Tuesday, March 24, 2015

Solar Dish Design: Continued

My Living Situation

I live in a 170-year-old house that we expanded, insulated and remodeled. I burn about 5-7 cords of wood for heat and stopped using oil many years ago. For hot water I use grid electricity. During the spring I harvest wood and spend a month working it into stove-size pieces that I stack in the back end of the garage to replenish what I used during the winter. We dedicate room for about 12 cords of wood, roughly a two-year supply. I would like to add a solar collector to my home to provide all my hot water and greatly reduce the amount of wood that I now process and burn. Photovoltaic panels mounted on the tracking structure will power the solar collector, its applications and controls.

As a solar designer I have a lot of material to work with that were left over from past or cancelled projects. I have aluminum extrusions, mirrors, and many other parts like motors, gear reducers, tubing, structural members, and the tools that enable me to work the materials into new assemblies. I’m trying to reduce my own fossil fuel footprint to minimize my contribution to global climate change and conserve fossil fuels. This project will build on years of working with models and prototypes of a new efficient solar dish design. This blog will walk through my approach addressing my situation.

A second goal is to interest others in developing a local organization to manufacture, assemble, install and maintain similar solar collectors. The starting materials are quite compact and can be readily produced in centralized locations: aluminum extrusions, glass mirrors, and hardware. Though quite easy to make from materials that readily fit in a pickup truck, once assembled, the gimbal, girders, trusses, mirror assemblies are quite bulky and would be expensive to ship long distances. An ideal model for this technology is community supported agriculture where consumers sustain those who locally grow their food.
Solar Dish - Concentrator Design

My primary goal is to make a point focus solar collector that can provide all my hot water, half the heat for my home, and a kilowatt of power for the solar collector and its applications and with some lighting and appliances. I will explain 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 what I expect my project to look like.
A 24 Mirror Assembly Dish Facing East with PV Panels On the Lower Portion

Size, Piers, Pivot and Gimbal

Designing concentrating solar collectors that follow the sun require choosing a size that optimizes effort, cost, available solar resources and energy needs. Solutions evolve as experience and technology progress. How much sunlight do you want to capture? A square meter of bright sun delivers a kilowatt: so 20 kilowatts would require 25 square meters of reflector (20/0.8 = 25) because reflectivity, dirt on mirrors and heat losses don’t let you collect it all. A new solar dish can deliver over 90% of the direct normal sunlight (the part that makes a sharp shadow and comes from the sun itself) which on clear days with blue sky is most of it, but haze, dirt, pollen and corrosion diminish performance to below 90%.

The sun doesn’t shine all the time so solar collectors have to be integrated into energy systems that continually provide benefits like hot water, space heating, air conditioning, clothes drying, dehumidification, cooking, lighting and other possibilities. Near the equator where it never gets cold and the sun shines for around 12 hours almost every day you may not need another source of energy but other regions probably require backup, usually by burning a fuel. In developed countries when connected to a power grid, one can use electricity that others generate with wind, hydro, coal or other fossil fuel. Or you can directly burn natural gas, propane, fuel oil or biomass in order of increasing effort involved. And there are solar energy storage options: batteries for power, tanks for hot fluids or ice (for cooling).

There is a very good modeling tool: SAM (System Advisor Model) available for free from the National Renewable Energy Laboratory that calculates performance of many types of solar collectors using solar energy available in every US state and Canada. Data from many other countries is also available or can be loaded into it. SAM uses hourly direct and diffuse sun intensity estimates and time it shines each day for a typical year for tracking and stationary solar collectors. It quantifies my experience that there are shorter and cloudier days here in upstate New York from October to December. When trying to align mirrors on solar collectors using the sun, I’ve been disappointed by months where the sun never peaked out! By including heat loss calculations for a specific building it becomes obvious that a solar collector large enough to collect enough heat during long sunny summer days and storing it in a huge container of hot water or other material isn’t rational nor does it make economic sense. On the bright side: it would not be difficult to store more than enough hot water for normal use year around. My philosophy has evolved to require that each solar collector provide its own power using photovoltaic panels.  A battery would enable a solar system to work when power lines are down.

Solar dish concentrators typically use a number of glass mirrors laid side by side either on a continuous parabolic surface shaped like a saucer or shallow dish or mounted separately on metal brackets so that they direct sunlight to a point.

A Solar Dish with Polished Aluminum Reflector with Receiver View on Left

Our mirror assemblies, MAs, use accurately curved aluminum extrusions mounted between two straight channels that enable aligning many mirrors so they act like a parabolic section of a dish. Features on the straight channels not only allow MAs to be stacked one on top of another during manufacturing, shipping and staging but also readily mounted on adjustment plates that extend above the concentrator structure so they can be aligned and fastened by one person in single operation.

Earlier Solar Dish Design That Had Different Size Mirror Assemblies

The concentrator made up of MAs directs intensified sunlight into the receiver. Shadows of the receiver and its structural supports falling on mirrors would diminish the amount of sunlight reflected. The next design will minimize this effect by moving the receiver down so that its shadow just misses the lower mirrors. The support structure will be arranged so its shadows do not fall on mirrors but on the two girders that support the trusses holding the MAs. In the illustration above, there is a space between rows of MAs where the receiver support structure shadow falls or reflected sunlight would be blocked by supports on its way to the receiver. 

Also illustrated above are seven piers that enable the gimbal to rotate around an axis that is parallel to the earth's axis. The developing design will probably use fewer supports in a different arrangement to make it easier to mow grass around the solar collector. The angle between the rotation axis and horizontal is equal to the latitude where the solar collector is erected. This way the Right Ascension Drive that is integrated into the rim of the gimbal can track the motion of the sun simply by rotating one degree every four minutes to achieve the one revolution per day of the earth, canceling it. The next post will describe structural components in more detail.


This effort will build a solar collector that should provide all my hot water and a significantly diminish how much wood we burn to keep our home comfortable. To repeat design parameters I mentioned in an earlier post, the resulting solar collector should:

   1. Intensify sunlight 1,000 times to power tiny receivers that delivers heat and power;

   2. Harvest more than 80% of available sunlight;

   3. Be made with home shop tools without welding or expensive equipment;

   4. Be installed using hand tools and then operate without expert attention;

   5. Replace in less than six months the energy invested in making all materials;

   6. Operate for 30 years: with parts easily repaired or replaced;

   7. Utilize only materials that can be readily reused or recycled;

   8. Power itself and connected systems so they all work after storms;

   9. Provide year-round hot water, air conditioning and space heating when integrated with backup systems; and

  10. Pay back investment in fewer than ten years, without subsidies.