capture

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There are multiple means of capturing the solar energy bombarding our planet. In addition to direct solar radiation, the wind, tides and the hydrological cycle are indirect results of sunshine and the energy available can be captured using different techniques. We will leave stories on solar hot water panels, wind turbines, wave buoys, hydro dams (rainwater storage), for others to discuss and instead focus on solar PV.

1. Solar PV

Whakatane area has high irradiance which makes capturing and transforming solar energy into usable electricity using Photovoltaic (PV) panels the preferred choice of local prosumers.

Check out Finn Peacocks beginner’s guide and Kristy Hoare’s website to discover how a PV system works. These knowledgeable people discuss the difference between system components, their availability, brands, and the science of photovoltaic technology.

Once you have an idea of the complexity involved in a rooftop system, Kristy has other discussion pages specific to New Zealand to further confuse or enthuse.

2. Shading and Panel Orientation

Our latitude of 37° south has a negative impact on the amount of energy that is available in winter. Ideally panels will face the sun directly (90°) to capture available energy.

Figure 1: North facing rooftop at 20° pitch, this system is rated 2.1 kWp

Having a roof at an angle of 25° facing north in NZ is ideal for fixing PV panels to. The examples below relate to panels fixed to a 12° pitch roof facing east-west. They will still deliver but obviously are less than the ideal. Frames can be added to alter the orientation but that exposes the panels to wind loads – they could blow away unless extra (expensive) structural measures are allowed for.

The way panels are wired together also has an impact. String (single inverter serving all panels) and optimiser (micro-inverter installed on each panel) systems are being installed in NZ. Kristy has some comments here. Apart from price, the main impact using a string array is that output from the array will be reduced to that available from the least producing panel. Shading of one panel is sufficient to disrupt the whole array.

In figure 2 a Norfolk Pine shades a panel reducing the string’s output to 70 W. The graph shows how on a sunny winter day the generation from the other 7 panels is interfered with for 2 hours.

Figure 2: Shade on a single panel reducing generation on an 8-panel string type system.

During the summer months the Norfolk Pine does not shade the panels and the output of the system is 3 times higher so the shade in winter is not significant. The tree’s value as an umbrella in summer is deemed more beneficial than the 1.4 kWh (23c) lost production over 30 sunny winter days ($9.60 lost per year). It has not got the chop!

Figure 3: West-East Optimiser arrays showing cumulative generation from sunrise to noon.

Figures 3 and 4 show 2 east-west facing arrays fitted with optimisers. The 8 panels on the east side of figure 3 have been benefiting most from the December morning sun. Two of the panels on the extreme right have produced less because they were shaded by a Pohutukawa tree at sunrise. Note how the other panels have not been limited to the output of the least productive, 927 kWh panel. The panels on the west side have yet to crank up.

Figure 4: Optimiser arrays showing cumulative production at the end of a sunny day.

Figure 4 shows the system at sunset on the same sunny day. The individual panels cumulative generation are about 2 kWh each. The west side caught up to east side as the day progressed. Note, the panel generating only 1.89 kWh is subject to a vent pipe’s shade. Chop it?

Figure 5: Enphase Q7 optimisers fixed to rails prior to panels being installed

Optimiser, or microinverter technology, is relatively new. Enphase, SolarEdge and SMA have been leap frogging each other making improvements to their products.

3. Tracking

Tracking involves a mechanical device which rotates the panels to face the sun directly. They are better suited to ground mounted arrays.

Single axis tracking will follow the sun’s path from east to west (sunrise to sunset). This can increase a panel’s generation capacity by 35%.

Dual axis tracking involves adding a component to capture the north-south seasonal (summer to winter) path of the sun offering a further 10% increase in capacity over a year.

Figure 6: NIWA solarview indicates the suns daily path and intensity above Ohope throughout a year.

NIWA has a calculator that estimates the solar energy that can be collected by a solar capture device (solar PV panel) at a given NZ address. The devices used to move tracker panels throughout the day use data sets like the NIWA figures to aim themselves at the sun.

4. Rooftop Solar PV in Eastern Bay of Plenty (EBOP)

Horizon Energy is our local lines company. Connected to their distribution network are 284 solar PV installations. 246 are residential rooftop, the remainder commercial installations.

Figure 7: Electricity Authority supply data on installed solar PV systems
Figure 8: Horizon Energy are also forecasting solar PV systems connecting to their grid.

As more grid tied rooftop PV is added, Horizon will have to make adjustments for electricity being exported back into their system.

5. Solar PV Farm

Instead of installing PV panels on rooftops, another option is to ground mount a larger number of arrays at ground level. Investors in a farm benefit from economy of scale at half the cost of a rooftop array. This discussion will be subject to a future Post: “Rooftop or Farmed solar PV?”

See also Post relating to a proposal to install Community Solar Farms on Whakatane District Council lands.

The next Page, Distribution, discusses transmitting electricity from a capture device (generator) to a consumer.