The PV Plan 

I really like the possibility of converting light to electricity. In the spirit of innovation I will simplify the PV system and reduce  the cost by returning to using DC power directly from the PV panels.  As I have no intention of transmitting power around the countryside this is another reason for staying with DC.

The passive house standard I feel helps in this approach by reducing the energy required in a house to a very low level which creates the synergy to make this leap for me to design a DC power heating system for our house.

Solar Background Information

The energy from the sun varies throughout the year due to cloud and the amount of air mass (AM) it has to pass through to reach the earth. When the angle of the sun is low in the sky the solar energy has to pass through more air which means less solar energy is available to convert light into electricity (example winter months).

When a PV manufacturer quotes the output power of their PV panel the international standard is to quote the output power at an air mass of 1.5 . An air mass of 0 is know as outer space where no air exists (AM0) . This is where the maximum energy can be captured if one lived in space. An air mass of 1 is when the sun is at its highest point in the sky (AM1) when on earth. The AM value of 1.5 is around 48 degrees off the highest point on earth.

Measuring the Solar Energy (W/m²)

In the winter months from tests I have carried out with my pyranometer ( a device that measures the solar energy-see below) the solar power I am recording with a data logger was between 0 and 400 watts per m2 (It can be higher on sunny days). In the spring/summer months the power can reach 1000 watts per m2 and more.

Harvesting Solar energy in the winter months for me is the priority which will entail the correct location and angle of the solar PV panel for the winter sun. The strategy is to try capture as much of the winter sun as possible  by balancing the solar gain of the glass in the south windows of the house (part of the passive house performance phpp calculations) and supplement this with the DC (Direct Current) electricity from solar PV panels to provide space and primary heating etc. It is very noticeable at this stage of the build the real benefit of gathering energy from the winter sun through the glass.  (See previous blog on performance data 23/05/2015).

In order to give an idea of the solar energy available I recorded the irradiance when the sun was behind a dark cloud (see image below). This equates to around 200 watts per m2 solar energy. When the sun came out from behind the cloud it reached over 1000 watts per m2 in the month of April.

Sun behind dark grey cloud is approximately 200 W/m2 of solar energy (above image). Most inverters start to loose their effeciencys at this point.

Sun behind dark grey cloud is approximately 112 W/m2 of solar energy on the typical overcast day. Most inverters would stop working at this level. 

Below is an example of the changes that take place on a sunny/cloudy day in May

In the above chart one can see an example of how difficult it is for an inverter to keep working efficiently (they work efficiently from approximately 200 w/m2).  The bottom line on the left is 100 watts/m2, The top line is 700 watts/m2 (click on the image to see more detail) .  In the winter time values from around 50w/m2 to 200w/m2 are the lower limits and the upper limits are around 600w/m2.

Self Build Homework (Develop a DC powered Solar Harvesting Unit)

For the above I do need to find a way of maximizing the output power of the PV panels as the iradiance varies. For this I need to develop a simple black box (a small amount of simple components) that will match the solar energy created by the PV panels and maximise the output over the winter months. I am close to having a working prototype to see this in action (all tests look good so far ).

The equipment to be purchased for the above will be 4 solar panels and the mounting brackets. 4 solar panels will provide around 1 KW of power (max). This will cost around €1000. More groups of these will be added in the future. (If any one has 4 spare panels to loan so that I can test the control unit I am building – please let me know.)

In essence I plan to create what I call a DC Solar Harvesting Unit (DCSHU) that will have specific electric power functions around the house.