Rooftop PV for water heating – a residential super saver


From pv magazine Australia

Self consumption of rooftop solar-generated electricity is Australia’s cheapest electricity bet, never mind that sucking up the solar onsite helps electricity networks manage grid voltage and power quality at times of peak solar generation.

Although batteries are the logical answer to saving lunchtime solar for later, they’re still too pricey for most homeowners. A new study led by Dr. Baran Yildiz of the University of New South Wales’ School of Photovoltaic and Renewable Energy Engineering (SPREE) shows that storing excess lunchtime solar in a giant thermal battery – an electric hot water tank – can save them an extra AUD 100 ($74.35) a year on their electricity bills for minimal upfront investment.  And homeowners can still have 20% to 30% of their excess solar to play with.

“Hot water storage tanks can store more energy than a Tesla Powerwall, in thermal form,” Yildiz tells pv magazine Australia. And although that energy is not recoverable as electricity, it serves a relentless residential need – water heating typically makes up around 25% of a household’s electricity demand.

For the purposes of the study, Yildiz and the Collaboration on Energy and Environmental Markets (CEEM) team developed an intelligent water-heating control (IWHC) tool in collaboration with Solar Analytics, which channels excess rooftop generation into household water heaters. The IWHC makes its decisions based on a household’s PV generation characteristics, electricity consumption and hot water use patterns, and real-time energy monitoring.

The IWHC – a complexity of algorithms in a simple piece of hardware – was pitted against two commercially available tools: timer-based switching of excess solar energy into water heating, and more sophisticated diverter technology, which has so far not been widely adopted. The three technologies were each installed and tested in around 10 households and the resulting data was validated and parlayed into simulation models that were then applied to a broader set of electricity data from 380 Australian households in the greater Brisbane, greater Sydney, and greater Adelaide areas.

Hot and strong

Although the diverter tech came out on top in terms of savings delivered, at an average of AUD 170 a year, its higher upfront cost means that the cheaper IWHC, delivering AUD 100 average savings, is a more cost-effective solution. The timer came in at AUD 80 of average annual savings.

The results indicate that “on average, households can utilize 2.4 kWh, 1.8 kWh and 3.4 kWh, of daily excess distributed-PV generation for water heating, using the IWHC, timer and diverter, respectively,” Yildiz says in his paper, “Assessment of control tools for utilizing excess distributed photovoltaic generation in domestic electric water heating systems,” which was published in the Oct. 15 edition of Applied Energy.

The outcomes were surprising to Yildiz on a number of fronts. Firstly, he says the research showed that “we can provide 48%, almost half, the daily requirement of energy for heating water using excess solar generation while providing 100% customer comfort.” That is, you’ll always get a nice hot shower.

Then, he says the fact that there’s still residual – between 20% and 30% – excess solar, opens up the possibilities of further deploying this cheapest energy source for other household duties, such as “pre-heating or pre-cooling the house, or for charging electric vehicles,” or for timely use of an electric pool pump or pool heater.

Politics past

Professor Alistair Sproul, the head of school at SPREE, emphasizes the benefits of using excess solar to keep hot water topped up compared to the now old-world idea of heating water with off-peak energy – at night, when demand used to be at its lowest.

Off-peak electricity tariffs (discounted between 10 p.m. and 7 a.m.) were introduced “when New South Wales had an abundance of coal-generated electricity and it wanted to keep them running – you don’t want to ramp these things down too much overnight,” Sproul told pv magazine Australia.

Off-peak electricity tariffs, he explains, were introduced to push gas-fired hot-water heating out of the game. “They were deliberately designed to cannibalize the gas industry, so the electricity industry could flourish,” he said.

Despite the fact that demand is now often overwhelmed during the middle of the day by an abundance of PV generation, and wholesale electricity prices can spin into the negative, electricity retailers are still propping up night-time demand by offering “off-peak” rates.

“The off-peak electricity tariff in New South Wales is probably now 12 to 14 cents per kilowatt-hour between 10 p.m. and 7 a.m.,” said Sproul. “But what’s the lifecycle cost of PV coming off your roof?”

He calculates that rooftop PV, installed at even AUD 2,000 per kilowatt (which is at the higher end of the range) “is about seven or eight cents a kilowatt hour – to the consumer who uses it.”

“Baran’s paper shows that if you can avoid buying electricity from the grid, even at 12 cents a kilowatt-hour off-peak, by using your PV excess in the middle of the day to heat your water, you can make savings,” said Sproul.

Self consumption of rooftop solar also has environmental benefits in that the electricity generated is immediately used on site rather than being exported to the grid for use elsewhere. That is, any transmission of electrons results in losses along the line, whittling away at the original bounty of renewable energy.

Yildiz said the outcomes of SPREE’s hot-water-as-a-super-solar-soaker study have already informed the Australian Energy Market Commission’s plans to better integrate energy storage systems into the National Electricity Market. And energy distribution network service providers, such as AusGrid, Energy Queensland, and Endeavour Energy are keen to apply the IWHC technology, which will likely be scaled up in the near future.

“With this paper, we wanted to show that it’s fine to invest in batteries,” said Yildiz. “But we already have hot water heaters – why don’t we implement this easy, low-hanging fruit first?”

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