The Duck Curve Dilemma
Copyright © 2024 Philip C. Cruver
In the realm of energy, California stands as a beacon of progress, with nearly 47 gigawatts of solar power installed, a monumental achievement capable of powering over 13.9 million homes and furnishing more than a quarter of the Golden State’s electricity needs.
However, this remarkable feat is accompanied by a paradoxical challenge: excess solar energy. With such abundance, the grid occasionally experiences negative electricity prices on sunny spring days when demand slackens. Gigawatts of solar energy are thus squandered, a phenomenon dubbed the "duck curve" by California’s grid operator, CAISO, owing to its resemblance to the belly of a duck.
The duck curve phenomenon reaches its peak during spring months when solar panels soak in sunlight but encounter diminished demand. Over the years, this curve has deepened into a daunting canyon, rendering solar power idle. In 2022, California frittered away a staggering 2.4 million megawatt-hours of electricity, with solar accounting for a whopping 95 percent. Last year, this amount of wastage occurred within the first eight months alone.
Discarding free energy inflates electricity prices and erodes the advantages of rooftop solar installation. Since the 1990s, California has incentivized rooftop solar adoption by compensating owners for exported energy at rates ranging from $0.20 to $0.30 per kilowatt-hour. However, a year ago, the state overhauled this system, transitioning to compensating new solar panel owners based on the grid's valuation of their power. During the deepest dips of the duck curve, this valuation can plummet close to zero. To counterbalance, customers are incentivized to install batteries, allowing them to supply power to the grid during peak hours.
In response, CAISO is exploring various strategies to alleviate the strain, including selling excess power to neighboring states. Additionally, California plans to bolster its energy storage infrastructure by deploying additional storage units and batteries to retain solar energy for later use. Moreover, the installation of transmission lines capable of transporting electricity to adjacent regions will alleviate congestion, as some of the forfeited power emanates from regions with insufficient transmission capacity to accommodate solar surges.
Amidst this conundrum lies a transformative solution: heat pumps. These devices hold the potential to revolutionize thermal energy storage, offering a sustainable avenue to harness surplus solar energy. Unlike conventional heating and cooling systems, heat pumps utilize electricity to transfer heat rather than generate it, making them ideal for integrating with excess solar power. During periods of solar glut, surplus energy can be diverted to operate heat pumps, which store thermal energy for later use.
A heat pump operates by transferring heat from a source to a destination. It uses a small amount of energy to move heat from a colder area to a warmer one, making the latter warmer and the former cooler. The efficiency of a heat pump is expressed as a Coefficient of Performance (COP), which is the ratio of the amount of heat moved to the amount of energy used to move it.
The operating efficiency of a heat pump can exceed 100% because it doesn't generate heat; it simply moves it. This efficiency is possible due to the thermodynamic properties of the refrigerant cycle it operates on. During the cycle, the refrigerant absorbs heat from a cold source and releases it to a warmer area through compression and expansion processes. In moderate climates, where there's sufficient heat available in the outdoor air, a well-designed heat pump can achieve COP values of 300-500%. Remarkably, this means that for every unit of energy consumed to run the heat pump, it can move 3 to 5 units of heat into the indoor space. In other words, for every kilowatt of electrical power used in the compressor, between 3 to 5 kilowatts of heat is produced.
These high COP values are made possible with Phase Change Materials (PCMs), which are substances that absorb or release a significant amount of heat when they change from one phase to another, such as from liquid to gas. This property makes them very useful in managing temperature, as they can store and release heat energy over a narrow temperature range. In the context of a heat pump, PCMs are used to enhance efficiency and performance. In the context of the duck curve for energy storage, PCMs can store excess heat produced by the heat pump during times when there is low demand. Later, when the demand increases, the stored heat can be released, reducing the workload on the heat pump, and saving energy.
The duck curve dilemma underscores the imperative of innovative solutions to address the challenges posed by renewable energy integration. Heat pumps emerge as a promising technology to transform surplus solar energy into a valuable resource, offering a pathway towards a more sustainable and resilient energy future and preventing the duck curve dilemma from becoming a debacle.
