Solar home pre-cooling cuts energy bills through summer, chills the solar duck

Using excess daytime rooftop solar to pre-cool homes can save households up to $185 dollars on energy costs over the hot summer months, a new study has found, while also helping to address both the low and peak demand challenges of the duck curve.

The solar duck curve is so-called because of the shape electricity demand profiles take when rooftop solar displaces all other power generation during the day, sometimes by more than 100%, until demand ramps back up as the sun goes down and solar fades out.

It’s a big problem for the energy market operator and for older fossil fuel generators like coal plants that are not renowned for their agile response to rapidly changing market conditions.

As University of WA’s Paul McCormick explains here, so far grid-level solutions have focused on treating the symptoms – like installing network batteries to soak up the excess solar – rather than the cause.

“Since the problem starts at the bottom (households) it makes more sense to fix the problem there, using a bottom-up solution.”

The solar pre-cooling study out of the University of New South Wales comes at the problem from that direction, looking at how solar self-consumption at times of peak generation can benefit both consumers and the grid.

The study from the team at the UNSW’s Collaboration on Energy and Environmental Markets (CEEM) and the School of Built Environment’s High-Performance Architecture (HPA) Research Cluster, uses measured residential PV and load data from Solar Analytics and thermal performance of buildings from CSIRO to analyse the impact of solar pre-cooling in Australian housing stock.

“When people first hear about using excess rooftop PV generation to pre-cool their homes, they often wonder if it will really reduce their electricity bills. Well, it turns out it can,” says Shayan Naderi, lead author of the study and a PhD candidate at UNSW.

“We studied this and saw that using excess rooftop PV generation to pre-cool your home can actually lower your
electricity bills by a good amount.”

The research, published in the journal Renewable Energy, is modelled on a mix of housing stock across nine different building types, three different energy star ratings (2-, 6-, and 8-star), three different construction weight (light-, medium- and heavy-weight), four capital cities – Adelaide, Melbourne, Sydney, and Brisbane – and four representative household load profiles.

For all the virtual buildings, a solar pre-cooling scenario is simulated for summer 2018-2019 – the summer of Black Saturday bushfires in Victoria – using surplus PV generation to run electric powered air-conditioning units.

The solar pre-cooling scenario was then compared with a baseline scenario to measure maximum demand reduction, minimum demand mitigation, peak import reduction, solar self-consumption improvement, and cost savings.

The results showed that 97% of the analysed solar homes had “a duck shaped AC excluded net demand”, contributing to the duck curve in the net demand profile of the National Electricity Market.

The researchers say these solar homes showed the highest solar pre-cooling potential, and significant potential to mitigate the duck curve, mostly by mitigating the minimum demand during the middle of the day by using up their own rooftop PV, with a maximum observed value of around 4kW.

The study also found the maximum observed cost saving at around $185 for the three months of summer, but this could only be achieved with a zero feed-in tariff. With a 6 c/kWh, cost savings are not as good, but can still be as high as around $125.

“The observed cost savings highlight the high potential for households to save money by changing their patterns of load consumption and using the surplus PV generation to pre-cool their buildings,” the report says.

Naderi says concerns about any negative impacts of solar pre-cooling on the thermal comfort of homes – “some folks worry they might get too chilly if their house is pre-cooled while they are at home” – were mostly allayed.

“From what we’ve seen, the cooling usually stops before it gets too cold, either because the extra solar power runs out or because of the programming of the cooling system.”

For those households that are empty during the day, pre-cooling might take some planning – either by programming the air conditioner to come on over the hours of, say, 10am-3pm, or if homes have a modern smart energy management system, appliances can be controlled remotely.

But the study is promising, considering the huge and growing uptake of rooftop solar and the current policy push for all homes to switch to efficient electric appliances, including heat-pump air conditioning.

And it offers just a small insight into how the smart, solar and electric homes of the future will manage energy supply and demand.