Tuesday, February 4, 2014

Solar Powered Window Socket Concept Or Reality?

Solar powered Windows Socket in concept for product that is getting lot of attention on the Internet. While concept idea is nice there is some technical problems with this concept.

In this article I will explain why I think this product concept is not reality with current technology. I decide to write this editorial because of the comments and reactions regarding this product on the internet.

The Solar Powered Windows Socket is designed by Kyuho Song & Boa Oh and I will not criticize design part of product but only technical and technology aspect of product.

The product

First I will talk about this product so you can get the basic idea what it does. It was first mentioned on yankodesign.com.

It is basically a wall plug that have solar cell on other side and suction plate that allows it to be mounted on windows or any glass surface.

Here are some images of product:

Fig 1: Solar powered windows socket [1]

Fig 2: Front of solar powered windows socket [1]
Only technical specification from design is that it needs 5 to 8 hours to charge and battery have capacity of 1000 mAh.

From the pictures above you can see it should power AC electrical devices.

Design flaws 

Power output 

Product have a power socket and power sockets are used for AC electric power and there is design flaw. 

Solar cells convert energy of the to DC electrical power and you need power inverter (DC to AC). The current inverters are not that small so inverter can't be placed in small container.
The better use with current design would be to use AC 5V power as an output and USB port as output. 

The USB requires less space than standard power socket outlet, and can provide power supply for portable mobile devices. With specified 1000 mAh battery Solar Powered Windows Socket would be perfect as an external battery for powering USB devices.

Size of the solar panel

Even if you cold put power inverter (DC to AC) to that small container or some kind of external power inverter size of the solar panel is too small to provide enough charge for some real use with even low powered devices.

Let us look the situation this way.
From design in figure 2 we can come to conclusion that size of solar panel is ~30% larger than standard power socket size.

The plug used in figure 1 is The Europlug (CEE 7/16) and size of  CEE 7 power socket is 3.6 cm [2].
By this we have solar panel in shape of the circle with diameter of 3.6 + ~30 % = 4.68 cm. For this post purpose I will use 5 cm circle diameter.

D – diameter of solar panel = 5 cm
A – area of solar panel = 19.63 cm2 = 0.0019630 m2

With assumption that the product is mounted on window, slope will be 90° and for azimuth angle we will assume 0° (product is orientated to south).

While there isn't specified type of solar cell we will assume that it will use CdTe type of solar cell. Current commercial CdTe solar cells have efficiency of 16.7% [3].

With this parameters we can calculate how much energy can this product produce.

Energy quantification

For energy quantification of product we will start with calculating total sun's irradiation for slope = 90° and for azimuth = 0°.

Table 1: Total sun's irradiance

Table 2: Average sun's irradiance, captured and converted energy by the product

Table 1. shows totals sun's irradiance in W.m-2 from 8:oo to 16:oo hours and Table 2 column 2 shows average irradiance in W.m-2 for slope of 90° and azimuth 0°.

In Table 2 column 3 you can see amount of irradtion that is product able to capture with it surface of 0.0019630 m2 in watts. And in column 4 total energy converted with CdTe solar cell with efficiency of 16.7% in watts.

Note: this is sun's irradiance for latitude N 48 43.972.

Fig 3: Total year irradiation

Figure 3 show total year irradiation from 0° to 90° slope angle.

Given the ration between theoretical and actual sunshine in Table 3 and data in Table 2 we can calculate total amount of energy that sun emits on earth surface, total amount of energy that product captures and total amount that product converters for 90° slope angle and 0° azimuth orientation.

Table 3: Ration between theoretical and actual sunshine

Table 4: Total amount of energy that product captures and total amount that product converters

Table 4 show total amount of energy that sun emits on earth surface in a day (column 2), total amount of energy that product captures in a day and total amount that product converters in a day for 90° slope angle and 0° azimuth orientation. All data in watts/hours.


With calculated energy quantification we got that Solar Powered Window Socket concept can only produce 1.69 W/h of electrical energy in December and 2.56 W/h of electrical energy in September in a day, Table 4. Given that December is worst and September is the best month for producing solar energy in given conditions.

This gives us an average of 2.25 Watts/hours per day and this is without any energy losses in inverter, cables, batteries and etc.

Giving that this product concept in this form and with current technology still can't be reality.
In current form and technology I only see this product concept as DC electrical device that can be used to power LED lights or as external battery or charging station for portable mobile devices.


[1] YANKO DESIGN, http://www.yankodesign.com/2013/04/26/plug-it-on-the-window/
[2] Specificationsfor GERMAN SCHUKO(CEE 7/7 ),
[3] Martin A. Green1, Keith Emery, Yoshihiro Hishikawa, Wilhelm Warta and Ewan D. Dunlop, Solar cell efficiency tables. Available on intenet: http://onlinelibrary.wiley.com/doi/10.1002/pip.2163/pdf
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