Thermal energy storage for buildings with PCM pellets
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This document discusses using phase change material (PCM) pellets for thermal energy storage in buildings. PCM pellets are produced from paraffin wax encapsulated in high-density polyethylene to provide a stable form. Field tests show PCM pellets added to building insulation can reduce peak heat flux by 33% and net heat gain by 13%. Non-passive storage systems using PCM pellets in fixed-bed tubes are also being developed and can provide cooling capacity equivalent to 1 ton of air conditioning. The document demonstrates commercial-scale PCM production and validates the thermal performance of PCM pellets for passive building envelope applications.
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Thermal energy storage for buildings with PCM pellets
- 1. PCM Pellets for Thermal Energy Storage in Buildings Ramin Abhari, P.E. July 22, 2013 Smart Building Construction Materials and Coatings Honolulu, HI
- 2. Thermal Energy Storage (TES) $$$?! $! Conventional Building System Building System with PCM Thermal Storage Day Night Air-Conditioning Natural Ventilation Night Night Day PCM Thermal Storage
- 3. The Prize for Storage Load,arbitraryscale (megawatts) 1:00 2:00 4:00 3:00 5:00 6:00 7:00 8:00 9:00 Time of Day 10:00 11:00 13:00 12:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 Peak Load Power PlantsIntermediate Load Power Plants Base Load Power Plants Typical summertime demand curve Typical demand curve with TES 24:00
- 4. 200 300 400 500 600 700 800 900 1,000 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 Enthalpy(J/g) Temperature (deg C) H-T Curves for Waterand OctadecanePhase Change Material (PCM)
- 5. Making PCM: Step 1. Paraffin Synthesis O O O O O O HC + 6 H2O + C3H8 + 15 H2 3 (octadecane) (veg oil) H2C H2C NiMo cat C16-C18 paraffin composition Melt point = 21-23 ºC Heat of fusion = 170-190 J/g
- 6. Making PCM: Step 2. Shape-Stable Pellets 70% paraffin, 30% HDPE Twin-Screw Extruder Under-water pelletizer
- 7. PVDC latex coating Ethyl cellulose pre-coat Wurster fluid-bed spray coater Making PCM: Step 3. Coated Pellets
- 8. 94% 95% 96% 97% 98% 99% 100% 101% 102% 0 1 2 3 4 5 6 7 PercentofInitialPCMPelletMassRemaining Heat/Wash Cycle Effect of PVDC Coating on Paraffin Seepage from PCM Pellets 5 kg scaleup coating lab coating uncoated Coating eliminates paraffin seepage from PCM pellets
- 9. Alternate Pellet Coating 6% oil-absorbing calcium silicate powder in V-blender SEM shows good two layer coverage No paraffin seepage, but not solvent resistant
- 10. 0 20 40 60 80 100 120 18 19 20 21 22 23 24 25 26 27 28 ThermalEnergyStored(J/g) Temperature (ºC) PCM pellet brick concrete PCM Pellet Thermal Properties
- 11. Thermal mass in a flexible form Compatible with sustainable architectural practices
- 12. Passive Storage: Building Envelopes ORNL field test 2012 Add PCM to insulation 33% ↓ peak heat flux 13% ↓ net heat gain
- 13. -10 -5 0 5 10 15 20 25 30 35 40 0 1 2 3 4 5 6 7 8 9 10 8/29 8/30 8/31 9/1 9/2 9/3 9/4 9/5 Temperature(T),'C HeatFlux(HF),W/m2 Heat Flux and Wall Cavity Temperatures: Aug 29 - Sept 4 HF cell HF cell+PCM HF cell/PCM/cell T wall ext 42% reduction Wall exterior temperature Heat Flux across Cavities Building Envelope Weekly Test Results Heat Flux thru Cellulose Control Heat Flux thru Cellulose+ PCM
- 14. PCM-Modified Insulation: Whole Building Model Addition of PCM pellets to attic insulation Up to 16% reduction annual electricity use 11-16 year payback
- 15. PCM-Modified Insulation: Flame Tests PCM pellets added to cellulose attic insulation Conformed to ASTM C739 flammability standard
- 16. Non-Passive Storage: Fixed-Bed Tubes 14” diam X 7.5’ PVC or PC pipe segment and a fan (cheap!) Reduces heat gain of the inhabited space (1 ton-hr cooling capacity) W arm air in (day) Cool air out (day) 1 2 2 Cool air in (night) Warm air out (night) 1 3 AirflowthroughbedofPCMpellets 7.7ft 14" OD PVC pipe Air Out Air Out Air In Air In Outside Wall Inside Inside 10X higher heat transfer rate than passive storage
- 17. Replacing Daytime AC: Tube Wall Visible energy conservation!
- 18. Summary Demonstrated PCM production using commercial-scale equipment PCM pellet performance validated in passive storage field test Fire test passed on PCM-enhanced insulation system Non-passive (PCM tube) application under development
- 19. Acknowledgements U.S. Department of Energy Southwest Research Institute Polymer Center of Excellence Advanced Fiber Technology The Coating Place Fraunhofer CSE
Editor's Notes
- Buildings are the Number 1 user of energy in the U.S.—ahead of transportation and manufacturing—accounting for 40% of total. Powering cooling systems is a major part of that energy. More energy efficient cooling systems clearly needs to be part of the global response to climate change– both in terms of mitigation and adaption.My presentation will discuss how energy storage with a novel and relatively low cost phase change material product can achieve this.
- This graphic shows how PCM thermal storage makes cooling a building more efficient. (Here arrows show flow of cool air; alternatively we could have shown removal of heat by reversing the direction of the arrow.)AC is used during hot days at high capacity to cool the inhabitants. At cooler nights the AC is run at reduced capacity (and higher Energy Efficiency Ratio). Depending on the region, sometimes natural ventilation is used at nights to cool. Although this graphic shows the extreme case of completely reducing the AC, for most climates what is achieved is that AC operation is shifted from days to more efficient nighttime operation.With PCM energy storage, the cooling provided by the AC and/or natural ventilation is “stored” at night and used during hot days. This cuts back on the daytime use of air-conditioning. Because cooling is more efficiently done at night, there is a corresponding net reduction in energy use.
- This is a typical summertime power load curve, similar to the ones utilities use to manage the electrical grid. At night time, when everyone is sleeping and offices are closed, the demand is at its lowest. As the day starts around 6 a.m. and workday begins, demand for electricity increases. The demand for electricity peaks as the day gets warmer and AC units start to run continuously. Three different types of power plants are used to meet this grid demand. The base load power plants run 24/7 and are the most efficient. These include nuclear, coal, and the combined-cycle gas power plants that are the most efficient… i.e. highest kWh per Btu fuel energy. These are also the least expensive to run. California Energy Commission report shows that shifting peak cooling load to off-peak electricity reduces source energy use by 20-43%. A UK study quotes 14-46% reduction in CO2 emissions as a result of such a “cooling shift.” And note that in these TES scenarios we are only taking credit for storing the cool from night-time AC, not from cooler night time ambient temperature or natural ventilation.
- This graph shows the amount of thermal energy absorbed by two different substances as they undergo phase change.During phase transition, i.e. from solid to liquid or liquid to vapor, Lots of energy is stored over a small temperature range. We are all familiar with how ice can keep a beverage at a constant temperature around 0 deg C no matter how hot the temperature is… until all ice is melted.PCM do the same but at a transition temperature that is the comfort temperature for human… in the 21-26 C range or 70’s Fahrenheit. Paraffin blends like hexadecane and octadecane are an example of such PCM. We see that as the PCM is exposed to more thermal energy its temperature increase… until it reaches its melt temperature. Then its temperature stays constant until it completes its melting. The molten paraffin liquid starts getting hotter as it is exposed to more thermal energy. At night time, the molten paraffin in frozen by removal of thermal energy, effectively “storing the cold” as it freezes into a solid wax. In some climates to get the paraffin to refreeze (i.e. drop below 21-22 C), AC is required. In other climates natural ventilation achieves this… essentially storing cold from renewable energy sources like wind and black body radiation from the building’s surface. In either case, PCM allow for storage of cold at night for use during hot days, or heat during day for use at hot nights, thus lowering energy consumption. Actual coated PCM pellet do not show as sharp a phase transition, but nevertheless have a much higher “energy storage capacity” as sensible thermal mass material over the comfort temperature range (70-79 F)
- This photo shows a field of canola plant across from the Stonehenge in Britain. Canola or rapeseed grows very well in the North Atlantic weather. Making the PCM paraffin is the first step in the manufacturing process. Paraffins have a number of advantages compared to other PCM material: they are non-toxic, non-corrosive, self-nucleating, highly stable, and water-repellant. In fact their only disadvantage is their flame properties.We have developed a process to make the perfect PCM paraffin composition from direct hydrodeoxygenation of vegetable oils such as canola oil. The reaction chemistry is shown in this slide too.Since canola oil is made mainly of C18 fatty acids, it makes a paraffin composition which is mainly octadecane. The small photo shows the paraffin as it starts to freeze below 23 C.
- The second step of the process is converting the PCM into the pellet form. This slide shows PCM pellets shooting out of a commercial scale plastic pelletizer. For PCM to be useful, they have to be shape-stable. That means, the shape of the PCM product should not change with paraffin phase transition. There are a number of ways of doing that, such as encapsulation. Our low-cost method compounds the PCM paraffin with HDPE at a ratio of 70/30 paraffin/HDPE in an extruder. This makes a homogeneous melt of very high viscosity, that can easily be pelletized using conventional plastic compounding and pelletizing equipment like extruders and underwater pelletizers. Here the molten compound extruded out of the die holes is chopped off into pellets under water which cools into spherical pellets. The pellets are about 2 to 4 mm in diameter… same as common plastic pellets that are sold for injection molding and other converting applications.
- The last step of the process is coating the pellet. This slide shows the sliced cross-section of the final pellet under a scanning electron microscope under 256X magnification. The small photo shows the spray coater we used to coat the pellets.Although the pellets out of the pelletizer are shape-stable, they do become oily when the pellets are heated above paraffin melting point. So the final step of making the PCM pellets is coating the pellets. Our coating formulation prevents seepage of paraffin, and imparts ignition resistance. The pellets are effectively coated with Daran Latex SL112, which is a polyvinylidene chloride copolymer. The PVDC has good ignition resistance and is an excellent barrier for oils. So the paraffin stays within the pellet even at high temperatures. However this polar polymer does not wet the surface of the non-polar paraffin/HDPE pellets well. To address this, an ethyl cellulose (Ethocel) pre-coat was used. So the batch coating process used a Wurster fluid-bed spray coater, where first the precoat was applied and then the PVDC coating layer. This was successfully scaled up from the lab coaters at Southwest Research Institute to the 5 kg scale at the Coating Place.This photo, an electron micrograph of the cross-section of a sliced pellet, shows how well the coating covers the pellets. The two layers of ethyl cellulose precoat and the PVDC coating are clearly visible.
- This slide shows the loss in pellet weight for the coated and uncoated pellets after several heating and hexane wash cycles. We can see that without the coating, 5% of the weight of the PCM pellet is lost after five of these heat and hexane wash cycles. However the coated pellets retain the original weight after multiple heat and wash cycles, confirming the effectiveness of the coating.
- This slide shows the measured heat storage capacity of the coated pellets. We saw a theoretical PCM thermal storage curve earlier that was based on thermodynamic properties such as heat capacity and heat of fusion. Here is the actual results measured on a coated PCM pellet sample. The points on the curve were generated using step-wise DSC analysis. We see here that while cycling within the comfort temperature range, we store and release about 100 J of thermal energy in each gram of PCM pellet. This is an order of magnitude higher than material of construction like brick and concrete. So in theory, a wall board that has PCM in it, has the same effective thermal mass as a wall 10 times the thickness.
- This is an ancient building with wind catching towers, located in the desert city of Yazd in central Iran. It is one of the sustainable architectural practices that allowed cities in the hot desert climate to flourish for many centuries before electricity. At nighttime, the desert wind cools the thermal mass of rocks inside the column. Then during the hot days, a draft is created where warm air is cooled as it passes over the cool mass of rocks from the previous night. It is a concept that can be enhanced with PCM… same thermal mass and cooling capacity as a wall of rocks, but thinner, lighter, and easier to build.
- We tested the concept of making a wood-frame wall act as a thick stone wall by adding PCM pellets to a 5.5” thick wall cavity filled with insulation. This photo is from Oak Ridge National Lab’s Natural Exposure Test Facility in Charleston, SC, which is where we field tested our PCM pellets in this passive thermal storage application.Passive thermal storage occurs when energy is naturally stored and released during diurnal cycles. For passive storage, building envelopes (wall and attic insulation) are a logical place for the PCM. Here the mechanism of heat transfer is conduction and radiation, and having the envelope of the building be the storage medium makes sense.To evaluate our PCM pellets, we combined it with cellulose insulation and installed it in wood frame wall’s cavity. The test was performed at Oak Ridge National Lab’s Natural Exposure Test Facility, and recorded heat flow across the wall and temperatures at various locations within and across the wall. The test was conducted over a one year period. The summertime average showed a 33% reduction in peak heat flux and 13% reduction in net heat gain for the wall that contained insulation and PCM pellets, compared to the wall that contained the insulation alone.
- Here is a typical weekly dataset from the ORNL field test. The PCM-enhanced wall insulation shows a 42% reduction in peak heat flux compared to the control. The net daily heat gain is the area under the heat flux curves. The area under the red curves was on average 13% less than the control.
- This is an output of a “whole building thermal modeling software” used to analyze the performance of the PCM in different U.S. climates. Attic insulation was chosen because it can easily be enhanced with PCM pellets as part of a retrofit project. The software used was called ESP-r. Annual electricity savings were predicted to be around 16% for Southwest U.S. climates, achieved by adding 22% pellets to the attic insulation.
- These photos are of the various fire tests that were performed as part of the project. The system that was modeled was for PCM pellets buried in attic insulation. This system was therefore tested according to the applicable fire standard. The PCM-modified attic insulation passed the fire specification in the cellulose insulation standard. We see in the sequence of photos on the top left that the PVDC-coated pellets self-extinguish after about 6 minutes. The other photos show the cellulose insulation tests for the PCM-modified samples, passing the smoldering ignition and the radiant flux fire tests.
- These drawings are for a non-passive energy storage unit using PCM pellets. One disadvantage of passive energy storage in the building envelope is that the rate of heat transfer when charging and discharging the PCM is fairly low. This makes the building performance heat transfer rate limited-- no matter how much PCM we add, we only gain the benefit of the fraction that freezes within the 8 hrs or so of off-peak cooling period. A non-passive storage system where a fan or blower is used to circulate the warming indoor air into a column of packed PCM pellets is a more effective system. This type of solid-gas heat transfer system is well-studied in the Chemical Engineering discipline. Our design shown in this slide achieves a heat transfer coefficient of 149 W/m2-K or 26.2 Btu/h-F-ft2… which is about 10X the heat transfer rate when heat stored in the wall or ceiling has to be removed by conduction and radiation alone. The columns shown can fit nicely in a home, apartment, or office, and each provide equivalent of 1 ton-hr of refrigeration energy capacity… a convenient unit for applying the rules of thumb used for specifying cooling systems for rooms (i.e. ton of refrigeration per square foot inhabited area) for installing these PCM tubes and quantifying the savings in terms of hours of AC operation replaced.
- For example, for a 1000 sq. ft. space, 1.3 ton of refrigeration AC capacity is needed. Assuming this runs for 8 hrs per day, the 1.3 ton AC provides about 10 ton-h of cooling energy. In order not to run the AC during day time, we need 10 of the 1 ton-hr columns. This can be in the form of the decorative “tube wall” shown in this drawing. It is very easy to quantify savings when an AC unit is replaced or its operation is moved mainly to off peak hours. Note that without any colored additive, the PCM pellets are opaque when frozen, and clear/translucent when melted. So a clear tube can show how much cooling has been charged or discharged. This can be added to any home, office, or public place.