The Arithmetic of Sustainability - Part 7: Beyond Fossil Fuels

The preceding analysis has established the contours of a sustainable energy supply for Britain: a mix of indigenous renewables (wind, tide, solar thermal), imported solar power, and dense nuclear fission. However, the arithmetic of generation is only half the challenge. Electricity from fluctuating sources (wind, solar) does not always match the moment-to-moment demand of consumers and industry.

This post addresses the fundamental issue of variability and the strategies required to manage it: storage and demand management.

I. The Fluctuation Problem

Wind power output is inherently variable. Actual production swings wildly: from near-zero during calm periods to full capacity during optimal conditions. Solar power follows a predictable diurnal cycle but is also subject to cloud cover.

1. The Scale of Variability

An examination of historical UK wind power data reveals the challenge:

• Wind variability: On calm days, UK wind power output can drop to less than 5% of installed capacity. This can happen for multiple consecutive days.
• Solar variability: Even in summer, UK solar output fluctuates significantly based on cloud cover. In winter, the daily duration and intensity of sunlight are drastically reduced.

UK wind output can swing from 5% to 90% of capacity within days

Any national grid relying heavily on wind and solar must have a robust mechanism to fill the gaps during periods of low renewable output and absorb surplus during periods of high output.

2. Options for Balancing Supply and Demand

There are fundamentally three approaches to managing the mismatch between variable supply and fluctuating demand:

1. Backup generation: Maintain standby power stations (typically gas turbines) that can fire up quickly to fill gaps.
2. Energy storage: Store surplus energy when production exceeds demand, and release it when demand exceeds production.
3. Demand management: Actively modify the timing of electricity consumption to match available supply.

Relying solely on backup generation from fossil fuels would undermine the sustainability goals. This compels serious consideration of storage and demand management.

II. Energy Storage: Technologies and Limits

Storing electrical energy at the scale required by a national grid is one of the most significant engineering challenges facing the transition to a sustainable energy system.

1. Pumped Hydroelectric Storage

Pumped hydro is the only proven, large-scale electricity storage technology currently available. It works by pumping water from a lower reservoir to an upper reservoir during periods of surplus power, then releasing the water through turbines to generate electricity during periods of high demand.

UK capacity: Britain’s existing pumped hydro facilities have a total storage capacity of approximately 30 GWh (enough to supply the national demand for only about half an hour at current consumption levels).

UK pumped hydro: 30 GWh storage—only 30 minutes of national demand

Expansion potential: Significant expansion of pumped hydro capacity in the UK is limited by geography. Suitable sites with large height differences and sufficient water capacity are scarce. Doubling or tripling current capacity would require major new infrastructure projects in Scotland and Wales, facing significant environmental and planning challenges.

2. Battery Storage

Batteries store electrical energy chemically. While technology is advancing rapidly (driven largely by the electric vehicle market), current battery technology faces significant constraints at the grid scale.

• Energy density: The best lithium-ion batteries store approximately 0.1 kWh/kg of mass.
• Cost: At current prices, providing even a few hours of national backup via batteries would cost tens of billions of pounds.
• Lifespan: Batteries degrade over time, requiring periodic replacement.

Battery storage is valuable for short-duration smoothing (minutes to hours) but is not economically viable for seasonal storage or for covering multi-day periods of low wind.

3. Hydrogen as a Storage Medium

Hydrogen offers a potential pathway for long-term energy storage, sometimes called “power-to-gas-to-power.”

The process:

1. Surplus electricity powers electrolyzers to split water into hydrogen and oxygen.
2. Hydrogen is stored (in tanks, underground caverns, or pipelines).
3. Hydrogen is later converted back to electricity via fuel cells or burned in gas turbines.

Efficiency penalty: The round-trip efficiency of this cycle is low:

• Electrolysis: ~70% efficient
• Compression/storage: ~90% efficient
• Fuel cell/turbine: ~50–60% efficient
• Total round-trip: ~30–40%

Hydrogen storage round-trip efficiency: only 30–40%

This means that for every 3 kWh of electricity stored as hydrogen, only about 1 kWh is recovered. This massive efficiency loss makes hydrogen better suited for specific applications (e.g., long-term strategic reserves, industrial feedstock) than for routine grid balancing.

III. Demand Management: Shifting When We Use Energy

If storing electricity is expensive and inefficient, an alternative strategy is to shift when electricity is consumed to better match when it is produced.

1. Smart Grids and Dynamic Pricing

A “smart grid” incorporates information technology to communicate real-time supply and demand conditions, potentially allowing consumers and automated systems to adjust behavior.

• Dynamic pricing: Electricity prices fluctuate based on supply. Consumers (or their smart appliances) shift non-time-sensitive loads (water heating, EV charging, laundry) to periods of low prices (high renewable output).
• Potential impact: Studies suggest that 10–20% of residential and commercial demand could be made flexible through smart grid technology and economic incentives.

2. Electric Vehicles as Distributed Storage

The transition to electric vehicles creates a large, distributed fleet of batteries. A national fleet of, say, 30 million EVs, each with a 40 kWh battery, represents a total storage capacity of 1200 GWh—far exceeding all pumped hydro.

30 million EVs with 40 kWh batteries = 1200 GWh distributed storage

Vehicle-to-Grid (V2G): Technology is being developed to allow EVs not only to draw power from the grid but also to supply power back. This could turn the EV fleet into a giant distributed battery, smoothing supply and demand.

Limitations:

• EVs are not always plugged in.
• Battery cycling for V2G accelerates degradation.
• Infrastructure and protocols for V2G are still developing.

Despite limitations, the EV fleet represents a significant potential resource for grid flexibility.

3. Industrial Demand Response

Large industrial consumers (aluminum smelters, data centers, chemical plants) can potentially adjust their power consumption on short notice in exchange for favorable electricity tariffs.

• Smelter flexibility: Aluminum smelting is highly electricity-intensive. Smelters can potentially reduce load for hours when grid stress is high.
• Data center load shifting: Computation can, in some cases, be scheduled during periods of low electricity cost.

Engaging large industrial loads in demand response programs could provide significant grid flexibility.

IV. The System-Level View: Balancing the Grid

Achieving a stable, sustainable electricity grid requires integrating all these elements: diverse generation sources, storage technologies, and demand management, orchestrated by intelligent grid management systems.

1. The Interconnected Grid

Britain’s connection to the European grid via interconnectors provides another layer of flexibility. During periods of low UK renewable output, power can be imported from continental sources, and vice versa. The planned expansion of interconnector capacity supports this balancing role.

2. The Need for Baseload

Despite advances in storage and demand management, there remains a fundamental need for reliable, dispatchable, baseload power that can operate continuously regardless of weather. Nuclear fission fulfills this role.

Renewables provide the bulk of energy over time, but nuclear (and potentially some gas with CCS as a bridge) provides the steady, predictable foundation upon which the variable sources can be layered.

V. The Bottom Line on Balancing

The transition to a grid dominated by variable renewables is possible but requires:

1. Massive investment in storage (pumped hydro expansion, grid-scale batteries, hydrogen infrastructure).
2. Deployment of smart grid technology and incentive structures to unlock demand flexibility.
3. Integration of the EV fleet as a distributed storage resource.
4. Retention of reliable baseload from nuclear power.
5. Expansion of interconnectors to leverage the diversity of the European weather system.

Without these complementary investments and policy measures, a sustainable energy system will remain an arithmetic impossibility.