How Renewables Work

At Good Energy Generation, the renewables we source power from include wind, wave, hydro, solar and biogeneration. Although we work with over 650 independent renewable generators, we are only touching the tip of the iceberg when it comes to all the weird and wonderful types of renewable technology. In this section we start by explaining the basics of electricity and how it is made and we go on to look at how the technologies that we source power from (and a couple that we don’t) work.

What is electricity?

Electricity is a form of energy. Electricity is the flow of electrons. All matter in the Universe is made up of atoms, and an atom has a center, called a nucleus. The nucleus contains positively charged particles called protons and uncharged particles called neutrons. The nucleus of an atom is surrounded by negatively charged particles called electrons. The negative charge of an electron is equal to the positive charge of a proton, and the number of electrons in an atom is usually equal to the number of protons. When the balancing force between protons and electrons is upset by an outside force, an atom may gain or lose an electron. When electrons are "lost" from an atom, the free movement of these electrons creates an electric current.

How do you make electricity?

An electricity generator converts mechanical energy into electrical energy. The process is based on the relationship between magnetism and electricity. When a magnet moves across the surface of a conductor, often a tightly wound reel of copper wire, an electric current occurs in the conductor.  Within the workings of a wind, hydro or conventional turbine, the conductor remains stationary while the energy embodied in the wind, water or steam drives the turbine which rotates the magnet around the conductor and this creates an electrical current.

Biogeneration – a clean, renewable energy

When plant life grows it absorbs carbon dioxide (CO₂) from the atmosphere. When a plant stops living, it decomposes and eventually it returns the same amount of CO₂ to the atmosphere as it absorbed during its life.

This plant life or organic matter can be processed to create fuels that can be used to generate heat and power. This is known as biogeneration. When these biofuels are burned to create energy, they return the same amount of CO₂ to the atmosphere as if they decompose naturally. This means that properly managed biogeneration does not disturb what is known as the carbon balance and does not contribute to climate change.

There are two main forms of biogeneration used for providing electricity and or heat.

Biomass

Materials such as waste wood, straw and other crop residues and crops grown specifically for energy production such as willow and miscanthus grass are used as fuel in biomass systems. Having been harvested and dried, the biomass is burned at high temperatures. The heat produced is used to turn water into steam. This steam turns a turbine which generates electricity.
The most efficiency biomass systems are Combined Heat and Power (CHP) systems. CHP takes the residual heat from the electricity generation process and puts it to good use. The Centre for Alternative Technology (CAT) is installing a CHP system that will be fueled by waste wood and will provide electricity and heat for the site. The surplus electricity they generate will be sold to Good Energy. For more information see the case study at the end of this document.

Biogas

Biogas is produced through a process called Anaerobic Digestion (AD). This is a natural biological process carried out by bacteria in the absence of air, by which organic material is broken down into stable fertiliser and useful biogas. These anaerobic bacteria are an integral component of nature’s waste management and are commonly found in soils and deep waters, as well as in landfill sites.

How does Anaerobic digestion work?

The break down of organic compounds is achieved in a soup of many types of bacteria including those that generate carbon dioxide and methane.
The organic waste is broken down into sugars and amino acids by enzymes similar to those found in our mouths that help digest our food. These sugars are then fermented to produce volatile fatty acids and then converted by various types of bacteria into biogas, a mixture of carbon dioxide (40%) and methane (60%) and other trace elements.

While there are many ways of building anaerobic digesters (AD) the basic principle takes a feedstock that is fed into a completely enclosed tank, which is heated and regularly mixed.

What do you need to make Biogas?

You need what are known as feedstocks. Organic feedstocks are very flexible, ranging from farm manures and sewage sludge to catering wastes and food wastes comprising uncooked and cooked food, meat and bone. The feedstock then goes through the process of anaerobic digestion. This is the process of creating optimum conditions for bacteria growth. As time goes by the feedstock is digested (eaten) by these bacteria that in turn generate two byproducts - digestate and biogas.

The biogas is then ready to be burned in a combined heat and power unit to produce electricity and heat. Normally a biogas engine can gain an electrical conversion efficiency of up to 35% with the remainder being available as heat.

Hydropower

Hydraulic power can be captured wherever a flow of water falls from a higher level to a lower level.  This may occur where a stream runs down a hillside, or a river passes over a waterfall or man-made weir, or where a reservoir discharges water back into the main river.  As the water moves towards sea level as a result of gravity, its kinetic energy is consolidated. As the water flows through the turbine the kinetic energy is harnessed and used to turn the turbine and generate electricity.

Generating electricity from hydropower involves harnessing the kinetic energy contained in water that is being forced by gravity from a high place to a lower place. Humans have used hydropower for centuries and many of the micro hydro projects being installed today use the existing infrastructure of a Victorian mill, such as the original weirs and mill races for example and adapt it for electricity production. This is common practice because building, new infrastructure is often too expensive to make micro hydro projects viable.

Hydropower is a very constant, reliable and long-lasting technology (there are many hydro-electric turbines from the early 20th century that have been reconditioned and are in operation today). If you have a suitable body of water, you should consider hydro.

The vertical fall of the water, known as the “head”, is essential for hydropower generation; fast-flowing water on its own does not contain sufficient energy for useful power production except on a very large scale, such as offshore marine currents.  Hence two quantities are required: a Flow Rate of water Q, and a Head H. It is generally better to have more head than more flow, since this keeps the equipment smaller.
 
The Gross Head (H) is the maximum available vertical fall in the water, from the upstream level to the downstream level.  The actual head seen by a turbine will be slightly less than the gross head due to losses incurred when transferring the water into and away from the machine.  This reduced head is known as the Net Head.
 
Sites where the gross head is less than 10 m would normally be classed as “low head”.  From 10-50 m would typically be called “medium head”.  Above 50 m would be classed as “high head”.
 
The Flow Rate (Q) in the river, is the volume of water passing per second, measured in m³/sec.  For small schemes, the flow rate may also be expressed in litres/second where 1000 litres/sec is equal to 1 m³/sec.

Energy is an amount of work done, or the ability to do work, measured in Joules.  Electricity is a form of energy, but is generally expressed in its own units of kilowatt-hours (kWh) where 1 kWh = 3,600,000 Joules and is the electricity supplied by 1 kW working for 1 hour.
 
Power is the energy converted per second, i.e. the rate of work being done, measured in watts (where 1 watt = 1 Joule/sec. and 1 kilowatt = 1000 watts). 
 
Hydro-turbines convert water pressure into mechanical shaft power, which can be used to drive an electricity generator, or other machinery. The power available is proportional to the product of head and flow rate. The general formula for any hydro system’s power output is:
 
   P = h r g Q H   
 
Where:

• P is the mechanical power produced at the turbine shaft (Watts).
• h is the hydraulic efficiency of the turbine.
• r is the density of water (1000 kg/m³).
• g is the acceleration due to gravity (9.81 m/s²).
• Q is the volume flow rate passing through the turbine (m³/s).
• H is the effective pressure head of water across the turbine (m).
The best turbines can have hydraulic efficiencies of over 90% (higher than all other prime movers), although this will reduce with size.  Micro-hydro systems (<100kW) tend to be 60 to 80% efficient.
 
If we take 70% as a typical water-to-wire efficiency for the whole system, then the above equation simplifies to:
P (kW) = 7 x Q (m³/s) x H (m)

(Source BHA)

Wind Power

How wind turbines work

Most wind turbines consist of rotor blades which rotate around a horizontal hub. The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle houses the electrical components and is mounted at the top of the tower. This type of turbine is referred to as a 'horizontal axis' machine.

The rotor diameters of horizontal axis turbines range up to 80 metres, while micro wind turbines can have a rotor diameter of just 2 meters. Typically, wind turbines have three blades but some designs have, two or just one. The blades are made of fibreglass-reinforced polyester or wood-epoxy. They rotate at 10-30 revolutions per minute at constant speed, although an increasing number of machines operate at a variable speed.

Power is controlled automatically as wind speed varies and machines are stopped at very high wind speeds to protect them from damage. Most have gearboxes although there are increasing numbers with direct drives. Sensors are used to monitor wind direction and the tower head is turned to line up with the wind as its direction changes.

Towers are usually cylindrical, made of steel and generally painted light grey however some towers are constructed from a steel lattice framework. Towers range from 10 meters for small scale turbines to 75 meters for large scale turbines.

Commercial wind turbines range in capacity from around 100 kilowatts to over 2 megawatts. The crucial parameter is the diameter of the rotor blades - the longer the blades, the larger the area 'swept' by the rotor and the greater the energy output. At present the average size of new machines being installed is now super megawatt, 1.3-2.3MW, and there are larger machines on the market. The trend is towards moving to these larger machines as they can produce electricity at a lower price.

The most important factor affecting the performance of a wind turbine is the windiness of the site. Most wind turbines start operating at a speed of 4-5 metres per second and reach maximum power at about 15 m/s. The power available from the wind is a function of the cube of the wind speed. Therefore a doubling of the wind speed gives eight times the power output from the turbine. All other things being equal, a turbine at a site with an average wind speed of 5 meters per second (m/s) will produce nearly twice as much power as a turbine at a location where the wind averages 4 m/s. Another factor affecting performance is the availability of the equipment. This is the capability to operate when the wind is available - an indication of the turbine's reliability. This is typically over 98% for modern machines.

And a third factor affecting performance is turbine arrangement. Turbines need to be positioned very precisely to ensure they gain the maximum energy from the wind. This is so they harness the available wind resource to the best of their ability without causing ‘wind shadow’ or introducing turbulence to the wind that will then blow through the surrounding turbines. This may reduce productivity and increase wear on the wind turbines.

(Source BWEA)

Wave and Tidal Energy

The British coastline is over 11,000 miles long and experiences some of the highest tidal ranges in the world. The ebb and flow of these tides hold a huge amount of energy. Technology to harness this energy has been developed and now exists on a commercial scale. In comparison to wind energy technology marine renewables are still immature but it is attracting investment.

Wave power

The power of the waves is readily visible on nearly every ocean shore in the world. There has been much research to harness the power of these waves, and various machines have now been developed. These fall broadly into three categories:

1. Machines which channel waves into constricted chambers. As the waves flow in and out of the chamber, they force air in and out of the chamber. These airflows are in turn channelled through a specialised turbine, which is used to drive a generator. This type of machine is principally designed for use on or near the shore, or for incorporation into breakwaters. Commercially, this kind of machine is the most advanced and is particularly advantageous when incorporated into coastal protection. One such example is WaveGen

2. Fixed or semi-fixed machines which utilise the pressure differential in the water that occurs at a submerged point as the wave passes over that point. The pressure differential is used by a variety of means to cause a fluid to flow in a circuit, which is then used to drive a turbine and generator.
3. Machines which utilise their buoyancy to cause movement in a part of the device as it moves up and down in the wave. The movement is used either directly or indirectly to drive a generator. One such example is Pelamis.

Tidal stream energy

This involves using tidal energy that occurs due to large movements of water within in the sea which are caused by the gravitational pull of the moon. Tidal power is a more predictable and reliable source of renewable energy than some alternatives, such as wind or solar power, as tide movements can be calculated more reliably than weather. The department of Business Enterprise and Regulatory Reform has estimated that 10% of the UK's electricity needs could be met by tidal power.

1. Tidal power can use either conventional or new technology to extract energy from a tidal stream. It is usually deployed in areas where there is a high tidal range.

Typically a barrage with turbines is built across an estuary or a bay. As the tide ebbs and rises, it creates a height differential between the inner and outer walls of the barrage. Water can then flow through the turbines and drive generators. Some tidal barrages operate on both the rising and falling tide, but others, particularly estuarine barrages, are designed to operate purely on the falling tide.

2. It is also possible to make use of the tidal flow that occurs between headlands and islands or in and out of estuaries. It is this application that is the focus of much research and development, and new products for this purpose are now being commercialised. These “in-flow” tidal turbines can be arranged singly or in arrays, allowing a range of power outputs to be produced.

(Source REA) 

Solar Power

There are a number of ways of harnessing the sun’s energy. The more established solar technologies tend to be applied on a smaller, domestic scale but there are large scale commercial systems in operation. Solar power can be harnessed directly for its heat. The technologies below convert solar energy into electricity.

Solar PV

Photovoltaic (PV) cells convert radiation from the sun into electricity.  A typical PV cell consists of a wafer of semi-conducting material, usually silicon, manufactured with two electrically different layers.  When sunlight hits the cell it excites the electrons within the silicon, creating an electric field across the layers and causing a flow of electricity.

There are several types of PV cell. Monocrystalline cells are made from a single large crystal of silicon - they are more efficient and slightly better in low light conditions, but they can be more expensive. Polycrystalline cells are made from cast blocks of silicon that contain many small crystals and are slightly less efficient than monocrystalline cells. In practice, for a typical residential property, there is little difference in the performance of these different products.

A solar PV panel (or module) is made up of a series of cells and the greater the light intensity on the panels the more electricity will be produced.  A typical panel used in a residential PV system will have the capacity to generate between 160 and 190Wp (Watt-peak). A number of PV panels can be linked together to make a PV array.  So, for example, a system consisting of 10 x 180Wp panels would have a peak output of 1,800Wp – or 1.8kWp (kiloWatt peak).

PV can be assembled in panel form and mounted on properties or on a larger scale as a solar farm. it can also be applied to glass even to woven materials as used on the tents of the US military. A PV array generates direct current (DC) electricity. This current can be changed using an inverter to create Alternating Current (AC) electricity to synchronises the generation with the grid fed AC electricity.

Concentrating Solar Power (CSP)

Concentrating Solar Power (also referred to as solar thermal electricity) plants produce electricity in much the same way as conventional power stations: by raising steam to drive turbines and generators. Instead of burning fuel to create heat, the heat is generated by using mirrors to reflect sunlight which is directed at a tower. This concentrated sunlight is used to heat water to create steam and the steam is used to drive a conventional turbine that generates electricity.

This technology needs to be sited where ther is excellent solar resource such as in deserts. Estimates have been made that it would take just 1% of the worlds deserts to be developed with CSP plants to meet current global electricity demand. 

CSP can be stored overnight by holding the heat energy in a saline thermal store. It is cost competitive with nuclear and fossil fuel. It has great potential and sites have already develobed in the Mohave Desert in the USA and in the South of Spain. The is consideration of using the Sahara for CSP to supply Africa and Europe but the political and civil unrest is a sticking point. In addition to the benefits CSP can bring in terms of clean energy it can also deliver clean water. It is very feasible to use sea water in the system and desalinate it in the energy generation process, thereby delivering fresh water supplies to desert regions without incurring addition cost or pollution.