Phase Changing Material in Solar Thermal Energy Storage Tiffany Wu Energy Technology and Policy University of Texas at Austin (www.powerfromthesun.net/chapter1/Chapter1.htm )

Contents Introduction Benefits and Drawbacks of PCM PCM Options Encapsulation Increasing Thermal Conductivity Conclusion

Introduction Most systems have a disconnect between supply and demand Intermittent solar energy supply can be maximized with a heat storage system Thermal energy can be stored both as sensible and latent heat Continued efforts to find a phase changing material is currently underway The system operates in three modes. During times of sunshine and when heating is required, air is passed through the collector and subsequently into the home. When heating is not required air is pumped into the thermal storage facility, melting the PCM, charging it for future use. When sunshine is not available, room air is passed through the storage facility, heated and then pumped into the house. When the storage facility is frozen an auxiliary gas heater is used to heat the home. Adequate amounts of fresh air are introduced when the solar heating system is delivering heat into the home as shown in Fig. 5. (Fath, 1998; Kousksou, 2007; Pasupathy, 2008)

Benefits and Drawbacks of PCM Higher storage density than sensible heat Smaller volume Smaller temperature change between storing and releasing energy Drawbacks: High cost Corrosiveness Density change Low thermal conductivity Phase separation Incongruent melting Supercooling Stability of properties under extended cycling, phase segregation, and subcooling of the PCM (Pasupathy, 2008)

PCM Options (Pasupathy, 2008)

PCM Options Inorganic Glauber’s salt, calcium chloride hexahydrate, sodium thiosulfate penthydrate, sodium carbonate decahydrate Benefits: Low cost and readily available High volumetric storage density Relatively high thermal conductivity Drawbacks: Corrosive Decomposition Incongruent melting Supercooling (Pasupathy, 2008; Farid, 2004)

PCM Options Organic Paraffin waxes and fatty acids Benefits: Melts congruently Chemically and physically stable High heat of fusion Drawbacks: More expensive and flammable Low thermal conductivity in solid state Lower heat storage capacity per volume (Pasupathy, 2008; Farid, 2004)

PCM Options

Encapsulation Prevents reactivity towards environment Compatible with stainless steel, polypropylene, and polyolefin Controls volume as phases change Prevents large drops in heat transfer rates (Farid, 2004)

(Kenisarin, 2007)

Increasing Thermal Conductivity Metallic fillers Metal matrix structures Finned tubes Aluminum filling with VSP 25 and VSP 50 The typical volume fractions in the PCM–graphite composition were 10% in volume of graphite, 75–85% in volume of a PCM, with the remaining volume filled with air. Depending on the volume fraction of the graphite in the composition, the effective thermal conductivity was increased from 0.2 W/(mK) for pure PCM to 25–30 W/(mK). This increase is greater by factor of 10 than that achieved by using metallic rings and it is up two orders of magnitude in comparison with pure PCM. Instead of an increase by a factor of 3, the heat flux in LHTS with similar composite materials was increased by a factor of about 10. At the same time, the heat storage density went down by only about 20%. Finned Tubes PCM-Graphite Matrix (Farid, 2004; Kenisarin, 2007)

Total solidification time of PCM is shorter with fins and lessing rings, but the total quantity of stored heat is slightly smaller The VSP25 filling provided the highest thermal conductivity of 1W/(mK), which is about six times that of pure paraffin (Kenisarin, 2007)

Conclusion Thermal energy storage is imperative to make solar energy more reliable and competitive Further research in phase changing material can improve the efficiency of energy storage Design of the system is also important in optimizing energy storage

References Aghbalou, F., F. Badia, and J. Illa. “Exergetic Optimization of Solar Collector and Thermal Energy Storage System.” International Journal of Heat and Mass Transfer 49.7-8 (Apr. 2006): 1255-1263. ScienceDirect. Elsevier. 16 Nov. 2007 <http://www.sciencedirect.com/>. Badescu, Viorel. “Model of a Thermal Energy Storage Device Integrated into a Solar Assisted Heat Pump System for Space Heating.” Energy Conversion and Management 44.10 (June 2003): 1589-1604. ScienceDirect. Elsevier. 16 Nov. 2007 <http://www.sciencedirect.com/>. Denholm, Paul, and Robert M. Margolis. “Evaluating the Limits of Solar Photovoltaics (PVs) in Electric Power Systems Utilizing Energy Storage and Other Enabling Technologies.” Energy Policy 35.9 (Sept. 2007): 4424-4433. ScienceDirect. Elsevier. 16 Nov. 2007 <http://www.sciencedirect.com/>. Farid, Mohammed M., et al. “A Review on Phase Change Energy Storage: Materials and Applications.” Energy Conversion and Management 45.9-19 (June 2004): 1597-1615. ScienceDirect. Elsevier. 17 Nov. 2007 <http://www.sciencedirect.com.ezproxy.lib.utexas.edu/>. Fath, Hassan E. S. “Technical Assessment of Solar Thermal Energy Storage Technologies.” Renewable Energy 13.1-4 (Summer 1998): 35-40. ScienceDirect. Elsevier. 17 Nov. 2007 <http://www.sciencedirect.com.ezproxy.lib.utexas.edu/>. Kenisarin, Murat, and Khamid Mahkamov. “Solar Energy Storage Using Phase Change Materials.” Renewable and Sustainable Energy Reviews 11.9 (Dec. 2007): 1913-1965. ScienceDirect. Elsevier. 17 Nov. 2007 <http://www.sciencedirect.com.ezproxy.lib.utexas.edu/>. Koca, Ahmet, et al. “Energy and Exergy Analysis of a Latent Heat Storage System with Phase Change Material for a Solar Collector.” Renewable Energy (May 2007): 1-8. ScienceDirect. Elsevier. 16 Nov. 2007 <http://www.sciencedirect.com/>. Kousksou, T., et al. “Second Law Analysis of Latent Thermal Storage for Solar System.” Solar Energy Materials and Solar Cells 91.14 (Sept. 2007): 1275-1281. ScienceDirect. Elsevier. 19 Nov. 2007 <http://www.sciencedirect.com.ezproxy.lib.utexas.edu/>. Pasupathy, A., R. Velraj, and R. V. Seeniraj. “Phase Change Material-based Building Architecture for Thermal Management in Residential and Commercial Establisments.” Renewable & Sustainable Energy Reviews 12.1 (Jan. 2008): 39-64. ScienceDirect. Elsevier. 18 Nov. 2007 <http://www.sciencedirect.com/>. Regin, A. Felix, S. C. Solanki, and J. S. Saini. “Heat Transfer Characteristics of Thermal Energy Storage System Using PCM Capsules: A Review.” Renewable and Sustainable Energy Reviews (Aug. 2007): 1-14. ScienceDirect. Elsevier. 20 Nov. 2007 <http://www.sciencedirect.com.ezproxy.lib.utexas.edu/>.

Other Applications Cooling of heat and electrical engines Cooling: use of off-peak rates Cooling: food, wine, milk products (absorbing peaks in demand), greenhouses Heating and hot water: using off-peak rates Medical applications: transportation of blood, operating tables, hot–cold therapies Passive storage in bio-climatic building/architecture (HDPE, paraffin) Safety: temperature level maintenance in rooms with computers or electrical/electronic appliances Smoothing exothermic temperature peaks in chemical reactions Solar power plants Spacecraft thermal systems Thermal comfort in vehicles Thermal protection of electronic devices (integrated in the appliance) Thermal protection of food: transport, hotel trade, ice-cream, etc. Thermal storage of solar energy (Kenisarin, 2007)