PCM battery shows promise in home heating system
Ben Welter - Tuesday, September 15, 2020
A home heating system equipped with a PCM battery put up encouraging numbers in a small trial in northern Italy earlier this year. The monthlong test “shows that is possible [for such a system] to supply the whole heating demand of a house reducing close to zero the costs,” says Domenico Feo, who developed the system. The full report, prepared by ThermaLink, a trademark of Sunservice Srl of Treviso, Italy, is available here.
The system is composed of a 6 kW solar array, an air/water heat pump and a thermal battery filled with BioPCM Q42, a biobased phase change material supplied by Phase Change Energy Solutions of Asheboro, N.C. A control unit, PCDCube, monitors temperatures and manages energy flow to optimize system performance. In an interview with Phase Change Matters, Feo answered questions about the heat battery, recently rebranded as ThermaCube.
Q: Thermal conductivity has proven to be a challenge in thermal energy storage across most types of PCM. Was conductivity an issue with the required charge/discharge cycles?
A: "As already known, the thermal conductivity of the PCM, especially of the organic one that we use, is low. This is, at the same time, a disadvantage and an advantage. The disadvantage has been minimized in using a heat exchanger made with a flat aluminum panel with the various layers just over 1 inch of distance; in this way we are able to make the entire volume of material work in a very efficient way. Vice versa this is an advantage for the very low heat losses during the non-work phases; in conditions of installations inside the buildings, the temperatures can be maintained for several days."
Q: If faster heat transfer is required, do you envision a redesign of the heat exchanger or a modification of the PCM to meet the demand of the energy transfer?
A: "As manufacturer, we can customize the heat exchanger dimensions and shape in order to achieve our goals, but as always in the industrial scalability, the quantities are mandatory for a good price. We designed a single heat exchanger dimension and multiply it in order to be able to produce the different storage capacities and realize a complete modular range. The transfer of energy can be however accelerated by increasing the flow rate of the fluid and the delta T between the fluid and the melting point, of course there is always a limit to this possibility. There are still studies around this point that need to be better understood."
Q: Would implementing several PCMs of different melting temperatures aid in the overall performance of the TES?
A: "Let's say that we expect a slight drop in performance at lower temperatures, but we don't have many other tests to give you a more general opinion."
Q: How is the PCM temperature measured throughout the entire storage to provide input to the PCDCube? Since the melting/solidification of PCM is not a homogenous process, are multiple measurements necessary to get a realistic status of the PCM?
A: "In the documents that we have published, we do not specify that we have installed two temperature probes, one in the center and another one near the walls and it is evident that, when the final phase of the discharge of the PCM approaches, the perimeter probe shows a decrease of more sudden temperature than the central one. We, however, assume that the fact of having installed our thermal battery outside the building in a very cold climate, has exaggerated the heat loss despite the low heat transmission."
Q: Have you considered other PCM suppliers/products, since many are located in Europe and the RAL quality organization is based there?
A: "This is a possibility, at least for the distribution of our products in Europe and Asia. Currently we try to separate the supply of the case with exchangers (that we manufacture) from the PCM with direct purchase of the client from the supplier, especially in the larger units."
Q: What is the customer value proposition for this product? What is the expected return on investment/payback period?
A: "This is a $1 million question; the proposal is certainly into the growing wave of the storage market, even the electrical ones for PV systems. (A Tesla Powerwall is cool but not economically advantageous if you don’t pay attention at the payback that is higher than the life of the battery). The best value proposition is to separate production from the consumption of the energy and take advantage of renewable energies which, unfortunately, are linked to the day / night cycle and the outdoor temperature conditions. We see perfect the combination HP + Thermal Battery because we can concentrate the HP working hours during the best outdoor conditions that, for many reasons, are affecting and reducing the performances. Furthermore, we also see a large market in the use of these thermal batteries in all these existing systems which are undersized or which, due to specific situations of changing use of the building, are no longer able to provide the necessary heating or cooling to the whole plant. About the ROI, this really varies a lot country by country because the cost of the electricity has different rates and peak hours schedules. The industrial needs are sometimes much more interesting and attractive than the residential ones."
Q: What are your plans for a commercial rollout?
A: "We are completing the Italian sales force and are looking for international distributors. Our goal is to propose the range of products at an international level with distribution agreements for the case only with exchangers; it will obviously be our responsibility to direct the partner to some reliable and high-quality PCM manufacturers. Obviously, those who will quickly give us the opportunity to build pilot plants, demonstrating the technology, will have an advantage in introducing them into the market and in the exclusivity of the contractual relationship."
"The PCM will be distributed throughout a graphite matrix in the form of small beads. Flow channels will run axially down the length of the cylinder for the propellant to pass through. The design is based loosely on the old NERVA fuel elements from the nuclear rocket program in the 1970s, with the UC kernels being replaced with our PCM. The drawing here shows a cutaway of the thermal capacitor surrounded by the optical system.
