Notes from Windward: #68

Solar Boiler

taking the next step towards solar steam

  September 27:

     There are lots of ways to generate renewable energy, but some routes make more sense for us than others. In the long run, we see our primary energy source as coming from the solar energy that falls on Windward at about one kilowatt per square meter. Lots of people use photovoltaic panels, and we do so some degree, but they're spendy. Moreover, they actually function more as a battery than as an energy source in that they require huge amounts of energy and environmental resources to create, energy which you can then recover over the years by exposing them to sun light. That makes sense for remote locations where you only need a modest amount of power, but when looking for energy at the small community scale--then not so much.

     Instead, Windward's community-scale energy system will be based on solar-generated steam power. We'll concentrate solar energy using horizontal parabolic trough reflectors to generate steam to power a modernized steam engine driving an axial flux alternator. There are well-proven examples of the mega version of Solar Energy Generating Systems that have been reliably generating megawatts of energy for the past two decades.

     Windward's approach to renewable energy isn't focused on inventing a new way to generate energy, but rather on finding ways to (1) build energy and food systems that reuse materials which would otherwise go to landfills or scrap-yards, and (2) to miniaturize commercially proven systems so that they can function at the scale of a urban neighborhood or rural village.

     While there certainly are economies of scale involved in the mega-systems, the micro route offers real advantages too. For example, the methane recovery system at our county's landfill burns 10,000 btu's worth of methane in huge 12-cylinder engines to generate a kilowatt/hour (3,400 btu's) of electricity. The math shows that the majority of the energy contained in the methane fuel is lost as heat from the engine's exhaust--an inefficiency that can be mined at the community level by using that otherwise lost energy to heat water and homes, run air conditioning in summer, operate a greenhouse in winter, etc.

     An efficient steam engine system generates electricity with an efficiency of around 20%, a figure which we hope to double by using secondary systems such as an atmospheric steam engine/condenser and eventually tertiary systems using refrigerant driven engines to tap even more energy out of the gathered solar heat. But even without the enhancements, since we're using sunlight which we have in abundance some 200 days a year, efficency isn't the critical issue it would be if we were running a fossil-fuel based system.

     Again, none of the technology we're working with is cutting-edge new, none of it is theoretical--it's just counter to the "bigger is better" mantra that drives commercial projects.

Zack and Oana prepare the fittings

     The next step in that process for us involves mounting a solar boiler on the roof of the PowerLab. For that, we picked up two ten-foot-plus lengths of used 2" diameter, schedule 40 steel pipe. Zack and Oana hoisted the pipe up onto the roof, and fitted them together to create the center-fed boiler that will be heated by the solar trough reflectors. The trick here is that the pipe won't be mounted at the focal point of the two 10'x8' parabolic reflectors--a design that would require the reflectors to be quite strong--but rather it's the other way around in that the reflectors will be hung from the pipe using steel cables to support and focus the reflectors.

Joining the sections

  October 10:

     With the 20' foot long boiler pipe assembled on PowerLab's roof, the next step was to start testing the boiler's safety using compressed air. A steam engine operates when the pressure on one side of its piston is greater than the pressure on the other side--it really doesn't care what's creating that pressure. That indifference will allow us to test out the axial flux alternator and the engine control systems by feeding the steam engine with room-temperature, compressed air instead of 300°F live steam.

     Research necessarily involves going beyond the known--it's what you do when you don't know what you're doing. Given that, it's important to take small, manageable steps that will allow us to address safety issues as fully as we can. For example, the solar boiler is made from Schedule 40 materials which are rated for up to 150 psi steam load. On the other hand, castings sometimes have flaws, and reused material may have been subjected to some sort of stress or internal corrosion that could cause a failure well below the 150 psi mark. And so we're always looking for ways to minimize the risk inherent in energy/pressure work by breaking the process down into small steps so that when some component fails, we'll get a small pop instead of some huge BANG!

     Just as we'll do our test runs on the axial flux alternator using compressed air to drive the steam engine, we'll use compressed air to test our boiler for leaks and flaws.

Oana mounts the hose connection, valve and guage

     We connected the high-pressure, portable air compressor that operates our heavy-duty construction staple guns since it's set up to build pressure to 130 psi. When everything was in place, we climbed down from the roof, entered the PowerLab and threw the switch. The decision to mount the solar boiler well above PowerLab's roof is another aspect of our effort to physically isolate pressurized systems from humans. That way there's little chance that a failure up there could harm the folks monitoring the operation from inside PowerLab--which is essentially a solid steel box.

holding pressure just fine

     After lunch we came back to check on the boiler. Everything was as it should have been, and the pressure showed a comfortable 130 psi.

  November 24:

     There's lots of routine annual chores taking up our time as we come down to the last few days of clear weather before snow shuts down our outside work for the year. Still, we endeavor to do what we can to push various projects forward. We intentionally work with small systems because our goal is to make the smallest mistakes possible as we figure out how to make the transition from theory to practice.

     Another key practice we embrace is re-use, as in finding ways to make use of things that would otherwise be headed for the landfill, and there are lots of places where used stuff works fine, but live steam isn't one of them. Indeed, anyone who doesn't find working with live steam to be a scary proposition needs to stick to photovoltaic panels.

     Now that we assembled our boiler and pressure tested it, the next step is to mount it high enough above PowerLab so that the horizontal parabolic trough reflectors can swing underneath the boiler. We're going with a passive design in which the trough reflectors are oriented east-west which means that the reflectors will only need to be adjusted about every other week during the spring and fall when the angle of the sun changes relatively quickly.

     While the trough's ability to collect solar energy is what interests us in this route, the critical factor is the trough's ability to stand up to strong winds. When the reflectors are in place, there'll be lots of "sail" area for the wind to play against, so the supports needed to be extra strong. Rather than cobble together something, we decided to wait for the next time we ran the work truck into Portland to pick up what our best guess said would be strong enough for a first run, which is the reason why this particular projects has been on hold for more than a month.

     There's a saying that suggests that if you want to hear God laugh, just tell Him your plans. Nature is a lot that way as well, and so any system built to work with natural systems such as wind is at best a compromise between construction cost and replacement cost. In this case we wanted to construct the main supports for our boiler from 2" wide, heavy-guage steel box tubing that could be bolted to the sides of the PowerLab. That way, the weight of the lab itself would help stabilize the troughs.

      The prevailing winds here come out of the northwest, and we carefully located Windward so that we're protected from most winds by a ridge, but "most winds" falls short of "all winds." We enjoy a beautiful view to the south out across the expanse of the Klickitat terrace, some twenty miles of fields of golden wheat, but a couple times each year, a high pressure system will come down out of Canada into Idaho and really nail us. In time, the weather system lets go and slides away to the east allowing the moderate temperatures of the Pacific Northwest to return, but that's the sort of experience that caused us to spend the money to get the extra strong supports for our troughs.

      As soon as we can, we'll switch from using solar heat to generate live steam to using a heat transfer fluid or "HTF" for short. The HTF is then held in an insulated tank that functions as a thermal battery. That will allow us to gather solar heat in the day time and use it to generate steam even after the sun sets. But that's a way's down the road since HTFs cost better than two grand per 55 gallon drum. That's another reason why we'll initially use compressed air to work out the kinks in our control software and hardware.

     A tube-built solar boiler has to be level in order to work, so we needed a way to make the solar boiler mount both strong and readily adjustable. To do that, we used a trick we learned when making shop equipment out of square tubing--to weld a nut into the end of the tube so that a bolt could be used to level the equipment. Given the thickness of the tubing we were working with, the right size bolt was an inch and an eighth in diameter. The distace from one face of the nut to the opposite face exactly fit inside the tube once the two opposing facets where ground down about an eighth of an inch on each side.

the 1 1/8" nut with two opposite edges ground away

     The first step was to drill a 1/2" hole in each of the four sides of the box tubing. The next step was to tap on the bolt head and drive the nut into the tubing until the nut filled the holes.

driving the nut into place

     The final step was to weld through the holes to secure the nut in place.

the nut welded in place

  November 26:

     In order to bolt the vertical boiler supports to the shipping container, we needed some sort of over-grown washer, so we cut off four 5" long sections of 1x2" u-channel, and then drilled a 1/2" hole in the center of each section. The next step was to "hang" each of the supports from the top of the container using a large C-clamp as a temporary support.

Andrew positions the support

     Each corner of a shipping container has three large holes which can be used, for example, to lift the container. That made it a simple matter to slide the u-channel in through the adjoining hole, slide the bolt through the u-channel and the box tubing, securing it with a lock washer and nut.

a top bolt in place

     Three more bolts, and the installation is done.

Onana points to a installed boiler support

Notes From Windward - Index - Vol. 68