The overarching objective was to demonstrate that Phase Change Material (PCM) in a heat exchanger design and integrated into building ventilation system can result in significant energy savings on DoD installations while:

a) Maintaining cooled spaces within a comfortable temperature and humidity ranges (71-73oF, ≤ 65%), and

b) Reducing facility energy consumption and shave peak cooling electric demand. 

This technology integrates a PCM heat exchanger to current air conditioning technologies. The medium of heat transfer is air, where in the cooling mode warm room air flows through the PCM unit releasing its heat to the PCM and cools down to near the PCM melting temperature. While in the regeneration mode, cold air flows through the PCM unit absorbing the heat stored and dropping the PCM temperature to below its solidification temperature. The objective of the demonstration is to show the energy savings and energy cost savings of the Hybrid Environmental Control Unit (HECU) technology.

This work investigated phase change material’s ability to deliver energy and cost savings in fixed buildings. PCM is a substance that absorbs/releases substantial thermal energy as it changes phase within a specific temperature range. For example, water is a PCM that absorbs/releases 334 kJ/kg as it melts/freezes at around 32℉. More sophisticated PCM’s may change phase over a range of temperatures, and the melting and freezing temperature ranges may not be identical. 

This work’s general methodology was to integrate PCM into building’s Environmental Control Unit (ECU) systems. PCM would be frozen using the ECU, and then melted later to provide space cooling without using the ECU. This methodology was expected to yield both energy and cost savings. Two distinct technology approaches were taken.

The first approach was to integrate multiple small PCM filled heat exchangers with the ECU air delivery system. These heat exchangers were installed under ceiling air registers, so cold air passed through them just before entering the conditioned space. These heat exchangers provided additional cooling at each cooling cycle’s end, thereby increasing time between subsequent cooling cycles saving energy.

The second approach was to integrate one large PCM filled heat exchanger with the ECU air delivery system. This heat exchanger was installed just after the ECU system’s air handler. This approach sought to shift ECU usage from afternoon (peak hours) to early morning (non-peak hours). Shifting hours in this way leads to energy savings in addition to energy cost savings when variable price plans are employed. These two approaches are henceforth referred to as Ceiling Coils and Peak Load Shaving (PLS).

Both approaches have four main technology components: 1) PCM, 2) Heat Exchanger(s), 3) Controls, and 4) Integration Hardware. PCM stores thermal energy so that space-cooling may be achieved without using ECU system’s condensers. Heat exchanger coils house PCM, and facilitate heat transfer between air and the PCM. This process is critical in both storing and releasing thermal energy from PCM. In the demonstration, controls and hardware are required to modify ECU system operation to account for the inclusion of PCM filled coils. Integration Hardware is all peripheral materials. 

CEILING COILS

The demonstration showed a 19% reduction in air conditioning energy use compared to the baseline and reduced peak demand by two hours. Also, the PCM-filled coils maintained the room temperature and humidity within the criteria and did not increase the fan energy consumption or decrease the air flow rate. However, the PCM ceiling coils could not meet the six year payback period.

PEAK LOAD SHAVING

The PLS coil was selected to cover only two hours of the six hour period due to the size of the PLS coil and the space availability in the demonstration site. The demonstration showed a 1.47% energy savings and, based on Tyndall AFB electricity pricing, reduced the air conditioning energy cost by 6.2%. The PLS coils maintained the room temperature and humidity within the criteria and did not increase the fan energy consumption, however, it decreased the air flow rate by about 100 CFM (from original air flow rate of 1600 CFM). Also, the PLS coils could not meet the six year payback period.

CEILING COILS

The coils were suspended directly beneath the air registers in the drop ceiling, allowing for natural convection cooling when the air handler’s fan was off. The coils were mounted flush with the existing drop ceiling and sealed with a gasket to ensure that all air exiting the ceiling registers passed through the coils. The 40-pound (when filled) coils were suspended from ceiling joists in the attic to support the weight. No changes were made to the building’s existing duct system to accommodate the ceiling coils.

PEAK LOAD SHAVING

The peak load shaving coil can be installed using a process similar to installing an air handler. Space limitation in the test facility was the primary factor that prohibited the installation of a PLS unit large enough to cool the room for the entire six hour peak period. Tripling the footprint was impractical in an existing facility of this size (1,200 square feet) but could be factored into the building design for full-scale implementation. Due to these reasons, a PLS coil that covered only two hours of the six hour peak period was installed.