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Solar & Alternative Energy

Emissivity-modulating electrochromic device for satellite thermal control

A variable emittance device, vacuum-deposited as a solid-state monolithic thin-film system, can function as an electrically-controlled dimmable mirror in the IR region with reflectance in the 10-90% range.
5 March 2008, SPIE Newsroom. DOI: 10.1117/2.1200802.1011

Spacecrafts and satellites have very challenging thermal management requirements because of the extreme temperature changes involved in sunlight-to-eclipse transitions. A satellite must be capable of changing the absorbance/emittance properties of its surface depending on prevailing thermal conditions. The modulation of these properties can effectively be achieved by covering satellite surfaces with variable emissivity (VE) micro-structures or coatings. Recently, we demonstrated excellent thermal management control in an electrochromic device (ECD) that functions as a variable emissivity device (VED).1,2

An ECD is typically a multi-layered system incorporating an active element consisting of optically and electrochemically active layers separated by an electrolyte layer sandwiched between two electrodes. The ions in the active element move through the electrolyte from one active layer to the other by application of small voltages to the electrodes. Ion intercalation and ion extraction from the active layers changes the optical properties of the overall system.

Our device, the EclipseVED™ system,1,2 exhibits two emissivity (e) modes: a transparent, non-absorbing low-e mode and a highly absorbing, low-reflectivity high-e mode. The transition between the two modes is reversible and can be controlled by varying the applied voltage level and pulse duration. The fabrication of our device required a special focus on configuring the thermal control region, the reflectance/emittance modulation system, the response speed, and the memory requirements for optimal performance.

The thermal control region for satellites and spacecrafts is close to room temperature for the 8-12μm wavelength range. This region defines the maximum emissivity modulation achievable, where a broad colored-state reflectance curve near the minimum region is desired. A fast color/bleach response, referred to as ‘speed’, is another important property of a VE-ECD. A speed of 4min is usually considered a good target time for operation at 300K (9.7μm) with an emissivity modulation range of 0.9.

Our device activates upon application of ±1V and can be configured with short- or long-term memory. In the first option, failing occurs in the bleached condition and in the second, the device can sustain a given reflectance condition for months. Our design features a monolithic solid-state thin-film system that is space-qualified, durable, and light (5g/m2). It can also be directly deposited on a satellite surface or on lightweight substrates applied to the satellite.

Figure 1. Reflectance spectra of two of our variable-emissivity electrochromic devices (VE-ECDs) in their high-emissivity (high-e) modes, optimized for 8μm (black) and 10μm (green). The blue line denotes a device in its lowest emittance (low-e) state.

Figure 2. Comparison of cooling rates for two VE-ECDs under low-emissivity (bleached) and high-emissivity (colored) conditions.

Figure 3. Results of the MidSTAR-1 experiment measuring the heat dissipation rates of colored (blue) and bleached (green) devices. Reference gold plate (red) and black surface (black) present low-e (emissivity=1) and high-e surfaces respectively.

The emissivity modulation capacity of our device was investigated in laboratory and space tests. Figure 1 shows the reflectance spectra of two different devices optimized for 8μm (350K) and 10μm (290K) respectively, with an emissivity modulation of 0.7 through the 4 to 26µm region. Reflectance minima are near zero with maxima near 90%. Switching speeds vary between 1 and 10min, depending on device active area and total system thickness. One sample (black trace) is optimized for the ‘hot’ 5-12μm region while the other (green trace) is optimized for a colder region, i.e. for 8-18μm. Spectral region preference depends on the spacecraft mission.

The heat dissipation rates of our system in the colored (high-e) and bleached (low-e) states were measured in satellite experiments performed under the Midshipman Space Technology Applications Research (MidSTAR) program using MidSTAR-1.3 The function of the devices under space-based conditions were remotely controlled from the earth. Results are shown in Figures 2 and 3.

To conclude, our technology is very attractive for satellite thermal control applications because of its light weight (5g/m2), low voltage requirement (±1V), extremely good emissivity control properties (from 0 to 0.9 at 300K), highly repeatable deposition process, and ability to cycle under space conditions. Our system represents the first ECD ever reported as fully operational in space with emissivity modulation functions verified by MidSTAR1 heat dissipation tests.

Hulya Demiryont
Eclipse Energy Systems
St. Petersburg, FL

Hulya Demiryont received her BS, MS, and PhD in physics from Istanbul University, where she later obtained a professor appointment, also heading the Physics Department. With over 41 years of combined academic and industrial experience, Hulya Demiryont is currently chief scientist at Eclipse Energy Systems, Inc.