Power Factor Issues with Smart Glass

In a Nutshell

Some smart glass technologies can auto-tint, absorbing solar heat on the building facade. 

This prevents steep temperature rises inside the building, reducing air conditioning loads and saving a sizeable chunk of the annual operational costs.

If the smart glass, however, ‘misuses’ electrical energy, the drop in efficiency can offset any financial benefits, and can sometimes result in fines from your utility.

This is what happens when a smart glass installation exhibits poor ‘Power Factor’.

What Is Power Factor?

The Power Factor (PF) of an electrical load (e.g. a washing machine, a refrigerator or a smart glass window) is a number, from 0 to 1, which describes how efficiently the load converts electrical energy into useful work. 

• PF= 1 → an ‘ideal citizen’ on the electrical grid with maximum efficiency
• PF= 0 → a ‘poor citizen’ that wastes energy

The lower this number, the lower your ‘credit rating’, so to speak, on the power grid.

Impact of Low Power Factor

Simply stated, this means more power is wasted in the electrical load, resulting in a greater burden on the grid. 

This can result in fines from your utility, since they are forced to deliver more current than is necessary to power the installation.

More current means larger, more expensive cables, more resistive (heat) losses, and more deterioration to equipment due to potential overcurrents.

Which Smart Glass suffers from Low Power Factor?

The smart glass technologies that suffer most are either capacitive or inductive in nature. 

By ‘capacitive’, we mean that the smart glass appears to the power grid as an electrical capacitor. 

Capacitive smart glass loads include SPD and PDLC smart glass.

We do not know of any smart glass which is inductive in nature (at this time), but some electronic controllers that drive smart glass can exhibit inductive behaviour.

A capacitor has parallel facing conductors enclosing a dielectric material that polarises when we apply an electric field across it.

Both SPD and PDLC smart glass have this structure, and operate with alternating (AC) currents, normally at 65 – 110 Vac (at 50 or 60 Hz).

The diagram below shows the capacitor-like structure of PDLC smart glass, where the parallel Indium Tin Oxide (ITO) transparent conductors form a sandwich around the liquid crystal dielectric. This is similar in structure to SPD smart glass.

Capacitor-like structure of PDLC smart glass (SPD glass is similar)

When you connect a capacitive load to the power grid, some of the energy is used to create the electric field inside the glass which causes the optical change:

• In PDLC smart glass, this is a change in its light scattering ability (translucent to transparent)
• In SPD smart glass, this is a change in its opacity (opaque to transparent)

This is not an issue with passive smart glass, such as photochromic or thermochromic glass, since they are not driven electrically. 

Nor does this issue pose a problem in DC (direct current) technologies, such as electrochromic smart glass, electrophoretic smart glass, embedded LED smart glass or augmented-reality smart glass. 

Electronic smart glass drivers often reduce such risks due to internal active correction circuitry.

Water-Based Analogy

Let’s look at a simplistic water-based analogy of power factor in the diagram below, which shows two scenarios, labelled (a) and (b). 

Scenario (a):

Water from a tap falls down onto a water wheel which rotates to produce useful work, before returning back to the source. No water is consumed or delayed by this arrangement (assuming minimal water leakage). 

Scenario (b):

If we add a temporary water storage element, shown as a drinking cup, it will fill up with water before overflowing onto the water wheel. In this case, the drinking cup is not doing any useful work and is actually hampering the flow of water. More water must be delivered (equal to the capacity of the drinking cup) to drive the wheel, and the arrangement suffers a time delay. 

Scenario (a) is analogous to a resistive electrical load, like a light bulb, which has a high power factor, since it suffers no delay or distortion in the delivered electrical power.

Scenario (b) is analogous to a capacitive (or inductive) storage element in an electrical circuit. The loss of energy in the electric (or magnetic) field of the storage element results in a low power factor.

Reasons for Low Power Factor

There are two principal types.

Displacement Power Factor

In this case, the device draws current out of phase with the supply voltage. The diagram below shows what this looks like when viewing the electrical waveforms.

• Resistive Load: the current is ‘in phase’ with the voltage (i.e. the peaks and troughs of the red and blue lines match in time).
• Inductive Load: the blue current waveform appears displaced to the right of the red voltage waveform, meaning that it occurs later in time. The current is said to ‘lag’ the voltage.
• Capacitive Load: the blue current waveform appears displaced to the left of the voltage waveform, meaning that the current ‘leads’ the voltage.

Both inductive and capacitive loads are linear in nature, resulting in this displacement. 

Examples include inductive loads (e.g. large motors) and capacitive loads (e.g. SPD or PDLC smart glass when driven by a step-down transformer).

Distortion Power Factor

In this case, the current is not proportional to the voltage, and gives rise to waveform distortions and harmonics (i.e. higher frequencies) which damage cables through overcurrent and overheating. 

This can result in equipment malfunctions and even fire risks. 

Examples include SPD / PDLC smart glass when driven by a switched-mode power supply (SMPS) which incorporates non-linear internal components such as diodes to rectify the voltage.

How to Calculate Power Factor

This measures the real work done by your device, compared to the total (apparent) power available from the grid.

Power Factor = Real Power / Apparent Power

• Real Power: The power that actually performs useful work, like heating, lighting or motion. 
• Reactive Power: The power required by capacitive or inductive devices to create internal electric or magnetic fields, and which doesn’t perform productive work. This is analogous to the drinking cup in the example above.
• Apparent Power: The combination of real and reactive power, which represents the total power supplied by the grid.

Outlook

This article has looked at how certain smart glass technologies, specifically SPD and PDLC smart glass, can suffer from poor Power Factor, due to their inefficient usage of electrical power.

A low Power Factor reduces the energy efficiency of the smart glass installation, increases operational building costs and can potentially lead to utility fines.

References

1. Modelling and electro-optical testing of suspended particle devices, Vergaz Benito, Universidad Carlos III de Madrid, URL

2. Power Efficiency Project, Kansas State University Engineering, URL