's get to work and see what can be accomplished this year.