Thermal management of facilities and processes on military installations requires the continual pumping of large volumes of a heat-transfer fluid (HTF). Laboratory studies have shown that HTFs infused with microencapsulated phase-change materials (MPCMs) have a higher heat-carrying capacity than conventional HTFs, which could reduce the required fluid-flow rate and thus system power costs. The objective of this project was to demonstrate the performance of MPCM slurries under continuous pumping conditions in full-scale applications at two U.S. military installations. An instrumented test loop was temporarily integrated with ground-source heat-exchange systems at Fort Hood, Texas, and Fort Dodge, Iowa, to compare the thermal performance and pumping-power requirements of conventional HTFs and MPCM slurries.
The MPCM species used in this demonstration were developed by Thies Technology, Inc., Henderson, Nevada. A significant performance improvement in pumped-flow conditions has been achieved with new MPCM slurry formulations that contain a high payload (85 wt%) of MPCMs. The demonstrated PCM is an engineered, biodegradable wax-like material that is contained in durable microcapsules. When transported as part of an aqueous slurry in a heating or cooling loop, the PCM changes between solid and liquid phases, melting as the slurry absorbs heat and solidifying as it releases heat through thermal cycling. The PCM has higher heat-carrying capacity than water because of the material’s latent heat of fusion—the heat required to change material phase before the bulk material temperature begins to rise.
Over the past 10 years, Thies Technology has developed a process for fabricating more durable microcapsules that are suitable for use with micro-volumes of PCM in pumped flow conditions. In collaboration with Texas A&M University, the company has developed MPCMs that can be pumped continuously for thousands of cycles with little degradation. This accomplishment represents a significant advance because capsule durability is essential for economical operation and to avoid fouling the pumps, pipes, and exchangers. With support from the National Science Foundation, Texas A&M researchers recorded data using an instrumented laboratory unit to establish that the subject MPCM improves thermal performance for heat-exchange applications.
Tests were conducted to study the enhancement of heat-transfer performance of systems in place at Fort Hood and Fort Dodge using MPCM slurries as compared with the traditional HTFs used there (i.e., water and 80/20 water/propylene glycol, respectively). First, the tests were run with the traditional HTFs in the instrumented test loop to obtain the baseline data needed for comparison with the results of the MPCM slurries. In addition, the operating temperature range obtained from the existing HTFs tests provided guidance to decide the melting and solidifying temperatures of the MPCM capsules. Then, different MPCM slurries with different mass fractions of PCM were used and a certain flow rate of the MPCM slurries was selected, which could carry the same amount of heat-transfer rate as the traditional HTFs tests to compare with the baseline.
Premature capsule breakage during early tests was addressed by thickening capsule walls and replacing the centrifugal circulation pump with a progressive cavity pump to reduce mechanical shear stresses. MPCM heat-carrying capability varied with mass ratio between capsule wall and phase-change material. The heat-transfer coefficient of performance for MPCMs was seen to improve at least 10% over conventional HTFs, with system power-consumption reductions of at least 3%. An economic assessment indicates that implementation of MPCMs as a drop-in application could provide a 1.77 return on investment ratio. However, results suggest that the technology is not mature enough to recommend for widespread drop-in implementation at this time.
This project demonstrated enhanced thermal performance and net energy savings through the use of pumped MPCM slurries for both heating and cooling applications. Using an instrumented intermediate test loop, real-time variations in thermal load demand were consistently met, and characterized, for the thermal energy transfer rate range of 1.7 to 10 kW. The improved thermal performance of the ground source system resulted from the slurry’s higher heat-transfer rate, which reduced the required flow rate of the heat-transfer fluid and, consequently, saved energy through decreased pumping power. Results indicate that MPCM slurries have the potential to become a viable HTF in heating and cooling applications if capsule durability can be improved and assured over long enough time scales to provide a significant economic return on investment.
Lessons learned include improved knowledge of the impact of pump type on effective capsule service life, and challenges with custom formulation of first-generation commercial MPCM capsules. Although some batches of capsules used in this study were not adequately durable, increasing capsule wall thickness and using a progressive cavity pump were found to improve MPCM capsule service life. For drop-in applications, a more complete characterization of the full thermal operational envelope is necessary. This will help to better engineer and optimize the operational phase change temperatures for melting and solidification. In future systems designed specifically for use with MPCM slurries, multiple other improvements should be possible. Designing for complete PCM melting and solidification entirely within the intended heat exchanger or delimited piping network region is critical.
While with further development pumped convective thermal transfer using MPCM slurries promises significant benefits, based on the results of this demonstration, they are not recommended for immediate widespread implementation.