Today we have been working with two customers, both of which are considering using ITO coated Polycarbonate sheet as a transparent heater for windows. One of the customers currently uses wires laminated in the sheet to heat the windows. They have recognized that using ITO coated Polycarbonate could be a cheaper option than laminating the wires into a window and the visual appearance of the product would also be much better.
The question that keeps coming up for this application is: “If I need to heat a window with X Watts/square inch, can I use ITO coated Polycarbonate?” Typically the value for X is between 0.2 and 0.8 depending upon the customer’s requirements.
As you might expect, this question is not a yes-no type question, but it involves some simple calculations. In order to carry out the calculation we need some simple information: the voltage (V) that is available for heating and the size of the window to be heated (both the width (W) between the two bus bars and the length (L) of the window/bus bar.
The first step is to calculate the total power requirement for the window, we will assume for this example that 0.5 Watts/square inch is needed.
Power (Watts) = 0.5 (watts/square inch) x W (inches) x L (inches)
We then need to calculate the Heater Resistance (R) where:
R (ohms) = [V (volts)]2 / Power (Watts)
We then need to calculate the Surface Resistance (SR) of the sheet where:
SR (ohms/sq) = L (inches) x R (ohms) / W (inches)
Combining these equations into one simple equation:
SR (ohms/sq) = [V (volts)]2 / ( 0.5 (watts/square inch) x [W (inches)]2
The limiting factor for Polycarbonate is that the minimum practical Surface Resistance is 10 Ohms/sq. This limitation means that reasonably high voltages will be required for wide heating elements. Smaller heating elements can be achieved with correspondingly lower voltages.
As an example, we will calculate whether a 9” wide x 12” long window requiring 0.5 watts/square inch heating from a 24 volt circuit can be produced from ITO coated Polycarbonate:
Surface Resistance (ohms/sq) =(24 volts x 24 volts) / (0.5 watts/square inch x 9” x 9”)
= 14 ohms/sq
Also the power requirement would be around 40 watts.
The 14 ohms/sq is easily achievable with ITO coated Polycarbonate in this application.
At HighLine Polycarbonate LLC we have a simple Excel spreadsheet to carry out this calculation. If you would like a free copy, please send us an email at firstname.lastname@example.org requesting a copy and we will send you one.