Phase change technology has been used in a very basic sense for hundreds of years, predominantly in the form of melting ice to keep food and people cool. Ice box refrigerators and artificial refrigeration date back to the 1750s, although it wasn’t until 1834 that the first vapor-compression refrigeration system was built.
Thermal energy storage and phase change technology have evolved since those early iterations. Today, more sophisticated materials enable stable temperatures to be maintained for long periods. They also take up considerably less space and are easily reusable.
Thermal energy storage through phase change materials (PCMs) can store and release large amounts of energy. It works by the change in the phase of the material as it holds and releases energy. Processes such as melting, solidifying, and evaporation require energy. Heat is absorbed or released as the material changes from solid to liquid or vice versa. PCMs change their phase when energy is input, and release this energy at a later time.
While similar to the effect of thermal mass, which also stores heat during the day and releases it at night, PCMs allow large amounts of energy to be stored in relatively small volumes, resulting in some of the lowest storage media costs. It works through direct contact between the phase change material and heat transfer fluid, by macroscopic capsules, or by micro-encapsulation, a new technique.
Phase change energy storage allows for storage of thermal energy as latent heat, which allows higher thermal energy storage capacity per unit weight or material without a change in temperature, storage from a thermal energy source or electrical energy source, and storage of thermal energy at the temperature of process application. Stored thermal energy is portable and rechargeable.
Better than Ice
Water storage is cheaper in the beginning, says Scott Queen, regional vice president of Phase Change Energy Solutions . . . except for the real estate required. Added to the size of a tank is a beautification aspect on many colleges that requires a structure around the tank. The amount of real estate required expands as the system grows.
By using a module system, Phase Change Energy Solutions can increase thermal capacity 8–10 times in existing tanks. Special modules are filled with material, sealed, put in a racking system, and dropped into concrete or metal tanks. “It works on cooling towers, chillers, and hot water tanks,” explains Queen, with a range of below 0°F to above 300°F. This space-saving option can be particularly beneficial if a school
He explains that their product is 100 BTUs per pound of energy, while water is 1 BTU. “With our product, you can reduce the size of a storage tank as much as from 100,000 gallons to 5,000 gallons,” estimates Queen. For customers who want to store hot water in a solar system, he says they’re 10–20 times smaller.
Advantages of the module system compared with ice storage include the fact that it doesn’t require an additional chiller. “Ice storage requires an additional chiller and is inefficient,” points out Queen. This works with any chiller system and can double the efficiency.
Ice systems require glycol, but this module system doesn’t need it. And, whereas water systems require a lot of water and treatment, the materials in the module system do not go bad. “They’ve been tested for 85 years to hit a set [temperature] point,” explains Queen. “Our competitors’ material doesn’t last; it’s like a rechargeable battery that loses a charge.”
Maintenance is near-zero, he adds, because the material doesn’t expand or contract like ice, so there is less wear and tear on the tanks: fewer stress fractures and ruptures. “Ours contracts when it freezes, but only slightly.”
In addition, unlike with ice, this material can be set at specific temperatures. “It’s custom-tailored within a range for each application to hit a set temperature,” affirms Queen. This can be especially important when shipping items such as medicines and vaccines that must be kept at specific temperatures. “We can ship cold items at -30°F, so instead of next-day air, we ship in 3–5 days, saving money.” Also, they can ship to areas where ice is not readily available.
Like Phase Change Energy Solutions, Entropy also works with shipping companies. “Ice is too cold for most medicines and vaccines,” notes Rusty Sutterlin, chief science officer. “We designed a plethora of temperature ranges, using different formulations, blends, and different chemistries.”
Entropy provides the largest assortment of solid-state PCMs, according to Sutterlin. A phase change material is a substance with a high heat of fusion that, by melting and solidifying at specific temperatures, has the capacity to store and release large amounts of energy.
PCMs are categorized as inorganic or organic. The commonly used phase change materials for technical applications are: paraffins (organic), salt hydrates (inorganic), and fatty acids (organic). Traditional PCMs have been formulated from salt hydrates or paraffin wax, but both materials have issues.
Inorganic PCMs are an engineered hydrated salt solution made from natural salts and water. The chemical composition is varied to achieve the required phase change temperature. Special nucleating agents are added to the mixture to minimize phase change salt separation and supercooling, which are characteristic of hydrated salt PCM.
Salt hydrates are non-toxic, non-flammable, and economical. However, they are susceptible to corrosion and are harder to contain. They can also break down with repeated thermal cycling, meaning that their storage density decreases with cycling because hydrated salts melt congruently with the formation of the lower hydrated salt, making the process irreversible and leading to the continuous decline in their storage efficiency. “Salt-based products have a lifecycle problem,” sums up Sutterlin.
According to Climate Tech, latent heat storage materials used inorganic PCMs such as salt hydrates because they had high latent heat values and were non-flammable, low-cost, and readily available. However, their corrosiveness, instability, improper re-solidification, and tendency to supercool led many to consider organic PCMs. Subcooling occurs when a salt hydrate begins to solidify at a temperature below its congelating temperature.
Bio-based PCMs are organic materials that are naturally existing fatty acids such as vegetable oil. Based on their chemical composition, their phase change temperature can vary. They are non-toxic, non-corrosive, and have infinite life cycles. However, they can be expensive and are flammable at high temperatures.
Organic PCMs are petroleum byproducts that have a unique phase-change temperature. Organic PCMs are more chemically stable than inorganic substances, they melt congruently, and supercooling does not pose a significant problem. They have infinite life cycles, but because the price varies with changes in global petroleum pricing, they can be expensive. In addition, paraffin wax is petroleum-based, so it’s not environmentally favorable—but it is flammable and may generate harmful fumes on combustion, making it potentially dangerous.
Entropy uses fatty acids—natural-based, typically of vegetable oil or animal fat. Both are safe, environmentally friendly, and neither breaks down, Sutterlin says.
The classic material uses melting technology. Phase change by melting and solidification of material can store large amounts of heat or cold. Melting typically involves a small change in volume—usually less than 10%. If the material can fit in a container while it’s liquid, the pressure is not changed significantly, allowing melting and solidification of the material to proceed at a constant temperature. Once it has melted, the material maintains its temperature constant at the melting temperature, also called phase change temperature.
The material absorbs a large amount of heat to change it from a solid to a liquid state, pulling energy. Sutterlin says it requires a lot of energy to raise the temperature 1° by releasing heat. “To heat 1 milliliter of water 1°C (from 25 to 26°C or from 30 to 31°C) requires 4.18 joules. To go from 20 to 22°C, it takes twice as much; you must double for 2°. That’s sensible heat: put in energy to raise the temperature. The reverse is the same.” But, he adds, to go from 1 to 0°C requires 330 joules. “Freezing is harder to reach.”
If the melting is completed, further transfer of heat results in sensible heat storage. The heat supplied upon melting is called latent heat; the process is known as latent heat storage. Sensible heat is the most common method of thermal energy storage. Heat is transferred to the material, increasing its temperature. Hot water storage is a common example of heating.
Phase change is latent heat, and there is a lot of energy to take advantage of as the material changes. Sutterlin uses a coffee cup as an example. If you put PCM that melts/freezes at 150°F into the outside wall of the cup and pour in coffee at 200°F, it would melt the PCM, cooling the coffee in 1–2 minutes and holding the temperature there. He calls PCM a “temperature speed bump” because it slows temperature change.
Similarly, buildings and rooms can be kept at steady temperatures in all seasons if PCM is installed into the wall board. “The idea of using PCMs in buildings is not new,” Sutterlin states, “but there are new ways to contain PCMs in walls.”
Modern lightweight construction lacks thermal mass, meaning that buildings can overheat in the summer and can’t retain heat in the winter. In order to maintain comfortable temperatures, heating and cooling systems are installed. However, it is also possible to replicate the effect of thermal mass of the building using phase change materials.
During a retrofit, if you don’t want to destroy the walls, you can add the material in a dropped ceiling for similar effect as part of a passive system in which heat or cold is stored automatically and released when indoor or outdoor temperatures rise or fall beyond the phase change point of the material. Phase Change Energy Solutions is putting it in schools. “We put it in the white boards if we can’t put it in the ceiling,” says Queen. “We can come up with some pretty creative placement without seeing it.” This makes retrofits easy. In fact, Queen says, retrofits are a major part of what they do.
Instead of being located in building components such as walls and ceilings, PCMs can be arranged in separate heat or cold stores as part of an active system. The stored heat or cold is in containment, separated from the building, and heat or cold transfer is performed on demand.
Although Sutterlin clearly differentiates this from the use of insulation in a building, he does consider insulation as indirect competition because people “use what’s familiar” to maintain desired temperatures.
There are “thousands of uses” of PCM, Queen says. But what’s the cost of adding them?
“When electricity is cheap, people waste it instead of investing in technology,” observes Sutterlin. Nevertheless, the lower cost of PCMs, along with wider education about their use, has increased acceptance. The public is beginning to understand the benefits of using this technology. For example, incorporating PCMs into a building system leads to more consistent thermal cycling of the HVAC compressor, making it more efficient, extending its lifetime, and lowering heating costs, particularly if you are peak load shifting by charging at night.
Many industrial centers, schools, hospitals, and airports run chillers at night for their thermal energy storage tanks, pumping water during the day. As buildings are expanded or campuses grow, the original tank is no longer big enough to service all the units. By putting PCM inside the tanks to increase energy storage density, building owners can avoid additional costs and energy shortages. It’s an easy retrofit, Sutterlin says.
This technology works with solar power used to heat tanks. “The PCM stores and transfers energy,” explains Sutterlin. “PCMs keep solar panels and lithium batteries cool for better efficiency so the batteries don’t overheat.” It could even contribute to LEED certification.
Other economic benefits of PCMs include storing thermal energy during off-peak hours for use during peak demand hours. In addition to helping building managers save on energy costs, this also helps stabilize grid load.
Shifting heating and cooling load reduces stress on the equipment, which could contribute to lower operating and maintenance costs. Building managers can size HVAC equipment for average loads rather than peak loads, which enables them to buy smaller equipment at less cost.
“We’re working with banks to achieve a 35% reduction in HVAC,” says Queen. In some cases, that results in a 10% reduction of total building energy costs. “Typical ROI is 10 cents per kW, but it depends on the location.” He prefers to calculate it in years, estimating that for many, the payback period is 5–7 years, with some meeting it in just 2–3 years.
There are many ways to save. Phase Change Energy Solutions recently introduced a phase storage product line. One, consisting of a small 5-gallon tank with food-grade material and stainless-steel coils inside, was designed for the beer industry. “Otherwise, they needed lots of bags of ice to do an event,” says Queen. “One brewery buys 75 bags of ice for an event.” This “ice-less” power allows them to pour beer for 8 hours at the perfect temperature.
Another example Queen gives is reducing chilled water storage by 10 times and cutting demand charges to run a chiller for cooling ethylene oxide when coming off a rail car. “It’s very explosive; it must be cooled.”
A Thousand Uses and Benefits
PCMs also carry the important task of keeping food products hot. As Queen explains, “Ninety percent of pizza markets worldwide use them for individual pizza delivery.” In fact, he says, that’s how the product started: keeping things hot.
More commonly used to cool products and spaces today, PCMs can be found in a surprising variety of applications. They’re effective in roofing applications, Queen says. “When placed under shingles or on a metal roof, they reduce heat load by 85%.” They can also be placed on the back of solar panels to help them maintain efficiency. “They keep buildings warm in winter, cool in summer, and are especially useful in regions where there’s no air conditioning.”
Economic benefits aside, PCMs are environmentally friendly because by reducing energy demand, they consequently reduce the building’s carbon footprint. Some material is food grade, made from a vegetable base and thus, a renewable resource that can qualify for LEED credits. “There are no environmental concerns,” says Queen. “It lasts forever—and you can eat it!” There are no risks, he believes. “The price point was a negative, but no longer.” Even installation takes only 4–5 hours, making it a quick fix.
As good as it is, Queen believes the best use of this material has not been invented yet. Their lead scientists are testing its use with hybrid batteries to increase car mileage, among other uses.
Three advantages of PCM over conventional water/ice storage techniques include:
- Higher thermal energy storage capacity
- Relatively constant temperature during charging and discharging
- Burner cycles for backup power generation, with reduced CO and HC emissions
However, four main disadvantages counter those somewhat:
- Higher cost of investment
- Peak power during discharge is limited due to limited heat conduction in the solid state of PCM
- Limited experience with long-term operation of charge-discharge cycles
- Risk of loss of stability and deterioration of the material