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Blog > Addressing Intermittency

Addressing Intermittency

March 17, 2022 by VCT Group

What is intermittency? 

Intermittency is the presence of variability combined with a lack of predictability.  

Electricity generation technologies are classified into three broad categories: 1) Baseload, which provides the bulk of the grid’s supply, (this currently includes nuclear, hydroelectric, and fossil fuels); 2) Dispatchable, which can be brought online within minutes to address peak demand, (often methane gas-powered generators); and 3) Intermittent, which has unpredictably variable output, (such as wind and solar). 

Renewable distributed energy resources, like wind and solar, are commonly dismissed as viable options for baseload grid power. The argument is that their intermittency (i.e., they only work when the sun shines and the wind blows), makes them unsuitable to address the constant baseload power demand, currently supplied by centralized technologies. There are three basic solutions to intermittency: 1) Ramp the production from baseload generators to match the demand; 2) Use secondary dispatchable generators to match supply to peak demand over short timescales; and 3) Store surplus energy during peak production times and utilize it during lows.  

The key to enabling greater adoption of renewables as baseload is energy storage. Solar, Wind and Battery (SWB) is a common phrase in our industry for good reason. Many jurisdictions use a net-metering (NEM) approach where solar operators feed surplus power into the grid, earning credits against the purchase of electricity from the grid in times of low generation. In effect, owners of solar arrays are using the grid as a “virtual” battery to store power. 

To completely achieve full electrification and transition to 100% renewable power, we need to increase the amount of renewable energy that can be stored for use as dispatchable power. We are going to need bigger batteries. 

The challenges of matching electricity supply & demand. 

As distributed energy resources (DERs) are added to the aging electricity grid, several complexities come into play. The primary obstacle is that the grid was initially designed to distribute power from a small number of high-capacity power stations, not to accept power generated from many smaller sources. The ability of the grid to accept feed-in power from renewable sites is geographically limited and can stop a DER project in the initial planning stages if the local grid lacks the capacity. Modernization of the grid is a high priority for the renewable energy sector. 

An innovative approach for addressing shortfalls in the grid is to use DERs in tandem with micro-grids. A micro-grid is a smaller, local electricity transmission system on the scale of an industrial facility or community. The micro-grid can be connected to the larger grid to take advantage of net-metering and grid backup but can also be isolated and independent. Such “islanded” micro-grids can continue to supply power even in the event of a large-scale blackout of the main grid. 

Large, centralized power plants can take up to 20 years to construct. In contrast, solar and wind installations can be constructed over much shorter timescales. The scalability of these technologies is well-suited for pairing with micro-grids, especially in remote regions where a connection to the grid is economically unfeasible. 

Traditional power plants, such as coal and nuclear, are not designed to ramp up and down quickly enough to address the fluctuating demand. By design, they operate most efficiently at full capacity and may suffer increased wear and tear if operated at lower power levels. These baseload resources rely on dispatchable power to match supply to demand over short timescales. 

To accelerate the adoption of wind and solar, some jurisdictions have enacted “always on” legislation that mandates that renewables are treated as baseload resources. This forces traditional power plants to ramp down during peak periods of excess renewable supply. Running these plants at partial power when they are designed for continuous base load can negatively impact the economic model of the businesses operating them. The lower efficiency from running at partial power can also increase greenhouse gas emissions. 

The demand for electricity fluctuates on differing timescales, adding additional complexity to the problem. In Ontario, the Independent Energy System Operator (IESO) instantaneously matches supply and demand every 5 minutes. On a daily timescale, electricity use tends to peak in the afternoon. Seasonally, peak demand is greater in summer than winter, as air conditioning consumes more energy than heating. Renewable solutions to match supply and demand must address the issue at all timescales. 

Currently, a mix of storage and non-storage solutions are utilized to match supply and demand. If renewable energy storage can be increased, it will take the place of gas-fired dispatchable power, allowing solar and wind to effectively transition from the intermittent category to baseload. 

Intermittency solutions – non-storage based 

Demand-side Management 

One strategy for matching supply and demand is simply to reduce the demand. Through better design many technologies can achieve greater efficiency and use less power. Variable electricity pricing – increasing pricing during peak demand and decreasing it in low-demand periods– can help shift consumption behaviour and smooth the demand curve.  

Unfortunately, the electrification of transportation and heating/cooling means that our demand for electricity is only going to grow over the short and medium term. Ontario’s Independent Energy System Operator (IESO) predicts the transportation sector’s energy needs for electrification alone are entering a period of demand growth – from 0.9TWh in 2023, to 26TWh in 2042 – an average of 20% growth year-over-year. IESO projects that if existing supply contracts are not renewed Ontario will begin to experience an energy shortfall by 2026. If existing supply is maintained, the shortfall is forecast to begin in the late 2030s. (2021 Annual Planning Outlook). 

Demand-side management can only slow rising demand, not arrest it. 

Gas-fired Generators 

Gas-fired generators, which typically burn methane, are a widespread solution. They have many positive attributes, but by burning fossil fuels they contribute to air pollution and climate change through greenhouse gas emission. 

  • They are quickly ramped and cycled, providing almost instantaneous response. 
  • While they are non-renewable, they may be cleaner in the future if they are combined with carbon capture and storage (CCS). 
  • Many large power plants, such as nuclear stations, already have gas generators installed on site already to provide operating power during outages. 

Co-location 

Solar peaks production midday when demand is highest. In contrast, wind peaks overnight when demand is low. These differing times of peak production complement each other and help to even out power generation when the technologies are used in tandem. 

Overcapacity 

Variable weather conditions mean that some wind and solar installations will always be offline. Overcapacity seeks to maintain production grid-wide by deploying a larger fleet than required to ensure that there are always enough sites online to meet demand.  

  • Expensive. 
  • Increases scale of electrification. 

Geographic averaging 

This approach is related to overcapacity. The sun is always shining somewhere, the wind always blowing. By deploying solar and wind over a widespread geographic area and averaging over the entire fleet the total generation can be made more predictable.  

  • Remote locations require investment in new transmission infrastructure. 
  • Climate and weather patterns are rapidly changing, decreasing long-term predictability. 

 

Storage-based Solutions. 

By sizing a renewable system to ensure there is a surplus generation capacity, the excess energy can be stored in batteries for dispatch when the system is producing less than the required demand. 

What do you think of when you hear the term “battery”? If you are like most people, you picture what you are familiar with – lead-acid car batteries, lithium-ion phone batteries, or the kind of alkaline batteries you use in a flashlight. However, a battery can take the form of any system that can convert energy into a stable, storable form for use later. In the case of the batteries above, these store and convert chemical energy into electricity. 

Chemical battery arrays are highly scalable, from residential systems with 10-15kWh capacity with enough to power a single house for several hours; to grid-scale factories with over a Gigawatt/hr of capacity, able to instantaneously dispatch 300MW of power – enough storage to power 300,000 homes for a few hours. 

Lithium Ion (Li-ion) Batteries 

Lithium-ion batteries are ubiquitous, used everywhere from smartphones to electric cars, to industrial energy storage. They have a high energy density and can store a large amount of charge compared to other options. They charge quickly but have a limited number of charge/discharge cycles before they degrade. 

  • Limited recharge/discharge cycles, total lifecycle of ~10 years 
  • Li-ion chemistry is flammable 
  • Expensive 

Flow batteries 

Flow batteries are an innovation that uses external tanks of liquid to generate electricity via redox (reduction-oxidation) reactions. By continuously pumping the liquids through a device containing electrodes separated by a thin membrane, electricity can be generated at the interface by the exchange of ions between the liquids. The liquids can either be recharged in situ, using surplus renewable electricity, or replaced with fresh electrolytes. Because the generation capacity is limited only by the surface area of the membrane, and the total run time is dependent on the size of the tanks used to hold the liquids, they are highly scalable. Power output can be modulated by adjusting the flow rate of the liquid past the electrodes. Many flow-batteries are based on the element vanadium, which is safer when compared to lithium. Current research is aimed at developing cheaper, iron-chemistry flow batteries. 

  • High capacity and scalable 
  • Safer than Li-ion (chemicals are unreactive, do not mix, cannot catch fire) 
  • Currently more expensive than Li-ion upfront, but their long lifecycle of 20-30 years makes them competitive 

Gas Production – Power to Gas (P2G) 

Surplus power can be used to produce either hydrogen or methane, which can be stored. Hydrogen can be created directly through electrolysis of water and utilized as is, or it can be combined with CO2 to produce methane. Methane is safer to store but produces CO2 when burned.  

  • Hydrogen can be used in fuel cells 
  • The methane gas can be used to power existing gas-fired generators 
  • Combined with carbon capture, this can potentially be developed to be carbon neutral 

Thermal Energy Storage (TES) 

Thermal energy storage uses excess power to heat up a reservoir of material, such as molten salt, or molten sand. Power is extracted by using the heat to generate steam and drive conventional steam turbines. The stored heat can also be used directly as a heat source for space heating. In a complementary concept, if the electricity is used to make ice, then it can serve to chill water for air conditioning use. 

Flywheel Inertial systems 

Flywheel storage uses surplus electricity to spin a large cylindrical mass to high speeds, storing the energy as angular momentum. Electricity can be regenerated by using the stored energy to drive a generator, which slows the rotation of the cylinder. Depending on the mass of the cylinder, minutes to hours of generation capacity are possible. 

Gravity Storage

This unique approach to uses surplus energy to power a crane that stacks large weights, such as concrete blocks, to convert electricity into gravitational potential energy. To extract the stored power, the weights are lowered in a controlled manner and used to drive a generator as they descend. To extend the power generation by extending the drop distance for the weights can be extended by utilizing underground vertical shafts, such as disused mines. 

A similar system has been proposed that works in reverse by reeling large inflatable balloons down to the seafloor. To generate electricity, their buoyancy can be utilized to drive a generator connected to their unspooling mooring cables as they rise. 

Pumped Water Storage 

Pumped water storage stores excess energy by pumping water to higher elevation. The system can either pump water from a flowing river to a reservoir or can be designed to pump water between a lower and upper reservoir. At its heart, the system is the same as a traditional hydroelectric system, using the energy of the falling water to turn a turbine. Pumped water storage is a mature technology that has been in use for decades. 

  • Closed-cycle with two reservoirs, or open-cycle located on a flowing river system 
  • Like hydroelectric power, the main challenges are regulatory and the geographic availability of suitable sites 

Compressed Air Energy Storage (CAES) 

Another pumped storage concept is compressed air energy storage. Surplus power is used to pump air into a closed cavern, such as an abandoned salt mine, raising its pressure. This high-pressure air can be released to drive a turbine, generating electricity on demand. 

  • Requires large airtight storage cavities, which are geographically limited, or expensive to create 

Vehicle-to-grid (V2G) 

The most innovative storage option, Vehicle-to-Grid bypasses the need to construct large, centralized battery storage. It relies on connecting electric vehicles (EVs) to the grid. Like traditional internal combustion engine vehicles, it is projected that EVs will be idle and parked most of the time. By designing vehicles with bi-directional charging, cars connected to the grid can be utilized as a giant distributed battery. During periods of peak demand, they can return their stored energy to the grid. While there have been pilot projects, bi-directional charging is not yet standard on all EVs. V2G would require upgrading our infrastructure to a smart electrical grid. EV owners may earn revenue by charging their vehicles during off-peak pricing periods and selling the power back to the grid at a profit during peak periods. 

  • Natural evolution – EV mandates are already in place 
  • The ability of EV owners to charge off-peak, then sell back to grid during peak pricing may act as an economic incentive to participate in V2G 
  • Bi-directional charging is not yet standard 
  • V2G requires an available charging station for every connected vehicle 

Conclusion 

A common objection to renewable energy technology is that it is not ready to act as the baseload power supply because of its intermittency. To complete the green energy transition, renewables must become the baseload power source. By combining renewables with energy storage this intermittency can be addressed. It will take us decades to meet the goals set forth at COP26 and the commitments to decarbonize our supply chains. All technologies will be required as we transition; however, solar and wind – combined with energy storage – are an agile solution that can be deployed now.  

There are many innovative solutions available for energy storage, some of which have been in use for several decades. The antiquated nature of the electricity grid means that it is currently possible for renewables to be curtailed during times of peak production as the grid does not have the capacity to feed in all the generated power. Increasing energy storage capacity not only addresses intermittency but can easily be paired with micro-grids to make locations energy independent. 

To achieve full electrification, the grid will need to be modernized and combined with energy storage. There are already diverse technological solutions for storing power. The future of baseload power is renewable. 

At VCT Group, we are focused on the design, engineering and deployment of innovative energy products that advance electrification. Talk to us today about how we can put the power of the sun to work for you. Powering everyone.  

 

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