Hydropower & Nuclear
Next: Geothermal and Heat Pumps
Hydropower
Capacity of hydropower dwarfs solar and wind today. It’s a very good control energy to balance intermittent solar and wind in places where it is available. Examples of large power plants include Itaipu in Brazil (76 TWh yearly generation) and Three Gorges (101 TWh yearly) in China.
Basic principle is to use the potential energy in the water level difference between before and after the dam. In the hydro plant potential energy turns to kinetic energy of flowing water that that turns turbines and generates electricity. Efficiencies are hight, somewhere between 80-90%. Hydro plants once built have low operating costs and are adjustable generation capacity. On the negative side the construction costs are high.
Hydro power plants can be categorized by regulation type. Run-of-river powered stations are facilities where flowing water of a river is channelled through a canal to spin a turbine. They typically have little or no storage facility. Regulation power plants are large systems where dam is used to store water in a reservoir. Electricity is produced by releasing water from the reservoir through a turbine that turns a generator. In pumped storage units there are two reservoirs. When there is excess of electricity, water is pumped from the lower reservoir to the upper one and released through a turbine when the need is biggest. They can act as one form of energy storage for intermittent electricity generation like wind and solar.
Tidal wave power plants can also be classified as hydroelectric power. The principle being different, utilising tidal variation in sea levels. Tidal stream generators are similar in principle to wind power, but the turbine is under sea. They utilize the flow of water as it rises and lowers. Tidal barrage units store water behind a high water and it gets used during receding tide. Tidal barrage power stations can also be built to bridges. Sometimes an artificial lagoon is built to store the water, then it can be called a tidal lagoon. Dynamic tidal power is yet another type that is still in research phase, none have been built so far.
For very small scale hydro-power projects there is at least Daniel Connelly’s design. and an open source tool from Oregon State University to assess potential generation capacity.
Turbines in hydropower are of three different types radial flow (pelton and turgo). Axial flow (Kaplan) and mixed flow (Francis)
Pelton is a jet turbine where falling water is led through a nozzle like in a jet. Its applicable to high heads, and is easy adjustability with high efficiency around 90%. Unit size typically up to <200 MW
Francis turbine are often installed vertically and are used in medium and large plants with unit size up to 800 MW. Typical efficiency 70-90%
Propeller turbines with adjustable blades are called Kaplan. These are axial turbines where water flows parallel to the shaft and have 3-6 blades typically. They are applicable to heads below 50 m and have unit size up to 400 MW. Efficiency 90% at power range 40-100% of capacity
Nuclear Power
In fission uranium u235 is split when a neutron hits its core and energy is released. The splitting of the U235 core releases 2-3 neutrons (depending on how the split happens – the core can split into different atoms). The released energy is in the kinetic energy of the released neutrons. This can lead to a chain reaction as these newly released neutrons hit new uranium atom nuclei in turn releasing more neutrons and so forth. The uranium atoms initially move at a high velocity and need to be slowed down to increase probability of a nuclear reaction. This can be done with light water, graphite or heavy water.
Chain reactions needs to be controlled to prevent overheating. This is done with control rods that absorb neutrons slowing down reaction. This can be done with light water or also graphite or heavy water (heavy water is a form of water that contains only deuterium rather than the common hydrogen-1 isotope)
Cooling in nuclear reactors can be done with many means such as light or heavy water, liquid metal, sodium, lead, quicksilver or molten salt. Based on the moderation substance and cooling media they get classified into different groups. PWR - pressurised light water cooled and moderated – is the most common design. BWR is a variant using boiling light water. PHWR uses pressurised heavy water. Finnish reactors are a variant called LWGT (light water graphite moderated) . GCR are gas cooled, graphite moderated reactors etc.
Natural uranium is mostly u238, only about 0,72% is u235 used in nuclear reactors. Natural uranium needs to go through enrichment process. Reaction grade is 3-4% of u235. Various techniques for this exist such as diffusion, centrifugal enrichment and laser methods. These are rather advanced technologies meaning that just a handful of countries have commercial enrichment plants.
Enriched uranium is made into pellets that are placed in airtight fuel rods. These in turn are made into bundles. A large commercial rector can have tens of thousands of rods when it is fully loaded.
In the nuclear sector a new trend is emerging where small modular reactors are made based on a productised design in contrast to previous method of designing each large installation separately. These small modular reactors (SMRs) are small enough that they can be manufactured in a factory and just transported to the final installation site. This allows all the benefits of mass production – lower cost, faster delivery times, fleet learning, skills transportable between units in operations and maintenance etc.
Benefits can also expand to regulators. Today large units are subject to a lot of demands in terms of documentation to a degree where one wonders if the true purpose is just to make construction more expensive. With small, standardized units the design is always the same and regulators could give type approvals rather than separate approvals for each instance. This is not true today as the regulators tend to be slow to changes.
Bigger units can be made from SMRs just by installing several small reactors on the same site. When one of the units has used all of its fuel, the whole thing can be transported back to the factory where fuel rods are removed. As the power station is potentially made of many such small reactors, there is no need to shut down electricity (or heat) generation at all. The only difference is a dent in production capacity due to removal of one unit. This is in contrast to large installations where the whole e.g., 1000 MegaWatt power station is shut down for the duration of maintenance.
There is nothing new in SMR as a principle. Similar small nuclear reactors have been built an installed into nuclear ice breakers since the 1970s but only a few units. SMRs may also become more common in ships in general. Benefit would be that with a single load of fuel, the ship can operate for 3-7 years using clean energy source. The designers can opt to use the energy amount to speed up boats. Now in order to meet environmental requirements, most ships are actually starting to sail slower.
Nuclear reactors are also finding new uses as a power source in transportation. For example a container ship cold could run for year without re-fueling.