Friday, June 26, 2015

Mirror Assemblies That Simply Warp Flat Glass Mirrors

In our first attempt to eliminate the backing glass layer and use flat mirrors alone, we mounted them directly to a parabolic surface. We had developed a technique to accurately curve aluminum extrusions by systematically hitting the outboard leg with a ball peen hammer. Increasing the number of hits per length or the force used reduces the focal length of the parabola until it matches a template. The photo below shows a truss with a curved top member.
Aluminum Truss With Parabolic Top Member and Discretely Bent Bottom Member

A number of these trusses made up the concentrator structure by attaching them to two girders that were discretely bent so the parallel trusses formed a parabola in the transverse direction. This parabolic surface was then lined with corrugated polycarbonate roofing material, see photo below.
Parabolic Concentrator Structure, with Left Side Covered with Corrugated Roofing

We directly attached mirror facets to the corrugated sheet using a variety of methods but none were satisfactory. Also, solar images reflected by these mirror facets mechanically attached to the corrugated sheet did not all superimpose at the target focal length hit each so did not concentrate sunlight enough to warrant continuing this option. Another drawback to this approach: we found no way to ascertain which mirrors needed correcting nor was there any way to tweak mirrors that did not direct sunlight to the target center.

Mirror Facets Attached to the Corrugated Parabolic Surface

We next tried two methods to form plastic materials into mirror panels: heat-forming thermoplastic structural sheets and molding fiberglass and resin, see photo below. Although both these methods may work in dedicated mass-assembly lines with multiple copies of dedicated tooling, neither approach proved workable as a boot-strap process because they required too much labor that would take too long to produce adequate results. Also, these materials were expensive and would probably not weather well for thirty years.

Thermo-formed Twin-wall Polycarbonate Panel-Based Mirror Assembly with Reflection on Target

Male Mold for Both Heat-forming and Casting Mirror Assemblies

A series of mirror assemblies that used the hammering technique to stretch aluminum material behind the parabolic surface delivered ever-improving performance.  These fastened curved mirror support members between straight end pieces with features that enabled stacking mirror assemblies one on top of another.
Mirror Assembly Intensifies Sunlight to Target the Same Size

Mirror Assembly Using Aluminum Rod Pullback Supports

Rear View of a Mirror Assembly with Four Curved Mirror Support Members Between Straight Ends with Stacking Features, One Foot Square Mirror Facets and Angle Mirror Pullback Supports

Stacked Mirror Assemblies with Candidate Mirror Pullback Support Options

One Foot Diameter Image of 3 x 3 Array of Warped One Foot Square Mirrors

Computer Model of a Dish Solar Collector Showing Where Concentrator Offset Alignment Features Are Required at Each Corner to Attach Mirror Assemblies to Underlying Trusses: Corners of Four Separate Mirror Assemblies Meet at a “Quad”

Mirror Assembly Fabrication Fixture That Insures Mirror Supports Form Parabolic Surface with a Mirror Assembly Clamped In Place

Stacked Mirror Assemblies With One on Top That Has Three 1 x 3 Foot Mirrors and Offset Alignment Features That Enable One Person Align and Fasten Them Close Together from the Front

A 3 x 3 Foot Mirror Assembly with Two 1 x 3 Foot Mirror Facets Mounted Against Their Respective Curved Members Using Three Pullback Fasteners (Bicycle Spoke Ferule + Machine Screw)

Mockup of a “Quad” Mirror Assembly Alignment Feature That Allows One Person to Mount and Adjust Each of Four Corners So That a Reflected Image Hits a Target at the Concentrator Focal Length. The Curved Mirror Support Member Shows Hammer Marks that Stretched the Leg Behind the Mirror Supporting Surface. Alternate Columns of Mirror Assemblies Are Offset So That One Can Be Removed, Replaced and Realigned Without Moving Another

Saturday, June 20, 2015

Mirror Assemblies That Used Sealed Warped Glass Mirrors

Before 1984, our solar collectors either directly generated low pressure steam for heating, distilling alcohol, cleaning aircraft parts, and curing concrete blocks or remotely boiled water using heat-transfer oil. When converting thermal energy into power, higher temperature is better within the physical limits of materials. Also, tiny hot bodies lose less energy than bigger ones at the same temperature, so smaller is better (and often less expensive).

High performance solar collectors typically direct sunlight hitting a large area of mirrors into cavity receivers, insulated vessels open at one end. One optical goal for developing solar concentrators: direct as much sunlight as possible into the smallest hole. Since the sun has a finite diameter, sunlight reflecting off flat mirrors continues to spread and images get larger farther away. Flat mirrors that are much smaller than a target can reflect sunlight inside the target area but making and aligning each one takes time. Heat or other forming larger parabolic mirrors that would reflect more than 90% of the sunlight hitting it into an opening is difficult, and probably expensive. We found it straightforward and easy to warp flat glass mirror facets so each delivers, some distance away, an image of the sun much smaller than itself.

This Small Receiver Intercepts Sunlight Reflected Off a Large Mirror Area. Note: The Receiver Allows Only Brightness from Around the Sun in the Image. The Diameter of the Actual Receiver Opening Is Half the Diameter of the Insulated Cylinder.

These Mirror Assemblies Directed Concentrated Sunlight Into a Receiver That Generated Power with a Turbine

In 1983 we built our first solar collector where we bent flat mirrors into the shape that approximated a parabolic surface. Because we wanted our mirrors to last more than 30 years, we developed a technique to encapsulate the silver reflective surface inside glass (to isolate the silver from the environment). We modified approaches used to manufacture insulated glass windows that also have to perform well for a long time. These glass-mirror sandwiches (360 @16.5 x 36.5 inches in the first) used spacer strips along the two long sides between the mirror and glass. A thinner strip of adhesive was then applied midway between the spacer strips. This warped both the double-strength (0.12 inch thick) backing glass and the single-strength  (0.09 inch thick) mirror so each formed shallow troughs. We used insulated glass techniques to absorb moisture and seal the volume between the sheets of glass. The solar image on a target at the focal length of a mirror facet at this stage was a narrow line, brighter than the sun but longer than the facet. An aluminum strip to attach a “pull-back” was bonded on the back of the mirror facet down the long centerline. When mounted in a mirror assembly, the corners of each mirror facet were fixed so facet images were superimposed on a target at the concentrator focal length. A turnbuckle connected to the center of each facet was then adjusted until its long solar reflection became round with an area less than one tenth the size of a mirror facet. “Pull-backs” warped the trough-shaped facets into parabolic shapes.

Front View of the Above Solar Collector Showing Warped Glass Mirrors Mounted on 24 Mirror Assemblies, 12 with Extensions

In 1986 we began building a larger concentrator (3,200 square feet) that used larger glass-mirror facets (392 @ 24.5 x 48 inches). We used the same techniques as above but integrated an aluminum frame on each facet to make them much more rugged, able to be stacked for staging and shipping, and easier to mount on mirror assemblies.

View of Mirror Facets Mounted on Mirror Assemblies of the Solar Collector Shown at the Top of This Post

Close-up Showing a Reflection of the Receiver in One of the Mirror Facets. The Actual Receiver Opening Is Too Bright to See But the Aperture Skirt Tubing That Protects the Receiver Structure Is Visible. Curvature of Straight Elements in Reflections Illustrates Mirror Facet Are Slightly Curved

In 1990, in our final experiment with laminated glass facets, we adhesively bonded a large sheet (4 x 7 feet) of double-strength glass to a parabolic structure formed out of grid of aluminum extrusions. We bonded a single-strength mirror to this and used insulated glass techniques to seal the volume between the sheets of glass. This large assembly intensified sunlight over 30 times but proved cumbersome. Safely handling such large sheets of glass requires special fixtures and would be difficult to do without breaking some. Losing even a small corner of either a mirror or backing would probably require recycling it. This test also uncovered another limit: bending glass breaks if the tension at any surface exceeds the tensile strength of the material. Any flaw, such as a scratch or chip, concentrates local forces and greatly reduces how much stress can be applied without breaking. Thin glass bends easily and can safely establish a spherical (or parabolic) surface with a much tighter radius (focal length) than thicker glass. The large mirror facet above was aimed at concentrators larger than 6,000 square feet (able to direct more than 500 kilowatts of sunlight into a receiver).

Worldwide, serious research into high performance solar thermal technologies dwindled in the late 1980s. We had to find work in other arenas to put our kids through college. In my spare time I continued to make tabletop models over the next two decades to tease out solutions to many problems and awkward approaches we encountered in earlier work. Some of our mirror assemblies had been outdoors since the early 1970s and most of their facets were still in good shape more than thirty years later. With the right edge treatment and organic coatings, thin glass mirrors can weather outdoors in upstate New York for many decades. Since we have more than a thousand new and used glass mirrors from earlier work, I developed a technique for mounting them without a backing glass in an array to form a mirror assembly. The next piece will cover Mirror Assemblies That Warp Simple Glass Mirrors.


Thursday, June 18, 2015

Mirror Assemblies That Used Flat Glass Mirrors

To intensify sunlight, we typically use thin glass mirrors, that have a silver coating on the back protected by paint, especially at the edges. In the 1970s, we mounted individual flat mirror facets on aluminum tubes (see the photo below) to make initial solar collectors. It took days to attach and orient 480 of these facets on our first, a task that could only be done when the sun was shining, since we used each mirror’s image on a target so each facet would reflect the center of its solar image to the same point.
Solar Collector: 1975 Student Competition on Relevant Engineering

A second generation mirror assembly, see photos below, enabled mounting and aligning mirror facets in less than half the time but these mirrors, and those of the prior approach were difficult to clean.  Each mirror facet had to be polished individually. Although the materials cost only a few dollars, these mirror assemblies had to be put together on the job site because they were fragile, difficult to handle and impossible to ship. Exposed mirror edges and corners too often tore clothing and skin and bumping a mirror usually required it to be realigned.
Adding Brackets to Hold Mirrors Reduced Assembly Time (1976)

To solve these glass mirror problems, we tried substituting reflective aluminum sheet for the next solar collector. We formed fiberglass bodies to hold reflectors that were easy for two people to handle (at ten feet: ungainly for one person). These assemblies required special racks for stacking and shipping. Although they had the proper parabolic shape along the length, these mirror assemblies performed poorly because the aluminum sheet that came rolled in a coil made the image of the sun too large. A straightedge placed across the width of the aluminum showed that the reflective front was convex, not flat or concave, even though we established an accurate concave parabola along its length. This distorted sheet material elongated the image of the sun, reducing sunlight concentration. It was easy to squeegee clean the aluminum reflective surface but aluminum is softer than glass and easily scratched. After only months of use, the reflectivity of aluminum deteriorated, and when new it reflected significantly less sunlight than glass mirrors that reflect over 90% of the sunlight.
Two Views of 1977 Solar Collector Showing 20 Aluminum Reflector Facets

Our next approach used the technology of a local swimming pool company that had developed a pool structure made from panels formed out of polystyrene foam with aluminum skins sandwiched between aluminum extrusions. We worked with them to develop an ever-improving series of mirror assemblies that began with bodies that had edges folded to form the sides and required separate tabs to hold the mirrors. We then tried capturing mirrors with a feature on curved aluminum side extrusions. Difficulty in sliding mirrors without breaking any led to side extrusions that enabled any mirror to be installed or replaced in less than a minute, see evolution in photos below.
Three Versions of Foam Core Mirror Assemblies

Eighteen solar collectors that we shipped to Saudi Arabia in 1982 for desalinating water from the Red Sea used the stainless steel screw/clip version shown above. Each of these had 108 of these eight-foot long mirror assemblies that could be stacked for shipping/staging and handled by one person. Photos below show views of this project.
12 of 18 Solar Collectors on Red Sea, 1983
Solar Collector Showing Receiver With Alignment Target Above
Mirror Assemblies Turned to Enable Cleaning

These foam-core mirror assemblies fulfilled most of the design features for early solar collectors but using flat, one-foot square mirrors was not appropriate for making larger solar collectors that required higher performance at lower cost. Newer industrial designs required larger mirror facets that were curved along two axes to make them concave so that each delivered an image of the sun that was much smaller than the mirror itself. Surplus mirror assemblies from the early era made great raised beds for growing flowers and vegetables, see below. 
View Showing Two Versions of Foam Core Mirror Assembly Bodies Used to Make Raised Flower Beds That Work Well After More Than 30 Years

These mirror assemblies firmly surrounded flat glass mirrors and were easy to handle, store, and ship compactly. They were also easy to clean and survived hail, frost and snow by facing the ground when not operating. Because the image each mirror made at the opening of the receiver was larger than the size of the mirror, a typical 864-mirror concentrator intensified sunlight only about 500 times; good for producing low pressure steam but not superheated steam above 60 atmospheres. For that we needed a concentrator that delivered over 1,000 suns which requires accurately aligned concave mirrors, the subject of the next installment, Mirror Assemblies That Used Warped Glass Sealed Mirrors.

Tuesday, June 16, 2015

Building High Performance Solar Collectors

Oil embargoes and hour-long lines to purchase gas in the 1970s started me developing high performance solar collectors that harness todays energy, reducing need for ancient fuels. The primary design challenge was making solar equipment affordable, with efficiency a secondary goal. The reason: fossil fuels were becoming ever more difficult to procure. These relics inherited two hundred million years of chemically processing ancient plants to become highly concentrated stored energy, readily available for power. Digging and selling coal, oil and gas have been very profitable. Those profits helped grow government, build highways and airports, manage world trade markets, and provide food, jobs, education, and healthcare. And we’ve burned a lot more of these resources since the 1970s.

Each US person currently consumes about 50 pounds of fossil fuels every day. Remaining fossil fuel supplies cannot sustain this demand and highly profitable fossil fuel reserves are almost gone. The oil price needed to extract a barrel of oil is double today’s $60 price. Yet low cost renewable solutions are not available for heating homes, driving industrial thermal processes, or simply harvesting solar energy to minimize burning fossil fuels.
Forty years ago, we had to build every solar collector module from scratch. There still are no “off-the-shelf”, affordable solar concentrating modules to work with. After building a few prototypes that used flat mirrors to direct sunlight to a receiving area, intensifying it up to 500 times, I learned that it was better to slightly warp flat mirrors so that each image intensified sunlight around 10 times. An array of hundreds of mirrors then readily magnified the sun’s radiant energy 1,000 times or more, without adding cost. The concentrator with its array of mirrors follows the sun so its reflections enter the opening in a receiver that converts sunlight into other energy forms such as heat and/or electricity. A low cost, high performance concentrator, like a “Swiss Army Knife”, should become a primary tool for using solar energy cost effectively today.

Why not just use solar panels mounted on roofs?
Rooftop solar collectors are expensive and typically deliver less than 20% of what’s available. Those that convert sunlight into electricity cover the area with semiconductor material. Heat collectors use insulated metal and fluid lines for transferring heat that have to be protected from weather. Both types deliver rated outputs only when the sun shines directly on them. Solar collectors that are fixed to a roof miss sunlight that does not directly shine on the front. When covered with snow, they collect nothing at all.

Won’t high performance solar collectors that track the sun be too expensive?
High performance solar concentrators do not have to cost much. Mirrors for a solar collector that can supply half the energy for a typical home can cost $300 to $500. The structure, controls, tracking, foundation and heat transfer equipment collectively cost about the same amount. A module that converts intensified sunlight into six kilowatts of power along with 12 kilowatts of heat in the area the size of a typical frying pan should become available for less than $3,000. But it will take a few teams tending engineering prototypes to prove simple equipment that runs itself and can be maintained by anyone handy.

Can high performance solar collectors diminish our consumption of fossil fuels?
High performance solar collectors can certainly reduce our fossil fuel use and one with three hundred square feet of mirrors can deliver the energy required for a typical home. However, the timing of sunshine does not exactly match a home’s heating need. Night, bad weather, and winter diminish how much sun we get. Much comes during long summer days, not long and cold winter nights. A logical goal when adopting solar energy might be to displace half the electric power and conventional heat (gas, propane, fuel oil) of a home, cutting in half the carbon footprint.  High performance solar collectors should be able to do almost anything fossil fuels do now but it will take time to commercialize technologies and prove energy storage strategies. Oil, gas and coal burning power plants have more than a century of development and we all expect that level of performance and cost. Many do not realize the potential nor support research into alternatives that enable living without altering climate. It’s easier to kick the oilcan down the road until it’s empty rather than develop equipment that allows living well without transferring underground energy resources into the atmosphere.

Processes that utilize intense solar power are shown below along with an option of substituting conventional photovoltaic panels for mirrors until modules that deliver both heat and power become available. Tracking these aims them directly at the sun enabling them to collect over 40% more energy and turning them away from the sky when the sun is not out keeps them clean and frost/snow free.
Point-focus Solar Concentrators Can Power a Variety of Processes
What is the current program for making the next solar collector?
Now that the maple-sugaring season is over, seven cords of firewood for heating my home are drying, and an acre of garden mulched and growing; I’ll be building our next solar collector. Over the next weeks I’ll present short histories of each module: mirror assembly, tracking structure, drives, foundation, receiver/pump module and controls before detailing the latest design. Over the years I’ve made lots of missteps while trying to develop cost effective solar collectors and these histories may enable others to pursue more promising possibilities. My next effort will be to build a solar collector that provides our hot water year-round and to significantly diminish how much wood we burn to keep our home comfortable. I’ll also use it to preserve food, dehydrate fruit and vegetables, sterilize soil, dehumidify our basement, transform sap into maple syrup and other heat intensive tasks.

This next solar collector should:
1.    Intensify sunlight 1,000 times so it can power tiny receivers that deliver both heat and power;
2.    Harvest more than 80% of available sunlight;
3.    Require home shop tools (no welding or costly equipment) to build it;
4.    Utilize only materials that can be readily reused or recycled;
5.    Go up using only hand tools and follow the sun by itself;
6.    Provide year-round hot water, air conditioning and space heating when integrated with backup systems;
7.    Power itself and connected systems that work in any weather;
8.    Return the energy invested in materials used in fewer than six months;
9.    Operate for 30 years: with any part easily repaired or replaced; and
10. Pay back money invested in fewer than ten years, without subsidies.

My wife and I are bootstrapping this effort. To minimize cost, I’ll primarily use materiel left over from past projects. The mirrors and aluminum extrusions will prove function but an optimized design will require new extrusions that incorporate features that reduce labor and minimize material.