PCM hot water storage: the next energy efficiency super star?

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For hot water storage, phase change materials should rank as the most important household energy efficiency product after insulation, heat pumps and solar.

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Heat storage and heat recovery from phase change material (PCM) systems could make hot water services a lot more efficient and that’s important – if you consider that every household in Australia has a hot water service and the entire stock in the country is replaced every nine years or so.
The move away from dual fuel homes with polluting fossil gas to environmentally friendly all-electric homes provides an opportunity to rethink how we produce, store and deliver hot water. Hot water is a huge energy efficiency opportunity and a must given its significant share of household energy consumption.
Today, the following hot water services are most common; fossil gas instantaneous, fossil gas storage, electric (resistive element) storage. Then add to those common conventional types – heat pump and various solar options, which are combined with conventional storage tanks.
With the exception of the gas instantaneous option, which loses about 30% of its energy up the exhaust flue, all the other options include a storage tank.
Storage tanks – unbeknownst to most Australians – lose about 50% of their stored energy to the atmosphere, that’s before you’ve even taken a shower. If a system is oversized for a given household they may lose even more.
There is another solution: Japanese consumers can purchase and install amazing “EcoCute” instantaneous heatpumps from Panasonic and Hitachi that have no flue losses as there is no liquid or gaseous fuel and no tank eliminates storage losses. Being heat pumps, their current draw is not massive either so the impact on the grid isn’t too huge. However without a mass market these units are being sold for about $6000-$8000 each.
While instantaneous heat pump hot water remains out of reach for Australians for lack of an importer and high price, there is something significant we can do to increase the efficiency and utility of our existing units while reducing the costs.
PCMs or phase change materials can be used in our hot water services instead of water to store most of our domestic hot water requirements, they work through the latent heat effect and by careful application of material science and engineering can be chosen for ideal hot water delivery temperatures between 45°C and 90°C.
In effect, appropriately selected Phase Change Materials can achieve seven times the heat storage capacity per unit volume than water. However we can’t deliver PCM material out of our taps so enough contact surface area between the PCMs and town or rainwater supply must be available so that heat can be given up by the PCMS fast enough to ensure continuous hot water supply.
A UK company PCM Products (which as far as I’m aware doesn’t have an Australian distributor) offers balls for new tanks and retrofit situations. They claim that the optimum size for a ball of organic PCM is 40mm diameter.
A higher tank PCM to in-tank water ratio can be achieved by making sure houses are equipped or upgraded to include high performance water fittings such as the world leading (from NZ) Methven Kiri Satinjet ultra low flow 4.5L showerhead (which this author thoroughly enjoys showering under every day), Hansa Polo Green 3.3L kitchen sink mixer, 4.5L bathroom mixers and 7.5L laundry mixer.
Depending on the PCM and its latent heat capacity it can store a lot more energy than a 1degree rise in the same volume of water.  A good rule of thumb is that a good PCM can store seven times the energy when compared to water for the same volume.
A ratio of around 80% PCM to 20% water should easily be achievable with efficient fittings.
This means that overall volume of a tank can reduce to 20% + (80%*0.14) = 30% a saving on tank volume of 70 per cent.
For suppliers, this means smaller tanks, less materials, easier to automate production lines with robotics, cheaper to store and cheaper logistics, pluses all round! And for consumers and the environment it means lower heat losses, so less energy consumed and lower bills and – as a bonus – less space taken up, in and around houses.
In the past, PCM storage options had really only been thought of as a way to decrease tank volume. But now we must think of them as a major energy efficiency product, possibly the most important energy efficiency product after insulation, heat pumps and solar.  To reiterate, by reducing tank surface area we are reducing tank losses, which means less energy.
It could mean even more for the poorer in our community, living in rental accommodation, as the common tank option is a small one with an electric element that is always active, which means they don’t get to take advantage of off-peak tariffs which are around 40-50 per cent cheaper than day tariffs in most distribution areas.
Finally, the ideal hot water service is a heatpump such as the Sanden CO2 heat pump optionally coupled with a set of heat-pipe evacuated on-roof solar tubes all connected to a 70 per cent smaller tank, packed with PCMs. Alternatively, a tank similar to those installed today (200L-400L) could provide for significantly more available hot water helping systems with solar ride through multi-day periods with little or no resource (cloud cover) and smart heatpump systems to time household water production to the warmest periods (taking into account tariff conditions as well, of course).
Serious research needs to take place as a collaboration between research institutions, universities and industry players to make this happen in a relatively short timeframe – and minimum energy performance standards around losses should be introduced to force a move away from conventional low storage density high loss tanks.
This can all be done and given there are already companies doing this some modest government stimulus would make this happen. In fact it already has happened. The Commonwealth games village in Parkville, Melbourne, had its solar hot water heaters retrofitted with PCM’s when it was discovered there wasn’t enough water for the athletes – who obviously liked a couple of long showers a day to freshen up.
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17 Comments
  1. RobS 4 years ago

    The comparison now for the cost effectiveness of any electric energy efficiency device should not be other equivalent devices but solar PV. With PV panel prices falling so far we now find more and more efficiency measures cost far more per Kwh saved then solar PV costs per KWh produced. For example if a high efficiency water heater saves 200,000kwh over its lifetime at a cost of $2,000 but $2,000 worth of PV panels will save 300,000KWh over the same time span and you have $2,000 to spend you would obviously be better sticking with the cheaper inneficient heater and installing the additional solar panels.

    • adam 4 years ago

      I suppose these two would work well integrated because we can store heat to be used when the sun isn’t shining, thereby avoiding batteries for hot water heating which could be used for space heating and washing obviously.

      Matt would know off the top of his head I’m sure whether there is a lot of “low hanging fruit” with energy efficiency (e.g., lighiting, heating) that would beat PV on a dollar basis – I suspect that there are though.

      Also given avoided energy is “dispatchable” and PV is not so say a lighting efficiency measure would be effectively available at night. True PV cost comparison would need storage in that case.

      Any safety risk on the PCM’s? What if they break – is the working fluid toxic?

      • RobS 4 years ago

        Yes once we get to the levels of solar penetration that storage becomes necessary then efficiency measures only have to beat the cost of solar and storage combines which adds about ~20c/Kwh presently, but rapidly falling, to the cost of the stored power. Completely agree that at that point a number of more expensive efficiency options come into play again to offset the size and therefore the cost of the storage capacity required. However the more we learn about renewable integration the more we find that extraordinary amounts of “intermittent generation” can be added to the grid before stability becomes an issue, South Australia we have seen in the latest AEMO reports is now about to pass 40% wind and solar without the need for any storage.

        • Matthew Wright 4 years ago

          Hi Rob,

          Two things –

          #1 Consumers will start adding storage shortly at today’s modest renewable penetration levels even though it would be much better value for society as a whole to increase penetration to above 60% before it is worthwhile adding storage.

          The reason is that producers of solar will not be (and are not) remunerated appropriately for the value of thier solar production.

          #2 There are still great opportunities for energy efficiency and many households have space constrained roofs. i.e. using a heatpump + SolarPV extends the amount of onsite renewable production / energy efficiency versus not doing the heatpump or other energy efficient options.

          The only difference is as costs of PV reduce it will make it pretty clear that from a Return on Investment perspective PV is the first thing you do until you’ve maxed out your roof. In other articles I discuss virtual battery options with PV including supersizing your solar whee

      • Matthew Wright 4 years ago

        The PCM companies (and there aren’t many of them) are tight lipped on what constitutes the PCM’s that act at various temperatures. I noticed PCM products offers a 42C and 85C. It would probably better to have a 50-55C PCM. My understanding is that they are mostly vegetable oils/fats and encased in balls (the ones that go in hot water services that is.

        I think oversizing is the go 300% panels on 100% inverter 3:1 ratio which gives you an intermediate generator most days of the year.

        Reverse Cycle air conditioners such as Daikin Ururu Sarara can cut a consumers energy consumption (When compared to gas) by 90% due to no longer having flue losses, humidity control and high C.O.P. of 6x.

        • Askgerbil Now 4 years ago

          A very cheap PCM is available to store thermal energy at 30C. This allows very high efficiency for adding solar thermal energy.
          A high-efficiency heat pump can raise the stored heat energy temperature to transfer some of it into a hot water tank.
          For building heating only energy to circulate the stored thermal energy is needed.

          • Matthew Wright 4 years ago

            So what is the makeup of the PCM? 30C isn’t very useful as you’d have to add heat to it and then draw from it.

          • Askgerbil Now 4 years ago

            Matthew,

            Heat pumps don’t “add heat”. They use a small amount of electrical energy to pump heat energy from one temperature to another temperature.
            An air-source heat pump transferring heat at 10C to a 60C hot water system needs a minimum of 1 kWh (3.6MJ) for pumping 24 MJ of heat energy.

            To pump heat energy from 30C to 60C, up to 40 MJ of heat energy could be pumped with the same 1 kWh (3.6 MJ) of electrical energy.

            Adding solar thermal energy to a 60C phase change material energy store would be less efficient than adding it to a 30C phase change material energy store. This is because a solar thermal collector radiates more heat back to the environment as its temperature increases.

          • Matthew Wright 4 years ago

            An air source heat pump is not a water source heat pump. So the only advantage of the 30C PCM as opposed to a 50-60C PCM is to deposit the heat that is transferred from the outside ambient into it.

            I’m quite aware of how heatpumps work – do a google search for “Matthew Wright” heat pump or “Matthew Wright” reverse cycle air conditioner and you’ll find I’ve written dozens of articles push air source heat pumps for hot water and space heating.

        • JohnRD 4 years ago

          Mathew: Hot water storage systems must operate with a minimum temperature of 60 deg C to avoid the risk of Legionnaires disease. (See for example: http://www.actpla.act.gov.au/__data/assets/pdf_file/0006/2130/Pn18.pdf)

          In a heat pump driven system I can imagine energy consumption being reduced if a PCM is available that operates just above this temperature because both the energy required by a heat pump and the rate of heat loss are a function of the temperature difference between the stored water and the relevant outside temperature.

          I can also see potential gains from the use of PCM’s if the PCM’s allow the storage tank to be closer to the main use point (avoids pipe losses), an existing system to increase heat storage capacity or a lightweight tank to be more easily put on a roof.

          However, PCM’s are not dirt cheap. It would be useful to see some rough figures for costs and benefits.

          I also think that the use of PCM’s in air conditioning systems may be more important than their use in hot water systems. Targetted installation of PCM heat/cold storage would have rendered most of the recent grid upgrades unecessary. (See:http://www.climateplus.info/2014/05/19/energy-storage-using-phase-change-materials/)

          • JohnRD 4 years ago

            Note that the requirement that hot water must be stored at a min of 60deg C does not mean that the PCM phase change has to take place above this point. It is OK as long as the water temperature reaches 60 deg by the end of a heating cycle.
            If heat pumps are being used it will take less energy if the phase change takes place at lower temperatures. All that is necessary is that the phase change temp is above the water temp required by the user.

  2. Jennifer Gow 4 years ago

    Very interesting as far as it goes but I’m not sure of its relevance to solar hot water systems. We live in Brisbane and along with our solar PV have installed solar hot water using evacuated tune collectors linked to a ground located tank with a small (solar powered) circulation pump. This allows our household to run 100% solar hot water for 10 months a year.In this context what does this technology offer?

    • Matthew Wright 4 years ago

      10/12 = 83% of the time you’re able to do solar in Qld and Qld is the best state for it! What about the 8 out of 10 Australians that don’t live in your sunny climate?

      For one PCMs – May offer the other two months powered completely by solar
      Or a smaller tank which is cheaper to manufacture / transport and install at your house!

      And there is more to it than this – I encourage you to retread the article.

  3. Simon Baird 4 years ago

    America’s phasing out their electric hot water systems from next year. You can now buy a G.E Heat Pump for less than $900 USD. Our Hydrotherm models sell for less then $800 after STC are taken off, http://www.hydrothermhotwatersystems.com.au
    You going to get far more bang for your buck with a heat pump then phase change in existing tanks, the cost of implementing this into the build for electrics would be significant.

    • Matthew Wright 4 years ago

      Actually Sanden CO2 heat pumps are by far the best as they operate really well in cold climates due to using CO2 as the working fluid. CO2 is the right working gas as it can work well with a large delta T.

      PCM balls would compliment a solar system or heat pump by reducing the size of the tank or increasing the size of the storage to ride through the coldest period.

  4. Jenny 4 years ago

    I understand how PCMs could be used in solar hot water systems to help bridge the gap between solar availability and demand (e.g. collecting additional solar heat during day for use in the evening or following morning). However, I don’t understand how they could be used to reduce heat losses from the tank. The increased latent heat capacity of PCMs means they effectively increase the heat storage capacity of the tank – but I’m not sure how this corresponds to reduced losses / energy savings. Could you explain this a bit more? I have read the article through a few times but feel like I’ve missed something.

    • Carl Raymond S 2 years ago

      Heat loss occurs through convection, conduction and radiation. With insulation the hot surface should not be exposed to air, so it’s the latter two to worry about, and they are both proportionate to surface area. Smaller tank, less surface, less heat loss.
      Further, because you need less insulation, you can use the best available without much affect on product cost.
      It’s not intuitive because most people confuse heat with temperature. This phase change material is no hotter than the water in a water tank system, but contain 7 times the heat.
      If you understand, you will get why a cheese and tomato toasted sandwich burns your tongue, but a cheese only variety does not.

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