Frequently asked questions (FAQs) about Renewable Energy
Renewable energy includes any form of energy that is derived from the sun. It is renewable in the sense that, on human timescales, the sun will shine for ever. The amount of energy from the sun is also very significant. The net solar power reaching the earth is more than ten thousand times humanity’s current rate of energy use from nuclear and fossil fuels and as much energy reaches the earth from the sun in an hour as is used by mankind in a year. Direct energy is collected by solar panels that convert solar radiation into hot water or electricity. Indirect energy comes from the wind, ocean waves and currents, and rivers, which depend for their existence on the movements of air or water driven by temperature differences. Plants, which can be burnt or ‘digested’, are the products of photosynthesis.
Tidal currents and geothermal energy are other forms of renewable energy. Tides draw their almost inexhaustible energy from the motion of the moon around the earth. Geothermal energy uses heat from the deep interior of the earth which often has been focussed into volcanic zones or comes from the radioactive decay of ancient granite intrusions.
Renewable energy has become very important for two reasons. First and foremost, the burning of fossil fuels is largely responsible for global warming and climate change which, if left unchecked, will lead to global average surface temperatures more than 2°C warmer than before the industrial revolution (mid-18th century). Policy makers have identified 2°C warming as the threshold of dangerous warming. Global warming of around 0.8°C is already leading to changing seasons, droughts and heat waves , heavy precipitation (floods), rising sea-level and more energetic storms in many parts of the world. Also fossil fuels (coal, oil and gas), on which our society depends for heating, electricity and transport, are finite. Oil and gas in particular are becoming increasingly expensive to extract as the easy-to-access reserves are worked out and globally the production of oil may already have peaked. Renewable energy has the great attraction that it draws on inexhaustible and free supplies of energy which means that, other than the cost of manufacture, installation and maintenance, it is not exposed to fluctuating prices.
As well as producing greenhouse gases fossil fuels produce considerable amounts of pollution when they are burnt. This includes oxides of sulphur and nitrogen as well as soot and ash. These emissions damage our health and our environment.
One further disadvantage of fossil fuels is that much of our oil, gas and coal has to be imported, which can be expensive and makes us reliant on other regimes for our sources of energy.
So today we are at a transition between a fossil-fuel driven society and a carbon-free society which will become largely dependent on renewables.
The main sources of renewable energy are solar panels, wind turbines, water turbines in rivers and the sea, devices to extract energy from waves, anaerobic digesters and geothermal energy in the form of heat from underground. Other than tidal currents and geothermal energy, these sources are derived directly or indirectly from solar radiation arriving at the earth’s surface. The different sources of renewable energy are described at greater length in other answers to these FAQs.
Years ago Hampshire had hundreds of water mills and tens of windmills but these mostly fell into disuse when coal became widely available.
Hampshire is a relatively large county with a coast and extensive downlands rising to 286m above sea-level. There are several rivers but none is particularly large. Hampshire, especially along the south coast, experiences some of the highest hours of sunshine in the UK. However, of all Hampshire’s renewable sources, wind has the greatest potential for generating electrical energy. Thus it should be no surprise that the greatest interest in, and most proposals for, renewables in Hampshire today relate to solar panels and wind turbines. Tidal currents are also being exploited off St Catherine’s Point on the Isle of Wight. Although river-based hydropower has been looked at the total potential in Hampshire is relatively low and those schemes that are currently going ahead appear to be more for the purposes of publicity and bolstering a ‘green’ image.
Solar panels that generate electricity depend on a special photovoltaic (or PV) effect. Two bonded layers of silicon, that have been ‘doped’ with small amounts of particular impurities, produce an electrical current and voltage when light strikes the junction between the layers. Silicon is the very common element contained in sand (silicon dioxide). Large numbers of these junctions are contained in a panel to produce a useful DC voltage and current which can then be inverted to generate mains AC voltage suitable for use. PV solar panels do not depend on accessing direct sunlight, they respond to scattered light, and so a southerly orientation is not essential although this remains the optimum direction.
Research is continuing to make solar panels more efficient in converting light to electricity and in reducing the cost to the point where PV electricity could become cheaper than fossil-fuel electricity. In the USA this has already happened.
Today solar panels are often installed on the roofs of houses and larger non-domestic buildings and in fields (solar farms). Because it is more efficient to consume the electricity on site, solar panels are particularly suited for installation on the roofs of offices, educational establishments, shops, hospitals and other buildings which have a fairly constant daytime demand. Solar farms are often more remote from consumers and so usually they feed into the national grid.
Modern solar panels that heat water usually contain a number of evacuated glass tubes each enclosing a pipe through which water flows. Specially treated black fins attached to the pipes absorb solar radiation and heat the water. For optimum operation the fins need to lie at right-angles to the sun and so the panels are ideally fitted to south-facing roofs or walls. The solar panels are part of a hydraulic circuit that includes a pump and a hot-water storage tank. When the temperature difference between the panel and the tank exceeds some pre-set value the pump switches on and circulates water. The tank contains a coiled pipe which acts as a heat exchanger. Usually the storage tank is shared with a separate heat exchanger connected to a gas boiler. Obviously the solar panels work best in summer but even in winter on a cold but sunny day some pre-heating of water occurs which gives a boost to the boiler system.
Wind turbines can be thought of as the more modern and efficient equivalent of a windmill. Windmills used a simple gearing arrangement to drive a shaft used to grind corn. Wind turbines often have gears at the top of the mast which amplify the speed of the slowly rotating turbine blades to drive a generator that produces electricity. Modern turbines are usually much taller than windmills to take advantage of the fact that wind speed increases with height above ground. This is especially important because the output of a turbine depends on the third power (cube) of the wind speed; in other words, if the wind speed doubles the output goes up by a factor of eight. However, to protect the turbine, above a certain wind speed the rated power output is kept constant to a point where, in a storm, the turbine is shut down altogether.
The output of individual wind turbines varies from a few hundred kilowatts (kW) up to 2-5 MW (1 MW = 1000 kW) with the largest turbines being sited offshore. A one MW turbine, for example, running on average for 27% of the time (to take account of less windy days and maintenance) will deliver around 2.4 GWh in a year, enough to support 600 average homes. Hampshire is favourably situated with regard to wind resources especially along the coast, offshore and on the downs. Nevertheless the number of operating turbines in the county is presently very small.
It is well known that planning applications for onshore, and even offshore, wind turbines may evoke vociferous opposition very often from those who live closest to the proposed site. It is very easy to attribute this opposition to nimbyism and indeed that may be at the source of some of the opposition. But frequently the only substantial basis for opposition comes down to arguments about the visual impact of turbines. Such arguments are based solely on personal opinions and are hard to counter – some people like to see turbines turning in the wind and the thought of the free electricity being produced whereas others apparently are appalled by the sight.
Arguments for wind turbines are;
· Turbines are a ‘mature’ or off-the-shelf technology.
· Wind turbines help the UK to reduce its carbon emissions by burning less fossil fuel in power stations.
· Wind turbines enhance the UK’s energy security because they reduce our reliance on imported fossil fuels.
· Wind turbines have one of the lowest lifetime emissions (g CO2/kWh) of all sources of renewable energy; the emissions are far less than for natural gas or coal.
· All areas of the UK should contribute to renewables as best they can – wind is one of Hampshire’s best renewable resources.
· Wind turbines replace fossil fuels and therefore reduce the pollution caused when fossil fuels are burnt.
· Compared to say a solar farm, wind turbines take up around one tenth of the land for the same output.
· Normally wind turbine planning proposals are for a fixed period (often 25 years) which means that at the end of its life a turbine could be dismantled and removed leaving little impact on the land.
· The price of wind energy is not exposed to global markets whereas the price of fossil fuels is expected to rise. Parity of wind and fossil fuel electricity prices is possible particularly if the cost of fossil fuels rises as it has in the past.
The principal arguments proposed against wind turbines are;
· Wind turbines are ‘inefficient’ because they don’t generate electricity continuously and so need expensive centralised back-up generation facilities. Efficiency of a turbine means how well it turns wind energy into electricity and the theoretical limit is 59% (for a car engine it is 37%). In practice a wind turbine has an actual efficiency of 33%. However more often this argument relates to the fact that a turbine doesn’t deliver electricity continuously at its rated value; the output is variable. See FAQs 9 [link] and 14 [link] for further discussion of this topic.
· Wind turbines only turn 70-85% of the time. Yes, but when they do turn they use a free fuel, the wind, and use it fairly efficiently (33%; see above).
· Turbines are noisy – this depends on circumstances. Specialist investigation of each site is required but normally a rotating wind turbine is considered to be ‘quiet’ by most observers standing a few hundred metres away.
· Turbines can have an impact on birds if sited on major migration routes and important feeding, breeding and roosting areas. The RSPB say they object to only about 6% of planning applications on this basis.
· Turbines are visually intrusive –fanciful claims have been made about turbines being visible from great distances but rational consideration indicates this would have to be with the aid of a powerful telescope, even very good eyesight, and a complete lack of intervening vegetation.
· Wind turbines receive subsidies paid for by electricity consumers. But so do nuclear power plants. Onshore wind farm will receive less subsidy than the new generation of nuclear power plants and for a shorter period of time. It is important to understand that in the past public money was provided for then new technologies such as nuclear power and continues to be provided today both to the oil and gas industries in the form of tax breaks and other supportive measures and to pay for the clear-up costs of nuclear power.
Wind turbines are sometimes said to be ‘inefficient’ because they don’t generate electricity continuously and so need expensive centralised back-up generation facilities. Efficiency of a turbine means how well it turns wind energy into electricity and the theoretical limit is 59% (for a car engine it is 37%). In practice a wind turbine has an actual efficiency of 33%. However more often this argument relates to the fact that a turbine doesn’t deliver electricity continuously at its rated value; the output is variable.
Other forms of electricity generation are not much more or less efficient. In the UK for example gas-fired power stations in 2013 achieved an overall efficiency of about 48% and a load factor of less than 30% i.e. it is used less than 30% of the time. The thermal efficiency of our nuclear power plants was even worse. In 2013 they achieved an efficiency of just 39.3% and a load factor of 73.8%. So older forms of generation are relatively inefficient at turning expensive fuels into usable electricity whereas a wind turbine is only slightly less efficient at turning a completely free fuel into electricity.
On average over a year an onshore turbine produces electricity equivalent to about 27% of what it would if it ran continuously at its rated value (the so-called load or capacity factor) and this is allowed for in planning and designing wind farms and calculating their costs and benefits. Many studies have been carried out of the amount, and type, of standby capacity required for wind. These studies generally conclude that, at least if wind energy contributes less than one fifth of the UK’s total electricity supply, there would not be a need for additional standby capacity to be constructed. However, other work shows that variable energy sources can be inserted into a grid at levels of more than one fifth of the total if more sophisticated systems of monitoring real time power demand and generation capacity are incorporated into the grid system. Such monitoring would reduce the amount of standby capacity required. So on less windy days, some thermal power stations will be required at certain times to operate at low outputs, and therefore not at their optimum efficiencies. But, if that is the price to be paid for reducing the burning of fossil fuels and the release of greenhouse gases into the atmosphere, many people think it is worth it (after B. Shorter, www.winacc.org). See the answer to FAQ 14 [link] for ‘what can be done about the variability of renewable energy?’
Hampshire has a number of small rivers that flow into the sea; from west to east these are the Avon, Test, Itchen, Hamble and Meon. The ability of a river to generate power for a mill or electricity generator at any point depends on the flow rate and the head or vertical fall in the river at that point. Although a number of small hydroelectric schemes exist, or are planned, on Hampshire’s rivers one study estimated the total potential from ten sites on the river Itchen to be around 600 MWh/year, enough to serve only about 150 average homes.
If organic waste, so called biomass, is digested under certain conditions it will generate biogas which can be used to generate heat or electricity or act as a source of hydrocarbons. This process can include the generation of gas from landfill but it is more efficient to use municipal solid waste (MSW) directly. Similarly wet feedstock (dung or sewage), when mixed with water, can be used in a temperature-controlled digester to also produce biogas. Although anaerobic digesters are a ‘mature’ technology they do depend on having a regular source of waste. They operate best in a combined heat and power mode. As is apparent from the above, many combinations of waste type and digestion products are possible.
Hampshire has a small number of operational agricultural and community combined heat and/or power (CHP) schemes (excluding water treatment plants).
Potentially the best areas for wave energy in the UK are off the west-facing coasts which are exposed to Atlantic waves. Hampshire’s coast lies too far up the relatively shallow English Channel to experience high-energy Atlantic waves and it is likely that it will be more rewarding to consider other sources of renewable energy.
Tidal energy is predictable, even years in advance, and varies with the state of the moon – the largest, spring tides occur at new and full moons and the smallest, neap tides occur half-way between spring tides. The height of the tide also depends on location – tidal height can be amplified in certain coastal locations according to the shape of the coast – as does the strength of tidal currents.
Tidal energy is accessed in two ways. The first depends on tidal height or range. If water is impounded at high tide in an enclosed basin and then released at a lower state of the tide, when a sufficient height difference exists, then this difference can be used to drive a mill or, using modern technology, a turbine. The Eling tide mill at the head of Southampton Water is one early example of this technique. As far as is known no other site along the Hampshire coast has been used, or even proposed, for the use of this method. The Hampshire coast has been rated as having relatively low tidal range potential.
The second way of accessing tidal energy is to install submerged turbines at locations where strong tidal currents occur. Suitable locations are where the tide flows in a constricted channel and around headlands. Two sites are known to be of interest in the Hampshire area, both off the Isle of Wight. One is 2.5 km south of St Catherine’s Point where it is estimated 50 GWh will be generated each year (enough electricity for 15,000 homes) by 2017/2018. The other site is close to Yarmouth where the flow is constricted between Hurst Castle and the Island. This site has been used to test prototype turbines.
Renewable energy that depends on solar or wind clearly will have variable output. Many studies have been carried out of the amount, and type, of standby capacity required for wind. These studies generally conclude that, at least if wind energy contributes less than one fifth of the UK’s total electricity supply, there would not be a need for additional standby capacity to be constructed. However, other work shows that variable energy sources can be inserted into a grid at levels of more than one fifth of the total if more sophisticated systems of monitoring real time power demand and generation capacity are incorporated into the grid system. Such monitoring would reduce the amount of standby capacity required. So on less windy days, some thermal power stations will be required at certain times to operate at low outputs, and therefore not at their optimum efficiencies. But, if that is the price to be paid for reducing the burning of fossil fuels and the release of greenhouse gases into the atmosphere, many people think it is worth it (after B. Shorter, www.winacc.org). The same arguments can be applied to electricity generated by solar panels and even by wave energy. As the capacity of renewables is ramped up beyond one fifth of the UK’s total generating capacity so progress will need to be made in developing the national grid and in matching supply and demand.
Another approach is to store surplus energy. This is addressed in FAQ 15 [link].
Storage of energy from renewables is of considerable importance since it can be used to smooth out any mismatch between supply and demand particularly as regards electricity. Energy can be stored over various timescales from minutes, hours and days to months or even between seasons. Energy can also be stored in a variety of forms such as potential energy (e.g. pumped hydro), kinetic energy (e.g. flywheels), as heat and as electricity. The long-term storage technologies tend to store larger quantities of energy and vice versa. The principal short-term (minutes to days) modes of storage are pumped hydro, flywheels, batteries, capacitors, hot water tanks, tidal waters, vehicles to grid (car batteries), compressed air, thermal storage and digital quantum batteries. Modes of energy storage over months to between seasons include hydrogen gas, biodiesel, synthetic methane, natural gas and underground water (Aquifer Thermal Energy Storage).
Clearly there is a range of ways to store energy depending on requirements and circumstances. Many of the above modes of storage are still in development whereas others, such as pumped hydro and hot water tanks, have been available for years. The challenge today is to maximise the efficiency of storage and to rapidly realise systems at a commercial scale.
Heat that arrives at or near the earth’s surface from the deep interior is described as geothermal energy. Heat is generated by radioactive decay mainly of uranium, potassium and thorium bound up in the rocks. In most parts of the earth this heat is very small, around one five-thousandth of the heat arriving from the sun. However, geothermal energy can be locally concentrated in volcanically active areas, such as Iceland or the Azores, or above large granite intrusions, such as in Cornwall. It is here that potentially geothermal energy can play an important role.
In Hampshire, and some other parts of the UK, deep sedimentary basins contain saline aquifers of warm water. One estimate put the resource of the Wessex Basin which underlies Hampshire at 33 GWthermal supposedly enough to heat 16.5 million homes. The Southampton District Energy Scheme has made use of such warm (ca. 76°C) water from a depth of 1800m under the Wessex Basin since the late 1980s. In 2011 the scheme was said to provide 23MW heat, 10.5MW chilled water and 24.1MW of electricity. The relatively low, but still significant, energy output may relate to difficulties in extracting heat at a rate sufficient to meet the demand. This is a problem common to many geothermal schemes.
The fact that the Wessex Basin geothermal resource has not been tapped elsewhere in Hampshire suggests that there are problems of cost or practicality in making use of this resource at present.
With many thanks to Bob Whitmarsh for the content of this page.