Phase change and alternative materials for domestic thermal energy storage

Abstract

In Europe, households account for 24.7% of the total energy consumption. Sanitary hot water and space heating represents more than 80% of total domestic energy consumption. The major share of this is covered by heaters operated either by gas, oil, or electricity. To decarbonise the domestic sector, renewable energy penetration of the electricity grid using heat pumps (HP) is an option. HP with an efficient thermal energy storage device can effectively solve the mismatch between supply and demand of energy and provides significant economic benefits and reduced associated CO2 emissions. Thermal energy storage (TES) technology has gained great popularity as an effective method for demand-side management of energy of heating. TES has the potential to harvest, store and save thermal energy for short or longer periods. Scientists and energy technologists are investing their efforts to develop efficient, reliable, and cost-effective TES systems which can be integrated with heat pumps (HP) and solar panels to store off-peak renewable energy and deliver thermal energy to decarbonise the heating sector. Among the various TES methods, latent heat storage (LHS) materials offer the most flexible operating ranges, comparatively high energy densities, and high durability at reasonable costs, which has opened a market for phase change materials (PCM). Thermal energy storage using PCM has been widely investigated for various applications from very low to very high temperatures. The net-zero carbon policy of the UK government ensures that all new dwellings should be energy self-sufficient with onsite energy production to use for domestic heating, lighting, and ventilation to target an 100% reduction in CO2 emission by 2050. The use of PCM in building components and hot water production can reduce the building energy demand, and indoor temperature fluctuations, and improve demand-side management by utilising available renewable energy and off-peak electricity.

The primary aim of the research is to design and develop a novel phase change material thermal energy storage system for the domestic thermal energy storage application. The TES system should be able to capture the excess amount of thermal energy from the electricity grid or the renewable resources and discharge it for a sustained period when there is a demand for thermal energy. Considering this, an extensive literature review has been conducted on the application of PCM-based TES in domestic heating applications. Commonly studied methods for overcoming the limitations of PCM systems, recent research on PCM integrated under floor heating, building walls, DHW tanks, and energy storage units ESU for hot water production are extensively reviewed. Commercially available PCM for the above applications are listed with their major thermophysical properties and supplier details.

Based on the findings from the literature review conducted, an initial study was conducted to investigate the effect of various heat transfer enhancement methods on the charging and discharging cycle thermal performance of an organic PCM available in the Uster University laboratory. Then the study was extended by selecting a promising commercial PCM (Croda60) and developing a macro encapsulated Croda60/EG TES unit to overcome the leakage of PCM from the EG. Based on the studies conducted on the Croda60/EG TES system at various operating conditions of heat transfer fluid, a multi-pass Croda60/EG 3kWh TES unit was designed and developed. The charging and discharging period analysis of the multi-pass system was conducted and compared with that of a commercial PCM heat battery provided by Sunamp Pvt. Ltd. Then a techno-economic comparison of the Sunamp UniQ heat batteries and hot water tank was conducted when both are operated with a high-temperature air source HP and evaluated the economic benefit and reduction in carbon emission with Sunamp heat battery across Europe.

It was found that heat exchanger design for discharging is more important than that for the charging process. The thermal energy when required for domestic use tends to be required quickly for a sustained period, which leads the authors to suggest systems needed to be designed around discharging performance rather than charging. The current global political and economic situation (Spring 2022) is seeing energy prices rise significantly and this, if sustained, will further enhance the economic advantages of thermal stores. Large thermal stores may be a simple technology that is readily understood, but the use of PCM battery stores can significantly reduce the space required, making them easier to retrofit into existing buildings.

Date of Award	Nov 2022
Original language	English
Supervisor	Philip Griffiths (Supervisor) & Ming Jun Huang (Supervisor)