Phase Change Material - Basic

Phase Change Properties

Phase change material

When the Phase change material check box is selected, this material is simulated as having the following temperature dependent material properties.

Phase change method

Choose from two methods to model PCMs:

• 1-Basic the method described in this section where the same melting/freezing temperature/enthalpy curve is used
• 2-Hysteresis which allows the melting/freezing process to follow different temperature/enthalpy curves, representing an effect that is commonly seen in actual building envelope phase change material applications.

Temperature coefficient for thermal conductivity

This field is used to enter the temperature dependent coefficient for thermal conductivity of the material. Units for this parameter are W/m-K². This is the thermal conductivity change per unit temperature excursion from 20°C. The conductivity value at 20°C is the one specified with the basic material properties of the regular material specified in the name field. The thermal conductivity is obtained from:

k = k_o + k₁(T_i − 20)

where:

k_o is the 20°C value of Thermal conductivity entered on the Material properties tab of the Materials dialog.

k₁ is the change in conductivity per degree temperature difference from 20°C (this field).

Temperature-Enthalpy Curves

The temperature – enthalpy set of inputs specify a two column tabular temperature-enthalpy function for the basic material. Sixteen pairs can be specified. Specify only the number of pairs necessary. The tabular function must cover the entire temperature range that will be seen by the material in the simulation. It is suggested that the function start at a low temperature (-20°C is typical), and extend to 100°C. Note that the function has no negative slopes and the lowest slope that will occur is the base material specific heat. Enthalpy contributions of the phase change are always added to the enthalpy that would result from a constant specific heat base material. Examples of simple generic enthalpy temperature function and also manufacturers data is shown below.

Generic Data from EnergyPlus

Example of Manufacturers PCM Data

Number of points on the curve

Select a number from 1 to 16. The number of points selected here will determine the number of points available for data entry below.

Temperature x

This field is used to specify the temperature of the temperature-enthalpy function for the material. Units are °C or °F.

Enthalpy x

This field specifies the enthalpy that corresponds to the previous temperature of the temperature-enthalpy function. Units are J/kg or Btu/lb.