Photovoltaics will generate more energy than we will use each year,
Photovoltaics (PV)
Many of the design features (including the building envelope and systems) help to minimise the energy demand of the house. We expect these features to allow us to need between a third and a half of the energy of an average New Zealand house.
The average house uses 10,660 kWh of energy, we expect to use 3,500 kWh.
Energy usage and generation
The BRANZ HEEP Study1 shows that the average Auckland home uses 10,660 kWh per year. Of this over 30% is for water heating and about 28% is for space heating. Our thermal modelling results show that we will be able to avoid space heating on typical winter days. Systems modelling has shown that with a solar hot water system we can reduce electrical energy requirements for water heating by over 80%. When taking into account all of our predicted energy uses we expect that we will consume approximately 3500 kWh/a.
The chart above shows the following:
- Red. Expected energy use by month (the effect of solar hot water generation has been taken into account in the energy usage).
- Blue. Energy generation from solar PV.
- Green. Net energy exported to the grid. As we have more PV than we need for our energy usage we will, over the course of a year, generate more energy than we use.
The PV system
We have installed a grid connected PV array. Rather than store energy onsite in batteries, any energy that is not immediately used in the house is exported back to the electricity grid. When there is insufficient energy generated by the PV array for the needs of the house, energy is imported from the grid.
This type of system is more cost effective both initially and over time as no batteries are required for set-up or replacement. By not storing batteries on-site we are also avoiding toxic elements such as lead and acid.
With a grid-connected system the grid effectively works as a battery. The chart below shows the energy exported vs the energy imported (note this is different to the energy we used), as well as the cummualative net energy exported to the grid (energy exported - energy imported).
Proposed solar array layout as of 25/05/2010
The North roof is complete covered with solar, split between with solar hot water and PV as shown above. We expect the PV array to generate at least 1250kWh/year for every kWp installed. So to meet our requirement of ~3500kWh we will need to install ~2.8kWp. We have actually installed a 4.16kWp PV array consisting of 88 x C21 roof tiles.
There are two technology options that we are considered. Amorphous thin film silicon and crystalline thin film silicon. There are some pro's and con's with each technology but the largest determining factor is that thin film has a lower efficiency and therefore requires a large area to achieve the same power output. Using a standard PV module size we can only fit 2.3 kWp of thin film. To provide enough flexibility with the size of the array we need to use mono or poly crystalline. With these technologies we can fit up to 4.2kWp.
Meeting the project goals:
Energy Efficiency and Green House Gas:
- Electricity generated by photovoltaics has zero operating CO2 emissions per kWh compared to electricity from the grid which is ~180kg/kWh on average in New Zealand (note that embodied carbon has not been taken into account in these figures).
Footnotes:
- Information about the BRANZ HEEP Study can be found here. You can also download the full study report (134 pages, 6.8MB) here. Table 4 on page 14 lists the average energy use by city.