A vision for PV systems

This Issue This is a part of the High-performance buildings feature

By , and - , Build 185

The use of solar photovoltaic systems could contribute significantly to achieving the high-performing buildings we aim for while reducing greenhouse gas emissions and reducing power bills.

WHEN TALKING ABOUT high-performing buildings in New Zealand, one probably thinks intuitively about the thermal performance of the dwelling. Can it reach a comfortable temperature throughout at a reasonable heating cost? This is traditionally where our homes have been failing – cold, damp houses with all the detrimental consequences are still regularly in the media limelight.

Need to aim higher and be more aspirational

While getting this right as soon as possible is a priority, when looking to the future, we also need to be more aspirational and visionary.

A high-performing building in Europe these days is designed and built in such a way that it doesn’t import much energy for heating and electrical appliances. It might even be an Energy-Plus House – electricity is generated on site, stored on site, used on site and at times, when there is plenty of it, exported into the wider grid.

Proven technology

Possibly the easiest, most reliable and cheapest way to generate electricity at home is by using photovoltaics (PV). Roof-mounted or ground-mounted PV panels convert the solar energy directly into electricity. The technology has improved dramatically since the early 2000s when guaranteed feed-in tariffs for electricity generated by PV sparked a boom in Germany.

Individual panels can now generate more than 400 watts, and panel manufacturers usually issue a 25-year performance guarantee – few other products get such a warranty. The technology has proven itself all over the world.

Common myths

There are still some common misconceptions when it comes to photovoltaic systems. One is that the manufacture of the panels requires more energy than what they will generate over their lifetime. This is incorrect.

State-of-the-art solar cell technologies (mono-PERCell) have an energy returned on energy invested (ERoEI) time of around 1 year, meaning they will generate 20–30 times more energy than is required to produce them.

While solar panels do not produce green-house gases (GHG) during operation, some CO2 is released during production, depending on the electricity mix during the production stage. Recent estimates put the life cycle GHG emissions for roof-mounted PV systems in Germany at around 55 g CO2 equivalent/kWh.

Fossil fuels used for electricity generation are a factor of 10–40 above that. As the electricity mix becomes increasingly greener, the production of solar cells will proportionally emit less GHG. For contrast, the figure for the New Zealand electricity mix is given by the Ministry for the Environment as 101 g CO2 equivalent/kWh.

Opportunities for PV growth in NZ

At around 82%, New Zealand already has a high proportion of renewable energy in the electricity grid. In recent years, this number has been dropping, and generation from fossil fuels such as coal and gas has been increasing. More electric vehicles will see a further need for increased supplies of renewable electricity.

With no generation support or subsidy available for solar energy in New Zealand, PVs are not yet common, but they have the potential to significantly contribute to a future sustainable electricity mix.

The average solar irradiation levels are around 3.7 kWh/day/m² in Wellington, which compares to around 2.9 kWh/day/m² for Berlin. Even with this less-favourable irradiation figure, PV panels in Germany generated more than 11% of its electricity demand in the first half of 2021.

PV also effectively reduces the midday demand peaks, and there are many days when it generates more than half of Germany’s 60 GW electricity demand. It is fair to say that PV has grown out of its infancy.

In economic terms, at present, feeding electricity into the New Zealand grid only yields a low monetary payback rate. One of the authors of this article receives 8c/kWh from his system.

To improve the economics of a domestic photovoltaic system, the aim must be to consume the generated electricity in house rather than to export it. Load matching – where devices operate when there is power available – is one possibility for optimising own consumption. Community or cooperative power schemes are also possible. However, storing the generated electricity in house – or in a vehicle battery – may offer the most flexibility.

Systems becoming financially viable

With the costs of PV systems decreasing substantially over the last few years and electricity prices in New Zealand reaching around 30c/kWh or more, adding solar PV on rooftops has reached a tipping point for financial viability.

For residential houses with annual loads ranging from 4,000 kWh to 12,000 kWh, it is becoming viable to install 3–5 kW of solar PV on the rooftop to offset 30–40% of the grid electricity purchased. In these cases, a payback period of around 7 years is achievable.

For larger offsets, the morning and evening peak loads of residential houses need to be addressed. In these cases, some form of heat or electricity – or both – storage is required. This comes at a higher upfront cost but can help achieve greater savings.

Overseas studies have shown that, for small residential PV systems (around 4 kW installed), own consumption can be boosted around 30–60% when adding a moderate amount of battery storage with usable capacity – for example, 2 kWh. BRANZ is currently also investigating if photovoltaic electricity can be stored cost-effectively in domestic hot water cylinders.

Even more advantageous with other buildings

For commercial, educational and industrial buildings, the situation is even better than residential, given the higher coincidence of the solar PV output with the commercial loads during daylight hours. At some commercial sites, up to 50% offset is achievable with the new cost-reduced PV technologies.

Of course, on a seasonal basis, solar output decreases during winter, so accurate assessment of the local loads and the size of PV and storage is essential to ensure good matching between them and correct sizing of the PV system. This way, the benefits from renewable technology can be maximised for the end user.

Note that the ancillary benefits of solar PV, such as carbon offsets and the strengthening of distribution grids, are not captured in New Zealand. However, with new regulations, climate imperatives and the continuing lowering of the cost of PV and batteries, this technology is certain to usher in a bright and sunny future across New Zealand.

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