Hydropower

Overview

Hydroelectric power or hydropower technologies generate electricity from flowing water. In many ways, hydropower represents an attractive form of renewable energy. It has high reliability, high efficiency and the power generated can be regulated to meet the demand. Furthermore, the cost of electricity is the lowest among all renewable sources and the facilities are able to last for over 100 years with very low maintenance and operation costs.

By far, hydropower is the largest source of renewable energy currently in use. It accounts for 57% of the renewable energy produced. This in turn represents 16% of the world’s electricity and 6% of the global energy supply. Despite all of its advantages, the relative contribution of hydropower is likely to shrink in the future, because of its low abundance factor. It is the low-hanging fruit of the renewable energy world and has therefore already seen large-scale development. Water is only replenished so fast through the hydrological cycle and there are only so many places where the supply is large enough and at a sufficient elevation to justify the huge investment costs of building hydropower plants.

Types

All hydropower systems use the energy of flowing water to produce electricity. There are three broad classes of hydroelectric power generation: impoundment, diversion and pumped storage.

• Impoundment – Impoundment facilities involve storing river water in a reservoir and releasing it through a turbine to produce electricity as needed. It is the most common type used for large-scale hydropower.
• Diversion – Diversion facilities redirect the flow of all or a portion of a river through a turbine. They involve lower investment costs since there is no need to build a dam. However, without a reservoir to buffer the seasonally dependent water supply, the reliability factor is lowered and the electricity production cannot be regulated according to demand. This type is therefore mostly used for small-scale electricity generation.
• Pumped storage – A pumped storage facility moves water from a low reservoir to a higher one when power supply exceeds the demand. When the demand is higher, the water is released back into the lower reservoir through a turbine. This system is an economical means of storing energy and can be used to improve the reliability factor of other renewable energy sources, such as wind and solar. Impoundment facilities can also use this method for storing excess energy produced from other sources.

Placement

The number of sites that can be economically developed for hydroelectric production are limited. The ideal place to build an impoundment reservoir is on a river on sloped terrain. If there is a river but no slope a diversion facility can be built. Conversely, if there is a slope but no river a pumped storage can be built, provided there is a body of water available. With either of these natural resources hydropower is a possibility, even for domestic use. Though not everyone can have their own Hoover Dam, there are many areas where small hydroelectric plants can be cost-effective.

Reliability

An impoundment facility can have a reliability factor that exceeds even that of the local power grid. As long as the yearly supply of water is greater than the energy demand, and the reservoir is large enough to buffer seasonal supply variations, a hydropower plant can produce energy all year long. Likewise, a pumped storage facility can reliably generate energy throughout the year, as long as the reservoir never becomes empty. For this to work, whatever energy system is used to refill the basin – whether solar, wind or something else – needs to be sufficiently sized to meet the annual demand. Both of these types of hydropower plants can regulate the energy production to meet changes in demand within a matter of seconds.

The diversion type plant is much more subject to seasonal variations in river water flow than the impoundment type. It is also more subject to freezing, as even if the topmost layer of a reservoir freezes it is still able to produce energy. Over the course of a year, the capacity factor for a diversion plant may be 50% or less. In particular, during the summer months there will likely be less flow and therefore less power output. The peak energy season is the winter months, which is fortunate as energy demand is highest in this season.

Durability

Hydroelectric developments are long-term investments that can benefit several generations. Large impoundment facilities are typically designed to last 100 years and beyond, which makes it among the longest lasting of all renewable energy technologies. What will eventually cause a dam to fail is the accumulation of sediment at the bottom of the reservoir. This results in permanent loss of storage volume. For a small reservoir removal of the sediment can be economically feasible, but for large-scale facilities the cost i prohibitive. Therefore, the lifetime of a large-scale dam is decided when the dam is built by the dam’s storage volume. A way to economically extend the lifetime of a dam is to filtering out much of the sediment before it enters the reservoir and then diverting it to the downstream of the dam. As for diversion and pumped storage facilities, their lifetimes have virtually no upper limit, as long as the facilities are properly maintained.

Maintenance

While a hydropower plant has a high capital cost, the maintenance costs are fairly low – between 1.5% and 2.5% of the initial cost per year. Small-scale hydropower plants can function on their own for a considerable number of years, though at least quarterly maintenance is recommended. Typical maintenance tasks involves cleaning filters, removing debris and oiling moving joints. As equipment is worn out – such as turbines, generators, valves and control equipment – they need to be repaired or replaced. For example, a water turbine has a life expectancy of 50 years and the bearings need to be changed every 7-10 years.

Efficiency

By far, hydropower is the most efficient way to generate electricity through renewable means. Large scale water turbines are capable of converting 90-95% of the available energy into mechanical energy. Smaller turbines with output less than 5 MW may have efficiencies between 80-85%. To achieve efficiencies as high as this, the turbine must be carefully matched to the head (height difference) and flow of the system.

Price

Hydropower is the least expensive source of renewable energy for generating electricity. Building a diversion hydropower system can cost between $2 000 to $10 000 per kW, depending on the river’s flow rate and the slope of the terrain. The main equipment – including the turbine, generator and control systems – typically accounts for only 10% of the cost. Site development, including the building of the pipeline, make up the other 90%. Typically, the engineering work accounts for 70% of the cost and efforts to meet environmental criteria represents the remaining 20%.

As for pumped storage facilities, the cost is usually lower than for diversion facilities. Given suitable terrain – such as a natural reservoir situated above the open sea – the cost for a pumped storage facility may be between $500 to $2 000 per kW, depending on the vertical height and the distance between the lower and higher reservoir.

Performance

The electric power production for a hydropower plant can be approximated using the following formula:

• Effect = Flow Rate (kg/sec) * Head (m) * Turbine Efficiency (%) * Friction Factor (%) * Conversion Factor (%)

The Head is the vertical distance between the intake and the turbine. It is the least expensive way to increase power generation, as it has minimal effect on the turbine’s size. While the head remains constant once the system is built, the flow rate will vary over the course of a year. For a diversion type system, it may not be cost-effective to size the turbine for the maximum, flood-level flow rate. A high flow rate can compensate for a low head, but a larger and more costly turbine will then be required.

The Friction Factor comes from friction losses occurring in the pipeline, and effectively lowers the water pressure at the turbine. Longer pipelines and smaller diameters create greater friction. Since larger pipelines are more expensive, a good tradeoff is often to size the pipes so that no more than 10% of the energy is lost as pipeline friction. Finally, the Conversion Factor is the percentage of mechanical energy from the turbine that is turned into electricity. For a small scale system, friction in the drive system (5%) and generator losses (5%) typically results in another 10% of the energy being lost. Given these estimates, the combined water-to-electricity efficiency for small-scale hydropower is 80% x 90% x 90% = 70%.

As an example, say we have a sloped river that we want to start utilizing for hydropower. The most economical and environmental-friendly method would be to build a diversion facility. We want to place the facility on the section of the river that has the greatest slope, as this reduces the length of the pipeline and thereby the cost and friction associated with it. From the river, we divert part of the flow through a pipeline and down to the turbine on the facility. The reason only part of the river is diverted is so as to allow fish and other aquatic life passage. A filter at the entrance to our canal prevents anything but water from entering the pipeline.

In this example, we measure the head to be 50 meters and the average flow rate to be 30 kg/sec. The effect generated by this plant will then be: 30 kg/sec x 50m x 70% = 1050 watt. This yields 1.05 kWh * 24 = 25 kWh per day. If we assume that the river flows 50% of the year (capacity factor), the amount of energy produced will be around 25 kWh x 365 days x 50% = 4500 kWh per year.

Cost per Energy

To give a cost of energy estimate, the lifetime of the system needs to be determined. Since a hydropower diversion facility consists of multiple components with varying lifespans, keeping the system going requires that different parts are repaired or replaced every few years. Since the changes can be done without having to replace the whole system, the system can last virtually forever as long as this maintenance continues. The most expensive and longest lasting component of a diversion facility is the plastic pipeline, which is expected to last up to 100 years. When it fails it can be said that the lifetime of the system is over.

A small-scale 10 kW diversion facility in a favorable spot may cost $50 000 to build. Maintaining the system over its 100 year life, at a cost of 2% per year, will cost $100 000. This adds up to a total lifetime cost of $150 000. Assuming that the river runs for 50% of the year, the facility will produce 10 kW x 24 x 365 x 40% = 35 000 kWh per year, or 3 500 MWh over its lifetime. The cost per energy then becomes $150 000 / 3 500 000 kWh = $0.04/kWh. Typical cost estimations vary between $0.02 and $0.08 per kWh.

Advantages

• High reliability – A hydropower plant with a reservoir can generate power whenever needed.
• Adjustability – With a reservoir, energy production can be quickly regulated to meet variations in electricity demand.
• Storability – Water can be pumped into the reservoir to store access energy.
• High efficiency – Hydro is the most efficient means of generating electricity among all energy sources.
• Low cost – At $0.04/kWh, hydropower represents the cheapest renewable source of electricity.
• High durability – Hydropower plants are the only plants able to last beyond a century.

Disadvantages

• Potential flow shortage – The river’s flow may be lowered because of drought, climate change or diversions.
• Environmental effects – Reservoirs will flood the surrounding areas of land and prevent aquatic life from passing through.
• High initial cost – Building a hydropower dam represents a huge initial cost.
• Security risk – If a dam breaks, the flood unleashed will destroy everything in its way.