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Product Guide

The Potential of Phase Change Materials

Phase change materials got off to a slow commercial start but have emerged in a revised form to promise reduced energy use

These mats of phase change material encased in plastic and foil help regulate temperature swings and lower cooling costs when installed in an attic, according to the manufacturer. Photo courtesy QE2.

National Gypsum caught the attention of energy-efficiency advocates a dozen years ago with an announcement that it would soon introduce a new type of gypsum drywall capable of absorbing and releasing heat in a cyclical process that would lower heating and cooling costs.

The wallboard, which National Gypsum planned to call ThermalCore, was developed around a wax-based material from the chemical giant BFAS. When the compound reached a certain temperature—in this case 73°F—it began to melt and, in the process, absorbed heat. When the material cooled below that transition temperature later in the day, it would solidify and release the stored heat. This “phase change”—from a solid to a liquid and back again—looked like the foundation for a new family of building materials, a passive process that could moderate temperature swings and lower demand on heating and cooling systems.

ThermalCore was announced at a Greenbuild conference in Phoenix in 2009. According to a report from Alex Wilson at the time, the phase change material (PCM) consisted of tiny beads of paraffin wax encapsulated in acrylic shells, which BSAF called Micronal. Wilson thought the product had real potential, assuming the cost wasn’t too high; “I look forward to tracking ThermalCore’s roll-out,” he wrote.

National Gypsum wasn’t alone in exploring PCMs. Knauf, DuPont, Armstrong, and Saint Gobain reportedly were developing their own versions of paraffin-based PCM building materials. It looked like the start of something big. And then, nothing. ThermalCore was never fully commercialized. Knauf, a multinational firm with operations in the U.S. and several European countries, does not offer its PCM drywall here, and said it had been withdrawn from the U.K. market “due to a lack of demand.” If there were other PCM drywalls on the cusp of commercial development, they didn’t seem to materialize.

What happened?

“It was very, very heavy, and it only achieved a nominal 8 or 10% savings,” said Gary Gray, head of Research and Development and Product Design for a PCM company called QE2. “What they had when they had the sheetrock that was impregnated with paraffin wax was a candle. It burned like a torch, and they couldn’t figure out how to stop that without making it weigh 150 lb. per sheet and cost $150 per sheet. It’s just not practical.”

It was a lot more expensive than conventional drywall, and not easily integrated into energy modeling software. Nor was the performance equal across all climate zones. It worked better in regions like the Southwest, where temperature fluctuations between night and day were more dramatic but not as well in climates like South Florida’s where it might not get cool enough at night to regenerate the PCM to a solid state.

Today, there are no known PCM drywall products on the market.

New directions

Although National Gypsum’s enhanced drywall never took off, the idea of using PCMs to improve the energy performance of residential and commercial buildings is anything but dead. Research is active and ongoing, and a number of companies have commercialized products that do the same thing as the drywall was supposed to do—store latent heat energy to reduce run times for HVAC equipment and save energy dollars. But instead of using encapsulated paraffin, many of these newer products use hydrated salts as the phase-change agent. At the same time, work continues on developing new types of PCMs that will be especially helpful as the country relies more heavily on renewable energy sources and needs more effective ways of storing energy for off-peak use.

Typical of the newer generation of salt-based PCMs is QE2 Platinum, a 1-1/2-ft. by 4-ft. blanket that can be placed on top of existing attic insulation or installed in wall cavities. The 3/8-in.-thick blanket, which weighs about 1 lb. per sq. ft., consists of sodium sulfate encased in a plastic and foil film. The blanket retails for $2 per sq. ft. and the casing is self-healing in the event it’s punctured—by a nail, for example, when a homeowner is hanging a picture on the wall.

In a telephone interview, Gray and QE2 National Accounts Manager Preston Nix Jr. said the salt compounds cost less and have a higher latent heat capacity per unit of weight than paraffin, and they don’t carry the same fire risks. By modifying the crystal size and shape, the temperature at which the material changes phases can be adjusted to suit specific building needs, they said.

Gray explained that in a cooling climate, blankets placed in an attic absorb heat in the air and then give it up when temperatures cool, reducing the amount of time an air conditioning system needs to run to keep indoor temperatures comfortable. In a heating climate, Gray said, the PCM blanket can be placed between the room interior and the insulation in a stud cavity so it absorbs energy generated by the mechanical heating system during the day and radiates it back into the room when temperatures cool.

The company recommends that the PCM “always communicate with the hot side” of the enclosure. In a cooling climate, the material should be exposed to the ambient air/sunlit surface, and in a heating climate with the conditioned space.

The blankets can be stapled into wall cavities and cut to size along welded seams in order to fit into odd spaces.

Insolcorp’s “Templok” product line is similar. The North Carolina company sells 16-in. by 24-in. and 24-in.-sq. panels that can be used in wall and ceiling framing. Buyers can choose one of several temperatures at which the phase change occurs. In addition, the company sells PCM commercial roofing panels.

ENRG Blanket” is among the PCMs sold by Phase Change Solutions. It’s intended for installations in drop ceilings. The company said the phase-change material, which it calls BioPCM, is plant-based and non-toxic, although it’s not clear exactly what it is (the company did not respond to phone messages or emails). The company also sells PCMS intended not only for buildings but also for food delivery, and pharmaceuticals.

Dörken, a German-based company that makes a variety of building materials, also has an entry into this market with its Delta Cool 24, a salt hydrate available in several forms, including pouches, balls, and dimple sheets. The product is 100% recyclable, non-toxic, and is a flame retardant, according to Dörken.

Claims for energy savings

These products are designed to moderate temperature ups and downs by behaving as thermal mass—absorbing and releasing energy at different times of the day. According to the Phase Change website, using the 2-ft. by 4-ft. blanket in a dropped ceiling would reduce HVAC costs by between 25% and 35%, cut run times for HVAC equipment by between 15% and 20% and reduce HVAC cycling by 20% to 25%. A PCM blanket stores as much heat as a 24-in.-thick block of concrete, the company says, and has a 100-year-plus lifespan. It’s not clear what third-party testing if any is behind those statements.

QE2 made similar claims, and Phase Change Solutions has posted a number of case studies at its website detailing energy savings with the use of its products.

“What we can say and what we’ve been able to demonstrate in about 40 houses or so in Louisiana and Texas is between 20% and 30% average reduction year over year in kilowatt hours used,” Gray said. “That payback time varies.” Asked if the same energy savings would be possible in a heating climate, Gray said, “I think that could be achieved, yes.”

photo showing inside of phase change packet
QE2 Platinum blankets comprise a number of connected 3-in. by 5-in. packets containing sodium sulfate, a material that goes from a solid state to a gel depending on the temperature. The foil-facing of one cell has been cut back to show the PCM inside. Image credit: Scott Gibson.

Gray said laboratory tests are currently underway that would confirm those claims, but he added, “The short answer is no, we haven’t done any proof of testing in the real world. That’s a function of lack of focus in that area. We’d love to do that.”

Dörken said that using Delta Cool 24 in the ceiling of a room kept at 77°F reduced required mechanical cooling from 50 watts per sq. m. to 20W/m2 while flattening out the temperature profile inside.

David Toups, a marketing consultant for Insolcorp, said that installing PCM tiles in the ceiling could reduce HVAC costs by between 20 and 30%. Several studies with the Department of Energy are underway to verify energy savings, he said.

But choosing the right transition temperature is the key to performance, Toups added. In a cold climate, the right temperature might be 69°F, while in Houston or Arizona he would lean toward a higher transition temperature. In a cold spot like northern New England, having a PCM with a temperature that’s too high means the material never gets the chance to store heat because the PCM can’t melt. In a cooling-dominated climate, too low a transition temperature means the PCM never turns back into a solid.

“The magic of a phase change material is when it actually melts and freezes,” he said in an interview. “As long as you can get that material to transition from a liquid state to a solid state and back again, that’s when it acts like a rechargeable battery.”

The road ahead

Although the mechanics of PCMs seem relatively simple, integrating them into building envelopes is complex, says Marcus Bianchi, senior research engineer and business development lead at the National Renewable Energy Laboratory in Golden, CO, who has studied PCMs for more than a decade.

Current building energy codes do not have a way to account for the phase change effect of these materials, Bianchi said in a telephone call, and they are effective only when there is a cyclical temperature swing allowing the PCMs to melt and then reset themselves when temperatures fall below the melt point.

“If you have, let’s say, a very cold temperature outside, day in and day out, and you have a hot temperature inside, day in and day out, the product sits there doing nothing,” Bianchi said. “You need to have the cyclical temperature change that makes the PCM change phase back and forth. If that doesn’t happen, the product doesn’t give you anything. Building codes don’t have a prescriptive path, a way, to accommodate this transient nature of PCMs.”

One reason that houses typically don’t have PCM materials in building assemblies today is that building codes don’t recognize them as an alternative to insulation. PCMs become what Bianchi called a “product of choice,” akin to the granite countertops that are derided as an unimportant frill that still help to sell houses. PCMs are not a “pretty product,” and it can be hard to explain their benefits while cosmetic features like stone countertops and fancy  bathrooms can be immediately appealing.

“This has to be done at the builder level,” he said of incorporating PCMs into the building envelope. “Maybe there are some very good, sophisticated builders who would do it, yes, but there are few who are interested in explaining the benefits of such a system. It’s too complex. And I think that complexity is what makes this product difficult.”

What might help make PCM building materials more prevalent in the years ahead, however, is the explosive growth of renewable energy in the U.S. pushed forward by the increasingly dire warnings of climate change caused by burning fossil fuels. What’s the connection? PCMs can become thermal batteries, storing heat energy when it’s cheap to produce and releasing it when needed later.

“The whole push for renewables is changing the game because renewables are not dispatchable,” he said. “You have renewables at certain times, then the wind dies down. You have to have storage. When you have a lot of electricity available you charge a system, and when you don’t’ have a lot of electricity available you use the charge that you have to actually run the systems. This is true for electricity, but it’s also true for thermal. Thermal is about 50% of the loads of a U.S. building, and batteries are not cheap. If you can’t have multiple batteries to store all of the energy you need, creating a thermal battery is pretty attractive.”

The concept is especially appealing for people who have time-of-day electric rates. For example, during the morning when rates are low you crank up the air conditioning and charge any PCMs  in the building envelope—that is, bring the PCM to a solid state. As the day progressed, and temperatures begin to go up, the PCM is able to absorb heat and go to a gel or liquid state. That helps to minimize temperature fluctuations and delay the need for mechanical cooling inside.

Bianchi continued, “There is an aspect of the envelope that’s very important that maybe we’re not touching yet. There is the energy piece of it, but there’s also the comfort piece of it. You could actually create an environment that provides more comfort by having a larger thermal mass, so the temperature swings outside don’t translate into temperature swings on the inside.”

But, Bianchi said, choosing the transition temperature of the PCM is key. If the material melts at, say, 76°F and the homeowner keeps the thermostat set at 72°F, there’s no benefit because the material never gets the chance to go through a phase change. In fact, an article that Bianchi helped write says that incorporating PCMs into building walls doesn’t always lead to improved performance. “Incorrect applications of PCMs can substantially increase energy use in buildings,” this report says.

At the moment, salt hydrates seem to have several advantages over paraffin wax PCMs, including lower cost and a higher energy density, but they have their own disadvantages and researchers continue to look at these and other possibilities. Among ongoing projects, Bianchi said, is the development of a new generation of PCMs whose transition point can be adjusted, making them as valuable in winter as they would be in summer and in a range of climates and building types.

Chart comparing phase change materials
Salt hydrates have several advantages over other phase change materials. Graphic courtesy Oak Ridge National Laboratory.

Optimizing the material

There may be many ways in which PCMs as part of the building enclosure or its mechanical systems can bolster energy efficiency. Some of them build on the original but ill-fated enthusiasm of PCM drywall. For example, researchers have looked at composites in which microencapsulated PCMs could be mixed with foamed concrete, opening the possibility of a new kind of lightweight cladding panel that could be used on building exteriors to reduce external heat gain.

PCMs may prove useful in HVAC systems, a path now being explored in commercial cold-storage systems by companies such as Viking Cold Solutions. In the U.K., a company called Sunamp sells a PCM thermal battery that can be charged with different energy sources—solar panels, ground- and air-source heat pumps—and used for  space heating or to produce domestic hot water. The devices use a PCM material called “Plentigrade,” typically a salt.

Sunamp, launched in 2009, is well established in Europe and last month opened offices in New York. General Manager Tom Sottile said in a telephone call that the company hoped to begin selling its thermal batteries in the U.S. in the fourth quarter of this year. The Sunamp water heater, called a Thermino, is made in four sizes, the equivalent of 20, 40, 60, and 80 gal. All of them have a footprint of 14-1/2 in. by 23 in. and can be charged either with an electric heating element or a heat pump. When fully charged, the 80-gal. Thermino will produce 80 gal. of water at 140°F before it needs recharging, a process that takes between two and three hours.

This photo shows a Thermino water heater
Sunamp’s PCM water heaters come in several sizes and are sold under the Thermino label. They are not yet available in the U.S. but are expected to be on the market later in the year. Image credit: Sunamp.

The water heaters store only a couple of quarts of water. Heat loss is half or less that of a conventional tank-style water heater, Sottile said, and the PCM has been tested through 40,000 cycles. That’s enough for two charging cycles per day for 50 years. Larger models could be used for space heating.

Sottile said the company is still working on pricing but he expected the biggest water heater would sell for between $3000 and $4000 when it becomes available here.

Sunamp’s products address that market niche that Bianchi thinks will be key in the years ahead—storing energy produced off-peak when it’s cheaper and used during peak loads when energy is more expensive. One model designed for space heating, for example, can store 375,000 Btu of energy.

“We’re not proposing it as an alternative to the central heating system,” Sottile said. “We’re proposing it as a way to either store heat when it’s less expensive to make it, or to help with peak loads.”


Scott Gibson is a contributing writer at GBA and Fine Homebuilding magazine.

43 Comments

  1. Paul Pfeiffer | | #1

    I always thought PCMs would be nice for a lightly conditioned space like a greenhouse where you want to keep the temperature above some minimum. Or perhaps have one PCM at a desired min and another at a desired max?

  2. Dan S | | #2

    > Sunamp’s products address that market niche that Bianchi thinks will be key in the years ahead—storing energy produced off-peak when it’s cheaper and used during peak loads when energy is more expensive. One model designed for space heating, for example, can store 375,000 Btu of energy.

    > “We’re not proposing it as an alternative to the central heating system,” Sottile said. “We’re proposing it as a way to either store heat when it’s less expensive to make it, or to help with peak loads.”

    I've been watching Sunamp for a bit, and I think they'd be crazy not to find a way to tightly integrate this with a heat pump and air handler to make a drop in replacement for a existing forced air furnace. 375KBTU is enough for multiple days' heat in many climate zones in the US, and pairing heating loads with cleaner but intermittent renewables by dispatching heat pumps to store heat to be used later (as is starting to happen with heat pump water heaters) will probably be a game changer for the electric grid - especially if it can be done for existing buildings and ductwork.

    An additional challenge is getting the utilities/regulators/consumers to agree to the more variable rate structures that would create the shared incentives to adopt these approaches.

  3. Expert Member
    Peter Engle | | #3

    It seems to me that the products designed to be placed within the thermal envelope to store heat all suffer from the same issue: You have to allow the interior temperature to vary by a relatively large margin (several degrees) to charge and discharge them. Most people are not comfortable with temperature changes more than a degree or so. Once they find their comfortable temperature, they set the thermostat and forget it. With minisplits, this is almost a requirement. Large temperature changes significantly lower their efficiencies. There's also the issue of the rate of heat transfer. If you're looking at a temperature change of only a degree or so between the PCM and the indoor air, there just isn't much convection, radiation or other heat exchange.

    That said, I can see the benefit of placing it as a blanket on top of attic floor insulation in the SW or other are with significant daily temperature swings, so long as a significant part of the year has swings above and below the materials transition temperature and also so long as the buildings don't already have a lot of thermal mass to even out those swings. Might actually be pretty limited utility and/or payback.

    1. Charlie Sullivan | | #4

      The above-insulation blanket is an interesting concept, but it still only evens out the heat load over the course of a day, rather than reducing it. It's going to be hard to justify that vs. spending the same money on another layer of insulation in the same spot, which reduces the load all day not just during peaks.

      The use in storage tanks for hydronic heat pump systems seems to me to have more potential. The temperature swing there can be 10 to 20 F while maintaining strict control over the temperature in the conditioned space.

  4. DCContrarian | | #5

    I was very excited about PCM a few months ago, I even started a thread in the Q&A:
    https://www.greenbuildingadvisor.com/question/calcium-chloride-hexahydrate-for-phase-change-heat-storage

    My fellow posters quickly disabused me and made me see the error of my views.

    Everything that PCM wants to do can be done with insulation. Insulation is boring, but it's the best way.

    1. Dan S | | #6

      > Everything that PCM wants to do can be done with insulation.

      Except serve as heat storage right? For that you need a medium: water/PCM/masonry/drywall/whatever, plus insulation to keep the heat in. If Sunamps #s are to be believed, PCMs can be a comparatively dense heat storage mechanism.

      1. Expert Member
        Malcolm Taylor | | #7

        Dan,

        I guess then the question becomes: is storing energy (in the form of heat) in building materials a useful strategy in most situations.

        1. Dan S | | #8

          It's probably not a useful strategy to store heat in building materials (although I heard Joe Lstiburek recently suggest doubling drywall as a way to do that via thermal mass).

          However, a dedicated heat storage device could make sense. The financial viability depends on the price of of the heat storage equipment and also of the energy used to generate or move that heat into the equipment.

          In places like the UK consumers can elect plans where the overnight electricity rates are a fraction of the daytime rates (I think as low as 4p/kWh). I believe that is the target scenario for a devices like Sunamp's heat battery.

          Here in the US in Washington state the standard off-peak price per kWh is only 7c/kWh, a price at which storing heat generated with electricity (either resistance or heat pump) could make sense.

          1. Expert Member
            Malcolm Taylor | | #10

            Dan,

            I don't know enough about this to comment usefully, but I wonder if it wouldn't be more efficient to simply store the electricity rather than convert it to heat?

          2. Dan S | | #11

            Malcolm,

            > I wonder if it wouldn't be more efficient to simply store the electricity rather than convert it to heat?

            Storing electricity in most homes today doesn't make any financial sense. Those home batteries have a very high upfront cost per unit of energy storage ($15000 or ~$1000/kWh installed for a Tesla Powerwall, for example), and arguably are a still a luxury item, and not really a sound investment unless you have regular power cuts (or fear thereof). I know because I have a similar battery and I spent last fire season waiting for the grid to go down, and it never did. The only value it has had so far has been showing it off to energy nerdy friends and neighbors, and teaching me about how such systems work.

            If the electricity is going to be converted to heat eventually anyways (i.e. for domestic hot water), storing it as heat can make a lot of sense from an equipment cost perspective, since simple PCMs like wax are far cheaper and more plentiful than Lithium/Cobalt/etc.

            This is already what electric tank water heaters (whether heat pump or resistance) do today at 1/5 to 1/10 the cost/kWh of energy storage vs an electrochemical battery. PCMs would just increase the density of heat storage medium, so you can store more heat in the same volume of space.

          3. Expert Member
            Malcolm Taylor | | #12

            Dan,

            Perhaps I put that poorly. I'm not sure it makes much sense to store either heat or electricity at the individual level. The problems with peak electricity demand and renewables might be best resolved at the grid level instead. But again, I'm probably not someone who should even be commenting at all on this stuff.

            "The only value it has had so far has been showing it off to energy nerdy friends and neighbors"

            Still much more useful that my friend Mike who put the money into a '61 Corvette that sits unused in his garage :)

          4. Dan S | | #13

            > The problems with peak electricity demand and renewables might be best resolved at the grid level instead.

            Malcolm,

            We're both wildly ideating, so let me continue :)

            I'm less convinced that the grid will be able to handle or must handle the variability of both supply and demand on its own.

            In nearly the entire country, peak demand is the result of summer cooling loads:
            https://www.eia.gov/todayinenergy/detail.php?id=42915

            This peak demand disproportionately contributes to CO2 emissions because it's usually provided by natural gas "peaker" plants that run continuously. Natural gas would have to multiply in price many more times over for any current electric grid storage tech to replace those.

            To the extent that the peak cooling load can be shifted by storing "cold" - either by pre-cooling a tightly sealed house or by storing "cold" in a PCM (like ice) and using it later - peak cooling demand could be shifted significantly.

            Commercial facilities (which tend to have electric rates signals closer to the spot market prices) already do this. In theory there isn't a reason that homes can't do it either. We just haven't had the incentive yet.

      2. DCContrarian | | #14

        PCM's only make sense for daily storage in places where the days are warm and the nights are cool and the average daily temperature is close to room temperature. No one is proposing anything that will have near the heat capacity to take summer heat and store it through the winter. And if you look at the math of doing that it's easy to see why it's prohibitive.

        Keep in mind that a typical house weighs on the order of 100,000 lbs, and is built from materials that have decent heat capacity. A super-insulated house will have plenty of heat capacity to stay comfortable with little energy use on a day when it's hot during the day and cold at night. The advantage of super-insulation is that not only does it work on those days but it also works on the hottest days, the coldest nights, and everything in between. PCM's do absolutely no good on days when you have to heat all day or cool all day.

        1. Paul Pfeiffer | | #15

          Let's say you either have an existing home with excessive southern glazing or for aesthetic (not energy) purposes are building a new home with the same problem. Now you have a house that overheats on sunny winter days. Isn't this a good candidate for PCMs?

          1. DCContrarian | | #16

            What you're describing is a twist on passive solar. While it's hard to make blanket generalizations because climate varies so dramatically, a lot of passive solar was built in the 1980's and 90's that just didn't work. And not only did it not work, post-build analysis shows that there never was any reason to expect that it could have worked, if the analysis had been done pre-build it would have been clear and the house never would have been built.

            I would recommend doing the back-of-the-envelope calculations for a specific location before getting too excited.

        2. Tyler Keniston | | #17

          "PCM's do absolutely no good on days when you have to heat all day or cool all day."

          I'm not saying its practical/economical, but if you placed a PCM on the exterior side of an assembly (where the temp swings are, such as just beneath roofing) you wouldn't need the temp to drop below interior setpoint (during cooling season) to see the benefit-- you only need some temp swing, period (doesn't need to swing past set-point). It would lower the maximum delta T across the assembly. Again, not suggesting it is preferable to adding more insulation, just saying the physics work.

          1. DCContrarian | | #18

            If you're heating all day or cooling all day the heat is flowing in one direction only.

            A PCM is like a reservoir. Once it fills up the flow in equals the flow out. If your flow is in one direction only the reservoir never empties. If you have a long heating season or cooling season you're much better off putting that money into insulation.

          2. Tyler Keniston | | #19

            "If you're heating all day or cooling all day the heat is flowing in one direction only."

            Yes, but not at a constant rate, assuming any variation in exterior conditions. Especially when radiative energy is considered (sun beating down).

            Consider a mid-day sun beating down on a roof. It will get very hot-- much hotter than ambient temp in the evening, even if that ambient evening temp is still warmer than interior set-point. The PCM, while melting, would maintain a constant temp since its undergoing a phase transition. This mean the effective delta T across the roof envelope is lower. Its like a peak shave. That constant temp will be held for longer, but the maximum delta T will be lower so total energy throughput would be lower. No?

            Not suggesting the practicality is there, just talking theory.

          3. DCContrarian | | #20

            If you're cooling all summer, the PCM melts once and then stays melted until fall.

            If you're melting and freezing and the melt point is at room temperature you're not cooling continuously, when the PCM is freezing you're heating.

            If the melt point is above room temperature and you're melting and freezing, you're still cooling continuously. You are delaying the inflow of heat somewhat. But you know what's a really good way to not only delay the inflow of heat, but prevent it all together? And a proven technology? Insulation.

            Do this experiment: figure how much material you'd need to store a quantity of heat using PCM. Then figure out how much heat you can block using the same amount of material as insulation.

          4. Tyler Keniston | | #21

            DC,
            In case I wasn't clear, I'm not arguing in favor of using PCM materials in building envelopes vs using insulation.

            I am only pointing out (why? not sure) that there is a theoretical example of when a PCM could provide SOME thermal benefit (perhaps not a cost effective one), even when 'heating or cooling all day.' For sure, the greatest chance for a PCM to offer real benefit would come from when there are larger temp swings above and below set-point.

            In my example, the band of conditions and the 'tuning' of the PCM would have to be just right, and thus not easily applied across a range of conditions.

            "If you're cooling all summer, the PCM melts once and then stays melted until fall."

            I think this is our point of confusion. If the PCM melts and stays melted until fall, then I agree with all your statements. At that point, it's not functioning as a PCM at all. But your above statement is not necessarily true. Let's say a PCM melts at 88 F. Interior set-point of 70F. Temp of a roof on a hot sunny day, maybe 130F or higher. Temp of ambient conditions later that night, perhaps 75F. 75 is still higher than the interior set-point of 70, but it is lower than the melting point of 88. Whether or not a PCM could be properly tuned to effectively remain a PCM throughout summer, I don't know, but it seems it should be possible in theory.

            Analogies never work out perfectly, but if we are to use water, I like to picture a house as a 'hole' in an ocean (cooling season). The hole is dammed by semi-permeable materials that let water trickle trough. That's the insulation. Note insulation does NOT stop heat flow, it slows the rate. Over time, the hole will become filled with water (will level with the ocean). The reason it doesn't is become we pump water out.

            The deeper the hole, the larger the delta T (the higher the pressure).
            Oceans also have tides.
            Exterior thermal mass would be like some spongy material that creates lag with the tides (and thus the applied pressures on the dam). A PCM, if functioning as such, would not only create this lag but would dictate a set level (pressure) against the dam.

            This would, at the least, shave peak load (power). My feel is that it would also shave total energy, as the PCM would effectively be slowing the rate of energy flow across the assembly. Perhaps it averages? I admit to not being sure what math to use to prove or disprove this. My feel is that it does not average, as heat is re-emitted from a material in all directions, not solely in one direction towards the interior (we are effectively taking the suns heat that would have added pressure across our assembly and are then letting it bleed off under the night sky. In other words, the time delay allows for more ambient dispersal of the heat. I'm not overly confident in that assessment, but its where I'm at for now. I am confident it would reduce peak load.

          5. Trevor Lambert | | #22

            My understanding is that the benefit comes from the energy exchanged for the phase change. If it's not changing phase, seems to me it would behave exactly the same as any other material of equal heat capacity.

  5. David_King | | #9

    I'm thinking this material might be useful in a ground source heat pump loop to minimize the length of the loop and thus reduce the installation cost but there would need to be a way to move stored heat or cool away from the material and into the surrounding earth more quickly than the current systems allow. Alternatively the storage sump would need to be large enough to store summer heat for winter use etc. If the salts are cheap enough and you only needed a moderate size container it might be useful that way. Some article exist because nothing is new under the sun. https://www.um.edu.mt/library/oar/bitstream/123456789/25537/1/EGC2013_SG1-05.pdf

  6. DCContrarian | | #23

    (This is a reply to Tyler's post #21, we can't thread any deeper)

    In the example you provide note that the total number of BTU's of cooling is the same with or without the PCM. I will concede that there are circumstances where it is useful to time-shift your BTU's. For example, in some places electricity is cheaper at night, so if you can shift your usage into the night you save money. This idea -- along with PCM -- is actually something people do already, with ice chillers that make ice during the night and then use it for cooling during the day. Similarly, the efficiency of heat pumps is higher when the outside temperature is closer to the inside temperature, so moving those same BTU's when it's cooler outside uses less electricity. And if by time-shifting you can smooth out variation in utilization and use smaller, cheaper equipment that's also a win.

  7. DCContrarian | | #24

    (This is a response to Trevor's post #22) He wrote:
    "If it's not changing phase, seems to me it would behave exactly the same as any other material of equal mass."

    I want to pick a nit here. I assume what you really meant is "it would behave exactly the same as any other material of equal heat capacity." The reason I want to flag that is there's a quite common misconception that mass and heat capacity are the same. It's one of the reasons that I get huffy when people talk about "thermal mass" because I feel it feeds that misconception. And that misconception exists even among people who should know better, and it leads to a particularly damaging practice, which is overusing concrete in residential construction because of the belief that since it's heavy it must hold heat. That's not how it work, and production of concrete is incredibly damaging to the atmosphere.

    OK, rant over. You are correct (in the edited version.)

    1. Trevor Lambert | | #25

      Yes, I should not have said mass. My point was that without the phase change, there's nothing special about these PCM. Whatever their heat capacity is, it's probably less than water. Might as well just put a bunch of jugs of water in the attic, it would be a lot cheaper.

      1. Tyler Keniston | | #26

        Trevor, I'm not sure if your original comment was in response to one of my statements, but hopefully it was clear that my use of the term "set point" was to describe the thermostat set point and not the melting point of the PCM. I agree, a PCM that doesn't change phases is no PCM at all.

        I still contend that in my example, there is net energy savings, but I think everyone's basically on the same page.

        The PCM with the adjustable melting point could make a big difference for climate applicability, but I imagine the complexity of such a set up would still be a big downside vs just improving insulation.

        1. Tyler Keniston | | #27

          What we really need are PCM's to cool our drinks.

          1. Expert Member
            Malcolm Taylor | | #28

            Tyler,

            Reminds me of the expression: Save your breath to cool your porridge.

          2. Expert Member
            Peter Engle | | #30

            We already use a PCM to cool our drinks. It's called "ice."

          3. Tyler Keniston | | #31

            You got it Peter. And thank god it floats.

        2. DCContrarian | | #29

          In post #5 I link to an earlier thread exploring the use of calcium chloride as a PCM. One of the nice things about calcium chloride is you can change the phase change point just by adding or removing water.

          While calcium chloride is probably an ideal PCM, what that thread convinced me of is that storing heat on a residential level just doesn't make sense.

  8. Jaccen | | #32

    Note exactly "apples to apples," but Chlupp's storage battery at the Sunrise House was a theoretically good idea w/ practical issues:

    https://www.greenbuildingadvisor.com/article/the-sunrise-house-in-fairbanks

    Of note:
    "Chlupp hoped that his home’s elaborate solar thermal system would provide a significant percentage of the home’s space heat. I asked McDonnell how the system is working. “Between mid-November and mid or late February, our heat is primarily—really, our heat is exclusively from burning wood,” McDonnell told me. “If you calculate how much heat energy can be stored in 5,000 gallons of water, it’s not really that much. Between mid-November and the end of February, during the winter in Fairbanks, the solar gain in Fairbanks never gets high enough, at no time during those months, to justify pumping the water stored in the thermal storage tank. We start to do that around the first of March.”"

    "(3) A problematic plastic liner in the 5,000-gallon solar thermal storage tank. McDonnell told me, “The thermal storage tank is a lined steel tank with a plywood exterior. Thorsten installed spray foam on the interior of the steel tank. Then he installed a cylindrical liner from some company, I think it’s called FlexiLiner or something like that. It’s a thick plastic liner intended for high temperatures. There are some issues with that. It’s an open-topped tank. In the summer, the vapor pressure of the water is high. It’s steaming above the water. Water is condensing on the ceiling of the tank and dripping down. It’s dripping down the edge of the liner, and a fair amount of water is getting outside the liner—probably under the liner by now, and against the spray foam. At one point, the pump wasn’t able to suck water out of the thermal storage tank, because the bottom of the liner had floated up from the bottom of the tank and blocked the inlet on the pipe. I had to replace the drop tube and relocate it, and also install new side inlets. It was a pain in the ass, because I had to get my hands inside the tank.”"

    "(3) The large water tank isn’t big enough for seasonal storage."

    "(4) The thermal storage tank is inside the home’s thermal envelope."

    The stone/concrete mass worked well. The water....not so much. While these are not PCM, they do show that the design of energy storage matters. PCM's could work if they are designed well into the "whole package." Getting the design right will be the stickler.

    1. Dan S | | #33

      Unless you have a private lake or ground source geothermal heat pump, seasonal heat storage like they attempted seems a bit ambitious. Storing heat in PCM based devices seems more appropriate for shifting heating loads by hours or maximum a day or so.

      For seasonal energy storage chemical storage in synfuels like hydrogen or ammonia seems like the most sensible mechanism, but that can only be safely done at the grid scale.

    2. DCContrarian | | #34

      Houses like this are what I was thinking of in post #16 when I said, "a lot of passive solar was built in the 1980's and 90's that just didn't work. And not only did it not work, post-build analysis shows that there never was any reason to expect that it could have worked, if the analysis had been done pre-build it would have been clear and the house never would have been built." It sounds like the water heat storage works exactly as analysis would have predicted; the leakage and condensation are issues that could be fixed if the principle were sound to begin with.

      I have to challenge the assertion that "The stone/concrete mass worked well." More likely, they don't create obvious problems like the water storage tanks. I suspect if you were to do a rigorous analysis you'd find they don't really contribute to reducing the annual heating bills.

      1. Bas Sommer | | #35

        That is too negative. It is fair to say that Fairbanks is a tough case.
        In more moderate climates a season-storage water based solar system can work, but is at today's prices not sensible.
        PGH + winters with some sun can work with 100% solar heat.

        1. Expert Member
          Malcolm Taylor | | #36

          Bas,

          "In more moderate climates a season-storage water based solar system can work"

          Theoretically, or are there systems currently working? I think I read about one in a neighbourhood near Calgary, Alberta, but if there are effective examples it would be interesting to hear about them.

          1. Bas Sommer | | #37

            Hi Malcolm,
            Sure. Mr. Jenni in Switzerland is building the stuff, actually has several multi-family houses which are at 100% solar utilization. An old solar-guru...
            please check https://www.jenni.ch/publications-448.html
            For sure, his favourites are not really 100% designs but something like 80% with some wood boiler back-up. In some winters even 80% is enough, some years you need some chords of wood.

          2. Expert Member
            Malcolm Taylor | | #43

            Bas,

            Thanks for the interesting link. I'll enjoy exploring it tonight.
            Here is one for the project I mentioned in Alberta: http://www.dlsc.ca/how.htm

        2. Dan S | | #38

          I don't understand how water heat storage can store enough energy in one season to be utilized in another at least at the household scale.

          Water stores 4.2kJ/L*C (AKA the specific heat of water).

          Take a 5000L tank (the smallest one listed on Jenni's site, so nearest sized one for a single-family-home). Assuming a 40C temperature rise (from 10C to 50C) in that volume of water induced via solar heating during the summer, that would store

          4.2 kJ/L*C * 40C * 5000L = 840000 kJ = 796kBTU

          So to heat a modest and very efficient single family home in a cold climate that required heat at a rate of 48kBTU/hr, the tank would only provide 16 hours of heating, and that's not accounting for inevitable and significant standby losses.

          That might be enough to bridge a few sunless days, but it is nowhere near enough to shift heat from summer to winter on its own. It seems like at best it's a bit of a heat capacitor to help dampen peak heating loads, but the vast majority of heat required would have to be added to the house in real time, either by direct insolation, via a heat pump, or by burning a fuel of some kind.

          Am I missing something?

          1. Brendan Albano | | #39

            My understanding is that seasonal energy storage using water does it by freezing the water. The enthalpy of fusion of water is 333.55 kJ / kg, so you can theoretically store lots more energy in the same quantity of water if you are doing it with a phase change. If you do a google image search for "eisspeicher" you'll find lots of neat looking pictures of huge tanks full of ice on German websites that I can't read.

            The practicality of it, especially at a residential scale, is certainly another question, but it is a real thing that is done!

          2. Dan S | | #40

            Brendan,

            Extracting energy from the water until it freezes is interesting, but I don't see how that works with Jenni's storage tanks which look too closely coupled to the conditioned space to allow to freeze.

            I'll check out the eisspeicher thing though.

          3. DCContrarian | | #41

            The heat of fusion of ice is 80 times the heat needed to raise it by 1C. So raising it by 40C has about half the heat per unit of mass or volume as freezing/melting. So twice as much, which I wouldn't call "lots more."

            The way to look at it is to look at how much energy it takes to heat or cool a house, and how much that energy costs. Then look at how much material it takes to store that much heat from season to season, and how much that material costs. I've never heard of a residential heat storage system that makes sense when looked at this way.

        3. DCContrarian | | #42

          It's not too negative. People shouldn't be encouraged to think that these systems can work, it's just a question of working out the details. When people go in with that approach they waste a lot of money, and do environmental damage in the process.

          People should be encouraged to do the math before building anything.

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