A new rooftop solar mandate in California could help set the stage for renewable energy to replace natural gas in space and water heating systems
In theory:
Solar photovoltaic —including rooftop installations — can help to decarbonise heating systems
In practice:
Yes, especially in warmer countries with relatively high amounts of sunshine in the winter
A word of caution:
“If we are proposing electrification as a solution for heating buildings in colder climates, we will see a lot more peak demand for electricity and it certainly will not be possible to satisfy this with solar alone.”
Key quote:
“The contribution solar can make to decarbonising heating will be extremely limited without a sharp reduction in energy consumption through efficiency improvements to buildings.”
As temperatures in parts of Australia went shooting up past 45°C in January 2019, air conditioners and the country’s significant and rapidly growing fleet of rooftop solar plants were working at full steam. As power demand surged, behind-the-meter rooftop solar photovoltaic (PV) installations were credited with helping to significantly reduce peak demand and soften the blow from coal plants breaking down. “Solar delivers when power is needed most, during peak demand on heatwave days,” says the Australia Institute, a think tank. Over two million Australian households, about 20% of the total, have rooftop solar, and the commercial and industrial sector rooftop sectors have also posted solid growth. Australia has some 70,000 small-scale commercial and industrial rooftop solar plants, over 20,000 which were installed in 2018 alone. “Using solar PV for cooling is easy since peak demand coincides with air conditioning and you produce energy when you need it,” says Jenny Chase from Bloomberg New Energy Finance. The storage capability of buildings also means air conditioning can easily be used for pre-cooling hours before the temperature spikes, she adds. And demand for cooling will increase as living conditions in hot countries improve and more people have the means to buy air conditioners, and climate change causes higher temperatures everywhere. Global energy demand from air conditioners could triple by 2050, forecasts the International Energy Agency. 
Heat pump potential
While its role in decarbonising cooling is clear-cut, solar PV —including rooftop installations — may also help to satisfy heating needs, particularly in locations with a relatively high amount of sunshine in winter. One example is California, where a rooftop solar mandate for new housing will go into effect in 2020. Supporters of the standards agreed in December 2018 believe they could help set the stage for renewable energy to replace natural gas in space and water heating systems through the electrification of heating, using heat pumps powered by solar PV plants. Heat pumps can be used for space heating and air conditioning, and for heating water. “What we see for the future is a reduction in carbon dioxide emissions because we will increasingly stop using fossil fuels and because heat pumps use much less energy than furnaces or boilers for the same amount of heat produced,” explains Chiara Dipasquale from the Institute for Renewable Energy at research group Eurac in Italy. Air conditioning with modern heat pumps is also more efficient than conventional air conditioners. Bundling heat pumps with rooftop solar makes a cost-effective investment for customers, in addition to contributing to decarbonisation, the US non-profit Rocky Mountain Institute (RMI) found in a 2018 report on the economics of electrifying buildings. Oakland, California, one of four US cities examined in the report, has strong solar resources and relatively high electricity prices. Reductions in energy bills from replacing natural gas heating with heat pumps and rooftop solar means customers save money after initial investment costs. RMI estimates the 15-year net cost of a natural gas retrofit at $16,100, electrification at $22,900, and solar and electrification at $13,500. One factor that should encourage the use of the heat pump-solar PV combination, and has already driven interest in battery storage in some places, is a growing tendency in the US and Europe for utilities to cut the prices paid for surplus rooftop solar power exported to the grid. It increasingly makes economic sense for customers to maximise self-consumption. Solar PV is, however, far from a one-size-fits-all solution for heating. “If we are proposing electrification as a solution for heating buildings in colder climates, we will see a lot more peak demand for electricity and it certainly will not be possible to satisfy this with solar alone,” says Mike Henchen, a manager on RMI’s electricity team. In fact, the contribution from solar PV in these places is likely to be slim.
Efficiency important
One solution is to combine solar with other zero-carbon sources like wind, which tends to be more plentiful in the winter, and hydro. “We also have to be careful about how much winter demand we are adding to the system, particularly in a colder climate,” says Henchen. In Northern Europe, power demand already peaks in winter and this will be magnified with the electrification of heating. “In Oakland, it is probably not such a big deal,” he adds, because of its milder climate. Gerhard Stryi-Hipp of the Fraunhofer Institute for Solar Energy Systems ISE, says at least 50% of heating in Europe will need to be derived from electricity from renewable energy by 2050 to meet decarbonisation objectives. It will largely be fuelled by wind and solar. He believes most European cities will seek to tap into local solar resources and “try to use as much rooftop space as possible”. In sun-rich Italy, falling costs and efficiency improvements of solar PV modules has meant that solar PV for heating makes increasing sense, says Gianni Silvestrini, scientific director of the Kyoto Club, an Italian non-profit. “But the contribution solar can make to decarbonising heating will be extremely limited without a sharp reduction in energy consumption” through efficiency improvements to buildings, he stresses. 
Rooftop versus utility-scale
Italy’s solar PV market, the capacity of which now stands at about 20 gigawatts (GW) and which satisfied about 7% of all electricity demand in 2018, has been largely driven by small-scale industrial and residential rooftop PV projects. In a 2011 law, Italy introduced a renewable energy obligation for new buildings and those undergoing major renovations which in practice means rooftop solar is now mandatory. Nonetheless, extensive rooftop potential still remains untapped; Silvestrini expects new rules aimed at facilitating on-site consumption of rooftop solar generated by multi-dwelling apartments to give rooftop solar a fresh push. While utility-scale solar PV projects at grid parity are a regular feature in Italy, they can be difficult to implement. “There is no problem of incentives, but of social acceptance and of where to put these plants,” says Silvestrini. Given frequent public and local government opposition to large-scale plants, the country is increasingly looking to place utility-scale plants on former industrial sites or waste dumps and to co-locate solar PV facilities and farming. Larger projects will be crucial to meeting Italy’s 2050 target for 50 GW of solar PV, an objective Silvestrini expects may be raised further. Given the urgent need to accelerate the energy transition, Silvestrini says Italy and other countries must tap into the potential of both rooftop and utility-scale solar PV projects. “From the point of view of decarbonisation and the grid, it doesn’t matter” if you get the power from rooftop solar or utility-scale projects, adds BNEF’s Chase. Lack of unutilised land area is likely to be a bigger driver for rooftop solar in Europe than it is in the US, believes Ric O’Connell, executive director of US electricity grid advisory GridLab. “But in the US there is a heated debate between those who would only do utility-scale solar projects because they cost less and [rooftop] solar PV because there is customer engagement,” he says.
Distributed systems
O’Connell does not want rooftop solar to be considered in isolation, but as an element in a distributed energy system that also encompasses storage, electric vehicles and demand response. With rooftop solar and other distributed resources: “We are adding a lot of dynamic, flexibility capacity that allows the grid to use more utility-scale power,” he says. “Rooftop solar and utility-scale solar are complementary.” By reducing peak demand, rooftop solar can help avoid or defer grid distribution and transmission upgrades. Rooftop solar can also help stabilise voltage problems as smart solar inverters can add or subtract reactive power used to regulate voltage to the grid, and provide frequency control and other services to the grid, Henchen says. There may also be intangible benefits, he adds, as resources like rooftop solar may be particularly important for communities who want local control over where energy comes from or have suffered health problems due to proximity to polluting power stations. The expansion of rooftop solar and solar PV in general is not without its challenges, however. “Rooftop solar in small amounts is great, but as you get more and more you will eventually reach a point when you have too much energy in the middle of the day and you have to think about how you will handle this,” points out O’Connell. 
Home storage
As battery prices decline, one of the options increasingly being considered is storage. “This should help make it easier to manage the grid, although it won’t be automatic,” says Henchen. “Utilities will need to roll out programmes.” Every battery that is added to the grid is not only a source of storage, but can also provide valuable technical services such as voltage regulation, he says. Some power utilities and system operators have spotted an opportunity. In a capacity market auction, ISO New England, an independent system operator based in Massachusetts, US, in February 2019 awarded home solar and battery group Sunrun a contract to supply 20 MW of peak capacity beginning in 2022. That capacity will come from the home solar-plus-storage systems of some 5000 customers aggregated as a virtual power plant (VPP). It marked the first capacity contract in the US awarded by an ISO to a home solar and battery operator. And in the state of Vermont, the utility Green Mountain Power (GMP) has offered customers, many of whom have rooftop solar, subsidised batteries. In exchange, GMP can access the batteries via a software platform and has called on them to supply power at times of peak demand, saving on costs. The utility also views behind-the-meter storage as a tool for optimising voltage and managing the grid more efficiently. Meanwhile, as Australia’s rooftop solar boom continues, BNEF has forecast the country will also be the biggest market for residential battery storage in 2019, accounting for some 30% of global demand. As it deals with the near term problem of meeting peak demand during a heatwave, the Australian Energy Market Operator is also looking ahead to 2050, when it and Commonwealth Scientific and Industrial Research Organisation forecast that up to 45% of all electricity generated in the country will come from distributed energy resources such as rooftop solar and battery storage.
TEXT Heather O’Brian