The Mithras Concentrator is a lightweight, rapidly deployable 100W solar power device, utilizing concentrated photovoltaic technology. CPV introduces a low-cost option to solar power generation, using reflective surfaces and a Fresnel lens to consolidate sunlight from a large area into a tighter beam. This reduces the cost of the final product, decreasing the number of PV cells necessary to capture a given amount of light energy, while producing a consistent voltage output and making more efficient use of space.
Solar cell performance is inversely proportional to temperature, and falls off rapidly after a certain threshold (around 110 degrees C). Black PV cells experience high temperatures because they absorb infrared and ultraviolet light. Our PV harnesses energy from the visible spectrum only, so we strip off the frequencies we are not using before they hit our cells.
Construction is simple. We use an inflatable spheroid dish (from a truck inner tube) which forms a parabolic surface, a Fresnel lens for a secondary focus, a 6” 90° glass triangular prism for “unwanted” frequency stripping, mylar, collapsible rods, a high-quality solar cell and a heat sink. The mylar is cut in a circular shape to fit the inner tube, and attached with adhesive, forming an airtight seal. This is done on both sides to create a torus. When the tube is inflated, it stretches the mylar into a bowl shape since there is a partial vacuum now in the “donut hole”. Rods are attached to the tube to form a tripod, angled so that the PV is 30” away (the focal point) from the bottom of the parabola. The Fresnel is then suspended in the middle of the tripod 19.5" from the bottom. The prism is then suspended between the lens and the PV and disperses the concentrated beam so that only wavelengths within the visible spectrum (the “rainbow”) strike the cell, while other wavelengths are reflected off into empty space, missing the top and bottom of the cell. The efficiency is increased, and large areas can be concentrated onto a single PV cell as long as you keep it cool. Our purpose-built SunChaser MPPT controller acts as a voltage spike and droop buffer when clouds go by. Our experience has shown that a 40x concentration is the maximum you can get before you need active cooling on the solar cells. This simple construct offers a very lightweight solar concentrator which is easily portable and much faster to construct than most competing approaches.
We have experimented with alternate materials since metalized mylar tends to tear. We have further refined our design with the prism arranged on a series of gears so that it will reactively (or proactively) move to accommodate the changing position of the sun. As a practical matter, the unsophisticated model shown here has proven to be the most popular model at our Solar Lunch events, seeing as most usage comes in the form of quick power requirements rather than long term unattended charging needs.