Thermal generation can smooth the negative impacts of wind power intermittency
Rupert Soames OBE, chief executive of British power rentals supplier Aggreko, discusses the cost-effectiveness of installing power rentals over power plant purchases to provide back-up power for wind farms and other sources of renewable power generation.

Rupert Soames, Aggreko, UK

Rupert Soames, CEO of Aggreko
The daunting task of decarbonizing the way we generate electricity has become a global priority, as the world’s power producers look to make significant cuts in power plants’ CO2 emissions to avoid global climate change. Determined policies with effective implementation and tough enforcement during the next few years will determine the future environmental impact of our energy structure.

The task of decarbonizing power generation means power grids will require a far higher proportion of renewable energy capacity than they currently have. With the exception of renewable energy resources such as biogas and other unconventional gases, all other forms of renewable power generation depend heavily upon the forces of nature for the power plant’s own mechanical energy needs.

However, renewable energy resources such as hydro, solar, tidal and wind power all experience varying levels of intermittency. This sporadic nature does not mean they cannot be used; it means grids must manage this irregular supply as these natural resources vary throughout the year.

The instability experienced in hydropower tends to be focused around the El Niña and El Niño effects, which are felt most strongly in countries located between the Tropic of Cancer and the Tropic of Capricorn. These global regions have a wide variety of rainfall typically experienced throughout a five-year cycle in which they go through periods of ample rainfall and then periods where they do not have enough rainfall to keep their hydropower coffer dams at operating levels. The countries affected by these varying conditions are mainly in Africa, Asia and South America, where they often experience extended periods of a year or 18 months of inadequate rainfall. This shortfall in hydropower generation may occur every five or six years in some regions where hydropower is the main form of power generation.

The limit to solar power is clearly between night and day, although it also faces similar difficulties with generation capacity decreasing in the evenings and during cloudier parts of the day. Solar power is a perfect example of renewable power intermittency, with power generation available during the day but not after night has fallen.

Meanwhile, tidal power also has intermittency issues, with low power output occurring at slack water. This is the midway point between the spring and neap tides, when virtually no tide is running at all, so there is no power generation available. However, wind power has an entirely different nature as the patterns and strength of wind force are extremely unpredictable.

Power utilities can fairly accurately predict local wind conditions for a day or even a week ahead. However, while they can also average out wind availability over a year, in some weeks or months there are extended periods of high output and then extended periods of low input and these periods of low input often coincide with periods of high energy demands.

The problems of wind

Wind power becomes really hard to manage above a 10 per cent penetration. So if there is a power grid of, say, 10 000 MW and it has 3000 MW of wind power there can be swings of power capacity of anywhere from 0 to 3000 MW coming onto the grid. An abrupt swing of 3000 MW is a serious amount of swing which becomes extremely difficult to manage. This is a prime example of the need to effectively manage sudden drops in power output when the wind speed drops.

It is equally difficult to manage the strong surges in wind output when the gear units of the wind turbines cut out for safety. It is at these times that the utility must have some thermal generation to smooth the negative impacts of wind power.

One instant benefit is the elimination of high labour costs associated with plant operations and maintenance

What the utility will aim to do is to run the thermal power plant as little as possible, as thermal power has fuel costs whereas the marginal cost of wind production is zero. So the power utility needs to have a thermal power plant acting as backup that has certain characteristics.

These characteristics are, firstly, that the reserve power plant will need to have a low capital cost per MW as it is going to be operating for a relatively small amount of time in its reserve mode. This means that it will most likely have a load factor of around 10-20 per cent so the utility cannot afford expensive pieces of capital equipment.

Secondly, the reserve power plant must have fast ramp-up and shut-down capabilities. This capability for a fast start-up and stop for short periods means that the back-up power plant cannot be based on a gas turbine prime mover, which is kept running all the time for baseload purposes.

It is essential that the back-up power plant must have excellent part load acceptance characteristics and have the ability to be switched off and on when it is needed. It also needs to be flexible as the grid will not need to bring in 350 MW in one go. The grid will normally require 10, 15, 50 and then 100 MW brought on gradually, so part-load capability is critical.

We believe power rentals are well suited for this reserve power purpose as it has low capital costs per MW, as well as good part-load characteristics. Flexible fast starts mean 100 MW can be brought on in less than a minute so they are perfect for solving many of wind power’s capacity problems.

These problems include when there are extreme storm conditions with high wind speeds which are disastrous for wind power generation. At high wind speeds, wind turbines switch off their load carrying as the drives are made to disconnect to avoid serious damage to the wind power equipment. Wind power equipment disconnects when the wind speed is above 35-40 knots so it moves from full load to no power at all within seconds as the wind speed increases. Grid managers need to have virtually instant thermal power on hand so they can manage the intermittent nature of natural energy resources but not necessarily throughout the year.

Additional thermal power will be needed in peak periods such as the seasonal heavy demands for energy during the winter months so they need to bring in additional load to supplement the wind power. For this back-up power capacity requirement, temporary power rentals are ideal.

In most countries with winter peak demands, which often coincide with periods of very high or low wind speeds, grid managers need to have some short term resources on hand to provide additional power generation for when the wind is blowing too strongly or too weakly. Power transmission utilities need to carefully plan for the advent of renewable power in their networks as renewable power will be a principal source of power in years to come.

The benefits of power rentals

In recent years, Aggreko has seen its power rentals business increase significantly around the world. In the last few years the company has opened up in fast developing regions of the world such as China, India, Brazil and South Africa. There are many drivers of this swift growth, with the main ones being the urgent requirement for fast-track power supplies and the need for financial control over working capital.

Companies with urgent power supply problems need to have control of their capital expenditure while not neglecting their company’s operating needs. These factors are creating an increased interest in power rentals. The need for reliable energy to satisfy urgent demands is found across a broad range of industries, including renewable energy projects.

Temporary power rentals are already providing valuable support for both the construction and commissioning of many renewable power projects around the world. This supply of essential energy enables many renewable energy projects to be completed and commissioned on time and within budget.

The services that a specialist temporary power company offers power projects are not just for power generation; they are more to do with a range of energy solutions including climate control. This supply of power and climate control equipment offers a number of instant benefits as the equipment is delivered in a comparatively short time-frame.

The range of financial benefits to the customer includes the early completion of the project, release of capital for other core activities, an agreed price for power supplies over a given term and the ability to increase or decrease the size of the equipment at short notice.

Other instant benefits are the elimination of high labour costs associated with power plant operations and maintenance plus the purchasing and warehousing of the spares and other equipment that needs to be available at all times.

A power rental package also includes all of the other added value services such as technical advice, power plant design, installation, engineering, commissioning and decommissioning, in addition to operations and maintenance of the power plant. Financially, power rentals for both large and small renewable power projects are potentially an immediate benefit as they avoid the need for large capital expenditure. In most cases the financial benefits will compare favourably against the large fixed costs of a power plant that will tie up large amounts of working capital.

Hidden Costs and Human Capital

When a power company is deciding whether to purchase a power plant or to rent a temporary power plant, a power producer should first evaluate the many hidden costs that are involved with a power plant purchase. Insurance is one essential item that is often overlooked by power companies when they are assessing the commerical cost and viability of running a power plant.

The significant cost of fuel for power generation units is often overlooked when companies are considering their options for a reserve power plant. The process of continuously re-fuelling the generating units is an expensive and time-consuming task critical for stable power plant operations. Therefore, the use of a temporary power rentals specialist that includes fuel deliveries as part of its power rentals package is an attractive option.

A critical factor in the power rentals versus power plant purchase debate is the human capital issue. A power plant today, with its modern automation and control systems, requires a large number of skilled engineers and technicians for the operations and maintenance of the complex equipment so that it can be relied upon.

If the purchasing company does not have the skilled personnel with the technical knowledge to operate and maintain the power plant then they will have to hire new staff. However, these skilled people are hard to find and even harder to hire if the power plant is in a remote location. This staffing problem is solved with a power rentals company that supplies operations and maintenance services for the duration of the hire period.

There are many invaluable features that make the proposition of renting power supplies for supporting renewable power projects increasingly attractive, such an improved cash flow and the speed by which the much needed energy can be supplied to the renewable power project.

The decision to rent power for a renewable energy project simply means that the company transfers the costs and uncertainties and the risks of power plant ownership onto the power rentals company. This shift of liability allows the power producer to concentrate on its core business knowing they can be confident of continuous supplies of power generation, so all the renewable power producer does is switch on their equipment and get on with their business.

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