Conference Agenda

Future smart and efficient energy management systems for space cooling and heating in building applications require novel solutions to store heat to decouple the energy demand and the availability of renewable energy sources. Latent thermal energy storages represent one of the most promising solutions; however, their cost-effective implementation in terms of energy and cost savings and payback time needs to be verified case by case. Moreover, the design of these components is not simple because the heat transfer process involves the solid/liquid phase change that deeply affects the storage heat transfer capability. This work presents an outlook on the importance of the scaling up of the research to deploy efficient and smart solutions for novel latent thermal energy storage systems. The paper explores the work done by the Thermal Energy Innovation research group from a few grams to hundreds of kilograms of organic phase change materials showing original and novel experimental and numerical results.

Latent thermal energy storages (LTES) with phase change materials (PCMs) are key to decarbonize heating and cooling loads by enabling the utilization of renewable energy sources with high variability. Smart energy management systems based on model predictive control can play a decisive role in optimizing the utilization of renewable energy in systems that incorporate LTES. However, smart energy management with model predictive control requires to accurately monitor the total energy stored in the LTES, which is determined by the liquid fraction of the PCM. Measuring the liquid fraction is challenging because solid-liquid phase change processes occur at nearly constant temperature. Hence, diverse approaches for determining the liquid fraction pose a trade-off between accuracy and ease of implementation. The present study aims to quantify the effect of the liquid fraction sensing accuracy on the performance of model predictive control strategies for systems with LTES. For this purpose, a residential heating application with an energy management system is analyzed. The heating system consists of a heat pump, an LTES and a photovoltaic array. The heat pump can be driven by the photovoltaic array and the electric grid. The energy management system uses model predictive control based on Mixed-integer Linear Programming (MILP). Representative daily profiles for the heating loads and weather conditions are used as forecasts for the model predictive control. The performance of the energy management system is assessed in terms of photovoltaic energy utilization for varying degrees of uncertainty in the estimation of the liquid fraction of the PCM in the LTES. From the results, a minimum acceptable accuracy for the liquid fraction sensing is determined.

A novel pressure-based state of charge sensor (P-SOC) for latent thermal energy storage systems has been developed. The P-SOC is multifaceted in its ability to determine the latent-energy stored in PCMs, require an off-the-shelf pressure sensor, and is a global measurement. As opposed to temperature-based measurements in the PCM, the P-SOC is unaffected by the PCM's sub-cooling. Heat transfer fluid flow and temperature-based measurements are expensive to achieve high accuracy in the field. In this manuscript, the experimental apparatus for evaluating the SOC is discussed, and the prototype is explained. The mathematical formulation for the relationship is also derived. The results show a good relationship between the pressure of the closed vessel and the amount of phase change material in the liquid or solid state. The accuracy and repeatability of the measurements for one organic phase change material are discussed in this manuscript. Future work includes additional testing of PCMs of different types and improved sensor accuracy and repeatability.

Phase change material (PCM) based thermal energy storage (TES) technologies are promising for building heating and cooling applications. PCMs integrated with building envelopes could enable flexibility of buildings thermal demand, shave peak load, and provide energy savings to the end user. Although PCMs have large latent heat storage capacity, their low thermal conductivity values limit their charge and discharge rates and their overall efficiency. Herein we investigate the thermal performance of a PCM based heat exchanger that is designed to offset buildings heating and cooling loads.

TES integrated with thermally anisotropic building envelope (TABE) offers unique benefits by redirecting natural thermal energy from a building to TES system using hydronic loops. In this work, we describe an experimental apparatus for two 5-gallon fin tube heat exchanger system integrated with a commercially available organic PCM that is subjected to heated and cooled boundaries. We investigated the thermal performance of fin tube heat exchanger by varying the volumetric fluid flow rate and temperature difference of the PCM heat exchanger. Furthermore, we demonstrate cyclic stability of PCM heat exchanger system using the developed experimental apparatus over many cycles. The developed method and apparatus provide a means to investigate and characterize thermal storage systems at gallon scale, which can help in the design of future energy storage systems optimized for cost and energy savings.

Sustainable energy generation and storage are key factors in the transformation of society towards a carbon-free future. While great progress has been made in the development of renewable energy generation systems, there is still a mismatch between the global energy supply and demand. Renewable energy sources, such as solar and wind energy, are subject to variable efficiencies that depend heavily on local weather conditions. Thus, energy storage is necessary for a sustainable energy grid to meet the demand of high usage phases during periods of lower energy production. Although many systems currently depend on chemical batteries for energy storage, these systems face issues regarding the limited availability of materials needed for fabrication as well as the energy intensive production and recycling processes tied to such systems. Thus, the feasibility, scalability and use cases of simplified and environmentally friendly alternative energy storage options must be investigated. Subsequently, a feasibility study was conducted on the use of a gravity battery as a form of domestic energy storage in Purdue University’s DC Nanogrid House, an ongoing project that aims to convert a residential property to run solely on DC power whilst operating predominantly independent of the grid.

Gravity batteries store energy in the form of potential by lifting a weight using a motor-winch combination. When needed, the battery can be discharged by lowering the weight and utilizing a generator to convert the potential energy back into electricity. This is an attractive form of energy storage for its simplicity and longevity without the need for chemical components. In the presented paper, the energy consumption data of the DC Nanogrid House was first analyzed to set goals for the required storage capacity of the system, this was followed by the development of an initial design that was later modeled using CAD. This design went through a techno-economic analysis and was optimized to meet the building and safety specifications. Finally, a scale model prototype was constructed to confirm the physical feasibility of the design and help communicate the concept. The case study, prototype and the techno-economic analysis performed were all used in determining that while gravity batteries continue to show great promise in industry, the insufficient volumetric energy density and efficiency of the systems make the technology currently unviable to be effectively utilized on the residential scale.

Nowadays, most countries are seeking to transition their energy matrix towards renewable sources. To achieve this goal, energy storage systems play a crucial role in compensating for the intermittency inherent of renewable sources. On the other hand, one of the largest consumers of primary energy are buildings, due to their heating demand and to their seasonal behavior. Therefore, to move towards renewable energies and to increase the role of buildings in this challenge, this paper presents a pioneering approach aimed at integrating renewable energy sources for feed a Carnot Battery (CB), to heat an abandoned mine flooded with water, produce electricity, and distribute the heating and cooling in a district heating network. This study is conducted in a real context of an abandoned mine in Martelange, Belgium, where three different size caverns are employed for store energy: 800, 6840, and 20000 m³ for hot (90°C), medium (50°C), and cold (8°C) water temperature, respectively. The renewable source corresponds to 500 m² of Photovoltaic Panels (PV) coupled a 550 Ah Ion-cell stack to supply energy to the Heat Pump (HP) of the CB and to the electrical resistances (ER). The configuration allows the caverns to be charged during the summer, when the photovoltaic production (84 MWh) is not consumed by the heating demand of a 50-apartment building complex, whose annual heat demand is 380 MWh. This Seasonal Underground Stratified Thermal Energy Storage (SUSTES) is discharged in winter to cover the heating demand and produce electricity by the Organic Rankine Cycle (ORC), as well as the cold reservoir can function as a cold source capable of transporting cold water at 8°C all year round to potential users of cooling energy (data centers, office buildings, hotel, hospital, schools). The system is modelled in a modular approach and then simulated in Dymola, for a period of 3 years with a 15-minute time step. This study highlights the potential of reusing abandoned mines as energy storage systems, capable of generating a benefit to adjacent communities by integrating diverse energy demands within a single system. In addition, this study stresses the energy integration of multiple systems, being able to optimally control the storage, conversion, and management of energy. This generates new perspectives for investors and residents, enabling the possibility of connecting the system to the grid, to extract electricity when the price is negative or low, storing it, and then producing electricity when the price rises.