During the Asia-Pacific Solar Energy Research Conference in Melbourne, Australia, in December, the IEA's SHC Task 69 "Solar Water Heating 2030" team organized a special session on photovoltaic hot water systems. The conference, entitled "Key Considerations for Adopting Photovoltaic Water Heaters", was presented by Dean Clift, who recently completed his PhD at RMIT University. Clift shared some advice on how to design and operate a photovoltaic hot water system to reduce peak loads on the public grid and provide maximum value to households. The presentation is the culmination of several years of Clift's efforts as a researcher at RMIT University and R&D manager for water heater manufacturer Rheem. One of his conclusions is that, in an Australian context, a 6.6 kWp PV generator, together with a heat pump and a simple timer, can reduce the total 10-year electricity cost of a household with a hot water system (capital and use) by 27%. The above scheme shows the complex interactions that occur when photovoltaic hot water is integrated into the public grid.
Robert A. Taylor, IEA SHC Task 69 Co-Task Manager and Workshop Initiator, said: "During the Asia Pacific Solar Energy Research Conference, we hosted a wonderful face-to-face session that brought together researchers and stakeholders from Australian industry, including representatives from hot water equipment manufacturers and grid service providers. "We were able to show some best practice examples of how to configure and operate a smart PV water heater to significantly reduce peak loads. ”
Clift began by describing water heating in Australia, which, like most developed countries, accounts for about 25% of household energy consumption. Therefore, it is impossible to achieve a 100% renewable grid without taking domestic hot water into account. Despite Australia's relatively small population, its 10.9 million households consume 16.7 TWh of hot water heat per year (about 10% of Australia's electricity consumption). Clift said the current storage type electric water heater represents a huge controllable load of 10.8 gigawatts.
Clift's simulations were conducted in Australia's National Electricity Market, with a focus on South Australia, which has the highest rooftop PV penetration in the world at 11 kWp/capita. "In this leading region, you can showcase interesting trends that can be pilot projects elsewhere," Clift said.
Figure 1: Different PV hot water installations and control strategies and their PV self-consumption rates. Source: RMIT University
Clift's first focus was to optimize the self-consumption of photovoltaic electricity from a home water heater without batteries (see Figure 1), and in this area, "our group's scientific and industrial colleagues in Task 69 were very interested in studying the different control strategies for photovoltaic water heaters and their impact on the national grid," explains Clift, who co-led subtask C on solar photovoltaic hot water systems in Task 69.
According to simulations, a standard electric water heater receives only 12.7% of the electricity it uses from photovoltaic electricity generated on the rooftops of private homes. When combined with improved water heater design and intelligent control, a simple storage electric water heater can reduce the annual grid power load to 0.4 MWh, with 84.2% of the hot water electricity demand coming from a 3.6 kWp photovoltaic generator on the homeowner's roof.
According to Clift, this improvement can only be made with an adjustment that includes a double heating zone and an immersion element. This allows priority heating in the upper part of the tank to reduce emergency heating through a common grid, while also utilizing the full capacity of the tank for optimal storage capacity. The control strategy guarantees an uninterrupted heat supply to the house as well as meeting the requirements of the corps hygiene requirements, while removing any excess PV available from the modest grid integrated 3.6 kwp PV array.
He compared these results to a system that uses an integrated air-source heat pump, which adds cost and complexity, but can reduce hot water power consumption to one-third of that of a standard electric water heater. From a homeowner's perspective, a simple timer with a heat pump is very effective, radically reducing grid consumption and comparable to the complex dual-zone storage electric water heater discussed above, Clift suggests.
Figure 2: Most domestic hot water heat pumps operate during peak load hours of the day Source: RMIT University
"The biggest battery we have in Australia is collective control of the humble water heater in our home"
However, from the utility's point of view, the situation looks very different. As the permeability of heat pump water heaters increases, timers can be critical for maintaining grid power quality. As Figure 2 shows, heat pumps without incremental timer control tend to result in the highest power consumption during peak grid demand periods. To help manage the grid, it may be advantageous to add variable capacity heat pumps, batteries, and the ability to aggregate and control such systems centrally or through local microgrids. This state-of-the-art system can further reduce grid consumption by 28% compared to using a standard heat pump. Compiled by Chen Jiaoyun
When we look forward to the adoption of photovoltaic hot water systems, Clift makes the following points:
- Water heaters can interact positively with the grid, dramatically increasing the amount of PV self-consumption on the network, thereby increasing the ability of our existing grid infrastructure to adopt renewable energy and phase out fossil fuel generators. However, we must carefully consider their optimal configuration (resistance heating, standard heat pumps, variable speed heat pumps, and the inclusion of batteries and advanced control schemes).
- In Australia, a 6.6 kWp PV generator, together with a heat pump and a simple timer, can reduce the total 10-year electricity cost of a household with a hot water system (capital and use) by 27%.
- Given the high capital cost, it is not cost-effective to include batteries in such a system. Efficient and simple use of the heat storage inherent in water heaters requires little to no battery energy storage. The decision to use batteries can reduce household grid consumption by about 80%, which is independent of water heating delivery.
- Currently, the best consumer cost solution for heat pumps uses fixed-speed heat pumps with a simple timer. However, this can change quickly as the capital cost of variable speed heat pumps decreases.
Ultimately, the special session showed that we can't forget about hot water systems because, as Clift says, "the biggest battery we have in Australia is collective control over the humble water heater in our home".