Phase-Change Material Thermal Energy Storage in HVAC&R Systems for Utility Load Balancing

Summary

Nearly half of US states are currently generating more than 25 % of their in-state electricity from renewables, and grid stability is an issue important to both utilities and consumers.  With this advent of significant fractions of electricity coming from renewables, particularly wind and solar, time-of-day pricing and subsequent load-shifting are important. One method of achieving load-shifting is thermal energy storage via phase-change materials integrated with HVAC&R systems. A potential added benefit of phase-change materials is a decrease in equipment cost since the HVAC&R system could theoretically be decreased in size. Nonetheless, a significant thermal resistance exists to the transfer of heat to and from the phase-change material.  This project will investigate methods of enhancing this heat transfer to make thermal storage equipment more cost-effective and efficient. 

Description

Objective
To facilitate the integration of phase-change materials (PCM) with HVAC&R equipment to enable cost-effective and efficient thermal energy storage for load shifting and stabilization of the electrical grid as well as cost savings for electricity rate payers.

Technical Idea
As found in earlier projects, integration of phase-change materials with HVAC systems can increase the system efficiency and shift thermal loads.  This is useful to the electric utility to reduce peak demand and make the electrical grid more stable.  It is also useful to building owners as it 1.) reduces the size of the HVAC equipment needed, and 2.) allows load shifting to times of day with more favorable electricity rates.  A constraint, however, has been the resistance to heat transfer in the phase-change material. Direct-contact heat exchange (DCHX) between the refrigerant and a phase-change material has the potential for vastly higher heat transfer rates as compared to traditional fin-tube heat exchangers, that will facilitate the use of phase-change materials with HVAC systems.

As identified in the FY17 EL Exploratory Project, DCHX between PCM and refrigerant for TES, when applied to residential air conditioning, can use 6 % to 33 % less energy.  Direct-contact heat transfer between the PCM material and the refrigerant provides a design with the smallest possible thermal resistance because it eliminates intermediate heat-exchange structures (e.g., tube walls).  This project was followed by the FY19 EL Exploratory Project, which produced a detailed engineered design that can be used to build a thermal energy storage system once a suitable PCM is found.  An EL Exploratory Project for FY22 identified a PCM, that is stable, nonflammable, and nontoxic for use in the initial tests, paving the road for further development of DCHX.

Research Plan
In FY25, the project will build a test apparatus to study various aspects of the charging, discharging, and cycling processes for thermal energy storage.  These include, among others, the multi-phase flow and heat transfer characteristics, the thermal energy storage efficiency, the energy and power density, the charging/discharging rate, the performance of the PCM/refrigerant separator, and distribution of the PCM and refrigerant in the system during operation.  The project will further seek to answer research questions such as: Will PCM be entrained with refrigerant and travel through the system? If so, how much PCM will be entrained and what are the factors influencing the entrainment? Can the entrained PCM effectively return to the DCHX section without clogging? Is there a safe limit for PCM entrainment for the refrigerant system?

The new test apparatus design will significantly improve the capabilities of previous setups by having an instrumented test chamber to observe and measure the DCHX heat transfer for various operating conditions.  To enable visualization, the chamber will be made of two layers of different transparent materials with special design that ensures pressure, sealing, and insulation. In addition to the test chamber, ancillary components will be designed and fabricated to deliver and maintain the operating conditions. Data obtained from the apparatus will be used to design and size DCHXs for residential energy storage applications.