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4..Does the PV have to go on the roof?

If you look at the housing around the suburbs of Cairns, you will be sure to notice that a proportion of the photovoltaic arrays on the roofs are not anywhere near facing north and they are placed on roofs that you could bet will be screaming hot - especially in our tropical summer.

To improve the performance of the PV, three factors should be considered:

TEMPERATURE: All crystalline photovoltaic panels suffer from higher temperatures because basically, the greater the temperature, the more the electrical resistance and less electricity is produced. The Australian Government website "Your Home" has this statement in its technical manual: "Crystalline modules perform best in cooler temperatures. The output of crystalline PV modules is rated for a given test temperature of 25°C in the cells; output reduces by approximately 0.5% for each degree Celsius in the module above the standard test temperature (this percentage reduction can vary 0.3–0.9%, which can make a material difference at temperatures significantly higher than 25°C)."

Therefore, the removal of a lot of this heat is both desirable and possible by placing the panels on the top of a pergola.

ORIENTATION: The PV panels should face within at least 20 degrees of true north. The performance of the PV can be affected throughout the day if the panels are too much off true north. (See the note 1 link below and look at the tables and discussion) However, if the panels are facing north east, these panels will generate more electricity in the morning and less in the afternoon. But, if there are high trees to the west of the block, then it may be desirable to actually have these panels facing north east. All this should be considered before placing the panels.

HORIZONTAL ANGLE: Elevating your PV array to suit the latitude, you should have your array's horizontal angle set at the mid-angle, that is, at an angle midway between the sunray paths for December the 21st and June the 21st. The horizontal angle for the mid-angle in the Cairns region is 17 degrees. For ease of construction when building the pergola, the horizontal angle should be 15 degrees.

 

One way of getting around all of the above would be to have the PV panels on top of the pergola. It then ought to be an easy matter to have the pergola facing north and also have these panels at the desired pitch. And, if such a design allowed spacing between the panels, this would allow breezes to flow through and would assist in the removal of heat. Also, to prevent cable problems, the pergola should be placed as close as possible to the dwelling.

Such a design of a solar pergola that not only allows all this, but can be expanded so that more solar panels can be added in the future, is shown below:

 

To reduce the construction cost of the pergola, this design is to have a DIY component, that is, the pergola will have timber beams that can be cut by saws at the specified angles. In addition, as previously mentioned, if you want to increase the number of PV panels at a later date, this design makes this possible. This is done by the sharing of support legs between the adjacent beams.

So to answer the question raised in the title: "Does the PV have to go on the roof?", the answer would have to be a resounding no, especially when you consider the modern trend for outdoor dining in the backyard and how useful a pergola would be there.

To print out all drawings associated with this design at an A4 size, please click here for a perspective view of the plan, here for a drawing showing various views and here for the plan view.

After the timber frames are complete and everything is ready for painting - if you think that temperature may be a problem for the panels - the colour of choice ought to be white. The importance of using white, especially one that has heat resistance properties, is discussed in more detail in the next section which deals with that most neglected space in modern housing design - the roof.

NOTES:

1... It is possible to calculate the angles of incidence of the sun rays upon the PV throughout the day and for various latitudes. From this, the power loss caused by the angle of incidence can be derived. In addition, a model of Sola Pergola Design 1 that I've built has delivered insights on how this pergola should be constructed and on things to look out for. Click here to find out more.

2...For the original webpage, click here.

3... In the temperate zone the sun paths have a greater curvature than in the tropics. This curvature will lessen the performance of the PV in the mornings and afternoons, especially in the Summer. Could this be a solution?

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