Dish Solar Concentrator Design

Over the next few months I will describe how I design point focus solar collector that will provide all the hot water and half the heat for my home. I will document how I manufacture the parts, assemble them, erect the structure, mount and align the mirror assemblies, connect the drives and plumb the receiver. The illustration below shows a computer model of an early version that has evolved over the past year.

Mirror Assemblies: Size, Quantity and Configuration

I will start by describing how I make mirror assemblies. They make up the concentrator and focus sunlight on a receiver designed to harness concentrated sunlight efficiently. Later I will cover the design of girders and trusses that support these components, the foundations required, and the gimbal that carries the concentrator and receiver. These all rotate together to follow the daily motion of the sun as well as allow all to follow the sun as it goes higher and lower in the sky through the seasons.

Computer Model of a Solar Dish with Mirror Assemblies and Receiver

A solar dish collector uses a concentrator to reflect sunlight into a receiver that transforms radiant energy into hot water or steam. The area of the concentrator may be 500 to 1,000 times larger than the opening of the receiver to minimize energy losses.  The receiver can be mounted close to the reflector, far away, or somewhere in between. Dividing the focal length, the distance from the mirrors to the receiver, by the concentrator diameter describes this (f/d) relationship. I work with an f/d ratio of 0.8, knowing that this works well with very high performance cavity receivers. Shorter focal lengths, say 0.6, require shallow cavities that lose more energy than deeper ones because hot surfaces inside lose more heat to wind and radiation. A longer focal length, say 1.0, where the receiver aperture is the same distance from the mirrors as the concentrator is wide, extends the receiver farther out from the main structure and requires more material and higher quality mirror assemblies.

Illustration Showing Various Focal Length to Diameter Ratios

Many point focus solar collectors have individual mirrors mounted on a structure to reflect images of the sun on a target. Attaching and aiming each one takes time and these individual mirror facets are difficult to clean. We made early solar collectors in the 1970s this way. We progressed to mechanically curved mirror assemblies that placed eight one-foot square mirrors end to end and were handled as a larger unit. Correct curves insure mirrors reflect sunlight to a spot. Mirrors are then continuous and easy to squeegee clean. Accurately curving aluminum extrusions is straightforward and simply attaching mirrors to them can establish a parabolic reflective surface that intensifies sunlight 30 to 60 times without having to adjust any mirrors. Attaching a few dozen of these assemblies to a structure and aligning them so they reflect sunlight to a single point creates the point focus concentrator. Permanently mounting mirrors in arrays minimizes labor when erecting a collector. An ideal: assemble and erect a solar collector in a single day. Accurately aligning mirror assemblies is easier and quicker than individually mounting nine to 16 times as many smaller mirror facets.

A specific design begins by choosing an appropriate size mirror assembly and then settling on a suitable array of these assemblies to make up the concentrator. Mirror facets come in many sizes and what I have on hand are 12”x12” and 12” x 36” mirrors that readily fit into square panels that are either three or four feet square or rectangles that are three by four foot. One person can readily handle these mirror assemblies. A 3’ x 3’ weighs 10 pounds and the larger ones weigh less than 20 pounds.

The table below shows a few ways that mirror assemblies can be arranged in a concentrator. A roughly circular arrangement gives each mirror a good view of the inside of the receiver. The farther away from the optical axis (a line through the centers of the concentrator and receiver) a mirror is, the poorer view it has of the inside of a cavity receiver, making it less effective than those closer to the center.

Mirror Assembly Configurations with Table of Mirror Area & Quantity

At this point I’m leaning toward configuration “C” that has 16 four-foot square mirror assemblies. In an hour in bright sun these would reflect 24 square meters of sunlight, 20 or more kilowatts, into a receiver that would displace burning five pounds of fuel oil and releasing over 11 pounds of carbon dioxide when I burned oil for heat and hot water. Over a year it should halve how much wood I have to gather and work into pieces for the stove: maybe by three cords, or six tons. I should be able to manufacture 16 mirror assemblies in a day or two and to fix and align them on the concentrator structure in one night so they hit a target.

It is easier to work in the dark using an LED flashlight shining at a mirror assembly, MA, from the center of a target at twice the focal length. Tighten the bolts at the four corners when it reflects the image to the target center. One could use sunlight during the day to do this but after aligning a few mirror assemblies the target becomes so bright it becomes impossible to see the reflection of a single MA. One could cover each fixed MA but this is cumbersome. Also, using the sun during the day works well around noon, when the concentrator faces up and is level, but because the structure must be tracking the sun for the process to work, climbing around the tilted moving structure in the morning or late afternoon becomes more difficult. And sunny days without clouds often don’t occur when you want them. Every night is dark, though, and working close to the ground with a stationary structure looks easier so we will be exploring this new MA mounting and alignment technique for this project.