Notes from Windward: #68


Jay's Heat Transfer Fluid Find

moving our solar steam project ahead

Walt writes:

     Score a big one for Jay!

     A key element in Windward's approach to building sustainability lies in seeing this organization as part of an interconnected web of people with whom we share common goals and connect on some level. Windward isn't a religion, and we don't believe that there's only One True Way in which to build sustainability. It's an ecumenical sort of approach that allows us to build productive relationships with most anyone working on solving various parts of the sustainability puzzle.

     We're fortunate to have an outer circle of such folks who we think of as our "Away Team," and they let us know when they come across some resource that's relevant to our work. Jay, who's currently off trouble shooting a commercial biodiesel plant in Portland, came across two drums of heat transfer fluid--HTF, for short--which just happened to be ideal for use in the solar steam system we're developing.

     There's an American saying that notes that opportunity doesn't knock twice; when it comes to finding key resources in the used market place, Windward's more in tune with the Chinese perspective that you have to go out, find opportunity, grab it with both hands and drag it home. And so, Andrew and I strapped the loading ramp to the back of the work truck and headed that way as soon as possible since special resources such as this don't come along very often.

     We picked Jay up and headed for the Ship Yard to load the two 400 pound-plus drums. Our 12' long portable loading ramp came in handy, allowing us to lay the drums on their side and roll them up the ramp and into the truck. A turn or two with rope to secure them, and we were on our way to pick up other gear that had been awaiting the truck's next run, and were headed for Windward well before dark.

two drums of HTF safely unloaded

     The two drums of HTF will enable us to take our solar steam system on to the next level. Instead of generating steam in the two inch diameter pipe that will be suspended at the focal point of the parabolic trough, we'll be able to pump HTF through copper tubing mounted on the "shady" side of the pipe. Sunlight will bounce off the reflector and heat both the copper tubing and the black pipe so that sunlight will heat one side of the pipe while conduction heats the other.

      A steam engine is driven by the pressure created when one gallon of water is transformed into 1,600 gallons of live steam. The hotter the water, the greater the pressure it exerts on the piston, as well as the boiler and all the piping in between. Every component is a potential failure point, a place where live steam could escape and cause damage--consequently, fewer components makes for a safer system.

     A twenty-one foot length of two inch diameter pipe filled half-way up contains a lot of water, and if some component failed, all of that water would turn to steam rushing out of the breach at a pressure of more than a hundred pounds per square inch. That's a key reason why the solar boiler was mounted on the roof of the PowerLab, so that in the event of some component's failure, the operators would be fully shielded from the live steam.

     The use of a Heat Transfer Fluid essentially eliminates that risk. Instead of heating water in the boiler, the sun heats the HTF flowing through the copper tubes and back into an insulated holding tank. As the control computer sees the HTF temperature pass the 350 °F mark, it starts feeding water into a mono-tube boiler formed out of a coil of tubing and mounted inside the HTF holding tank. That way there's only a small amount of very hot water involved at any time. When the pressure in the monotube boiler reaches 150 psi, the control computer opens the steam engine's throttle, and away we go.

     Using a HTF system has other benefits as well. Boilers heated by direct sunlight are subject to losing pressure when a cloud blocks the sun. A hot tank of HTF will keep generating steam even when the sun is no longer heating the fluid, and by adding thermal mass to the tank, you can increase the amount of heat stored by the hot oil considerably. Also, you can use alternative ways such as gasified wood chips, or the exhaust from an internal combustion engine, to heat the HTF on a cloudy day.

     Acquiring two drums of an HTF that's designed to work at any temp between 550 °F and -15 °F is an important step towards our sustainable energy goals!

     Well done Jay!

Notes From Windward - Index - Vol. 68