The best way to explain a PCM may be to think about what happens when you boil a pot of water. Initially, the water absorbs heat from the burner under the pot, and the water heats up. When the temperature of the water reaches 212°F (100°C), the water starts to boil. That is, the water begins to change from a liquid state to a gaseous state, or steam. This conversion from one state to another is known as a "phase" change.

But what happens once you have reached 212°F, the temperature at which the phase change occurs? At this point, continued heat energy from the burner stops increasing the temperature of the water. Instead, the heat energy from the burner is used to overcome the molecular attraction of the water molecules and convert them into steam.

As long as the water is being converted into steam, heat energy from the burner contributes only to the change of liquid into gas; the temperature of the water and the pot stays at 212°F. It is only after all the water has been converted to steam that the temperature starts to rise again. If not removed from the burner or refilled with water, the pot can become hot enough to destroy it.

In this example, water, or H₂O, acts as a PCM by using heat energy to convert water into steam. H₂O also undergoes a phase change when the steam cools and reverts to water, or when water freezes into ice, and when ice melts into water. Each of these phase changes requires energy. The more energy used to convert a material from one state to another, the higher its "latent energy".

When ice melts into water, the ice absorbs heat from the environment, giving the molecules enough energy to overcome the attraction that holds them together as a solid. Similarly, the phase change that takes place in the Transformer Extender converts Trexco's proprietary PCM from solid to liquid. Again, heat energy is used to overcome the attraction of the molecules of the PCM in its solid form, causing the melting from solid to liquid.

For the functioning of the Transformer Extender, Trexco's proprietary PCM has two unique and important characteristics:

• It changes states (from solid to liquid) at a relatively low temperature, and



• It has a very high "latent heat of fusion". That is, it takes a lot of heat energy to convert the material from its solid to its liquid form at a fixed temperature.

The PCM plays several important and interrelated roles in the operation of the Transformer Extender. Three of these roles are storage, modulation, and load-shifting:

1. Storage. The PCM stores and reuses the cheapest off-peak station electric to supplement waste transformer heat for driving a chiller to cool transformer oil.

2. Modulation. The PCM allows efficient interaction between various elements of the Transformer Extender – and provides substantial control over transformer operating temperature.

3. Load Shifting. The PCM turns the conventional refrigeration expense concept on its head. Typically, the greatest load requirement for chiller operation comes when the demand for electricity is greatest and the cost of electricity is the highest – resulting in the most destructive possible impact on system economics. In contrast, by using the cheapest waste or off-peak energy at times of high demand, the Transformer Extender runs the chiller with the least expensive possible sources of energy.