Solar panels aren’t just available as large pieces of infrastructure to be placed in a field or on a roof for example. They now come in all shapes and sizes from ones that attach to your backpack to tiny ones for charging your phone. This increased portability and number of options finds us wanting to use solar panels in all sorts of places to get power. One of those places may be inside a building or vehicle and you may therefore ask, can you use solar panels through glass windows?
Well, the short answer is yes you can use solar panels through glass windows but they will be nowhere near as effective as when placed outside.
So it isn’t all bad news then, but why won’t they work optimally? Well to answer that we first need to look at how solar panels work.
How do solar panels work?
Solar panels are made up of lots of ‘photovoltaic cells’ which are housed between a material which is semi-conductive. The semi-conductive material is usually silicon (the material common inside all your electronics) or in older models could even be glass.
An electrical field needs to be established in these photovoltaic cells. To do this the silicon is altered by adding some other elements, phosphorus to one end to give a negative charge and boron to the other to give a positive charge. This creates an electrical equivalent to a magnet with one end being positive and one being negative and this is an electrical field.
So you now have electrons flowing around within these photovoltaic cells. So how does this generate power?
Well, light is made up of millions of tiny particles known as photons. These photons from sunlight hit the photovoltaic cells and this knocks one of the electrons flowing around free. These electrons are then collected by metal conductive plates and can then be utilized to create electricity.
Why might you want to put solar panels behind glass?
There are a number of reasons why you might want to put solar panels behind glass.
- With more and more portable options now available such as small battery packs for charging phones or laptops, you may want to leave these charging in your car or caravan whilst driving.
- If you live in an area where they could get stolen. Solar panels aren’t cheap and if you just want to use a small panel but don’t want to fix it securely to your roof, you may be worried about leaving it outside in the garden where it could get stolen.
- One of the most common places people use solar panels behind glass is in a caravan or motorhome. Many people simply don’t have the room to install panels, or don’t fancy drilling a hole in the roof to fix them. However, when parked up the large windows can make a great place for a panel.
How do glass windows affect this process?
You might think that being behind glass might actually be better for solar panels. We’ve all been in a greenhouse or building with lots of windows on a sunny day and the temperature can get much much higher than outside. This is mainly due to a lack of airflow inside making it feel much hotter than if you were standing outside with a breeze.
But regardless of this fact, to get solar panels to work we aren’t interested in temperature and the amount they heat up, we are interested in the number of photon particles which reach the photovoltaic cells (as explained above).
So now you know solar panels are reliant on these photons reaching the photovoltaic cells to work it will be no surprise to learn that the more photons that reach the panels, the more effective they will be.
Putting something such as a sheet of glass in the way will affect this. Even though glass lets some photons through it also reflects some as well. Those photons that do pass through the glass slow down and therefore change direction (are refracted).
This reflection and refraction is shown simply in the image below, the incident ray is the light coming from the sun and then once that hits the glass (the light blue section) some light is reflected back at an angle and some that passes through is refracted and changes direction slightly. :
This will always occur no matter how clean or transparent the glass but by different amounts. So for tinted glass more reflection will occur.
Those photons that are reflected away never reach the solar panel and therefore this decreases the number of photons and thus the likelihood of an electron being knocked out of the electrical field in the photovoltaic cells.
If your windows are in a house or another fixed building the orientation will cause further issues.
Outdoor solar panels are usually fixed to a south facing surface in the northern hemisphere. This is so they are exposed to sunlight for the most amount of time. Studies showed this actually makes quite a big difference to efficiency, with east and west facing panels averaging around 20% and north facing around 40% less efficient than south facing panels. In the southern hemisphere the opposite is true and panels should face north to get the most sunlight and thus generate the most energy.
In a house you are unlikely to have windows that face directly south and even they are unlikely to be in the roof (or facing directly up) and so will only receive sunlight through them either earlier in the day or later in the day when the sun is not at its strongest. The sun is at its strongest (highest irradiance) when at its highest point, i.e directly overhead.
A car or caravan will give you slightly more flexibility as you can turn it to face the correct direction, even moving it to maximise the amount of sunlight entering throughout the day. But if you are driving then this is going to keep changing and windows tend to be small in size not letting much light through.
Being inside means you are more likely to encounter shading for many reasons. This might be a window frame or a branch of a tree outside the window. Every time your solar panel ends up in the shade this is time when it will not be able to generate electricity.
Outside, a south facing panel will still receive some sunlight even when the sun is shining to the east or west. But inside, unless you are in a greenhouse or room with windows all around you are likely to find the panel is in the shade for a large proportion of the day.
Higher temperatures can make the panels less effective
You may or may not have heard that solar panels actually work better in lower temperatures. This is partly true and very high temperatures can actually cause solar panels to work less effectively. This is because the silicon in the photovoltaic cells is an excellent conductor of heat. Studies have shown that efficiency decreases by 0.5% per degree increase over 25oC.
So as you can imagine inside a car or house (without the air-conditioning) it can get extremely hot due to the insulation of the external materials. It is not unusual for temperatures inside a parked car to get very high indeed and therefore this will cause even more inefficiencies on top of all the other factors already mentioned in this article.
How to optimize panels for use behind glass?
If you still insist on trying to use your solar panel behind glass then there are a few things you should do to optimize your chances of getting useful amounts of electricity:
- Use a south facing window if you have the option to get as much direct sunlight onto the panel as possible.
- If the panel is a small portable one that can be easily moved around then you can move it to different windows depending on where the sun is at that time of day. This will give you the maximum amount of sun throughout the day.
- Place the panels as close to the glass as possible to minimise loss from refraction and also to prevent shading from window frames or other objects in the room.
- If you can open a window and allow the sunlight through then that is, of course, the best option but you are unlikely to be asking the question if that was an option.
So there you have it, solar panels will work when placed behind glass but don’t expect any great results.
That said it really depends on what you need. Obviously, if you are attempting to power your entire house, then putting panels behind glass isn’t really feasible. However, if you just want to charge a mobile phone or some small appliances in a caravan then you should be able to do that.