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 will allow us to use between a third and a half 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% of the energy usage is water heating and about 28% is space heating. Our thermal modelling results show that we will be able to avoid space heating on typical winter days. Systems modelling has show 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 expected energy usage by month in red (the effect of solar hot water generation has been taken into account in the energy usage) and energy generation from solar PV in blue. The green area shows the net energy exported to the grid. As we have more PV than we need for our energy usage we will ove the course of a year generate more energy than we use.
The PV system
We are planning to install 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; batteries need to be periodically replaced, and contain toxic elements such as lead and acid. With a grid-tie system the grid effectively works as a battery.
Proposed solar array layout as of 25/05/2010
We are proposing to cover a large percentage of the North roof 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.
There are two technology options that we are considering. 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 3.6kWp.
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).
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