Wietze Post on Energy: Base load, reliable supply and risk of generators

Base load, the constant demand for electricity, forms the backbone of a stable grid, vital for powering critical loads. Often met by coal and nuclear plants, Eskom’s unreliability has led to the rise of backup systems, primarily diesel generators. Yet, the global shift towards renewables, like solar and wind, backed by massive battery banks, is reshaping the energy landscape. As coal and nuclear units decline, a renewable-dominated grid proves more stable, offering a glimpse into a sustainable energy future.

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Post 3 of 4 – By Ir. Wietze Post

What is “Base load”?

Base load is the demand that’s always present, at a fairly constant level, day and night. It’s the least consumed portion of the total electricity demand.

In everyday talk, base load also refers to the power supply needed to meet the lowest grid demand. This creates confusion because power demand, and supply, are very different things.

At all times, satisfying the power demand with an equal or greater power supply is essential. This maintains a stable and reliable electricity grid.

I will refer to the power supply needed to meet the base load as the base power supply.

The load is the sum of all demands. The demand can come from a phone charger, pool pump, or aluminium smelter. Together, all the demands add up to the load on the grid.

Please note that we’re talking about power, not energy. Power is instantaneous and is measured in kW (kilowatt) or kVA (kilo-volt-ampere). Energy is an accumulation of power over time and is measured in kWh (kilo-watt-hour).

To visualise base load, we can think of all the things that run at night. All the street lights, homes, fridges, bakeries, supermarkets, and petrol stations. But also heavy industry, mines, smelters, factories, and many more. For some of these, the power supply must not be interrupted. Critical loads must always receive power.

To date, base power supply is usually provided by large power plants. They’re designed to operate continuously and efficiently at a relatively constant output. In South Africa, base power plants have included coal-fired, nuclear, and hydropower plants.

Coal-fired plants are designed to meet an average runtime of 80% to 90% over their entire lifespan. Nuclear plants aim for 95%. In South Africa, the average for the coal-fired fleet is now in the 50% to 60% range. The Koeberg nuclear units have been down for many months. I expect their lifespan runtime will be in the 80% to 85% range. The actual lifespan runtime achieved only becomes clear when retiring the plant.

Only after the plant has been cleared away can we calculate the actual lifetime cost per kWh. Clearing away the plant and remediation of the area can cost a great deal. Waste removal and storage of spent nuclear fuel can cost an extraordinary amount.

Eskom is no longer reliable.

Eskom’s supply capacity is less reliable than it used to be. The power plants, transmission lines, switch yards, and transformers – all are unreliable. Part of this is due to Eskom’s management. But many thieves and other rogue elements damage Eskom’s equipment. Together these aspects cause an unreliable power supply.

As a result, critical loads now have backup power supply systems at hand. These are usually diesel/gas generators. Hydropower reservoirs are also used for backup energy. 

Running diesel generators is expensive (in the region of R10-R20 per kWh). So the operators are turning to renewable energy plants. These are usually solar and wind farms that can cover vast expanses. The solar and wind generators are backed up by massive battery banks. This development is speeding up as solar, wind, and battery equipment becomes cheaper.

Solar, wind, and battery plants are becoming heavy industry’s prime energy suppliers. Diesel/gas generators and the grid back up the batteries. Batteries stationed close to the load guarantee power supply and quality. This transition is playing out globally.

When coal and nuclear units trip.

Coal and nuclear generation units are much more powerful than solar or wind units. Thus the impact on the grid of a sudden loss of a coal or nuclear plant is far greater than the loss of a renewable plant.

When a coal or nuclear unit trips, that’s an instantaneous loss of hundreds of MW from the grid. When a solar or wind power plant trips, it’s a loss of ±5kW to 1MW. The latter makes virtually no difference to the grid. That’s why a renewable-dominated grid is more stable than a coal-based grid.

Solar and wind-powered plants now usually incorporate (battery) storage to provide reliable power. Thousands of dispersed solar, wind, and battery plants connect to power grids. In SA too, although the scale is yet small. Thus the grid becomes more stable and reliable than with 10 to 40 coal-fired power plants.

Adding hundreds of coal-fired plants to the grid would improve stability and reliability. Unfortunately, a coal-fired plant’s typical unit size does not fit such an approach. Larger coal and nuclear units are more cost-effective than smaller units.

Of course, the system must be able to reliably supply far more than only the base load. The grid must supply the total demand 24/7/365. To date, that’s been done by a mix of large generator technologies. Those were: nuclear, coal, hydro, and gas/diesel turbines. Some plants run constantly, while others ramp up and down.

Gas/diesel turbines usually provide quick-response services. Batteries can readily do that too. Batteries are cheaper to own and operate than gas-peaker plants. Batteries can manage a lot of instantaneous services on the grid that gas plants cannot. Besides, batteries are quick to install. A further advantage of a battery is that a coal plant can charge the battery for later use. This allows the coal plant to run continuously instead of ramping up and down. Thus the use case for gas peaker plants has eroded. There’s usually no point in even considering a gas plant.

Energy consumers are moving towards cheaper sources of power. This invariably means a mix of renewable energy sources. Unfortunately, not all customers have space for a mix of renewable plants on-premise. A possible solution is to install solar power on the premises. They complement this by importing wind power from a wind farm in the vicinity. Besides the power supplies, they can install big batteries to buffer their energy.

A renewable energy system can adequately supply the facility during 95% to 99% of the time. 90% of the time it can export a large amount of excess energy to the grid. But how to ensure enough energy supply during the uncertain 1% to 5% of time?

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