The good news is: solar. The bad news is: uncontrollable rooftop solar. How do we utilize the production of rooftop solar in the middle of the day?
This is an especially significant problem for California. The California ISO coined the term “duck curve” to describe that state’s net load – that is, load minus renewable production from solar photovoltaic (PV), wind, and run-of-river hydro production. On a mild sunny day, the problem that the duck curve illustrates is that net load falls so low that other generation cannot follow it.
The same challenge is emerging in other places, as well. In Australia, for example, Paul Simshauser of Griffith University, South East Queensland, describes the same situation there in his warm, northern state. (Click here to download his paper.)
Interestingly, Simshauser’s data shows that air-conditioning (AC) is a significant contribution to the rising neck of the magpie goose during critical summer days (that is, a significant contribution to the increase in net load during the evening of such days after the sun goes down). Although I do not have the specific data on hour-to-hour consumption for inland California, I believe that a similar pattern would apply there and in many regions with high AC consumption. In Texas, for example, residential consumption in summer greatly exceeds that in spring and fall. It is well understood that summer peak consumption is driven by AC load during times of high temperatures that persist into evenings.
To illustrate, Figure 1 shows net load for Central Texas during August 2018 (data courtesy of Grant Fisher and Esha Choudhary of Pecan Street Inc.). The blue line shows net load (consumption, including AC, minus rooftop PV production) for a sample of Austin homes. The data points are 15-minute average power consumption, averaged over the sample of homes with both AC and rooftop PV that Pecan Street monitors. Each day, net load falls (becoming negative, implying net export to the electric distribution system) during the day, but then rises again and experiences a peak in the late afternoon and evening, with solar production decreasing just as AC consumption increases: the upward sloping “neck” of the duck or magpie goose.
This combination of AC needs that persist after the sun goes down and PV production that falls precipitously at sundown suggests a way forward: to pre-cool houses in the hours before sundown. Researchers working under the U.S. Department of Energy’s Building America program have modeled two example homes for several regions using EnergyPlus. (Click here to download the report.) They found that pre-cooling is an effective way to reduce peak residential load.
However, pre-cooling will result in higher energy consumption by 2% to 8% overall. This increase in energy consumption can be thought of as analogous to “round-trip losses” in a battery storage system, implying that storing energy involves overall more energy consumption than using the energy when it is produced. Results depend on both weather patterns and the thermal insulation and thermal mass of the housing stock.
How does pre-cooling interact with the duck curve? Pre-cooling can increase consumption when the sun is shining and decrease it after the sun goes down. This has two advantages. First, with significant rooftop solar, there is significant export during the day to the electric distribution system. There are limits to the amount of such exports, and California is heading toward a situation where PV production may otherwise have to be curtailed during the day. Therefore, pre-cooling could offer significant benefits by increasing utilization of renewables during the day, while also reducing non-renewable production in the evening. Second, pre-cooling will reduce the “ramp rate” of net load; that is, the rate of increase in net load over time, which is represented by the upward sloping neck of the duck. Because net load must be matched by other generation, and because generation has limited ability to ramp, reducing the slope of the “neck” can ease the need for ramping capacity.
Sometimes chemical battery storage is advocated as a solution to the mismatch between PV production and electrical demand. Interestingly, the higher energy consumption with pre-cooling found by the DOE Building America program is similar in magnitude to the round trip losses of a Tesla battery. In contrast to a chemical battery, pre-cooling does not require (much) capital investment, at least for a well-insulated home. While pre-cooling might not work for typical current Queensland housing stock, it might be effective in regions where there is already significant investment in insulation. Much housing stock in Texas, for example, has double-glazing as well as ceiling and wall insulation, and further investments in building efficiency would not only help with improving prospects for energy storage but also pay dividends in overall energy savings. I understand from Scott Jarman of Austin Energy that this Austin utility already practices pre-cooling in some of its controlled residential thermostats in preparation for critical peaks.
So, could we pre-cool all residences all the time? Pre-cooling homes could effectively be practiced more widely and not just on critical peak days. The idea would be to significantly pre-cool well-insulated homes while PVs were still producing significant power, and then to allow indoor temperatures to drift upward as the sun goes down. This would facilitate better utilization of PV production and reduce the slope of net load in the evening.
I have not performed the detailed modeling to evaluate the potential explicitly, but figure 1 suggests what might be possible for Austin. I considered shifting the AC consumption represented in the Pecan Street data to occur three hours earlier. I accounted for the round-trip losses by assuming that 10% more electricity for AC would be required when shifting consumption by three hours. The result is shown in the orange line, which has less variation than the blue line: peak consumption is significantly lowered, there is lower net export of solar to the grid, and the ramp rate of the net load is significantly reduced. To be clear: the blue line simply shows the effect on net load of bringing forward AC consumption by three hours and increasing it by 10%, whereas a more careful simulation is required to obtain actual results with a real home.
What does data for a single day tell us? Figure 2 depicts net load for the specific day of August 1, 2018. We can see that the general duck-like shape of the net load as shown by the blue line has been flattened by bringing AC consumption forward in time: as shown by the orange line, peak of net load is lower, net electricity exports from homes have been eliminated for this day, and the “neck” of the duck rising to the peak has a lower slope. Simulation of a pre-cooling strategy would undoubtedly show a different detailed pattern of net load, but a similar general effect could be expected.
Won’t consumers balk at spending more money on higher electricity usage to pre-cool their homes? California is addressing this problem by introducing new lower time-of-use (TOU) rates for electricity during sunny hours.
Traditional TOU rates were designed to shift consumption to nighttime, say after 10pm or 11pm, when load is typically lowest. Some argue, however, that these traditional TOU rates are ineffective, and recent evidence from Bruce Mountain, Victoria University, Melbourne, (click here to download the presentation) supports that claim, by suggesting that such traditional rates, with low prices overnight, have not convinced homeowners to shift their consumption to nighttime in the state of Victoria. No one wants to do their laundry in the middle of the night to save a few pennies.
But with the new TOU rates they would be willing to do their laundry, dishes, and electric-vehicle charging – and pre-cool their homes — in the afternoon. The new, improved version of TOU with lower prices during middle hours of the day was mentioned in the DOE Building America study, and that’s exactly what California is doing.
Renewables challenge us to rethink our basic assumptions. To mix metaphors, there is more than one way to skin a duck — or a magpie goose. With high PV penetration, we cannot always control supply to meet demand. We need to change demand to follow supply. And that’s what pre-cooling will achieve.
Next time: more ways to change demand to follow supply.