All passive-heat storage is not created equal: The case for phase-change materials

Anyone who has sat on a sun-warmed rock on a cold night has experienced the effects of passive-heat storage. The effect occurs regularly in nature with anything that collects solar heat during the day and releases it at night. In this case, rock is a type of thermal mass — material that absorbs and retains heat for release later. With indoor spaces, many different types of thermal mass can be utilized.

Good passive-solar building design means that the walls, floors and windows can collect, store and give off heat during cold temperatures and repel heat during warm periods. Examples of commonly used thermal mass are brick, rock, concrete, tile and, more recently, various types of phase-change materials.

In poorly insulated structures such as greenhouses, thermal mass is used to trap solar heat during the day for night release, preventing damage and promoting growth in plants and veggies. It also is used to decrease overheating by providing a place for the sun’s energy to accumulate, creating a more stable thermal environment. A typical thermal mass found in greenhouses is a Trombe wall of dirt, concrete, brick and/or barrels of water. 

Normally, the larger the thermal mass in a space, the greater its ability to store heat and stabilize temperatures, resulting in less energy consumption, and savings on heating and cooling costs.  Any thermal mass is good for heat storage, but some work better than others. 

Phase-change materials as thermal mass

Phase-change denotes materials that change from a solid to a liquid and liquid to a solid while absorbing and releasing thermal energy. Some commonly utilized PCMs are water, paraffins, fatty acids and salt hydrates. PCMs are an ideal solution for passive temperature control, especially when there are large variations between outside day and night temperatures.  PCMs made from salt hydrates serve as practical and efficient thermal mass in buildings, homes and greenhouses because they are nontoxic and nonflammable, require less space than other types of thermal mass, and are simple to employ.

A good example of how a 72-degree-Fahrenheit salt-hydrate PCM works can be observed at Sycamore greenhouse at A-B Tech’s main campus in Asheville. The installation process was simple. The PCM was poured into 5-foot-long, 1.24-inch PVC pipes. Wooden supports were drilled into the existing cinder-block wall inside the greenhouse, and the pipes were horizontally bound. The rows of pipe were framed with wood and covered in a half-inch insulation board. An attic fan was attached beneath the row of pipes for thorough air circulation. The fans pull the air from above, through the PCM pipes, releasing the heated or cooled air below. This ultimately stabilizes the temperatures inside the greenhouse.

A total of 274 pipes were installed, with each pipe containing 5 pounds of PCM, for a total of 1,370 pounds. This system provides 30 kilowatt-hours of heat-storage capacity. During the day (the heat-gain cycle), the PCM pipes absorb 30 kwh of heat, reducing its cooling load by 30 kwh. At night, during the heat-loss cycle, the system will release the PCM’s absorbed heat and reduce the heating load by 30 kwh. This provides 60 kwh of free energy within a 24-hour day.

All the materials above, excluding the PCM, can be bought locally. This 72-degree, salt-hydrate PCM can fill any container made from HDPE, PP plastic or stainless steel. Placement inside a space can be against, on or inside a wall; above the ceiling; in between floors; or independently standing. The PCM is melted by warm daytime temperatures and begins to release the absorbed heat as the temperature falls below 70 degrees. The PCM then solidifies. During solidification, a constant temperature of 72 degrees is maintained. Once it’s completely solid, it acts as a heat sink as temperatures rise above 74 degrees and continue the cycle of absorbing heat.

Without the PCM installation, the greenhouse would have been intolerably warm during the summer months. The temperatures inside the greenhouse are consistently 15-20 degrees lower with the PCM than without it. The phase-change material in the greenhouse eliminates temperature extremes that would normally occur with our changing seasons, not to mention that it retains optimal conditions for plant growth.

Due to the simple nature of phase-change materials, there are numerous methods of storing heat in both passive and active solar applications. All types of buildings and enclosed spaces can benefit from incorporating PCMs. Computer and communications facilities already utilize PCMs to absorb heat and maintain the recommended temperatures to protect their electronics. Thermal heat storage can, at the same time, shift the load on heating and cooling equipment and serve as backup during power failures. The possibilities of heat storage for enhancing active solar applications are currently being explored and implemented by professionals in various industries worldwide. Thermal heat-storage solutions will become an important objective of future energy-conservation efforts with the goal of preserving natural resources and lowering energy consumption and costs.

Nina Reinhart is a partner at RGEES. To find out more about phase-change materials, visit