This document serves as a write-up of one product option, an inexpensive indoor-rated high-bay fixture, which is one potential offshoot of the project. From initial feasibility studies and prototyping on the project, it is clear that heat pipes are well suited as a thermal management technology for high power density LED arrays where the thermal load can be dissipated to the immediate environment of the fixture.
- ~18,000 lumen output (400W metal-halide replacement).
- System efficiency of up to 113 lumens/Watt
- Small fixture size (approx. 12cm x 15cm x 25cm including a 10 cm reflector).
- Restricted to indoor environment as a first product. Weather-proof outdoor versions are certainly feasible, but would be a more expensive product and require longer R&D cycle time to bring to completion.
- System cost of approx. $360 (exclusive of manufacturing cost).
A prototype system was created as part of the project (Figure 1). This prototype was constructed using four CXA2011 LED arrays mounted directly to the cooling plate of a Zalman CNPS 10X Performa heat pipe cooling assembly designed for CPU cooling. With this prototype, the CXA chips were driven at 48.5V and 0.900A each, producing a total thermal load estimated at 130W to 140W with a steady-state LED substrate temperature of approx. 35°C in ambient conditions of 22°C. CXA2011 chips do not provide the highest lumen density (lm/mm2), and a better choice for a production device would be to use a tightly packed array of Cree XT-E LEDs driven at 350mA, as detailed in Table 1. It is expected that the junction temperature for a tightly packed array of Cree XT-E LEDs in the design detailed in Table 2 would be approximately 50°C.
Design and Cost Analysis
The cooling modules for this design are mass produced for the computer cooling industry, and are available at a much lower price than a completely custom design. Retail price for the Zalman CNPS 10X Performa cooling package used in the prototype was $39.99 from Memory Express, and included an assortment of mounting brackets for various CPU formats. The OEM cost for a stripped down version of this assembly should be somewhat cheaper. The UNVLT D700C150UV10F power supply is an inexpensive 150W constant current supply, selling for approximately $32. The LED array was sized with 12 rows of 12 chips (144 in total) to match the capacity of readily available power supplies. This results in a module output of close to 18k lumen. Since we require a larger PCB size than a standard CPU format (approx. 4cm x 4cm) we would either need a custom cooling plate base from the thermal module manufacturer, or we would need to use a thick MCPCB with good heat spreading ability (such as the 1.6mm thick copper MCPCB from SinkPad).
The LED cost is the largest single cost of the system, with 144 LEDs costing $151.20 at the volume pricing available from Arrow (November 2012). This system needs only a simple casing, which is envisioned to be hung from a ceiling mount that enclosures the power supply. With a tightly spaced array of LEDs, optical control is limited to a simple reflector/cut-off shield and an optional lens at the end of the reflector. If a lens is used at the end of the reflector, the optical losses will be higher than for the no-lens case detailed in the table. Using very conservative cost estimates for the reflector and case, the total system cost comes out to $358.20 (Table 2). If inexpensive cases and reflectors can be sourced, the total system cost may be close to $300/unit.