Trough Lens Cone
This section provides specific optimization examples of solar energy derived from trough-lens-cone technology.
TLC Overview
Ultra-efficient, durability PV
Trough-Lens-Cone PV, or TLC, is a novel PV architecture that will bring ultra-efficient, durability PV to a cost well below that of ordinary silicon flat panels.
TLC uses three-stage optics to concentrate sunlight 1500X onto microcell arrays on small heat-spreader substrates. As illustrated above, a parabolic trough (T) concentrates ~40X on the first axis onto a long, narrow module (shown on the left side of the illustration). The module's cover is a glass lens tile that has a series of linear lenses on its front, and each lens (L) concentrates ~10X on the 2nd axis to form a short 400X focal line (as shown in the illustration's inset). Each 400X line is further focused by a row of silicone total-internal-reflection cones molded on the back of the lens tile; each cone (C) further concentrates 2.5X on the 1st axis and 1.5X on the 2nd axis, and homogenizes the 1500X light onto a tandem microcell.
Even with three optical stages, TLC is very low cost and high efficiency. Field-proven parabolic trough mirrors are inexpensive and very efficient, and the trough's initial 40X concentration reduces the sealed module size by 97%, making high-efficiency module materials highly affordable. The lens tile can be roll-formed from low-iron glass (with 80 times less area per Watt than textured flat-panel cover-glass), and molding the cones in optical silicone uses 40 times less silicone than silicone-on-glass lenses for Fresnel CPV. Molding the cones right on the back of the lens tile also avoids interfaces with refractive index changes, keeping optical efficiency very high.
While microcells normally have high assembly costs from placing and interconnecting vast numbers of tiny cells spread over large areas, in TLC the initial concentration from the trough packs a 96-cell array onto a compact substrate that fits high-speed assembly equipment from the electronics industry for very low receiver assembly cost. The receivers then auto-interconnect as they are placed along the lens tile, keeping module assembly costs very low as well. High concentration keeps cell cost low, tandem microcells retain >40% efficiency at 1500X, and the short electrical paths have very low resistive losses, keeping electrical efficiency high as well.
The compact module supports the cells with thermal-expansion-matched materials, and hermetically seals the cells between glass and low-expansion stainless steel. The mirror's silver and the lens glass filter out complementary portions of the harsh UVB, and the cones prevent protect the cells from sodium from the glass. Small cells survive extended thermal cycling well, and the cells are well-cooled with high-surface-area aluminum fins. TLC will thus be highly resistant to the main causes of PV module degradation and failure (including LID, yellowing, delamination, cracked isolation, corrosion, PID, diode fail, interconnect breakage, contact failure, glass breakage, loose frame, thermal cycling, oxidation, moisture, and hot spots), so TLC modules will have low degradation and long life.
Projections are for >38%-efficient modules with >50-year durability at a manufacturing cost of 16 ¢/Watt in moderate quantity (with a target of 10 ¢/W in very large quantity). That's roughly twice the efficiency and durability of silicon PV at half the cost (and a 4X cost reduction from standard high-efficiency CPV).
Energy flow in TLC
Energy flow in TLC
Energy flow in TLC:
T: A parabolic Trough mirror focuses 40X onto a long, narrow focus (which the TLC module intercepts).
L: Lenses on the front of the module refocus the trough’s focus to a series of short focal lines.
C: Cones (Winston CPC TIR cones) funnel the light from the focal lines into a regular array of tiny foci.
µCA: Microcell arrays on small receivers match the tiny foci. A receiver’s cells are in parallel.
e-: Electron path through a receiver is low resistance. Receivers are in series along the focus.
TLC Cooling
Cells need good cooling to be efficient and to have long life. CPV cells are typically mounted on highly thermally conductive, low-thermal-expansion Aluminum Nitride (AlN), but even with the trough’s initial concentration, AlN is too expensive for a substrate that holds a whole array of microcells. TLC therefore uses Aluminum / Silicon Carbide, or AlSiC, which is also low expansion and highly thermally conductive, but costs 30 time less per area than AlN.
Microcells shed heat well, so with TLC’s microcells mounted directly on the AlSiC the heat is rapidly spread from 1500X at the cells out to the 40X of the trough’s focus. The much-lower thermal flux then lets low cost materials provide 4500V isolation needed for safety in 1500V solar systems, while the 40X concentration still keeps the area, and thus the cost, very low. Heat then flows through a 1.5-mm-thick low-thermal-expansion (alloy 410) stainless steel module lid to reach aluminum fins brazed to the back of the lid, and the highly-conductive aluminum fins then further spread the heat. The fins have a total surface area ~1.6 times the trough mirror’s light-gathering area, so the heat is then simply carried away by the surrounding air. The fins are always vertical, so they transfer heat well through natural convection when there is no wind. This provides better cooling than traditional CPV while using less aluminum. (TLC can also transfer the heat to a heat-transfer fluid when heat is desired as a by-product.)
Cross-section of TLC module (showing cooling fin)
The heat only has to flow a few millimeters to reach the cooling fins on the back of the module.
Finite-element modelling shows the temperatures
Even with full sun and no wind, the cells are cooled well, with the maximum temperature being only 50ºC above ambient