The Basics of Phase Change Materials
Originally authored By Kristof Irwin, edited for blogging format
With our upcoming Building Science Happy Hour focusing the topic of phase change materials, we wanted to provide some information on the basics. This blog post will take a look at phase change materials (hereafter referred to as PCM) in lightweight building assemblies.
PCM Technology Background Information
PCMs are latent heat storage materials. Unlike insulation, which only retards the flow of thermal energy through a structure, PCM absorbs heat and stores it in the molecular structure of the material. If this heat can later be shed passively to the exterior, building energy use can be decreased. PCM can also be used at interior surfaces to stabilize interior temperatures by absorbing and releasing heat as it stays in solid (frozen) state at room temperatures.
Research and development on the application of both active and passive PCM to the built environment has been ongoing for years. There are both organic and inorganic PCM products either available commercially or under development. Inorganic compounds include salts and salt hydrates and eutectics. Organic materials include paraffins, fatty acids and sugar alcohols. PCM selection depends on many factors, including: phase change temperature and transition characteristics, energy storage density, thermal conductivity, material stability, compatibility with construction materials and process, long lifetime, non- toxicity and inflammability, and pricing and availability.
Recent R&D work in organic PCMs based on proprietary mixtures and processing of organic extremely pure fatty acids has created a product with precise, controllable transition temperatures, natural fire suppression characteristics and a high energy/enthalpy storage density. The product also integrates easily with existing building techniques and is priced to make it cost effective, with reported ROIs of 2-4 years.
The candidate product currently under consideration is called BioPCM and is manufactured from a combination of soy and palm oil by Phase Change Energy Solutions Inc. The product has 3 lines of PCM with increasing energy storage densities. These are referred to as M-27, M-51 and M-91, where the “M-factor” is similar to R-value but include latent heat storage effects (these behave similarly to a mass effect, thus the “M”). M-27 is specified to store 27BTU of latent heat per square foot. In addition, each of these products can be specified with a phase transition temperature of 73, 76, 79 or 84° F, these are known as Q-values. Selection of M-factors and Q-values is based on building load characteristics, installation considerations, local diurnal temperature cycles and interior set point temperature. The M-27 and M-51 products with Q-values of 79 and 84 are the primary selections for local designs. These materials can be installed on any building envelope surface.
Improving Comfort & Performance
PCM acts to absorb heat flow through the building envelope and to stabilize interior temperatures, particularly surface temperatures. These effects improve building comfort and reduce the energy needed to run mechanical systems to heat/cool the building. By delaying and offsetting thermal load to the building, PCM also performs peak load shifting. Studies have shown energy savings of 20-50% relative to reference buildings without PCM. The chart below show the results of energy monitoring in a side-by-side study summertime study near Phoenix, Arizona. Note that Arizona typically has less humidity than central Texas typically and there for experiences a wider range of diurnal temperatures, which allow for full recharge of the PCM each night. Full recharge, physically re-freezing into solid form, of the PCM is a prime performance consideration in central Texas.
PCM & Modeled Energy Performance
Positive Energy is currently investigating the capacity of PCM to offset the increased cooling due to high SHGC values and radiant heating effects, most notably in the rooms with significant glazing loads, as these are dynamic and tend to dominate comfort during critical times of the day.
As a reminder, under the Performance Path the IECC2009 preserves three mandatory prescriptive requirements:
1. Envelope leakage (ACH50 < 7)
2. Duct leakage (8/12 CFM25 of leakage per 100 SF CFA)
3. Area-weighted SHGC of 0.5 (relaxed from 0.3 in Prescriptive Path).
The strategy for projects has been to show the ability for PCM to offset the glazing load by storing incident radiant energy and re-radiating this heat to the exterior.
In addition to reviewing the open literature on PCMs, we have been working with University building science labs to understand PCM function in our climate zone, and with energy modeling software vendors to model the impact of PCMs on energy use.
Consultations with PCM researchers at UT’s Cockrell School of Engineering offer confirmation of the effectiveness of the both the passive and active PCM scenarios, and that more quantitative research is needed to validate the passive PCM concept in our local climate zone (2A). One specific concern is the relation of the material transition temperature and local diurnal temperature swings. This concern is directly related to both the sharpness of the phase transition and the transition temperature. Based on local TMY data the Q-84 product will be able to recharge (re-freeze) during Austin summer conditions, where the highest night time low temperatures are typically near 79°F.
Repeated interactions with Architectural Energy Corporation (AEC) have led to the conclusion that their REM/Rate software can not accurately account for the latent storage of PCM. The recommended strategy has been to model PCM as additional R-value. Because the Treehouse envelope already has envelope R-value above R-21 in all non-glazed surface, the effect of additional R-value is extremely limited. We are hitting the flat part of the hockey stick curve as shown below. Because of this, the actual performance of PCM will exceed the modeled performance shown in the next section.