Solar at High Noon


Surplus generation at mid-day calls for long-duration energy storage.

Surplus generation at mid-day calls for long-duration energy storage.

Fortnightly Magazine - December 2015

Goals and mandates for renewable are growing by leaps and bounds. One hundred percent for Hawaii by 2045; seventy-five percent for Vermont by 2032; fifty percent for California by 2030; 1 eighty percent for Germany by 2050 2 - all are in place. And these long-term goals are supported by interim goals for 2020, 2025, etc. Many other countries and U.S. states have adopted renewables goals nearly as aggressive.

Improving market economics also are in play in driving rapid solar adoption. We've seen utility-scale solar photovoltaic (PV) projects with contracts for less than four cents per kWh ($40 per MWh). 3 This price undercuts even the total operating and capital costs for fossil generation fired by natural gas. Sometimes it runs less than just the operating cost of gas generation.

Meanwhile, rooftop PV solar generation (supported by net metering policies) is growing rapidly and stressing the distribution grids in some locations. The situation will soon become even more challenging, especially in areas such as Hawaii and California where retail electricity rates are relatively high. In Hawaii, some homeowners have been prevented from interconnecting more rooftop solar because of overgeneration. These bans prevent users from escaping high-cost monthly utility bills. Voters become irate. They begin to put political pressure on their public utility commissions (PUCs)to break the logjam.

Figure 1 - Supply and Demand

For all these reasons, utility executives and PUCs should plan now to handle extensive solar penetration, including the significant overgeneration that can occur at mid-day.

California presents a good case study to illustrate the problem. The Golden State already is facing stresses from high solar. In fact, California Governor Jerry Brown has signed a bill mandating by 2030 that 50 per cent of the load served by electric utility resources must originate from renewables. In other words, this 50 percent under this new renewable portfolio standard (RPS) will come on top of hydroelectric generation and renewable energy provided independently via rooftop solar owned by retail customers.

Figure 1 shows the projected California mid-day overgeneration expected in 2030 with 50 per cent renewables. 5 Overgeneration is the energy above the black  curve indicating load. The solid green area shows the mandated RPS renewables. The hatched green area shows the amount of rooftop solar energy covered by Net Energy Metering (NEM). 

We see three basic ways to deal with overgeneration:

  • Curtailment: Turning off excess renewables. That means even more renewables must be built to meet the 50 percent mandate. However, the additional renewables may mostly be solar, which just adds to the overgeneration that must be curtailed. 
  • Load management: Shifting