Solar & Wind
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First let’s consider photovoltaics, concentrated solar power and solar heat collectors. Solar energy is also stored indirectly in biomass, wind etc. but we’ll cover indirect forms separately.
You can view this post and the following posts more as a checkup how the technology works and how readily it supports decentralisation.
Solar cells
Solar cells convert energy from sun contained in light directly to electricity by the photovoltaic effect. Today best cells in R&D labs reach near 40%, in practice efficiency is around 20%. This is still a lot better than chlorophyll in plants (efficiency in plants being just a few percent).
Solar cells are typically connected together in a larger panel or array of panels to increase capacity. Silicon is the most often used material, alternatives like perovskite are under intense development. Silicon based materials generate electricity up to 18% of incoming solar energy but with some enhancements up to 23%.
Typical cell construction is such where p and n-type silicon are sandwiched together and a photon hitting causes an electron on n-side to loosen (exited) and a hole forms on the p side. The exited electron is directed through an electric circuit causing a flow of electricity. This current then can power up electric devices or its energy stored in batteries . Solar cells produce direct current (DC, non-alternating). Normally it needs to be changed into alternate current (AC) used in everyday devices by an inverter.
Price of solar cells has been dropping dramatically over a longer period.
A home scale solar systems main components are photo voltaic (PV) panels, installation kit, a control unit controlling the charging of batteries (MPPT charge controllers being most common), batteries and an inverter turning the DC into regular AC. In addition, there often is a control unit that the manufacturer uses to collect information of the system and present a user interface to user for following generation status, state of batteries, consumption and for controlling the system.
Commercial solar power stations contain large arrays of PV panels and much more advanced control and monitoring logic to detect faults and optimise the operations.
There are open-source designs for building solar systems and for solar tracking (keeping automatically the direction of panels optimal), but production of actual cells is technology intensive and does not really lend itself to decentralised manufacturing. In itself solar is a good solution for local power generation especially in areas with abundant sunshine. Also large areas that are currently uninhabited like hot deserts could house big solar generation setups, but in practice this is often not possible due to political instability. Also transfer of energy from generation to consumption may require turning the energy to chemical form. There is quite a lot of enthusiasm around hydrogen as a carrier. But other options like turning it to methane, petrol or ammonium are possible.
Solar also works well on rooftops that are otherwise unused.
On negative side, solar cells require a lot of space leading to environmental destruction.
Some emerging approaches however use polymer-based liquids to print PV cells on practically any surface. Their efficiency is however low compared to silicon-based cells and they remain a research and niche option.
Concentrated solar power
Another option to generate electricity from sun is concentrated solar power mirrors. They are used to collect sun’s rays to one point for heating some liquid (water, molten salt etc). This hot liquid is either directly or indirectly used to generate steam that turns a traditional steam turbine.
Main architectural choices are
Point where solar rays collected to one point. These can either be parabolic or solar tower where rays hit the top of the tower. Can use water or molten salts, hundreds of Celsius temperatures reached
Linear. These are longitudinal and they can either be parabolic (long open “pipe” collecting rays to one pipe containing oils to be heated) or Fresnell collectors. Fresnell collectors adjusting collection so that in the morning one collector receives solar radiation, in mid-day it is divided and in the afternoon another depending what is most favourable.
CSP power tends to be in the range of 100-150 MW. Apart from the heating mechanism they are normal steam engines. The solar collector replaces the boiler but there is a turbine, cooling system and pumps to ensure the flow of the liquid.
A third way to store solar energy (quite different) is Solar Thermal Advanced Reactor System (STARS). It uses solar heat to turn methane and water to syngas. The point is that syngas has higher energy contents than methane.
Heat collector
These are simple constructions where solar energy is used to heat up a liquid and that heat is the directly used or stored. As constructions there are pipes with liquid going into the collector, glass on top of the collector structure, absorptive material collecting heat at bottom. This heats the liquid. A solar heat collector can be active or passive. In active systems there is a pump circulating the fluid. The liquid is often a water glycol mixture.
The hot liquid goes from the heat collector to a heat exchanger and gets stored into a tank. Hot water can then be used from that tank to heat a house for example. A controller ensures proper operations.
With active systems the tank with hot liquid can be on top, with passive tank is below
One option for the heat collector is to store excess heat to the ground and use the stored heat in colder period of the year to increase the efficiency of a ground heat pump. Some systems “drive” heat directly to ground wells (some tens of meters deep) or it can also be used in an insulated sand storage.
Hybrid systems contain both a heat collector and PV cells generating both heat and electricity at the same time. This is advantageous because heat tends to decrease the efficiency of solar cells.
Wind
Wind is used for turning turbines to turn mechanical energy into electricity. The main components of a wind power unit are the tower, rotor (housing the blades), generator turning rotational movement into alternate current and nacelle.
There are several types of wind power stations, the most common one being Horizontal Axis Wind Turbine (HAWTs). Most often such power stations have 3 blades, more uncommon ones have single or dual blade. Taller units with bigger blades product more electricity. Size of wind mills has been growing rapidly (now 5 MW, biggest 8MW). World’s largest 16 MW. The tower can be as high as 250 m high with 100 meter long blades. The reason for size is that wind speeds higher are stronger and more even as there is more variation in wind speed near ground.
Thorntonbank Wind Farm, using 5 MW turbines REpower 5M in the North Sea off the coast of Belgium. Image Hans Hillewaert. CC BY-SA 4.0
The other option is Vertical Axis Wind Turbine (VAWT) where blades attached to top and bottom. Most common design is called Darrieus. These do not produce as well as HAWT
A Darrieus wind turbine once used to generate electricity on the Magdalen Islands by aarchiba. Public Domain
Life time of wind mill is 25 years, after that maintenance costs will increase too much. Generation capacity depends on location and wind speeds (contrast to solar that is available roughly everywhere). Often wind power stations are placed at sea, sea shores, hills, open fields etc.
Wind power is an intermittent energy source requiring control energy or ability to store excess for times when wind is not blowing. Most common control energy has been natural gas. This means that increase in wind power has directly meant long term commitment to fossil fuels for the duration of the unit (25 years).
In theory wind power stations can utilize 60% of energy, in practice it is closer to 50%. Units do not produce energy when wind speed is over 25 m/s and they require minimum wind speed or e.g. 3-5 m/s. Wind farms are remotely controlled and many newer units have adjustable blades increasing efficiency. Automation takes care of operating them.
Sandia National Laboratories has created an open-source design for a wind turbine. https://github.com/ckelley2/NRT It could be used as a basis for further creation of designs that fit the needs and skills of decentralized communities.
The main benefits of wind mills are that when the wind is blowing, the generated energy is cheap and during operations clean. They are also creating jobs wherever they are hoisted.
Disadvantages
Wind power also has quite a few disadvantages.
Good sites located far away from consumption. Lots of forest and other nature is cleared to make way for the power cables.
Blades impact local wildlife. The tip of the blade in large wind power stations moves faster than cars on motor ways and the wild life is not prepared for this. The blades kill large numbers of insects and different bugs. Small birds gather to feed on then just to be guillotined by the blades. Larger birds of prey get attracts to easy dinner when they see the dead bird and then it is their turn to be either killed or maimed by the wind power station.
Wind power stations also create noise pollution that disturbs the population living nearby. For best performance these tall structures are often placed at hill tops. This maximizes the area where noises penetrate. As well as humans, wind turbine noise (WTN) has detrimental effect on nearby wildlife harming. Planning guidelines today do not address these adverse effects. Comparing this to strict guidelines placed over other energy generation technologies like hydro or nuclear power shows how bias has affected legislative efforts.
In Finland and other cold countries freezing causes problems. That’s why they are equipped with diesel generators to prevent freezing when wind is not blowing.
For wind energy some of these ill effects are minimized when constructed at sea. Winds are strong and prevalent with little or no wild life or humans to be affected. For example, in Finland the coastal region in the west is well suited as it is very shallow long into the sea. In fact land is still raising from the sea in those areas albeit slowly. Placement at sea makes them however vulnerable to purposeful damage by an adversial actor as no one can guard the long transmission links un the sea bed.
There are also other parts of the world very suited for wind power. For example, in the East African Rift at lake Turkana winds are strong and consistent making it a perfect location.
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