Phase-Change Media for CSP Thermal Energy Storage

As a simple case study of the potential of PCMs for thermal energy storage at any temperature range, we consider the Micronal PCM incorporated within National Gypsum's ThermalCORE drywall sheets (Carnett 2010). ThermalCORE represents the most advanced--that is, closest to commercial viability--attempt at a concept that has been in development for some time; basically, a paraffin-wax type substance suspended within the bulk matrix of the wall board allows latent heat storage at a temperature close to the comfort level for human habitation (Goswami, Kreith et al. 2000).

Micronal encapsulation scheme; from (NationalGypsum 2011).

In the case of Micronal PCM, the phase transition occurs at approximately 73^oF (23^oC). Micronal is basically a paraffin wax microencapsulated by an acrylic polymer. The Micronal PCM in the drywall sheets will absorb heat as the day increases the house's temperature from cooler night-time temperatures; conversely, as the night brings on cooler weather, the PCM will release its heat back into the home. The net effect is to lessen the amplitude of the temperature fluctuations in the house, reducing the heating and/or cooling load, depending on the time of year (NationalGypsum 2011).

Hypothetical home temperature fluctuations; from (NationalGypsum 2011).

According to the ThermalCORE website (www.thermalcore.info), the overall latent heat capacity of the drywall amounts to 22 BTU/ft² (NationalGypsum 2011). We can calculate quite easily the overall cooling load saved with these wall boards for a home in Florida which would otherwise require its air conditioner to regulate the higher temperature fluctuations (it is assumed no heat is required at lower temperature fluctuations, as would be the case in warmer weather). Assuming a total wall/ceiling area of 5,000 ft²:

Taking the average home energy needs to be 30kWh/day, this would seem to suggest that more energy is saved than would be expended by the home in the first place! Of course, this simplistic analysis assumes a perfectly insulated system and a 100 percent efficient HVAC system--rather faulty assumptions. Perhaps a better indicator: real-world testing of the wall boards in warmer southern European climes suggests a savings of as much as 20 percent of the electricity needed for air conditioning (Bourzac 2010).

The potential for savings with such technologies is clear. This is why the scale-up and implementation of the concept for use in CSP plants is so enticing.

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